Current status and future potential of MRI‐guided focused ultrasound surgery

Abstract

The combination of the imaging abilities of magnetic resonance imaging (MRI) with the ability to delivery energy to targets deep in the body noninvasively with focused ultrasound presents a disruptive technology with the potential to significantly affect healthcare. MRI offers precise targeting, visualization, and quantification of temperature changes and the ability to immediately evaluate the treatment. By exploiting different mechanisms, focused ultrasound offers a range of therapies, ranging from thermal ablation to targeted drug delivery. This article reviews recent preclinical and tests clinical of this technology. WHILE ITS POTENTIAL has been recognized for decades (1-4), focused ultrasound (FUS) therapy has been in the incubation phase for a long time. Although it has been tested in numerous animal experiments, its widespread clinical realization has lagged, primarily due to the lack of intraprocedural accurate temperature monitoring. When the idea of MRI-guidance was introduced in connection with FUS and MRI thermometry during sonication was demonstrated (5,6), it was anticipated that MR-guided FUS surgery (MRgFUS) would be widely accepted clinically within a few years. Despite these early expectations, there has been relatively slow progress in the clinical implementation of MRgFUS.Early development concentrated primarily on breast tumors (benign fibroadenoma (7,8) and malignant breast cancer (9,10)). These early implementations of the MRgFUS technology motivated the improvement of transducer technology from single-element transducers to MRI-compatible multielement phase arrays (11). There was also significant progress in MRI thermometry and its use to monitor and control sonications. Importantly, too, methods were developed to robustly quantify temperature changes (12). At first, only 2D temperature maps were obtained by MRI to allow for accurate targeting of the beam (13). Later these thermal images were used to generate maps of the thermal dose (14) that could then be used to tailor the energy deposition using closed-loop feedback control (15). This preclinical work led to the development of the first commercial system by InSightec (Haifa, Israel), the Exablate 2000, the first implementation of a fully integrated MRgFUS system that combined a phased array transducer, a computer-controlled robotic positioner, and a workstation that integrated control based on MR thermometry.The early limited clinical trials of breast tumor treatment (8 -10,16,17) were followed by the first major clinical application: the noninvasive therapy of benign uterine fibroids (18,19). The successful Phase I trial was followed by a multicenter . These clinical trials led to the approval of this technology for the treatment of uterine fibroids in several countries, including the United States. Examples of MR thermometry and postablation imaging of a uterine fibroid ablation are shown in Figure 1.The success of uterine fibroid ablation has invigorated the entire field. Technology has rapidly improved and clinical applications ke...

Imaging Findings in MR Imaging-Guided Focused Ultrasound Treatment for Patients with Essential Tremor

“…MR imaging allows precise intraprocedural localization of the ablation target, verification of safety margins for the sonography treatment, and real-time monitoring of thermal ablation dynamics. [1][2][3][4][5][6][7] MR imaging-guided focused sonography is now accepted in the treatment of soft-tissue disorders, including prostate cancer and uterine fibroids. Intracranial applications for brain tumors 8,9 and neuropathic pain syndromes 10,11 are currently under investigation.…”

MRI-Guided Focused Ultrasound Surgery

“…As part of MRgFUS, this ability allows the complete ablation of the gross target volume (GTV), which includes the tumor with additional margins as deemed appropriate by the treating physician, while sparing the nontargeted tissue. As in radiosurgery, to optimize energy delivery, preoperative treatment planning is essential, but unlike radiosurgery, MRgFUS can be monitored and controlled in real time by MRI thermometry (12)(13)(14)(15)(16). Thermal ablation also enables immediate evaluation of the treatment response and allows repeated treatments.…”

“…The result of this integration is arguably the most complex image-guided and controlled therapy delivery system available today: MRI-guided focused ultrasound (MRgFUS) (12)(13)(14). So far, only MRI offers the best combination of tumor margin detection, depictions of details of surrounding anatomy, and the monitoring of temperature changes during therapy (15,16).…”

“…As such, MRgFUS is safe and effective for noninvasive surgery that can replace invasive surgery and ionizing radiation-based therapies, such as radiosurgery or brachytherapy, and can provide treatment for patients for whom invasive surgery or radiation may not be options. The feasibility and effectiveness of MRgFUS are being tested in several clinical applications, which include the ablation of benign and malignant tumors and palliative therapy of bone pain due to metastasis (12)(13)(14). Nonthermal effects of FUS are also being utilized for various novel clinical applications from targeted drug delivery to gene therapy (17,18).…”

Robotically assisted velocity-sensitive triggered focused ultrasound surgery

“…The major challenges of Magnetic Resonance guided Focused Ultrasound Surgery (MRgFUS, [1]) of abdominal organs are breathing motion and limited patient access [2]. Recently, a triggered segmented echo planar imaging (sEPI) sequence was introduced employing a velocity-sensitive navigator for motion-compensated MR temperature imaging [3,4].…”