Joshua Coon Thesis Defense 12/7/11

Thesis Defense

Joshua Coon

Wednesday, December 7, 2011
3:00pm (334 JFB)

Title: Treatment Time Reduction through Parameter Optimization in Magnetic Resonance Guided High Intensity Focused Ultrasound Treatments


Magnetic Resonance guided High Intensity Focused Ultrasound (MRgHIFU) treatments are a promising modality for cancer treatments in which a focused beam of ultrasound energy is used to ablate tumor tissue. However, obstacles still exist to its widespread clinical implementation. One of the foremost among these is that treatment times tend to be long, with large tumors often requiring several hours to treat. This research centers on efforts to reduce treatment times through intelligent selection of the user-controllable parameters. These parameters include the treatment path that the focal zone follows through the tissue, the size of the focal zone, the spacing between focal zones, and the packing of focal zones. To do this, several treatments using various combinations of these parameters were simulated using a finite difference method on a tissue region with a wide range of acoustic, thermal, geometric, and tumor properties. For each of these treatments, the total treatment time was iteratively optimized using either a heuristic method or routines included in the Matlab software package, with constraints imposed for patient safety and treatment efficacy.

The results demonstrate that large reductions in treatment time are possible through the intelligent selection of user-controllable treatment parameters. For the treatment path, treatment times are reduced by as much as an order of magnitude if the focal zones are arranged into stacks along the axial direction and a middle-front-back ordering is followed. For situations where normal tissue heating constraints are less stringent, these focal zones should have high levels of adjacency to further decrease treatment times; however, adjacency should be avoided in cases where normal tissue constraints are more stringent. Also, the use of smaller, more concentrated focal zones produces shorter treatment times than larger, more diluted focal zones, a result verified in an agar phantom model. Further, focal zones should be packed for a small amount of overlap in the axial direction and with a small gap in the transverse direction. Finally, increased packing of focal zones axially has diminishing returns in terms of treatment time reduction. These studies suggest that all treatment time reductions occur due to selection of parameters that advantageously use of mechanisms of increasing thermal superposition in the tumor, decreasing thermal superposition in the normal tissue, and advantageously using non-linear rates of thermal dose deposition with increasing temperature.

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