Variation in Tube Voltage for Adult Neck MDCT: Effect on Radiation Dose and Image Quality
Abstract
OBJECTIVE. The purpose of this study was to assess the effect of peak kilovoltage on radiation dose and image quality in adult neck MDCT.
MATERIALS AND METHODS. An anthropomorphic phantom with metal oxide semiconductor field effect transistor detectors was imaged with a 64-MDCT scanner. The reference CT protocol called for 120 kVp, and images obtained with that protocol were compared with CT images obtained with protocols entailing 80, 100, and 140 kVp. All imaging was performed with automatic tube current modulation. Organ dose and effective dose were determined for each protocol and compared with those obtained with the 120-kVp protocol. Image noise was evaluated objectively and subjectively for each protocol.
RESULTS. The highest organ doses for all protocols were to the thyroid, ocular lens, skin, and mandible. The greatest reductions in organ dose were for the bone marrow of the cervical spine and mandible: 43% and 35% with the 100-kVp protocol and 63% and 53% with the 80-kVp protocol. Effective dose decreased as much as 9% with the 100-kVp protocol and 12% with the 80-kVp protocol. Use of the 140-kVp protocol was associated with an increase in organ dose as high as 64% for bone marrow in the cervical spine and a 19% increase in effective dose. Image noise increased with lower peak kilovoltage. The measured noise difference was greatest at 80 kVp, absolute increases were less than 2.5 HU. There was no difference in subjective image quality among protocols.
CONCLUSION. Reducing the voltage from 120 to 80 kVp for neck CT can result in greater than 50% reduction in the absorbed organ dose to the bone marrow of the cervical spine and mandible without impairment in subjective image quality.
Current techniques of dose reduction for neck MDCT protocols include thyroid bismuth shields and automatic tube current modulation (ATCM). Use of these techniques has been found to result in 47% and 78% dose reduction to the thyroid gland [1, 2]. Another means of reducing the radiation dose and tube output, which to our knowledge has not been evaluated for neck MDCT, is to lower the peak kilovoltage of the tube [3]. Tube voltage adjustments have been used clinically for pediatric CT to lower radiation dose with a reduction in effective dose of 48–73% [4]. Lowering the peak kilovoltage for adult CT is not common in clinical practice, but this technique may be especially suited to the neck because it is a smaller anatomic region than the chest and abdomen.
Radiation dose reduction must always be balanced with its effect on image quality. Using lower peak kilovoltage for neck MDCT may both decrease radiation dose and improve image quality. Improved image quality may be seen in contrast-enhanced studies because imaging is closer to the K-edge of iodine [5]. In the neck this property may be especially advantageous for the assessment of vascular structures, soft-tissue structures, and mucosal abnormalities for which IV contrast administration is necessary for proper diagnosis.
The purpose of this phantom study was to assess radiation dose and image quality in adult neck CT protocols entailing varying peak kilovoltage settings in combination with ATCM. We hypothesized that use of a lower peak kilovoltage setting in combination with ATCM would result in dose savings without compromising image quality.
Materials and Methods
Neither institutional review board approval nor HIPAA compliance was required for this phantom study.
Anthropomorphic Phantom and Dosimetry
A commercially available anthropomorphic female phantom (model 702-D, CIRS) (Figs. 1A and 1B) was used for measurement of absorbed organ dose. The phantom was equivalent to an adult measuring 160 cm in height and 55 kg in weight. Twenty metal oxide semiconductor field effect transistor (MOSFET) dosimeters (model 1002RD, Best Medical) with active detector areas of 200 × 200 μm (total dimensions, 2.5 mm width × 1.3 mm thickness × 8 mm length) were placed in defined anatomic locations in the neck, chest, abdomen, and pelvis. The detectors were in locations representing the following organs: breast; skin over mandible; cerebellum; ocular lens; mandible and salivary glands; cervical spine; thyroid; esophagus; lung; sternum; thymus; thoracolumbar spine; lower esophagus and heart; liver and gallbladder; kidney; spleen and adrenal glands; and stomach, colon, and small bowel. Each detector represented a point dose in the organ with the exception that three detectors were placed in distinct locations in the lung (upper, mid, and lower), two in the esophagus (upper and lower), and two in the spinal bone marrow (cervical and thoracolumbar). In regions with more than one detector, the values were averaged to represent a single organ dose for calculation of effective dose.
