Wehner J, Weissler B, Dueppenbecker P M, Gebhardt P, Goldschmidt B, Schug D, Kiessling F, Schulz V
Department for Physics of Molecular Imaging Systems, Institute for Experimental Molecular Imaging Systems, RWTH Aachen University, Pauwelsstrasse 19, 52074 Aachen, Germany.
Phys Med Biol. 2015 Mar 21;60(6):2231-55. doi: 10.1088/0031-9155/60/6/2231. Epub 2015 Feb 16.
PET (positron emission tomography) with its high sensitivity in combination with MRI (magnetic resonance imaging) providing anatomic information with good soft-tissue contrast is considered to be a promising hybrid imaging modality. However, the integration of a PET detector into an MRI system is a challenging task since the MRI system is a sensitive device for external disturbances and provides a harsh environment for electronic devices. Consequently, the PET detector has to be transparent for the MRI system and insensitive to electromagnetic disturbances. Due to the variety of MRI protocols imposing a wide range of requirements regarding the MR-compatibility, an extensive study is mandatory to reliably assess worst-case interference phenomena between the PET detector and the MRI scanner. We have built the first preclinical PET insert, designed for a clinical 3 T MRI, using digital silicon photomultipliers (digital SiPM, type DPC 3200-22, Philips Digital Photon Counting). Since no thorough interference investigation with this new digital sensor has been reported so far, we present in this work such a comprehensive MR-compatibility study. Acceptable distortion of the B0 field homogeneity (volume RMS = 0.08 ppm, peak-to-peak value = 0.71 ppm) has been found for the PET detector installed. The signal-to-noise ratio degradation stays between 2-15% for activities up to 21 MBq. Ghosting artifacts were only found for demanding EPI (echo planar imaging) sequences with read-out gradients in Z direction caused by additional eddy currents originated from the PET detector. On the PET side, interference mainly between the gradient system and the PET detector occurred: extreme gradient tests were executed using synthetic sequences with triangular pulse shape and maximum slew rate. Under this condition, a relative degradation of the energy (⩽10%) and timing (⩽15%) resolution was noticed. However, barely measurable performance deterioration occurred when morphological MRI protocols are conducted certifying that the overall PET performance parameters remain unharmed.
正电子发射断层扫描（PET）具有高灵敏度，与能提供具有良好软组织对比度的解剖信息的磁共振成像（MRI）相结合，被认为是一种很有前景的混合成像方式。然而，将PET探测器集成到MRI系统中是一项具有挑战性的任务，因为MRI系统对外部干扰很敏感，并且为电子设备提供了恶劣的环境。因此，PET探测器必须对MRI系统透明且对电磁干扰不敏感。由于各种MRI协议对MR兼容性提出了广泛的要求，必须进行广泛的研究以可靠地评估PET探测器与MRI扫描仪之间的最坏情况干扰现象。我们使用数字硅光电倍增管（数字SiPM，型号DPC 3200-22，飞利浦数字光子计数）构建了第一个专为临床3T MRI设计的临床前PET插入件。由于迄今为止尚未报道对这种新型数字传感器进行的全面干扰研究，我们在这项工作中展示了这样一项全面的MR兼容性研究。对于安装的PET探测器，已发现B0场均匀性的可接受失真（体积均方根 = 0.08 ppm，峰峰值 = 0.71 ppm）。对于高达21 MBq的活度，信噪比下降保持在2%至15%之间。仅在具有由PET探测器产生的额外涡流引起的Z方向读出梯度的苛刻回波平面成像（EPI）序列中发现了重影伪影。在PET方面，主要发生在梯度系统和PET探测器之间的干扰：使用具有三角形脉冲形状和最大 slew 率的合成序列执行极端梯度测试。在这种情况下，注意到能量分辨率（⩽10%）和时间分辨率（⩽15%）有相对下降。然而，当进行形态学MRI协议时，几乎没有可测量的性能下降，这证明总体PET性能参数未受损害。
