Principles of X-Ray Imaging 1
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Principles of X-Ray Imaging 1 Already a few weeks after the discovery of X-rays in 1895 3. The X-rays are attenuated differently by the various body by Wilhelm Conrad Rontgen€ the first medical images with tissues. photographic plates and fluorescent screens were made. This 4. Scattered radiation, which impairs image contrast, is was the origin of projection radiography and fluoroscopy. reduced. The greatest steps forward in X-ray diagnostic radiology 5. The transmitted photons are detected. since Roentgen’s observations were the development of the 6. The image is processed and – in the case of CT – image intensifier systems and then above all the announce- reconstructed. ment of computed tomography (CT) in a clinical environment This makes it possible to discuss the aspects of image by Hounsfield at the 1972 British Institute of Radiology quality and radiation exposure for both systems together in annual conference. A further important step was the introduc- the main parts of the book (cf. Chap. 2). tion of digital image receptors in projection radiography In radiography/fluoroscopy with digital image receptors during the last years. Compared to conventional film-screen and in computed tomography the digital image consists of a systems these receptors allow the separate optimisation of (typically square) matrix of picture elements (pixels) which photon detection and image processing, resulting in signifi- represent the corresponding volume elements (voxels) and – cant advantages for image quality and dose. after the exposure – carry the local intensity information (gray Although today projection radiography is still the most scale value). Quality of digital images depends primarily on frequent examination with X-rays the use of computed tomo- the image matrix size, i.e. the pixel size (cf. Chap. 9). As the graphy increases rapidly, and – because it involves larger matrix size is increased resolution improves but the number radiation doses than the conventional imaging procedures of photons in each pixel must be increased in order to maintain (cf. Table 10.1) – contributes significantly to the annual a certain minimum noise level. collective dose (see Fig. 1.1). Therefore CT also obtains growing attention in radiation protection (Brenner and Hall 2007). 1.1 Projection Radiography In X-ray diagnostic radiology the image is generated by and Fluoroscopy the interaction of X-ray photons, which have transmitted the patient, with a photon detector. These photons can either be In projection radiography and fluoroscopy the image is a primary photons, which have passed through the tissue with- two-dimensional projection of the attenuating properties of out interacting, or secondary photons, which result from an all the tissues along the paths of the X-rays. The components interaction along their path through the patient. The second- of a typical radiographic/fluoroscopic system are shown in ary photons will in general be deflected from their original Fig. 1.3. direction and result in scattered radiation. The photons emitted by the X-ray tube are collimated by The basic principles of projection radiography/fluoroscopy a beam-limiting device. Then they enter the patient, where and CT are shortly explained in Sects. 1.1 and 1.2 respec- they may be scattered, absorbed or transmitted without tively. Although totally different in image character, both interaction. The primary photons recorded by the image imaging systems have in common certain features, which receptor form the image. The secondary photons create a can be recognised in Fig. 1.2: certain amount of background radiation which degrades 1. X-rays are produced in an X-ray tube. contrast. If necessary, the majority of the scattered photons 2. The energy distribution of the photons is modified by can be removed by placing an anti-scatter device between inherent and additional filtration. the patient and the image receptor. This device can simply be H. Aichinger et al., Radiation Exposure and Image Quality in X-Ray Diagnostic Radiology, 3 DOI 10.1007/978-3-642-11241-6_1, # Springer-Verlag Berlin Heidelberg 2012 4 1 Principles of X-Ray Imaging Remainder Dental CT 0.7% 0.2% Angiography and 7% intervention Remainder 1% 3% Thorax 2% Mammography 9% 4% Skeleton GI and urogen 3% Dental and bile tract GI and urogen 8% 37% and bile tract 1% Mammo- graphy CT 18% Skeleton 60% Angiography 33% and intervention Thorax 13% Fig. 1.1 Contribution of various examination types to total frequency (left) and to collective effective dose (right) in 2006 for Germany adapted from BMU (2009) X-ray tube x Collimation Production of X-rays Filtration Object transmission Patient Scatter reduction Photon detection Patient support Anti-scatter device AEC system Image reconstruction and processing Image receptor Fig. 1.2 Basic principles of radiography/fluoroscopy and CT imaging Fig. 1.3 Typical arrangement of a radiography/fluoroscopy system 1.2 Computed Tomography 5 an air gap or a so-called anti-scatter grid formed from a reconstructed as image signal by computation. In practice series of parallel metal strips. An automatic exposure control CT numbers or Hounsfield units are used instead of mtissue system (AEC) provides for the correct exposure of the image where the Hounsfield unit HU is defined by: receptor. Today digital image receptors predominate in radi- ðm À m Þ ography and fluoroscopy, but film-screen systems and image ¼ Á tissue water HU 1000 m (1.1) intensifiers are also still in use. water where mwater is the linear attenuation coefficient of water. 