Personalization of 177Lu-based radionuclide therapy requires implementation of dosimetry methods that are both accurate and practical enough for routine clinical use. Quantitative single-photon emission computed tomography/computed tomography (QSPECT/CT) is the preferred scanning modality to achieve this and necessitates characterizing the response of the camera, and calibrating it, over the full range of therapeutic activities and system capacity. Various methods to determine the camera calibration factor (CF) and the deadtime constant (τ) were investigated, with the aim to design a simple and robust protocol for quantitative 177Lu imaging. The SPECT/CT camera was equipped with a medium energy collimator. Multiple phantoms were used to reproduce various attenuation conditions: rod sources in air or water-equivalent media, as well as a Jaszczak phantom with inserts. Planar and tomographic images of a wide range of activities were acquired, with multiple energy windows for scatter correction (double or triple energy window technique) as well as count rate monitoring over a large spectrum of energy. Dead time was modelled using the paralysable model. CF and τ were deduced by curve fitting either separately in two steps (CF determined first using a subset of low-activity acquisitions, then τ determined using the full range of activity) or at once (both CF and τ determined using the full range of activity). Total or segmented activity in the SPECT field of view was computed. Finally, these methods were compared in terms of accuracy to recover the known activity, in particular when planar-derived parameters were applied to the SPECT data. The SPECT camera was shown to operate as expected on a finite count rate range (up to ~ 350 kcps over the entire energy spectrum). CF and τ from planar (sources in air) and SPECT segmented Jaszczak data yielded a very good agreement (CF < 1% and τ < 3%). Determining CF and τ from a single curve fit made dead-time-corrected images less prone to overestimating recovered activity. Using triple-energy window scatter correction while acquiring one or more additional energy window(s) to enable wide-spectrum count rate monitoring (i.e. ranging 55–250 or 18–680 keV) yielded the most consistent results across the various geometries. The final, planar-derived calibration parameters for our system were a CF of 9.36 ± 0.01 cps/MBq and a τ of 0.550 ± 0.003 μs. Using the latter, the activity in a Jaszczak phantom could be quantified by QSPECT with an accuracy of 0.02 ± 1.10%. Serial planar acquisitions of sources in air using an activity range covering the full operational capacity of the SPECT/CT system, with multiple energy windows for wide-spectrum count rate monitoring, and followed by simultaneous determination of CF and τ using a single equation derived from the paralysable model, constitutes a practical method to enable accurate dead-time-corrected QSPECT imaging in a post-177Lu radionuclide therapy setting.

中文翻译：

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带有死区校正的177Lu定量SPECT（QSPECT）的全面SPECT / CT系统表征和校准。

基于177Lu的放射性核素治疗的个性化需要剂量学方法的实施，这些方法既准确又实用，足以用于常规临床应用。定量单光子发射计算机断层扫描/计算机断层扫描（QSPECT / CT）是实现此目的的首选扫描方式，并且有必要在整个治疗活动和系统容量范围内表征相机的响应并对其进行校准。研究了确定相机校准因子（CF）和停滞时间常数（τ）的各种方法，目的是设计一种简单而强大的定量177Lu成像方案。SPECT / CT相机配备了中能准直仪。使用了多个幻像来再现各种衰减条件：空气或水等价介质中的棒状光源，以及带有插入物的Jaszczak幻像。采集了各种活动的平面和断层图像，其中包括用于散射校正的多个能量窗口（双或三能量窗口技术）以及对大范围能量的计数率监控。死区时间使用可瘫痪模型进行建模。CF和τ可通过两步曲线拟合来推导（首先使用低活动性采集的子集确定CF，然后使用活动的全部范围确定τ）或一次（使用CF的全部范围确定CF和τ）活动）。计算了SPECT视野中的总活动或分段活动。最后，在准确性方面比较了这些方法以恢复已知的活动，尤其是在将平面派生的参数应用于SPECT数据时。SPECT相机在有限的计数率范围内（在整个能谱范围内最高〜350 kcps）可以正常工作。来自平面（空气源）和SPECT分段Jaszczak数据的CF和τ产生了很好的一致性（CF <1％和τ<3％）。从单个曲线拟合确定CF和τ可以使经停滞时间校正的图像不太容易高估恢复的活动。使用三重能量窗口散射校正，同时获取一个或多个其他能量窗口以实现广谱计数率监视（即55-250 keV或18-680 keV范围），可在各种几何结构上获得最一致的结果。对于我们的系统，最终的，源自平面的校准参数是9.36±0.01 cps / MBq的CF和0.550±0.003μs的τ。使用后者，Jaszczak体模中的活性可以通过QSPECT进行定量，准确度为0.02±1.10％。使用活动范围覆盖SPECT / CT系统的全部操作能力的空气范围内的源平面连续采集，具有多个能量窗口，用于广谱计数率监测，然后使用从下式导出的单个方程式同时测定CF和τ这种可麻痹的模型构成了一种实用的方法，可以在177Lu后放射性核素治疗环境中进行精确的经死时间校正的QSPECT成像。