MR-guided Focused Ultrasound Surgery

The feasibility of focused ultrasound for thermal therapy has been well established for more than 50 years. However, it has yet to reach wide application in the clinic. The main reason for this has been the lack of monitoring of the procedure. The earliest attempts at focused ultrasound therapy were simply targeted by x-rays or ultrasound imaging. While these imaging modalities allow for targeting of the ultrasound beam, they do not yet have the capability to monitor the thermal exposure or to image the resulting tissue response with acceptable accuracy.

Magnetic Resonance Imaging (MRI) has been recently applied to focused ultrasound thermal therapy. It offers the following capabilities:

Superior soft tissue contrast: MRI can distinguish the desired target tissue (cancer, etc.) with high resolution and contrast.
Imaging in any orientation: MR images can be acquired in any plane in two and three dimensions. This capability allows for superior treatment planning ensuring that surrounding tissues are not in the path of the ultrasound beam.
Temperature sensitivity: Several MRI parameters, including T1, the diffusion coefficient, and the proton resonant frequency (PRF) of water, are temperature sensitive. Currently the PRF method is the most sensitive method (<1°C sensitivity) and is apparently independent of the tissue type and the tissue state (thermally coagulated or not).
Superior targeting: Because very low temperatures can be imaged with MRI, the precise location of the ultrasound beam can be visualized with a low power exposure. This ensures that the ultrasound is going to the correct target.
Thermal quantification: With its accuracy in thermometry, the thermal exposure (dose) can be quantified. This allows the capability of monitoring online whether the tissue at the target is receiving a sufficient thermal dose to cause the desired damage and whether the surrounding tissue is receiving a safe dose. This exposure control is essential for successful thermal therapies.
Post-therapy imaging: The resulting tissue damage (or lack thereof) can be imaged after the treatment with standard MR sequences. Contrast agents can be given to establish whether the tissue's blood supply has been compromised (a good indicator of thermal damage).
Repeatability: Because MRI does not use ionizing radiation (unlike x-rays and CT scans), it can be repeated multiple times. Thus, many images can be safely acquired during the therapy, and the patient can be repeatedly imaged after the therapy to establish proper treatment.

Current Topics

	Optimizing the MRI scan parameters for thermal therapies
	Developing MRI-compatible treatment devices
	Demonstrating the feasibility of targeting the ultrasound beam with temperature-sensitive MRI and of predicting the tissue damage with thermal dosimetry
	Developing new MRI sequences that can be used to overcome the current limitations of MRI thermometry (imaging in fatty tissues, improving the motion sensitivity)
	Using rapid MRI to control online the power deposition
	Using the sequences during tumor treatments