A new method to measure thyroid uptake with a gamma camera without routine use of a standard source

Gamma camera-specific reference standards for radioactive iodine uptake measurements

BACKGROUND

Current guidelines of the radioiodine uptake (RAIU) test allow the use of different equipment, isotopes, activity and region-of-interest (ROI). We evaluated presence and extent of these differences in clinical practice and evaluated the effect of some of these variations on RAIU outcomes. Also, gamma camera-specific reference standards were calculated and retrospectively compared with measurements obtained during clinical RAIU tests.

MATERIALS AND METHODS

First, questionnaires were sent to Dutch nuclear medicine departments requesting information about equipment usage, isotope, isotope formulation, activity and measurement techniques. Secondly, a neck phantom containing a range of activities in capsule or water-dissolved formulation was scanned. Counts were measured using automatic ROI, square box ROI or all counts in the image. Thirdly, clinical RAIU data were collected during 2015-2018 using three different gamma cameras. Reference standards for each scanner were calculated using regression analysis between reference activity and measured counts. Uptake measurements using this gamma camera-specific reference standard were compared with original measurements.

RESULTS

The survey demonstrated significant differences in isotope, isotope formulation, activity, use of neck phantoms, frequency and duration of reference measurements, distance to collimator, use of background measurements and ROI delineation. The phantom study demonstrated higher counts for the water-dissolved formulation than capsules using both automatic and square box ROI. Also, higher counts were found using a square box ROI than an automatic ROI. The retrospective study showed feasibility of RAIU calculations using camera-specific reference standards and good correlation with the original RAIU measurements.

CONCLUSIONS

This study demonstrated considerable technical variation in RAIU measurement in clinical practice. The phantom study demonstrated that these differences could result in differences in count measurements, potentially resulting in different dose calculations for radioactive iodine therapy. Retrospective data suggest that camera-specific reference standards may be used instead of individual reference measurements using separate activity sources, which may thus eliminate some sources of variation.

Comparison of 99mTc Pertechnetate Thyroid Uptake Rates by Gamma Probe and Gamma Camera Methods for Differentiating Graves' Disease and Thyroiditis

BACKGROUND

Technetium-99 m (^99mTc) pertechnetate thyroid uptake rate can be measured with a gamma probe or gamma camera. This study was performed to compare their diagnostic accuracy for evaluating patients with thyrotoxicosis.

METHODS

We retrospectively reviewed ^99mTc pertechnetate thyroid uptake rate records of patients by both gamma probe and camera methods at a tertiary center in Korea from November 2019 to June 2020. We determined the normal reference ranges of thyroid uptake rate by two methods and compared both methods for differentiating Graves' disease and thyroiditis in patients with thyrotoxicosis by receiver operating characteristic curve analysis.

RESULTS

A total of 371 patients (euthyroid 89, Graves 167, and thyroiditis 115) were included. The normal reference range for thyroid uptake rate in euthyroid patients was 0.3-1.9% and 2.0-4.7% for the camera and probe methods, respectively. For differentiating Graves' disease and thyroiditis, the area under the curve of the camera method (0.988) was significantly greater than that (0.975) of the probe method (p = 0.030). The sensitivity and specificity for the probe method were 92.2% and 91.3%, respectively, with a cut-off value of 3.0%. Those for the camera method were 93.4% and 94.8%, respectively, with a cut-off value of 0.7%.

CONCLUSION

^99mTc pertechnetate thyroid uptake rate measured by the camera method had higher diagnostic accuracy than the probe method for evaluating patients with thyrotoxicosis. Considering the diagnostic accuracy and patient convenience, the thyroid uptake rate measured by the camera method would be a good substitute for the probe method.

Monte Carlo simulation of energy spectra for123I imaging

(123)I is a radionuclide frequently used in nuclear medicine imaging. The image formed by the 159 keV photopeak includes a considerable scatter component due to high energy gamma-ray emission. In order to evaluate the fraction of scattered photons, a Monte Carlo simulation of a scintillation camera used for (123)I imaging was undertaken. The Monte Carlo code consists of two modules, the HEXAGON code modelled the collimator with a complex hexagonal geometry and the NAI code modelled the NaI detector system including the back compartment. The simulation was carried out for various types of collimators under two separate conditions of the source locations in air and in water. Energy spectra of (123)I for every pixel (matrix size = 256 x 256) were obtained by separating the unscattered from the scattered and the penetrated photons. The calculated energy spectra (cps MBq(-1) keV(-1)) agreed with the measured spectra with approximately 20% deviations for three different collimators. The difference of the sensitivities (cps MBq(-1)) for the window of 143-175 keV was less than 10% between the simulation and the experiment. The partial sensitivities for the scattered and the unscattered components were obtained. The simulated fraction of the unscattered photons to the total photons were 0.46 for LEHR, 0.54 for LEGP and 0.90 for MEGP for the 'in air' set-up, and 0.35, 0.40 and 0.68 for the 'in water' set-up, respectively. The Monte Carlo simulation presented in this work enabled us to investigate the design of a new collimator optimum for (123)I scintigraphy.