May 2018, VOLUME 210
NUMBER 5

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May 2018, Volume 210, Number 5

Medical Physics and Informatics

Original Research

Dynamic CT for Parathyroid Adenoma Detection: How Does Radiation Dose Compare With Nuclear Medicine?

+ Affiliations:
1Department of Radiology, The Royal Melbourne Hospital, Parkville, Victoria, Australia.

2Present address: Frankston Hospital, 1 Hastings Rd, Frankston 3199, Victoria, Australia.

3Present address: Epworth Medical Imaging, Epworth Richmond, Victoria, Australia.

4Endocrine Surgery Unit, The Royal Melbourne Hospital, Parkville, Victoria, Australia.

5Department of Nuclear Medicine, The Royal Melbourne Hospital, Parkville, Victoria, Australia.

Citation: American Journal of Roentgenology. 2018;210: 1118-1122. 10.2214/AJR.17.18674

ABSTRACT
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OBJECTIVE. Dynamic CT is increasingly used for preoperative localization of parathyroid adenomas, but concerns remain about the radiation effective dose of CT compared with that of 99mTc-sestamibi scintigraphy. The purpose of this study was to compare the radiation dose delivered by three-phase dynamic CT with that delivered by 99mTc-sestamibi SPECT/CT performed in accordance with our current protocols and to assess the possible reduction in effective dose achieved by decreasing the scan length (i.e., z-axis) of two phases of the dynamic CT protocol.

MATERIALS AND METHODS. The effective dose of a 99mTc-sestamibi nuclear medicine parathyroid study performed with and without coregistration CT was calculated and compared with the effective dose of our current three-phase dynamic CT protocol as well as a proposed protocol involving CT with reduced scan length.

RESULTS. The median effective dose for a 99mTc-sestamibi nuclear medicine study was 5.6 mSv. This increased to 12.4 mSv with the addition of coregistration CT, which is higher than the median effective dose of 9.3 mSv associated with the dynamic CT protocol. Reducing the scan length of two phases in the dynamic CT protocol could reduce the median effective dose to 6.1 mSv, which would be similar to that of the dose from the 99mTc-sestamibi study alone.

CONCLUSION. Dynamic CT used for the detection of parathyroid adenoma can deliver a lower radiation dose than 99mTc-sestamibi SPECT/CT. It may be possible to reduce the dose further by decreasing the scan length of two of the phases, although whether this has an impact on accuracy of the localization needs further investigation.

Keywords: dynamic CT, nuclear medicine, parathyroid adenoma, radiation dose

Dynamic CT has become a viable alternative and adjunct to nuclear medicine imaging in the localization of parathyroid adenoma and has shown similar or higher accuracy [14]. CT has the additional benefit of excellent anatomic delineation, which can guide targeted surgery, with reduced morbidity compared with traditional bilateral parathyroid exploration. Initially, CT came at the cost of a higher radiation dose than that associated with nuclear medicine studies [5, 6]. Improvements in the detector design of modern CT scanners and reconstruction algorithms, such as iterative reconstruction, have in some cases significantly reduced the radiation dose delivered by CT studies. Also, an increase in obtaining 99mTc-sestamibi parathyroid studies with coregistration CT to improve anatomic localization of suspected adenomas has been noted [7, 8]. Therefore, it would be prudent to reassess the radiation dose delivered by each modality with the use of modern scanners and SPECT/CT.

Dynamic CT was introduced on the premise that parathyroid adenomas can be distinguished from thyroid nodules and lymph nodes on the basis of the typical temporal enhancement pattern of each. Parathyroid adenomas typically appear hypoattenuated on unenhanced imaging, hyperattenuated on arterial phase imaging, and hypoattenuated on delayed phase imaging relative to thyroid tissue. By contrast, parathyroid adenomas typically appear hypervascular relative to lymph nodes on arterial studies and may also be morphologically different. These findings have been described in numerous studies [14]. The scan that is typically obtained covers the entire range of the expected ectopic location of parathyroid adenomas, from the base of the skull to the upper mediastinum; by convention, each phase has included the same scan range. Because three-phase imaging typically is helpful only when scanning through the thyroid to distinguish between exophytic thyroid nodules and parathyroid adenoma, the unenhanced and delayed phases could be limited to this anatomic region, therefore reducing the scan range of two of the three phases and reducing the dose delivered by a dynamic CT study.

In this retrospective study, we compared the radiation dose delivered by dynamic three-phase parathyroid CT with that delivered by 99mTc-sestamibi SPECT/CT performed for the localization of suspected adenomas, using our current dynamic CT protocol. We also assessed a potential reduction in effective dose by decreasing the scan length (i.e., z-axis) of the unenhanced and delayed phases of the CT examination.

