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Guiding Lasers To Their Target

Like most treatments, laser therapy can benefit from image guidance. A Houston-based company has developed an MRI-guided system that has been tested and is now FDA-approved.

Laser induced thermal therapy (LITT) destroys unhealthy tissue, like cancer, with the intense heat supplied from a laser. The laser light is channeled through an optical fiber that can be inserted practically anywhere in the body. One of the biggest challenges in LITT is that the target cannot get too hot, otherwise it will char - thus preventing further laser light from penetrating into the tissue.

To avoid this, physicians have traditionally measured the temperature at some point near the target using a thermometer-like probe. But real-time imaging could provide a non-invasive means to monitor the temperature throughout the target region. In particular, MRI provides a sensitive temperature probe. The frequency of the MRI signal, which depends on the magnetic properties of water molecules, shifts as the temperature of the corresponding tissue changes.

Using this effect, Visualase, Inc., has developed a closed-loop MRI guided LITT system that provides the user with a temperature map of the target region and calculates the corresponding dose (i.e. the likelihood that cells in some region will die from the applied heat). If nearby healthy tissue is receiving too much heat, or if the temperature is approaching the charring temperature, the user can respond by changing the laser power or shutting it off, which helps to increase both the safety and efficacy of the procedure.

Trials of the system were independently performed on canines by R. Jason Stafford from the University of Texas M.D. Anderson Cancer Center, and his collaborators. Several LITT lesions were made in the brain, spine and prostate with MRI guidance. The results showed that the machine's calculated dose matched up well with a post-operation assessment. Initial safety studies have also been performed in human patients and the device has recently received FDA approval. Stafford thinks MRI-guided LITT will provide a less invasive alternative to conventional surgery. He is currently working to improve the real-time targeting and heat delivery.

The research was described in the talk, "Closed-Loop Guidance of Laser Induced Thermal Therapy Using MRI," presented July 31, 2008 at the 50th meeting of the American Association of Physicists in Medicine.


Source : American Institute of Physics. August 2008.


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