Non-invasive Characterization of Tissue Thermal Conductivity, Blood Perfusion and Elasticity in vivo using Magnetic Resonance Imaging and High Intensity Focused Ultrasound

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.