Phantom Study Evaluating Sensitivity of a Cardiac Gated Snapshot MR-OEF Rosette Pulse Sequence for Ischemic Stroke
Abstract
Purpose
This study evaluates the sensitivity of two cardiac-gated rosette pulse sequences for MR-based measurement of the oxygen extraction fraction (OEF), which is predictive of recurring ischemic stroke in intracranial atherosclerotic disease (ICAD). The present work serves as proof of concept for future optimization studies, with the goal of developing a rapid, non-invasive, widely available method of imaging OEF in patients.
Methods
Because blood magnetic susceptibility increases with decreasing oxygenation, a cylindrical agarose phantom was prepared containing six 5mL tubes with concentrations of [0, 0.005, 0.01, 0.015, 0.02, 0.025] mg/mL Feraheme in distilled water. These concentrations correspond to susceptibility values of [0.15, 0.17, 0.33, 0.53, 0.74, 0.85] ppm. T2-weighted trueFISP and T2*-weighted FLASH sequences with rosette sampling trajectories were acquired on a Philips Ingenia 3T scanner. Acquisition flip angles ranged 10°-70°, and the number of rosette petals ranged 4-13. Susceptibility weighted imaging (SWI) and quantitative susceptibility mapping (QSM) were performed to provide reference visuals and susceptibility values. Rosette phase images were reconstructed using an iterative penalized weighted least squares algorithm from the Michigan Image Reconstruction Toolbox (MIRT). Background artifact correction was performed by fitting and subsequent subtraction of a polynomial to the agarose background.
Results
After correcting background artifact, the tube phase shifts observed for both rosette acquisition types exhibit linearity with respect to QSM reference measurements, with Pearson’s r = 0.92 (R2=0.84) for an example four-petal trueFISP acquisition and r = 0.94 (R2=0.89) for an example nine-petal FLASH.
Conclusion
This study demonstrates that FLASH and trueFISP cardiac gated rosette sequences are sensitive to a range of susceptibilities, which is a crucial prerequisite for OEF measurement. Next steps for this work include generating OEF maps for ICAD patients, thereby bringing MR-OEF closer to clinical deployment and enabling improved assessment of & treatment planning for recurring ischemic stroke.