Digital Radiography Image Quality: Image Acquisition Mark B. Williams, PhDa, Elizabeth A. Krupinski, PhDb, Keith J. Strauss, MSc, William K. Breeden, III, MSd, Mark S. Rzeszotarski, PhDe, Kimberly Applegate, MD, MSf, Margaret Wyattg, Sandra Bjork, RN, JDg, J. Anthony Seibert, PhDh

This article on digital radiography image acquisition is the first of two articles written as part of an intersociety effort to establish image quality standards for digital and computed radiography. The topic of the other paper is digital radiography image processing and display. The articles were developed collaboratively by the ACR, the American Association of Physicists in Medicine, and the Society for Imaging Informatics in Medicine. Increas- ingly, medical imaging and patient information are being managed using digital data during acquisition, transmission, storage, display, interpretation, and consultation. Data management during each of these oper- ations has a direct impact on the quality of patient care. These articles describe what is known to improve image quality for digital and computed radiography and make recommendations on optimal acquisition, processing, and display. The practice of digital radiography is a rapidly evolving technology that will require the timely revision of any guidelines and standards. This document provides a basis for the technologies available today in clinical practice and may be useful in guiding the future clinical practice of digital radiography. Key Words: Digital radiography, image quality, image acquisition J Am Coll Radiol 2007;4:371-388. Copyright © 2007 American College of Radiology

INTRODUCTION AND DEFINITIONS There are several types of detectors that can be classified as DR systems, including automated (cassetteless) CR Computed radiography (CR) and digital radiography (DR) systems. In addition, some DR systems are adapted to a are the commonly used terms for digital radiographic cassette-based x-ray system. Thus, the historical nomen- detectors. Computed radiography uses a photostimu- clature becomes less accurate as technology advances be- lable storage phosphor that stores the latent image with cause distinct classification into the two broad categories subsequent processing using a stimulating laser beam and of CR and DR is no longer possible. More appropriate is can be easily adapted to a cassette-based system analo- the distinction based on cassette vs cassetteless operation. gous to that used in screen-film (SF) radiography. His- In this article, digital radiography is used to refer to all torically, DR has been used to describe a digital x-ray types of digital radiographic systems, including both imaging system that reads the transmitted x-ray signal those historically termed CR and those historically immediately after exposure with the detector in place. termed DR. There are many ways to categorize the current state- aDepartment of Radiology, University of Virginia, Charlottesville, Va. of-the-art digital radiography technology. One categori- bDepartment of Radiology, University of Arizona, Tucson, Ariz. zation considers (1) form factor, (2) image acquisition cDepartment of Radiology, Children’s Hospital, Harvard University, Boston, time, and (3) x-ray signal conversion methodology. The Mass. concept of cassette-based vs cassetteless operation is de- dSt. Vincent Hospital, Indianapolis, Ind. fined using the term form factor. A cassette-based digital eDepartment of Radiology, Case Western Reserve University, Cleveland, detector uses the SF paradigm, which allows the use of Ohio. existing imaging modality infrastructure and provides fDepartment of Radiology, Indiana University, Indianapolis, Ind. excellent positioning flexibility. On the other hand, the g American College of Radiology, Reston, Va. labor-intensive handling of cassettes and the need to wait h Department of Radiology, University of California, Davis, Sacramento, for the image, often with batch-mode processing, leads to Calif. a loss of time efficiency. Cassetteless operation indicates Corresponding author and reprints: J. Anthony Seibert, PhD, University of California, Davis, Department of Radiology, 4860 Y Street, Suite 3100, Sac- the ability to acquire the x-ray signal and, without sub- ramento, CA 95817; e-mail: [email protected]. sequent user intervention, produce an image at a local

© 2007 American College of Radiology 371 0091-2182/07/$32.00 ● DOI 10.1016/j.jacr.2007.02.002 372 Journal of the American College of Radiology/Vol. 4 No. 6 June 2007 workstation for review and manipulation. By image ac- 5. the easier and more reliable generation of accurately quisition time, a distinction is made regarding the ability labeled and identified image data; to acquire an image nearly instantaneously over the full 6. the ability to electronically transmit data to an ap- field of view for a large area detector vs the sequential, propriate storage medium from which it can be elec- longer scan time acquisition of a slot-scan device. X-ray tronically retrieved for display for formal interpreta- signal conversion refers to the method by which the x-rays tion, review, and consultation; and are converted to the output signal of interest and is usu- 7. the ability to transmit data to and from remote sites ally described as being either an indirect or direct radia- for consultation, review, or formal interpretation. tion detection scheme. All digital detectors produce an Components of the performance of high-quality digi- output signal, usually in the form of electrons, which tal radiography include, but are not limited to: represent a quantity of charge that corresponds to the number of x-rays absorbed in a given detector element 1. the development of validated imaging protocols so (del) in the detector. The charge is then converted to a that the consistency of image quality and radiation dose can be established and maintained between digital value for storage in the image matrix. Indirect rooms and between sites; refers to the conversion of x-rays into secondary informa- 2. the use of appropriate compression of image data to tion carriers, such as light via a scintillator, before con- facilitate transmission or storage, without loss of version to charge. Direct refers to the conversion of x-rays clinically significant information; into electron-hole pairs and the direct collection of 3. the archiving of data to maintain accurate patient charge using semiconductors as the x-ray converter, with- medical records in a form that may be retrieved in a out a secondary conversion event. There are advantages timely fashion; and disadvantages that can be ascribed to each of the 4. the ability to retrieve data from available prior imag- digital detectors in these broad categories, and they de- ing studies to be displayed for comparison with a termine the best solution for a given clinical task or current study; imaging application. 5. the ability to apply image processing for better dis- This article is applicable to the practice of cassette- play of acquired information; based and cassetteless digital radiography. It defines mo- 6. adherence to applicable facility, state, and federal tivations, equipment guidelines, specifications of data regulations; manipulation and management, and quality control and 7. maintenance of patient confidentiality; quality improvement procedures for the use of digital 8. minimization of the occurrence of poor image qual- radiography that should result in high quality radiologic ity; patient care. It also includes a brief review of the differ- 9. minimization of the delivery of inappropriate ioniz- ences between digital radiography and SF radiography in ing radiation dose to patients; and terms of imaging characteristics such as sensitivity, dy- 10. the promotion of clinical efficiency and continuous namic range, noise, and spatial resolution. A brief over- quality improvement. view of current receptor technologies available for the clinical practice of digital radiography also is included. A GENERAL REQUIREMENTS glossary of commonly used terminology is included in the companion paper on image processing and display. Image acquisition should be performed in accordance Motivations for using digital radiography in clinical with the appropriate ACR modality or examination practice include, but are not limited to: guideline or standard. At the time of acquisition, the system must have capabilities for capturing demographic 1. the significantly larger range of x-ray intensities that as well as imaging information such as accession number, can be imaged by digital receptors compared with patient name, identification number, date and time of analog systems; examination, name of facility or institution of acquisi- 2. the independence of displayed contrast from the tion, type of examination, patient or anatomic part ori- x-ray tube potential setting (kVp) through adjust- entation (eg, right, left, superior, inferior), amount and ment of the display window width; method of data compression, and display of the total 3. the independence of displayed brightness from mil- number of images acquired in the study. This informa- liampere-second (mAs) setting through adjustment tion (except for the number of images acquired) must be of the display window level; associated with the images when transmitted. These 4. the availability of image processing and computer- fields should be formatted according to the Digital Im- aided diagnostic algorithms for image enhancement aging and Communications in Medicine (DICOM) and analysis; standard. Williams et al/Image Acquisition 373

