Bryant Svedin Thesis Defense 2/17/17

Thesis Defense

Bryant Svedin

Friday, February 17, 2017
10:30AM (334 JFB)

Title: Improving Magnetic Resonance Thermometry Using Respiration Correction, Ultrasound Focus Prediction and Golden Angle Stack of Stars Sampling

This dissertation presents original research that improves the ability of magnetic resonance imaging (MRI) to measure temperature using the proton resonance frequency (PRF) shift in aqueous tissue and T1 measurements in fat tissue in order to monitor focused ultrasound (FUS) treatments. First, the inherent errors involved in measuring the longitudinal relaxation time T1 using the variable flip angle method with a 2D acquisition are presented. The edges of the slice profile can contribute a significant amount of signal for large flip angles at steady state, which causes significant errors in the T1 estimate. Respiration motion causes phase artifacts, which lead to errors when measuring temperature changes using the PRF method. A respiration correction method for 3D imaging temperature of the breast is presented. Free induction decay (FID) navigators were used to measure and correct phase offsets induced by respiration. A rapid method for predicting the ultrasound focus position in MR coordinates using 3 tracker coils with a special MRI pulse sequence is presented. The focus position was predicted to within approximately 2.1 mm in less than 1 second. A method for acquiring a large field of view with high spatial and temporal resolution is presented. This method used a multi-echo pseudo-golden angle stack of stars imaging sequence to acquire the large field of view with high spatial resolution and k-space weighted image contrast (KWIC) to increase the temporal resolution. The pseudo-golden angle allowed for removal of artifacts introduced by the KWIC reconstruction algorithm. The multiple echoes allowed for high readout bandwidth to reduce blurring due to off resonance and chemical shift as well as provide separate water/fat images, estimates of the initial signal magnitude M(0), T2* time constant, and combination of echo phases. The combined echo phases provided significant improvement to the PRF temperature precision, and ranged from ~0.3-1.0 °C within human breast.

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