Studies on measurement of thyroid uptake rate on ¹³¹I scintigraphy images

Objectives It is important to evaluate accurately ¹³¹I uptake in the thyroid gland before the internal ¹³¹I therapy for patients with Graves’ disease. The purpose of this study was to compare measurements of ¹³¹I thyroid uptake rate (TUR) before and after ¹³¹I therapy by using thyroid phantom and clinical data.

Methods An original thyroid phantom was designed and the ¹³¹I solutions were enclosed into the phantoms. In the phantom study, TUR was calculated with the data acquired from planar and SPECT images and the TUR measurement system (AZ-800, Anzai Medical). The TUR values obtained by these three kinds of methods were compared with that measured by the automatic well counter (ACCUFLEXγ 7001, Hitachi-Aloka Medical) as the standard. In the clinical study, the equation for expressing the relationship between the ¹³¹I radioactivity and the counts on the planar images was established using the ¹³¹I capsule and planar images. The TUR values in clinical cases were measured and compared with those measured by the TUR measurement system before ¹³¹I therapy.

Results In the phantom studies, the maximum errors of the TUR values obtained using planar and SPECT images, and the TUR measurement system were 4.79, 4.91, and 9.86%, respectively. The results show that the TUR values with errors less than 5% can be obtained by planar and SPECT images. In the human studies, there was a close correlation between the TUR values measured by the TUR measurement system before ¹³¹I therapy and those measured on planar images after ¹³¹I therapy. There was up to 25% differences between the TUR values measured before ¹³¹I therapy and those after therapy.

Conclusions The phantom studies show that TUR can be measured on planar and SPECT images with errors less than 5%. The human studies show that the TUR values measured on planar images after ¹³¹I therapy correlate closely with those measured before the therapy, although there are differences up to 25% between them.