The ability to record patient date of birth and gender costs for film materials and labor. The chief advantages and a brief patient history is recommended. The ability are high spatial resolution, consistency of image appear- to record peak kilovoltage (kVp), tube current (mA), ance, and the accumulated experience of radiologists, exposure time, beam filtration, and radiation exposure medical physicists, and technologists in utilizing SF ra- indicator is imperative, particularly for cassette and cas- diography in an optimal manner. setteless digital radiography systems that have a direct Screen-film images can be converted to a digital form interface to the x-ray generator information. The use of using a film digitizer to produce an equivalent digital the DICOM DX object is recommended instead of the image. The characteristics of the output digitized image more limited CR object for digital radiography. It is also are chiefly those of the SF image, with its inherent ad- recommended that gray-scale presentation state objects vantages and disadvantages as listed above. Two major be used to transmit annotations, shutter, and display digitizer types are based on laser point scanning and lookup tables (LUTs). optical line transmission scanning, using a light collec- tion guide or an array of charge-coupled device (CCD) COMPARISON OF SCREEN-FILM AND photosensors, respectively [1]. In general, these digitizers DIGITAL RECEPTORS provide good gray-scale rendition, with the laser systems typically having more accurate conversion of high optical There are several fundamental differences between SF density values and the CCD array scanners able to have a systems and digital (CR or DR) systems in terms of the more compact, higher resolution scanner capability. physical processes involved in image acquisition. The Film digitizers should output the DICOM radiographic different processes introduce different constraints on the imaging information object descriptor with patient and factors determining image quality, such as spatial resolu- acquisition method demographic data and must meet the tion, contrast, and noise. Here we briefly describe the minimum spatial frequency resolution requirements of basic operation of each system and then discuss the most 2.5 lp/mm at the film plane for conventional digital important acquisition-related factors affecting image radiography images. In general, film digitizers introduce quality for each. additional noise in the digital image, especially in regions of high optical density. Screen-Film Computed Radiography In SF radiography, an x-ray photon absorbed in the screen deposits energy, which is converted into the pro- In photostimulable storage phosphor (CR) systems, x- duction of multiple visible light photons that travel ray photons are absorbed in the storage phosphor, also through the screen and in turn deposit energy in the film known as the imaging plate (IP). Unlike conventional grains in the emulsion layers of the film. The image screens, the deposition of x-ray energy in storage phos- information is stored in the film emulsion during an phors results in the storage of a portion of the energy in exposure through the accumulation of exposed grains. highly localized, metastable areas called f-centers, which Regions of the film with a greater density of exposed serve as energy wells. During an x-ray exposure, the im- grains have higher optical density and appear dark after age is built up in the phosphor through the accumulation film processing and thus correspond to regions of greater of f-centers. After the exposure, plate readers scan a (red) x-ray absorption in the screen. However, the relationship laser beam over the surface of the IP. In one implemen- between optical density and the number of absorbed tation the laser beam sweeps across the IP (scan direction) x-rays is nonlinear. For analog SF radiography, the acqui- and in combination with IP translation (subscan direc- sition, display, and storage of the image are intrinsically tion) the beam extracts the latent image from the IP. The nonseparable. This means that acquisition techniques plate reading time depends on the size of the detector and must be used that result in both acceptable film darken- the scan speed of the reader. Some newer readers use a ing while maximizing image contrast. Because the range line-scan approach (the laser illumination on the plate is of acceptable optical density is limited by view box lumi- a line) rather than a point-scan approach to increase nance and the response of the human visual system, max- readout speed, but the principle is the same in both imization of image contrast occurs at the expense of ex- approaches. At each location, the energy of the incident posure dynamic range. This is referred to as the exposure laser photons triggers a deexcitation of the f-centers, re- latitude, and it is quite narrow in SF radiography com- sulting in the prompt emission of (indigo) light photons. pared with digital radiography. A portion of these photons is detected by a photosensitive Limitations in SF radiography include narrow expo- device (usually a photomultiplier tube for point-scan sure latitude, the need for chemical processing, ineffi- readers or a linear solid-state photodiode array for line- cient mechanical handling, incompatibility with elec- scan readers), whose output is then digitized and stored. tronic transmission and image enhancement, and higher Although the amount of photostimulated light detected 374 Journal of the American College of Radiology/Vol. 4 No. 6 June 2007

is proportional to the number of f-centers and thus to the edge of the array for digitization and storage. The entire number of absorbed x-rays at that location (ie, the rela- detector is read out by successively turning on all of the tionship between the photomultiplier tube signal and the rows in a sequential fashion and storing the digital data in number of absorbed x-rays is linear), nonlinear amplifi- corresponding locations in the output digital image ma- cation of the photomultiplier tube signal before digitiza- trix. tion is often used. The resulting raw pixel values are subsequently processed for display using a combination Advantages and Limitations of Digital of segmentation, rescaling, and filtering algorithms. The Radiography size of the image matrix is a function of the IP dimensions The main limitations of CR and DR are higher initial ϫ ϫ ϫ ϫ (18 24 cm, 24 30 cm, 35 35 cm, 35 43 cm, and cost (especially for DR), a lack of familiarity on the part custom sizes) and the pixel sampling pitch (typically 100 of both radiologists and technologists with electronic ␮ to 200 m in general radiography). Output image sizes image display and with online soft copy reading (com- vary between 8 and 16 MB for radiography with a typical pared with alternator-based batch-mode reading), and ϫ matrix size of approximately 2,000 2,500 pixels. A the lack of consistent feedback to technologists concern- detailed description of technical issues associated with ing the use of optimal acquisition techniques. The latter CR imaging is given in the report of American Associa- problem, along with the much larger dynamic range of tion of Physicists in Medicine Task Group 10 [2]. digital systems, has led to a gradual increase in patient radiation dose, an issue discussed in more detail below DR Systems [6-8]. The advantages of CR and DR include the separa- DR systems encompass a number of different technolo- tion of acquisition, display, and archiving, allowing tre- gies that are rapidly evolving. At this time, the majority of mendous flexibility using image-processing functions DR systems use thin-film transistor (TFT) arrays, com- such as those that adjust the level (analogous to the monly known as flat-panel arrays. Systems based on brightness) and window width (analogous to the con- CCDs are available for general-purpose and chest radi- trast) of the image gray-scale presentation. However, dis- ography, using scanned, slot-shaped detectors [3-5];a play contrast is limited by the inherent image signal-to- single CCD; or tiled arrays of CCDs optically coupled to noise ratio (SNR), because as the signal contrast is larger area x-ray-to-light converters. Thin-film transistor increased, so is the visibility of noise. Other advantages arrays are composed of a matrix of discrete dels, each of include: anatomy-specific presentation and disease-spe- which contains a transistor. The transistors operate as cific algorithms; in most cases better x-ray detection effi- gates, permitting an electric charge to flow through them ciency and higher detective quantum efficiency (DQE), only when they are turned on. During an x-ray exposure, permitting lower doses to patients; the ability to use a the gates are turned off, and the image is built up in the second computer reader to assist the radiologist; a reduc- dels in the form of an electric charge, with the amount of tion in the number of image retakes due to underexpo- charge in each del proportional to the number of x-rays sure or overexposure; and the elimination of labor-inten- absorbed in that region of the detector (again, a linear sive handling and distribution of images during the relationship). The means by which x-ray energy is con- acquisition process. verted to a stored electrical charge varies somewhat be- tween manufacturers but can be broadly classified ac- FACTORS AFFECTING IMAGE QUALITY cording to whether it involves an intermediate conversion of x-ray energy to visible photons (indirect Just as with SF radiography, a number of factors affect flat-panel devices) or not (direct flat-panel devices). In an the quality of the image in digital radiography. Contrast, indirect device, an x-ray-to-light converter, similar to detail, and noise are the primary factors associated with that used in SF imaging, is placed in contact with the image quality, and they play a major role in CR and DR. TFT array. Each del of the TFT array contains a light Some additional factors are the result of the digital nature sensor (a photodiode) to convert the fluorescent light to a of the process, and they are discussed below. stored electric charge. In a direct device, a layer of mate- Detection Efficiency rial is deposited directly onto the TFT array. When an x-ray photon is absorbed in this photoconductor mate- For SF, CR, and DR, the efficiency with which incident rial, an electric charge is generated and collected in the x-rays are absorbed is determined by the absorber thick- dels of the TFT array. For both indirect and direct flat- ness, density, and composition. Efficiency can be in- panel detectors, after the x-ray exposure, the TFT array creased by increasing material density or absorber thick- gates are turned on one row at a time, and the amount of ness (usually at the expense of spatial resolution) or charge stored in each del of that row is transferred through the incorporation of materials having atomic through drain lines to a row of charge amplifiers at the numbers that provide a good match between the x-ray Williams et al/Image Acquisition 375

