Ultrashort echo‐time MRI versus CT for skull aberration correction in MR‐guided transcranial focused ultrasound: In vitro comparison on human calvaria. Issue 5 (13th April 2015)

• Record Type: Journal Article
• Authors: Miller, G. Wilson; Eames, Matthew; Snell, John; Aubry, Jean‐François
• Is Part Of: Medical physics. Volume 42:Issue 5(2015)
• Page Start: 2223
• Page End: 2233
• Publication Date: 2015-04-13
• Subjects: biomedical MRI -- biomedical ultrasonics -- brain -- computerised tomography -- image segmentation -- medical image processing -- phantoms -- transducers; Magnetic resonance imaging -- Biological effects of ultrasound, ultrasonic tomography -- Computed tomography; Involving electronic [emr] or nuclear [nmr] magnetic resonance, e.g. magnetic resonance imaging -- Computerised tomographs -- Diagnosis using ultrasonic, sonic or infrasonic waves -- Biological material, e.g. blood, urine; Haemocytometers -- Digital computing or data processing equipment or methods, specially adapted for specific applications -- Image data processing or generation, in general; transcranial MR‐guided focused ultrasound -- ultrashort echo time (UTE) MRI -- bone segmentation -- skull aberration correction; Computed tomography -- Optical aberrations -- Medical magnetic resonance imaging -- Medical image segmentation -- Brain -- Temperature measurement -- Transducers -- Ultrasonography; Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1118/1.4916656
• Languages: English
• ISSNs: 0094-2405

Abstract

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 patient's 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 tissue‐mimicking 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, threshold‐based 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 acoustic watts at several different target locations within the electronic steering range of the transducer. Each target location was 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. MR thermometry 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 using UTE MRI to compute skull‐induced ultrasound aberration corrections. Their results suggest that UTE MRI could be used instead of CT to implement such corrections on current 0.7 MHz clinical TcMRgFUS devices. The MR image acquisition and segmentation procedure demonstrated here would add less than 15 min to a clinical MRgFUS treatment session.