Signal-to-noise improvements in three-dimensional NMR microscopy using limited-angle excitation

The 3D FT variant of spin-echo imaging has previously been successfully used to yield images at microscopic resolution. In obtaining such high resolution, optimization of signal to noise for a given acquisition time is crucial. Using a limited-angle (<60) slice-selective pulse one can improve the effective signal to noise or reduce the experimental time. The spin echo is generated through gradient refocusing. A phantom with T₁ of 800 and 1200 ms was used to simulate white and gray matter. Signal intensity was modeled by the expression $\text{M=}\text{M}{}_0\text{sin}\text{θ[1−}\text{exp}\text{(−}\text{T}{}_{\mathrm{R}}\text{T}{}_1\text{)]}\text{1−}\text{exp}\text{(−}\text{T}{}_{\mathrm{R}}\text{T}{}_1\text{)}\text{cos}\text{θ}$ For T_R < 200 ms, limited-angle excitation can yield improvements in signal to noise by greater than a factor of two. These results were experimentally verified on a 1.5 T prototype system (General Electric, Milwaukee, Wis.) configured with gradient and rf coils designed for NMR microscopy. In those cases where signal-to-noise concerns do not require signal averaging beyond that which is inherent in the 3D FT technique, the limited-angle approach can reduce the acquisition time by as much as a factor of four. These results were verified in small animal studies of the brain of a 200 g rat.