Evaluation of the radiofrequency performance of a wide-bore 1.5 T positron emission tomography/magnetic resonance imaging body coil for radiotherapy planning

Abstract

Background and purpose The restricted bore diameter of current simultaneous positron emission tomography/magnetic resonance imaging (PET/MRI) systems can be an impediment to achieving similar patient positioning during PET/MRI planning and radiotherapy. Our goal was to evaluate the B 1 transmit (B 1 + ) uniformity, B 1 + efficiency, and specific absorption rate (SAR) of a novel radiofrequency (RF) body coil design, in which RF shielded PET detectors were integrated with the specific aim of enabling a wide-bore PET/MRI system. Materials and methods We designed and constructed a wide-bore PET/MRI RF body coil to be integrated with a clinical MRI system. To increase its inner bore diameter, the PET detectors were positioned between the conductors and the RF shield of the RF body coil. Simulations and experiments with phantoms and human volunteers were performed to compare the B 1 + uniformity, B 1 + efficiency, and SAR between our design and the clinical body coil. Results In the simulations, our design achieved nearly the same B 1 + field uniformity as the clinical body coil and an almost identical SAR distribution. The uniformity findings were confirmed by the physical experiments. The B 1 + efficiency was 38% lower compared to the clinical body coil. Conclusions To achieve wide-bore PET/MRI, it is possible to integrate shielding for PET detectors between the body coil conductors and the RF shield without compromising MRI performance. Reduced B 1 + efficiency may be compensated by adding a second RF amplifier. This finding may facilitate the application of simultaneous whole-body PET/MRI in radiotherapy planning.

Minimizing the need for coil attenuation correction in integrated PET/MRI at 1.5 T using low-density MR-linac receive arrays

“…An example of a system for which no dedicated PET/MRI arrays are available is the new 1.5 T PET/MRI system that is currently being developed and constructed at our institute. This wide-bore system is designed for use as a single-exam planning tool for MRI-guided radiotherapy (MRIgRT) treatments on the 1.5 T MR-linac (MRL), thereby eliminating the need for separate MRI and PET/CT scans and improving the geometric accuracy compared to 3 T systems (Branderhorst et al 2021, UMC Utrecht 2021. Interestingly, receive arrays of the MRL can also be connected to the new PET/MRI system and these arrays have been designed for optimal photon transparency (radiolucency) to limit dose deviations during MRIgRT treatments.…”