Radiation Dose Modulation. the Multidetector CT Era: From Basics to Practice 1

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1 Note: This copy is for your personal non-commercial use only. To order presentation-ready copies for distribution to your colleagues or clients, contact us at EDUCATION EXHIBIT Radiation Dose Modulation Techniques in the Multidetector CT Era: From Basics to Practice TEACHING POINTS See last page Chang Hyun Lee, MD Jin Mo Goo, MD Hyun Ju Lee, MD Sung-Joon Ye, PhD Chang Min Park, MD Eun Ju Chun, MD Jung-Gi Im, MD Radiation exposure to the patient has become a concern for the radiologist in the multidetector computed tomography (CT) era. With the introduction of faster multidetector CT scanners, various techniques have been developed to reduce the radiation dose to the patient; one method is automatic exposure control (AEC). AEC systems make use of different types of control, including patient-size AEC, z-axis AEC, rotational or angular AEC, or a combination of two or more of these types. AEC systems operate on the basis of several methods: standard deviation, noise index, reference milliamperage, and reference image. A clear understanding of how to use different AEC systems on different multidetector CT scanners will allow users to modulate radiation dose, reduce photon starvation artifacts, and maintain image quality throughout the body. Further development of AEC systems and their successful introduction into clinical practice will require user education and good communication between users and manufacturers. RSNA, 2008 radiographics.rsnajnls.org Abbreviations: ACS = automatic current selection, AEC = automatic exposure control, DLP = dose-length product RadioGraphics 2008; 28: Published online /rg Content Codes: 1 From the Department of Radiology, Seoul National University College of Medicine and Institute of Radiation Medicine, 28 Yeongeon-dong, Jongno-gu, Seoul , South Korea (C.H.L., J.M.G., H.J.L., C.M.P., E.J.C., J.-G.I.); Gangnam Healthcare Center, Seoul National University Hospital, Seoul, South Korea (C.H.L.); and Department of Radiation Oncology, Seoul National University Medical Research Center, Seoul, South Korea (S.-J.Y.). Presented as an education exhibit at the 2006 RSNA Annual Meeting. Received April 17, 2007; revision requested July 23 and received September 4; accepted November 7. All authors have no financial relationships to disclose. Address correspondence to J.M.G. ( jmgoo@plaza.snu.ac.kr). RSNA, 2008

2 1452 September-October 2008 RG Volume 28 Number 5 Introduction Multidetector computed tomography (CT) scanners have led to greatly expanded clinical use of CT. With the simultaneous scanning of up to 64 sections, technologic advances have further improved the volume coverage, z resolution, and scanning speed of CT scanners. The result has been a considerable increase in the number of examinations and in the average scanned volume obtained per examination. In the past 25 years, the number of nuclear medicine studies has nearly tripled, but the number of CT studies has increased more than twentyfold (1). Unfortunately, these improvements in imaging have also inevitably led to an increase in radiation exposure to the patient. In the United States, citizens are being exposed to nearly six times more radiation from medical devices compared with 1980 levels, and CT accounted for 45% of the U.S. population s collective medical radiation exposure, even though it made up only 12% of all medical radiation procedures performed in the United States (2). This radiation exposure from CT is especially critical in pediatric patients and young adult females (3 5). Therefore, the increase in radiation exposure from CT examinations has been of concern to radiologists, medical physicists, government regulators, and CT equipment manufacturers. To cope with this increase in radiation exposure, scanner manufacturers have implemented their own dose modulation techniques to appropriately manage or reduce radiation doses. Apart from the various scanner geometry and noise filtering techniques, tube current modulation is one of the more effective methods of controlling radiation dose (6). However, with no set standard, CT equipment manufacturers currently provide different methods of applying automatic exposure control (AEC) (7). Thus, if these AEC systems are to be used properly, radiologists must be aware of the characteristics of dose modulation techniques with different multidetector CT scanners and learn how to apply these techniques to the patient. In this article, we discuss and illustrate the basic radiation dose parameters on multidetector CT scanners, types of AEC, control of AEC systems on different scanners, determination of protocols with AEC systems, and clinical applications of AEC systems. Reading Radiation Dose Parameters on CT Consoles Radiation Exposure The radiation exposure (expressed in coulombs per kilogram) is defined as the total charge produced in dry air when all electrons liberated by photons in a unit mass of air are completely stopped in air. The quantity must always be defined with respect to the specific material in which the interactions are taking place (eg, air kerma [kinetic energy released per unit mass], water kerma). From radiation exposure, one can calculate the skin entrance dose, which is important for deterministic effects such as skin erythema. These effects can be of potential concern in CT fluoroscopy or repeated multidetector CT and angiography, although they are not usually encountered at routine CT (8,9). Absorbed Dose For diagnostic x-rays, air kerma is the same as the absorbed dose delivered to a given volume of air in the absence of scatter. Absorbed dose represents the energy (in grays) absorbed per unit mass within an object. Absorbed dose does not take into account the differences in radiation sensitivity between organs and cannot provide an estimate of whole-body radiation risk. Equivalent dose (in sieverts) in tissue that incorporates tissue weighting factors has the same numeric value as absorbed dose because the radiation weighting factor is 1 for x-rays (5). CT Dose Index The CT dose index (10) is the most commonly used dose indicator. Display of this information on the user interface of modern CT scanners is required by European Union regulations, but outside the European Union some manufacturers provide display of the CT dose index only when required by the user (Fig 1). This allows direct comparison of the radiation dose at different scanning parameter settings, even between scanners made by different manufacturers (10). However, the CT dose index does not indicate the precise dose for any individual patient, but is rather an index of the dose as measured and calculated in a polymethylmethacrylate phantom. Although the CT dose index is a valuable tool for protocol comparison, it does not take into

3 RG Volume 28 Number 5 Lee et al 1453 Figure 1. Chart shows the basic dose parameters (as determined with an AEC system) displayed on the console of a Siemens multidetector CT scanner. Note that the reference milliamperage has been set at 100. From the volume CT dose index and dose-length product (DLP), the effective dose can be roughly calculated as DLP conversion factor. csl = section collimation (not section width), mas = average applied milliamperage, ref. = quality reference milliamperage, TI = rotation time, Total DLP = DLP value of the entire examination, Total mas = actual value of the entire examination in milliamperes. weighted CT dose index has to be corrected by the pitch factor (dose index divided by pitch) and is then termed the volume CT dose index. Figure 2. Photograph shows instruments used to determine the CT dose index. Plexiglas body and head phantoms are placed on a CT table. The ionization chamber probe is inserted into the center of the body phantom to measure the central CTDI 100 (CT dose index; subscript denotes that the measurement was made with a 100-mm ionization chamber). account patient-associated parameters such as size, shape, and inhomogeneous composition. For any given scanning technique, patient dose depends on the size and attenuation of the patient (ie, the greater the patient attenuation, the smaller the patient dose). Therefore, the displayed CT dose index is smaller than the actual dose delivered to young children and infants (11). The CT dose index is now commonly measured from one axial rotation of the scanner with use of a 100-mm pencil ionization chamber (Fig 2). An average or weighted CT dose index is calculated by adding one-third of the central value and two-thirds of the peripheral values. For scanning with a pitch that does not equal 1, the Dose-Length Product DLP is an indicator of the integrated radiation dose of an entire CT examination (5). The DLP incorporates the number of scans and the scan width (DLP = volume CT dose index total scan length). For conventional (nonspiral) scanning, the scan length is the sum of all section collimations for example, 25 mm (25 1 mm) for high-resolution CT. However, spiral and multidetector CT oversample data at the beginning and end of the scanning range because these data are needed for raw data interpolation of the first and last sections. There are differences between the various manufacturers, but approximately one half-rotation at the beginning and another halfrotation at the end have to be added to calculate the radiation exposure to the patient. Thus, the scan length, as provided by the scanner, should be increased by at least one table feed. Effective Dose The radiation risk to the patient can be estimated from the effective dose (in sieverts), which can be calculated by summing the absorbed doses to individual organs weighted for their radiation sensitivity (5). However, because we cannot obtain accurate measurements of all pertinent organ doses and the risk coefficients specific to age, gender, and organ being irradiated, the estimated dose is calculated for an idealized 70-kg, 30-year-old Teaching Point

4 1454 September-October 2008 RG Volume 28 Number 5 Teaching Point patient (5,12). In spite of these limitations, effective dose is the most widely used quantity for comparison between radiologic procedures. There are various computer programs that can calculate dose for individual organs using the volume CT dose index and organ weighting factors from International Commission on Radiological Protection Publication 60 (5). From effective dose, the risk estimates for stochastic effects can be determined with linear extrapolation of radiation exposure data from Japanese atomic bomb survivors (5,13). Calculation of the effective dose is usually made with a mathematic anthropomorphic phantom or computer-simulated irradiation Monte Carlo techniques (statistical calculations of photon interactions) (12). However, a reasonable approximation of the effective dose can be obtained with a conversion factor k (msv mgy -1 cm -1, where msv = millisieverts and mgy = milligrays) that varies depending on the body region being imaged (Fig 3, Table 1) (5,12). Figure 3. Diagram illustrates a simple algorithm for estimating radiation risk from CT examinations. Data in box are provided by the CT scanner. Teaching Point Automatic Exposure Control AEC systems for multidetector CT scanners are now available from all major scanner manufacturers under different names (3,14). These dose modulation systems operate in a variety of ways, but their main purpose is to adjust radiation dose according to the patient s attenuation and ultimately to reduce the radiation dose to the patient while sustaining diagnostic image quality (6). AEC systems have a number of potential advantages, including better control of patient radiation dose, avoidance of photon starvation artifacts, reduced load on the x-ray tube, and the maintenance of image quality in spite of different attenuation values on CT scans (15,16). With these benefits of AEC systems in mind, users should learn how to use and apply the systems properly. However, concerns about the routine use of AEC still remain. Although AEC systems generally reduce radiation dose, image noise inevitably increases, particularly in the region adjacent to contrast material and prosthesis-related artifacts. Table 1 Conversion Factors for Calculating Effective Dose Anatomic Region Conversion Factor (msv mgy -1 cm -1 ) Head Neck Chest Abdomen Pelvis Source. Reference 18. Note. mgy = milligrays, msv = millisieverts. control of tube current (milliamperage) (7). This tube current modulation can be performed using one or more of three basic methods: patientsize AEC, z-axis AEC, and rotational AEC (Fig 4). Most recent multidetector CT software and technology makes use of a combination of these methods. Teaching Point Types of AEC Reduction of radiation dose and variations in image quality in patients of different sizes and attenuations are generally achieved through the Patient-Size AEC and Z-axis AEC. Scan projection radiographs (topographic views) are applied in patient-size and z-axis AEC and are used mainly for the assessment of the size and attenuation of the patient. In reality, AEC systems tend to operate on the basis of a single anteropos-

5 RG Volume 28 Number 5 Lee et al 1455 Figure 4. Drawings illustrate types of AEC systems. With patient-size AEC (a), tube current is modulated according to patient size; with z-axis AEC (b), tube current is modulated according to patient attenuation along the z-axis; with rotational or angular AEC (c), tube current is modulated according to the asymmetry at each z-axis position; with combination AEC (d), tube current is modulated according to the combined effects of two or three of these types of AEC. terior topographic view to achieve compatibility with existing clinical practice (7). In patient-size AEC, the tube current is adjusted based on the overall size of the patient to reduce the variation in image quality between small patients and large patients. For a given patient size, the appropriate milliamperage is selected and is used for the entire examination or scan series. The AEC system (DoseRight) from Philips Medical Systems (Best, the Netherlands) has automatic current selection (ACS), which provides patient-based AEC. This system automatically suggests a milliamperage to be used throughout the examination for each patient depending on his or her size as seen on the topogram. The milliamperage is set with use of a reference image, whose noise value the system attempts to duplicate. The reference image is prestored at the factory and is based on a patient size of 33 cm. The suggested milliamperage will produce CT scans with image noise similar to that of the reference image. The system displays the volume CT dose index with ACS prior to scanning. Thus, the user can select the suggested milliamperage or individual settings at his or her discretion, with the dose difference between scans obtained with and without ACS technique. AEC systems from GE Medical Systems (Waukesha, Wis) (AutomA), Siemens Medical Solutions (Forchheim, Germany) (CARE Dose 4D), and Toshiba Medical (Tokyo, Japan) (SureExposure) provide both patient-size and z-axis AEC. The aim of z-axis AEC is to reduce the variation in the quality of images from the same series. AutomA (GE) adjusts the tube current to maintain a user-specified noise level in the image data. Z-axis AEC determines the tube current

6 1456 September-October 2008 RG Volume 28 Number 5 on the basis of projection data obtained from a topogram and a set of empirically determined noise prediction coefficients with use of the reference technique (17). These projection data can be used to determine the attenuation, size, and shape of the patient. AutomA provides a noise index to allow users to select the level of quantum noise that will be present in the reconstructed images, and the system modulates the tube currents to maintain the noise level in all images irrespective of patient size and attenuation. SureExposure (Toshiba) is also based on the projection data obtained from a topogram. For z-axis modulation, the water equivalent diameter at each level of the patient based on a topogram is calculated relative to the maximum attenuation (17). The appropriate tube current is applied at the maximum water equivalent diameter to achieve the selected standard deviation (noise level). Tube current is then modulated to maintain the standard deviation throughout the examination. Consequently, the milliamperage would be low through the thorax and high through the abdomen, which would be helpful when scanning both anatomic areas because, unlike the thorax, the abdomen contains solid organs. Rotational or Angular AEC. Rotational or angular dose modulation involves varying the tube current to equalize the photon flux to the detector as the x-ray tube rotates about the patient (eg, from anteroposterior to lateral). Angular dose modulation was first introduced for a single detector row helical CT scanner (SmartScan, GE) in This technique makes use of the information obtained from two topograms (anteroposterior and lateral views). With this information, sinusoidal modulation of tube current is achieved during 360 rotation for equalization of x-ray absorption. In a noncircular cross-sectional geometry, attenuation varies in different projection angles. At angular projections with a small patient diameter or body region, the tube current can be reduced without substantially increasing the image noise. Generally, lateral projections are more attenuating than anteroposterior projections, particularly in asymmetric regions of the body such as the shoulder or pelvis (compared with the head). With the advances in AEC techniques, an online modulation technique has been developed that does not require the information provided by topograms. This technique calculates the modulation function data (an objective image quality parameter) from the online patient attenuation. These data are sent to the generator control for dose modulation with a delay of 180 from the x-ray generation angle. Thus, the system makes use of attenuation data from the previous rotation and modulates tube current to accommodate patient attenuation on the fly (15,16). In angular dose modulation, more dose modulations occur in asymmetric regions and the variation in image noise throughout the examination can be minimized. This rotational AEC is also helpful in reducing photon starvation artifacts, especially in the shoulder (15). CT equipment manufacturers usually provide AEC systems with a combination of two or three of these types of AEC (3). Most 64-row scanners now provide these AEC systems, which include near real-time modulation techniques requiring x-ray tubes and generators to vary their output rapidly for subsecond rotation times (Table 2) (14). Operation of AEC Systems on Different Multidetector CT Scanners Standard Deviation based AEC. With Sure- Exposure 3D (Toshiba), users have to provide a standard deviation of pixel value for specifying image quality. A high standard deviation value will produce noisy images, whereas a low standard deviation value will produce less noisy images. SureExposure has a reference position selector for a given region, and the mean attenuation within this scanning region is used to calculate the tube current. The system makes use of a topogram to obtain the attenuation information used to calculate the tube current for each rotation. Next, on an image-by-image basis, the scanner sets the tube current that is required to achieve the requested standard deviation value. This method allows constant and consistent image quality for all patients and patient shapes. Therefore, the milliamperage is controlled by standard deviation and is dependent on the kernel for primary image reconstruction. Sure- Exposure aims to match the image noise to the target standard deviation value; however, with use of a standard kernel, the images are noisier than requested for higher target standard deviation values (7).

7 RG Volume 28 Number 5 Lee et al 1457 Table 2 Features of AEC Systems in a 64-section CT Scanner Feature Automatic ma control software Method for operator control of AEC Method for system control of ma ma adjustment for patient size ma adjustment along the z axis ma modulation during rotation GE LightSpeed VCT Philips Brilliance CT 64 CT Scanner Siemens Sensation 64 Toshiba Aquilion 64 Single or dual scout image, online control Source. Modified from reference 14. Note. ma = milliamperage. Single or dual scout image Single scout image, online control Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes SmartmA Doseright ACS, Doseright DOM (D DOM, Z DOM) CARE Dose 4D SureExposure Noise index Reference image Reference ma Standard deviation Single or dual scanogram Noise Index based AEC. SmartmA (GE) allows the user to set diagnostic image quality by entering Noise Index and a range of acceptable tube current settings (minimum and maximum milliamperage) (7). The current AEC system from GE has two elements: AutomA and SmartmA. AutomA provides the patient-size and z-axis AEC elements, and SmartmA provides the rotational AEC element. Users need to understand that image noise is inversely proportional to the square of the tube current. The noise index is used to set the estimated tube current, producing a noise level in reconstructed images that is based on anticipated patient attenuation from topograms. This system also aims to maintain a constant image noise level in each section. In this system, the standard deviation of images reflects the noise index most closely when GE s Standard reconstruction kernel is used. Images acquired with sharper or smoother kernels will result in images with standard deviations that are higher or lower than the noise index (7). Reference Milliamperage based AEC. CARE Dose 4D (Siemens) makes use of the effective milliamperage to compensate for the helical pitch for a given tube milliamperage. The effective milliamperage is defined as (tube current [in milliamperes] gantry rotation time [in sec- onds])/pitch factor. After selecting peak voltage, table feed, and detector configuration, the user must enter a reference effective milliamperage for the examination. This is the value that would be used with an average-sized patient. CARE Dose 4D assesses the size of the patient cross section being scanned and adjusts tube current relative to the reference effective milliamperage. It aims to provide adequate image noise, which varies depending on the size of the patient. This system operates on the principle that different-sized patients require different levels of noise to maintain adequate image quality. The degree to which tube current is adjusted for patient size can be selected using weak, average, or strong compensation settings. Reference effective milliamperage is designed for an average-sized patient (70 80 kg for adults, 20 kg [or 5 years of age] for pediatric patients). The scanner could be set up to provide average reductions in the reference effective milliamperage for slim patients and strong increases in the reference effective milliamperage for obese patients. These compensations result in less tube current adjustment than would be necessary to keep image noise constant for all patient sizes. With this approach, smaller or larger patients can be dealt

8 1458 September-October 2008 RG Volume 28 Number 5 with before the minimum or maximum tube current is reached. CARE Dose 4D adjusts the tube current over the patient s z-axis (one dimension) and in the x- and y-axis (three dimensions) based on the topogram. Real-time four-dimensional modulation is available during scanning and is controlled by using feedback from the previous rotation to set the tube current according to the attenuation measured at each tube angle. Reference Image based AEC. DoseRight by Philips consists of two types of dose modulations: DoseRight ACS, which provides patient-based AEC; and DoseRight DOM (dose modulation), which provides z-axis (Z DOM) and rotational (D DOM) AEC. DoseRight ACS makes use of a topogram to assess the patient s attenuation to set the tube current, which is used at all z-axis positions. Within a particular protocol, the user can select DoseRight ACS and DoseRight DOM either singly or together. On the basis of the system s prior attenuation measurements in crosssectional anatomy, ACS automatically suggests the lowest milliamperage settings to maintain consistent image quality at a low radiation dose throughout the examination. Tube current is set so that 90% of the images will have noise equal to or lower than that of the reference image, with the remaining 10% of images having noise equal to or higher than that of the reference image. In this system, the required image quality is expressed as an existing clinical image rather than an abstract standard deviation value or noise index. Therefore, it is difficult to compare scanning protocols, since there is no value associated with the image quality in the reference image. There is also the possibility of obtaining a better image with lower noise than one that has been deemed good enough for evaluation because of the temptation to pick a nice image. This could potentially lead to the system attempting to match images with higher quality than is needed and using higher doses than are necessary. Determining Protocols with AEC Systems Optimization of scanning protocol involves many parameters, including tube voltage, tube current, section thickness, collimation, and pitch. The Table 3 European Guidelines for the Diagnostic Reference Levels in Various CT Examinations Examination Diagnostic Reference Level CTDIw (mgy) DLP (mgy cm) Routine head Face and sinuses Vertebral trauma Routine chest High-resolution CT (lung) Routine abdomen Liver and spleen Routine pelvis Osseous pelvis Source. Reference 18. Note. CTDIw = weighted CT dose index, mgy = milligrays. image reconstruction kernel is also important. Changing even one of these parameters can affect different AEC systems depending on the CT scanner being used. For example, changing the tube voltage and section thickness will not affect the tube current on Siemens CARE Dose systems, which permit variable levels of image noise for different-sized patients, but will do so on other AEC systems (7). Changing the reconstruction kernel will alter the tube current used by the Toshiba SureExposure system, which tries to maintain the image noise in response to variations in scanning and reconstruction parameters, but will not alter the tube current used by other systems (7). Therefore, users should know about each of their system s characteristics and the effect that changing scanning and reconstruction parameters will have on the system. Nevertheless, in the use of AEC systems, selecting an appropriate noise index, standard deviation, reference milliamperage, or reference image is vital. This process is not straightforward, however. There are two ways to determine appropriate values for the use of AEC systems. The European Guidelines on Quality Criteria for Computed Tomography are a good standard for optimizing scanning protocol with reasonable radiation dose (Table 3) (18). These guideline values are provided to the extent available in relation to technique for a standard-sized patient for each type of CT examination considered

9 RG Volume 28 Number 5 Lee et al 1459 Teaching Point (Table 3). Another way to optimize protocol is to use simulation software before scanning, although this technique is not currently available to most CT practitioners. This software can simulate the effect of increasing image noise, and the resultant simulated data can be reconstructed. Thereafter, users will be able to evaluate image quality with radiation dose modulation. Clinical Application of AEC Systems AEC systems are now installed in all CT scanners from major manufacturers to reduce the radiation dose to the patient. Although the use of these systems generally leads to a decrease in radiation exposure, there remains the possibility of higher exposure than would occur without the application of an AEC system (7). Therefore, it is paramount that radiologists know how to use the AEC systems in their own scanners. AEC systems do not reduce radiation dose per se; rather, they control radiation exposure relative to the required image quality (7). Conclusions Reductions in radiation dose inevitably result in a corresponding reduction in image quality. However, an optimal diagnostic image can still be acquired with the aid of AEC systems. Unfortunately, with no set standard, CT equipment manufacturers have created their own ways of modulating radiation dose, thereby creating the need to understand how to use different AEC systems and what kinds of values or information the user must predetermine. Therefore, users must possess a good understanding of the concepts of noise index, standard deviation, reference images, and reference milliamperage as they relate to each AEC system. For further development of AEC systems and their successful introduction into clinical practice, user education and good communication between users and manufacturers are needed. A consensus regarding good default AEC systems may also help users more easily achieve diagnostic image quality with reduced radiation dose. References 1. Amis ES Jr, Butler PF, Applegate KE, et al. American College of Radiology white paper on radiation dose in medicine. J Am Coll Radiol 2007;4: Mettler FA Jr. Magnitude of radiation uses and doses in the United States: NCRP scientific committee 6-2 analysis of medical exposures. Fortythird NCRP annual meeting program, 2007; Brenner D, Elliston C, Hall E, Berdon W. Estimated risks of radiation-induced fatal cancer from pediatric CT. AJR Am J Roentgenol 2001;176: Khursheed A, Hillier MC, Shrimpton PC, Wall BF. Influence of patient age on normalized effective doses calculated for CT examinations. Br J Radiol 2002;75: Recommendations of the International Commission on Radiological Protection. Ann ICRP 1991;21(1-3): Mulkens TH, Bellinck P, Baeyaert M, et al. Use of an automatic exposure control mechanism for dose optimization in multi detector row CT examinations: clinical evaluation. Radiology 2005;237: Keat N. Report CT scanner automatic exposure control systems. London, England: Im- PACT, Available at: org/reports/report Accessed June 16, Golding SJ, Shrimpton PC. Commentary. Radiation dose in CT: are we meeting the challenge? Br J Radiol 2002;75: Imanishi Y, Fukui A, Niimi H, et al. Radiationinduced temporary hair loss as a radiation damage only occurring in patients who had the combination of MDCT and DSA. Eur Radiol 2005;15: Shope TB, Gagne RM, Johnson GC. A method for describing the doses delivered by transmission x-ray computed tomography. Med Phys 1981;8: Fearon T. CT dose parameters and their limitations. Pediatr Radiol 2002;32: McCollough CH, Schueler BA. Calculation of effective dose. Med Phys 2000;27: National Council on Radiation Protection and Measurements. Risk estimates for radiation protection. NCRP report no Bethesda, Md: National Council on Radiation Protection and Measurements, Lewis M, Keat N, Edyvean S. Report 06013: 32 to 64 slice CT scanner comparison report version 14. London, England: ImPACT, Available at: impactscan.org/reports/report Accessed June 16, Kalender WA, Wolf H, Suess C. Dose reduction in CT by anatomically adapted tube current modulation: phantom measurements. Med Phys 1999;26: Gies M, Kalender WA, Wolf H, Suess C. Dose reduction in CT by anatomically adapted tube current modulation: simulation studies. Med Phys 1999;26: Kalra MK, Maher MM, Toth TL, et al. Techniques and applications of automatic tube current modulation for CT. Radiology 2004;233: Jessen K, Panzer W, Shrimpton P, et al. EUR 16262: European guidelines on quality criteria for computed tomography. Luxembourg: Office for Official Publications of the European Communities, Also available at: /guidelines/ct/quality/. Accessed June 16, 2008.

10 RG Volume 28 Volume 5 September-October 2008 Lee et al Radiation Dose Modulation Techniques in the Multidetector CT Era: From Basics to Practice Chang Hyun Lee, MD, et al RadioGraphics 2008; 28: Published online /rg Content Codes: Page 1453 DLP is an indicator of the integrated radiation dose of an entire CT examination. Page 1454 A reasonable approximation of the effective dose can be obtained with a conversion factor k (msv mgy [minus]1 cm [minus]1, where msv = millisieverts and mgy = milligrays) that varies depending on the body region being imaged. Page 1454 AEC systems have a number of potential advantages, including better control of patient radiation dose, avoidance of photon starvation artifacts, reduced load on the x-ray tube, and the maintenance of image quality in spite of different attenuation values on CT scans. Page 1454 Tube current modulation can be performed using one or more of three basic methods: patient-size AEC, z-axis AEC, and rotational AEC. Page 1459 Unfortunately, with no set standard, CT equipment manufacturers have created their own ways of modulating radiation dose, thereby creating the need to understand how to use different AEC systems and what kinds of values or information the user must predetermine. Therefore, users must possess a good understanding of the concepts of noise index, standard deviation, reference images, and reference milliamperage as they relate to each AEC system.