Feasibility of ultrashort echo time images using full-wave acoustic and thermal modeling for transcranial MRI-guided focused ultrasound (tcMRgFUS) planning.

Transcranial MRI-guided focused ultrasound (tcMRgFUS) is increasingly used to non-invasively treat a wide variety of neurological disorders including essential tremors, Parkinson's disease, and neuropathic pain. Although this treatment is an MRI-guided procedure, the current pre-treatment screening and planning involve a CT of the head to obtain 3D skull images. These images are necessary for estimating the proportion of absorbed energy and the acoustic phase shift associated with the skull and determining the transmit energy of ultrasonic waves to create thermal lesions at a desired focal spot. Ultrashort echo time (UTE) MR sequences can capture signals from tissues such as bone which have a very short transverse relaxation time. In this manuscript, we assess the use of a UTE based sequence to image the calvarium and test the feasibility of obviating the need for CT based imaging during an MR-guided focused ultrasound therapy. We demonstrate that the segmentation of bone using UTE images leads to similar skull density ratio values as determined from CT with high correlation (r  =  0.88; p   <  0.0001). Furthermore, through treatment specific modeling we demonstrate that the thermal profiles and focal locations are in concordance with the actual treatment plan when using the UTE based skull intensity information suggesting the possibility of replacing the CT scans with UTE based skull imaging in all tcMRgFUS procedures, potentially eliminating unnecessary radiation exposure. Overall, the results indicate that UTE MR imaging may serve as an effective and accurate alternative to CT imaging for both screening and pre-treatment planning on patients undergoing the tcMRgFUS procedure.