Detectors were placed in the head and neck of the phantom to maximize characterization of the absorbed organ doses directly in the path of the primary x-ray beam. Additional detectors were placed in organs outside the primary beam in organs important for determining the effective dose. For organs that would be partially scanned and with more than one detector (lung, esophagus, and spine), at least one of the detectors was placed strategically in the primary beam of the x-ray tube. As a result, the measured organ doses could represent the high end of dose value rather than the mean from the multiple spatial locations. This method provides a conservative measurement of dose appropriate for radiation protection determinations. In CT, partial irradiation of organs occurs regularly, and to the best of our knowledge, no standard exists on how to handle partial volume irradiation of organs in the calculation of effective dose.
Each MOSFET detector was calibrated for the appropriate beam energy, and individual calibration factors for all 20 detectors were stored in a laptop computer. Detailed calibration methods and validation of MOSFET methods have been described previously by Yoshizumi et al. [6]. The lower limit of absorbed dose detected with the MOSFET detectors with an AutoSense system is 1.40 mGy.
MDCT Protocol
Helical acquisitions were performed with a 64-MDCT scanner (LightSpeed VCT, GE Healthcare). After acquisition of initial scout views in the lateral and anteroposterior projections, the phantom was scanned from the sella turcica to the thoracic inlet with our standard clinical neck CT protocol (reference) of 120 kVp and ATCM (Smart mA, GE Healthcare) with a noise index of 8 and minimum and maximum tube currents of 100 and 500 mA. Other parameters were as follows: gantry rotation time, 0.8 second; table speed, 20.25 mm/rotation; pitch, 0.516:1; acquisition collimation, 64 × 0.625 mm; z-axis, 25 cm. Images were reconstructed with the standard algorithm (soft tissue) and collimation, 2.5 mm; interval, 2.5 mm; FOV, 22 cm. To evaluate the effect of peak kilovoltage on radiation dose and image quality, we modified the reference protocol to use voltage settings of 80, 100, and 140 kVp while keeping all the other CT parameters the same as the reference protocol.
The phantom was scanned three times with each of the four protocols to obtain mean organ doses. Dose-length product and volume CT dose index were recorded from the CT scanner for each protocol before scanning. The minimum and maximum tube current values reached for each of the protocols were also recorded.
Radiation Dose
The mean absorbed organ doses were obtained from the MOSFET dosimeters by averaging three organ doses from three consecutive scans with the same protocol. The mean absorbed organ doses for 80, 100, and 140 kVp were compared with 120 kVp as absolute values in milligrays and as percentage organ dose reduction. The equation was percentage organ dose = [(organ dose for 120 kVp – organ dose for 80, 100, or 140 kVp) × 100] ÷ organ dose for 120 kVp.
We calculated the effective dose (ED), an equivalent uniform dose to the entire body from the measured organ doses (D) by applying tissue-weighting factors (WT) in accordance with publication 103 of the International Commission on Radiologic Protection [7] and by assuming a radiation-weighting factor (WR) of 1.0 for x-rays. The effective dose was computed with the following equation:where Hi is the equivalent dose for organ i.
Image Quality Assessment
Image quality was evaluated by quantitative and qualitative analyses at a workstation (Advantage Windows Workstation, version 4.2, GE Healthcare). For the quantitative analysis, noise measurements were made by recording the SD of attenuation for a circular 1-cm2 region of interest placed in homogeneous soft-tissue structures in the phantom. Noise was measured in the soft tissues in the anterior and posterior aspects of the neck at three selected locations along the z-axis of the neck. The levels were the upper neck (immediately below mandible), thyroid gland, and lower neck–upper mediastinum (Figs. 2A, 2B, and 2C). The noise values determined for each location at each level were averaged to give the mean noise level for upper neck, mid neck, and lower neck and mediastinum.
For qualitative assessment, images were reviewed independently by two fellowship-trained neuroradiologists with 8 and 18 years of experience. The neuroradiologists were asked to compare images obtained at 120 kVp to those obtained at 80, 100, and 140 kVp. Two images were placed side by side on the same viewing screen. The readers were told which image was obtained with the reference protocol but were blinded to the peak kilovoltage of the comparison protocol. All CT parameter information was removed from the images, which were reviewed with soft-tissue window settings (width, 300 HU; level, 40 HU). Images were graded on a 5-point scale for relative amount of noise and streak artifact compared with the 120-kVp reference protocol. Streak artifact was caused by MOSFET detectors and bone density structures. A score of 5 represented moderately better image quality with moderately less noise and streak artifact than the 120-kVp protocol. A score of 4 represented slightly better image quality with slightly less noise and streak artifact than the 120-kVp protocol. A score of 3 represented no significant difference in image quality compared with the 120-kVp protocol. A score of 2 represented slightly inferior image quality with mildly more noise and streak artifact than the 120-kVp protocol. A score of 1 represented moderately inferior image quality with moderately more noise and streak artifact than the 120-kVp protocol.
