[Thyroid uptake error in four different gamma camera systems].

The thyroid uptake of (123)I and (131)I is generally measured by a gamma camera system. We evaluated the error in determining thyroid uptake caused by different methods of calculation among four gamma camera systems with various collimators. We first designed an original thyroid phantom that consisted of the thyroid and a body containing various levels of radioiodine activity. The applications for thyroid uptake equipped in two gamma camera systems performed calculations by the automatic method with background counts not subtracted from the capsule counts. When the size of the rectangular region of interest (ROI) for the capsule was set at 10x8 cm (a typical ROI size for the thyroid), percentages of thyroid uptake as calculated by the manual method with background counts subtracted from the capsule counts and thyroid counts were 52% to 57% when the value was set at 55% for (123)I; and 54.2% and 58.7%, respectively, when the value was set at 60% for (131)I. On the other hand, the percentages of thyroid uptake calculated by the automatic method with the application using two gamma camera systems with non-subtraction of background counts from the capsule counts were 46% and 50.5% when the value was set at 55%; and 49.6% when the value was set at 60%. The values calculated by the automatic method were underestimated as a result of background counts that were not subtracted from the capsule counts. When ROI size for the capsule was set at 4x4 cm, which is slightly larger than the capsule size, even thyroid uptake as determined by the automatic method using the application showed a difference of less 2% from the set values. There was no difference in thyroid uptake among the various kinds of collimators, high-resolution collimators, all-purpose collimators, and a suitable collimator for gamma-ray energy of (123)I.