isoPhasor: a generic and precise marker visualization, localization, and quantification method based on phase saddles in 3D MR imaging

Abstract

Purpose To derive a generic approach for accurate localization and characterization of susceptibility markers in MRI, compatible with many common types of pulse sequences, sampling trajectories, and acceleration methods. Theory and Methods A susceptibility marker’s dipolar phase evolution creates 3 saddles in the phase gradient of the spatial encoding, for each sampled data point in k‐space. The signal originating from these saddles can be focused at the location of the marker to create positive contrast. The required phase shift can be calculated from the scan parameters and the marker properties, providing a marker detection algorithm generic for different scan types. The method was validated numerically and experimentally for a broad range of spherical susceptibility markers (0.3 < radius < 1.6 mm, 10 < |∆χ| < 3300 ppm), under various conditions. Results For all numerical and experimental phantoms, the average localization error was below one third of the voxel size, whereas the average error in magnetic strength quantification was 7%. The experiments included different pulse sequences (gradient echo, spin echo [SE], and free induction decay scans), sampling strategies (Cartesian, radial), and acceleration methods (echo planar imaging EPI, turbo SE). Conclusion Spherical markers can be identified from their phase saddles, enabling clear visualization, precise localization, and accurate quantification of their magnetic strength, in a wide range of clinically relevant pulse sequences and sampling strategies.

Development of an MRI-Based Workflow for Post-implant Dosimetry of Prostate Low-Dose-Rate (Ldr) Brachytherapy

“…Previous works in MR-based fiducial marker detection are mainly based on template matching using either signal voids on magnitude images or field distortions on phase images; although the aforementioned methods have been effective under certain conditions (orientation, dimensions, shape), clear marker depiction with positive contrast similar to CT was not achieved (Bouwman, Custers, Bakker, Viergever, & Seevinck, 2019;Ghose et al, 2016;Jonsson, Garpebring, Karlsson, & Nyholm, 2012). Our results with QSM showed that the relative location of the markers was identical with the CT image indicating the possibility of using QSM in MR-guided EBRT for the disease sites like prostate, liver and lung.…”