Room: Karl Dean Ballroom C
Purpose: There is great interest in using MRI-guided radiotherapy (MR-IGRT) for noninvasive cardiac radiosurgery of tachycardias due to MRI's excellent soft tissue contrast. However, most patients with tachycardia have cardiovascular implantable electronic devices (CIEDs). CIEDs produce significant magnetic susceptibility artifacts especially when using steady-state free precession MRI sequences. Typically, the CIED is located near the heart resulting in signal dephasing and geometric distortion near the targeted tissues. Low-field MR-IGRT is an attractive solution since Bâ‚€ inhomogeneities decrease at low field. However, traditional Bâ‚€ maps are more challenging at low field due to the large echo time difference between fat and water. In this study, we implemented a fast Bâ‚€ map at low field and used it to assess geometric distortion associated with a CIED.
Methods: The studies were performed on a ViewRay MR-IGRT. We modified the field map sequence to disable the fat-water separation and minimize the echo time difference (Î”TE) between the first and second gradient echoes. A Medtronic Virtuoso CIED (Minneapolis, MN) with leads was placed in a sodium polyacrylate hydrogel phantom and imaged at 0.32 T. Field maps were created with various matrix sizes and Î”TEs. Field maps were converted to geometric shifts based on the typical 2D MRI parameters used during treatment.
Results: Short Î”TE significantly reduced phase aliasing without a sacrifice in Bâ‚€ map quality. Shortening the TR from 1000 to 244 ms did not significantly degrade the quality of the phase map. There is significant signal dephasing associated with the ferrous components (e.g., battery) in the CIED. Field inhomogeneities drop off rapidly outside the CIED leading to geometric distortions of ~1 mm or less at 5 cm from the CIED.
Conclusion: A short Î”TE phase map sequence was successfully used at low field to assess the field inhomogeneities associated with a CIED.
Funding Support, Disclosures, and Conflict of Interest: Washington University in St Louis receives research funding, and consulting and speaking fees from ViewRay. Dr. Gach owns common stock shares in ViewRay.