We describe the use of MRI-compatible focused ultrasound (FUS) for selective fast focal switching of high-contrast-ratio magnetocaloric MRI particle labels. Lanthanum-Iron-Silicon (La-Fe-Si) particles that have sharp first-order magnetic phase transitions at physiological temperatures and MRI field strengths were used as MRI labels. Non-invasive rapid thermal switching of these magnetocaloric MRI particle labels with focused ultrasound was clearly demonstrated at physiological temperatures within a clinical 1.5T MR scanner. We have thereby shown that high differential contrast ratio magnetocaloric MRI labels can be switched non-invasively, rapidly, and selectively under conditions relevant to clinical MRI.
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