Fig. 5

1.5-T MRI-tracked HIFU-triggered release of three wax-coated capsules (n = 3, separate experiments shown on three different layers). The position of the wax-coated capsule prior to the application of the HIFU pulse was determined using a 3D T2-weighted turbo spin echo sequence (repetition time 1000 ms, echo time 130 ms, a). The intact wax-coated capsule showed no hyperintensity on a 3D T1-weighted fast field echo sequence (repetition time 6.75 ms, echo time 3.3 ms, flip angle 10°), demonstrating that the non-hydrated lyophilised GBCA is not T1-hyperintense (b). T2-weighted MRI was used for guidance of the HIFU pulse and MR thermometry (colour mapping reveals thermal exposure, c). After the highly localised application of a 200-W HIFU pulse, a distinct T1-hyperintense signal was observed (d) using the same sequence as in b. The local temperature increase most probably melted the wax coating, allowing water to penetrate the capsule core and hydrate the encapsulated lyophilised GBCA, and thus resulting in the T1-hyperintensity. Hyperintensity varies depending also on the dissolution kinetics of the GBCA in the surrounding water, the amount of surrounding medium, and the imaging delay after HIFU-induced disintegration. GBCA: Gadolinium-based contrast agent; HIFU: High-intensity focused ultrasound