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Special Review |
1 From the Department of Radiology, Massachusetts General Hospital, Harvard Medical School, White 270-E, 55 Fruit St, Boston, MA 02114 (M.K.K., M.M.M., L.M.H., M.A.B., J.A.S., S.S.); and General Electric Medical Systems, Milwaukee, Wis (T.L.T.). Received December 19, 2002; revision requested February 21, 2003; final revision received April 18; accepted May 1. Address correspondence to S.S. (e-mail: ssaini@partners.org).
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ABSTRACT |
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© RSNA, 2004
Index terms: Computed tomography (CT), image quality • Computed tomography (CT), multi–detector row, **.12115, **.121182 • Computed tomography (CT), radiation exposure, **.47 • Radiations, exposure to patients and personnel, **.47 • Review
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INTRODUCTION |
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CT SCANNING: DATA AND RISK PROJECTION |
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While CT accounts for only 11% of x-ray–based examinations in the United States, it delivers over two-thirds of the total radiation dose associated with medical imaging (5). In the United Kingdom, the population-averaged effective dose comprises x-ray procedures in hospitals (87%), nuclear medicine examinations (11%), mammography screening (1.5%), and extramural dentistry (0.2%) (6). The contribution of CT to the collective effective dose from medical exposure to the population increased to an estimated 40% in 1999, in comparison with 20% in 1990. Similarly, in Poland the number of CT examinations increased from 170,000 in 1995 to 460,000 in 1999, accounting for a fourfold increase in collective effective radiation dose and nearly a threefold increase in CT examinations in this period (7). In the Netherlands, the annual effective dose from diagnostic medical exposure in 1998 increased to 0.59 mSv per capita, reflecting an increase of 26% since the previous inventory in the Netherlands a decade earlier (8). The increase in patient dose was attributed to the upsurge in frequency of CT examinations and vascular radiologic procedures. The United Nations Scientific Committee on the Effects of Atomic Radiation 2000 report on medical radiation exposure stated that, worldwide, CT constitutes 5% of radiologic examinations and contributes 34% of the collective dose (9).
Owing to the burgeoning application of CT, there is an emergent need for radiation dose reduction to avoid a reversal of the risk-benefit ratio associated with this imaging modality (10). Risks associated with radiation exposure can be considered with regard to two main categories: namely, deterministic effects or stochastic effects. Deterministic risk results from cell death and is quantified in terms of radiation dose to a particular region that has a threshold level beyond which these effects generally occur. Deterministic risks are rarely seen with diagnostic x-ray–based examinations, including CT, because radiation doses typically do not reach the threshold level. Indeed, the main risks to the subject undergoing a diagnostic x-ray–based examination are due to stochastic effects, which may result in cancer, and genetic effects, which occur in the offspring of the irradiated subject. The probability of stochastic effects depends on the amount of absorbed dose. In an American College of Radiology publication, Gray (11) emphasized the need for radiation reduction in the following manner:
The estimated risk of cancer death for those undergoing CT is 12.5/10,000 population for each pass of the CT scan through the abdomen. This risk compares with 12 cancer deaths/10,000 population from 1 year of smoking in a similar population (however, one should take into account that the risk of smoking may be much greater when considered throughout a lifetime, in which case the risks from CT examinations become much smaller than those from smoking).
In addition, an International Commission on Radiological Protection Special Task Force report on CT radiation exposure (10) stated that CT radiation doses are relatively high. The radiation doses from CT to tissues can often approach or exceed the levels known to increase the probability of cancer. Indeed, Brenner et al (12) projected an increased risk of cancer mortality in children as a result of CT radiation exposure. They estimated the lifetime cancer mortality risks attributable to CT radiation exposure in a 1-year-old to be 0.18% (for abdominal CT) and 0.07% (for head CT), which represents a small increase in cancer mortality over the natural background rate. In the United States, where 600,000 abdominal and head CT examinations are performed annually in children younger than 15 years, an estimated 500 of those scanned may ultimately die of cancer attributable to CT radiation (12).
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IMPORTANT CT SCANNING PARAMETERS |
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In conventional radiography, radiation dose decreases continuously from the beam’s entrance into the body to its exit, whereas in CT the dose is distributed more uniformly across the scanning plane because the patient is equally irradiated from all directions. In a head CT examination, for instance, the dose is uniform across the field of view. In larger objects such as the abdomen, the dose is equally distributed around the periphery of the scanned object and decreases by a factor of only two near the center of the object. Hence, dose comparisons between CT and conventional radiography in terms of skin dose are not appropriate. Furthermore, the radiation energy delivered by CT is not fully contained within the scanning volume. Scattered radiation, divergence of the radiation beam, and limits to the efficiency of beam collimation all contribute to the radiation exposure beyond the boundaries of the scan volume. In the case of the multiple scan acquisitions required to image some length of a patient’s anatomy, it becomes essential to consider the effect of the radiation dose delivered beyond the boundaries of a single scan.
The radiation dose descriptor known as the CT dose index, or CTDI, integrates the radiation dose delivered both within and beyond the scan volume. CTDI is the principle dose descriptor in CT. The average across the field of view to take into account variations in absorbed dose from the periphery to the center of the object results in a dose descriptor known as the weighted CTDI, or CTDIw. CTDIw represents the average dose in the scan volume for contiguous CT scans.
In the case when there is either a gap or an overlap between sequential scans, CTDIw must be scaled accordingly, resulting in the dose descriptor volume CTDI, or CTDIvol. CTDIvol represents the average dose within a scan volume (relative to a standardized CT phantom) and is now required to be displayed on the user interface of the CT scanner. CTDIvol is presented in milligrays. While it is not the dose to any specific patient, it is a standardized index of the average dose delivered from the scanning series. Intuitively, a longer series imparts a higher total radiation dose to the patient than does a shorter series. The term dose length product is used to represent the integrated dose and is equal to the average dose within the scanning volume (CTDIvol) times the total scan length (in centimeters). This parameter is also displayed on some CT systems.
