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1 All authors: Department of Radiology, Massachusetts General Hospital and Harvard Medical School, White 270-E, 55 Fruit St., Boston, MA 02114.
Received March 13, 2002;
accepted after revision May 13, 2002.
Address correspondence to S. Saini.
Abstract
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SUBJECTS AND METHODS. Using a multidetector CT scanner, we prospectively studied 39 patients who were 65 years and older who had a known history of cancer. After performing a diagnostic contrast-enhanced CT examination, we obtained four slices each (centered at the top of the right kidney) at a standard radiation dose (240-300 mA) and at a 50% reduced dose (120-150 mA) at a constant kilovoltage of 140. Scans were obtained during a single breath-hold, with a 2.5-mm detector configuration and a slice pitch of 6:1. Reconstructed slice thickness was 5 mm. In a blinded review, two radiologists rated the randomized CT scans for overall image quality and anatomic details of liver, spleen, adrenal glands, kidneys, pancreas, and abdominal wall, using a 5-point scale (1 = unacceptable, 2 = substandard, 3 = acceptable, 4 = above average, and 5 = superior). Patients' weight and abdominal circumference, area, and anteroposterior and transverse diameters were correlated with image quality of scans obtained at standard-dose and 50% reduced—dose CT. Statistical analysis of the data was performed using Wilcoxon's signed rank test.
RESULTS. Overall, the image quality score was significantly higher (p < 0.005) on the scans obtained with standard-dose CT. No statistically significant difference in image quality was noted in the 50% reduced— and standard-dose CT scans in patients who weighed less than 180 lb, or 81 kg, (p > 0.05) and who had a transverse abdominal diameter of less than 34.5 cm (p > 0.05), an anteroposterior diameter of less than 28 cm (p > 0.05), a cross-sectional circumference of less than 105 cm (p > 0.05), and a cross-sectional area of less than 800 cm2 (p > 0.05). Good interobserver agreement (p > 0.5) was found between the two reviewing radiologists.
CONCLUSION. Abdominal CT scan quality appears to be acceptable even with a 50% reduction in radiation dose except in patients with large anthropometric measurements. A reduction in CT radiation dose is possible if the tube current is optimized for the patient's weight and abdominal dimensions.
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Although use of low-dose radiation has been advocated for CT [2], a systematic evaluation of reduced-dose abdominal CT in adults has not been thoroughly investigated. Therefore, we sought to study the impact on image quality of a 50% reduction in the radiation dose of abdominal CT. We also analyzed the influence of a patient's weight and cross-sectional abdominal dimensions—such as anteroposterior diameter, transverse diameter, and cross-sectional area and circumference as independent factors—on the optimization of CT scanning parameters (tube current) for abdominal studies.
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CT examination of all patients was performed with a multidetector CT (MDCT) scanner (LightSpeed QX/i; General Electric Medical Systems, Milwaukee, WI). After performing standard contrast-enhanced abdominal CT, we obtained two sets of four additional scans during the equilibrium phase (approximately 3-5 min after the injection of contrast material); in each set, scanning began at the upper pole of the right kidney. The first set was obtained at the standard tube voltage (140 kVp) and tube current (240-300 mA) settings. These were identical to the settings with which the just-concluded diagnostic study had been obtained, settings that had been selected by the technologist on the basis of the size of the patient, according to existing departmental guidelines. Immediately thereafter, a second set of four images was obtained at the identical anatomic location, but these (50% reduced—dose) CT scans were obtained by reducing the standard-dose tube current by one half (120-150 mA) while maintaining the tube voltage at a constant 140 kVp. The remaining parameters common to both techniques were identical for both image sets and included a detector configuration of 2.5 mm, table speed of 15 mm/rotation based on a nonoverlapping slice pitch of 6:1, and a tube rotation time of 0.8 sec. All scans were obtained in a single breath-hold. Five-millimeter-thick contiguous slices were reconstructed for image analysis. The weighted CT dose index from the standard-dose and 50% reduced—dose abdominal CT ranged between 17.76 and 22.20 mSv and 8.37 and 10.47 mSv, respectively.
Two subspecialty trained radiologists (reviewer 1 and reviewer 2) who were unaware of the technical scanning parameters qualitatively evaluated the 78 randomized image sets on a digital picture archiving and communication system (PACS) diagnostic workstation (Impax RS 3000 review station; AGFA Technical Imaging Systems, Richfield Park, NJ), using a 5-point scale. Image sets were scored as 1, unacceptable; 2, substandard; 3, acceptable; 4, above average; and 5, superior. The 78 image sets were first assessed separately and then again, during a second interpretation session, in a direct side-by-side comparison of the standard-dose and 50% reduced—dose CT scans. The qualitative image quality score was based on image noise, soft-tissue contrast, and sharpness of organ boundaries for the liver, adrenal glands, kidneys, pancreas, and abdominal wall. In addition, the SD of gallbladder density (measured in Hounsfield units) was measured in both scan sets in those patients (n = 9) in whom the gallbladder appeared on the images. The SD served as quantitative marker of image noise. Patients were weighed, and their demographic data, including age, sex, and clinical diagnosis, were recorded on the day of the CT examination. Using the CT scans, we measured the cross-sectional abdominal area and circumference and the anteroposterior and transverse diameters of abdomen at the level of upper pole of right kidney in all patients with the PACS diagnostic workstation software.
