Johns Hopkins University, Baltimore MD, USA.
Phys Med Biol. 2010 Feb 7;55(3):817-31. doi: 10.1088/0031-9155/55/3/018. Epub 2010 Jan 14.
Whole-body (WB) planar imaging has long been one of the staple methods of dosimetry, and its quantification has been formalized by the MIRD Committee in pamphlet no 16. One of the issues not specifically addressed in the formalism occurs when the count rates reaching the detector are sufficiently high to result in camera count saturation. Camera dead-time effects have been extensively studied, but all of the developed correction methods assume static acquisitions. However, during WB planar (sweep) imaging, a variable amount of imaged activity exists in the detector's field of view as a function of time and therefore the camera saturation is time dependent. A new time-dependent algorithm was developed to correct for dead-time effects during WB planar acquisitions that accounts for relative motion between detector heads and imaged object. Static camera dead-time parameters were acquired by imaging decaying activity in a phantom and obtaining a saturation curve. Using these parameters, an iterative algorithm akin to Newton's method was developed, which takes into account the variable count rate seen by the detector as a function of time. The algorithm was tested on simulated data as well as on a whole-body scan of high activity Samarium-153 in an ellipsoid phantom. A complete set of parameters from unsaturated phantom data necessary for count rate to activity conversion was also obtained, including build-up and attenuation coefficients, in order to convert corrected count rate values to activity. The algorithm proved successful in accounting for motion- and time-dependent saturation effects in both the simulated and measured data and converged to any desired degree of precision. The clearance half-life calculated from the ellipsoid phantom data was calculated to be 45.1 h after dead-time correction and 51.4 h with no correction; the physical decay half-life of Samarium-153 is 46.3 h. Accurate WB planar dosimetry of high activities relies on successfully compensating for camera saturation which takes into account the variable activity in the field of view, i.e. time-dependent dead-time effects. The algorithm presented here accomplishes this task.
全身（WB）平面成像一直是剂量学的主要方法之一，其定量方法已由 MIRD 委员会在小册子第 16 号中正式化。形式化中没有具体解决的一个问题是，当到达探测器的计数率足够高以致于导致相机计数饱和时。相机死时间效应已经得到了广泛的研究，但所有开发的校正方法都假设是静态采集。然而，在 WB 平面（扫描）成像期间，由于时间的原因，探测器视场中的成像活性量是变化的，因此相机饱和是时间相关的。开发了一种新的时间相关算法，用于校正 WB 平面采集期间的死时间效应，该算法考虑了探测器和成像物体之间的相对运动。通过在体模中成像衰减的活性并获得饱和曲线来获取静态相机死时间参数。使用这些参数，开发了一种类似于牛顿法的迭代算法，该算法考虑了探测器随时间变化的可变计数率。该算法在模拟数据以及在椭球体模中的高活性钐-153 的全身扫描上进行了测试。还获得了从未饱和体模数据获得的一整套用于计数率到活性转换的参数，包括建立和衰减系数，以便将校正后的计数率值转换为活性。该算法在模拟和测量数据中都成功地证明了可以对运动和时间相关的饱和效应进行校正，并可以达到任何所需的精度。从椭球体模数据计算得出的清除半衰期，在进行死时间校正后为 45.1 小时，未校正时为 51.4 小时；钐-153 的物理半衰期为 46.3 小时。准确的高活性全身平面剂量学依赖于成功补偿相机饱和，相机饱和考虑了视场中的可变活性，即时间相关的死时间效应。这里提出的算法完成了这项任务。
