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Gastrointestinal Imaging |
1 From the Department of Diagnostic Radiology, Graduate School of Medical Sciences (Y.N., K.A., M.H., M.I., T.N., D.R., S.M., S.S., Y.Y.), and Department of Radiological Sciences, School of Health Sciences (Y.F.), Kumamoto University, 1-1-1 Honjo, Kumamoto 860-8556, Japan; and Philips Medical Systems, Minato-ku, Tokyo, Japan (N.S.). Received September 26, 2004; revision requested November 30; revision received February 12, 2005; accepted March 8. Address correspondence to Y.N.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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MATERIALS AND METHODS: The institutional review board approved this study. Prior informed consent was obtained from all patients. Forty patients (24 women, 16 men; mean age, 62 years) underwent initial abdominal CT at 120 kV with 100 mL of contrast material (protocol A). Then all patients were randomly assigned to one of two protocols (protocol B, CT at 90 kV with 100 mL contrast material; protocol C, CT at 90 kV with 80 mL contrast material). The CT numbers of their abdominal organs were assessed quantitatively and qualitatively. Statistical analysis was performed by using the two-tailed paired t test, Kruskal-Wallis test, and 
test of interobserver agreement. The radiation dose was measured with a phantom that consisted of glass-rod dosimeters.
RESULTS: Quantitative analysis revealed that protocols B and C yielded significantly better enhancement of the aorta, liver, pancreas, spleen, and kidney than did protocol A (P < .05). With qualitative analysis, the difference among the three protocols in regard to image quality was not significant. At 90 kV versus 120 kV, the radiation dose reduction in the center of the phantom was 56.8% (6.3 vs 14.6 mGy); in the periphery, it was 46.2% (13.6 vs 25.3 mGy).
CONCLUSION: By decreasing the tube voltage, the amount of contrast material can be reduced without image quality degradation. In scans obtained with a low tube voltage, the radiation dose can be reduced as much as 56.8%, and higher contrast material enhancement can be achieved.
© RSNA, 2005
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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According to the current literature, the theoretical risk for radiation-induced cancer from CT examinations to patients (4–6), as well as to the medical radiologic workers (7,8), is not negligible. Therefore, a reduction in the radiation dose delivered at CT has become an important issue, and various techniques and patient-based strategies have focused on optimizing and reducing the radiation dose delivered during CT studies (9,10). Sigal-Cinqualbre et al (11) reported that chest multi–detector row CT scans obtained with a low tube voltage not only helped to reduce the radiation dose but also yielded images with improved contrast enhancement. Although findings in their report suggest that scanning techniques with low tube voltage may hold promise for abdominal dynamic CT, to the best of our knowledge, the benefits of abdominal dynamic CT scans obtained with low tube voltage have not been reported to date.
Thus, the purpose of our study was to prospectively investigate the effect of low tube voltage on radiation dose, contrast enhancement, image quality, and image noise at abdominal dynamic CT.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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In advance, we informed the volunteers and patients that their agreement to undergo scanning at 120 kV and 90 kV would expose them to radiation doses of approximately 13 and 6 mGy in weighted average CT dose index, respectively, and that these doses are within the range of exposure experienced by persons who undergo abdominal CT. We also explained the risks related to radiation exposure to all study participants. The institutional review board of the Graduate School of Medical Sciences, Kumamoto University, Kumamoto, Japan, approved both the volunteer and patient studies. Prior written informed consent was obtained from all volunteers and patients. All CT examinations were performed with a 16–detector row CT scanner (IDT16; Philips Medical Systems), which provides 90, 120, and 140 kV options. The maximal tube current–time product limit was 415, 350, and 300 mAs at 90, 120, and 140 kV, respectively, at a constant beam pitch of 0.659:1 and gantry rotation of 0.75 second. The voltages we compared were 120 kV and 90 kV. Before scanning at 120 kV and 90 kV, zero calibration was performed to compensate for any deviance between the two tube voltage settings.
Preliminary Studies
First, we performed studies in a phantom and in volunteers. The scanning parameters were tube current–time product of 300 mAs, rotation time of 0.75 second, beam collimation of 16 (detector rows) x 1.5 mm (section thickness), reconstruction section thickness of 5 mm, table feed per rotation of 21.1 mm, beam pitch of 0.659:1, field of view of 36 cm, and pixel matrix size of 512 x 512. The scans were obtained at 120 kV and 90 kV. In the phantom and volunteer studies, the constant tube current–time product level was set at 300 mAs to evaluate the effect of changing only the tube voltage.
