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Potential Applications of the Susceptibility-Based Positive Contrast in MR-Guided Radiation Therapy

R Nosrati1,2*, M Paudel3,4 , A Ravi3,4 , A Pejovic-Milic1 , G Morton3,4 , G Stanisz1,2,4 , (1) Ryerson University, Toronto, Canada (2) Sunnybrook Research Institute, Toronto, Canada (3) Sunnybrook Odette Cancer Center, Toronto, Canada (4) University of Toronto, Toronto, Canada


(Sunday, 7/14/2019) 2:00 PM - 3:00 PM

Room: 221CD

Purpose: To study the feasibility of Quantitative Susceptibility Mapping (QSM) for visualization of some MR-invisible radiation therapy devices including fiducial markers, High-Dose-Rate (HDR) interstitial brachytherapy CT-marker and obturators, and biopsy needle.

Methods: Two agar-based tissue mimicking phantoms consisting were prepared. Three carbon coated zirconium markers (Carbon Medical Technologies) were implanted into a prostate phantom (CIRS); three gold fiducial markers (CIVCO Radiotherapy) were placed at random orientations in an agar-based phantom (Fig.1). Two plastic needles with CT-marker stylet (Elekta), two plastic needles with ProGuide obturators (Elekta) and one pure titanium biopsy needle were inserted into the second agar phantom (Fig.2c). Also, two hollow needles, one needle with CT-marker and the other needle with ProGuide obturator were inserted into a meat phantom (Fig.2b). Phantoms were scanned using a 3T MR scanner with a 3D multi-echo gradient echo sequence. Our recently proposed QSM-based algorithm was modified and applied to all investigated potential applications. The QSM results were compared to CT for spatial accuracy analysis.

Results: QSM successfully generated positive contrast for both types of investigated fiducial markers with high spatial accuracy. Zirconium is strongly paramagnetic hence BiomarC markers had excellent contrast on QSM, while gold is 3.5 times more diamagnetic than soft tissue thus inverted QSM was calculated for gold marker visualization. As shown is Fig.1c-h there was no spatial difference between the markers on CT and QSM. The Aluminum-based CT-marker stylet contains nitinol (nickel+titanium) which generated strong susceptibility contrast on QSM (Fig.2d-e). Given the strong paramagnetic properties of titanium, the biopsy needle and the titanium-based ProGuide obturators were clearly depicted on the reconstructed QSM (Fig.2d-e).

Conclusion: QSM resolves the lack of MR positive contrast for conventional paramagnetic radiation therapy devices only through post-processing. Upon validation of the technique in-vivo, QSM has the potential to replace CT towards MR-only guided radiation therapy workflows.


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