the charge-holding capacity of the dels. When this capac- ity is filled, the detector must be read out to remove the charge before any subsequent exposure. The dynamic range of typical TFT arrays used in general radiography, with del sizes of 150 to 200 ␮m, is approximately 3 to 4 orders of magnitude (a ratio of 1,000:1 to 10,000:1) [11]. Spatial Sampling All digital detectors sample the continuously varying x- ray fluence at their input at discrete locations separated by some interval called the sampling pitch. In CR sys- tems, the sampling pitch is the distance between adjacent laser beam positions during plate readout. In DR sys- Fig 1. The photon absorption fractions for computed tems, the pitch is the center-to-center spacing between radiography and rare-earth x-ray phosphors are plot- dels. The spatial frequency at which sampling occurs ted as a function of energy. Phosphor thicknesses are determines how well high-frequency fluctuations in x-ray representative of a standard 400-speed conventional fluence (those that occur over short distances in the im- screen, a standard resolution computed radiography aging plane) are imaged by the digital system. The sam- 2 phosphor detector (100 mg/cm ), and a cesium io- pling frequency of a system with sampling pitch p is 1/p. dide phosphor commonly used in indirect thin-film For example, if the pitch is 100 ␮m (0.1 mm), the sam- Ϫ transistor array and optically coupled charge-coupled pling frequency is 10 per millimeter (10 mm 1). The device camera digital radiography systems [2]. highest spatial frequency in the incident x-ray fluence that can be reliably imaged (the Nyquist frequency) is spectrum exiting the patient and the absorption charac- half the sampling frequency, or 1/(2p). Thus, a system ␮ teristics of the material. Figure 1 shows the absorption with 100- m pitch can image frequencies up to 5 cycles efficiency of 3 materials commonly used as x-ray scintil- per millimeter. If the x-ray fluence incident on the recep- lators, plotted against incident x-ray energy [2]. tor contains information at frequencies higher than the Nyquist frequency, and the presampling modulation Dynamic Range In digital chest radiography, the ratio of the maximum to minimum x-ray exposures incident on the detector sur- face can be greater than 100:1 [9]. For high-quality dig- ital radiography, the receptor must be able to maintain good contrast resolution over this wide dynamic range. The dynamic range of an x-ray imaging system is the ratio of the largest and smallest input x-ray intensities that can be imaged. For all systems, the smallest useful intensity is determined by the intrinsic system noise. The signal must be large enough to exceed this noise, com- bined with the x-ray quantum noise. The largest intensity is determined by receptor saturation. The dynamic range of SF systems, often referred to as the latitude, is deter- mined by the loss of contrast at low and high exposure levels (the toe and shoulder, respectively, of the H&D Fig 2. The characteristic response of a 400-speed curve). For SF systems, the useful dynamic range is de- rare-earth screen-film (solid S-shaped curve) and the pendent on the screen and film characteristics, ranging computed radiography photostimulable phosphor from approximately 10:1 to 100:1 [10]. By comparison, (PSP) detector (dashed curve) are compared. Expo- the response curve of CR is linear (the photostimulated sure ranges indicated by double arrows roughly indi- light output is directly proportional to x-ray exposure) cate the exposure ranges characterized as underex- over approximately 4 orders of magnitude (a ratio of posed, correct, or overexposed. Useless areas depict 10,000:1) [2]. Figure 2 compares the response curves for system responses that do not contain information a 400-speed SF system with that of a typical CR system useful for diagnosis because of either excessive [2]. The dynamic range of DR systems is determined at quantum noise at the low-dose end of the scale or the low end by the system noise and at the high end by saturation of the PSP detector [2]. 376 Journal of the American College of Radiology/Vol. 4 No. 6 June 2007 transfer function (MTF) is nonvanishing at these fre- In CR imaging, the primary source of spatial resolu- quencies, the result is false image signals occurring at low tion loss is scattering of the laser light beam during image frequencies. This phenomenon, called aliasing, produces plate readout. Laser light scatter results in the deexcita- large-scale artifacts in images and reduces the ability to tion of locations in the phosphor that are somewhat see fine details in the images. larger than the size of the laser beam and potentially larger than the separation between laser positions. As a Spatial Resolution result, the actual blur extends beyond the pixel size. Be- Spatial resolution is the ability of an imaging system to cause thicker phosphors result in a greater scattering allow two adjacent structures to be visualized as being depth and more blur, CR systems have been introduced separate, or the distinctness of an edge in the image (ie, that permit the plate to be read from both sides [12]. sharpness). Spatial resolution losses occur because of Also, CR systems with a structured crystalline geometry blurring caused by geometric factors (eg, the size of the allow a greater thickness and enhanced detection efficiency x-ray tube focal spot, light diffusion in the receptor), del without the corresponding loss of spatial resolution [13]. effective aperture size, and motion of the patient relative Spatial resolution in DR systems depends primarily on to the x-ray source and image receptor. The measure- two factors. The first factor for indirect systems is the ment of spatial resolution is performed by subjective or spread of light photons in the x-ray-to-light conversion objective methods. Subjective measurement is achieved process, and this can be important. This results in blur- with a bar pattern of alternating radio-opaque bars and ring similar to that described above because of the spread radiolucent spaces of equal width, imaged to determine of photons within the screen of an SF system. To mini- the limiting resolution in line pairs per unit of distance, mize visible photon spread, several manufacturers of in- usually expressed in units of line pairs per millimeter. direct DR systems use structured converters, in which the Intrinsic detector resolution measurements are per- converter material (usually cesium iodide) is formed into formed by fixing the bar pattern to the receptor surface to narrow, parallel columnar structures, oriented so that the eliminate focal spot blurring. System resolution (includ- x-ray photons are incident along the long dimension of ing the effects of the focal spot blur) uses a bar pattern the columns. The advantage of this structure is that the placed at a clinically relevant distance above the receptor. majority of the light exiting the converter onto the TFT For digital systems, the resolution may be different in the array has been internally reflected along the length of the row and column directions, often requiring separate eval- columns. The internal reflection process thus confines uations. The limiting resolution is the frequency at which the light photons to a region close to the actual location the bars and spaces can no longer be visualized. Objective of the x-ray absorption. This approach permits improved measurements by the MTF are obtained by measuring absorption efficiency through thicker absorbers (longer the transfer of signal amplitude (contrast) of sinusoidal columns) without as much spatial resolution degradation patterns (of various frequencies) from incident x-rays to as seen in unstructured converters constructed using the the output. The system MTF is determined by the prod- same thickness. Direct-conversion DR systems do not uct of the individual MTF components along the signal suffer from this effect, because the spread of the electrons chain. The system MTF can be measured by imaging a within the photoconductor material as they are acceler- test object containing a narrow slit or a sharp edge. For ated toward the TFT array is minimal. most digital radiography examinations (except possibly The second factor affecting spatial resolution in DR is digital fluoroscopy), a limiting system spatial resolution the size of the del itself. Because all x-rays absorbed Ϫ of at least 2.5 mm 1 is essential and higher resolution within a single del during an exposure contribute to a Ϫ (e.g. 5.0 mm 1) is desirable for specialized applications. single quantity (the summed charge read from that del), In SF imaging, spatial resolution is determined pri- there is no way to distinguish different absorption loca- marily by the thickness of the screen, because the light tions within a del. Therefore, structures in the patient produced when an x-ray photon is absorbed is emitted in smaller than the del size are smeared out, and their con- all directions from that point. Thicker screens provide trast is reduced (this is known as the partial volume more distance for the light to spread before reaching the effect). Such structures may thus be undetectable unless film and have poorer spatial resolution. The film has very they are inherently high contrast objects (eg, a microcal- high resolution and is not responsible for spatial resolu- cification smaller than a del may be recognized as a cal- tion degradation. A conventional 400-speed SF system cification because its attenuation properties are so differ- for general radiography has a spatial resolution limit (the ent from the other tissue in the del). smallest visible set of bars in a bar pattern image) of Ϫ Noise approximately 7 lp/mm (7 mm 1). In mammography, in which very thin screens are used, the limiting resolu- In radiography, noise can be defined as any fluctuations Ϫ tion is often greater than 15 mm 1. in an image that do not correspond to variations in the Williams et al/Image Acquisition 377 x-ray attenuation of the object being imaged. Image noise For example, an 8-bit analog-to-digital converter en- is typically measured by illuminating the receptor with a codes 28 or 256 discrete digital values (0-255) over the uniform x-ray fluence, then measuring the variance (the signal amplitude range (these values are often called an- square of the standard deviation) in selected regions of alog-to-digital units). An insufficient number of quanti- the resulting image. A more informative measure of noise zation steps will introduce signal-encoding errors that can be obtained by estimating the noise power spectrum increase the quantization noise. Digital detectors for pro- (NPS), which characterizes the spatial frequency depen- jection radiography typically use from 10 to 14 bits dence of the noise [14-16]. Knowledge of the frequency (1,024 to 16,384 unique analog-to-digital units) in the response of noise in an imaging system is important be- output image to keep quantization noise small. Some cause there are a number of additional noise sources in systems use nonlinear (eg, logarithmic) amplification be- digital radiography, such as aliasing and electronic noise, fore digitization, which can further reduce the quantiza- that are not present in SF systems. tion error, particularly in the low output signal response Ideally, image noise is dominated by the x-ray quan- range of the detector [17]. tum noise. However, all image receptors contain some Scatter radiation is another source of image