Phantom study.—Five plastic test tubes filled with varying concentrations of iodinated solution (2.31, 3.46, 5.20, 7.80, and 11.71 mg of iodine per milliliter) and one tube containing only purified water were used. The iodinated solutions were prepared with iterative dilutions by using the formulated concentrate of iopamidol (Iopamiron 300; Nihon Schering, Osaka, Japan), with an iodine concentration of 300 mg of iodine per milliliter. Each test tube was separately placed in the center of the gantry and scanned. With a 1-cm2 circular region-of-interest cursor, the CT number and image noise (defined as the standard deviation of the mean CT number) of each of the six tubes were measured on images obtained at 120 kV and 90 kV. Our study in the phantom showed that the mean CT numbers of all six test tubes that contained different concentrations of iodine or purified water were higher at 90 kV than they were at 120 kV. Figure 1 shows that, at 120 kV and 90 kV, there was a significant linear correlation between the mean CT number of each of the five tubes of the phantom and increased iodine concentrations (r = 0.999, P < .001). The mean CT number of the tubes of the phantom increased with increasing iodine concentrations; the increase was greater at 90 kV than it was at 120 kV.
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Clinical Study
Between January and April of 2004, 40 consecutive patients with chronic hepatic disease and known hepatocellular carcinoma (24 women and 16 men; mean age, 62 years; range, 28–79 years) were enrolled in our clinical study. All examinations were performed for clinical purposes.
Triple-phase dynamic CT scans were obtained. The parameters were as follows: rotation time, 0.75 second; beam collimation, 16 x 1.5 mm; reconstruction section thickness, 5 mm; table feed per rotation, 21.1 mm; beam pitch, 0.659:1; field of view, 36 cm; and matrix size, 512 x 512 pixels. All patients underwent initial studies at 120 kV and 300 mAs, with 100 mL of iopamidol (Iopamiron 370, Nihon Schering), with a total of 37 g of iodine, injected during 30 seconds (flow rate of 3.3 mL/sec at 100 mL and 1.23 g of iodine per second [protocol A]). The scanning delay time from the start of contrast material delivery was 25 and 35 seconds in the early and delayed arterial phases, respectively, and 180 seconds in the equilibrium phase.
Within 3 months of undergoing studies with protocol A, 20 (12 women and eight men; mean age, 64.2 years; range, 30–79 years; mean body weight, 56.7 kg; range, 35–81 kg) of 40 patients were randomly assigned to protocol B (90 kV and 300 mAs, with 100 mL of iopamidol and 37 g of iodine injected as in protocol A). The other 20 patients (12 women and eight men; mean age, 59.1 years; range, 28–74 years; mean body weight, 58.5 kg; range, 36–78 kg) were randomly assigned to protocol C (90 kV and 300 mAs, with 80 mL iopamidol and 29.6 g of iodine injected intravenously within 30 seconds at a flow rate of 2.4 mL/sec).
Because the calculation of the effective doses to patients could not be made with routine clinical conditions, we based our evaluation of radiation dose on the estimation with a weighted average CT dose index. The effective radiation doses were also measured by using a weighted average CT dose index and were compared on the scans obtained with 120 kV and 90 kV.
Image Analysis
For quantitative analysis, a radiologist (D.R.) who had 6 years of experience with CT examinations measured the CT number and image noise with a 1-cm2 circular region-of-interest cursor in the aorta, portal vein, liver, pancreas, gallbladder, kidney (cortex and medulla), muscle, and subcutaneous fat in both the early and delayed arterial phases. The measurements were performed on CT sections at the level of the first vertebral body where these abdominal organs could be observed. The signal-to-noise ratio (SNR) of the aorta, or SNRaor, in the arterial phase was computed by dividing the mean CT number, or CTNaor by the standard deviation from that mean value, or SDaor (SNRaor = CTNaor/SDaor). After placing a 1-cm2 region of interest in the anterior segment of the liver, the SNR of the liver in the delayed arterial phase was also calculated in the same way. The relationship between SNR values and each patient's body weight was correlated for protocols A and B.
