MR-guided focused ultrasound

Indications investigated

Preliminary and follow-up examinations

In order to select patients for treatment in the best possible way and for rigorous quality control, various examinations are required before the operation and 6 months afterwards. The aim is to ensure that treated patients have the best possible chances of achieving the best possible result with as few side effects and complications as possible.

* The L-dopa test examines whether Parkinson’s symptoms improve significantly with high-dose levodopa medication. The result of this test allows an assessment of the potential benefit of ultrasound therapy and thus plays an important role in the decision about the therapy. The alcohol test examines the extent to which a tremor changes after consuming alcohol.

The examination results are discussed in detail with patients and their loved ones before the operation and they are informed about the advantages and disadvantages, opportunities and risks. Only then will a final decision be made about the operation.

Procedure

Before the treatment:

Aspirin (ASA/acetylsalicylic acid) and other blood-thinning medication must be completely discontinued in good time in accordance with the neurosurgeon’s instructions (e.g. aspirin: 7 days before surgery).

Change of medication as determined by the treating neurologist.

Admission to hospital:

1 day before the procedure.

Treatment:

One-sided focused ultrasound, e.g. for the side that is more affected or more important for the patient.

Dismissal:

Usually 1 day after the procedure, usually at home.

If necessary, the procedure can be performed on the second side at a later date.

On the day of treatment:

The aim of the treatment is to concentrate sound waves in a tiny spot in the middle of the skull in order to create a permanent lesion by heating the tissue. To do this, sound waves must first pass through skin, bone and tissue from the outside. One problem here is the air surrounding the head, which is kept around the skin by the head hair as an insulating layer for heat (but also for sound waves). For this reason, the scalp hair must be completely shaved off for the treatment.

Under local anesthesia, a metal ring is attached to the skull with four screws through the skin. This is not painful, but is briefly perceived as a feeling of pressure.

The patient is now positioned lying on their back on the couch of the MRI machine. The actual device for generating sound waves is placed over the head like a helmet. The space between the “helmet” and the skin is filled with water to improve scarf conduction and to cool the scalp.

A single sound wave treatment (“sonication”) only takes a few seconds. First, the accuracy of the system is checked several times with only low energy. The energy supplied is then slowly increased, heating the tissue in the target area to around 50°C.

At this temperature, the lesion is not yet permanent, but the neurologist can already determine whether the treatment will be effective and whether any unexpected side effects will occur. If the treatment were stopped at this point, all effects and side effects would disappear within a short time.

For the final treatment, the energy of the device is increased again and the target area is heated to approx. 60°C. At this temperature, cells are permanently damaged, which is intended to bring about a lasting effect of the treatment.

Finally, the retaining ring is removed from the skull and the treatment is complete. It takes about 2 hours in total.

Possible risks of MR-guided focused ultrasound

Although the skin and skull are not opened for treatment with focused ultrasound, permanent damage to brain tissue is caused with high energy. This can lead to very rare but potentially serious complications.

• The vibrations generated by ultrasound can theoretically lead to the rupture of fine blood vessels. A cerebral hemorrhage, like a stroke, can cause permanent neurological disorders or even be fatal. The risk is extremely low and cannot yet be quantified with certainty. However, it is certainly less than 0.5%.
• If the device malfunctions, the target region could be missed and damage could be caused elsewhere in the brain. This would also lead to neurological disorders. However, as the therapy is carried out with real-time monitoring, this risk is very low.
• Healing processes of the brain can partially repair the lesion or compensate for it by restructuring the brain. As a result, the symptoms that were initially successfully treated recur in some patients after a few weeks to months (loss of symptom control).

Possible side effects of MR-guided focused ultrasound

The following disorders can occur temporarily or permanently, depending on the location of the lesion in the brain:

Legend: STN: subthalamic nucleus, Vim: ventromedial nucleus of the thalamus, GPi: globus pallidus internus. +++: >20%, ++: 10-20%, +: <10% (Zurich experience). As a rule, the Vim or immediately adjacent structures are primarily treated for focused ultrasound.

In contrast to deep brain stimulation, the effects and side effects of focused ultrasound are generally irreversible, i.e. they can no longer be influenced or improved after the treatment.