degrada- internal sources of noise. Random noise (noise not cor- tion in radiography, and it acts in the same manner as related with particular locations on the receptor) typi- other noise sources. Scatter reduces the available dynamic cally cannot be corrected and adds to the random noise of range of x-ray intensities at the exit side of the patient. the x-ray quanta. Examples of random noise include film The primary effect of scatter is the reduction of subject granularity noise in SF and electronic noise in CR and contrast. In addition to reducing subject contrast, scatter DR. Internal noise that has a fixed correlation to loca- decreases the SNR because it contains no signal but does tions on the receptor is called fixed-pattern noise. Sources contain Poisson quantum noise. Scanned slot DR detec- of fixed-pattern noise include spatial variation in screen tors possess inherent scatter rejection capability and do thickness in SF, position-dependent light collection effi- not require the use of a grid. Area detectors used in CR, ciency in CR plate readers, and variations among pream- DR, and SF radiography should be used with an antiscat- plifier gains in DR. One advantage of digital imaging is ter grid in clinical imaging situations in which scatter that fixed-pattern noise can be largely eliminated dominates (usually patient thicknesses in excess of 10 through digital postprocessing. This is typically per- cm). They are often not used with pediatric and extrem- formed by obtaining a set of nominally identical images ity imaging. The use of grids is especially important in using a uniform x-ray flood field. Averaging the flood CR because of the increased scatter sensitivity of barium images results in a single image in which the magnitude halide (k-edge approximately 35 keV) compared with SF of the random noise is small compared with the magni- tude of the fixed-pattern noise. This image is stored and screens such as gadolinium oxysulfide (k-edge approxi- used as a map to remove the fixed-pattern noise from all mately 50 keV) (see Figure 1). When using stationary subsequent images in a process called flat fielding. Detec- grids (eg, snap-on grids used in portable x-ray and for tor artifacts are common in digital systems as a result of lateral tabletop exposures), consider the use of high-fre- spatial variations in the sensitivity of the receptor, caus- quency grid strips of greater than 60 lines/cm (140 lines/ ing the image to have structure that is unrelated to the in) to avoid grid aliasing patterns caused by insufficient tissue being imaged. Bad dels, typically aligned in col- sampling. Consultation with the vendor for an appropri- umns and rows, may occur during the manufacturing ate grid is recommended. process. The locations of such point and line defects are When using scatter grids with digital detectors, any typically identified and stored, and adjacent neighbor adjustment in exposure technique (mAs) should be made response values are averaged and used as substitute values in the interest of maintaining SNR. Insertion of the grid for the missing data. The number and proximity of de- reduces both the signal (incomplete transmission of the tector defects that is allowable without affecting image primary radiation by the grid) and noise (removal of quality remains to be specified. scattered radiation), with the relative amount of reduc- Digitization of the analog detector output voltage to tion dependent on both the scatter-to-primary ratio in form discrete pixel values introduces a type of noise called the beam (higher for larger patients) and the design of the quantization noise. Digital imaging systems use an ana- grid. This is different than in SF radiography, in which log-to-digital converter to sample and quantize the image upward mAs compensation is required both for the re- data into discrete integer (gray-scale) values, with the moval of primary and scattered radiation to achieve the range of values determined by the number of electronic correct film optical density range. For this reason, the binary (on/off) channels within the ADC, also known as adjustment of radiographic technique upward according bits. The maximum number of gray-scale values that can to the Bucky factor (defined as the increase in exposure be encoded is equal to 2#, where # is the number of bits. required to obtain the same optical density as with no 378 Journal of the American College of Radiology/Vol. 4 No. 6 June 2007 grid) overestimates the appropriate technique increase for digital receptors. Contrast Contrast (radiographic contrast) is proportional to the magnitude of the signal difference between the struc- ture of interest and its surroundings in the displayed image, which is expressed in terms of the optical den- sity difference between two adjacent areas on the SF film or as the relative brightness difference between the corresponding areas in a digital image displayed on a monitor. For both SF and digital imaging, radio- graphic contrast is influenced by subject contrast and receptor sensitivity. However, in digital imaging, con- trast in the displayed image can also be altered by the adjustment of display parameters independent of the acquisition parameters. Subject contrast is propor- tional to the relative difference in x-ray exposure on the exit side of the patient and is the result of the attenuating properties of the tissues under study. At- Fig 3. Detective quantum efficiencies of screen-film, general-purpose computed radiography (GP-CR), tenuation is strongly dependent on the x-ray energy high-resolution computed radiography (HR-CR), and spectrum and is determined by the target material, selenium-based digital radiography (DR) systems. kilovoltage, and total beam filtration. Subject contrast The detective quantum efficiencies (DQEs) were mea- is further reduced by the presence of scatter. Receptor sured using 4 different standardized radiation quality sensitivity is defined as the amount by which the out- beam qualities, with higher radiation quality (RQA) put (optical density for SF or analog-to-digital unit number corresponding to higher average beam en- value for CR and DR) changes per unit change in ergy [23]. exposure to the receptor. In digital detector design, sensitivity can, to a certain extent, be selected through electronic gain settings, but this must be done with an Detective Quantum Efficiency awareness of the range of exposures that must be si- multaneously measured, taking into account the de- The DQE is a useful descriptor of image receptor perfor- tector response energy dependence. With a properly mance because it takes into account detection efficiency, designed image acquisition system, the dynamic range spatial resolution, and noise. The DQE describes the should be adequate to measure the entire range of relative efficiency in maintaining the signal-to-noise level intensities from that of the unattenuated beam outside available in the imaging process and is defined as 2 2 2 the patient to that through the densest, thickest part of SNR out/SNR in, where SNR in is the SNR of the expo- the anatomy. The signal stored in digital form is di- sure incident on the receptor, numerically equal to the rectly (or logarithmically) proportional to the amount input fluence. SNRout is the ratio of the relative magni- of radiation transmitted through the patient. For this tudes of the image signal to the noise. Thus, DQE can be reason, the stored for-processing image reflects the considered the system’s SNR transfer efficiency. Efficient inherent subject contrast very faithfully, provided the SNR transfer is important because the detection perfor- image receptor is operated in the linear (or log-linear) mance of both human and ideal observers improves with portion of its response curve. increasing SNR [18-22]. For CR and DR, image processing (usually proprietary The DQE is generally plotted against spatial fre- to the device’s manufacturer and not under control of the quency, DQE( f ). A perfect system has a DQE( f )of radiologist) is used to determine display contrast by es- unity at all spatial frequencies. Real systems lose effi- tablishing the relationship between raw pixel values and ciency at higher spatial frequencies because of decreasing gray-scale levels. This produces the final for-presentation spatial resolution and increasing noise sources. DQE( f ) image for interpretation either on hard-copy film or soft- is determined by quantitative measurements of the mod- copy displays. Discussion of image processing, including ulation transfer function, MTF( f ), to determine the contrast enhancement and spatial frequency processing, spatial resolution and the noise power spectrum, is presented in the companion paper on image processing NPS( f ), to determine the noise characteristics, both as a and display. function of spatial frequency, using an accurately known Williams et al/Image Acquisition 379 input exposure to the detector. The exposure required to EXPOSURE INDICATORS FOR DIGITAL achieve a given SNR in the image is inversely related to RADIOGRAPHY the system DQE. Therefore, systems with higher DQE Currently, most CR manufacturers provide exposure indi- require less exposure for a given image quality (SNR) cators on images displayed on the image reader workstation. than systems with lower DQE. The DQE is important in However, this information may not be transferred over to CR and DR because these technologies can offer signifi- the picture archiving and communication system (PACS) cantly improved DQE compared with SF systems. How- or may appear buried in the DICOM information page of ever, it is important to remember that the DQE is an the patient. Most PACS have the ability to display the ex- energy-dependent quantity. Figure 3 compares the DQE posure indicator on the image. Check with your PACS of a general-purpose radiographic SF system with those vendor and insist that this information be displayed on each of a general-purpose CR system, a high-resolution CR image. Today, the various numerical exposure indicators system, and a DR system. Plots are shown for 4 different used by different CR manufacturers are not easily compara- standardized radiation quality x-ray beams, with the ble. To compound the problem further, some DR manu- higher radiation quality number corresponding to higher facturers do not provide an exposure indicator on their pro- beam energy. Figure 3 demonstrates that the DQEs of cessed images, or the information is not transferred to the the CR and DR systems decrease compared with that of PACS and is not recoverable. SF with increasing beam energy. As pointed out by the The American Association of Physicists in Medicine study authors, this suggests that automatically