Image noise, an important determinant of CT image quality, is inversely related to the x-ray beam energy. Although a decrease in tube current or tube voltage results in a reduction in radiation dose, such a decrease is also associated with an increase in image noise, which may compromise the image quality to a variable extent. Thus, while CT radiation dose reduction is a crucial issue given the risks of radiation exposure, it is equally essential to realize the benefit of a "quality CT examination" that adequately addresses pertinent clinical issues affecting patient care (10). Therefore, radiation dose reduction, although prudent when appropriate, must not compromise the diagnostic outcome of a clinically relevant examination. It is worthwhile to remember that in most circumstances, strategies should be directed toward radiation dose optimization rather than dose reduction per se, so that the image quality maintains a diagnostic standard. For instance, high radiation dose may not necessarily provide substantially improved image quality and increased lesion conspicuity in comparison with standard or even low-dose scanning (Fig 1). Research on dose reduction must, therefore, focus on image quality and standard practice. The challenge to practitioners is to identify acceptable thresholds of image quality so that the minimum radiation doses needed to achieve these can be determined. Definition of image quality must extend to issues of lesion detection so that the goal of radiation dose optimization can be achieved. The challenge to CT scanner manufacturers is to improve the dose efficiency of CT systems and to provide features that allow practitioners to further reduce the dose needed while achieving the required diagnostic confidence.
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Scanner Geometry
The distance between the focal spot of the x-ray tube and the isocenter of the scanner depends on scanner geometry, since single– or multi–detector row helical CT scanners can have a long or a short geometric configuration. According to the inverse square law, radiation intensity varies with the inverse of the squared distance between radiation source and patient. Thus, if all other scanning parameters are identical, a short-geometry scanner can produce more interaction of radiation with the patient and lower image noise than a long-geometry scanner can. This underscores the fact that the "transfer" of scanning parameters from one scanner type to another should be performed with caution, so that image quality can be maintained with identical or reduced radiation dose, depending on scanner geometry and other attributes (eg, reconstruction algorithms) (15).
Tube Current and Potential
Reduction in tube current is the most practical means of reducing CT radiation dose. A 50% reduction in tube current reduces radiation dose by half. The beam energy and photon fluence of an x-ray beam varies with the tube potential and the current used during the particular examination. Tube current–time product settings are proportional to the number of photons in the defined exposure time (photon fluence). Authors of previous studies (16–25) on CT of the head, neck, chest, abdomen, and pediatric pelvis have suggested that it is possible to reduce tube current without markedly affecting image quality. Any decrease in tube current should be considered carefully, because such reduction causes an increase in image noise, which may affect the diagnostic outcome of the examination. This is especially true in abdominal studies, where low-contrast areas are severely affected by an increase in image noise (10). The pelvis, however, with its greater inherent contrast is usually not noticeably affected. According to a recent review of scanning protocols (24), diagnostic-quality abdominal CT scans can be obtained at lower tube currents with a four–detector row scanner.
Tube potential (peak voltage) determines the incident x-ray beam energy, and variation in tube potential causes a substantial change in CT radiation dose. The effect of tube voltage on image quality is more complex, since it affects both image noise and tissue contrast. An important outcome that may be associated with decreased tube voltage is a notable increase in image noise. This occurs if the patient is too large or the tube current is not appropriately increased to compensate for the lower tube voltage. The dose change is approximately proportional to the square of the tube voltage change (ie, square of the ratio of final and initial peak voltage), and the noise change is approximately inversely proportional to the tube voltage change (10).
Image quality ramifications of a decrease in tube voltage to reduce radiation exposure must be carefully examined before this strategy is implemented. For most patients, abdominal CT can be optimally performed at 120 kVp instead of 140 kVp, resulting in a 20%–40% reduction in radiation dose (24). For very large patients, a higher tube voltage is generally more appropriate. There is a need for further research on the use of lower tube voltage for dose advantages, because of the complex relationship between tissue contrast, image noise, and radiation dose that depends on patient size. According to preliminary results reported by Lieberman et al (26), head CT performed in children at a substantially reduced tube voltage (if performed with increased tube current) may result in the lowest possible patient dose with no decrease in image contrast-to-noise ratio. However, further studies should precede such a reduction in the tube voltage used to acquire CT scans.
Scanning Modes
Use of a multi–detector row CT scanner results in some amount of unused radiation extending beyond the beginning and end of the imaging region (27). This occurs because, at the start of the acquisition, only the first detector row is contributing to the image. As the acquisition proceeds, additional detector rows enter the imaging region until all rows are contributing. A similar effect occurs in reverse at the end of the acquisition. As a result, it is generally more dose efficient to use a single helical scan rather than multiple helical scans if there are no overriding clinical considerations, such as breath holding, for the patient. The need to prescribe multiple contiguous helical scans should be infrequent with modern high-speed multi–detector row scanners.
Scanning Length
With the widespread availability of helical CT scanners, there is a general tendency to increase the area of coverage (to include regions beyond the actual area of interest in the chest, abdomen, or pelvis), which increases effective radiation dose to the patient (10). Therefore, it is essential to draw the attention of referring physicians and radiologists to dose consequences and to establish scanning protocols that restrict the examination to what is absolutely essential.
Collimation, Table Speed, and Pitch
For helical CT scanners, pitch is defined as the ratio of table feed per gantry rotation to the nominal width of the x-ray beam. An increase in the pitch decreases the duration of radiation exposure to the anatomic part being scanned. With helical CT scanners, beam collimation, table speed, and pitch are interlinked parameters that affect the diagnostic quality of an imaging study.
Faster table speed for a given collimation, resulting in higher pitch, is associated with a reduced radiation dose (especially if other scanning parameters, including tube current, are held constant) because of a shorter exposure time, whereas narrow collimation with slow table speed, resulting in a longer exposure time, is associated with a higher radiation dose. This is not true for scanners that use an effective milliampere-second setting (defined as milliampere seconds divided by pitch) and maintain a constant value for effective milliampere seconds. In such scanners, the effective milliampere-second level is held constant irrespective of pitch value, so that radiation dose does not vary as pitch is changed. For a given collimation, an increase in table speed increases the pitch and reduces the radiation dose by 1 divided by the pitch (10,28–30). Modern multi–detector row scanners may automatically recommend the appropriate tube current adjustment to maintain a given image noise level when pitch is changed.