Statistical Analysis
Data were entered into a Microsoft Excel spread-sheet. Statistical analyses were performed with SAS Enterprise (version 4.2, SAS Institute) and R (R Project) software. The mean organ doses for the 140-, 100-, and 80-kVp protocols were compared with the organ dose from the reference 120-kVp protocol with two-factor analysis of variance with all possible pairwise interactions. The brain was used as the reference organ, and 120 kVp as the reference dose for treatment type interaction contrasts. With this method, comparisons of organs and an overall comparison of all organs were derived. The unpaired Student t test was used to compare the mean noise values for the 140-, 100-, and 80-kVp protocols with the mean noise values for the reference 120-kVp protocol. A two-tailed value of p < 0.05 was considered statistically significant.
Results
Tube Current Range
Change in peak kilovoltage was inversely related to the maximum tube current used during the study: As peak kilovoltage decreased, the maximum tube current increased, and as the peak kilovoltage increased, the maximum tube current decreased. Figure 3 shows that the relation was nonlinear and that the maximum allowable tube current set for the CT scanner (500 mA) was not reached in any of the protocols. The maximum tube currents reached in the 80-, 100-, 120-, and 140-kVp protocols were 431, 219, 120, and 100 mA. The maximum tube current for each protocol was reached at the level of the lower neck and upper mediastinum, near the level of the thyroid. The minimum tube current used was at the upper neck immediately below the level of the mandible for all protocols. The minimum allowable tube current of 100 mA was reached for the 100-, 120-, and 140-kVp protocols (Fig. 3). The minimum tube current for the 80-kVp protocol was 111 mA. For the high-voltage protocol of 140 kVp, the tube current was constant at 100 mA (same minimum and maximum tube current) along the entire z-axis.
Organ Dose
The average measured absorbed organ doses are listed in Table 1. The organ with the highest dose for all the protocols was the thyroid, followed by the ocular lens, skin, bone marrow of the mandible, bone marrow of the sternum, and bone marrow of the cervical spine (all greater than 19 mGy at 120 kVp).
The absorbed organ dose reductions for the 80-, 100-, and 140-kVp protocols compared with the reference neck 120-kVp protocol are shown in Table 1. Across all organs, the 80- and 100-kVp protocols were associated with 20% (p = 0.025) and 12% (p = 0.06) reductions in dose. Conversely, the 140-kVp protocol was associated with a 39% overall increase in dose (p < 0.0001). The reference organ, the brain, had reductions of 33% (p = 0.001) and 22% (p = 0.03) with the 80- and 100-kVp protocols. There was significant variability in mean dose across organs (p = 0.001). The organ regions with the greatest dose reduction were the bone marrow of the cervical spine and bone marrow of the mandible: 43% and 35% with the 100-kVp protocol (p = 0.004 for cervical spine) and 63% and 53% with the 80-kVp protocol (p < 0.005). The doses to the thyroid and ocular lens were reduced with use of a lower-voltage protocol, but these reductions were less than 13%. The upper lung dose at 120 kVp was extrapolated from 80 and 100 kVp because the x-ray beam missed the MOSFET detector, thus the dose was grossly underestimated. The greatest increase in dose with the 140-kVp protocol (64%) was recorded for the bone marrow of the cervical spine.
Effective Dose
Table 2 shows the organ doses and tissue weighting factors used to derive the equivalent dose and the effective dose. Table 3 shows the effective dose calculated from measured organ dose, CT dose index, and dose-length product. All of these values decreased with reductions in peak kilovoltage. Effective dose decreased 12% with the 80-kVp protocol compared with the 120-kVp protocol. The effective dose decreased 9% with the 100-kVp protocol. There was a 19% increase in effective dose when the voltage was increased from 120 to 140 kVp.
Image Quality Assessment
The noise values were statistically significantly higher for the 80-kVp protocol than for the 120-kVp protocol at all three levels of the neck, but the absolute differences in image noise were less than 2.5 HU (Table 4). Lowering the tube voltage to 100 kVp did not produce a significant increase in noise measurements in the mid or lower neck, but there was a statistically significant increase in noise values in the upper neck. However, the absolute difference in image noise at this level was only 1.1 HU. By subjective assessment of image quality, a grade 3 score (no difference) was given to the images obtained with the 80-, 100-, and 140-kVp protocols compared with the 120-kVp protocol (Figs. 4A, 4B, 4C, and 4D).
Discussion
To our knowledge, this study is the first evaluation of the effect of variable peak kilovoltage in conjunction with ATCM on radiation dose and image quality in neck MDCT. In this adult phantom study we found that for neck MDCT protocols, reducing the peak kilovoltage while using ATCM reduced individual organ doses and the effective dose without a significant increase in noise or decrease in subjective image quality.