The image quality scores for the standard-dose and half-dose CT scans were correlated with patients' weight and the previously specified cross-sectional abdominal dimensions. We determined the range of dimensions at which acceptable image quality could be obtained with reduced-dose CT by analyzing image quality scores for individual anthropometric parameters with Wilcoxon's signed rank test. For each anthropometric measurement, we categorized the study population on the basis of reduced-dose CT scans of acceptable image quality compared with standard-dose CT scans (average score > 3; p > 0.05) and with reduced-dose CT scans of significantly compromised image quality (average score < 3; p < 0.05). We performed statistical analysis of the data using the Wilcoxon's signed rank test. A p value of less than 0.05 was considered significant (95% confidence level).
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In the randomized review, the mean scores for standard-dose abdominal CT
scans across the two weight categories for reviewer 1 were 3.9 (<180 lb)
and 3.7 (
180 lb). The mean scores for standard-dose abdominal CT scans
across the two weight categories for reviewer 2 were 3.8 (< 180 lb) and 3.4
( 180 lb). Average mean scores for the same categories of patient weight
for the 50% reduced—dose CT scans were lower than the mean scores for
standard-dose CT scans for both reviewers
(Table 1). However, although
the mean image quality score for standard-dose CT scans was higher than that
for the 50% reduced—dose CT scans in both weight categories, the mean
image quality of the 50% reduced—dose CT scans was acceptable for
patients in the low-weight category. For patients in the high-weight category,
the mean image quality of the 50% reduced—dose CT scans was less than
acceptable.
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Similarly, for cross-sectional abdominal parameters, mean image quality scores of the 50% reduced—dose CT scans were acceptable for patients with a cross-sectional area less than 800 cm2, a circumference less than 105 cm, an anteroposterior diameter less than 28 cm, and a transverse diameter of less than 34.5 cm (Table 1). Good interobserver agreement was found in the image quality scores in all the categories of measurements (p > 0.5).
In the direct comparison of the standard-dose and 50% reduced—dose CT scans, both reviewers preferred the standard-dose CT scans for 37 (94.9%) of 39 patients; for the remaining two patients, the reviewers considered image quality of both sets to be equivalent (Figs. 1A,1B,2A,2B,3A,3B,4A,4B). In the direct comparison review, the mean image quality score for the standard-dose abdominal CT scans were 3.80 for reviewer 1 and 3.75 for reviewer 2, with the standard error of the mean of 0.054 and 0.049, respectively. For the reduced radiation—dose CT scans, the average scores of image quality were 3.02 for reviewer 1 and 3.05 for reviewer 2, with the standard error of the mean being 0.052 and 0.062, respectively.
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The SD of gallbladder density increased in the 50% reduced—dose CT scans by between 3 and 9 H. The low-dose SD was 19.9 H, and the standard-dose SD was 14.2 H, with a relative increase in SD of 5.7 H.
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With the widespread availability and use of CT, the radiation dose from CT to the population continues to rise. Globally, CT contributes one third of the radiation dose that the population receives from medical X-ray sources, although it accounts for only 5% of examinations [5]. The continued technologic improvement in helical CT—which permits rapid multiphase imaging with thinner slices—brings with it the potential to further increase patient radiation doses [6]. In addition, technical parameters in CT, as opposed to other X-ray—based examinations, are often not appropriately adjusted for the size of the patient, the body part examined, and the type of information required from the study [5, 7].
Several strategies can help to reduce population-based radiation dose from CT. These include limiting the use of CT to carefully identified indications, avoiding multiphase protocols, adequately addressing specific clinical issues, making judicious choices on when to repeat or to suggest follow-up CT, and adjusting technical scanning parameters appropriately [2, 5]. Tube voltage, tube current, scanning time, pitch, slice thickness, and scanning volume are some of the major technical factors that influence radiation dose from CT. Radiation dose is linearly related to tube current, scanning time, and scanning volume [5]. Although modern helical CT scanners have decreased scanning times, the radiation dose associated with helical CT scanners is greater than that of other imaging procedures because of the increased tube current and increased volume of irradiated tissue. Moreover, the patient's body habitus and the part of the body being examined are frequently ignored when setting the tube current [5, 7].
Practical ways to decrease radiation dose in helical CT involve reducing tube current and increasing pitch. Previous studies of CT of the head and neck, chest, and pediatric pelvis have suggested that it is possible to reduce tube currents used without jeopardizing image quality [8,9,10,11]. Several studies of chest CT performed for cancer screening have shown no significant difference in detection of nodules on CT scans obtained at 10-30% of the standard tube currents [12,13,14,15,16]. Cohnen et al. [9] concluded that no loss of diagnostic quality in CT scans of the head occurred when the radiation dose was reduced by 40%. Sohaib et al. [17] obtained acceptable CT scans of the sinonasal region with a 75% dose reduction. In addition, several studies have implied that increasing pitch also reduces patient radiation dose by cutting down the scanning time [18, 19]. When the pitch is doubled, radiation dose is reduced by half [20]. This method is especially valuable in patients undergoing survey or follow-up CT examinations. However, using a higher pitch can result in lesions being missed because of slice broadening and the consequent volume-averaging effects as well as an increase in artifacts.
