MRI-guided focused ultrasound surgery

Abstract

MRI-guided focused ultrasound (MRgFUS) surgery is a noninvasive thermal ablation method that uses magnetic resonance imaging (MRI) for target definition, treatment planning, and closed-loop control of energy deposition. Integrating FUS and MRI as a therapy delivery system allows us to localize, target, and monitor in real time, and thus to ablate targeted tissue without damaging normal structures. This precision makes MRgFUS an attractive alternative to surgical resection or radiation therapy of benign and malignant tumors. Already approved for the treatment of uterine fibroids, MRgFUS is in ongoing clinical trials for the treatment of breast, liver, prostate, and brain cancer and for the palliation of pain in bone metastasis. In addition to thermal ablation, FUS, with or without the use of microbubbles, can temporarily change vascular or cell membrane permeability and release or activate various compounds for targeted drug delivery or gene therapy. A disruptive technology, MRgFUS provides new therapeutic approaches and may cause major changes in patient management and several medical disciplines.

Figure 1

Pre- and post-MRgFUS imaging of uterine fibroid. (a) Sagittal T2W image of the uterus shows a single dominant uterine fibroid, measuring 5.5 cm in AP dimension. (b) Sagittal T1W image after intravenous administration of gadolinium shows homogeneous nonenhancing tissue in the area of treatment. This corresponds to the area of thermal necrosis. (c) Sagittal T2W image 24 months after MRgFUS shows the fibroid, now reduced in size (measuring 4.8 cm in AP dimension). (d) Sagittal T2W image at 36 months after MRgFUS shows continued shrinkage, with the AP dimension now measuring 3.8 cm. Note also the entire uterus is shrinking. MRgFUS is an effective, noninvasive, FDA-approved treatment for benign uterine fibroids.

Figure 2

Commercial system for MRgFUS treatment of brain tumors (Exablate 4000, Insightec, Haifa, Israel). (a) MR thermometry image showing focal heating during a clinical MRgFUS brain tumor treatment. (b) T2-weighted MRI with the focal heating superimposed. Exablate 4000 uses a hemispherical 1000-element phased-array transducer to sonicate through the intact skull, providing a completely noninvasive ablation method. The region between the scalp and the transducer is filled with chilled, degassed water to provide acoustic coupling and cooling. The phased array is used to steer the focal point to different locations and to correct skull-induced aberrations.