Materials and Methods
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Ethics approval for this study was obtained from the ethics committee at Royal Melbourne Hospital.

The dynamic CT scan protocol performed at our institution involves obtaining an unenhanced scan followed by arterial and delayed phase scans. The scan range is from the angle of the mandible to the top of the aortic arch in all three phases. All three phases are performed using automatic dose modulation (Care kV, Siemens Healthcare) with use of tube current modulation technology in the x-, y-, and z-axis directions (CareDose 4D, Siemens Healthcare). Images are obtained using a dual-source CT scanner (Somatom Definition DS, Siemens Healthcare). The reconstruction algorithms used to produce the CT images are based on the iterative reconstruction method.

All nuclear medicine parathyroid imaging is performed using a SPECT system (Symbia T16, Siemens Healthcare). The nuclear medicine department uses a scanning range from the angle of the mandible to the diaphragm in the coregistration CT, to avoid missing ectopic adenomas located along the pericardium. For all patients, coregistration CT is performed using a 16-MDCT scanner (Somatom Emotion Duo, Siemens Healthcare) operated at 130 kV with automatic dose modulation performed using CareDose 4D, 70 Quality Reference mAs (Siemens Healthcare), and a pitch of 1. The reconstruction algorithms used to reconstruct the CT images are based on the filtered back-projection method.

Table 1 summarizes the CT parameters used for both dynamic CT and nuclear medicine coregistration CT. Figure 1 shows the typical scan range for the dynamic CT and nuclear medicine coregistration CT.

TABLE 1: CT Parameters for Dynamic CT and Nuclear Medicine Coregistration CT
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Fig. 1A —69-year-old man with lymphoma.

A, Scan range (area between two horizontal lines) for all three phases of currently used dynamic CT protocol (A) and currently used nuclear medicine coregistration CT protocol (B).

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Fig. 1B —69-year-old man with lymphoma.

B, Scan range (area between two horizontal lines) for all three phases of currently used dynamic CT protocol (A) and currently used nuclear medicine coregistration CT protocol (B).

Median CT exposure and dose data were collected for 30 consecutive patients (10 male patients and 20 female patients) who, from July to November 2015, underwent dynamic CT for suspected parathyroid adenoma performed using our institution's standard three-phase protocol. The effective dose of this scan was calculated. All dose calculations were performed by the department's medical physicist.

The effective dose for a 99mTc-sestamibi parathyroid examination, both alone and in combination with coregistration CT, was calculated using data from 30 consecutive patients (nine male patients and 21 female patients) who underwent this examination between July and September 2015 with use of the mean effective dose conversion coefficients for the 99mTc-sestamibi examination (for a resting subject), as tabulated by International Commission on Radiological Protection publication 80 [9]. The effective dose for coregistration CT was estimated using the commercial dosimetry program for evaluating exposure (CT-Expo, version 2.3).

A dynamic CT protocol with a reduced scan length was then devised in which the scan lengths of the unenhanced and delayed phases were restricted to a z-axis from the hyoid to the top of the manubrium. These landmarks were chosen because they are easily identified by radiographers using lateral scout imaging, and a review of the CT scans of 30 patients showed that the range included the entire thyroid in all but one patient. The scan length for the arterial phase remained unchanged and extended from the mandible to the aortic arch.

The estimated effective dose for the proposed dynamic CT protocol with the reduced scan length was then calculated using a dosimetry program for evaluating exposure (CT-Expo, version 2.3, SASCRAD). This was achieved by reviewing the CT images and noting the tube current (expressed as milliamperes) per rotation selected by the scanner in the region from the angle of the mandible to the hyoid and from the manubrium to the top of the aortic arch (i.e., the excess scanning region). The mean tube current for this excess region was then determined. Using the estimated tube current per rotation, the rotation time, and the peak kilovoltage, the dosimetry program for calculating exposure was used to estimate the volume CT dose index (CTDIvol) and the dose-length product (DLP) and to calculate the effective dose for the excess region. This was then subtracted from the unenhanced and delayed phase CT examinations to estimate the effective dose associated with the reduced scan length.

Given the difference in scan length between nuclear medicine coregistration CT and dynamic CT, the effective dose that would be delivered by SPECT/CT if it were to use the same scan length as a dynamic CT was also estimated in the same manner as the effective dose for the dynamic CT protocol with the reduced FOV.

Calculated effective doses were derived using tissue weighting in accordance with International Commission on Radiological Protection publication 103 [10] and were presented as sex-averaged (not sex-specific) doses with the use of median CT exposure parameters. The median dose was chosen because it is more likely to represent the average patient size. The effective dose estimates for the current dynamic CT protocol and the proposed dynamic CT protocol with the reduced scan length were then calculated using the commercial dosimetry program for evaluating exposure.

