Sensitivity and power deposition in a high-field imaging experiment

Image signal-to-noise ratio and power dissipation are investigated theoreti cally up to 400 MHz. While the text is mathematical, the figures give insig hts into predictions. Hertz potential is introduced for probe modeling wher e charge separation cannot be ignored. Using a spherical geometry, the pote ntial from current loops that would produce a homogeneous static B-1 field is calculated; at high frequency it is shown to create an unnecessarily inh omogeneous field. However, a totally homogeneous field Is shown to be unatt ainable. Boundary conditions are solved for circularly polarized fields, an d strategies for limited shimming of the sample B-1 field are then presente d. A distinction is drawn between dielectric resonance and spatial field fo cusing. At high frequency, the region of maximum specific absorption is sho wn 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 traditi onal, low-frequency formulation. On average, it is mostly found to be sligh tly larger at high frequency. Nevertheless, the free induction decay is som etimes found to be annulled. (C) 2000 Wiley-Liss, Inc.