Referenceless Thermometry Using Bloch-Siegert Phase Shift and Auto Regressive Model

Manivannan Jayapalan¹

Thermal monitoring in Magnetic Resonance guided Focused Ultrasound (MRgFUS) treatments is a crucial step where the phase images from MR images are used to get thermal maps. One of the widely used techniques is proton resonance frequency (PRF) shift technique that involves some form of image subtraction using a baseline pre-treatment image. Subject motion and tissue deformation due to coagulation can severely distort these techniques. Self-referenced methods help to overcome this hurdle which requires some area of tissue around the area of treatment, hot zone, for polynomial fitting to estimate the baseline phase. The accuracy of the temperature map from this method mainly depends on how close the estimated phase is with baseline phase. In this work, a new technique is described, where the equivalent-baseline phase values are generated/ estimated by applying the current phase values to a model that is based out of Bloch-Siegert phase Shift obtained in run-time along the same location. The phase estimated using Bloch-Siegert phase shift matches well with the baseline phase and so the temperature variations. This method not only eliminates the need for baseline subtraction but also produces better results as the model is generated in runtime against the known values. In clinical scenario, temperature measurement at any location, at any point of treatment could be obtained without the baseline information by using the Bloch-Siegert phase shift from a known RF pulse and the current phase values. This new technique would be amenable for the MRgFUS treatments, in particular moving organs where subtraction methods fail.