MR-ARFI and SWI to Detect Calcifications in the Brain in MRgHIFU Treatments

Introduction Calcifications in the brain can be associated with several intracranial pathologies including tumor growth, epilepsy, hyperthyriodism, and infectious diseases [1][2]. Recent advances have shown that MR guided High Intensity Focused Ultrasound (MRgHIFU) can be used in the brain, as a non-invasive functional neurosurgery [3]. Calcifications exhibit a high acoustic attenuation coefficient compared with surrounding brain tissue thus, having the potential to interfere with an MRgHIFU treatment. Prostate cancer HIFU studies have deemed major prostatic calcifications a contraindication to prostate HIFU treatment [4]. For these reasons, it is of interest to visualize the presence of calcifications in brain during an MRgHIFU treatment. While calcifications may be identified in pre-treatment CT scans, they are not registered to the MR images or the in vivo HIFU focal spot. SWI has identified calcifications in the brain by exploiting the diamagnetic properties of calcium, although problems of aliasing and overestimation of size exist [5]. Additionally, MR-Acoustic Radiation Force Imaging (MR-ARFI) can exploit the ultrasonic interaction with calcium by mapping local displacements, and has been explored in phantoms [6]. In this study we present the use MR-ARFI imaging to detect a calcification in ex-vivo swine brain by creating a displacement weighted map of an ROI larger than the focal spot. This technique is proposed as an adjunct to SWI in order to visualize calcifications.