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Experimental Studies |
1 From the Mallinckrodt Institute of Radiology, Washington University School of Medicine, 510 S Kingshighway Blvd, St Louis, MO 63110 (M.J.S., C.H.); and Siemens Medical Corporation, Forcheim, Germany (B.S., D.B., C.S.). Received December 29, 2003; revision requested March 2, 2004; revision received April 23; accepted May 24. Address correspondence to M.J.S. (e-mail: siegelm@mir.wustl.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Four round lucite phantoms with 8–32-cm diameters were scanned with multi–detector row computed tomography (CT) and 80–120 kVp. Radiation dose was based on CT dose index, image noise, and iodine contrast and measured with constant and variable tube currents that were age appropriate for each tube voltage. Radiation dose and image noise and contrast were compared in round and oval 24-cm phantoms. For various combinations of technical factors and phantom sizes and shapes, percentage differences were calculated for radiation dose and image noise and contrast. Associations between tube voltage and radiation dose, image noise, and image contrast in round and oval phantoms were determined by fitting second-degree polynomials to data. Differences in radiation dose and image noise and contrast, which were attributable to differences in tube voltage, were tested with paired t tests.
RESULTS: With 165-mAs tube current, radiation doses with 140- and 80-kVp tube voltages were 103% ([41.9 mGy – 20.6 mGy]/20.6 mGy) and 58% ([10.2 mGy – 4.2 mGy]/10.1 mGy) higher in the 8-cm phantom than in the 32-cm phantom. When tube current was adapted for phantom size, radiation dose at 80 kVp in the 8-cm phantom was reduced by 82% ([10.1 mGy – 1.8 mGy]/10.1 mGy). In the 8-cm phantom, tube voltage was decreased from 120 to 80 kVp and tube current remained at 165 mAs, resulting in a 68% noise increase ([3.1 HU – 1.8 HU]/1.8 HU). With variable tube current, 80-kVp tube voltage in the 8-cm phantom led to a 138% noise increase ([7.3 HU – 3.1 HU]/3.1 HU). With reduced tube voltage, image contrast increased. In the 8-cm phantom, with a constant 165-mAs tube current and a decrease in tube voltage from 120 to 80 kVp, there was a 35% ([333 HU – 217 HU]/333 HU) increase in contrast. No difference was noted in radiation dose or noise between round and oval phantoms (P = .604 and P = .06, respectively), but a small statistically significant difference (1%) in contrast attenuation was demonstrated (P = .025).
CONCLUSION: Reduced tube voltage for pediatric contrast material–enhanced CT reduces radiation dose and maintains image contrast. Image noise increases, but the effect is minimal in smaller phantoms. An additional reduction in tube current further reduces radiation dose.
© RSNA, 2004
Index terms: Computed tomography (CT), image quality • Computed tomography (CT), in infants and children • Computed tomography (CT), radiation exposure • Computed tomography (CT), technology • Phantoms • Test objects
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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As the percentage of CT examinations is increasing in the general population, the percentage of CT examinations is rapidly increasing in children as well. In 1989, approximately 4% of CT examinations were performed in pediatric patients; in 1993, this percentage increased to 6% (4). Currently, about 10% of all CT examinations are performed in pediatric patients, and they deliver about 67% of the overall collective radiation dose to this population (5). The radiation dose from CT remains a major concern, especially in pediatric applications, because of the potential carcinogenic effects of relatively low levels of ionizing radiation exposure (4,6). With future advances in scanner technology, the number of CT examinations will likely continue to increase, as will the collective medical radiation dose to the pediatric population.
