Comparative Evaluation of 24-Hour Thyroid 131I Uptake Between Camera-Based Method Using Medium-Energy Collimator and Standard Uptake Probe-Based Method

Quantifying partial volume effect in SPECT and planar imaging: optimizing region of interest for activity concentration estimation in different sphere sizes

INTRODUCTION

To quantify the partial volume effect in single photon emission tomography (SPECT) and planar images of Carlson phantom as well as providing an optimum region of interest (ROI) required to more accurately estimate the activity concentration for different sphere sizes.

METHODS

131 I solution with the 161.16 kBq/ml concentration was uniformly filled into the different spheres of Carlson phantom (cold background condition) with the diameters of 7.3, 9.2, 11.4, 14.3, 17.9, 22.4 and 29.9 mm, and there was no background activity. In the hot background condition, the spheres were filled with the solution of 131 I with the 1276.5 kBq/ml addition to the background activity concentration of 161.16 kBq/ml in all the phantoms. The spheres were mounted inside the phantom and underwent SPECT and planar images. ROI was drawn closely on the boundary of each sphere image and it was extended to extract the true count.

RESULTS

In the cold background condition, the recovery coefficient (RC) value for SPECT images ranged between 0.8 and 1.03. However, in planar imaging, the RC value was 0.72 for the smallest sphere size and it increased for larger spheres until 0.98 for 29.9 mm. In the hot background condition, the RC value for sphere diameters larger than 20 mm was overestimated more than in the cold background condition. The ROI/size required to more accurately determine activity concentration for the cold background ranged from 1.18 to 2.7. However, in the hot background condition, this ratio varied from 1.34 to 4.05.

CONCLUSION

In the quantification of partial volume effects, the spill-out effect seems to play a crucial role in the distribution of the image counts beyond the boundaries of the image pixels. However, more investigations are needed to accurately characterize limitations regarding the object size, background levels, and other factors.

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.

A SIMPLE AND NOVEL APPROACH TO STUDY KINETICS AND ESTIMATE RADIATION DOSES FROM INTERNALLY ADMINISTERED RADIOPHARMACEUTICALS USING AN EXTERNAL DOSE MEASUREMENT SYSTEM

Various methods have been reported to study radiotracer kinetics and make internal dosimetry feasible in the routine clinical nuclear medicine practice. The aim of the present study was to quantify cumulative activity and organ doses using an indigenously designed and fabricated external dose measurement system. The measurement was demonstrated on patients undergoing whole-body (WB) 18F-FDG (Fluorine-18-fluorodeoxyglucose) direct positron emission tomography/computed tomography investigations. An external dose measurement system comprising of an ionisation chamber-survey meter and the movable focussing collimator was used to quantify the uptake of 18F-FDG in liver and brain. Cumulative activity and normalised cumulative activity in these organs were calculated. The results were validated by performing measurements on a phantom uniformly filled with known activity of 18F-FDG.The difference in the absorbed dose estimated with and without collimator was statistically significant (p < 0.05). The external dose measurement technique is relatively novel, convenient and reliable for the assessment of internal absorbed dose of organs.

Quantification of radioactivity by planar gamma-camera images, a promoted method of absorbed dose in the thyroid after iodine-131 treatment

Iodine-131 (¹³¹I) is an essential and widely used radioisotope in thyroid diseases and animal experiments. Planar imaging has been considered the most popular method for ¹³¹I thyroid uptake radioactive activity quantification. The ROI defining section is essential and can affect the accuracy of quantitative results. However, a consistent method has not been proposed. In this study, a UC-ROI defining method based on ULWL setting and colour display grade was applied. Three steps were performed: image acquisition of five standard activity models and obtaining the exact value that the counts per radioactive activity contributes to the ROI; image acquisition of 20 rat thyroids and obtaining the counts of the ROI (thyroid); and calculating the rat thyroid radioactive activity and comparing these values with the true values. The accuracy of quantification activity of ¹³¹I in rat thyroid reached 2.62% ± 0.41%. The mean quantification within 5% could be achieved in 19 of 20 rat thyroids. No significant difference existed between calculated thyroid ¹³¹I activity and true values with a paired matched-test (t = -0.384, P = 0.706 > 0.05). The results indicated that with the UC-ROI defining method, more accurate thyroid uptake ¹³¹I radioactive activity quantification by SPECT planar imaging can be achieved in vivo rat study.

