Brain and Head-and-Neck MRI in Immobilization Mask: A Practical Solution for MR-Only Radiotherapy

Abstract

In brain/head-and-neck radiotherapy (RT), thermoplastic immobilization masks guarantee reproducible patient positioning in treatment position between MRI, CT, and irradiation. Since immobilization masks do not fit in the diagnostic MR head/head-and-neck coils, flexible surface coils are used for MRI imaging in clinical practice. These coils are placed around the head/neck, in contact with the immobilization masks. However, the positioning of these flexible coils is technician dependent, thus leading to poor image reproducibility. Additionally, flexible surface coils have an inferior signal-to-noise-ratio (SNR) compared to diagnostic coils. The aim of this work was to create a new immobilization setup which fits into the diagnostic MR coils in order to enhance MR image quality and reproducibility. For this purpose, a practical immobilization setup was constructed. The performances of the standard clinical and the proposed setups were compared with four tests: SNR, image quality, motion restriction, and reproducibility of inter-fraction subject positioning. The new immobilization setup resulted in 3.4 times higher SNR values on average than the standard setup, except directly below the flexible surface coils where similar SNR was observed. Overall, the image quality was superior for brain/head-and-neck images acquired with the proposed RT setup. Comparable motion restriction in feet-head/left-right directions (maximum motion ≈1 mm) and comparable inter-fraction repositioning accuracy (mean inter-fraction movement 1 ± 0.5 mm) were observed for the standard and the new setup.

Implementation of a dedicated 1.5 T MR scanner for radiotherapy treatment planning featuring a novel high-channel coil setup for brain imaging in treatment position

“…Additionally, positioning in a thermoplastic mask leads to reduced motion artifacts. In our case, no motion artifacts were observed in the mask setups, while some groups report movement to be less than 1.5 mm [20].…”

Patient positioning and immobilization procedures for hybrid MR-Linac systems

“…To date, there is limited evidence available from preliminary reports of MRgRT treatments for brain malignancies [ 18 ]. While there are some experiences using radiotherapy positioning devices in diagnostic MRI scanners to obtain diagnostic imaging in treatment position [ 19 ], evidence from hybrid MR-linac systems is scarce. Moreover, none of the available hybrid systems provides specific brain coils for dedicated imaging.…”

Magnetic resonance imaging for brain stereotactic radiotherapy

“…Performing MRI studies in the treatment position with mask immobilization, however, decreases patient motion, which may help with precise target volume definition ( Fig. 3d-e; [23]).…”

“…A few centers have established solutions for acquiring simulation MRIs with mask immobilization [21,23,24]. As most stereotactic mask systems will not fit into routine radiologic head coils, most groups have used a flexible coil setup, which could degrade image quality [6,23,25].…”

“…A few centers have established solutions for acquiring simulation MRIs with mask immobilization [21,23,24]. As most stereotactic mask systems will not fit into routine radiologic head coils, most groups have used a flexible coil setup, which could degrade image quality [6,23,25]. According to the current consensus imaging with mask immobilization as well as imaging in a routine radiologic setup are both appropriate strategies for MR simulation in intracranial radiotherapy if MR images are registered to a planning CT [6].…”