Introduction of ExAblate using MR guided Focused Ultrasound
Focused Ultrasound is a non-invasive, non-ionizing thermal ablation modality that destroys target tissue. Like a magnifying glass focuses the sun’s energy on a single point, focused ultrasound concentrates high intensity energy on a target causing coagulation and cell death. This demonstration on a phantom gel shows how focused ultrasound energy affects one focal point only while sparing surrounding tissue.
Focused ultrasound is readily absorbed by bone. A wide beam of focused ultrasound intersects the bone cortex producing a broad area of heating at the periosteal margin. Pain-transmitting endocortical nerve endings are destroyed, providing pain palliation.
Continuous MR imaging with ExAblate
Continuous MR imaging with focused ultrasound provides precise anatomical images in multiple orientations and slices. This sagittal pelvic image shows the spine, the pubic bone, and bladder. This axial pelvic image shows the femur, pelvis, bone tumor and the sciatic nerves. This visualization enables the physician to pinpoint the anatomy and treatment target for safe and effective treatment.
MRI provides real time measurement of thermal changes in the tissue. Every 5 seconds during the sonication, a new thermal MR image is acquired, and temperature rise is displayed. If the rise is insufficient, parameters can be adjusted to achieve the desired temperature.
The combination of MRI and focused ultrasound produces a closed loop system with careful tissue targeting coupled with ongoing tissue thermometry at the target site . At any point the physician knows what has been treated, the tissue effect, and if changes are required. Post -treatment, contrast MR images allow the physician to verify treatment outcome.
ExAblate
ExAblate consists of a patient table, a workstation, an equipment cabinet containing the electronics, and a water cooling system. The focused ultrasound source is an electronic phased array transducer immersed in a sealed water bath. The focal depth in the body is controlled electronically, to adjust the bone surface sonication size.
Treatment overview
Treatment planning and the entire treatment are conducted from the ExAblate workstation. The patient lies on the ExAblate table inside the MRI with the targeted lesion positioned above the transducer. The patient is prepared with an IV to deliver medication and MR contrast. Anesthesia is used for patient comfort according to hospital guidelines. The treatment goal is to fully ablate the targeted tissue in the defined region of treatment, by ensuring adequate temperature rise at the bone-soft tissue interface.
Treatment planning
Axial and sagittal MR images of the tumor area are acquired to localize and define the target lesion. They are displayed in two rows of strips.
The physician may upload CT images and register them to the MR images to enable optimal viewing of the bony tissue. This is overlaid in green.
The physician identifies and outlines the skin line, bone surface, and treatment area.
Based on this input, the system generates a treatment plan including energy levels, number and size of spots. At any point, the physician can add spots, delete or move spots, or change the angle, changing the energy to optimize treatment.
Verification
Targeting accuracy is verified using a low sub-therapeutic test sonication. This identifies the point of actual energy delivery. Adjustments can be made to correct tissue-specific beam deviations.
ExAblate treatment
ExAblate treatment consists of multiple sonications delivered until the entire targeted tissue is ablated. Each sonication lasts about 20 seconds.
When the sonicate button is pressed, the MR begins scanning, energy delivery begins, and thermal images are displayed showing the temperature buildup. The temperature in the bone-soft tissue interface should reach between 65-85° Celsius. After each sonication the accumulated dose is displayed.
The physician knows immediately if the focused ultrasound energy has reached the planned spot and temperature.
Sonications are repeated until the planned tumor area has been ablated. After each sonication the ablated area is displayed in blue as accumulated thermal dose.
The patient can immediately stop treatment at any time using a hand held stop sonication button.
Post ExAblate treatment
Post treatment contrast enhanced MR images enable the physician to verify the immediate treatment effects by depicting the ablated region.
After treatment the patient is observed for 2-3 hours and then released, according to site standards.
Summary
ExAblate’s combination of Focused Ultrasound and Magnetic Resonance guidance provides a non-invasive, ionizing radiation free personalized treatment, adjusted and optimized to the individual patient’s anatomy and response to treatment.
It provides MR visualization of anatomy and energy beam path for careful targeting, real-time MR thermometry for monitoring and control, and closed-loop feedback
All of these help ensure safety and efficacy of the treatment.
