Ultrashort echo-time MRI versus CT for skull aberration correction in MR-guided transcranial focused ultrasound: In vitro comparison on human calvaria

Abstract

Purpose: Transcranial magnetic resonance-guided focused ultrasound (TcMRgFUS) brain treatment systems compensate for skull-induced beam aberrations by adjusting the phase and amplitude of individual ultrasound transducer elements. These corrections are currently calculated based on a preacquired computed tomography (CT) scan of the patients head. The purpose of the work presented here is to demonstrate the feasibility of using ultrashort echo-time magnetic resonance imaging (UTE MRI) instead of CT to calculate and apply aberration corrections on a clinical TcMRgFUS system. Methods: Phantom experiments were performed in three ex-vivo human skulls filled with tissuemimicking hydrogel. Each skull phantom was imaged with both CT and UTE MRI. The MR images were then segmented into skull and not-skull pixels using a computationally efficient, thresholdbased algorithm, and the resulting 3D binary skull map was converted into a series of 2D virtual CT images. Each skull was mounted in the head transducer of a clinical TcMRgFUS system (ExAblate Neuro, Insightec, Israel), and transcranial sonications were performed using a power setting of approximately 750 acousticwatts at several different target locations within the electronic steering range of the transducer. Each target locationwas sonicated three times: once using aberration corrections calculated from the actual CT scan, once using corrections calculated from the MRI-derived virtual CT scan, and once without applying any aberration correction.MRthermometry was performed in conjunction with each 10-s sonication, and the highest single-pixel temperature rise and surrounding-pixel mean were recorded for each sonication. Results: The measured temperature rises were ∼45% larger for aberration-corrected sonications than for noncorrected sonications. This improvement was highly significant (p < 10?4). The difference between the single-pixel peak temperature rise and the surrounding-pixel mean, which reflects the sharpness of the thermal focus,was also significantly larger for aberration-corrected sonications. There was no significant difference between the sonication results achieved using CT-based and MR-based aberration correction. Conclusions: The authors have demonstrated that transcranial focal heating can be significantly improved in vitro by usingUTEMRIto compute skull-induced ultrasound aberration corrections. Their results suggest that UTE MRI could be used instead of CT to implement such corrections on current 0.7MHzclinical TcMRgFUS devices. TheMRimage acquisition and segmentation procedure demonstrated here would add less than 15 min to a clinical MRgFUS treatment session.