MR Guided Focused Ultrasound (MRgFUS)

MR-guided focused ultrasound (MRgFUS) is an incisionless treatment for tremor. Hundreds of ultrasound beams are focused, through the intact skull, onto a single point deep inside the brain, where their combined energy creates a small, precisely placed lesion. Each individual beam is far too weak to affect the tissue it passes through; only at the focal point does the combined energy become therapeutic.

Nothing is cut and nothing is implanted. The patient lies inside an MRI scanner wearing a helmet-shaped transducer, awake and in conversation with the treatment team throughout.

MRgFUS can be thought of as a modern, image-guided version of thalamotomy (a lesioning procedure performed for decades) with one crucial difference: the target can be tested at low energy first, and adjusted, before any permanent change is made.

The head is shaved in the treatment area, and a lightweight frame is fitted to keep the skull in a fixed position. Inside the scanner, MRI thermometry shows the treatment team the temperature at the target in real time; this is what makes the procedure controllable rather than approximate.

Treatment proceeds in stages. Low-energy “test” sonications warm the target just enough to produce a temporary effect. Between each one, the patient is asked to perform a simple task; drawing a spiral, pouring water, holding out a hand. If tremor improves and no unwanted effects appear, energy is increased in a further step. If something is not right (numbness, altered speech, unsteadiness) the target is adjusted before any permanent lesion is made. Most patients notice a reduction in tremor in the treated hand during the session itself. The full procedure typically takes two to four hours.

MRgFUS is most often considered for tremor disorders, particularly:

• Essential tremor, or tremor-dominant Parkinson’s disease, that has not responded adequately to medication
• Tremor that is disabling on one side of the body more than the other

MRgFUS is usually performed on one side of the brain, treating the opposite side of the body. Treating both sides in a single or staged procedure is approached with much greater caution, because of a higher risk of speech and balance side effects; where tremor significantly disables both hands, DBS is often the more appropriate option to discuss.

A specific factor assessed before treatment is the skull density ratio; a measure, taken from a CT scan, of how well ultrasound energy can pass through an individual patient’s skull to reach the target. In a minority of patients, the skull absorbs too much energy for the procedure to be effective, which is why this is checked before treatment is offered rather than after arrival for the procedure.

Other relevant factors include a patient’s preference to avoid, or unsuitability for, open surgery, and the ability to lie still inside an MRI scanner for the duration of the session.

There is no surgical wound and no implanted hardware to settle, and because the skull is never opened, travel restrictions after treatment are typically shorter than after DBS. Temporary side effects are relatively common in the first weeks after treatment, including unsteadiness, tingling in the lips or fingers, or mild gait disturbance, and these usually settle over time. Because the lesion created is permanent, however, any effect that persists cannot be reversed by adjusting a device, in contrast to DBS. This distinction is central to the choice between the two treatments.

• Detailed history and tremor assessment
• Neurological examination
• CT imaging to measure skull density ratio
• Brain MRI for treatment planning
• Assessment of general suitability for a procedure performed while lying still inside an MRI scanner

MRgFUS aims to meaningfully reduce tremor in the treated hand and improve the ability to carry out everyday tasks that the tremor had disrupted. As with all functional neurosurgery, treatment goals vary between patients, and individual results may differ; the degree and durability of benefit is not identical for every patient.

Risks include the temporary effects described above, as well as the possibility of a persistent side effect, since the lesion created is not reversible. A small number of patients do not achieve adequate tremor control from the procedure. Because MRgFUS is usually performed on one side, the untreated side is not addressed, which is an important consideration when both hands are significantly affected.

There is no wound to heal and no hardware to settle. Most patients are observed for one night after the procedure. Because the skull is never opened, clearance for air travel is usually possible within a few days, rather than the ten to fourteen days generally required after DBS. Follow-up continues to monitor tremor control and any late side effects.

Neither treatment is definitively “better”; they answer different clinical questions and suit different situations.

In practice, the choice usually turns on a few key questions: is the tremor mainly one-sided or two-sided, are there motor symptoms beyond tremor, and how much weight does the patient place on the treatment being adjustable or reversible. These are worked through individually with each patient, following full evaluation.