Radiation dose is affected by several scanning parameters, such as beam energy (tube voltage), tube current–time product (measured in milliampere-seconds), section thickness, number of sections, and pitch (7). Several authors have recommended reductions in the tube current–time product as a function of patient size to reduce radiation dose to pediatric patients in clinical settings (8–11). Boone et al (12) evaluated size-dependent technique factors, including varying the tube current and voltage, by using phantoms that ranged from 10 to 32 cm in diameter and reported CT techniques that allowed constant image quality and reduced radiation dose in pediatric patients. The limitation of this study was the failure to measure radiation doses in phantoms intended to simulate small infants. Nickoloff et al (13) analyzed the effect of phantom size, tube voltage, tube current, and scanner type on the CT dose index and found that CT dose index is an exponential function of phantom diameter and that it increases in phantoms with a smaller diameter. In this study, the effects of variable phantom size and technical factors on image contrast and noise were not evaluated. While Huda et al (14) did discuss how changes in tube voltage and tube current influenced the contrast-to-noise ratio, they used larger phantoms that ranged from 14.9 to 37.7 cm in diameter. Thus, the purpose of our study was to evaluate the effect that varying tube voltage and current in phantoms of different sizes and shapes, which were intended to simulate pediatric patients who were undergoing abdominal contrast material–enhanced CT studies, would have on radiation dose and image noise and contrast.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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CT examinations of the phantoms were performed with four tube voltages: 80, 100, 120, and 140 kVp. The first set of CT scans was obtained in the round phantoms, with tube current fixed at 165 mAs. This tube current is consistent with that in published reports (17,18), and it was chosen to ensure optimal image quality in the adult-sized phantom. Subsequent CT scans were obtained in the round phantoms with two fixed tube voltages (80 and 120 kVp) and with a reduced tube current that would be age and weight appropriate. The 8-, 16-, and 24-cm phantoms were scanned with 80 kVp and a tube current of 30, 50, and 110 mAs, respectively. These values are based on published reference standards (17,18). The largest phantom, which had a 32-cm diameter, was not scanned at 80 kVp because an adequate amount of tube current could not be generated to achieve acceptable image quality. The 8-, 16-, 24-, and 32-cm phantoms were scanned with a tube voltage of 120 kVp and a tube current of 20, 35, 85, and 165 mAs, respectively. The tube currents used with the 120-kVp setting are those that were used at our institution when we performed examinations with 120 kVp. Finally, the 24-cm oval phantom was examined with both 80 and 120 kVp; tube current settings appropriate for each tube voltage were used.
Radiation Dose Measurements
The term radiation dose refers to the absorbed dose or energy dose. The unit of measure for absorbed dose is joules per kilogram or grays (1 Gy = 1000 mGy = 100 rad). The absorbed dose describes how much energy from ionizing radiation has been absorbed in a specific point of a volume. We studied absorbed dose because it is usually used for calculation of organ and effective dose values (3,19).
In this study, we used the weighted CT dose index (CTDIw) for reporting scanner dose performance. The measurement of the CT radiation dose profile integrated along the 100-mm length of the pencil chamber (CTDI100) was measured at the center and edge of each phantom. The weighted CT dose index provides a weighted average of the center (CTDI100center) and the peripheral (CTDI100peripheral) dose measurements. This value is calculated by using the following equation: CTDIw = [(1/3) · (CTDI100center)] + [(2/3) · (CTDI100peripheral)] (20). The peripheral CT dose index was calculated as the average of the four peripheral locations.
Measurements were performed in each of the phantoms by using a dosimeter and an ionization chamber (Fig 2). The dose chamber was positioned in the center and 1 cm below the surface, and a dosimeter was used to convert the electric signals to dose readings calibrated in air kerma. The absorbed dose to water was calculated from the measured air kerma with a conversion factor of 1.05 (21).
Image Quality
In this study, iodine was used as a contrast agent to simulate contrast-enhanced abdominal scanning. Image contrast and noise in the center of the round and oval phantoms were selected as relevant criteria for optimization of image quality because, in general, the center of the phantom has the highest level of image noise and the lowest level of image contrast, which limits the use of these images for diagnostic purposes.
Four scans were obtained, and image noise and image contrast were measured to reduce statistical variation of the data. For measurement of image noise, we used two subtracted images from each of the four scans to avoid errors created by the drill holes and potential image nonuniformities and applied a factor of root two (22). Image noise in the central inserts was evaluated with a region of interest that had an area of 4 cm2 and was placed by an author (B.S.). These were placed in a position that allowed us to avoid measuring edge artifacts.
The central drill holes were filled with an iodine solution. The iodine concentration was adjusted, resulting in a value of 200 HU, and was measured with a 120-kVp tube voltage in the center of the phantoms. We selected this value because it resulted in image contrast values equivalent to those of typical CT scans in adults to optimize image quality. The Hounsfield unit measurements of the iodine contrast agent in the central insert were evaluated with a region of interest with a diameter of 10 mm, which was placed by an author (B.S.).