increasing formed Task Group 116 to address the lack of a uniform the beam energy when switching from SF to digital radi- exposure indicator on digital radiography images. Their re- ography may not always be the most appropriate course port, “Recommended Exposure Indicator for Digital Radi- of action [23]. ography,” will propose standardized radiation exposure DOSE CREEP IN DIGITAL RADIOGRAPHY conditions from which a relative exposure indicator can be AND ITS PREVENTION defined. Because the major manufacturers of digital radio- graphic image receptors participated in the task group, this Film-based imaging provides immediate feedback to tech- standardized exposure indicator should migrate into their nologists and radiologists concerning the delivery of the future products. See exposure value estimate in the glossary in proper radiation exposure to patients. If the optical densities the companion paper on image processing and display, in the film image are too great, the patient received too which summarizes several of the exposure indices currently much radiation, whereas reduced optical densities indicated used by different manufacturers. a reduced radiation exposure relative to the proper value. When digital image receptors replace SF image receptors, PEDIATRIC IMAGING ISSUES the brightness and contrast in the displayed image on the monitor are independent of the exposure used during acqui- As with all pediatric imaging examinations involving ion- sition. Only the noise in the image changes with the expo- izing radiation, CR and DR should be performed using sure. Radiologists react negatively to digital images with the lowest possible radiation exposure to the patient (as excessive noise (low patient exposure) but rarely complain low as reasonably achievable [ALARA]), because pediat- about images with reduced noise due to excessive patient ric patients are believed to be up to 10 times more sensi- exposures. Technologists quickly recognize this and gradu- tive to ionizing radiation than adults [25]. This suggests ally increase patient doses by adjusting radiographic tech- that the exposure delivered to the digital image receptor niques upward. This leads to the potential for dose creep in should be approximately 0.4 to 0.8 mR for chest and CR and DR [6-8]. The medical physicist and/or imaging abdominal examinations [26]. However, it should be specialist should monitor for dose creep on a consistent, noted that adopting a policy of reduced exposure levels to ongoing basis, because it is a recurring phenomenon. One the image receptor to achieve ALARA in pediatric imag- effective way to eliminate dose creep is to develop validated ing increases the possibility of unacceptably high quan- radiographic technique charts for all performed examina- tum noise and thus reduces the margin for error in the tions as a function of patient size. If these factors are entered selection of the parameters that affect the beam quality. into the anatomic programming of state-of-the-art x-ray Also, the achievable reduction in exposure at the plate is generator controls, the technologists simply select the radio- dependent on how well the digital system is optimized logic examination and patient size at the beginning of the for pediatric imaging and on the nature of the particular case and are assured of the use of standard radiographic disease being diagnosed [27]. technique factors and standard radiation exposure regardless of Commitment and effort on the part of staff members whether the examination is conducted with the image receptor is required to achieve ALARA in pediatric imaging. First, on the table top with manual techniques or in the Bucky tray the multiple x-ray machines of a given type in a depart- using the automatic exposure control (AEC) [24]. ment require validation, calibration, and matching to 380 Journal of the American College of Radiology/Vol. 4 No. 6 June 2007 ensure the same exposure for similar radiographic tech- which the wide exposure latitude makes it possible to obtain nique parameters. The actual exposure required to pro- clinical images that are severely underexposed, resulting in duce clinically acceptable images for a given digital recep- noisy images, or grossly overexposed, resulting in possible tor is a function of the digital receptor’s DQE. The goal data saturation as well as high patient doses. Improper ex- is to produce similar image quality in all images regard- posures in digital radiography are more likely to go unno- less of the device chosen and regardless of the size of the ticed because image processing can correct these deficiencies pediatric patient. Second, all CR readers or DR image to a much greater degree than SF systems because of the receptors should be adjusted to provide a uniform re- wide latitude of the image receptor. Exposure charts are sponse to the recorded x-ray image pattern in space. critical in the portable x-ray environment with CR, because Third, all monitors on all diagnostic display workstations manual technique factors are normally required. An under- should be tested and calibrated to provide a uniform exposed radiograph will have unacceptable noise, while an appearance of displayed images. These topics are ad- overexposed radiograph may appear to be of exceptional dressed in more detail elsewhere [28]. quality at the expense of patient dose. Pediatric patients provide unique challenges when im- The first challenge is to understand what constitutes a aged with digital radiography. Uncooperative pediatric properly exposed radiograph [29-31]. In SF systems, the patients may require sedation or anesthesia. The range in limited dynamic range of the film determines the exposure size from the neonate to the young adult patient requires latitude. In the digital world, wider exposure latitude can be a wide range in radiographic technique factors and a used to take into account the differing energy responses of digital image receptor that properly processes an x-ray the systems and the variability introduced in the dose esti- pattern in space that may or may not cover the entire mation scheme. Appendix A provides a suggested range of image receptor. The decision to use an antiscatter grid exposures to facilitate identifying whether a proper radio- must be carefully considered as a function of patient size. graph has been obtained. The log of median exposure, sen- Standard positioning aids used to immobilize pediatric sitivity number, exposure index, and sensitivity value are patients may generate unacceptable artifacts when used representative exposure indicators used by several CR ven- with digital image receptors. Because the x-ray pattern dors. The detector exposure column lists an approximate from a pediatric chest compared with an adult chest, for range of corresponding incident exposure values to the CR example, has a different dynamic range and other char- imaging plate. Note that repeating overexposures is not rec- acteristics [24], display parameters used by the digital ommended unless information has been lost. At the low acquisition device to properly display the digital image dose end of the scale, it is important for the radiologist and may need to be unique as a function of patient size. technologist to review underexposed radiographs to develop Display parameters require further alteration if gonadal a comfort level for which radiographs need to be repeated. It shielding or orthopedic implants appear in the displayed is useful to provide some guidance to the technologist with image. Finally, because scoliosis examinations are com- respect to how to correct an improperly exposed radiograph mon in children, the digital image receptor must provide (see Appendix B). Exposure charts must be tailored for each an efficient method to generate images up to 36 inches in digital radiography x-ray system and detector combination length, preferably without doubly exposing some sec- [32]. There is considerable variability in image receptor re- tions of the patient’s anatomy. sponse because of varying scatter sensitivity, the use of grids with different grid ratios, collimation, beam filtration, the PROPER EXPOSURE FACTORS FOR choice of kilovoltage, source to image distance, and image STANDARD EXAMINATIONS receptor size. Many CR and DR systems also permit the Exposure (technique) charts are part of the standard of choice of several exposure (speed) classes. Technique factors care expected by the Joint Commission on Accreditation for each clinically used exposure class should be constructed. of Healthcare Organizations and are required in many In addition, exposure charts should be designed to function states. For example, in Ohio, exposure charts are required over the wide range of adult patient sizes seen in our clinics and must include specification of the body part, thick- today and over the even wider range of sizes found in pedi- ness, and image receptor components to be used for the atric radiography. procedure. Internationally, most countries require regu- Given a table of body-part thicknesses with suggested lar use of exposure charts. It is necessary to check your technique factors, one can derive, using a spreadsheet pro- county, state, and local regulations for any specific re- gram, a full table of values using knowledge of the effect of quirements. You may also be required to compute esti- thickness and kVp on the penetration of the beam. There mates for entrance skin exposures for these charts. are also several commercial programs that can be tailored to Exposure charts are an essential quality assurance compo- digital radiography. Typically, the entire exposure chart is nent of any diagnostic x-ray imaging department. These calibrated to a single set of exposures based on a common exposure guides are especially important in CR and DR, in body part, such as the abdomen. A water tank or anthropo- Williams et al/Image Acquisition 381 morphic phantom can be used to develop appropriate tech- these requirements for adult chest portable imaging and niques for the abdominal radiograph. Water works better other adult examinations is shown in Appendix E. than acrylic or aluminum because the scatter characteristics are more comparable with the clinical situation. As dis- AUTOMATIC EXPOSURE CONTROL cussed previously, scatter characteristics can be quite differ- ent for digital radiography than for SF. It is also necessary to In CR, phototimers are frequently used with wall and un- measure Bucky factors for the undertable and wall Bucky dertable Buckys in emergency, inpatient, and outpatient assemblies as well as for any snap-on grids (also known as radiology department settings. These AEC units are de- grid caps, portable grids) that may be used with the x-ray