For qualitative analysis, three radiologists (Y.N., D.R., K.A.) with 6–18 years of experience in abdominal CT independently performed a blinded qualitative analysis of CT images obtained with each protocol during the delayed arterial phase. The criteria for image grading were consensually established among the three before the start of image reading. Parameters assessed with subjective CT image readings were overall image quality, enhancement of the abdominal organs, image noise, and streak artifacts caused by beam hardening. For analysis of overall image quality and organ enhancement, a three-point ordinal scale was used as follows: score 1, unacceptable; score 2, acceptable; and score 3, excellent. Diagnostic quality was recorded as excellent when the boundary of different structures was sharp and contrast resolution and lesion visualization were satisfactory. Diagnostic quality was considered unacceptable in cases in which these image attributes were unsatisfactory. Similarly, for image noise and streak artifact assessment a three-point scale was used, where score 1 was unacceptable; score 2, acceptable; and score 3, excellent. A rating of excellent with respect to image noise and artifacts was assigned when mottling or streaking was minimal or less than appreciable levels. Noise and artifacts were considered acceptable in cases with an average amount of mottle or graininess, satisfactory visualization of small anatomic structures such as blood vessels, and variable attenuation of the interface between structures. Image noise and artifacts were considered unacceptable in cases in which mottle and streaking interfered with visualization of these structures. All images were inspected in random order with a Digital Imaging and Communications in Medicine viewer that had a spatial resolution of 1600 x 1200 (RadiForce R22; Nanao, Ishikawa, Japan) by using our picture archiving and communications system (Image VINS Pro, version 3.01; Yokogawa Electric, Tokyo, Japan). A radiologist who did not participate in the qualitative evaluation initially reviewed all CT images obtained at 120 kV and 90 kV and determined the optimal imaging settings for both tube voltage settings. The evaluators were not allowed to change the optimal preset window width of 300 HU and window level of 50 HU.
Statistical Analysis
Statistical analyses were performed with commercially available software (SPSS, version 10.05; SPSS, Chicago, Ill). Among the 40 patients, we looked for statistically significant interprotocol differences in the CT number of the aorta, portal vein, liver, pancreas, spleen, gallbladder, and kidney by using the two-tailed paired t test. Statistically significant differences in the SNR at 120 kV and 90 kV were also measured by using the two-tailed paired t test. For qualitative analysis, we performed a power analysis to determine, at a 95% confidence level, the minimum sample size necessary to detect a one-point difference in the mean values of the three protocols (14). Our calculations revealed that the required minimum sample size was 18 patients.
The Kruskal-Wallis test was used to investigate statistically significant differences in the qualitative scores recorded by the three radiologists. If there was a statistically significant difference among the groups, pairwise comparisons were performed by using the Steel-Dwass test. For all studies, a difference with a P value of less than .05 was considered significant.
Interobserver agreement was measured with the test. The scale for the coefficients for interobserver agreement was as follows: less than 0.20, poor; 0.21–0.40, fair; 0.41–0.60, moderate; 0.61–0.80, substantial; and 0.81–1.00, almost perfect (15,16).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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= 0.67 and 0.61, respectively). The agreement with respect to image noise and streak artifact was moderate ( = 0.41 and 0.45, respectively).
Radiation Dose Measurement
The radiation doses for the phantom study at 300 mAs are shown in Table 5. At 90 kV, the doses delivered to the central and peripheral cavities were decreased by 56.8% and 46.2%, respectively, compared with those delivered at 120 kV (Table 5). According to the manufacturer's data, the weighted average CT dose index was 13.2 and 5.7 mGy at 120 kV and 90 kV, respectively. The calculated average dose was decreased by 56.8% at 90 kV compared with the average dose delivered at 120 kV.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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We used a constant tube current–time product at 300 mAs to evaluate the precise effect of the tube voltage. Although 300 mAs is slightly greater than the usual tube current–time product setting for abdominal scans, we confirmed that the actual radiation dose delivered was within an acceptable range by using phantom dosimetry. Results of our study in the phantom showed that it was possible to reduce radiation exposure substantially by decreasing the tube voltage from 120 kV to 90 kV; we obtained a reduction of 56.8% and 46.2% in the radiation dose delivered to the central and peripheral cavities of the phantom, respectively. Because dose measurements are difficult to obtain in patients, we substituted the weighted average CT dose index value for radiation exposure readings. In the preliminary measurement of the weighted average CT dose index, the radiation dose at the 300-mAs setting was 13.2 mGy at 120 kV and 5.7 mGy at 90 kV. The dose reduction estimated with the weighted average CT dose index was nearly equivalent to our dosimetric readings in the study with the phantom.
