Principles of ExAblate treatment
ExAblate is a hybrid technique using ultrasound’s heat-generating ability of combined with magnetic resonance imaging [MRI], which provides detailed anatomical images for guidance, allows for real-time thermal monitoring during the treatment, and post-procedural assessment of the extent of treatment after the administration of a gadolinium contrast.
MR imaging permits safe targeting of the energy, provides 3D anatomical information for exact tumour localisation and visualises the beam path to avoid surrounding tissue, nerves, scars and other organs. ExAblate involves real time and continuous MR thermal feedback, and determines whether the energy was directed correctly and the appropriate amount delivered. Post-treatment contrast imaging is used to evaluate the treatment outcome.
Focused ultrasound therapy directs high energy to a chosen point: on the near side and far side of this point, the energy is dissipated and is not sufficient to damage adjacent tissues. The energy causes the temperatures at the focal spot focal to rise to 60-80°C resulting in thermal ablation of a small area (1-2cc) of tissue. A larger volume is ablated by treating multiple adjacent small areas. The number of sonications and thus the length of the treatment depend on the size of the fibroid. The time required for the entire ExAblate procedure ranges from 120-180 minutes.
This use of MR with FUS provides the ability to get immediate feedback, react to that feedback, and know immediately what the outcome is. This provides the doctor with information that ensures efficacy and safety of the procedure.
Benefits of ExAblate treatment
Key benefits of ExAblate are:
- It is a non-invasive procedure replacing hysterectomy or myomectomy;
- It has a low rate of morbidity, associated adverse events and complications;
- It is an outpatient procedure,
- Only limited conscious sedation is required, thus avoiding the problems associated with anaesthesia;
- Return to normal activity is very rapid, usually within 1-2 days following the procedure;
- Little to no follow-up medical care or medications are required;
- The procedure is highly acceptable to patients;
These benefits are set out in more detail in the section headed “Key results from clinical studies showing benefits of ExAblate” below.
Evidence base for ExAblate
ExAblate is well established. At the time of writing, over 8000 patients have been treated worldwide. There is a broad evidence base supporting ExAblate in a variety of therapeutic settings. Over 80 peer reviewed papers have shown that ExAblate is safe and effective in the treatment of UF, with up to three year followup.
Outline of a ExAblate treatment
On the day of treatment, a peripheral intravenous cannula is inserted for the administration of moderate sedation. The patient’s blood pressure, heart rate, oxygenation, and comfort level are monitored throughout the treatment. A catheter is placed in the bladder drain and control the bladder during the procedure. Patients are provided with compression stockings for treatment. Patients are asked to shave their lower abdomen from the umbilicus to the pubic bone before arriving for treatment to avoid hair interfering with the direction of the ultrasound energy. On the day of treatment, the skin is cleaned with alcohol to remove any oils or powders that might be on the skin. Patients lie prone on the specialized MRI table that has an ultrasound transducer embedded in it, as shown in Figure 1 below.
Each 12- to 24-second ultrasound treatment is referred to as a sonication. Sonications are delivered through the skin, initially at low power. Once the focal point has been confirmed, higher power sonication is delivered to destroy UF tissue.
MR imaging is key to guiding and controlling the delivery of FUS to ensure effective and safe treatment, by:
- enabling the doctor to localise the tumour and verify the energy beam pathway using high precision anatomic resolution and tissue differentiation in three dimensions;
- enabling the doctor to adapt treatment parameters using real time thermometry to observe tissue temperature changes;
- providing an immediate assessment of treatment efficacy with contrast enhanced MR images.
The ultrasound beam is focused to small areas within the fibroid, as shown in Figure 2 below. Numerous small areas are treated within the fibroid to destroy a large volume of the fibroid tissue. Thermal monitoring during each sonication allows for real-time determination of temperatures obtained in the area of treatment. This allows treatment parameters to be optimized for maximum ablation of UF and to maintain the patient’s comfort. The patient is under conscious sedation and is therefore normally able to communicate with the operator about any adverse symptoms such as burning sensations or pain which might arise from damage to tissue adjacent to the target area. In this case, the sonication is immediately terminated and more images obtained.
The patient lies prone on the patient table entering the MR, with the transducer producing focused ultrasound beams, and its automated positioner correctly reaching each targeted spot.
Once the treatment is completed, additional imaging is obtained after the administration of a contrast medium to assess the effect of treatment. The contrast medium is no longer able to enter the areas of destroyed tissue and shows as areas of non-perfusion on MRI images.
Over time, the fibroid decreases in size, the dead tissue being resorbed by the body.
What is the ExAblate treatment?
It is the only non-invasive surgical procedure that has been approved by the FDA to treat uterine fibroids using focused ultrasound. Ultrasound waves are high frequency sound waves that the human ear cannot hear. When they are focused, similar to how a magnifying glass focuses light waves, heating of fibroid tissue occurs at the focus. The ExAblate device uses MR images to allow the doctor to see inside your body and locate where the fibroid is. During the treatment, MR images allow the doctor to see what temperatures the fibroid has reached, and after treatment, MR images allow the doctor to see how successful the treatment has been.
How do I know if I am eligible for treatment?
Your doctor is the best person to ask about this. He or she will be able to tell you if you are a suitable candidate for the ExAblate treatment. In general, people who cannot go into an MRI will not be able to have the ExAblate treatment – this includes patients with metallic implants, patients who are claustrophobic, and patients who are allergic to contrast agent, a dye used to see blood vessels during MR imaging. ExAblate treatment is not intended for women who are pregnant or desire future pregnancy. The effects of ExAblate on the ability to become pregnant and carry a fetus to term, and on the development of the fetus, have not yet been determined.