We assessed the accuracy of the image contrast measurements by assessing the CT numbers of air and water, which are commonly used in CT quantitative phantoms. We assessed the relative CT number of water by filling the central hole of each of the four phantoms with water and scanning them with four different tube voltages (80, 100, 120, and 140 kVp) and a constant tube current (165 mAs). Four examinations were performed with each set of parameters to reduce statistical variation of the data. CT scans were evaluated for the mean CT numbers of water, which was assumed to have an attenuation of 0 HU. The measured CT values of water varied from –2 HU to 2 HU, which validated the accuracy of the measurements. The values for air were also in the expected range (–1000 HU).
Statistical Analysis
For the round phantoms, the differences in tube voltage, radiation dose, image noise, and image contrast at fixed and varying tube currents were reported in simple percentiles. For both circular and oval phantoms, radiation dose, image noise, and image contrast were plotted against phantom diameter for various voltage settings. For some plots, tube current was fixed at 165 mAs; for other plots, tube current was set to be appropriate for phantom size. The effects of varying tube voltage, tube current, and phantom size on radiation dose, image noise, and image contrast were documented with calculation of percentage change.
Regression analyses were used to assess the associations between tube voltage and radiation dose, image noise, and image contrast. Differences in radiation dose, image noise, and image contrast—which were attributable to differences in tube voltage—between circular and oval phantoms were tested with paired t tests. Image noise and image contrast values from the centers of the phantoms were entered into these analyses because image noise and image contrast are most affected by beam attenuation at the center of the phantom.
Data distribution normality was determined with Shapiro-Wilk W Tests, and 
was set at .05. Data analyses were performed with JMP statistical software (version 5.1; SAS, Cary, NC).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Unnecessary radiation may be delivered when CT scanner parameters are not appropriately adjusted for patient size. Recommendations for reducing radiation exposure have stressed optimization of CT settings based on patient weight or diameter and the anatomic region of interest (16,25–32). Specific recommendations have included reduction of both the tube current and the number of examinations performed with contrast material, use of a relatively large pitch, and elimination of inappropriate referrals for CT. As a rule, however, these articles have not focused on the effect that varying tube voltage has on the radiation dose, which was the focus of our study.
A critical issue in the evaluation of the radiation dose used in pediatric CT examinations is the size of the patient, since radiation dose is related to patient size. Our results, which are similar to those of other studies, demonstrate that the weighted CT dose index increased in small-diameter phantoms when examined with identical tube energy and current settings. This is because there is less intervening material to absorb the radiation in smaller phantoms. Because ionizing radiation causes more biologic effects in children, it is important to adapt the CT technical parameters to minimize radiation dose. Thus, this study was undertaken to determine the relationship of radiation dose and image quality to the phantom diameter and shape and the tube voltage and tube current.
The most important aspect of this study was the attempt to explicitly address the effect that changing the tube voltage and using multi–detector row techniques has on radiation dose in pediatric-sized phantoms and whether lowering the voltage has any effect on image quality. The results of our study demonstrate that beam energy has a direct influence on radiation dose. When tube voltage is decreased, radiation dose decreases in phantoms of all sizes. Similar to the results of other studies, our results also confirm that tube current has a direct influence on radiation dose (8,12–14). In addition to confirming this effect, our results add new information about the effect that a reduction in both tube current and tube voltage has on the radiation dose. A reduction in both tube current and tube voltage decreases the radiation dose even further in a constant-sized phantom. More specifically, with tube current fixed at 165 mAs, the radiation doses in the 8-cm phantom when 140- and 80-kVp tube voltages were used were 103% ([41.9 mGy – 20.6 mGy]/20.6 mGy) and 58% ([10.2 mGy – 4.2 mGy]/10.1 mGy) higher than those in the 32-cm phantom. When the tube current was adapted for phantom size, the radiation dose with an 80-kVp tube voltage was reduced by 82% ([10.1 mGy – 1.8 mGy]/10.1 mGy) in the 8-cm phantom.
