Experimental investigation of thermal effects in HIFU-based external valvuloplasty with a non-spherical transducer, using high-resolution MR thermometry.

Real-time image-guided extracorporeal high intensity focused ultrasound (HIFU) has been suggested for minimally invasive treatment of valvular dysfunction in the saphenous vein. Local application of heat on the perimeter of the valve zone was previously reported to induce a partial shrinkage of the collagen, which may correct valvular function. In our study, a novel MR compatible HIFU device has been investigated. This device is based on a non-spherical geometry, with two active elements that create a focusing line which is orthogonal to the beam main axis, aiming to cover the valve longitudinally. The prototype performance was characterized by electro-acoustical measurements of the pressure field and by high-resolution MR thermometry. Pressure and thermal fields were found in good agreement with the theoretical predictions. To investigate the therapeutic potential, fresh samples of excised human veins were filled with an agarose gel, embedded in porcine muscle and exposed to HIFU. The power level applied during a fixed duration of 30 s was varied such that the absolute temperature at focus ranged between 52 degrees C and 83 degrees C. Targeting was achieved under MR guidance using a MR compatible XZ positioning system. A dedicated waterproof miniature loop coil was specifically built to achieve high-resolution MRI image-based targeting (0.25 mm x 0.25 mm x 3 mm voxel) and thermometry (0.4 mm x 0.4 mm x 4 mm voxel). The vein wall was clearly identified on MR images before and after HIFU treatment. The thermal buildup created by the non-spherical transducer could be characterized from MR thermometry data. Shrinkage of the vein wall (above 65 degrees C) was determined by absolute temperature and was not a cumulative thermal dose effect.