# Characterization of spatial integrity with active and passive implants in a low-field magnetic resonance linear accelerator scanner

## TL;DR

This study examines how metallic implants affect MRI accuracy in radiotherapy, showing that distortions near implants can be up to 3 mm but diminish beyond 20 mm.

## Contribution

Characterizes geometric distortions from active and passive implants in a low-field MR-LINAC scanner for radiotherapy planning.

## Key findings

- Passive implants caused up to 2 mm distortions near the largest banding artefacts.
- Active implants caused up to 3 mm distortions in the same region.
- Spatial integrity was recovered beyond 20 mm from the artefacts.

## Abstract

•Metallic implants compromise the spatial integrity of magnetic resonance images.•Phantom-based geometric distortion analysis with various implants.•Application in radiotherapy on a 0.35 T magnetic resonance linear accelerator.•Contouring rules accounting for geometric uncertainties near metallic implants.

Metallic implants compromise the spatial integrity of magnetic resonance images.

Phantom-based geometric distortion analysis with various implants.

Application in radiotherapy on a 0.35 T magnetic resonance linear accelerator.

Contouring rules accounting for geometric uncertainties near metallic implants.

Standard imaging protocols can guarantee the spatial integrity of magnetic resonance (MR) images utilized in radiotherapy. However, the presence of metallic implants can significantly compromise this integrity. Our proposed method aims at characterizing the geometric distortions induced by both passive and active implants commonly encountered in planning images obtained from a low-field 0.35 T MR-linear accelerator (LINAC).

We designed a spatial integrity phantom defining 1276 control points and covering a field of view of 20x20x20 cm3. This phantom was scanned in a water tank with and without different implants used in hip and shoulder arthroplasty procedures as well as with active cardiac stimulators. The images were acquired with the clinical planning sequence (balanced steady-state free-precession, resolution 1.5x1.5x1.5 mm3). Spatial integrity was assessed by the Euclidian distance between the control point detected on the image and their theoretical locations. A first plane free of artefact (FPFA) was defined to evaluate the spatial integrity beyond the larger banding artefact.

In the region extending up to 20 mm from the largest banding artefacts, the tested passive and active implants could cause distortions up to 2 mm and 3 mm, respectively. Beyond this region the spatial integrity was recovered and the image could be considered as unaffected by the implants.

We characterized the impact of common implants on a low field MR-LINAC planning sequence. These measurements could support the creation of extra margin while contouring organs at risk and target volumes in the vicinity of implants.