The study protocol was approved by the Swiss Animal Care and Experimentation Committee (approval number BE 14/10). All aneurysms were created in adult female New Zealand white rabbits (3–4 kg). A detailed description of the aneurysm creation is presented elsewhere[6]. In brief, general anesthesia was induced by administration of ketamine hydrochloride (Pfizer AG, Zurich, Switzerland) and xylazine hydrochloride (Vétoquinol AG, Ittigen, Switzerland) and continued intravenously.
Aneurysm creation
Under an operating microscope, a 1-cm segment of the external jugular vein was harvested and used as a venous pouch. Both common carotid arteries (CCA) were prepared, and after ligation, the stump of the right CCA was mobilized to the left CCA. After an elliptic arteriotomy, the stump of the right CCA and the venous pouch were sutured to the left CCA using an end-to-side anastomosis with 10-0 thread (Ethilon, Ethicon Inc., Somerville, NJ, USA), resulting in an artificial bifurcation aneurysm (Figure 1). In order to allow epithelialization of the vessel walls, the MRgFUS treatment was carried out a minimum of 3 weeks after aneurysm creation. Furthermore, skin wound healing had to be advanced to a point where the stitches were completely absorbed and no obvious scar formations were detectable that could provoke additional acoustic aberrations.
Magnetic resonance-guided focused ultrasound sonication
The MRgFUS procedure was performed using the InSightec ExAblate 4000 system (Insightec Ltd., Haifa, Israel) integrated into a clinical 3T MR system (GE Healthcare, Milwaukee, WI, USA). The hemispheric phased array ultrasound transducer with 1,024 elements was driven at 650 kHz and oriented face up, filled with degassed water, and covered with a transparent film. After hair removal, the neck of the anesthetized rabbit was positioned on the film such that the aneurysm was aligned with the acoustic focus of the transducer. Tape was used to facilitate appropriate positioning and to stabilize the animal torso (Figures 2 and3). In order to prevent the animals from aspirating water, acoustically transparent gel blocks supported the head. Liquid ultrasound gel and degassed water served as acoustic coupling media between the film and the skin of the animals. In the last two animals, the skin was opened through an incision to avoid sonication through the skin/fat layer. A clinical 7.5-cm surface coil (GE Healthcare, Milwaukee, WI, USA) was used for MR imaging. A flow-sensitive time-of-flight SPGR MR sequence was used to visualize the aneurysm before and after sonication. For MR angiography, a contrast agent (Omniscan, GE Healthcare, Milwaukee, WI, USA) was administered through a catheter in the ear vein. MR thermometry was based on phase-sensitive GRE sequences to evaluate the thermally induced proton resonance frequency shift. This method is known to be prone to movement and susceptibility artifacts. Since thermal measurements were not possible within the vessel or the vessel wall itself, readings were taken in stationary tissue close to the vessel. Estimating from the spectral Doppler velocimetry (SDV) in relevant regions of the phase images, uncertainty of thermal measurements under optimal conditions was <1°C. However, animal movement due to heartbeat and respiration as well as blood flow in the region of interest might have degraded measurement precision. Sonications were targeted at the border of the aneurysm wall (Figure 4). The focal spot of the ExAblate system used here has a full width at half maximum (FWHM) of 5–6 mm, which is sufficient to cover the vessel lumen. In order to facilitate thermal measurements, sonications were not always centered fully into the vessel but were also used to heat tissue adjacent to the vessel to obtain thermal readings from stationary tissue. For each animal, sonications started at 25-W acoustic power and 10-s duration. From there, the total acoustic energy was incrementally increased until temperatures above 50°C or an effect on the vessels such as alteration of shape or size could be detected by MR thermometry or angiography, respectively.