MR-guided focused ultrasound

MR-guided focused ultrasound (MRgFUS) enables targeted modification of tissue deep within the brain—without any incisions in the skull. Bundled ultrasound waves are directed through the skull to a tiny point in the brain that is causing the symptoms of the disease. The ultrasound waves are harmless to the surrounding tissue because they only meet at a precisely defined target point in the brain, where they heat up the tissue. The procedure is extremely precise and capable of heating the small target area to around 55–60 °C with millimeter accuracy. At this temperature, the tissue is destroyed in a targeted manner. This process is called thermocoagulation or ablation.

Magnetic resonance imaging (MRI) enables precise control throughout the entire procedure: it shows the exact position of the ultrasound waves in real time and simultaneously measures the temperature in the target tissue. This ensures that only the desired area is treated, while the surrounding healthy brain tissue remains unaffected.

MR-guided focused ultrasound is primarily used to treat movement disorders (essential tremor, tremor-dominant Parkinson's disease) or chronic pain conditions.

The most common indications include:

• Essential tremor: A common neurological disorder characterized by uncontrollable shaking of the hands or other parts of the body.
  Tremor
• Tremor-dominant Parkinson's disease: When the symptoms of the disease cannot be adequately controlled with medication and lead to a reduction in quality of life.
  Parkinson's disease
• Neuropathic pain: This is caused by damage to nerves or injuries to the spinal cord or brain, resulting in persistent, burning, or stabbing pain, often without any external injury. Neuropathic pain include
  • Phantom pain after amputation
  • Polyneuropathy (e.g., as part of diabetes)
  • Plexus rupture
  • Pain in paraplegia or tetraplegia
  • Pain after a stroke
  • Neuropathic trigeminal neuralgia
  • Pain after herpes infection (e.g., shingles)
  • Persistent pain after surgery for a herniated disc

Careful selection of suitable candidates is crucial for safety and treatment success. In cases of movement disorders, the indication is determined at the Interdisciplinary Center for Parkinson's Disease and Movement Disorders (ZfPB). In cases of neuropathic pain, the indication is also determined on an interdisciplinary basis following a pre-surgical assessment.

Important exclusion criteria are:

• intracranial vascular malformations
• blood thinning that cannot be paused for 2–3 days from an internal medicine perspective
• metal implants in the head area
• insufficient bone density of the skull bone or an excessively thick skull cap (upper part of the skull), as both can weaken the ultrasound waves

The preparation and assessment phase prior to the procedure is similar to that for deep brain stimulation. A complete pre-surgical assessment is carried out.

The only additional examination required is a computed tomography (CT) scan of the skull. This is needed to assess the thickness of the skull bone. In rare cases, the skull is structured in such a way that treatment with focused ultrasound is not possible.

In addition, the head must be completely shaved before treatment.

The actual treatment takes approximately two to four hours.

At the beginning, the head is fixed in a special stereotactic frame so that the ultrasound waves can be precisely aligned. This is done under local anesthesia.

An MRI is then performed to determine the exact target area inside the brain.

During the actual treatment, the patient remains awake and can talk to the doctors. Vital signs (heart rate, blood pressure, and oxygen levels in the blood) are monitored continuously throughout the treatment.

First, a test heating is performed, during which the temperature in the tissue is only slightly increased to approx. 41–43°C. If the result is satisfactory, the target area is further heated to 56–58°C, which finally deactivates a small area of about 2 mm in diameter.

The effect is immediate in cases of essential tremor, while symptom relief in neuropathic pain and Parkinson's disease develops gradually (progressively) over weeks to months.

After the procedure, our patients remain at the Inselspital for one to two nights for observation.

After discharge, follow-up checks are usually carried out after 1 month, 3 months, 1 year, and 3 years.

Possible side effects of MRgFUS treatment are usually mild and often temporary. They also vary depending on the target area. These include:

General risks

• Mild headache or fatigue after treatment
• Swelling or tightness in the head due to the fixation (subsides within a few hours after removal of the frame)
• Nausea or dizziness during or shortly after treatment
• Post-treatment bleeding (very rare)

Specific risks associated with movement disorders

• Sensory disturbances (e.g., numbness or tingling in the hands or face) depending on the target site
• Mild unsteadiness or balance problems, which usually subside within a few days to weeks
• Temporary speech problems (very rare)

In very rare cases, permanent side effects may occur if surrounding tissue is affected.

Both MR-guided focused ultrasound and deep brain stimulation (DBS) are established procedures for treating essential tremor, Parkinson's tremor, and neuropathic pain.

For neuropathic pain, spinal cord stimulation and dorsal root ganglion stimulation are also available as treatment options.

The procedure used depends primarily on medical factors and the individual needs of the patient and is always discussed in detail by our interdisciplinary team of specialists.