signed to turn off the x-ray generator when an appropriate machine. Once exposure factors have been determined that exposure level has been received at the image receptor. Au- result in an appropriate radiograph for the abdomen with tomatic exposure controls work well if properly calibrated the various image receptors, all other values for mAs are and properly positioned over clinically important areas of interpolated using corrections for distance, Bucky factors, the patient. They fail if placed incorrectly or if pathology in exposure class, patient thickness, and machine output. The the AEC region of interest causes a significantly different exposure chart is then cleaned up by rounding values of mAs density to be present than expected. Automatic exposure to match available mAs stations on the machine. An exam- control devices are energy dependent and may require cali- ple of a technique chart developed for CR is shown in bration at multiple kVps to function properly over a wide Appendix C. range of patient sizes. Some x-ray generators have prepro- Similar exposure charts should be constructed for the use grammed energy response curves, and these units may not of portable x-ray equipment. The x-ray output of a portable work as well with CR as with conventional SF receptors radiography unit is generally substantially lower than that of because they were probably manufactured to be matched to a 3-phase or high-frequency stationary x-ray machine. This typical SF energy and scatter characteristics. In this situa- reduced output capability results in increased exposure tion, calibrate the wall Bucky for chest work and the under- table Bucky for abdominal procedures, or whatever is the times compared with conventional radiography. As a result, most common procedure performed on that image receptor patient motion is a common image degradation factor. This at your institution. factor, coupled with the difficulty in positioning grids cor- In DR, the AEC is normally built into the image rectly in a portable setting, results in frequent exposures receptor assembly, and it works in a manner similar to the being taken without the use of a grid, also degrading overall phototimers used in SF radiography. However, the num- image quality. Manual techniques are used because photo- ber or area of the photosensitive elements may differ timers are usually not available in the portable setting. An from the configurations normally used with conven- example of an exposure chart suitable for portable radiogra- tional SF systems depending on vendor implementation. phy in an emergency department trauma setting is shown in Most x-ray generator phototimers are designed to be Appendix D. calibrated for multiple image receptor speeds, usually Another issue to consider, particularly for portable designated by names such as regular, chest, and detail radiography, is achieving reproducible image quality for cassettes. AECs can be calibrated to multiple exposure repeated studies such as portable chest images that are classes such as 400, 200, and 100 to cover a variety of used for radiographic detection and evidence of incre- different CR and DR receptors or specific clinical situa- mental changes. Important for the radiologist is consis- tions such as pediatrics [33,34]. Some CR vendors per- tent acquisition techniques by the technologist, includ- mit a variety of exposure classes in their software, so the ing kVp, mAs, geometry (source to image distance, exposure class on the x-ray console must be manually patient positioning), the orientation of imaging receptor selected to match the exposure class appropriate for that cassette, the use of a grid, technologist positioning mark- procedure. This can lead to errors if technologists are not ers, and the consistent use of image-processing parame- vigilant in matching the x-ray generator exposure class to ters that are optimized for the specific study. Acquisition the CR reader exposure class. With some implementa- protocols and technique charts with minimal parameters tions of CR, the AEC unit must be calibrated on the basis and variables (eg, using crosswise image acquisition for all of cassette size instead of exposure class. Check with the adult chests with decubitus grids, using a fixed source-to- vendor so that you know which method is most appro- image receptor distance (SID) of 127 cm [50 in], 110 priate for you. In DR, the exposure class may also be user kVp, and adjusting only mAs as a function of estimated selectable, and vigilance is necessary to ensure that tech- patient girth) are key to establishing the desired repro- nologists are using the appropriate exposure classes, es- ducibility of image presentation (orientation, magnifica- pecially for pediatrics. It is usually set automatically in tion, image enhancement) on the radiologist display digital radiography systems with an integral x-ray tube workstation. An example of an exposure chart specifying assembly. 382 Journal of the American College of Radiology/Vol. 4 No. 6 June 2007

SUMMARY: OBSERVATIONS AND Annual Meeting. Oak Brook, IL: Radiological Society of North America; CHALLENGES 1995:7-22. 14. Blackman RB, Tukey JW. The measurement of power spectra. New York: The rapid movement in radiography away from SF tech- Dover; 1958. nology to both CR and DR has created both improved 15. Rabbani M, Shaw R, Van Metter R. Detective quantum efficiency of efficiency and more options in radiology. Yet at the same imaging systems with amplifying and scattering mechanisms. J Opt Soc time, the increased complexity and lack of transfer of Am A 1987;4:895-901. skills learned in the SF era by both radiologists and tech- 16. Cunningham IA. Applied linear-systems theory. In: Beutel J, Kundel HL, nologists has produced a gap in the ability to perform Van Metter RL, eds. Handbook of medical imaging. Bellingham, WA: quality assurance and quality improvement. It is imper- SPIE Press; 2000:79-159. ative to understand these digital radiography systems to 17. Seibert JA. Digital image processing basics. In: Balter S, Shope TB, eds. provide quality radiology services and to protect patients Radiological Society of North American 81st Scientific Assembly and Annual Meeting. Oak Brook, IL: Radiological Society of North America; from unnecessary radiation dose. 1995:121-42. ACKNOWLEDGMENTS 18. International Commission on Radiation Units and Measurements. Med- ical imaging—the assessment of image quality. Bethesda, MD: Interna- This paper was written collaboratively by the ACR, the tional Commission on Radiation Units and Measurements; 1996. American Association of Physicists in Medicine, and the 19. Myers KJ. Ideal observer models of visual signal detection. In: Beutel J, Society for Imaging Informatics in Medicine according Kundel HL, Van Metter RL, eds. Handbook of medical imaging. Belling- to the process described in the ACR’s Practice Guidelines ham, WA: SPIE Press; 2000:559-92. and Technical Standards book. 20. 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Appendix A suggest improvements in technique factors with CR, as well as in deciding when to repeat a study. Use caution in apply- On the basis of the vendor’s recommendation for what consti- ing this chart to any specific CR implementation since x-ray tutes an appropriately exposed radiograph, one can use Table generator output characteristics and CR plate energy sensi- A1 or a similar table to provide guidance to the technologist tivities vary widely. regarding proper exposure levels. Several CR exposure indices are illustrated in this example. With too low an incident exposure, the image is domi- Appendix C nated by noise, the corresponding SNR is very poor, and the ability to detect subtle differences in x-ray attenuation is Table C1 is an example of an exposure chart for CR for a compromised. With too high an incident exposure, highly 3-phase, 12-pulse generator. Distances are in inches. When penetrated areas in the image may suffer from saturation constructing a clinical exposure chart, the mAs values effects and the loss of contrast that cannot be compensated should be adjusted to match the nearest mAs station avail- for with image processing. These recommendations depend able on the generator. on several issues, including the type of CR reader (newer systems, such as the dual-light collection systems, have a higher DQE and will consequently have a lower exposure Appendix D recommendation for all indications), the type of image plate (later-generation image plates may have better exposure Table D1 is a typical mobile radiography exposure chart for performance), the quality and calibration settings of the CR CR used in an emergency department trauma setting. This reader and image plate, and the tolerance of the radiologist example includes the use of 3 exposure classes, depending to image noise, among other considerations. Continuous on the clinical situation. Note the use of lower kVps and analysis and feedback are required to get the optimal range longer exposure times and the minimal use of grids in this of exposures and indications for a given examination at a portable trauma setting compared with the previous fixed given site. Similar guidance can be provided for digital 3-phase generator exposure chart (Table C1). Use caution radiography for those systems that provide an indication of in applying this chart to any specific CR implementation the exposure used to obtain a radiograph. Use caution in since x-ray generator output characteristics and CR plate applying this chart to any specific CR implementation since energy sensitivities vary widely. x-ray generator output characteristics and CR plate energy sensitivities vary widely. Appendix B Appendix E Table B1 is a conversion factor table that provides a rapid Table E1 is an example of an exposure chart developed for means for the technologist to understand how to correct for bedside CR with the standardization of parameters. In this an improperly exposed radiograph. This example is for one example, all AP chest examinations are performed crosswise CR vendor implementation (Agfa), using log of median (landscape orientation). All adult chest procedures are per- exposure as the exposure indicator. Similar tables can be formed using a decubitus grid. If their conditions allow, constructed using the information in Table A1 as a guide- patients are inclined to get as close to upright as is possible. line for other vendor exposure settings. Technologists are Use caution in applying this chart to any specific CR im- very familiar with the change required to obtain an appro- plementation since x-ray generator output characteristics priate SF radiograph, and the same concept is used here to and CR plate energy sensitivities vary widely.

