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

This study was designed to validate a reliable gamma camera-based method for measuring thyroid uptake. The method is based on a stable calibration procedure and does not require daily use of a standard source. In addition, the method is designed to overcome deadtime losses inherent in uptake probe measurements with iodine-123.

A calibration procedure was designed using I-123 capsules in a neck phantom and tested using two gamma cameras. Patient thyroid uptake measurements made with the proposed gamma camera method were correlated with measurements obtained using the traditional uptake probe technique. Image quality was compared between the low-energy, parallel-hole collimator used in this study and the pinhole collimator usually used for thyroid imaging.

Capsule count rates measured using the gamma camera are linear with activity, and the calibration procedure is reproducible when tests are performed several months apart. Thyroid uptake measured with the gamma camera correlated closely (R2 > 0.9) with measurements made using the probe technique. Image quality with the low-energy parallel-hole collimator was comparable to that obtained with the pinhole collimator.

The proposed camera-based method for measuring thyroid uptake is accurate and reproducible. It can be performed readily in conjunction with the routine scan procedure. Adoption of this technique reduces equipment requirements in the nuclear medicine department.