Although scanning at a higher pitch is generally more dose efficient, it also tends to cause helical artifacts, degradation of the section-sensitivity profile (section broadening), and decrease in spatial resolution. Hence, alterations in pitch can have varying effects on image quality in different situations. For instance, in CT colonoscopy image quality and reconstruction artifacts are less affected by pitch than by beam collimation, so that a higher pitch with narrow beam collimation may be preferable for reducing radiation dose (31,32). However, in situations such as imaging of metastatic liver lesions or pancreatic lesions, which generally require thin collimation, an increased pitch may affect detectability because lesions may be missed owing to degradation of the section-sensitivity profile (10). We have not noted any marked difference in the image quality of scans obtained at a pitch of 1.5:1 relative to images obtained at a pitch of 0.75:1. Hence, at our institution we acquire most abdominal scans with a pitch of 1.5:1, which results in up to 50% radiation dose saving in comparison with the dose with a pitch of 0.75:1, with other scanning parameters unchanged.
Owing to "overbeaming" in multi–detector row CT, some amount of the x-ray beam is incident beyond the edge of the detector rows (27,30). Generally, thicker beam collimation in multi–detector row CT results in a more dose-efficient examination, because overbeaming constitutes a smaller proportion of the detected x-ray beam. Depending on the scanner type, however, thick collimation limits the width of the thinnest sections that can be reconstructed. On the other hand, although thin collimation increases the proportion of overbeaming x rays, it allows reconstruction of thinner sections. Hence, beam collimation and pitch must be carefully selected to address specific clinical requirements. For instance, a thicker collimation and a pitch greater than 1:1 is usually sufficient for screening examinations such as CT colonography and CT for urinary tract calculus. However, CT scanning for certain clinical situations, such as liver resection or transplantation work-up, is frequently performed with thin collimation and a pitch of less than 1:1.
Gantry Rotation Time
There has been a dramatic decrease in tube rotation times with recent technologic innovations, most notably with the development of four–, eight–, and, recently, 16–detector row CT scanners. Whereas a four-row scanner with a 0.8-second rotation time requires a 16-second breath hold to scan the entire abdomen, an eight-row scanner covers this length in 8 seconds. If the tube rotation time is decreased (faster gantry rotation), the radiation exposure decreases, and tube current may thus have to be increased to maintain constant image quality (10). Modern 16-row scanners are capable of high scanning speeds and submillimeter section thicknesses. Thin collimation can lead to a higher dose, especially if tube current is increased to maintain image noise at a level similar to that of thicker sections. The contrast resolution of small lesions improves because of reduced partial volume effects; hence, greater noise on thinner sections may often be acceptable (33). In addition, submillimeter-collimation scans can usually be reconstructed as thicker sections, which reduces inherent noise. Thus, it is important to optimize beam collimation for different multi–detector row scanners on the basis of the clinical situation in question.
Shielding
Protection of radiosensitive organs, such as the breast, eye lenses, and gonads, is particularly relevant in pediatric patients and young adults, because these structures frequently lie in the beam pathway (10,34). Beaconsfield et al (35) studied the effect of shielding regions of the body that are not included in the direct path of the x-ray beam during CT. They reported that with lead protection, thyroid and breast radiation doses were reduced by an average of 45% and 76%, respectively, in 110 patients undergoing routine head CT. Therefore, external shielding may be helpful in reducing radiation exposure to parts that are not included in the examination field. In examinations where the gonads are included in the field but are not the organs of clinical concern, some form of shielding should be used. Hidajat et al (34) showed that in abdominal CT examinations, the testis capsule is an important instrument for reducing the dose absorbed by the testes (by up to 95%), whereas the lead apron is not appropriate for dose reduction to the ovaries (due to their inconstant position). Hein et al (36) reported that the use of a shield for protection of eye lenses in paranasal sinus CT is a suitable and effective means of reducing surface radiation dose by 40%.
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RADIATION DOSE REDUCTION AND JUDICIOUS PRACTICES |
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CT images are often acquired before, during, and after intravenous administration of contrast material. When medically appropriate, multiple exposures may be reduced by eliminating precontrast imaging. This may be especially relevant in the evaluation of liver and bowel wall conditions, where precontrast images can frequently be omitted without affecting the interpretation of the imaging study. As recommended by the International Commission on Radiological Protection, all CT performed for research purposes but without immediate benefit to the individual undergoing the examination should be subject to critical evaluation, since the doses can be markedly higher than those of conventional radiography (10). A critical step toward uniform optimization of CT radiation dose is the establishment of uniform protocols for all examinations on the basis of patient attributes (dimension, weight) and/or scanning features (image noise, automatic modulation of tube current). This will ensure that diagnostic quality images are acquired by using radiation doses that are reduced to the lowest levels possible.
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MODULATION OF CT PARAMETERS FOR DOSE REDUCTION |
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Weight
Several investigators have suggested that tube current settings can be substantially reduced for CT of the chest in both adults and children (20,21,37–40). Image quality identical to that in adults can be obtained in pediatric patients by using markedly reduced radiation exposure. For abdominal CT, Donnelly et al (25) described modulation of scanning parameters in children on the basis of weight. They documented that patient weight can be used to select an appropriate tube current that is much lower than the adult settings used earlier for pediatric abdominal CT. In addition, Donnelly et al suggested the use of a substantially reduced tube current for children weighing 4.5–68.0 kg. Recent studies have shown that for adult patients, too, radiation exposure from abdominal CT can be reduced substantially. For abdominal CT in adults, tube current can be reduced on the basis of patient weight (22). The quality of abdominal CT images obtained with a four–detector row scanner at a 50%-reduced radiation dose was compared with that of images obtained with a standard dose. Although standard-dose images were less noisy and more visually pleasing in patients weighing less than 81.6 kg, image quality at 50%-reduced tube current was acceptable. In contrast, for patients weighing more than 81.6 kg, reduced-dose CT images were found to be too noisy, and image quality was not acceptable. It follows that lighter patients should be evaluated with reduced radiation by changing the tube current according to the patient’s weight.
Cross-sectional Dimensions
Attenuation of the incident x-ray beam in CT depends on the size of the body portion being evaluated; that is, greater exposure is required in corpulent patients to attain image quality equal to that in slimmer patients (41). Selection of CT parameters on the basis of a patient’s weight can lead to large variations in image quality between, for instance, two persons with the same weight but different height. Scanning parameters can, therefore, be modified on the basis of cross-sectional body dimensions to optimize radiation exposure from CT.