Because the distance of the pathway traversed by the X-ray beam determines its attenuation, the diameter of the patient should provide better criteria for optimizing scanning parameters without losing relevant diagnostic information [21]. Haaga [21] and Haaga et al. [22] reported a linear relationship between the image noise and the mean square diameter of the patient and advocated use of a patient's diameter to determine the tube current requirement.
In our study, we reduced the tube current by 50% to determine whether CT radiation dose could be substantially reduced without significantly compromising image quality in certain categories of patients. We found that, depending on a patient's anthropometric parameters, CT tube current could be reduced by 50% while maintaining image quality that was acceptable compared with image quality achievable with the standard tube current. Thus, our study confirms the encouraging results of prior studies [9,10,11,12,13,14,15,16,17] undertaken concerning CT examinations of other body parts. The image quality of the CT scans of low-weight patients was acceptable when obtained with a 50% reduction in the CT radiation dose. However, the image quality of the CT scans of high-weight patients was below acceptable levels.
Similarly, no statistically significant difference in image quality was found between the standard- and reduced-dose CT scans in patients with smaller cross-sectional abdominal dimensions (Table 1). Substantial deterioration in image quality was noted in the reduced-radiation CT scans of patients with larger abdominal dimensions (e.g., a transverse diameter > 34.5 cm [p < 0.0008], cross-sectional abdominal area > 800 cm2 [p < 0.0002], and cross-sectional circumference > 105 cm [p < 0.009]). Although we measured the dimensions from CT scans, these measurements could be easily obtained using a simple measuring caliper before a patient undergoes CT, and, if necessary, the cross-sectional area of the abdomen could then be calculated using the formula for the area of an ellipse. Alternatively, the technologist could directly measure these dimensions at a fixed landmark from the CT console monitor on the scout, unenhanced, or automated bolus tracking images.
The fact that the CT scans of patients with smaller dimensions were of acceptable image quality when obtained with reduced-dose radiation suggests that the narrow range of tube current settings used in standard protocols should be expanded. It is possible that CT scanning of the low-weight patients could be undertaken using even lower tube current settings than used in our study. However, the 50% reduced—dose CT scans are not as visually pleasing because of the inherent problem of increased noise, which explains the reviewers' preference for the standard-dose CT scans in the direct comparison analysis. Therefore, in a competitive environment, an imaging center may be reluctant to follow reduced-radiation guidelines if a competing center is turning out more aesthetically pleasing scans using the standard dose of radiation. Nevertheless, documentation of the equivalent diagnostic quality of the reduced-dose images would make it imperative for, or even legally binding on, imaging centers to reduce the radiation dose for the greater benefit of patient population in compliance with the "as low as reasonably achievable" (ALARA) radiation guidelines set forth by the Nuclear Regulatory Commission [23]. Also, the incidence of artifacts, particularly beam-hardening artifacts, increases on 50% reduced-dose CT scans as does the possibility of missing low-contrast lesions.
There are some limitations to our study. We used the equilibrium phase scanning for obtaining the two sets of scans. Scanning during this phase may potentially obscure focal liver lesions [20]. Minor inaccuracies in our measurement of abdominal dimensions may have occurred, although we attempted to minimize the errors by taking the same measurement twice in the presence of two observers. Objective parameters of image quality and the relation of these parameters to the cross-sectional dimensions of patients were not evaluated. A more pertinent limitation was that we did not study the impact of 50% reduced—dose CT on diagnostic outcome. This is a critical study deficiency that should be addressed in a more detailed study so that the diagnostic implications of reduced-dose CT can be assessed. However, our approach was to first identify the threshold for simple patient anthropometric parameters at which reduced-dose CT could be adequately performed without loss of acceptable image quality. We achieved that goal with our study.
Our subsequent approach will be to compare the impact of reduced-dose CT in specific clinical situations involving the abdomen and pelvis. Reducing the radiation dose in CT without compromising diagnostic accuracy would be particularly advantageous for the many young patients currently undergoing multiple CT examinations in acute and oncologic settings. We have addressed the possibility of reducing CT radiation dose for relatively lighter (body weight < 180 lb) or thinner (smaller abdominal dimensions) patients. Further studies as well as technologic advances in CT are required to explore the possibility of reducing CT radiation dose for relatively larger patients.
To conclude, our study shows that radiation dose for routine abdominal CT may be reduced by 50% in patients weighing less than 180 lb and having smaller cross-sectional dimensions (e.g., abdominal circumference < 105 cm) without significant loss in the image quality of the resultant scans.
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