Author: Michael Evan Chaga, Timothy Chen, Darra M. Conti, Jing Feng, Wenzheng Feng, Joseph Hanley, Jeff Stainsby, Tingyu Wang 👨🔬
Affiliation: Synaptive Medical, Hackensack Meridian Health, Jersey Shore University Medical Center 🌍
Purpose:
The Synaptive MRI is a 0.5T superconducting head only system (first US install 2023). This is the first study that evaluates the geometric accuracy using a Sun Nuclear MR Distortion and Image Fusion Head Phantom.
Methods:
A Sun Nuclear head phantom with a skull containing a 3D grid of 2.5mm diameter cross-shaped rods (spaced 10mm I-S, 10.5mm A-P, and 11mm L-R) was scanned in a GE CT (0.625mm slices, 0.488x0.488mm in-plane resolution). Four MRI protocols, T1-Weighted Spoiled Gradient Recalled Echo (T1-SPGR), T2-Weighted Fast Spin Echo (T2-FSE3D), Steady State Free Precession (SSFP), Time of Flight MR Angiogram (TOF-MRA), customized for stereotactic treatment planning to cover the whole head were also used to scan the phantom. T1-SPGR used 1mm slices and 0.488×0.488mm in-plane resolution, SSFP used 0.5mm slices and 0.417×0.417mm in-plane resolution. All images were acquired with a 25cm FOV. CT and MRI images were rigidly registered utilizing the full phantom. All test points were localized. MRI test points were mapped to the CT coordinate system and deviations magnitude were calculated as distortion.
Results:
Average/maximum distortion between T1-SPGR, T2-FSE, SSFP, and TOF-MRA compared to CT were 0.21/0.62mm, 0.14/0.47mm, 0.17/0.52mm, and 0.26/0.73mm, respectively. Distortion increased approximately linearly from center to periphery. T2-FSE3D showed the least distortion, followed by SSFP, T1-SPGR, and finally TOF-MRA (largest distortion). This aligns with their receive bandwidths: T2-FSE3D has the highest, TOF-MRA the lowest. This implies T2 errors are likely due to gradient non-linearity, while other sequences suffer increasingly from B0-induced distortion.
Conclusion:
Distortion measured with SNC phantom were well within the 2mm tolerance. For stereotactic treatment planning, since Eclipse registers MRI to CT via anatomy including the skull and ear canal, registration will be more accurate than Gamma Knife style registration, which uses external fiducial box where distortion is the highest.