Haaga et al (42) reported that image noise was related to cross-sectional dimensions and advocated the use of cross-sectional measurements for optimizing scanning parameters and CT radiation dose. A new method has recently been reported (43), in which radiation dose is varied to achieve similar levels of image noise for patients with various abdominal diameters, thereby minimizing radiation dose in most cases. This method results in substantial radiation dose reduction for these patients. Modulation of scanning parameters by using the diameter of the anatomic cross section being evaluated resulted in a reduction in dose of up to 45%. The results suggest a potential for reduction of radiation exposure in slim patients on the basis of cross-sectional abdominal diameter. Similarly, a significant correlation has been reported (22) between reduced-dose image quality and abdominal cross-sectional parameters such as abdominal circumference, cross-sectional area, and anteroposterior and transverse diameters of the abdomen. At 50%-reduced tube current (half the radiation dose), image quality was acceptable in patients with a cross-sectional area of less than 800 cm2, a circumference of less than 105 cm, a root mean square diameter of less than 44 cm, an anteroposterior diameter of less than 28 cm, and a transverse diameter of less than 34.5 cm. Conversely, image quality with reduced–tube current CT was unacceptable in patients with larger abdominal dimensions (ie, exceeding the aforementioned measurements).
These dimensions can be easily calculated before the examination with a simple measuring caliper. Alternatively, the technologist can directly measure these dimensions at the CT console monitor by using a fixed landmark on a scout image, a single precontrast image, or automated bolus-tracking images. McCollough et al (44) evaluated the use of size-based CT technique charts for reducing radiation dose to pediatric and small patients and for improving image quality in large patients. They reported that modifications of tube current in proportion to patient width are feasible and result in a two- to fourfold dose reduction for small patients.
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LITERATURE ON CT RADIATION REDUCTION |
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Chest CT Scanning
Dose requirements for CT of the chest are much smaller than those for the abdomen because of low x-ray absorption in the lungs (14). As a consequence, chest CT can be performed at a lower tube current than abdominal CT can. For chest CT, Prasad et al (37) documented acceptable image quality for evaluation of normal anatomic structures with a 50% reduction in tube current. Low-dose CT with reduced tube current has been reported to be as effective as standard-dose CT performed with a higher tube current for demonstration of pathologic findings in the lung and mediastinum (20). Therefore, low-dose CT should be considered as a viable alternative to standard-dose CT, especially in young patients with benign disease (38).
Low-dose CT has been recommended for screening for lung cancer (Fig 2). Promising results have been shown with very low tube currents (38,45,46). Similarly, studies have shown that pulmonary nodules can be detected with equal effectiveness at low-dose CT performed with substantially reduced tube current (38). For detection of benign asbestos-related pleural-based plaques and thickening, Michel et al (47) have reported that low-dose high-resolution CT of the chest can give results that are equivalent to those of scans obtained with a standard higher radiation dose.
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CT for Urinary Tract Calculi
Unenhanced CT scans for evaluation of acute flank pain that are obtained with reduced radiation dose (low tube current) can give excellent diagnostic information with radiation exposure at least 50% lower than that of excretory urography (51,52). Spielmann et al (53) used an anthropomorphic torso phantom and reported unimpaired visualization of renal calculi on images obtained with markedly reduced tube current on single– and multi–detector row CT scanners, with a dose reduction of more than 75%. Similarly, some investigators have reported using unenhanced reduced-radiation-dose helical CT with a pitch of 2 or greater and obtaining satisfactory results in cases of suspected renal colic (Fig 3) (54,55).
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TECHNOLOGIC ADVANCES FOR RADIATION REDUCTION |
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X-ray Beam Utilization
Prepatient tracking, or control of x-ray tube focal spot motion and beam collimation, enhances scanner efficiency (by decreasing z-axis beam collimation) and thus reduces radiation exposure. With this technique, overbeaming is reduced by means of measurement of the beam position every few milliseconds and continual repositioning of a source aperture to hold a narrow beam fixed on the detector. Thus, the beam is stabilized on the detectors, allowing an x-ray exposure profile that is narrower than the detected x-ray profile, and the radiation dose associated with multi–detector row CT is reduced in comparison with that of systems with no focal spot tracking.
X-ray Filtration
X-ray filters decrease the "soft x rays" that constitute absorbed radiation that never reaches the detectors and thus does not contribute to the image. Efficient x-ray filters selectively remove these soft x rays and thus decrease absorbed radiation. Itoh et al (56) compared radiation exposure with an aluminum filter (5.8 mm thick at the center) and that with a conventional filter in a phantom and patient study. They noted a 17% reduction in radiation exposure and a 9% decrease in noise at very-low-dose CT with the new filter. Bow-tie filters or beam-shaping filters reduce the surface radiation dose by 50% compared with the dose with flat filters (27). Bow-tie and beam-shaping filters minimize radiation exposure in the thinner portions of patient anatomy, thus providing better noise consistency within the image while saving substantial amounts in radiation dose.
Automatic Modulation of Tube Current
Tube current modulation is a technical innovation that can substantially reduce radiation dose. The concept of automatic tube current modulation is based on the premise that pixel noise on a CT scan is attributable to quantum noise in the projections. By adjusting the tube current to follow the changing patient anatomy, quantum noise in the projections can be adjusted to maintain a desired noise level on the image and to improve dose efficiency.
There are two methods used on CT scanners today: z-axis modulation and angular (x- and y-axis) modulation. Both methods have a complementary role in minimizing patient dose. In z-axis modulation, tube current is adjusted to maintain a user-selected quantum noise level in the image data. Noise is regulated on the final image to a level desired by the user. In this sense, z-axis modulation is the CT equivalent of the autoexposure control systems used for many years with conventional x-ray systems. z-Axis modulation is an attempt to render all images with similar noise, independent of patient size and anatomy. The dose savings with z-axis modulation are expected to be greater than those with fixed–tube current methods, since the tube current will be automatically reduced for smaller patients and anatomic regions.
z-Axis modulation has been recently introduced for multi–detector row CT scanners (AutomA; GE Medical Systems, Waukesha, Wis). Tube current modulation is determined from the attenuation and shape of scout scan projections in the patient just prior to the CT examination. Clinical results of this technique have not yet been published in the literature.
