Optimizing the point spread function in phase‐encoded magnetic resonance microscopy

Abstract Three‐dimensional phase‐encoded magnetic resonance microscopy is the most promising method for obtaining images with isotropic spatial resolutions on the order of a few micrometers. The attainable spatial resolution is limited by the available gradient strength ( G max ) and the molecular self‐diffusion coefficient ( D ) of the sample. In this study, numerical simulations in the microscopic‐size regime are presented in order to show that for given values of G max and D , there exists an optimum number of phase‐encoding steps that maximize the spatial resolution in terms of minimizing the full‐width at half‐maximum (FWHM) of the image point spread function (PSF). Unlike the case of “macroscopic” imaging, in which diffusion plays an insignificant role in determining spatial resolution, acquiring data beyond this optimal value actually degrades the image PSF. An alternative version of phase encoding, using a variable phase‐encoding time rather than a variable gradient strength, is analyzed in terms of improvements in the image PSF and/or reductions in the data acquisition time for a given spatial resolution. © 2004 Wiley Periodicals, Inc. Concepts Magn Reson 22A: 25–36, 2004.