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首页> 外文期刊>Medical Physics >Digital count summing vs analog charge summing for photon counting detectors: A performance simulation study
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Digital count summing vs analog charge summing for photon counting detectors: A performance simulation study

机译：数字计数求和光子计数探测器的模拟电荷求和：性能仿真研究

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Purpose Charge sharing is a significant problem for CdTe‐based photon counting detectors (PCDs) and can cause high‐energy photons to be misclassified as one or more low‐energy events. Charge sharing is especially problematic in PCDs for CT because the high flux necessitates small pixels, which increase the magnitude of charge sharing. Analog charge summing (ACS) is a powerful solution to reduce spectral distortion arising from charge sharing but may be difficult to implement. We investigate correction of the signal after digitization by the comparator (“digital count summing”), which is only able to correct a subset of charge sharing events but may have implementation advantages. We compare and quantify the relative performance of digital and ACS in simulations. Methods Transport of photons in CdTe was modeled using Monte Carlo simulations. Energy deposited in the CdTe substrate was converted to electrical charges of a predetermined shape, and all charges within the detector pixel are assumed to be perfectly collected. In ACS, the maximum charge received over any 2?×?2 block of pixels was grouped together prior to digitization. In digital count summing (DCS), the charge was digitized in each pixel, and subsequently, adjacent pixels that detected events grouped their charge to record a single, higher energy event. All simulations were performed at the limit of low flux (no pileup). The default tube voltage was 120?kVp, object thickness was 20?cm of water, pixel pitch was 250?μm, and charge cloud modeled as a Gaussian with σ ?=?40?μm. Variation of these parameters was examined in a sensitivity analysis. Results Detectors that used no correction, DCS, and ACS misclassified 51%, 39%, and 15% of incident photons, respectively. For iodine basis material imaging, DCS exhibited 100% greater dose efficiency compared to uncorrected, and ACS exhibited an additional 111% greater dose efficiency compared to digital charge summing. For a nonspectral task, the dose efficiency improvement as estimated by improvement of zero‐frequency detective quantum efficiency, DQE(0) was 10% for DCS compared to uncorrected and 10% for ACS compared to DCS. A sensitivity analysis showed that DCS generally achieved half the benefit of ACS over a range of conditions, although the benefit was markedly less if the charge cloud was instead modeled as a small sphere. Conclusions Summing of counts after digitization may be a simpler alternative to summing of charge prior to digitization due to the relative complexity of analog circuit design. Over most conditions studied, it provides roughly half the benefit of ACS and may offer certain implementation advantages.
机译：目的电荷共享是基于CDTE的光子计数探测器（PCD）的重要问题，并且可以使高能量的光子被错误分类为一个或多个低能量事件。在CT的PCD中，电荷共享尤其有问题，因为高通量需要小像素，这增加了电荷共享的幅度。模拟电荷求和（ACS）是一种强大的解决方案，可以降低电荷共享引起的光谱失真，但可能难以实现。我们调查了比较器数字化后信号的校正（“数字计数求和”），只能纠正电荷共享事件的子集，但可能具有实现优势。我们比较并量化模拟中数字和ACS的相对性能。方法使用蒙特卡罗模拟模拟CDTE中光子的传输。将沉积在CdTe衬底中的能量转化为预定形状的电荷，并且假设检测器像素内的所有电荷都被完美收集。在ACS中，在任何2？×2个像素块上接收的最大电荷在数字化之前分组。在数字计数求和（DCS）中，在每个像素中将电荷数字化，随后，检测到事件的相邻像素分组它们的充电以记录单个，更高的能量事件。所有模拟均在低通量（无堆积）的极限下进行。默认管电压为120？KVP，物体厚度为20？Cm水，像素间距为250Ωμm，并且用σ=Δ40Ω·40Ω·40Ωμm的电荷云。在灵敏度分析中检查了这些参数的变异。结果探测器，无需校正，DC和ACS分别错误分类51％，39％和15％的入射光子。对于碘基础材料成像，与未校正相比，DC具有100％的剂量效率，与数字电荷求和相比，ACS额外的剂量效率呈现了额外的111％。对于非光谱任务，通过提高零频侦探量子效率的剂量效率改善，DCS（0）为DCS的10％与未校正和ACS相比的10％与DCS相比。灵敏度分析表明，DC通常在一系列条件下实现了ACS的一半，尽管如果电荷云被建模为小球，则效益明显较低。结论由于模拟电路设计的相对复杂性，数字化在数字化之前，数字化之后的计数可以是更简单的对电荷总结的替代方案。在大多数情况下学习，它提供了大约一半的ACS利益，并可提供某些实施优势。

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