Angular modulation has a different objective than z-modulation. In angular modulation, the tube current is adjusted to minimize x rays in projections (angles) that have less importance for the reduction of overall image noise content. In anatomy that is highly asymmetric (eg, the shoulders), x rays are much less attenuated in the anteroposterior direction than in the lateral direction (57–60). Thus, the overwhelming abundance of anteroposterior x rays can often be reduced dramatically without a marked effect on overall image noise. Angular modulation was first introduced on single–detector row scanners in 1994 (HiLight Advantage; GE Medical Systems) (61,62). Dose reductions of up to 25% were reported at that time, with virtually no change in image noise. On these early systems, both lateral and anteroposterior scout scans were required to determine angular modulation. More recently, angular tube current modulation has been introduced on multi–detector row scanners (CARE Dose; Siemens, Erlangen, Germany). In this implementation, the modulation is determined in real time by using projection data that lag 180° from the x-ray generation angle. A recent investigation of 100 helical CT imaging studies in children in whom angular modulation was used showed a 10%–60% decrease in dose, with a mean reduction of 22.3% (neck, 20%; thorax, 23%; abdomen, 23%; thorax and abdomen, 22%) without loss of image quality (63).
The ideal CT scanner will employ both z-axis and angular modulation techniques. When available in all commercial CT scanners, use of manual techniques, whereby a tube current value is selected on the basis of some simple measure of the patient (eg, weight or cross-sectional dimensions), will be replaced with this computerized objective approach. With these developments, tube current modulation in CT scanners will be comparable to photographic timing or automatic brightness controls currently used in conventional radiography. Indeed, automatic tube current modulation promises to be an important development in the optimization of scanning parameters that will help eliminate the guesswork involved in parameter selection.
Projection-adaptive Reconstruction Filters
A marked decrease in signal is common in regions such as the shoulders, owing to beam attenuation in a particular projection. This leads to increased image noise with substantial impairment of image quality and results from photon noise contamination with the electronic noise of the data-acquisition system. Projection space filters increase the filtration of signal-dependent noise in the reconstruction data and thus minimize the loss of resolution. Although there is some loss of image resolution (less than 5%) with the use of these filters, use of projection-adaptive reconstruction filters prevents an otherwise diagnostically compromised image. Kachelriess et al (64) investigated the use of multidimensional generalized adaptive filters for reducing image noise and patient radiation dose. They documented a 30%–60% reduction in image noise, typically along the direction of the highest attenuation in noncylindric body regions such as the shoulder and metallic implants, without an increase in radiation dose.
Computer-simulated Dose-Reduction Software
Evaluation of the effects of low-dose CT on image quality, lesion detection, and lesion conspicuity and comparison with standard-dose CT images is a fundamental part of dose-reduction research. This requires image acquisition with standard and reduced radiation doses, which frequently results in increased radiation dose to the volunteering subjects. Computer-simulated dose-reduction software adds noise to an image acquired at a particular tube current to simulate images acquired at a lower tube current (lower radiation). Mayo et al (65) reported that the technique provides realistic reduced-radiation-dose images of the chest without additional radiation exposure to patients. In a recent study, Frush et al (66) investigated the accuracy of computer-simulated dose reduction for evaluating systematic dose reduction for abdominal multi–detector row CT in pediatric patients. They reported that the technique could be applied to multi–detector row CT of abdominal organ systems for systematic evaluation of radiation dose reduction. Validation of this technique for simulating images acquired with reduced radiation dose in adult abdominal scans is needed to expand the applications and explore new areas of dose reduction. Leidecker et al (67) also investigated the feasibility of optimizing clinical CT protocols by providing tools for adding virtual noise to measured patient raw data in order to estimate the dose reduction potential for clinical CT protocols.
Filters
As discussed earlier, radiation dose reduction is limited by increased image noise that can obscure lesions otherwise visible on images obtained with standard higher dose parameters. Noise-reduction filters have been designed to decrease image noise on scans acquired with reduced radiation dose. Alvarez and Stonestrom (68) reported that two-dimensional linear filtering of the image may alter the spatial resolution and noise properties of CT images, depending on the knowledge of noise and imaging properties of the system. They developed filters that minimize the variation in noise subject to a constraint on spatial resolution, with a 17% reduction in noise variance in comparison with that of conventional filters. Use of nonlinear image-processing techniques, in particular smoothing, has also been reported (69) for creation of good-quality CT images obtained with lower radiation. Recently, Yu et al (70) reported use of a new algorithm for reconstruction of CT images with noise properties superior to those of images reconstructed with a conventional fan-beam filtered back-projection (FFBP) algorithm currently used in commercial CT systems, including multi–detector row scanners. This algorithm converts the fan-beam data to nonuniformly sampled parallel-beam data by invoking the Fourier shift theorem in the angular direction. The approach performs ramp filtration on nonuniform sampling grids along the radial direction before back projecting the filtered data to form the image. The decrease in noise with this algorithm may be translated into reduced x-ray dose delivered to the patient and enhanced detection of subtle lesions, compared with reconstructions based on the currently widely used FFBP algorithm.
Noise reduction filters have also been designed on the basis of the principle that a group of structural pixels representative of structures of interest and a group of nonstructural pixels representative of nonstructural regions are both present in any image (71,72). The structural pixels can be identified by determining gradient values for each pixel and by identifying pixels with a desired relationship to the gradient threshold value (73). The noise-reduction filter technique involves isotropic filtering of nonstructural regions with a low-pass filter and directional filtering of structural regions with a smoothing filter operating parallel to the edges and an enhancing filter operating perpendicular to the edges. A blending parameter regulates the recombination of the structural and nonstructural segments. Noise-reduction filters decrease noise on low-dose CT images but adversely affect contrast and sharpness and may therefore decrease lesion contrast and conspicuity (71). This was validated in a subsequent evaluation of lesion detection and characterization with low-dose CT images processed with noise-reduction filters (74). Although these noise-reduction filters decreased image noise on low-dose images, they also decreased lesion conspicuity and lesion-to-background contrast. Further improvement in the technique is needed, therefore, to maintain image contrast while decreasing image noise so that this concept can be adopted to optimize the quality of CT images acquired at reduced radiation dose and make them more acceptable.