Gamma camera based method for 131I capsule counting: an alternate method to Uptake probe method

The main objective of this study was to check the validity of using gamma camera as an alternate method to thyroid uptake probe, for counting 25uCi (0.925 MBq) and 50uCi (1.85 MBq) 131I capsules before administration to thyroid patients. Methods: - 10 sets each of 25uCi (0.925 MBq) and 50uCi (1.85 MBq) 131I capsules received from Board Of Radiation and Isotope Technology, Department Of Atomic Energy, India (BRIT, DAE) have been counted individually using thyroid uptake probe for 10 seconds following institutional protocol and also by keeping individual capsule of a set with 8cm gap between each of them .These capsules were also scanned by Scintillation gamma camera for 100 seconds. Capsules having counts within the range of mean ±2 Standard Deviation (SD) were accepted for patient administration. After analysing both the data, correlation coefficient between these two methods has been evaluated. Results: Scanned images were analysed by drawing Identical ROI around each set of 25uCi (0.925 MBq) and 50uCi (1.85 MBq) 131I capsules. Capsules with counts within 2 Standard Deviation from mean were accepted for patient administration. Good correlation coefficient (r >0.95) was observed between these two counts set. Conclusion: Gamma camera based 131I -capsule counting method is an easy and time saving method compared to probe based capsule counting method as we can scan a set of capsules in a single acquisition. It can provide uniformity information for a batch of 131I -capsules and avoid the time consuming method of individual capsule counting with the thyroid uptake probe.

Comparing pre-therapeutic 124I and 131I uptake tests with intra-therapeutic 131I uptake in benign thyroid disorders

PURPOSE

¹²⁴I-PET/CT can be used for pre-therapeutic assessment of radioactive iodine uptake in benign thyroid disorders, however systematic comparisons with intra-therapeutic uptake are still lacking for these disorders. The goals of this study were to compare ¹²⁴I RAIU and conventional ¹³¹I RAIU tests with each other; to compare both tests with intra-therapeutic uptake (reference); and to verify the time course of radioactive iodine uptake at three time points (30, 102, and 336 h [14 days] post administration; p.a.).

METHODS

Thirteen patients with benign thyroid diseases underwent ¹³¹I RAIU test and ¹²⁴I RAIU test one after another before the intra-therapeutic ¹³¹I uptake (reference) was measured via short-range and long-range measurements. After correction for decay, relative uptake differences were calculated and subjected to the Bland-Altman method for the evaluation of levels of agreement.

RESULTS

Radioactive iodine uptake tests with ¹²⁴I-PET/CT and ¹³¹I probe did not show systematic deviations at any time point. Likewise, at 30 and 102 h p.a. there was no systematic discrepancy between pre-therapeutic and intra-therapeutic uptake levels. At 14 days p.a., however, both pre-therapeutic tests tended to overestimate the uptake compared to reference. Findings showed, for the first time with ¹²⁴I, that radioiodine therapy has some early radiobiological effects possibly limiting the accuracy of pre-therapeutic dosimetry.

CONCLUSIONS

¹²⁴I RAIU tests represent a feasible alternative to standard ¹³¹I RAIU tests. The additional benefits of ¹²⁴I-PET/CT (e.g., functional topography, inclusion of retrosternal areas, possibility to enable fusion imaging) may thus increase the scope of this technology in benign thyroid disorders.

Relevance of a thyroid phantom in estimating thyroid radioiodine uptake values using a gamma camera

The objective of this work was to evaluate the necessity of a thyroid phantom in counting a standard capsule during estimation of iodine-131 thyroid uptake using gamma camera methods. For this, camera-based uptake was calculated taking a standard capsule within a thyroid phantom, as well as a standard capsule (without phantom) placed at 5, 10, and 15 cm from the face of the collimator. The values obtained in each setting were compared with the traditional standard thyroid probe-based method. Among these four sets of values, that with the phantom was the closest to the reference probe-based uptake values. Among those without the phantom, the camera-based uptake with the standard at 15 cm from the face of the collimator was closer to the standard probe method. However, as the image at 15 cm would give poor resolution, it would not be feasible to adopt this method for clinical routine. Thus, to conclude, for calculating camera-based uptake, a standard capsule in the phantom gives the best comparable values to the standard probe-based method, indicating the need for the phantom when adopting the gamma camera-based methodology.