Lee K H, Siegel M E, Fernandez O A
Department of Radiology, University of Southern California, Los Angeles, USA.
Clin Nucl Med. 1995 Mar;20(3):199-202. doi: 10.1097/00003072-199503000-00001.
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.
本研究试图找出使用甲状腺摄取探头和γ相机测量甲状腺摄取量时所观察到的差异的原因。使用带有针孔准直器的γ相机和两种市售的甲状腺摄取系统，对65例患者的甲状腺摄取量进行了测量。此外，还使用相同的探头系统和γ相机，对0.111至7.4 MBq（3至200微居里）的I-123进行了模拟甲状腺摄取测量。通过回归分析对患者和体模研究的结果进行了评估。γ相机记录的计数与体模中I-123的量成正比，但探头系统呈现出非线性关系。在1.48 Mq（40微居里）时观察到计数损失20%，在颈部体模中与临床给药剂量7.4 MBq（200微居里）相关的剂量下，计数损失高达50%。由于在对胶囊标准进行计数时，高计数率下存在严重的死时间损失，摄取率出现人为偏高。65例患者的研究显示出类似的趋势，探头技术得出的摄取率高于γ相机。甲状腺摄取系统的使用者应在临床使用的I-123活度剂量范围内校准其仪器的计数效率。为了获得准确的摄取率，必须通过与探头在多种计数率下的效率相对应的校准因子对计数进行校正，包括治疗前对预期剂量计数的效率。