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CONCLUSION |
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FOOTNOTES |
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REFERENCES |
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K. Matsubara, K. Koshida, K. Ichikawa, M. Suzuki, T. Takata, T. Yamamoto, and O. Matsui Misoperation of CT Automatic Tube Current Modulation Systems with Inappropriate Patient Centering: Phantom Studies Am. J. Roentgenol., April 1, 2009; 192(4): 862 - 865. [Abstract] [Full Text] [PDF]
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A. Sodickson, P. F. Baeyens, K. P. Andriole, L. M. Prevedello, R. D. Nawfel, R. Hanson, and R. Khorasani Recurrent CT, Cumulative Radiation Exposure, and Associated Radiation-induced Cancer Risks from CT of Adults Radiology, April 1, 2009; 251(1): 175 - 184. [Abstract] [Full Text] [PDF]
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T. A. Jaffe Reply Am. J. Roentgenol., March 1, 2009; 192(3): W141 - W141. [Full Text] [PDF]
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 J.S.P. Tan, K.-L. Tan, J.C.L. Lee, C.-M. Wan, J.-L. Leong, and L.-L. Chan Comparison of Eye Lens Dose on Neuroimaging Protocols between 16- and 64-Section Multidetector CT: Achieving the Lowest Possible Dose AJNR Am. J. Neuroradiol., February 1, 2009; 30(2): 373 - 377. [Abstract] [Full Text] [PDF]
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 W. De Wever, S. Stroobants, J. Coolen, and J. A. Verschakelen Integrated PET/CT in the staging of nonsmall cell lung cancer: technical aspects and clinical integration Eur. Respir. J., January 1, 2009; 33(1): 201 - 212. [Abstract] [Full Text] [PDF]
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 A N Desmond, K O'Regan, C Curran, S McWilliams, T Fitzgerald, M M Maher, and F Shanahan Crohn's disease: factors associated with exposure to high levels of diagnostic radiation Gut, November 1, 2008; 57(11): 1524 - 1529. [Abstract] [Full Text] [PDF]
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 J. E. Ngaile, C. B. S. Uiso, P. Msaki, and R. Kazema Use of lead shields for radiation protection of superficial organs in patients undergoing head CT examinations Radiat Prot Dosimetry, July 1, 2008; 130(4): 490 - 498. [Abstract] [Full Text] [PDF]
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A. B. Smith, W. P. Dillon, B. C. Lau, R. Gould, F. R. Verdun, E. B. Lopez, and M. Wintermark Radiation Dose Reduction Strategy for CT Protocols: Successful Implementation in Neuroradiology Section Radiology, May 1, 2008; 247(2): 499 - 506. [Abstract] [Full Text] [PDF]
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M.T. Russell, J.R. Fink, F. Rebeles, K. Kanal, M. Ramos, and Y. Anzai Balancing Radiation Dose and Image Quality: Clinical Applications of Neck Volume CT AJNR Am. J. Neuroradiol., April 1, 2008; 29(4): 727 - 731. [Abstract] [Full Text] [PDF]
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T. A. Jaffe, T. T. Yoshizumi, G. I. Toncheva, G. Nguyen, L. M. Hurwitz, and R. C. Nelson Early First-Trimester Fetal Radiation Dose Estimation in 16-MDCT Without and With Automated Tube Current Modulation Am. J. Roentgenol., April 1, 2008; 190(4): 860 - 864. [Abstract] [Full Text] [PDF]
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R. Grosjean, B. Sauer, R. M. Guerra, M. Daudon, A. Blum, J. Felblinger, and J. Hubert Characterization of Human Renal Stones with MDCT: Advantage of Dual Energy and Limitations Due to Respiratory Motion Am. J. Roentgenol., March 1, 2008; 190(3): 720 - 728. [Abstract] [Full Text] [PDF]
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D. Honnef, J. E. Wildberger, G. Haras, C. Hohl, G. Staatz, R. W. Gunther, and A. H. Mahnken Prospective Evaluation of Image Quality with Use of a Patient Image Gallery for Dose Reduction in Pediatric 16-MDCT Am. J. Roentgenol., February 1, 2008; 190(2): 467 - 473. [Abstract] [Full Text] [PDF]
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S. T. Schindera, R. C. Nelson, T. L. Toth, G. T. Nguyen, G. I. Toncheva, D. M. DeLong, and T. T. Yoshizumi Effect of Patient Size on Radiation Dose for Abdominal MDCT with Automatic Tube Current Modulation: Phantom Study Am. J. Roentgenol., February 1, 2008; 190(2): W100 - W105. [Abstract] [Full Text] [PDF]
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J. D. MacKenzie, J. Nazario-Larrieu, T. Cai, M. S. Ledbetter, M. A. Duran-Mendicuti, P. F. Judy, and F. J. Rybicki Reduced-Dose CT: Effect on Reader Evaluation in Detection of Pulmonary Embolism Am. J. Roentgenol., December 1, 2007; 189(6): 1371 - 1379. [Abstract] [Full Text] [PDF]
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H. T. Abada, J. Golzarian, R. Iezzi, A. R. Cotroneo, M. L. Storto, and L. Bonomo Multidetector CT in Abdominal Aortic Aneurysm Following Endovascular Repair: How to Consider the Value of a Delayed Phase Radiology, November 1, 2007; 245(2): 610 - 611. [Full Text] [PDF]
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E. Just da Costa e Silva and G. Alves Pontes da Silva Eliminating Unenhanced CT When Evaluating Abdominal Neoplasms in Children Am. J. Roentgenol., November 1, 2007; 189(5): 1211 - 1214. [Abstract] [Full Text] [PDF]
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A.B. Smith, W.P. Dillon, R. Gould, and M. Wintermark Radiation Dose-Reduction Strategies for Neuroradiology CT Protocols AJNR Am. J. Neuroradiol., October 1, 2007; 28(9): 1628 - 1632. [Abstract] [Full Text] [PDF]
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T.H. Mulkens, P. Marchal, S. Daineffe, R. Salgado, P. Bellinck, B. t. Rijdt, B. Kegelaers, and J.-L. Termote Comparison of Low-Dose With Standard-Dose Multidetector CT in Cervical Spine Trauma AJNR Am. J. Neuroradiol., September 1, 2007; 28(8): 1444 - 1450. [Abstract] [Full Text] [PDF]
