Sensitivity and power deposition in a high-field imaging experiment.
Image signal-to-noise ratio and power dissipation are investigated theoretically up to 400 MHz. While the text is mathematical, the figures give insights into predictions. Hertz potential is introduced for probe modeling where charge separation cannot be ignored. Using a spherical geometry, the potential from current loops that would produce a homogeneous static B1 field is calculated; at high frequency it is shown to create an unnecessarily inhomogeneous field. However, a totally homogeneous field is shown to be unattainable. Boundary conditions are solved for circularly polarized fields, and strategies for limited shimming of the sample B1 field are then presented. A distinction is drawn between dielectric resonance and spatial field focusing. At high frequency, the region of maximum specific absorption is shown to move inside the sample and decrease. From the fields in both rotating frames, the signal-to-noise ratio is derived and compared with the traditional, low-frequency formulation. On average, it is mostly found to be slightly larger at high frequency. Nevertheless, the free induction decay is sometimes found to be annulled.
Hoult
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Institute for Biodiagnostics, National Research Council of Canada, Winnipeg, Manitoba, Canada.
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D004574:Electromagnetic Fields / Q000009:adverse effects*; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000009:adverse effects / Q000379:methods*; D008962:Models, Theoretical; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted