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Review |
1 From the Division of Abdominal Imaging and Intervention, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Wht-270, 32 Fruit St, Boston, MA 02134 (M.K.K., M.M.M.); GE Medical Systems, Waukesha, Wis (T.L.T.); Siemens Medical Solutions, Forchheim, Germany (B.S.); Toshiba America Medical Systems, Tustin, Calif (B.L.W.); Philips Medical Systems, Cleveland, Ohio (H.T.M.); and Department of Radiology, Emory University School of Medicine, Emory University Hospital, Atlanta, Ga (S.S.). Received July 25, 2003; revision requested September 30; final revision received and accepted December 9. Address correspondence to S.S. (e-mail: mannudeep_k_kalra@yahoo.com).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONPRINCIPLES OF ATCM TECHNIQUESCLINICAL USE OF ATCM...LIMITATIONS OF AVAILABLE ATCM...CONCLUSIONESSENTIALSREFERENCES |
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© RSNA, 2004
Index terms: Computed tomography (CT), image quality • Computed tomography (CT), radiation exposure • Computed tomography (CT), technology • Radiations, exposure to patients and personnel • Review
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONPRINCIPLES OF ATCM TECHNIQUESCLINICAL USE OF ATCM...LIMITATIONS OF AVAILABLE ATCM...CONCLUSIONESSENTIALSREFERENCES |
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Amid growing concerns about CT radiation exposure, the adoption of ATCM techniques should permit overall reduction in radiation exposure in CT examinations. Unfortunately, owing to rapid technologic advances, different vendors have developed different ATCM techniques and use proprietary nomenclature.
Nevertheless, the introduction of ATCM techniques in modern CT scanners represents an important step toward standardization of tube current protocols, with elimination of arbitrary selection by radiologists and technologists. This article will describe the principles of ATCM techniques, their clinical use, and the advantages and disadvantages of application in diagnostic CT scanning.
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PRINCIPLES OF ATCM TECHNIQUES |
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TOPABSTRACTINTRODUCTIONPRINCIPLES OF ATCM TECHNIQUESCLINICAL USE OF ATCM...LIMITATIONS OF AVAILABLE ATCM...CONCLUSIONESSENTIALSREFERENCES |
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Authors of previous clinical and experimental studies have reported that satisfactory image quality can be obtained with a reduction of tube current on the basis of weight and cross-sectional dimensions of patients undergoing CT scanning (10–12). Results of these studies may be explained on the basis of variable attenuation of the incident beam traversing a particular cross-sectional dimension at a particular projection angle (1,2). The resultant attenuation determines image noise and is affected by scanning parameters, particularly tube current. For example, greater beam attenuation in a particular dimension or projection will result in greater noise and will require higher tube current than that needed by a beam undergoing less attenuation in another projection.
Manual adjustment of tube current based on patient weight or dimensions can aid in establishing an appropriate balance between image noise and radiation exposure (10,12). However, these adjustments do not guarantee constant image quality throughout the entire examination. For example, in CT scanning of chest, the choice of a fixed tube current does not account for differences in beam attenuation between the shoulder region and midchest region or between anteroposterior and lateral cross-sectional dimensions. ATCM techniques allow maintenance of constant image quality at a required radiation exposure level because ATCM rapidly responds to large variations in beam attenuation. ATCM is based on the fact that image noise is determined by x-ray quantum noise in the transmitted beam projections. This technique aims to modulate tube current on the basis of regional body anatomy for adjustment of x-ray quantum noise to maintain constant image noise with improved dose efficiency.
Currently, two distinct techniques are available for ATCM: angular (x-y) modulation and z-axis modulation. Both techniques modulate tube current in an effort to maintain constant image quality at the lowest dose while simultaneously reducing tube loading (heating) and minimizing streak artifacts caused by a minimal number of photons.
Angular Modulation
The angular-modulation technique was introduced in 1994 for a single–detector row helical CT scanner (SmartScan; GE Medical Systems, Waukesha, Wis) (13–15). This software-based technique modulated tube current on the basis of the measured density of regional structures and the absorption values of the object of interest. This information was obtained by measuring local x-ray beam absorption in 100 central channels on two localizer radiographs (lateral and anteroposterior views). Preprogrammed sinusoidal modulation of tube current was achieved during 360° rotation for equalization of local differences in beam absorption to obtain relatively constant noise content and reduce radiation exposure. A radiation dose reduction of up to 20%, depending on patient geometry (asymmetry), has been reported (14). A recent refinement of the angular-modulation approach is an online, real-time, anatomy-adapted, attenuation-based tube current modulation technique (CARE Dose; Siemens Medical Solutions, Forchheim, Germany) that does not need the information of radiographic localizer images to achieve ATCM (1–3,16–20).
Angular-modulation techniques automatically adjust the tube current for each projection angle to the attenuation of the patient to minimize x-rays in projection angles (eg, anteroposterior or posteroanterior angles are less important than are lateral projections because the former cause less beam attenuation and hence are associated with less noise) that are less important with regard to reducing the overall noise content (Fig 1). In phantom studies in which an online angular-modulation technique was compared with the previous angular-modulation technique, a substantially greater radiation reduction (up to 50%) with the online modulation technique was documented (1).
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The Dose-Right Dose Modulation, or DOM, technique (Philips Medical Systems, Eindhoven, the Netherlands) of angular modulation also adjusts tube current in asymmetric anatomic regions. This technique is based on the premise that tube current should be modulated according to the square root of the measured attenuation for that projection (1). The DOM technique modulates the current within a 360° tube rotation according to the square root of the attenuation measured from the similar and previous 180° or 360° views. In other words, the attenuation measured at an angular view (projection angle) is used to optimize the tube current later for a similar angular view.
Z-Axis Modulation
The z-axis–modulation (AutomA, GE Medical Systems; Real E.C., Toshiba Medical, Tokyo, Japan) technique functions somewhat differently than does angular modulation (21). The AutomA technique adjusts the tube current automatically to maintain a user-specified quantum noise level in the image data. It provides a noise index to allow users to select the amount of x-ray noise that will be present in the reconstructed images. Using a localizer radiograph, the scanner computes the tube current needed to obtain images with a selected noise level. Hence, z-axis modulation attempts to make all images have a similar noise irrespective of patient size and anatomy. The noise index value is approximately equal to the image noise (standard deviation) in the central region of an image of a uniform phantom.
In the z-axis–modulation technique, the system determines the tube current by using the patient’s localizer radiograph projection data and a set of empirically determined noise prediction coefficients by using the reference technique (Fig 2). The reference technique comprises an arbitrary 2.5-mm-thick section obtained at the selected peak voltage and 100 mAs for transverse reconstruction with a standard reconstruction algorithm. The projection data from a single localizer radiograph can be used to determine the density, size, and shape information of the patient (4,5). The total projection attenuation data of a single localizer radiograph contain the patient’s density and size information about the projection area, whereas the amplitude and area of the projection contain the patient’s shape information, which gives an estimate of the patient’s elliptic asymmetry expressed as an oval ratio at a given z-axis position. The oval ratio is the ratio of the a and b parameters (lengths of the long and short axes) of an ellipse. The ellipse parameters can be determined for the patient by using the equation for the area of an ellipse. The measured projection area and amplitude from the localizer radiograph give the area and the length of one axis, a, of the ellipse, allowing the length of the other axis, b, to be calculated. These characteristics of the localizer radiograph predict the amount of x-rays that will reach the detector for a specified technique and, hence, determine the image standard deviation due to x-ray noise for a given reconstruction algorithm. The predicted x-ray noise at a given z-axis position for the reference technique (ie, reference noise) is calculated from the projection area and oval ratio from the localizer radiograph by using the polynomial coefficients that were determined from the noise measurements in a set of phantoms representing a wide range of patient sizes and shapes.
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Often, the actual noise measured on the image by drawing a region of interest will differ from the noise index selected for scanning. This is due to the fact that noise index settings only adjust the tube current, whereas the standard deviation is also affected by other parameters, including the reconstruction algorithm, the reconstructed section thickness (if different from the prospective thickness), the use of image space filters, variations in patient anatomy and patient motion, and the presence of beam-hardening artifacts. Substantial differences between the selected noise index and the standard deviation can also occur in very large patients owing to insufficient signal strength at the detector and superimposition of electronic noise, which can be minimized by using a higher peak voltage. Likewise, improper centering of patients in the scan field of view can result in noisier images owing to inappropriate beam attenuation by the bow-tie filter (5). As bow-tie filter attenuation increases with distance from isocenter, the thickest part of the patient should be approximately centered in the scan field of view to prevent inappropriate attenuation compensation by the bow-tie filter due to incorrect patient positioning.
The Real E.C. technique implemented on Toshiba CT scanners is a z-axis–modulation technique that is also based on patient attenuation measurements acquired from a single localizer radiograph. Evolving from its original form, which provided operators with several options in the choice of nominal tube current to modulate on the basis of patient attenuation data, Real E.C. now offers four levels of image noise to match the diagnostic needs of the examination. The scanner software enables this by calculating the "water-equivalent" thickness of each section from the localizer radiograph (Figs 3, 4). The appropriate tube current is applied at the thickest section in the z-axis direction to achieve the selected standard deviation (noise level). Tube current is then modulated to maintain the selected noise level throughout the entire scan.
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CLINICAL USE OF ATCM TECHNIQUES |
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TOPABSTRACTINTRODUCTIONPRINCIPLES OF ATCM TECHNIQUESCLINICAL USE OF ATCM...LIMITATIONS OF AVAILABLE ATCM...CONCLUSIONESSENTIALSREFERENCES |
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Z-Axis Modulation
The AutomA z-axis–modulation technique is offered with recently available multi–detector row CT scanners (LightSpeed; GE Medical Systems). Although angular modulation has been compared with fixed-tube-current techniques, to date there are no studies of which we are aware in which angular modulation has been compared with z-axis modulation in terms of image quality or radiation dose efficiency (18–20).
In scanners with the AutomA feature, the operator can use either a fixed-tube-current technique or a z-axis–modulation technique. In fixed-tube-current scanning protocols, the technologist selects a suitable tube current for all examinations on the basis of his or her judgment or departmental guidelines. With this z-axis–modulation technique, instead of selecting a fixed tube current the technologist selects a noise index and a range of acceptable tube current (minimum and maximum milliampere) settings. As described in the preceding section, the noise index determines image quality. No unit of measure has been assigned to the noise index, but because it approximates the standard deviation (in Hounsfield units) on CT images of a phantom, the noise index can be expressed in Hounsfield units. From a practical standpoint, the technologist selects the noise index and the acceptable range of tube current settings for the scan (Fig 2b). A 5% decrease in noise index (in Hounsfield units) typically increases radiation exposure by 10% and vice versa.
Determination of the range of tube current values can sometimes be influenced by patient habitus. Because noise index and tube current range affect image quality and radiation exposure to the patient, these two parameters must be judiciously selected. A very low noise index may provide higher image quality but will also result in higher than necessary radiation exposure to the patient. Conversely, a higher noise index will result in radiation dose reduction at the price of noisier images. Thus, with the AutomA technique, radiation exposure depends on the selected noise index and patient size. Higher radiation exposure can be avoided by selecting a higher noise index or by setting the maximum tube current parameter to the same level used with fixed-tube-current protocols. Although image noise will increase in regions where the required tube current is limited by the maximum tube current, overall image quality would be similar to that with fixed-tube-current scanning. The vendor prescribes a noise index of 11–12 HU for routine abdominal and pelvic examinations and 10–11 HU for chest examinations, with a minimum tube current of 60–80 mA.
The protocol used at our institution for routine chest, abdominal, and pelvic CT examinations with a 16–detector row CT scanner and the AutomA technique of z-axis modulation is summarized in Table 1. Once the user has determined the desired parameter adjustments (including noise index), the parameters can be stored in the computer memory and recalled without need for modification in similar clinical cases.
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Conversely, with small patients, particularly when not correctly positioned, ATCM techniques can result in excessive reduction in tube current and very noisy images. Instructions to technologists about the necessity of proper centering of patients in the gantry isocenter and increasing the limit for the minimum tube current parameter usually help limit the probability that noisy images will be obtained in small patients. Selecting a lower noise index for small patients while capping the maximum tube current parameter to avoid higher than necessary radiation exposure can also help improve image quality (5).
Authors of a recent study (4) found that the AutomA technique for pelvic and abdominal CT resulted in a mean reduction in tube current–time product of 33% (range, 1%–91%), compared with manual selection of tube current, in 87% (54 of 62) of patients. Compared with manual selection of a fixed tube current, CT examinations of abdomen and pelvis performed with the z-axis–modulation technique provided images with similar noise, diagnostic acceptability, and lesion detectability. In a study of 153 patients who underwent abdominal-pelvic CT with the AutomA technique (5), a greater reduction in mean radiation dose was noted in smaller patients (mean weight = 72 kg ± 17 [standard deviation], mean anteroposterior abdominal diameter = 23.2 cm ± 3.5, mean transverse diameter = 30.9 cm ± 3.8) than in larger patients (weight = 82 kg ± 16, anteroposterior diameter = 26.9 cm ± 3.9, transverse diameter = 34 cm ± 3.4). Findings from a recent phantom study (6) showed that kidney stones smaller than 5 mm can be adequately evaluated by using AutomA technique, with 56%–77% reduction in radiation dose relative to the dose from a fixed-tube-current technique. In addition, the authors of that study reported that patients with kidney stones can be adequately scanned with a noise index of 20 HU.
With the Real E.C. technique of z-axis modulation, the noise level can be incorporated into the examination protocol, thus relieving the operator of any need to apply the technique to individual examinations. Should any protocol need adjustment to suit the needs of individual patients, the Real E.C. technique automatically evaluates the effects on the noise level of parameters such as peak voltage, section thickness, helical pitch, and reconstruction algorithm and selects the required tube current to deliver the desired standard deviation.
The availability of multiple choices of image noise is consistent with recent attempts to reduce patient dose by careful matching of technique not only to patient habitus but also to diagnostic needs. By incorporating the choices of image quality and dose within the stored protocol, the operator can make modifications at the time of scanning to ensure that radiation dose reduction is achieved while a specific image quality is generated.
Angular Modulation
In many studies, angular modulation (eg, CARE Dose) has been reported to help reduce radiation exposure in phantom experiments and clinical studies involving both adults and children (18–20). With angular modulation, the effective tube current–time product is the key difference. Scanners with the angular modulation technique offer users the option of scanning with angular modulation or fixed tube current (Fig 1b). After selecting other scanning parameters such as peak voltage, table feed, and detector configuration, the user enters a value for the effective tube current–time product for the scan. The effective tube current–time product (mAseff) is defined as follows: mAseff = (TC · GR)/PF, where TC is the tube current (in milliamperes), GR is the gantry rotation time (in seconds), and PF is the CT pitch factor.
The scanner maintains a constant effective tube current–time product irrespective of pitch value so that radiation dose does not vary as pitch is changed. This helps to maintain a constant image quality as the pitch factor (table feed) changes. Consequently, an increase in pitch leads to an increase in tube current, and a decrease in pitch leads to a reduction in tube current. After selection of the effective tube current–time product, the scanner performs real-time modulation of tube current in the x-y plane. The selected effective tube current–time product represents the maximum effective value that will be used for scanning. The angular-modulation technique reduces the effective tube current–time product to a value less than that selected for projection angles with a lower attenuation profile. The final displayed effective tube current–time product represents an average of various effective values used in different projections (Fig 5).
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The Dose-Right Dose Modulation technique of ATCM uses the milliampere value of the default protocol as a near-upper (within 10%) bound and modulates the tube current during gantry rotation on the basis of the system’s prior attenuation measurements in cross-sectional anatomy. The Dose-Right Dose Modulation technique produces an image signal-to-noise ratio and image quality that are very close to those of the full nonmodulated milliampere image quality of the system but with marked reduction in radiation dose. Within a particular protocol, the user can select both ACS and Dose-Right Dose Modulation independently or together. As described in a previous section, the ACS icon on the user interface automatically suggests a patient-tailored tube current–time product setting; Dose-Right Dose Modulation then modulates this setting downward during rotation to reduce dose without affecting the desired image quality. The volume CT dose index and dose length product are displayed according to the protocol tube current–time product and the selected protocol parameters. Radiation dose reduction with Dose-Right Dose Modulation ranges between 15% and 40%, depending on the patient’s size and the anatomy being scanned. If Dose-Right Dose Modulation is selected, the actual volume CT dose index and dose length product are displayed after the scan on the basis of the actual average tube current–time product used for the scan, which aids the operator in monitoring reductions in the volume CT dose index and dose length product by comparing the prescan (programmed) and postscan (actual) dose values.
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LIMITATIONS OF AVAILABLE ATCM TECHNIQUES |
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TOPABSTRACTINTRODUCTIONPRINCIPLES OF ATCM TECHNIQUESCLINICAL USE OF ATCM...LIMITATIONS OF AVAILABLE ATCM...CONCLUSIONESSENTIALSREFERENCES |
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Because z-axis modulation is a recent innovation, there is a noticeable lack of scientific documentation in the medical literature regarding appropriate noise indexes for specific sizes or ages of patients and specific clinical indications. In addition to selecting manufacturer-recommended noise indexes, however, users can restrict the range of tube currents that can be used for scanning in order to cap the maximum dose to ensure a minimum image quality. However, use of an arbitrary range of tube currents introduces subjective constraints to the technique. A lower minimum tube current may result in reduced patient exposure, which occasionally results in noisier images in small patients who are scanned at a substantially reduced tube current. Conversely, large patients occasionally receive higher tube current with z-axis modulation than they would receive if a fixed-tube-current technique was used in order to maintain the selected image noise. While ATCM results in better image quality in this setting, it can result in greater radiation exposure than would a fixed-tube-current protocol in large patients.
The foremost limitation of ATCM is the lack of uniformity between techniques developed by different vendors. With the addition of ATCM techniques to the technologic revolution in CT scanners, user comprehension is confounded by the separate protocols for systems from different vendors. Industry standards organizations need to build a consensus so that a more uniform ATCM technique is offered to the user in order to minimize confusion and ensure appropriate use of the technique. Further experience and research with ATCM techniques, regardless of type, will aid in optimization these techniques. In addition, the introduction of ATCM to an institution requires close communication between radiologists, medical physicists, and technologists, because a definite learning curve exists.
Presently, no vendor offers a combined ATCM technique that uses both angular and z-axis modulation as complementary approaches for maximum radiation benefits, although industry expects to release a combination technique by the end of 2003. A work-in-progress installation (Care Dose 4D; Siemens Medical Solutions), which combines the angular- and z-axis–modulation techniques, has been reported (22,23). This technique modulates the tube current within the section (angular modulation) and also effects a change of attenuation in different anatomic regions (z-axis modulation). To adapt the tube current in the z-axis, this technique uses a database that correlates image quality at a specific effective tube current–time product for different anatomic regions and a "standard" body with the effective tube current–time product that is necessary for individual patient body habitus to achieve similar image quality. For the examination, the user enters an effective milliampere-seconds setting for a "normal-sized" patient to obtain a specific expected image quality. The CARE Dose 4D protocol adapts the tube current to the patient’s individual anatomy and modulates the tube current in the section and the z-axis to obtain the desired image quality for all images at the lowest dose levels. Initial results (22,23) have shown a 20%–60% dose reduction, depending on the anatomic region and patient habitus, with improved image quality.
Authors of another study (24) in which combined angular and z-axis modulation (3D Auto mA; GE Yokogowa Medical Systems, Tokyo, Japan) was used have also reported dose reductions of 60% in abdominal-pelvic CT examinations. This technique uses a single localizer radiograph to determine patient asymmetry and appropriate angular and z-axis modulation for the patient. The investigators added noise (computer modification of original raw scan data to simulate lower tube current noise levels) to patients’ scan data to produce images and calculate the radiation dose reduction.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONPRINCIPLES OF ATCM TECHNIQUESCLINICAL USE OF ATCM...LIMITATIONS OF AVAILABLE ATCM...CONCLUSIONESSENTIALSREFERENCES |
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ESSENTIALS |
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TOPABSTRACTINTRODUCTIONPRINCIPLES OF ATCM TECHNIQUESCLINICAL USE OF ATCM...LIMITATIONS OF AVAILABLE ATCM...CONCLUSIONESSENTIALSREFERENCES |
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These techniques adjust tube current in an effort to maintain constant image quality at the lowest dose.
Automatic tube current modulation represents an exciting recent technologic innovation to enable radiation dose optimization.
