New approaches to magnetic resonance thermometry

Minimally invasive thermal therapy of benign and malignant diseases benefits from near real-time MR image guidance. The ability to construct maps of in vivo body temperature makes MRI particularly well suited for guiding and monitoring minimally invasive thermal therapy. For many applications, the proton resonance frequency (PRF) shift is the method of choice for temperature monitoring. In this thesis, several new methods are proposed that address problems of currently used techniques to make temperature measurement with PRF thermometry more robust and reliable for in vivo applications.; Conventional PRF thermometry relies on the subtraction of baseline images, which make it sensitive to tissue motion and frequency drift during the course of treatment. A new method is presented that eliminates these problems by estimating the background phase from each acquired image phase. In this referenceless method, a two-dimensional polynomial is fit to the background phase outside the heated region in a weighted least squares fit. Extrapolation of the polynomial to the heated region serves as the background phase estimate which is then subtracted from the actual phase. This referenceless method is demonstrated on a phantom during laser heating, zero degree temperature rise images of in vivo human liver, interstitial laser ablation of porcine liver, and transurethral ultrasound ablation of canine prostate. Due to its robustness to motion and tissue displacement, the referenceless method is well suited to measure temperature changes during in vivo thermal ablation procedures.; If the referenceless method is used to determine temperature changes during prostate ablation, phase gaps between water and fat in image regions used to determine the background phase can confound the phase estimation. An extension to referenceless reconstruction is described that allows background phase estimation in the presence of phase discontinuities between aqueous and fatty tissue. The modified referenceless reconstruction is evaluated in phantoms and during in vivo canine prostate ablation.; Experimental results have shown that the referenceless reconstruction methods provide reliable temperature maps in liver and prostate without suffering from frame-to-frame motion. Overcoming motion sensitivity, the referenceless methods provide a step towards more robust and more reliable temperature monitoring for in vivo minimally invasive thermal ablation procedures.