Results
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Table 2 presents the median dose and range and the median scan length for the three individual imaging phases in the currently used dynamic CT protocol.

TABLE 2: Dose and Scan Length for Each Phase of the Current Dynamic CT Protocol

With use of the Care kV reference value of 120 kV, the unenhanced phase typically requires 120 kVp to achieve an improved contrast-to-noise ratio, whereas the contrast-enhanced phases typical result in 100 kVp. The Quality Reference mAs value is automatically increased at the lower kilovoltage setting to maintain a constant signal-to-noise ratio compared with target values. Table 2 shows that the median values for the CTDIvol and the DLP for the contrast-enhanced phases of this examination were approximately 15–20% lower when compared with the unenhanced phase and resulted in a lower effective dose despite all three phases having a similar scan length.

For nuclear medicine parathyroid imaging performed at our institution, the 99mTc-sestamibi scans of the 30 patients were obtained using a median activity of 804 MBq. This was estimated based on the activity of and time that the radioisotope was drawn and was corrected for the time that the injection was injected.

The current nuclear medicine coregistration CT scan range extends to the diaphragm. The scan length from the aortic arch to the diaphragm is approximately 30% of the total coregistration CT scan length. Given the number of radiosensitive organs and the increased tube current required to achieve an acceptable signal-to-noise ratio through the thoracic cage, scanning through this region results in approximately 50% of the total effective dose. The CTDIvol value for the nuclear medicine coregistration CT is approximately 15–20% higher than that for the dynamic CT unenhanced phase and indicates that the DLP is approximately 40% higher. For the coregistration CT study, median (range) values were as follows: 139.5 Quality Reference mAs (87–211 Quality Reference mAs), a CTDIvol for a 32-cm phantom that was 17.4 mGy (10.9–26.3 mGy), a DLP of 555.0 mGy × cm (330–949 mGy × cm), and a scan length of 32.3 cm (24.8–37.1 cm). For the 99mTc-sestamibi study, the median (range) activity was 804 MBq (760–863 MBq).

The proposed reduced scan length for the unenhanced and the delayed phases of the dynamic CT protocol would reduce the z-axis scan length by approximately 25% in each of these phases.

Table 3 summarizes the effective dose delivered in each stage of the currently used dynamic CT protocol, the currently used nuclear medicine protocol, and the proposed reduced-FOV nuclear medicine coregistration CT and dynamic CT protocols.

TABLE 3: Effective Dose Delivered by Each Stage of the Currently Used and Proposed Reduced-Scan-Range Dynamic CT Protocol, and the Currently Used and Proposed Reduced-Scan-Range Nuclear Medicine Coregistration CT Protocol

With use of the current nuclear medicine and dynamic CT protocols, a typical nuclear medicine study delivers an effective dose that is higher than that of dynamic CT. Reducing the nuclear medicine coregistration CT scan range to be comparable to that of dynamic CT could result in a lower effective dose for combined SPECT/CT, compared with the currently used dynamic CT protocol. Using the reduced scan length for the unenhanced and delayed phases of the dynamic CT protocol could result in an effective dose that is lower than the effective dose of both the currently used dynamic CT protocol and the nuclear medicine SPECT/CT protocol.

Discussion
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Early studies indicated that nuclear medicine 99mTc-sestamibi scintigraphy delivers a lower radiation dose than dynamic CT [5, 6]. At our institution, we have shown that with the evolution of CT equipment and the move toward performing nuclear medicine SPECT with coregistration CT, dynamic CT can deliver a lower effective dose than parathyroid SPECT/CT (9.3 vs 12.4 mSv, respectively).

The use of coregistration CT in nuclear medicine imaging contributes a significant amount to the delivered dose. In our institution, the dose delivered by coregistration CT was higher than that delivered in any phase of dynamic CT. Two major factors contribute to the discrepancy. First, coregistration CT uses filtered back projection to reconstruct the CT images, whereas dynamic CT benefits from iterative reconstruction. The latter allows a lower CTDIvol and, therefore, DLP and dose than those that can be achieved with filtered back projection to obtain the same image quality. Our nuclear medicine department has a preference to maintain a level of image quality sufficient to accurately locate a suspected parathyroid adenoma. This requires a higher signal-to-noise ratio for improved spatial resolution visualization, further contributing to the high CTDIvol. This level of image quality typically is not desired or achieved for a CT examination that is performed for coregistration or attenuation correction purposes only, but it may reflect a push to obtain better anatomic differentiation by nuclear medicine physicians.