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONPRINCIPLES OF ATCM TECHNIQUESCLINICAL USE OF ATCM...LIMITATIONS OF AVAILABLE ATCM...CONCLUSIONESSENTIALSREFERENCES |
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K. Rothkamm, S. Balroop, J. Shekhdar, P. Fernie, and V. Goh Leukocyte DNA Damage after Multi-Detector Row CT: A Quantitative Biomarker of Low-Level Radiation Exposure Radiology, January 1, 2007; 242(1): 244 - 251. [Abstract] [Full Text] [PDF]
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P C Shrimpton, M C Hillier, M A Lewis, and M Dunn National survey of doses from CT in the UK: 2003 Br. J. Radiol., December 1, 2006; 79(948): 968 - 980. [Abstract] [Full Text] [PDF]
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M. K. Kalra and S. Saini Standardized Nomenclature and Description of CT Scanning Techniques Radiology, December 1, 2006; 241(3): 657 - 660. [Full Text] [PDF]
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S. Namasivayam, M.K. Kalra, K.M. Pottala, S.M. Waldrop, and P.A. Hudgins Optimization of Z-Axis Automatic Exposure Control for Multidetector Row CT Evaluation of Neck and Comparison with Fixed Tube Current Technique for Image Quality and Radiation Dose AJNR Am. J. Neuroradiol., November 1, 2006; 27(10): 2221 - 2225. [Abstract] [Full Text] [PDF]
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W. C. Chan, B. N. Joe, F. V. Coakley, E. L. Prien Jr, R. G. Gould, S. Prevrhal, W. C. Barber, K. S. Kirkwood, A. Qayyum, and B. M. Yeh Gallstone Detection at CT in Vitro: Effect of Peak Voltage Setting. Radiology, November 1, 2006; 241(2): 546 - 553. [Abstract] [Full Text] [PDF]
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A. Graser, B. J. Wintersperger, C. Suess, M. F. Reiser, and C. R. Becker Dose reduction and image quality in MDCT colonography using tube current modulation. Am. J. Roentgenol., September 1, 2006; 187(3): 695 - 701. [Abstract] [Full Text] [PDF]
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H. T. Abada, C. Larchez, B. Daoud, A. Sigal-Cinqualbre, and J.-F. Paul MDCT of the Coronary Arteries: Feasibility of Low-Dose CT with ECG-Pulsed Tube Current Modulation to Reduce Radiation Dose Am. J. Roentgenol., June 1, 2006; 186(6_Supplement_2): S387 - S390. [Abstract] [Full Text] [PDF]
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S. Rizzo, M. Kalra, B. Schmidt, T. Dalal, C. Suess, T. Flohr, M. Blake, and S. Saini Comparison of Angular and Combined Automatic Tube Current Modulation Techniques with Constant Tube Current CT of the Abdomen and Pelvis. Am. J. Roentgenol., March 1, 2006; 186(3): 673 - 679. [Abstract] [Full Text] [PDF]
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J. Campbell, M. K. Kalra, S. Rizzo, M. M. Maher, and J.-A. Shepard Scanning Beyond Anatomic Limits of the Thorax in Chest CT: Findings, Radiation Dose, and Automatic Tube Current Modulation Am. J. Roentgenol., December 1, 2005; 185(6): 1525 - 1530. [Abstract] [Full Text] [PDF]
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M. K. Kalra, S. Rizzo, M. M. Maher, E. F. Halpern, T. L. Toth, J.-A. O. Shepard, and S. L. Aquino Chest CT Performed with Z-Axis Modulation: Scanning Protocol and Radiation Dose Radiology, October 1, 2005; 237(1): 303 - 308. [Abstract] [Full Text] [PDF]
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J. C. Weinreb, P. A. Larson, P. K. Woodard, W. Stanford, G. D. Rubin, A. E. Stillman, D. A. Bluemke, A. J. Duerinckx, N. R. Dunnick, and G. G. Smith American College of Radiology Clinical Statement on Noninvasive Cardiac Imaging Radiology, June 1, 2005; 235(3): 723 - 727. [Full Text] [PDF]
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M. K. Kalra, M. M. Maher, R. V. D'Souza, S. Rizzo, E. F. Halpern, M. A. Blake, and S. Saini Detection of Urinary Tract Stones at Low-Radiation-Dose CT with Z-Axis Automatic Tube Current Modulation: Phantom and Clinical Studies Radiology, May 1, 2005; 235(2): 523 - 529. [Abstract] [Full Text] [PDF]
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S. M. R. Rizzo, M. K. Kalra, B. Schmidt, J.-F. Paul, A. Sigal-Cinqualbre, and H. Abada Automatic Exposure Control Techniques for Individual Dose Adaptation * Dr Paul and colleagues respond: Radiology, April 1, 2005; 235(1): 335 - 336. [Full Text] [PDF]
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