When selecting technique factors, one must also take into account whether there is improvement or degradation of image quality. The results of our study show that it is possible to use a tube voltage of 80 kVp and maintain image contrast in phantoms. We demonstrated that with a reduction in tube voltage, there was an increase in image contrast. In the 8-cm phantom, the tube current remained constant at 165 mAs, and the tube voltage was decreased from 120 to 80 kVp, which resulted in a 35% ([333 HU – 217 HU]/333 HU) increase in image contrast. Huda et al (14) also found that the CT contrast is improved, not only for iodine contrast but also for fat and muscle, with lower tube voltage values, which supports our findings. In a recent article, Cody et al (33) reported that use of a tube voltage of 80 kVp resulted in beam-hardening artifacts in patients 4 years of age and younger; thus, they recommended the use of 100–120-kVp settings in pediatric patients. Their study was performed with a 4 x 5-mm detector configuration that used an axial (contiguous) rather than a helical mode, and only the surface radiation dose was measured. Experience in adult-sized phantoms and patients also has shown that CT noise increases with lower tube voltage (14,28).
The results of our study confirm that image noise increases as tube voltage decreases. In our 8-cm phantom, when the tube voltage was decreased from 120 to 80 kVp and a 165-mAs tube current was maintained, noise increased by 68% ([3.1 HU – 1.8 HU]/1.8 HU). We have also shown, however, that the increase in image noise is more obvious in larger phantoms than in smaller phantoms. Of equal importance, we have demonstrated that there is no appreciable difference in image noise in the infant-sized (8-cm) phantoms at the 80- and 120-kVp settings. This likely reflects the fact that x-ray photons are able to better penetrate smaller phantoms because there is less attenuation caused by intervening material.
Additionally, we compared radiation dose, image noise, and image contrast in two differently shaped phantoms, one that was circular and one that was oval. The oval phantom more realistically simulates the human body. The results of the comparison show no significant difference in radiation dose or image noise because of phantom shape. Contrast values were higher (by about 1%) with oval phantoms than with circular phantoms. This difference, however, would not be detectable by observers; therefore, it is not of clinical importance. This information is important because it means that circular phantoms, which are widely available in comparison to oval phantoms, can be used to study methods of radiation dose reduction.
Spiral rather than axial (contiguous) techniques were used throughout this investigation. The results herein apply equally to conventional axial CT techniques, however, provided that the technical factors—including tube voltage, tube current, and section thickness—are the same and the pitch is 1:1 (34). The data in this study were acquired with a pitch of 2, which reduces the radiation dose by half when compared with a pitch of 1.
Among the limitations of our study is the fact that we did not address the effect that a reduction in tube voltage had on the radiation dose to individual body organs. This approach would require the use of an anthropomorphic phantom. We used an acrylic model, since our goal was to assess the interaction of phantom size and tube voltage on radiation dose, not the effect of radiation dose on body organs. In addition, the acrylic model allowed us to assess image quality more efficiently.
Another limitation of this study is that we have presented radiation dose only as it relates to reduction of the tube voltage and tube current. Other parameters, such as detector section thickness, table speed, and gantry cycle time also affect radiation dose. We selected a 2.5-cm detector configuration, high table speed (pitch of 2), and fast gantry cycle because these are commonly used parameters in clinical multi–detector row CT imaging (35). Even given the fact that only a single image thickness, table speed, and gantry rotation time were used, we believe that our data provide a reliable research method for further investigation of radiation dose reduction for CT examinations in children.
An argument also can be made that differences in image quality may not be as robust in patients because of differences in the administration of contrast medium or in breathing-related artifacts, both of which can affect the visibility of structures. The question of image quality will have to be answered in future studies conducted in pediatric patients.
Practical applications: The results of this study indicate that radiation dose can be decreased and acceptable image quality can be achieved in pediatric contrast-enhanced CT examinations by reducing tube voltage and tube current. An important clinical application of this research is its potential usefulness in investigations of other organ systems, such as the chest and the musculoskeletal and central nervous systems, in an attempt to determine guidelines for tube voltage and tube current in these organ systems. Perhaps it is of more clinical importance that these results could be used to develop clinical protocols in the pediatric population.
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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(CTDIw)]. Contrast-to-noise ratio, as a measure of image quality, increases with reduction of tube voltage and smaller phantom size.