Parl C, Kolb A, Schmid A M, Wehrl H F, Disselhorst J A, Soubiran P D, Stricker-Shaver D, Pichler B J
Department of Preclinical Imaging and Radiopharmacy, Werner Siemens Imaging Center, Eberhard Karls University Tuebingen, Roentgenweg 13, 72076 Tuebingen, Germany.
Phys Med Biol. 2017 Sep 1;62(18):7357-7378. doi: 10.1088/1361-6560/aa8384.
Preclinical imaging benefits from simultaneous acquisition of high-resolution anatomical and molecular data. Additionally, PET/MRI systems can provide functional PET and functional MRI data. To optimize PET sensitivity, we propose a system design that fully integrates the MRI coil into the PET system. This allows positioning the scintillators near the object but requires an optimized design of the MRI coil and PET detector. It further requires a new approach in realizing the radiofrequency (RF) shielding. Thus, we propose the use of an optically transparent RF shielding material between the PET scintillator and the light sensor, suppressing the interference between both systems. We evaluated two conductive foils (ITO, 9900) and a wire mesh. The PET performance was tested on a dual-layer scintillator consisting of 12 × 12 LSO matrices, shifted by half a pitch. The pixel size was 0.9 × 0.9 mm; the lengths were 10.0 mm and 5.0 mm, respectively. For a light sensor, we used a 4 × 4 SiPM array. The RF attenuation was measured from 320 kHz to 420 MHz using two pick-up coils. MRI-compatibility and shielding effect of the materials were evaluated with an MRI system. The average FWHM energy resolution at 511 keV of all 144 crystals of the layer next to the SiPM was deteriorated from 15.73 ± 0.24% to 16.32 ± 0.13%, 16.60 ± 0.25%, and 19.16 ± 0.21% by the ITO foil, 9900 foil, mesh material, respectively. The average peak-to-valley ratio of the PET detector changed from 5.77 ± 0.29 to 4.50 ± 0.39, 4.78 ± 0.48, 3.62 ± 0.16, respectively. The ITO, 9900, mesh attenuated the scintillation light by 11.3 ± 1.6%, 11.0 ± 1.8%, 54.3 ± 0.4%, respectively. To attenuate the RF from 20 MHz to 200 MHz, mesh performed better than copper. The results show that an RF shielding material that is sufficiently transparent for scintillation light and is MRI compatible can be obtained. This result enables the development of a fully integrated PET detector and MRI coil assembly.
临床前成像受益于高分辨率解剖学和分子数据的同时采集。此外，PET/MRI系统可以提供功能性PET和功能性MRI数据。为了优化PET灵敏度，我们提出一种将MRI线圈完全集成到PET系统中的系统设计。这使得闪烁体能够靠近物体放置，但需要对MRI线圈和PET探测器进行优化设计。这还需要一种实现射频（RF）屏蔽的新方法。因此，我们建议在PET闪烁体和光传感器之间使用光学透明的RF屏蔽材料，以抑制两个系统之间的干扰。我们评估了两种导电箔（ITO、9900）和一种金属丝网。在由12×12的LSO矩阵组成的双层闪烁体上测试PET性能，这些矩阵偏移了半个间距。像素尺寸为0.9×0.9毫米；长度分别为10.0毫米和5.0毫米。对于光传感器，我们使用了4×4的硅光电倍增管（SiPM）阵列。使用两个拾取线圈测量320kHz至420MHz的RF衰减。用MRI系统评估材料的MRI兼容性和屏蔽效果。靠近SiPM的那一层的所有144个晶体在511keV处的平均半高宽能量分辨率分别因ITO箔、9900箔、金属丝网材料而从15.73±0.24%恶化到16.32±0.13%、16.60±0.25%和19.16±0.21%。PET探测器的平均峰谷比分别从5.77±0.29变为4.50±0.39、4.78±0.48、3.62±0.16。ITO、9900、金属丝网分别使闪烁光衰减了11.3±1.6%、11.0±1.8%、54.3±0.4%。为了衰减20MHz至200MHz的RF，金属丝网的性能优于铜。结果表明，可以获得对闪烁光足够透明且与MRI兼容的RF屏蔽材料。这一结果有助于开发完全集成的PET探测器和MRI线圈组件。
