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  • Updated 01.15.2024
  • Released 12.05.2021
  • Expires For CME 01.15.2027

Use of focused ultrasound in neurologic disorders



Focused ultrasound is a noninvasive technique that uses acoustic waves to affect function or structure. High-intensity focused ultrasound (HIFU) utilizes high-energy ultrasound waves to precisely heat tissues, resulting in permanent therapeutic effects. Low-intensity focused ultrasound (LIFU) is administered at lower energy levels and exerts nondestructive mechanical pressure effects on cellular membranes and ion channels that can be used for neuromodulation or opening the blood-brain barrier to enhance drug effects with low central nervous system penetration and accelerate the excretion of toxic metabolites into the bloodstream.

Magnetic resonance–guided focused ultrasound (MRgFUS) has several different clinical applications in neurologic disorders, including ablative therapy for tremors and motor symptoms in Parkinson disease. MrGFUS LIFU is being researched in epilepsy, neuropathic pain, neuro-oncology, and neurodegenerative diseases. MRgFUS is generally considered to be safe and well tolerated and has become a favorable alternative to well-established treatment modalities, such as deep brain stimulation and stereotactic radiosurgery.

Key points

• Focused ultrasound is a noninvasive technique used to affect the structure or function of the nervous system.

• Focused ultrasound can be used for neuromodulation or permanent tissue ablation.

Historical note and terminology

Ultrasound is defined as mechanical waves with frequency higher than human audible sound (conventionally 20,000 Hz) that can travel through all types of matter: gases, liquids, solids, and plasma. Subsequently, the mechanical waves can be reflected, refracted, or attenuated by the medium. Although discovered in 1880, the effects of focused ultrasound on neuronal activity have been investigated since the 1920s. An efficient generator of focused ultrasound was designed, built, and successfully operated in the 1940s in the United States (21). HIFU was found to cause behavioral and histopathological changes (as seen on autopsy) in animals. However, use was limited by incidental skin injury during the procedure. Interest in focused ultrasound ablation increased with subsequent advances in medical imaging and technology. Extensive developments in the 1950s and 1960s led to applications in clinical treatments of neurologic disorders, such as Parkinson disease and brain tumors. Based on earlier successes, a computer-controlled focus ultrasound system guided by B-mode ultrasound, “The Candy machine,” was engineered in the 1970s to treat patients with brain tumors. This device could not be used through the skull, and its application required a craniectomy, which is a procedure that involves removing a portion of the skull. The results were controversial, although the safety and feasibility of the procedure were demonstrated (13). Ultrasound continued to be the only guidance modality until the early 1990s. By then, MRgFUS success had been demonstrated by Hynynen and colleagues and later by other groups (06).

The InsighTec ExAblate 2000 was the first MRgFUS system to obtain regulatory approval in the United States. In 2016, focused ultrasound received FDA approval to treat medication-refractory unilateral essential tremor. Regarding neurologic disorders, MRgFUS is now additionally FDA approved for bilateral essential tremor, tremor dominant Parkinson disease, as well as dyskinesias and other motor symptoms in Parkinson disease.

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