University Medical Centre, Utrecht, Netherlands.
Phys Med Biol. 2018 Mar 14;63(6):065006. doi: 10.1088/1361-6560/aaaf02.
In SPECT/CT systems x-ray and γ-ray imaging is performed sequentially. Simultaneous acquisition may have advantages, for instance in interventional settings. However, this may expose a gamma camera to relatively high x-ray doses and deteriorate its functioning. We studied the NaI(Tl) response to x-ray pulses with a photodiode, PMT and gamma camera, respectively. First, we exposed a NaI(Tl)-photodiode assembly to x-ray pulses to investigate potential crystal afterglow. Next, we exposed a NaI(Tl)-PMT assembly to 10 ms LED pulses (mimicking x-ray pulses) and measured the response to flashing LED probe-pulses (mimicking γ-pulses). We then exposed the assembly to x-ray pulses, with detector entrance doses of up to 9 nGy/pulse, and analysed the response for γ-pulse variations. Finally, we studied the response of a Siemens Diacam gamma camera to γ-rays while exposed to x-ray pulses. X-ray exposure of the crystal, read out with a photodiode, revealed 15% afterglow fraction after 3 ms. The NaI(Tl)-PMT assembly showed disturbances up to 10 ms after 10 ms LED exposure. After x-ray exposure however, responses showed elevated baselines, with 60 ms decay-time. Both for x-ray and LED exposure and after baseline subtraction, probe-pulse analysis revealed disturbed pulse height measurements shortly after exposure. X-ray exposure of the Diacam corroborated the elementary experiments. Up to 50 ms after an x-ray pulse, no events are registered, followed by apparent energy elevations up to 100 ms after exposure. Limiting the dose to 0.02 nGy/pulse prevents detrimental effects. Conventional gamma cameras exhibit substantial dead-time and mis-registration of photon energies up to 100 ms after intense x-ray pulses. This is due PMT limitations and due to afterglow in the crystal. Using PMTs with modified circuitry, we show that deteriorative afterglow effects can be reduced without noticeable effects on the PMT performance, up to x-ray pulse doses of 1 nGy.
在 SPECT/CT 系统中，X 射线和γ射线成像是顺序进行的。同时采集可能具有优势，例如在介入设置中。然而，这可能会使伽马相机暴露在相对较高的 X 射线剂量下，并使其功能恶化。我们分别使用光电二极管、PMT 和伽马相机研究了 NaI(Tl)对 X 射线脉冲的响应。首先，我们将 NaI(Tl)-光电二极管组件暴露于 X 射线脉冲下，以研究潜在的晶体余晖。接下来，我们将 NaI(Tl)-PMT 组件暴露于 10ms LED 脉冲（模拟 X 射线脉冲）下，并测量对闪烁 LED 探测脉冲（模拟γ脉冲）的响应。然后，我们将组件暴露于 X 射线脉冲下，探测器入口剂量高达 9nGy/脉冲，并分析了γ脉冲变化的响应。最后，我们研究了西门子 Diacam 伽马相机在暴露于 X 射线脉冲下对γ射线的响应。用光电二极管读取的晶体 X 射线暴露显示，在 3ms 后有 15%的余晖分数。NaI(Tl)-PMT 组件在 10ms LED 暴露后 10ms 仍会出现干扰。然而，在 X 射线暴露后，响应显示出升高的基线，衰减时间为 60ms。对于 X 射线和 LED 暴露以及基线扣除后，探测脉冲分析显示在暴露后不久，脉冲高度测量受到干扰。Diacam 的 X 射线暴露证实了基本实验。在 X 射线脉冲后多达 50ms，不会记录到任何事件，随后在暴露后 100ms 后观察到明显的能量升高。将剂量限制在 0.02nGy/脉冲可防止有害影响。传统的伽马相机在高强度 X 射线脉冲后，会出现长达 100ms 的显著死时间和光子能量错位。这是由于 PMT 的限制以及晶体中的余晖造成的。使用具有改进电路的 PMT，我们表明，在不显著影响 PMT 性能的情况下，可以减少有害的余晖效应，最高可达 1nGy 的 X 射线脉冲剂量。
