Technical Note: Extended field‐of‐view (FOV) MRI distortion determination through multi‐positional phantom imaging

Abstract

Comprehensive characterization of geometric distortions for MRI simulators and MRI-guided treatment delivery systems is typically performed with large phantoms that are costly and unwieldy to handle. Here we propose an easily implementable methodology for MR distortion determination of the entire imaging space of the scanner through the use of a compact commercially available distortion phantom. The MagphanRT phantom was scanned at several locations within a MR scanner. From each scan, an approximate location of the phantom was determined from a subset of the fiducial spheres. The fiducial displacements were determined, and a displacement field was fitted to the displacement data using the entire multi-scan data set. An orthogonal polynomial expansion fitting function was used that had been augmented to include independent rigid-body transformations for each scan. The rigid-body portions of the displacement field were thereafter discarded, and the resultant fit then represented the distortion field. Multi-positional scans of the phantom were used successfully to determine the distortion field with extended coverage. A single scan of the phantom covered 20 cm in its smallest dimension. By stitching together overlapping scans we extended the distortion measurements to 30 cm. No information about the absolute location or orientation of each scan was required. The method, termed the Multi-Scan Expansion (MSE) method, can be easily applied for larger field-of-views (FOVs) by using a combination of larger phantom displacements and more scans. The implementation of the MSE method allows for distortion determination beyond the physical limitations of the phantom. The method is scalable to the user's needs and does not require any specialized equipment. This approach could open up for easier determination of the distortion magnitude at distances further from the scanner's isocenter. This is especially important in the newly proposed methodologies of MR-only simulation in RT and in adaptive replanning in MR linac systems.

Optimizing imaging resolution in brain MRI: understanding the impact of technical factors

“…According to brain MRI studies, utilizing a FOV of 230 mm or less is common to achieve high spatial resolution on the screen. This is because the FOV increases with the number of pixels, improving the signal-to-noise ratio (SNR) and producing smoother and more detailed images [ 13 ]. Based on the results, the average FOV in this study was 217.67, which corresponds to the standard range for brain MRI and is consistent with other studies [ 14 ].…”

Evaluation of an MRI linac magnetic isocenter walkout with gantry rotation in the presence of angle-specific corrections

“…The cloud-based processing software (Image Owl Inc., Greenwich, NY, USA) compares the empirically found centroids of the spheres void of signal to the a priori known geometry of the phantom, thus estimating image distortion. The rigid shifts and rotations constitute the first term of the distortion series (Schüler et al 2020). This is an appropriate tool for quantitating the relative displacement of the magnetic isocenter with gantry angle as it eliminates the uncertainties of image registration.…”

“…The maximum geometric distortion values in three dimensions for MW and A3i system configurations were recorded in the MagPhan experiments for 12 gantry positions, within 200 and 350 mm diameter spheres following the methodology of Schüler et al (Schüler et al 2020).…”