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 t he gamma camera with a pinhole collimator and two commercially availab le thyroid uptake systems. Moreover, simulated thyroid uptake measurem ents using 0.111 to 7.4 MBq (3 to 200 muCi) of 1-123 were also taken u sing the same probe systems and gamma cameras. Results of the patient and phantom studies were evaluated by regression analyses. The gamma c ameras recorded counts in direct proportion to the amount of 1-123 in the phantom, but the probe systems exhibited a non-linear relationship . A 20% loss of counts was observed with 1.48 Mq (40 muCi), and up to 50% at the dose relating to a clinically administered dose of 7.4 MBq (200 muCi) in the neck phantom. Because of severe dead-time losses at high count rates associated with counting the capsule standard, the up take ratios were artifactually higher. The 65 patient study showed a s imilar trend with the probe technique yielding uptake ratios higher th an the gamma camera. Users of thyroid uptake systems should calibrate their instrument's counting efficiency in the dose range of the 1-123 activity to be used clinically. To obtain an accurate uptake ratio, th e counts must be corrected by calibration factors corresponding to the efficiency of the probe at multiple counting rates including that fro m counting the intended dose before therapy.