In 2016, the US Food and Drug Administration (FDA) approved focused ultrasound for treating essential tremor. Two years later, the FDA approved it for tremors in Parkinson’s disease. The minimally invasive procedure, which is performed by surgeons, although no incisions are made, uses MRI scans to identify the area in the brain responsible for the tremor and uses sound waves to burn the tissue in that area. For more information, read How Focused Ultrasound Works.

For those considering the roughly four-hour procedure, here’s what to expect.

First surgeons measure the patients’ skull shape, thickness, and density using a special MRI scan of the head called magnetic resource-acoustic radiation force imaging (MR-ARFI). These measurements are used to calculate a skull density ratio (SDR). The patient’s SDR helps determine the intensity of the treatment. If the patient’s SDR is below 0.35 it may reduce the chances of success and prolong the procedure.

Imaging can provide important pre-treatment information and account for variations in bone composition, says Paul S. Fishman, MD, PhD, professor of neurology at the University of Maryland School of Medicine and chief of neurology at the Veterans Affairs Maryland Health Care System in Baltimore.

The patient’s head is shaved, and after local anesthesia is administered, an intricate frame is attached around the head with four screws to help hold it still and keep the head and brain aligned within the MRI scanner where the procedure takes place. An elastic silicone bag of circulating water is wrapped around the head to keep it cool during surgery.

The patient lies on an MRI table, and the frame is locked into the helmet-like transducer, an electronic device that generates the ultrasound waves. Once the frame and transducer are in place, the patient, who remains awake during the entire procedure, is guided into the MRI machine.

Working from a nearby control station, the medical team spends 60 to 90 minutes delivering beams of ultrasound energy called sonications between 41 and 46 degrees Celsius (107- and 113-degrees Fahrenheit) to the brain that make no permanent impact on it. They do this to pinpoint the exact spot to receive the sonications, so surrounding brain tissue will not be damaged.

Once the target area is confirmed, technicians gradually increase the temperature of the ultrasound beams to between 46 and 50 degrees Celsius (115- and 122-degrees Fahrenheit), so they can assess the impact on the tremor and ensure an absence of adverse effects. Sonications last between 10 and 39 seconds.

In between sonications, the patient is brought in and out of the MRI and asked to perform a series of neurologic mini-tests, such as holding a cup, drawing a circle, or touching fingers together. Adjustments are made based on the patient’s feedback. “This is why the patient remains awake,” says Neal Kassell, MD, professor of neurosurgery at University of Virginia and chairman of the Focused Ultrasound Foundation. “Most [patients] can provide valuable information as we go through this process.” Increased temperature in the brain or skull may cause patients to experience discomfort. At any point in the process if they feel discomfort or anxiety, patients can press the “stop sonication” button they’re given at the beginning of the procedure.

Technicians slowly increase the temperature to between 55 and 60 degrees Celsius (150 and 160 degrees Fahrenheit) to induce a permanent lesion, or ablation, and destroy the identified brain tissue responsible for tremor. This process is repeated as many times as necessary to eliminate the tremor—or as much of the tremor as possible.