Discrepancies in thyroid uptake values. Use of commercial thyroid probe systems versus scintillation cameras.

This study attempts to find the cause of differences observed between measurements of the thyroid uptake using a thyroid uptake probe and a gamma camera. The thyroid uptakes of 65 patients were measured using the gamma camera with a pinhole collimator and two commercially available thyroid uptake systems. Moreover, simulated thyroid uptake measurements using 0.111 to 7.4 MBq (3 to 200 microCi) of I-123 were also taken using the same probe systems and gamma cameras. Results of the patient and phantom studies were evaluated by regression analyses. The gamma cameras recorded counts in direct proportion to the amount of I-123 in the phantom, but the probe systems exhibited a non-linear relationship. A 20% loss of counts was observed with 1.48 Mq (40 microCi), and up to 50% at the dose relating to a clinically administered dose of 7.4 MBq (200 microCi) in the neck phantom. Because of severe dead-time losses at high count rates associated with counting the capsule standard, the uptake ratios were artifactually higher. The 65 patient study showed a similar trend with the probe technique yielding uptake ratios higher than the gamma camera. Users of thyroid uptake systems should calibrate their instrument's counting efficiency in the dose range of the I-123 activity to be used clinically. To obtain an accurate uptake ratio, the counts must be corrected by calibration factors corresponding to the efficiency of the probe at multiple counting rates including that from counting the intended dose before therapy.