Zijlema Stefan E, Branderhorst Woutjan, Bastiaannet Remco, Tijssen Rob H N, Lagendijk Jan J W, van den Berg Cornelis A T
Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.
Computational Imaging Group for MR Diagnostics & Therapy, Center for Image Sciences, University Medical Center Utrecht, The Netherlands.
Phys Med Biol. 2021 Oct 11;66(20). doi: 10.1088/1361-6560/ac2a8a.
The simultaneous use of positron emission tomography (PET) and magnetic resonance imaging (MRI) requires attenuation correction (AC) of photon-attenuating objects, such as MRI receive arrays. However, AC of flexible, on-body arrays is complex and therefore often omitted. This can lead to significant, spatially varying PET signal losses when conventional MRI receive arrays are used. Only few dedicated, photon transparent PET/MRI arrays exist, none of which are compatible with our new, wide-bore 1.5 T PET/MRI system dedicated to radiotherapy planning. In this work, we investigated the use of 1.5 T MR-linac (MRL) receive arrays for PET/MRI, as these were designed to have a low photon attenuation for accurate dose delivery and can be connected to the new 1.5 T PET/MRI scanner. Three arrays were assessed: an 8-channel clinically-used MRL array, a 32-channel prototype MRL array, and a conventional MRI receive array. We experimentally determined, simulated, and compared the impact of these arrays on the PET sensitivity and image reconstructions. Furthermore, MRI performance was compared. Overall coil-induced PET sensitivity losses were reduced from 8.5% (conventional) to 1.7% (clinical MRL) and 0.7% (prototype MRL). Phantom measurements showed local signal errors of up to 32.7% (conventional) versus 3.6% (clinical MRL) and 3.5% (prototype MRL). Simulations with data of eight cancer patients showed average signal losses were reduced from 14.3% (conventional) to 1.2% (clinical MRL) and 1.0% (prototype MRL). MRI data showed that the signal-to-noise ratio of the MRL arrays was slightly lower at depth (110 versus 135). The parallel imaging performance of the conventional and prototype MRL arrays was similar, while the clinical MRL array's performance was lower. In conclusion, MRL arrays reducePET signal losses >10×, which decreases, or eliminates, the need for coil AC on a new 1.5 T PET/MRI system. The prototype MRL array allows flexible coil positioning without compromising PET or MRI performance. One limitation of MRL arrays is their limited radiolucent PET window (field of view) in the craniocaudal direction.
同时使用正电子发射断层扫描（PET）和磁共振成像（MRI）需要对光子衰减物体进行衰减校正（AC），例如MRI接收阵列。然而，对灵活的体表阵列进行AC很复杂，因此常常被省略。当使用传统的MRI接收阵列时，这可能会导致显著的、空间变化的PET信号损失。仅有少数专用的、光子透明的PET/MRI阵列，其中没有一个与我们用于放射治疗计划的新型宽孔径1.5T PET/MRI系统兼容。在这项工作中，我们研究了将1.5T磁共振直线加速器（MRL）接收阵列用于PET/MRI，因为这些阵列设计为具有低光子衰减以实现精确的剂量输送，并且可以连接到新型1.5T PET/MRI扫描仪。评估了三种阵列：一个8通道临床使用的MRL阵列、一个32通道的MRL原型阵列和一个传统的MRI接收阵列。我们通过实验确定、模拟并比较了这些阵列对PET灵敏度和图像重建的影响。此外，还比较了MRI性能。总体而言，线圈引起的PET灵敏度损失从8.5%（传统阵列）降至1.7%（临床MRL阵列）和0.7%（MRL原型阵列）。体模测量显示，局部信号误差高达32.7%（传统阵列），而临床MRL阵列和MRL原型阵列分别为3.6%和3.5%。对八名癌症患者的数据进行模拟显示，平均信号损失从14.3%（传统阵列）降至1.2%（临床MRL阵列）和1.0%（MRL原型阵列）。MRI数据表明，MRL阵列在深度处的信噪比略低（分别为每厘米110和135）。传统MRL阵列和原型MRL阵列的并行成像性能相似，而临床MRL阵列的性能较低。总之，MRL阵列可将PET信号损失降低10倍以上，从而减少或消除了在新型1.5T PET/MRI系统上进行线圈AC的需求。MRL原型阵列允许灵活的线圈定位，而不会损害PET或MRI性能。MRL阵列的一个局限性是其在颅尾方向上有限的射线可透过PET窗口（视野）。
