The lack of radiation-attenuating tuning capacitors in high impedance coils (HICs) make HICs an interesting building block of receive arrays for MRI-guided radiotherapy (MRIgRT). Additionally, their flexibility and limited channel coupling allow for low-density support materials, which are likely to be more radiation transparent (radiolucent). In this work, we introduce the use of HICs in receive arrays for MRIgRT treatments. We discuss the design and show the dosimetric feasibility of a HIC receive array that has a high channel count and aims to improve the imaging performance of the 1.5 T MR-linac. Our on-body design comprises an anterior and posterior element, which each feature a channel layout (32 channels total). The anterior element is flexible, while the posterior element is rigid to support the patient. Mockups consisting of support materials and conductors were built, irradiated, and optimized to minimize impact on the surface dose (7% of the dose maximum) and dose at depth ( 0.8% under a single conductor and 1.4% under a conductor crossing). Anatomical motion and the use of multiple beam angles will ensure that these slight dose changes at depth are clinically insignificant. Subsequently, several functional, single-channel HIC imaging prototypes and a 5-channel array were built to assess the performance in terms of signal-to-noise ratio (SNR). The performance was compared to the clinical MR-linac array and showed that the 5-channel imaging prototype outperformed the clinical array in terms of SNR and channel coupling. Imaging performance was not affected by the radiation beam. In conclusion, the use of HICs allowed for the design of our flexible, on-body receive array for MRIgRT. The design was shown to be dosimetrically feasible and improved the SNR. Future research with a full array will need to show the gain in parallel imaging performance and thus acceleration.

High impedance coils (HICs) are suitable as a building block of receive arrays for MRI-guided radiotherapy (MRIgRT) as HICs do not require radiation-attenuating capacitors and dense support materials. Recently, we proved the feasibility of using HICs to create a radiation transparent (i.e. radiolucent) window. In this work, we constructed a fully functional 32-channel array based on this design. The anterior element is flexible and follows the shape of the subject, while the posterior element is rigid to support the subject. Both elements feature a 2 × 8 channel layout. Here, we discuss the construction process and characterize the array’s radiolucency and imaging performance. The dosimetric impact of the array was quantified by assessing the surface dose increase and attenuation of a single beam. The imaging performance of the prototype was compared to the clinical array in terms of visual appearance, signal-to-noise ratio (SNR), and acceleration performance, both in phantom and in-vivo measurements. Dosimetry measurements showed that on-body placement changed the anterior and posterior surface dose by +3% and −16% of the dose maximum. Attenuation under the anterior support materials and conductors was 0.3% and ≤1.5%, respectively. Phantom and in-vivo imaging with this array demonstrated an improvement of the SNR at the surface and the image quality in general. Simultaneous irradiation did not affect the SNR. G-factors were reduced considerably and clinically used sequences could be accelerated by up to 45%, which would greatly reduce pre-beam imaging times. Finally, the maximally achievable temporal resolution of abdominal 3D cine imaging was improved to 1.1 s, which was > 5 × faster than could be achieved with the clinical array. This constitutes a big step towards the ability to resolve respiratory motion in 3D. In conclusion, the proposed 32-channel array is compatible with MRIgRT and can significantly reduce scan times and/or improve the image quality of all on-line scans.

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