Table A1. Suggested guidelines for QC action based on estimated incident exposure (Conventional CR system for an “equivalent” speed class ϭ 200) Fuji Agfa Kodak Detector Exposure (S Number) (lgM Value) (Exposure Index) (mR) Indication Ͼ1,000 Ͻ1.45 Ͻ1,250 Ͻ0.2 Underexposed: repeat if necessary 1000-601 1.45-1.74 1,250-1,549 0.2-0.3 Underexposed: QC exception 600-301 1.75-2.04 1,550-1,849 0.3-0.7 Underexposed: QC review 300-150 2.05-2.35 1,850-2,150 0.7-1.3 Acceptable range 149-75 2.36-2.65 2,151-2,450 1.3-2.7 Overexposed: QC review 74-50 2.66-2.95 2,451-2,750 2.7-4.0 Overexposed: QC exception Ͻ50 Ͼ2.95 Ͼ2,750 Ͼ4.0 Overexposed: QC repeat if necessary Note: lgM ϭ log of median exposure; QC ϭ quality control; S number ϭ sensitivity number. 384 ora fteAeia olg fRdooyVl o ue2007 June 6 No. 4 Radiology/Vol. of College American the of Journal

Table B1. Required change in milliampere-seconds (mAs) to achieve a target log of median exposure (IgM) value of 2.20 Underexposed: Increase mAs Proper Clinical Techniques Overexposed: Reduce mAs Difference Difference Difference LgM From Target Required mAs LgM From Target Required mAs LgM From Target Required mAs Value LgM Scale Factor Value LgM Scale Factor Value LgM Scale Factor Underexposed, 1.40 Ϫ0.80 6.31 2.00 Ϫ0.20 1.58 2.42 0.22 0.60 Overexposed repeat 1.42 Ϫ0.78 0.03 2.02 Ϫ0.18 1.51 2.44 0.24 0.58 1.44 Ϫ0.76 5.75 2.04 Ϫ0.16 1.45 2.46 0.26 0.55 1.46 Ϫ0.74 5.50 2.06 Ϫ0.14 1.38 2.48 0.28 0.52 1.48 Ϫ0.72 5.25 2.08 Ϫ0.12 1.32 2.50 0.30 0.50 1.50 Ϫ0.70 5.01 2.10 Ϫ0.10 1.26 2.52 0.32 0.48 1.52 Ϫ0.68 4.79 2.12 Ϫ0.08 1.20 2.54 0.34 0.46 1.54 Ϫ0.66 4.57 2.14 Ϫ0.06 1.15 2.56 0.36 0.44 1.56 Ϫ0.64 4.37 2.16 Ϫ0.04 1.10 2.58 0.38 0.42 1.58 Ϫ0.62 4.17 2.18 Ϫ0.02 1.05 2.60 0.40 0.40 1.60 Ϫ0.60 3.98 ϾϾϾ 2.20 0.00 1.00 ϽϽϽ 2.62 0.42 0.38 Overexposed, 1.62 Ϫ0.58 3.80 2.22 0.02 0.95 2.64 0.44 0.36 check for 1.64 Ϫ0.56 3.63 2.24 0.04 0.91 2.66 0.46 0.35 burnout, 1.66 Ϫ0.54 3.47 2.26 0.06 0.87 2.68 0.48 0.33 repeat if 1.68 Ϫ0.52 3.31 2.28 0.08 0.83 2.70 0.50 0.32 necessary 1.70 Ϫ0.50 3.16 2.30 0.10 0.79 2.72 0.52 0.30 1.72 Ϫ0.48 3.02 2.32 0.12 0.76 2.74 0.54 0.29 1.74 Ϫ0.46 2.88 2.34 0.14 0.72 2.76 0.56 0.28 1.76 Ϫ0.44 2.75 2.36 0.16 0.69 2.78 0.58 0.26 1.78 Ϫ0.42 2.63 2.38 0.18 0.68 2.80 0.60 0.25 Underexposed, 1.80 Ϫ0.40 2.51 2.40 0.20 0.63 2.82 0.62 0.24 review with 1.82 Ϫ0.38 2.40 2.84 0.64 0.23 radiologist 1.84 Ϫ0.36 2.29 2.86 0.68 0.22 1.86 Ϫ0.34 2.19 2.88 0.68 0.21 1.88 Ϫ0.32 2.09 2.90 0.70 0.20 1.90 Ϫ0.30 2.00 2.92 0.72 0.19 1.92 Ϫ0.28 1.91 2.94 0.74 0.18 1.94 Ϫ0.26 1.82 2.96 0.76 0.17 1.96 Ϫ0.24 1.74 2.98 0.78 0.17 1.98 Ϫ0.22 1.68 3.00 0.80 0.15 Example 1: LgM value was 1.80. To get a proper exposure of 2.2, increase the mAs by a factor of 2.5. If the mAs was 20, change it to 2.5 ϫ 20 ϭ 50 mAs. You could also increase the kilovolt peak (kVp). Remember, a 15% increase in kVp doubles the tube output. Example 2: LgM value was 3.02. To get a proper exposure of 2.2, decrease the mAs by a factor of 0.16. If the mAs was 20, change it to 0.16 ϫ 20 ϭ 32 mAs. You could also decrease the kVp to achieve the same effect. Table C1. Computed radiography exposure chart Total mAs for Different Body Part Thicknesses Speed Average Procedure View Distance Receptor class kVp cm ؊6cm ؊4cm ؊2 cm Average ؉2cm ؉4cm ؉6cm ؉8cm Chest PA/AP 72 W 200 106 72 1.25 1.67 2.50 3.33 5.00 6.67 10.00 13.33 LAT 72 W 200 116 30 2.50 3.33 5.00 6.67 10.00 13.33 20.00 26.67 AP 72 T 200 80 22 1.25 1.67 2.50 3.33 5.00 6.67 10.00 13.33 LAT 72 T 200 90 30 2.50 3.33 5.00 6.67 10.00 13.33 20.00 26.67 Supine chest AP 40 T 200 80 22 0.90 1.20 1.80 2.40 3.60 4.80 7.20 9.60 Ribs/sternum AP Above Diaph 72 W 400 60 22 10.00 13.33 20.00 26.67 40.00 53.33 80.00 106.67 OBL Above Diaph 72 W 400 64 24 15.00 20.00 30.00 40.00 60.00 80.00 120.00 160.00 AP Below Diaph 72 W 400 76 22 30.00 40.00 60.00 80.00 120.00 160.00 240.00 320.00 Abdomen AP/PA 40 B 400 80 22 7.50 10.00 15.00 20.00 30.00 40.00 60.00 80.00 30° OBL 40 B 400 60 24 11.00 14.67 22.00 29.33 44.00 58.67 88.00 117.33 Pelvis/hip AP 40 B 400 80 22 7.50 10.00 15.00 20.00 30.00 40.00 60.00 80.00 LAT 40 B 400 80 22 7.50 10.00 15.00 20.00 30.00 40.00 60.00 80.00 Sacrum AP 40 B 400 76 20 7.50 10.00 15.00 20.00 30.00 40.00 60.00 80.00 LAT 40 B 400 80 28 15.00 20.00 30.00 40.00 60.00 80.00 120.00 160.00 Lumbar spine AP 40 B 400 80 22 7.50 10.00 15.00 20.00 30.00 40.00 60.00 80.00 45° OBL 40 B 400 80 26 15.00 20.00 30.00 40.00 60.00 80.00 120.00 160.00 LAT 40 B 400 80 30 30.00 40.00 60.00 80.00 120.00 160.00 240.00 320.00 L5/S1 40 B 400 90 30 30.00 40.00 60.00 80.00 120.00 180.00 240.00 320.00 Thoracic spine AP 40 B 400 76 22 6.50 8.87 13.00 17.33 26.00 34.67 52.00 69.33 LAT 40 B 400 76 30 15.00 20.00 30.00 40.00 60.00 80.00 120.00 160.00 Cervical spine AP/Odon 40 B 400 76 13 2.50 3.33 5.00 6.67 10.00 13.33 20.00 26.67 AP/Odon 72 W 400 76 13 7.50 10.00 15.00 20.00 30.00 40.00 60.00 80.00 OBL/LAT 72 W 400 76 13 7.50 10.00 15.00 20.00 30.00 40.00 60.00 80.00 OBL/LAT 72 T 400 76 13 2.50 3.33 5.00 6.67 10.00 13.33 20.00 26.67 Skull PA/Caldwell 40 B 400 80 19 5.00 6.67 10.00 13.33 20.00 26.67 40.00 53.33 LAT 40 B 400 76 15 2.50 3.33 5.00 6.67 10.00 13.33 20.00 26.67 Townes 40 B 400 80 22 7.50 10.00 15.00 20.00 30.00 40.00 60.00 80.00 Sinuses/facial PA/Caldwell 40 B 400 76 19 5.00 6.67 10.00 13.33 20.00 26.67 40.00 53.33

bones Waters 40 B 400 80 20 5.00 6.67 10.00 13.33 20.00 26.67 40.00 53.33 Acquisition al/Image et Williams LAT 40 B 400 76 15 1.88 2.50 3.75 5.00 7.50 10.00 15.00 20.00 Hand PA 40 T 100 54 4 1.10 1.47 2.20 2.93 4.40 5.87 8.80 11.73 OBL 40 T 100 54 6 1.25 1.67 2.50 3.33 5.00 6.67 10.00 13.33 Fanned LAT 40 T 100 54 8 2.25 3.00 4.50 6.00 8.00 12.00 16.00 24.00 Wrist PA 40 T 100 60 5 1.10 1.47 2.20 2.93 4.40 5.67 6.80 11.73 OBL 40 T 100 60 6 1.85 2.20 3.30 4.40 6.60 8.80 13.20 17.60 LAT 40 T 100 60 7 2.25 3.00 4.50 6.00 8.00 12.00 18.00 24.00 Forearm AP 40 T 100 64 7 1.10 1.47 2.20 2.93 4.40 5.87 8.80 11.73 LAT 40 T 100 64 8 2.25 3.00 4.50 6.00 8.00 12.00 18.00 24.00 AP ϭ anterior-posterior; Diaph ϭ diaphragm; L5/S1 ϭ fifth lumbar vertebrae/first sacra vertebrae; LAT ϭ Lateral; OBL ϭ oblique; Ondo ϭ ondotoid; PA ϭ posterior-anterior. 385 386 ora fteAeia olg fRdooyVl o ue2007 June 6 No. 4 Radiology/Vol. of College American the of Journal

