Koenig Thomas, Zuber Marcus, Hamann Elias, Cecilia Angelica, Ballabriga Rafael, Campbell Michael, Ruat Marie, Tlustos Lukas, Fauler Alex, Fiederle Michael, Baumbach Tilo
Karlsruhe Institute of Technology (KIT), Institute for Photon Science and Synchrotron Radiation (IPS) and ANKA Synchrotron Radiation Facility, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
Phys Med Biol. 2014 Oct 21;59(20):6195-213. doi: 10.1088/0031-9155/59/20/6195. Epub 2014 Sep 26.
Spectroscopic x-ray imaging based on pixellated semiconductor detectors can be sensitive to charge sharing and K-fluorescence, depending on the sensor material used, its thickness and the pixel pitch employed. As a consequence, spectroscopic resolution is partially lost. In this paper, we study a new detector ASIC, the Medipix3RX, that offers a novel feature called charge summing, which is established by making adjacent pixels communicate with each other. Consequently, single photon interactions resulting in multiple hits are almost completely avoided. We investigate this charge summing mode with respect to those of its imaging properties that are of interest in medical physics and benchmark them against the case without charge summing. In particular, we review its influence on spectroscopic resolution and find that the low energy bias normally present when recording energy spectra is dramatically reduced. Furthermore, we show that charge summing provides a modulation transfer function which is almost independent of the energy threshold setting, which is in contrast to approaches common so far. We demonstrate that this property is directly linked to the detective quantum efficiency, which is found to increase by a factor of three or more when the energy threshold approaches the photon energy and when using charge summing. As a consequence, the contrast-to-noise ratio is found to double at elevated threshold levels and the dynamic range increases for a given counter depth. All these effects are shown to lead to an improved ability to perform material discrimination in spectroscopic CT, using iodine and gadolinium contrast agents. Hence, when compared to conventional photon counting detectors, these benefits carry the potential of substantially reducing the imaging dose a patient is exposed to during diagnostic CT examinations.
基于像素化半导体探测器的光谱X射线成像可能会对电荷共享和K荧光敏感，这取决于所使用的传感器材料、其厚度以及所采用的像素间距。因此，光谱分辨率会部分丧失。在本文中，我们研究了一种新型探测器ASIC——Medipix3RX，它具有一种名为电荷求和的新特性，该特性通过使相邻像素相互通信来实现。因此，几乎完全避免了导致多次命中的单光子相互作用。我们针对其在医学物理中感兴趣的成像特性研究这种电荷求和模式，并将其与无电荷求和的情况进行对比。特别是，我们考察了它对光谱分辨率的影响，发现记录能谱时通常存在的低能偏差会大幅降低。此外，我们表明电荷求和提供了一个几乎与能量阈值设置无关的调制传递函数，这与目前常见的方法形成对比。我们证明这一特性与探测量子效率直接相关，当能量阈值接近光子能量且使用电荷求和时，探测量子效率会提高三倍或更多。结果，在较高阈值水平下对比度噪声比会翻倍，并且对于给定的计数器深度动态范围会增加。所有这些效应都表明在使用碘和钆造影剂的光谱CT中进行材料鉴别时能力得到了提高。因此，与传统光子计数探测器相比，这些优势有可能大幅降低患者在诊断CT检查期间所接受的成像剂量。
