Brunner F Cassol, Clemens J C, Hemmer C, Morel C
Centre de Physique des Particules de Marseille (IN2P3-CNRS and Université de la Mediterannée), 13288 Marseille, France.
Phys Med Biol. 2009 Mar 21;54(6):1773-89. doi: 10.1088/0031-9155/54/6/024. Epub 2009 Mar 3.
Hybrid pixel detectors, originally developed for tracking particles in high-energy physics experiments, have recently been used in material sciences and macromolecular crystallography. Their capability to count single photons and to apply a threshold on the photon energy suggests that they could be optimal digital x-ray detectors in low energy beams such as for small animal computed tomography (CT). To investigate this issue, we have studied the imaging performance of photon counting hybrid pixel detectors based on the XPAD3-S chip. Two detectors are considered, connected either to a Si or to a CdTe sensor, the latter being of interest for its higher efficiency. Both a standard 'International Electrotechnical Commission' (IEC) mammography beam and a beam used for mouse CT results published in the literature are employed. The detector stability, linearity and noise are investigated as a function of the dose for several imaging exposures ( approximately 0.1-400 microGy). The perfect linearity of both detectors is confirmed, but an increase in internal noise for counting statistics higher than approximately 5000 photons has been found, corresponding to exposures above approximately 110 microGy and approximately 50 microGy for the Si and CdTe sensors, respectively. The noise power spectrum (NPS), the modulation transfer function (MTF) and the detective quantum efficiency (DQE) are then measured for two energy threshold configurations (5 keV and 18 keV) and three doses ( approximately 3, 30 and 300 microGy), in order to obtain a complete estimation of the detector performances. In general, the CdTe sensor shows a clear superiority with a maximal DQE(0) of approximately 1, thanks to its high efficiency ( approximately 100%). The DQE of the Si sensor is more dependent on the radiation quality, due to the energy dependence of its efficiency its maximum is approximately 0.4 with respect to the softer radiation. Finally, we compare the XPAD3-S DQE with published curves of other digital devices in a similar radiation condition. The XPAD3-S/CdTe detector appears to be the best with the highest DQE at low frequency, although some improvements are expected to reduce the increase of noise with the counts statistics and to guarantee a better stability of the detector response.
混合像素探测器最初是为高能物理实验中的粒子跟踪而开发的，最近已被应用于材料科学和大分子晶体学。它们能够对单个光子进行计数并对光子能量应用阈值，这表明它们可能是低能光束（如用于小动物计算机断层扫描（CT））中理想的数字X射线探测器。为了研究这个问题，我们研究了基于XPAD3 - S芯片的光子计数混合像素探测器的成像性能。考虑了两个探测器，分别连接到硅（Si）或碲化镉（CdTe）传感器，后者因其更高的效率而备受关注。使用了标准的“国际电工委员会”（IEC）乳腺摄影光束以及文献中公布的用于小鼠CT的光束。针对几种成像曝光（约0.1 - 400微戈瑞），研究了探测器的稳定性、线性度和噪声随剂量的变化。确认了两个探测器都具有完美的线性度，但发现对于计数统计高于约5000个光子的情况，内部噪声会增加，分别对应于硅传感器和约110微戈瑞以上、碲化镉传感器和约50微戈瑞以上的曝光。然后针对两种能量阈值配置（5 keV和18 keV）和三种剂量（约3、30和300微戈瑞）测量了噪声功率谱（NPS）、调制传递函数（MTF）和探测量子效率（DQE），以便全面评估探测器性能。总体而言，碲化镉传感器表现出明显优势，其最大DQE(0)约为1，这得益于其高效率（约100%）。硅传感器的DQE对辐射质量的依赖性更强，由于其效率的能量依赖性，相对于较软的辐射，其最大值约为0.4。最后，我们在类似辐射条件下将XPAD3 - S的DQE与其他数字设备已发表的曲线进行了比较。XPAD3 - S/CdTe探测器在低频下似乎具有最高的DQE，是最好的，不过预计还需要一些改进来减少噪声随计数统计的增加，并保证探测器响应具有更好的稳定性。