1.2 Computed Tomography The experimental set-up of Hounsfield corresponded largely to the arrangement sketched in Fig. 1.4. Whereas it is not possible in projection radiography to This set-up was termed the ‘first generation’ of CT gain any depth information from a single image, computed (Kalender 2006). To speed up scanning and to utilise the tomography separates the superimposed anatomical details available X-ray power more efficiently the first commercial and produces sectional or axial slice images with excellent scanners (the ‘second generation’) used some more detectors soft tissue contrast. Compared to projection radiography and and a small fan beam. The typical scan time for an 80 Â 80 fluoroscopy computed tomography is a rather new imaging image matrix was 5 min (Kalender 2006). technique. Therefore it seems to be reasonable to present its Continuously rotating CT systems (‘third generation’) fundamental principles in some more detail. according to Fig. 1.5 with a fan beam covering the total patient The principle of computed tomography is illustrated in cross-section and a corresponding detector array, consisting Fig. 1.4. of gas proportional detectors or scintillation detectors (cf. A well-collimated X-ray pencil beam is attenuated by Sect. 8.2), were introduced in the 1980s. Continuous rotation the tissues along its path and the transmitted radiation is was made possible by a slip-ring technology for electrical detected. In order to generate one projection the tube-detector power supply and data acquisition. Scan time was reduced assembly scans the object in a linear translatory motion. This down to 2 s for a single slice with a 256 Â 256 matrix. procedure is repeated at many viewing angles (typically at A major step forward in CT technology was the introduc- least 180 projections are received with a rotational increment tion of spiral or helical CT by Kalender and Vock in 1989 of 1). From these projections a two-dimensional discrete (Kalender et al. 1989; Vock et al. 1989): Slice-by-slice distribution of the linear attenuation coefficients mtissue is imaging was replaced by volume scanning. The principle X-ray tube Detector Fig. 1.4 Principle of data acquisition in CT imaging (Adapted from Fig. 1.5 Continuously rotating CT system with a fan beam and Bunke 2003) corresponding detector array (Adapted from Bunke 2003) 6 1 Principles of X-Ray Imaging Fig. 1.6 Principle of spiral CT imaging (From Bunke 2003) of this method is illustrated in Fig. 1.6: While the fan beam is [ mm ] 5 2.5 1.5 1 1 1.5 2.5 5 continuously rotating the patient is moved with constant velocity along his body axis (the z-axis) through the gantry; 2 × 8 this results in a spiral track of the focal spot around the patient and accordingly in a spiral data set. 4 × 5 Direct image reconstruction from these data would give rise to image artefacts (similar to motion artefacts). This 4 × 2.5 can mostly be avoided by data interpolation. The interpolation method developed at first was the 360 linear interpolation 4 × 1 (LI) algorithm, which used data from a full rotation of the tube-detector assembly. Since for a complete interpolated data 2 × 0.5 set at a definite slice position two successive 360 rotations on either side of the selected plane were necessary, considerable Fig. 1.7 Adaptive array detector with detector combinations for widening of the slice profile resulted, thus reducing image different slice thicknesses (from Bunke 2003), e.g. the uppermost quality. Therefore the 360 LI was soon replaced by a 180 combination allows slice widths in the longitudinal direction from LI where interpolation from opposing 180 points reduces the 1 to 5 mm at the isocentre spiral range used for reconstruction. This is possible since X-ray beam attenuation at a distinct rotation angle j is equiv- alent to the X-ray beam attenuation traversing the body from The MSCT detector arrays could be divided into two the opposite side, at 180 + j. As the distance of the data groups: Those with detector elements of unequal width along points is now smaller, effective slice width will be less. the z-axis (adaptive array detector) and those with elements of In 1992 CT scanners were introduced, which used two equal width (linear or matrix detector).