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 A. S. Brody, D. P. Frush, W. Huda, R. L. Brent, and and the Section on Radiology Radiation Risk to Children From Computed Tomography Pediatrics, September 1, 2007; 120(3): 677 - 682. [Abstract] [Full Text] [PDF]
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 A. Deetjen, S. Mollmann, G. Conradi, A. Rolf, A. Schmermund, C. W Hamm, and T. Dill Use of automatic exposure control in multislice computed tomography of the coronaries: comparison of 16-slice and 64-slice scanner data with conventional coronary angiography Heart, September 1, 2007; 93(9): 1040 - 1043. [Abstract] [Full Text] [PDF]
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C. L. Hollingsworth, T. T. Yoshizumi, D. P. Frush, F. P. Chan, G. Toncheva, G. Nguyen, C. R. Lowry, and L. M. Hurwitz Pediatric Cardiac-Gated CT Angiography: Assessment of Radiation Dose Am. J. Roentgenol., July 1, 2007; 189(1): 12 - 18. [Abstract] [Full Text] [PDF]
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G. Hur, S. W. Hong, S. Y. Kim, Y. H. Kim, Y. J. Hwang, W. R. Lee, and S. J. Cha Uniform Image Quality Achieved by Tube Current Modulation Using SD of Attenuation in Coronary CT Angiography Am. J. Roentgenol., July 1, 2007; 189(1): 188 - 196. [Abstract] [Full Text] [PDF]
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K. M. Kanal, B. K. Stewart, O. Kolokythas, and W. P. Shuman Impact of Operator-Selected Image Noise Index and Reconstruction Slice Thickness on Patient Radiation Dose in 64-MDCT Am. J. Roentgenol., July 1, 2007; 189(1): 219 - 225. [Abstract] [Full Text] [PDF]
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A. A. Bankier, C. Schaefer-Prokop, V. De Maertelaer, D. Tack, P. Jaksch, W. Klepetko, and P. A. Gevenois Air Trapping: Comparison of Standard-Dose and Simulated Low-Dose Thin-Section CT Techniques Radiology, March 1, 2007; 242(3): 898 - 906. [Abstract] [Full Text] [PDF]
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J. Li, U. K. Udayasankar, T. L. Toth, J. Seamans, W. C. Small, and M. K. Kalra Automatic Patient Centering for MDCT: Effect on Radiation Dose Am. J. Roentgenol., February 1, 2007; 188(2): 547 - 552. [Abstract] [Full Text] [PDF]
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T. H. Mulkens, S. Daineffe, R. De Wijngaert, P. Bellinck, A. Leonard, G. Smet, and J.-L. Termote Urinary Stone Disease: Comparison of Standard-Dose and Low-Dose with 4D MDCT Tube Current Modulation Am. J. Roentgenol., February 1, 2007; 188(2): 553 - 562. [Abstract] [Full Text] [PDF]
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J. D. Dodd Evidence-based Practice in Radiology: Steps 3 and 4--Appraise and Apply Diagnostic Radiology Literature Radiology, February 1, 2007; 242(2): 342 - 354. [Abstract] [Full Text] [PDF]
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D. V. Sahani, S. P. Kalva, P. F. Hahn, and S. Saini 16-MDCT Angiography in Living Kidney Donors at Various Tube Potentials: Impact on Image Quality and Radiation Dose Am. J. Roentgenol., January 1, 2007; 188(1): 115 - 120. [Abstract] [Full Text] [PDF]
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 N. C. Dalrymple, S. R. Prasad, F. M. El-Merhi, and K. N. Chintapalli Price of Isotropy in Multidetector CT RadioGraphics, January 1, 2007; 27(1): 49 - 62. [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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 E. P. Judge, J. D. Dodd, J. B. Masterson, and C. G. Gallagher Pulmonary Abnormalities on High-Resolution CT Demonstrate More Rapid Decline Than FEV1 in Adults With Cystic Fibrosis. Chest, November 1, 2006; 130(5): 1424 - 1432. [Abstract] [Full Text] [PDF]
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S. G. Silverman, S. A. Akbar, K. J. Mortele, K. Tuncali, J. G. Bhagwat, and J. L. Seifter Multi-Detector Row CT Urography of Normal Urinary Collecting System: Furosemide versus Saline as Adjunct to Contrast Medium Radiology, September 1, 2006; 240(3): 749 - 755. [Abstract] [Full Text] [PDF]
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V. Tsapaki, J. E. Aldrich, R. Sharma, M. A. Staniszewska, A. Krisanachinda, M. Rehani, A. Hufton, C. Triantopoulou, P. N. Maniatis, J. Papailiou, et al. Dose Reduction in CT while Maintaining Diagnostic Confidence: Diagnostic Reference Levels at Routine Head, Chest, and Abdominal CT--IAEA-coordinated Research Project Radiology, September 1, 2006; 240(3): 828 - 834. [Abstract] [Full Text] [PDF]
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E. Coche, S. Vynckier, and M. Octave-Prignot Pulmonary Embolism: Radiation Dose with Multi-Detector Row CT and Digital Angiography for Diagnosis Radiology, September 1, 2006; 240(3): 690 - 697. [Abstract] [Full Text] [PDF]
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J. D. Dodd, C. A. Souza, and N. L. Muller Conventional high-resolution CT versus helical high-resolution MDCT in the detection of bronchiectasis. Am. J. Roentgenol., August 1, 2006; 187(2): 414 - 420. [Abstract] [Full Text] [PDF]
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J. L. Hadley, J. Agola, and P. Wong Potential Impact of the American College of Radiology Appropriateness Criteria on CT for Trauma. Am. J. Roentgenol., April 1, 2006; 186(4): 937 - 942. [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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C. H. McCollough, M. R. Bruesewitz, and J. M. Kofler Jr CT Dose Reduction and Dose Management Tools: Overview of Available Options. RadioGraphics, March 1, 2006; 26(2): 503 - 512. [Abstract] [Full Text] [PDF]
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A. B. Wolbarst and W. R. Hendee Evolving and Experimental Technologies in Medical Imaging Radiology, January 1, 2006; 238(1): 16 - 39. [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. Macari and E. J. Bini CT Colonography: Where Have We Been and Where Are We Going? Radiology, December 1, 2005; 237(3): 819 - 833. [Abstract] [Full Text] [PDF]
