Non-invasive Characterization of Tissue Thermal Conductivity, Blood Perfusion and Elasticity in vivo using Magnetic Resonance Imaging and High Intensity Focused Ultrasound
Zhang, Jiming 1980-
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In Magnetic resonance imaging (MRI) guided high intensity focused ultrasound (MR-HIFU)surgery,high-intensity ultrasound beam is focused within the body under MRI guidance to create focal regions of thermal coagulation. MR-HIFU surgery is increasingly used to non-invasively ablate uterine fibroids, breast cancer, and liver metastases. Until now, MR-HIFU surgery used thermal dose as a sole measure of treatment effectiveness, without taking into account the bio-thermal and bio-mechanical properties of tissue, as well as tissue physiologic response to HIFU heating. In this dissertation, we propose methods to use the MR-HIFU system, to characterize tissue thermal (conductivity), physiological (blood perfusion), and mechanical (elasticity) properties. We have performed in vivo, non-invasive measurements of tissue thermal conductivity and studied the thermal response of blood perfusion to thermal ablation of pig muscle and uterine fibroids using MR-HIFU. We demonstrate the feasibility of measuring elasticity in phantom by MRI tracking the mechanic wave propagation introduced by a transient discharge of acoustic radiation force from HIFU. The results showed that the thermal conductivity of muscle tissue undergoes little variation at clinical MR-HIFU surgery temperature range (60oC~90degC) compared to the value at lower temperature (<40degC). However the local blood perfusion rate undergoes a fast (in tens of seconds) and large increase (~20 times) compared to that in the muscle without thermal ablation. The shear elasticity estimated by MR-HIFU agrees with conventional MR elastograhy method for estimating mechanical property of tissue mimicking phantoms. We demonstrate that the non-invasive, in vivo characterization of thermal conductivity, blood perfusion behavior and mechanical property can be carried out in one MR-HIFU platform. It will eventually benefit the planning, the heating stratagem, and the outcome of thermal surgery.