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CT Scan Parameters and Radiation Dose: Practical Advice for Radiologists Siva P

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CT Scan Parameters and Radiation Dose: Practical Advice for Radiologists Siva P CT Scan Parameters and Radiation Dose: Practical Advice for Radiologists Siva P. Raman, MD, Mahadevappa Mahesh, MS, PhD, Robert V. Blasko, BS, RT(R)(CT), Elliot K. Fishman, MD Although there has been increasing recognition of the importance of reducing radiation dose when performing multidetector CT examinations, the increasing complexity of CT scanner technology, as well as confusion about the importance of many different CT scan parameters, has served as an impediment to radiologists seeking to create lower dose protocols. The authors seek to guide radiologists through the manipulation of 8 fundamental CT scan parameters that can be altered or optimized to reduce patient radiation dose, including detector configuration, tube current, tube potential, reconstruction algorithm, patient positioning, scan range, reconstructed slice thickness, and pitch. Although there is always an inherent trade-off between image quality or noise and patient radiation dose, in many cases, a reasoned manipulation of these 8 parameters can allow the safer imaging of patients (with lower dose) while preserving diagnostic image quality. Key Words: Radiation dose, automated tube current modulation, low kVp, pitch, iterative reconstruction J Am Coll Radiol 2013;10:840-846. Copyright © 2013 American College of Radiology INTRODUCTION can affect radiation dose and image quality and examine Over the past several years, there has been increasing how these can be altered to reduce dose. recognition of the importance of reducing radiation dose In this review, we seek to provide radiologists with a in radiologic studies, particularly with regard to multide- simple approach to the manipulation of 8 CT parameters tector CT. The growth in the volume of multidetector that can be changed by the radiologist, while designing or CT studies performed in the United States, the increas- altering scan protocols, to reduce patient radiation dose: ing use of CT in susceptible populations (including chil- (1) detector configuration, (2) tube current, (3) tube dren), and growing concerns on the part of the general potential (kVp), (4) reconstruction algorithm, (5) pa- public with regard to radiation exposure have provided tient positioning, (6) scan range, (7) reconstructed slice an impetus for performing these studies with the least thickness, and (8) pitch. possible radiation dose [1]. It is now believed that as many as 0.4% of all malignancies in the United States can be traced back to radiation from CT studies per- DETECTOR CONFIGURATION formed between 1991 and 1996 and that radiation from Detector configuration is a term encompassing the num- CT studies currently being performed may ultimately ber of data channels being used in the z axis and the account for 1.5% to 2% of all cancers in the future [2]. “effective detector thickness” of each data channel. For Unfortunately, despite this growing awareness among example, a detector configuration of 64 ϫ 0.5 mm would radiologists, the introduction of each new generation of suggest the use of 64 data channels in the z-axis, each of CT scanners has resulted in scanning protocols that are which has an effective thickness of 0.5 mm. Notably, the increasingly complex and difficult to manipulate for a effective detector thickness represents the smallest possi- radiologist whose primary expertise lies in clinical imag- ble reconstructed slice thickness: if a study is acquired ing rather than medical physics. When facing increasing criticism regarding CT dose, the best way to tackle the with an effective detector thickness of 0.5 mm, images issue is to understand all the factors and parameters that cannot be reconstructed to any smaller interval (such as 0.25 mm). Both the number of channels used and the effective detector thickness can be varied depending on how many channels in a detector array are used, which Department of Radiology, Johns Hopkins University, Baltimore, Maryland. Corresponding author and reprints: Siva P. Raman, MD, Johns Hopkins channels in a detector array are used, and the manner in University, Department of Radiology, JHOC 3251, 601 N Caroline Street, which different channels are combined [3]. Different scan- Baltimore, MD 21287; e-mail: [email protected]. ners from different manufacturers have different included 840 © 2013 American College of Radiology 1546-1440/13/$36.00 ● http://dx.doi.org/10.1016/j.jacr.2013.05.032 Raman et al/CT Scan Parameters and Radiation Dose 841 detector channels, and the detector configurations available most applications. This computer software, which has on different equipment can vary widely [4]. various names depending on the specific vendor (ie, The product of the detector configuration (the num- CARE Dose 4D on Siemens, Dose-Right on Phillips, ber of data channels multiplied by the effective detector Auto mA/Smart mA on GE, and