Focused Ultrasound in Bone/ Musculoskeletal (HIFU)

Standard ultrasound (i.e. diagnostic ultrasound for imaging) uses low energy ultrasound waves to create images of structures in the body. The energy is not focused on a single spot and does not create high temperatures.

FUS has several attractive features when compared to other methods for non-invasive or minimally invasive tumor ablation such as vascular embolization, radiofrequency ablation, cryotherapy, and targeted radiotherapy. These advantages include:

• Unlike radiation therapy, the passage of ultrasound energy through intervening tissue has no cumulative effect on that tissue or tissue beyond the focus. Focused ultrasound treatment is extremely precise, with the boundary zone between necrotic treated and viable untreated tissue measuring less than 1 mm. Accordingly, FUS is easily repeatable, unlike radiotherapy where tissue toxicity makes repeated treatment problematic.
• FUS produces in-depth precise tissue necrosis using an external applicator, with no need to insert an instrument into the target tissue or blood vessels. That is, the procedure is completely incisionless and non-invasive and so carries negligible risks of inducing hemorrhage or infection.
• Patients treated for bone lesions or soft tissue tumors with FUS are able to return home within 1-2 hours of their procedure, experience rapid and substantial pain relief within a few days, and are often able to reduce or completely stop analgesic use shortly thereafter.
• FUS requires just a single session of treatment, unlike the multiple sessions required for traditionally fractionated radiotherapy.
• FUS employs a near-instantaneous delivery of focused energy and does not depend on heat conduction to treat the tumor. Therefore, FUS can be used even in the presence of large adjacent vessels which can act as heat-sinks and limit tumor killing by other modalities such as radiofrequency ablation.
• The tissue necrosis produced by FUS is immediate and efficacy can be checked by obtaining gadolinium enhanced MR images to show the non-perfused treatment volume at the end of the procedure.

MRI provides two key advantages for guiding focused ultrasound.

First, the ability to rapidly obtain multiplanar and multiparametric images allows for accurate tumor localization, improving FUS targeting.

Second, MR thermometry allows for real-time pixel-by-pixel temperature monitoring during the sonication, so that the extent and degree of tissue heating can be controlled and monitored. Other methods of guiding FUS, for example, the use of endorectal ultrasound for prostate therapy do not provide these advantages of precise tumor localization and treatment monitoring.

HIFU offers a very favorable safety profile. The most important risk is the risk of a skin burn. In order to minimize this risk, cooling devices and procedures are used to protect the skin during the treatment procedure.

Several tests and discussions are required before performing an MRgFUS procedure, in order to make sure that only appropriate patients who are likely to benefit from the procedure and who do not have any risk factors that would be a contraindication to performing the procedure are selected. The exact requirements vary with the condition being treated, but at a minimum MRI of the treatment target is required in all cases. It is preferable but not essential for this MRI to be performed at UCSF.

The length of the procedure is variable, depending on what condition is being treated, but most cases last at least 3 hours. In general, the area of treatment is positioned so that the skin overlying the target is centered on the treatment table. The patient receives general anesthesia so that they do not move during the procedure. (They may also receive a nerve block to help decrease post-procedure pain.)

Initial images are obtained to confirm the target is still accessible and appropriate. Then separate pulses (“sonications”) of focused ultrasound energy are applied through the skin using a transducer that is in the table top of the MRI scanner, each lasting about 20 seconds. At the end of the study, an injection of an MRI contrast dye (“gadolinium”) is injected to confirm that the target has been successfully destroyed.

After MRgFUS, patients are allowed to rest comfortably while the effects of any medication given are allowed to wear off. Pain is managed with pain medication. Most patients are able to return to normal activities a few days later. Although, patients who have treatments of bone lesions in the lower extremities may be asked to avoid high impact activities (e.g. running) for up to 4 weeks following the procedure to minimize the risk of bone injury.

Patients who are part of a clinical trial may be asked to fill up questionnaires or other forms in the weeks and months after the procedure.

• Rosanna Wustrack, MD
• Melissa Zimel, MD
• Richard O’Donnell, MD
• Kristopher Jackson, NP
• Jordan Kaitz, RN
• Lindsay Lawless, RN
• Ross Okimoto, MD
• Victoria Wang, MD, PhD
• Sorbarikor Piawah, MD
• Brian Schulte, MD
• Lawrence Fong, MD