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H. MacMahon, J. H. M. Austin, G. Gamsu, C. J. Herold, J. R. Jett, D. P. Naidich, E. F. Patz Jr, and S. J. Swensen Guidelines for Management of Small Pulmonary Nodules Detected on CT Scans: A Statement from the Fleischner Society Radiology, November 1, 2005; 237(2): 395 - 400. [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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S. M. R. Rizzo, M. K. Kalra, B. Schmidt, R. Raupach, M. M. Maher, M. A. Blake, and S. Saini CT Images of Abdomen and Pelvis: Effect of Nonlinear Three-dimensional Optimized Reconstruction Algorithm on Image Quality and Lesion Characteristics Radiology, October 1, 2005; 237(1): 309 - 315. [Abstract] [Full Text] [PDF]
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T. H. Mulkens, P. Bellinck, M. Baeyaert, D. Ghysen, X. Van Dijck, E. Mussen, C. Venstermans, and J.-L. Termote Use of an Automatic Exposure Control Mechanism for Dose Optimization in Multi-Detector Row CT Examinations: Clinical Evaluation Radiology, October 1, 2005; 237(1): 213 - 223. [Abstract] [Full Text] [PDF]
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H. M. Bonel, L. Jager, K. A. Frei, S. Galiano, S. K. Srivastav, T. Flohr, M. F. Reiser, and H.-P. Dinkel Optimization of MDCT of the Wrist to Achieve Diagnostic Image Quality with Minimum Radiation Exposure Am. J. Roentgenol., September 1, 2005; 185(3): 647 - 654. [Abstract] [Full Text] [PDF]
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N. C. Dalrymple, S. R. Prasad, M. W. Freckleton, and K. N. Chintapalli Informatics in Radiology (infoRAD): Introduction to the Language of Three-dimensional Imaging with Multidetector CT RadioGraphics, September 1, 2005; 25(5): 1409 - 1428. [Abstract] [Full Text] [PDF]
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J. C. O'Daniel, D. M. Stevens, and D. D. Cody Reducing Radiation Exposure from Survey CT Scans Am. J. Roentgenol., August 1, 2005; 185(2): 509 - 515. [Abstract] [Full Text] [PDF]
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V. M. Chapman, M. Kalra, E. Halpern, B. Grottkau, M. Albright, and D. Jaramillo 16-MDCT of the Posttraumatic Pediatric Elbow: Optimum Parameters and Associated Radiation Dose Am. J. Roentgenol., August 1, 2005; 185(2): 516 - 521. [Abstract] [Full Text] [PDF]
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J. Menke Comparison of Different Body Size Parameters for Individual Dose Adaptation in Body CT of Adults Radiology, August 1, 2005; 236(2): 565 - 571. [Abstract] [Full Text] [PDF]
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T. Dalal, M. K. Kalra, S. M. R. Rizzo, B. Schmidt, C. Suess, T. Flohr, M. A. Blake, and S. Saini Metallic Prosthesis: Technique to Avoid Increase in CT Radiation Dose with Automatic Tube Current Modulation in a Phantom and Patients Radiology, August 1, 2005; 236(2): 671 - 675. [Abstract] [Full Text] [PDF]
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S. J. Golding Multi-slice computed tomography (MSCT): the dose challenge of the new revolution Radiat Prot Dosimetry, May 17, 2005; 114(1-3): 303 - 307. [Abstract] [Full Text] [PDF]
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T. Irie and H. Inoue Individual Modulation of the Tube Current-Seconds to Achieve Similar Levels of Image Noise in Contrast-Enhanced Abdominal CT Am. J. Roentgenol., May 1, 2005; 184(5): 1514 - 1518. [Abstract] [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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 G. Brix, U. Lechel, G. Glatting, S. I. Ziegler, W. Munzing, S. P. Muller, and T. Beyer Radiation Exposure of Patients Undergoing Whole-Body Dual-Modality 18F-FDG PET/CT Examinations J. Nucl. Med., April 1, 2005; 46(4): 608 - 613. [Abstract] [Full Text] [PDF]
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S. M. R. Rizzo, M. K. Kalra, M. M. Maher, M. A. Blake, T. L. Toth, and S. Saini Do Metallic Endoprostheses Increase Radiation Dose Associated with Automatic Tube-Current Modulation in Abdominal-Pelvic MDCT? A Phantom and Patient Study Am. J. Roentgenol., February 1, 2005; 184(2): 491 - 496. [Abstract] [Full Text] [PDF]
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M. K. Kalra, M. M. Maher, T. L. Toth, B. Schmidt, B. L. Westerman, H. T. Morgan, and S. Saini Techniques and Applications of Automatic Tube Current Modulation for CT Radiology, December 1, 2004; 233(3): 649 - 657. [Abstract] [Full Text] [PDF]
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M. J. Siegel, B. Schmidt, D. Bradley, C. Suess, and C. Hildebolt Radiation Dose and Image Quality in Pediatric CT: Effect of Technical Factors and Phantom Size and Shape Radiology, November 1, 2004; 233(2): 515 - 522. [Abstract] [Full Text] [PDF]
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M. K. Kalra, M. M. Maher, R. S. Kamath, T. Horiuchi, T. L. Toth, E. F. Halpern, and S. Saini Sixteen-Detector Row CT of Abdomen and Pelvis: Study for Optimization of Z-Axis Modulation Technique Performed in 153 Patients Radiology, October 1, 2004; 233(1): 241 - 249. [Abstract] [Full Text] [PDF]
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M. K. Kalra, M. M. Maher, T. L. Toth, R. S. Kamath, E. F. Halpern, and S. Saini Comparison of Z-Axis Automatic Tube Current Modulation Technique with Fixed Tube Current CT Scanning of Abdomen and Pelvis Radiology, August 1, 2004; 232(2): 347 - 353. [Abstract] [Full Text] [PDF]
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M. K. Kalra, M. M. Maher, T. L. Toth, R. S. Kamath, E. F. Halpern, and S. Saini Radiation from "Extra" Images Acquired with Abdominal and/or Pelvic CT: Effect of Automatic Tube Current Modulation Radiology, August 1, 2004; 232(2): 409 - 414. [Abstract] [Full Text] [PDF]
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ABSTRACT
INTRODUCTION