For pediatric CT, select use of lower peak kilovoltage based on body weight has been successfully implemented in clinical practice at several institutions and has been found to lower radiation dose without hampering imaging quality [4]. For adult CT, low-voltage CT techniques with fixed tube current have been evaluated for contrast-enhanced chest and abdominal CT examinations [8–10]. Heyer et al. [10], comparing use of 100 kVp with use of 120 kVp for pulmonary CT angiography, noted a 43% reduction in effective dose by dose-length product estimations. Nakayama et al. [11] noted a 57% reduction in estimated effective dose for abdominal CT when using 90 kVp compared with 120 kVp. In this study, we used ATCM rather than fixed tube current and found more modest 12% reduction in effective dose for the 80-kVp protocol and 9% for the 100-kVp protocol compared with the 120-kVp protocol. The smaller dose reductions in our study are related to the compensatory increase in tube current that occurs with the use of ATCM to maintain image quality.
In designing the study, we considered that ATCM might nullify the dose savings associated with lower peak kilovoltage because of the increase in tube current. However, our results showed that the CT dose index, dose-length product, and absorbed organ doses all decreased with a decrease in peak kilovoltage. Our study showed a variable dose saving for different organs of the neck. The bone marrow of the cervical spine and mandible had the greatest dose reduction, more than 50%, when a lower peak kilovoltage was used. Because bone marrow has the highest tissue-weighting factor in the neck (a reflection of its radiosensitivity), this dose saving is important for minimizing stochastic risk of carcinogenesis from radiation during MDCT of the neck [12]. The thyroid is also considered an important target for dose reduction; it received the highest organ dose in all protocols in our study, but dose reduction to the thyroid was less than 10% with lower-voltage protocols. This result likely occurred because the thyroid lies in the lower neck close to the shoulders, where there is a compensatory increase in tube current with ATCM compared with the upper neck, which is of smaller diameter.
An important consideration for CT dose reduction techniques is the effect on image quality. The absolute differences in noise between the 80-kVp protocol and the reference protocol were small in our study and below the threshold of detection by readers. These results are similar to those of Matsuoka et al. [9], who studied low-voltage techniques for pulmonary CT angiography with ATCM. They found image noise was significantly higher for a 110- or 100-kVp protocol (mean noise, 25.3 HU) than for a 130- or 120-kVp protocol (mean noise, 18.7 HU), but these differences were not appreciated at subjective assessment.
There were several limitations to our study. A major limitation was the parameter limits placed on ATCM with a noise index of 8 and minimum tube current of 100 mA. If these limits had not been present, the tube current could have been less than 100 mA at 120 and 140 kVp, resulting in lower organ and effective doses. Although removing or altering these factors might have made the results more generalizable, we chose these limits of noise index and minimum tube current because they are part of our standard clinical protocol, and the purpose of the study was to evaluate the effect of peak kilovoltage on absorbed doses in our clinical protocol. Second, that we used only one phantom size limits the applicability of results across patients of varying habitus. Images of patients with large-diameter necks and shoulders may be of impaired quality because the tube current may reach the maximum level set or attainable with the CT scanner and be unable to compensate for the lower peak kilovoltage setting. We chose to use a smaller phantom because previous studies have shown that lower peak kilovoltage is most applicable in smaller patients. Third, we analyzed only CT images obtained with a single type of MDCT scanner from one manufacturer. The results may vary for other generations of CT scanners. Fourth, individual organ doses are point location–specific and do not represent the overall absorbed dose for the entire organ but are an estimate of the organ dose. Although multiple MOSFET detectors in the organ may better represent the mean dose (particularly for larger organs such as the lung and liver), placing multiple detectors in the same organ is a practical challenge owing to limits on the number of physical detectors, resulting artifact from wires to the detectors, and need to obtain doses in the primary beam and doses to organs that lie outside the primary beam. Use of one detector per organ in this study allowed comparisons between protocols because the dosimeter locations were constant between protocols. Finally, there were limitations to evaluating image quality in phantoms because iodinated contrast material cannot be administered and individual small soft-tissue structures are not separately identified, as they would on clinical images. It also is difficult to notice small noise changes to a homogeneous area with the human eye.
Conclusion
Our phantom study showed that reducing peak kilovoltage in conjunction with ATCM during neck MDCT reduces organ dose and effective dose. A reduction in voltage from 120 to 80 kVp reduces the radiation organ dose to bone marrow of the cervical spine and mandible more than 50% and reduces effective dose 12–20% without impairment of subjective image quality.
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History
Submitted: March 3, 2011
Accepted: July 6, 2011
First published: November 23, 2012
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