Institute of Medicine, FZ Jülich, D-52425 Jülich, Germany.
Med Phys. 2011 May;38(5):2795-805. doi: 10.1118/1.3582699.
The use of magnetic resonance (MR) radiofrequency (RF) surface coils is a prerequisite for high-quality positron emission tomography (PET)/MR imaging. In lack of in-gantry transmission (TX) sources, the exact position of the RF coils is unknown in PET/MR, and may, therefore, lead to false attenuation correction (AC) of the emission (EM) data. The authors assess lesion and background quantification in AC-PET by mimicking different PET/MR imaging situations using a whole-body (WB) PET-only tomograph.
Phantom experiments were performed on a PET tomograph with 68 Ge-rod TX sources. First, a 15-cm plastic cylinder was filled uniformly with [18F]-FDG to simulate a head study. Second, a NEMA NU-2001 image quality phantom (35 x 25 x 25 cm3) was filled uniformly with [18F]-FDG to simulate torso imaging. The phantom contained six lesions (10-38 mm diameter, lesion-to-background ratio 6:1) centred around a 5 cm diameter lung insert. EM and TX measurements were acquired with and without MR head (cylinder) and surface (NU-2001 phantom) RF coils in place. The following imaging situations were mimicked in both head and torso phantom studies: (1) PET scan without MR coils in EM and TX for reference, (2) PET scan with coils in both EM and TX, and (3) PET scan with coils in EM but without coils in TX. Two more set-ups were performed for the torso phantom: (4) PET scan with coils in EM only and phantom shifted slightly compared to (3), and (5) PET scan with coils in EM and TX following local displacement of the surface coils. PET EM data (1)-(4) were corrected for attenuation and scatter using cold TX data. Imaging situations (1)-(3) were repeated with the cylinder phantom and head coil in a combined PET/MR prototype system employing template-based AC.
Head phantom: In case the MR head coils were not accounted for during AC (3), central and peripheral background activity concentration was underestimated by 13%-19% when compared to the reference setup (1). The effects of MR coil omission during AC was replicated in the repeat study with the combined PET/MR prototype. Torso phantom: All lesions were equally visible on all AC-PET images. The effects of disregarding MR surface RF coils during AC [(3) vs (1)] were 4%, or less. A slightly higher bias was observed when accounting for the RF surface coils that were shifted between EM and TX (5). The effect of coil misalignment and neglect during AC on the quantification of the simulated lungs was insignificant compared to the noise levels in AC-PET.
Unaccounted attenuation from MR surface coils causes a regional bias of AC-PET data in body regions near the MR coils. Bias of central regions was more noticeable in smaller-size objects. In torso studies with body surface coils, the visibility of central lesions on PET was unaffected by MR coils following incomplete AC. Coil misalignment of several cm between emission and attenuation images causes an error that was comparable to that arising from unaccounted MR coil attenuation but small compared to the average standard deviation of the activity concentration levels.
磁共振（MR）射频（RF）表面线圈的使用是高质量正电子发射断层扫描（PET）/MR 成像的前提。在缺乏体腔内传输（TX）源的情况下，PET/MR 中 RF 线圈的位置未知，因此可能导致发射（EM）数据的错误衰减校正（AC）。作者通过使用全身（WB）PET 专用断层扫描仪模拟不同的 PET/MR 成像情况，评估 AC-PET 中的病变和背景定量。
在配备 68Ge 棒 TX 源的 PET 断层扫描仪上进行了一系列的体模实验。首先，一个 15cm 的塑料圆柱体内均匀填充 [18F]-FDG，以模拟头部研究。其次，一个 NEMA NU-2001 图像质量体模（35 x 25 x 25 cm3）内均匀填充 [18F]-FDG，以模拟躯干成像。体模内有六个病变（直径 10-38mm，病变与背景比 6:1），中心为一个 5cm 直径的肺插入物。在有和没有 MR 头部（圆柱）和表面（NU-2001 体模）RF 线圈的情况下，分别采集 EM 和 TX 测量值。在头和躯干体模研究中模拟了以下成像情况：（1）无 MR 线圈的 PET 扫描，用于参考，（2）EM 和 TX 均有线圈的 PET 扫描，（3）EM 有线圈而 TX 无线圈的 PET 扫描。对躯干体模还进行了另外两个设置：（4）仅 EM 有线圈的 PET 扫描，与（3）相比，体模略有移动，（5）EM 和 TX 有线圈的 PET 扫描，随后对表面线圈进行局部移位。使用冷 TX 数据对 PET EM 数据（1）-（4）进行衰减和散射校正。使用配备模板的 AC 的组合 PET/MR 原型系统，重复进行（1）-（3）成像情况。
头部体模：在 AC 期间未考虑 MR 头线圈的情况下（3），与参考设置（1）相比，中央和外周背景活性浓度被低估了 13%-19%。在使用配备模板的 AC 的组合 PET/MR 原型的重复研究中，再现了忽略 MR 线圈的 AC 效果。躯干体模：所有病变在所有 AC-PET 图像上均可见。在 AC 期间忽略 MR 表面 RF 线圈的影响为 4%或更小。当考虑在 EM 和 TX 之间移动的 RF 表面线圈时，观察到稍高的偏差（5）。在 AC-PET 中，忽略线圈失准和在 AC 期间忽略线圈对模拟肺的定量影响与噪声水平相比可以忽略不计。
MR 表面线圈未考虑的衰减会导致体部靠近 MR 线圈的区域的 AC-PET 数据出现区域性偏差。在较小尺寸的物体中，中央区域的偏差更为明显。在躯干研究中，使用体表面线圈，在不完全 AC 后，中央病变在 PET 上的可见度不受 MR 线圈的影响。在发射和衰减图像之间几厘米的线圈失准会导致与未考虑的 MR 线圈衰减引起的误差相当的误差，但与活性浓度水平的平均标准偏差相比很小。
