4. My MRI RF coil does not have the homogeneity I expect. What should I do?
Homogeneity is sometimes hard to evaluate on the bench and we need to have a quantitative indication to use. Look at the discussion on Coil Lengths for guidelines on what typical field profile to expect in small coils. We recommend a spin-echo mapping method (see S. Crozier, K. Luescher, L.K. Forbes, and D.M. Doddrell, “Optimized Small-Bore, High-Pass Resonator Designs,” J. Magn. Reson. Ser. B , 1995, 109 , 1-11) in which a series of slice-selective spin-echo images are acquired with varying pre-pulse lengths. The pre-pulse lengths should be (2*n*π ) where n is 0,1,2, and possibly 3. The resulting images will have varying degrees of inhomogeneity with rings of light and dark that can be used to quantitatively evaluate B1. Quantitative B1 mapping can also be done using a gradient-echo image, preferably with a flip angle under 60°, if an accurately calibrated grey scale or color scale can be supplied with the image.
