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Sirius MRI Markers for Prostate Post-Implant Assessment: MR Protocol Development

T Lim

T Lim*, J Wang , R Kudchadker , R Stafford , T Bathala , T Pugh , G Ibbott , S Frank , UT MD Anderson Cancer Center, Houston, TX


SU-C-17A-2 Sunday 1:00PM - 1:55PM Room: 17A

Purpose: Currently, CT is used to visualize prostate brachytherapy sources, at the expense of accurate structure contouring. MRI is superior to CT for anatomical delineation, but the sources appear as voids on MRI images. Previously we have developed Sirius MRI markers (C4 Imaging) to replace spacers to assist source localization on MRI images. Here we develop an MRI pulse sequence protocol that enhances the signal of these markers to enable MRI-only post-implant prostate dosimetric analysis.

Methods: To simulate a clinical scenario, a CIRS multi-modality prostate phantom was implanted with 66 markers and 86 sources. The implanted phantom was imaged on both 1.5T and 3.0T GE scanners under various conditions, different pulse sequences (2D fast spin echo [FSE], 3D balanced steady-state free precession [bSSFP] and 3D fast spoiled gradient echo [FSPGR]), as well as varying amount of padding to simulate various patient sizes and associated signal fall-off from the surface coil elements. Standard FSE sequences from the current clinical protocols were also evaluated. Marker visibility, marker size, intra-marker distance, total scan time and artifacts were evaluated for various combinations of echo time, repetition time, flip angle, number of excitations, bandwidth, slice thickness and spacing, field-of-view, frequency/phase encoding steps and frequency direction.

Results: We have developed a 3D FSPGR pulse sequence that enhances marker signal and ensures the integrity of the marker shape while maintaining reasonable scan time. For patients contraindicated for 3.0T, we have also developed a similar sequence for 1.5T scanners. Signal fall-off with distance from prostate to coil can be compensated mainly by decreasing bandwidth. The markers are not visible using standard FSE sequences. FSPGR sequences are more robust for consistent marker visualization as compared to bSSFP sequences.

Conclusion: The developed MRI pulse sequence protocol for Sirius MRI markers assists source localization to enable MRI-only post-implant prostate dosimetric analysis.

Funding Support, Disclosures, and Conflict of Interest: S.J. Frank is a co-founder of C4 Imaging (manufactures the MRI markers).

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