For these reasons, it is quite possible that the effective dose that we have obtained for combined SPECT/CT may be higher than that achieved at many institutions. However, it should also be noted that even if our mean DLP was halved, making it closer to more typical values for coregistration CT performed as part of a parathyroid SPECT/CT study, this would result in a median effective dose of 9 mSv for the combined SPECT/CT study compared with a median dose of 9.3 mSv for the current dynamic CT protocol and 6.1 mSv for the proposed dynamic CT protocol.

Another important and modifiable factor in the delivered dose from nuclear medicine coregistration CT is the scan range. Our nuclear medicine department uses a scan length for coregistration CT that is longer than that used for dynamic CT, to avoid missing ectopic parathyroid glands along the pericardium. It is estimated that by using a scan length similar to that used for dynamic CT, a reduction of approximately 50% of the effective dose of coregistration CT (6.8 vs 3.4 mSv) could be achieved. In doing so, the total effective dose would be approximately 9.0 mSv, which would be comparable to the effective dose of current dynamic CT. Given the findings of the present study, a review of the required CT scan length for coregistration purposes will be undertaken at our institution, which may result in a reduced effective dose.

Parathyroid imaging was initially described as a multiphase imaging technique, and published data suggest that multiple phases continue to be used in most parathyroid imaging [14]. Given that the unenhanced and delayed phases are helpful only in differentiating parathyroid adenomas from thyroid nodules, we propose that the unenhanced and delayed phases could be confined to the region of the thyroid gland in our modified protocol. We note that Hoang et al. [11] also restrict the unenhanced phase for a similar reason.

A recent study by Raghavan et al. [12] went further, suggesting that an arterial phase alone is adequate for detecting parathyroid adenomas. With the use of our CT parameters, this would result in an effective dose of only 2.9 mSv, which is much lower that of a 99mTc-sestamibi scan. A single phase would be a significant shift from the generally accepted multiphase paradigm, and Raghavan and colleagues admitted that an additional unenhanced phase might be helpful “to enable differentiation of an intrinsically hyperattenuated exophytic thyroid nodule from an enhancing parathyroid nodule” [12]. The use of more than one phase is supported by Hunter et al. [13], who used a logistic regression model to evaluate one-, two-, and three-phase protocols and found that three-phase imaging was superior to protocols using fewer phases.

This is indicative of the heterogeneity of the published data on dynamic CT protocols, which in turn reflects individual reporting experience as well as the lack of prospective studies with significant patient numbers. Our own study is limited by the small number of 30 patients per cohort, and we acknowledge that larger prospective studies are needed in this field.

Of note, there is a paucity of published data on the effective dose delivered by different dynamic CT protocols, and it would be reasonable to assume that a wide dose range is delivered by the wide range of CT protocols, but the effective dose of our current dynamic CT protocol is similar to that found at other institutions [6].

Although SPECT/CT and dynamic CT effective doses could, in theory, be comparable, the effective dose of dynamic CT still remains higher than that of 99mTc-sestamibi scintigraphy alone. This may not need be the case. We have proposed a protocol that could result in a dynamic CT dose similar to that of the 99mTc-sestamibi scan alone. This proposed technique requires investigation to ensure that the reduced scan length does not reduce sensitivity or specificity for localizing adenomas.

This study does not attempt to evaluate the sensitivity and specificity of dynamic CT for the detection of parathyroid adenoma or compare the accuracy of dynamic CT and nuclear medicine studies, because this has been done in previous studies [14]. Hunter et al. [4] showed a higher accuracy of dynamic CT, compared with 99mTc-sestamibi imaging, in detecting sporadic parathyroid adenoma. Rodgers et al. [1] reported a higher sensitivity in localizing hyperfunctioning parathyroid glands, compared with that of ultrasound and 99mTc-sestamibi imaging.

Improved preoperative imaging has significant implications for surgical planning where precise anatomic localization of suspected parathyroid adenomas afforded by CT can guide targeted surgery with reduced morbidity. Other benefits of CT include a reduced scan time compared with that required for nuclear medicine imaging and no need for precautions regarding exposure of others to radiation after the scan is complete.

Currently, CT is often used as a second-line problem solving tool because of the perceived higher radiation dose compared with the dose for nuclear medicine imaging. The current dose estimates provided in the present study may change the preoperative imaging paradigm to favor dynamic CT as first-line imaging.

Conclusion
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Dynamic CT, when used for the detection of parathyroid adenoma, can deliver a radiation effective dose lower than that associated with 99-mTc-sestamibi SPECT/CT. It may be possible to reduce the dose even further by reducing the scan length of two of the phases, although whether this has an impact on accuracy of localization needs further investigation.

References
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Address correspondence to C. A. Czarnecki ().

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