Svärd Staffan Jacobsson, Hindorf Cecilia, Nilsson Joachim N
Department of Medical Radiation Physics and Nuclear Medicine, Karolinska University Hospital, Solna, Sweden.
Department of Molecular Medicine and Surgery, Karolinska Institutet, Stockholm, Sweden.
EJNMMI Phys. 2025 Jan 26;12(1):8. doi: 10.1186/s40658-025-00718-7.
System calibration is essential for accurate SPECT/CT dosimetry. However, count losses due to dead time and pulse pileup may cause calibration errors, in particular for I, where high count rates may be encountered. Calibration at low count rates should also be avoided to minimise detrimental effects from e.g. background counts and statistical fluctuations. This paper aims to present experimental data illustrating count-rate dependencies and to propose practical routines to mitigate errors in the I calibration procedure without needing advanced analysis tools.
The sensitivities of two General Electric (GE) Discovery 670 Pro systems were assessed using two Jaszczak phantom geometries. SPECT/CT data were collected over two months, starting with an initial I content of > 2 GBq, decaying to approximately 20 MBq. This allowed for a detailed analysis of count losses due to dead time and pulse pileup. From the sensitivity analysis, it was shown that robust calibration was obtained for I phantom activities ranging between 250 and 1500 MBq.
The results show that adequate corrections for dead-time and pulse-pileup counting losses are essential for accurate calibration. It is argued that loss corrections should be based on total spectrum count rates in projections and not only on the 364.5 keV energy window data. The measurement campaigns presented in this paper, using basic tools and equipment, may serve as a model for establishing routines for count-loss corrections as well as for system calibration and regular control of system sensitivity. The data suggest that analysis of source and count concentration in a homogeneous Jaszczak phantom offers robust calibration, whereas analysis of source strength and counts in a delineated phantom insert offers a practical and robust method for regular quality control.
系统校准对于准确的SPECT/CT剂量测定至关重要。然而，由于死时间和脉冲堆积导致的计数损失可能会引起校准误差，特别是对于碘（I），可能会遇到高计数率的情况。还应避免在低计数率下进行校准，以尽量减少例如本底计数和统计波动等不利影响。本文旨在展示说明计数率依赖性的实验数据，并提出实用程序以减轻碘校准程序中的误差，而无需先进的分析工具。
使用两种Jaszczak体模几何形状评估了两台通用电气（GE）Discovery 670 Pro系统的灵敏度。SPECT/CT数据收集了两个月，初始碘含量>2GBq，衰减至约20MBq。这使得能够详细分析由于死时间和脉冲堆积导致的计数损失。从灵敏度分析可知，对于碘体模活度在250至1500MBq之间可获得稳健的校准。
结果表明，对死时间和脉冲堆积计数损失进行充分校正对于准确校准至关重要。有人认为，损失校正应基于投影中的总谱计数率，而不仅仅基于364.5keV能量窗数据。本文介绍的测量活动使用基本工具和设备，可作为建立计数损失校正程序以及系统校准和系统灵敏度定期控制程序的模型。数据表明，在均匀的Jaszczak体模中分析源和计数浓度可提供稳健的校准，而在划定的体模插入物中分析源强度和计数提供了一种实用且稳健的定期质量控制方法。