In scans obtained with a low tube voltage, the contrast is enhanced because iodine-based contrast material provides greater x-ray attenuation caused by the increase in the relative atomic number of iodine (Z = 53) at exposure to reduced x-ray energy. Swensen et al (17) reported that, with chest scans obtained with dual tube voltage settings, calcified benign nodules in the lung can be readily identified. The photoelectric effect in x-ray attenuation is increased at lower tube voltages, particularly in scans of structures with a high effective atomic number, such as bone, and in iodinated contrast material. Because of Compton scattering, most x-rays interact less with soft tissue as the tube voltage increases. Therefore, a reduction in the tube voltage leads to an increase in the attenuation of calcified structures and iodinated contrast material as the photoelectric effect increases and Compton scattering decreases. Consequently, as the K edge of iodine is closer to the reduced voltage, beam attenuation is increased and higher attenuation readings are obtained (18). With our instruments, the effective energies from 120 kV and 90 kV are 64 keV and 54 keV, respectively. Our results confirm that this physical principle can be applied to the abdominal dynamic CT studies: Images obtained with protocol C (90 kV, 80 mL of contrast material) were more highly enhanced than were images obtained with protocol A (120 kV, 100 mL of contrast material). We suggest that decreasing the tube voltage makes it possible to reduce the amount of contrast material. Although we used a 16–detector row CT scanner, we postulate that the same results will be obtained with four-, 40-, or 64-section scanners.
There are some limitations in our study. Scanning with low tube voltage results in the degradation of image quality from noise and artifacts. Although streak artifacts caused by beam hardening at the border of high-contrast areas were slightly increased on images obtained with low tube voltage, the results of qualitative analysis of artifacts were acceptable, and there was no significant difference between results for images obtained at 90 kV and those obtained at 120 kV. Furthermore, these artifacts rarely affected the diagnostic value of the scans. Image noise was increased in scans obtained with low tube voltage because decreasing the tube voltage leads to a direct reduction in photon flux. Image noise, however, is mainly related to the characteristics of the CT scanner used and to factors connected with individual patients, such as physique. Increased image noise has a greater effect on the quality of abdominal images because the abdominal region is inherently of lower contrast. In our study, the values of SNR of the aorta and SNR of the liver were significantly decreased on images obtained at 90 kV than they were on images obtained at 120 kV. This implies that noise has a greater effect on images obtained at 90 kV than it has on those obtained at 120 kV. Our qualitative analysis also disclosed a statistically significant difference between images obtained at 90 kV and those obtained at 120 kV. As the image noise increased, the SNR values were inversely correlated with the patient's body weight. The tube current–time product setting should consider individual anthropometric measurements to minimize radiation exposure without compromising diagnostic image quality. Therefore, a technique with low tube voltage should be used in patients whose body weight is less than 80 kg. Tube voltage reduction from 120 kV to 90 kV will lead to a reduction in the radiation dose by as much as 35% without impairment of the low-contrast detectability on CT scans (19).
Our next challenge is to adjust the tube current–time product setting to reflect the patient's physique and to develop new filters to reduce image noise. Kalra et al (20–22) reported results of studies with noise reduction filters that effectively helped to reduce the noise on CT images acquired with a radiation dose reduced by 50% without compromise of image quality. Their filters were developed to compensate for the degradation of image quality that results from the increased noise that accompanies the use of reduced tube current. Changes in tube voltage and associated changes in beam hardening, however, affect image quality in different ways. We are in the process of developing a new filter that addresses the issues raised by changes in tube voltage.
In conclusion, we found that the amount of contrast material can be reduced by at least 20% with a reduction of the tube voltage from 120 kV to 90 kV, without degradation in image quality. Findings in our study confirmed that, in scans obtained with low tube voltage, the radiation dose was reduced by as much as 57% and that these scans yielded higher contrast material enhancement. Decreasing the tube voltage and the iodine concentration of the contrast material particularly would benefit patients who may need to undergo multiple CT examinations and young patients who are at increased risk for developing cancer from medical radiation exposure (4).
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FOOTNOTES |
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Abbreviations: SNR = signal-to-noise ratio
See also the article by Funama et al in this issue.
See Materials and Methods for pertinent disclosures.
Author contributions: Guarantors of integrity of entire study, Y.N., K.A., Y.Y.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, Y.N., S.M., N.S.; clinical studies, Y.N., K.A., M.I., T.N., D.R.; experimental studies, Y.N., K.A., Y.F., M.H., M.I., D.R., S.S.; statistical analysis, Y.N., K.A., Y.F., M.H.; and manuscript editing, Y.N., K.A., Y.Y.
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