SUREExposure 3D on row thickness) is equivalent to the beam collimation. The Toshiba), automatically increases the mAs in those parts physical manifestation of the beam collimation is the of the body with the greatest attenuation and decreases x-ray beam size as defined at the gantry isocenter [5].In the mAs in those parts of the body with lower attenua- the example given previously, with a detector configura- tion. As a result, the software usually increases the mAs in tion of 64 ϫ 0.5 mm, the beam collimation would be 32 the shoulder and hips (which have relatively more atten- mm. This has a critical impact on patient dose because uation) and decreases the mAs in the abdomen and tho- the x-ray beam is actually slightly wider than the beam rax (which have relatively less attenuation) [1,7]. In most collimation as a result of a penumbra effect (also referred cases, automated tube current modulation reduces pa- to as “overbeaming”), resulting in a small amount of tient dose and minimizes photon starvation artifacts. “wasted” radiation dose with each rotation [6].Asare- Automated tube current modulation works on differ- sult, if a smaller beam collimation (4 ϫ 0.5 mm) is used, ent principles depending on the specific vendor, al- the relative wasted dose is relatively high compared with though a combination of 4 different strategies is generally the primary beam size. On the other hand, with a larger used: Patient size modulation varies the mAs on the basis 64 ϫ 0.5 mm collimation, the wasted dose is relatively of a global evaluation of the overall size of the patient as low compared with the primary beam size. However, it seen on the scout radiograph. Z-axis modulation changes should be noted that with the newest CT scanners, the the mAs constantly along the z-axis of the patient de- penumbra effect is much less apparent compared with pending on the patient attenuation at each point, as earlier generations of spiral CT technology because the determined using the scout image. Angular (x, y) modu- extent of the overbeaming or penumbra effect is less lation changes the mAs as the x-ray tube rotates 360° pronounced (relative to the beam width). Therefore, the around the patient to account for different attenuations need to obtain a smaller effective detector thickness to in different projections of the x-ray beam [1]. Combined improve spatial resolution in the z-axis must be balanced x, y, and z modulation adjusts the mAs in all 3 axes on the against the increased radiation dose associated with such basis of the patient’s attenuation [8]. The user must de- a detector configuration, as well as longer scan times fine a minimum acceptable image quality, the definition [3,5]. of which will vary depending on the specific vendor and The detector configuration should be determined on system. For example, GE users must specify a noise index the basis of the type of study performed, the necessary value that is maintained as a constant on every image in a slice thickness for multiplanar reformations (MPRs), and series, while Siemens users must specify an image quality the need for 3-D images. For routine acquisitions, with- reference mAs, the mAs that would be used for an aver- out the need for thin-section MPR images or 3-D imag- age-sized patient [1]. ing, an extremely thin effective detector thickness is In almost all cases, automated tube current modulation unnecessary. In such cases, if thick-section 5-mm images should be enabled and used by all users, although some in the axial, coronal, and sagittal planes are sufficient for significant caveats and exceptions must be kept in mind: radiologist review, the effective detector thickness can be ● Small or pediatric patients, when incorrectly positioned widened to 1.25 or 1.5 mm (rather than 0.75 or 0.5 or improperly centered within the CT gantry, can result mm), without a marked impact on image quality (either in excessively noisy images with artificially low mAs. Not for the axial images or MPR images). However, if thin- only must technologists be properly taught to correctly section MPRs or high-resolution 3-D images (with in- position patients, but it may also be helpful to modify the creased spatial resolution in the z-axis) are required, a noise index or increase the reference mAs for smaller thin effective detector thickness (0.5-0.75 mm) may be patients (while still capping the maximum mAs to avoid necessary, although there is an associated dose penalty. inadvertently high radiation doses) [8,9]. ● Many institutions do not use automated tube current TUBE CURRENT modulation when performing scans of the head, particu- Increases in tube current or the product of tube current larly in pediatric patients, because of the difficulty in and scan time (mAs) result in improved image quality, correctly placing the head in the isocenter of the gantry. A decreased image noise, and increased patient dose. In small deviation of the head from the isocenter can result general, the relationship between tube current and pa- in a significant increase in image noise.
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