Table C1. Continued Total mAs for Different Body Part Thicknesses Speed Average Procedure View Distance Receptor class kVp cm ؊6cm ؊4cm ؊2 cm Average ؉2cm ؉4cm ؉6cm ؉8cm Elbow AP 40 T 100 64 7 1.50 2.00 3.00 4.00 8.00 8.00 12.00 16.00 OBL 40 T 100 64 8 2.00 2.67 4.00 5.33 8.00 10.87 16.00 21.33 LAT 40 T 100 64 8 3.00 4.00 6.00 8.00 12.00 16.00 24.00 32.00 Humerus AP 40 T 100 72 8 1.10 1.47 2.20 2.93 4.40 5.67 8.80 11.73 LAT 40 T 100 72 9 1.80 2.40 3.60 4.80 7.20 9.60 14.40 19.20 Toes All 40 T 100 60 2 1.10 1.47 2.20 2.93 4.40 5.67 8.80 11.73 Foot AP 40 T 100 64 7 1.25 1.67 2.60 3.33 5.00 6.67 10.00 13.33 OBL 40 T 100 64 8 1.65 2.20 3.30 4.40 6.60 8.80 13.20 17.50 LAT 40 T 100 64 8 2.50 3.33 5.00 6.67 10.00 13.33 20.00 26.67 Calcanus PA 40 T 100 68 9 3.50 4.67 7.00 9.33 14.00 18.67 26.00 37.33 LAT 40 T 100 68 8 3.50 4.67 7.00 9.33 14.00 18.67 28.00 37.33 Ankle AP/OBL 40 T 100 64 9 1.80 2.40 3.60 4.80 7.20 9.60 14.40 19.20 LAT 40 T 100 64 8 2.50 3.33 5.00 6.67 10.00 13.33 20.00 26.67 Tibia/fibula AP 40 T 100 68 11 2.50 3.33 5.00 6.67 10.00 13.33 20.00 26.67 LAT 40 T 100 68 10 2.50 3.33 5.00 5.57 10.00 13.33 20.00 26.67 Knee AP 40 T 100 70 12 2.50 3.33 5.00 6.67 10.00 13.33 20.00 26.67 LAT 40 T 100 70 11 2.50 3.33 5.00 6.67 10.00 13.33 20.00 26.67 AP/OBL 40 B 100 70 12 2.50 3.33 5.00 6.67 10.00 13.33 20.00 26.67 LAT 40 B 100 70 11 2.50 3.33 5.00 6.67 10.00 13.33 20.00 26.67 Femur AP/LAT 40 B 400 76 16 6.00 8.00 12.00 16.00 24.00 32.00 48.00 64.00 Shoulder AP/Transax 40 B 400 76 18 3.00 4.00 6.00 8.00 12.00 16.00 24.00 32.00 Transthoracic 40 B 400 90 40 7.50 10.00 15.00 20.00 30.00 40.00 60.00 80.00 Clavicle AP/PA 40 B 400 74 16 3.00 4.00 6.00 8.00 12.00 16.00 24.00 32.00 Scapula AP 40 B 400 74 18 3.00 4.00 6.00 8.00 12.00 16.00 24.00 32.00 LAT 40 B 400 74 24 5.00 6.67 10.00 13.33 20.00 26.67 40.00 53.33 Note: AP ϭ anterior-posterior; B ϭ under table Bucky with grid; LAT ϭ lateral; mAs ϭ milliampere-seconds; OBL ϭ oblique; PA ϭ posterior-anterior; T ϭ tabletop nongrid; Transax ϭ trans-axial; W ϭ wall Bucky with grid. Williams et al/Image Acquisition 387

Table D1. Mobile computed radiography exposure chart for trauma radiology Body Part Position Patient Size kVp mAs Distance Speed Class Grid Chest PA/AP Small 80 5 72Љ 200 No Upright 72Љ Average 85 10 72Љ 200 No Large 90 16 72Љ 200 No PA/AP Small 70 5 40Љ 200 No Average 75 8 40Љ 200 No Large 85 10 40Љ 200 No Lateral-72Љ Small 85 10 72Љ 200 No Average 90 20 72Љ 200 No Large 95 32 72Љ 200 No Lateral Small 75 10 40Љ 200 No Average 80 16 40Љ 200 No Large 90 20 40Љ 200 No AP Newborn 65 0.5 40Љ 400 No Lateral Newborn 70 1 40Љ 400 No Abdomen AP-Grid Average 70 48 40Љ 400 Yes Ankle AP Average 55 12.55 40Љ 100 No Lateral Average 55 10 40Љ 100 No Cervical spine AP Average 60 7.5 40Љ 400 No Lateral-72Љ Average 65 15 72Љ 400 No Elbow AP Average 55 12.5 40Љ 100 No Lateral Average 55 10 40Љ 100 No Femur AP/Lateral Average 55 10 40Љ 400 No Foot AP Average 52 8 40Љ 100 No Hand/Wrist PA Average 50 5 40Љ 100 No Lateral Average 55 8 40Љ 100 No Humerus AP/Lateral Average 55 2.5 40Љ 100 No Knee AP Average 55 25 40Љ 100 No Lateral Average 55 16 40Љ 100 No Lumbar spine AP-Grid Average 75 48 40Љ 400 Yes Lateral-Grid Average 85 150 40Љ 400 Yes Pelvis/Hip AP Average 55 24 40Љ 400 No AP-Grid Average 70 50 40Љ 400 Yes Lateral-Grid Average 80 75 40Љ 400 Yes Shoulder/Clavicle AP Average 55 5 40Љ 400 No Skull AP Average 60 16 40Љ 400 No Lateral Average 55 10 40Љ 400 No AP-Grid Average 75 20 40Љ 400 Yes Lateral-Grid Average 65 10 40Љ 400 Yes Thoracic spine AP Average 65 16 40Љ 400 No Note: AP ϭ anterior-posterior; kVp ϭ kilovolt peak; mAs ϭ milliampere-seconds. 388 Journal of the American College of Radiology/Vol. 4 No. 6 June 2007

Table E1. Exposure chart: example of computed radiography exposure chart using grids and standardized parameters Measurement Small Medium Large Anatomic Region Examination (cm) SID Grid kVp (mAs) (mAs) (mAs) Chesta Chest portable AP 20-30 50Љ Yesa 110 6 10 12 Skull Skull PA/AP 18-21 40Љ Yes 80 12 20 30 Skull lateral 14-17 40Љ Yes 80 6 12 18 Spine Cervical AP 11-14 40Љ Yes 80 6 12 18 Thoracic AP 20-24 40Љ Yes 80 18 26 40 Lumbar AP 18-22 40Љ Yes 80 24 40 64 Abdomen Abdomen (KUB) 18-22 40Љ Yes 80 24 40 64 Pelvis Pelvis AP 19-23 40Љ Yes 80 24 40 64 Hip AP 17-21 40Љ Yes 80 20 35 50 Cross table lateral hip 25-31 40Љ Yes 85 30 42 55 Upper extremities Finger 1.5-4 40Љ No 60 1 1 2 Hand AP/OBL 3-5 40Љ No 60 1 2 4 Hand lateral 3-5 40Љ No 60 2 3 4 Wrist AP/OBL 3-6 40Љ No 60 1.5 2 3 Wrist lateral 3-6 40Љ No 60 1.5 3 4 Forearm AP 6-8 40Љ No 60 2 3 4 Forearm lateral 6-8 40Љ No 60 2 3 4 Elbow 6-8 40Љ No 60 2 3 4 Humerus 7-10 40Љ Yes 80 6 10 13 Shoulder 12-16 40Љ Yes 80 7 12 15 Lower extremities Toes 1.5-4 40Љ No 60 1.5 2.5 4 Foot AP/OBL 6-8 40Љ No 60 2 3 5 Foot lateral 6-8 40Љ No 60 2.5 4 6 Ankle AP 8-10 40Љ No 60 3 5 7 Ankle lateral 8-10 40Љ No 60 2 3 6 Tib/Fib AP 10-12 40Љ No 60 2 4 6 Tib/Fib lateral 10-12 40Љ No 60 2 4 6 Knee AP/lateral 10-13 40Љ No 60 6 12 20 Knee AP/lateral 10-13 40Љ Yes 60 3 5 8 Femur AP/lateral 14-17 40Љ Yes 80 15 25 40

Note: AP ϭ anterior-posterior; Fib ϭ fibula; KUB ϭ Kidneys, Ureter, Bladder; kVp ϭ kilovolt peak; large ϭϾ30 cm; mAs ϭ milliampere-seconds; medium ϭ 25 to 30 cm; OBL ϭ oblique; PA ϭ posterior-anterior; SID ϭ Source to Image receptor Distance; small ϭ 20 to 25 cm; Tib ϭ tibia. aUse short dimension (decubitus) grid, position cassette always cross-wise (landscape).