  Magnetic resonance microscopy images at cellular resolution (< 10 microns)
are limited by diffusion. SNR and spatial resolution suffer from the dephasing
of transverse magnetization caused by diffusion of spins in strong gradients.
Such effects may be reduced by using phase encoding instead of frequency
encoding readout gradients. Demonstration of the benefits of phase encoding are
lacking, and the conditions in which it is preferred are not clearly
established. We quantify when phase encoding outperforms a readout gradient
with emphasis on the detrimental effects of diffusion on SNR and resolution. A
15.2T MRI scanner, with 1 T/m gradients, and micro solenoid RF coils < 1 mm in
diameter, were used to quantify diffusion effects on resolution and SNR of
frequency and phase encoded acquisitions. Frequency and phase encoding
resolution and SNR per square root time were calculated and measured for images
at the diffusion limited resolution. The point-spread-function was measured for
phase and frequency encoding using additional constant time gradients with
voxels 3-15 microns. The effect of diffusion during the readout gradient on SNR
was experimentally demonstrated. The achieved resolutions of frequency and
phase encoded acquisitions were measured via the point-spread-function. SNR per
square root time and actual resolution were calculated for a wide range of
gradient amplitudes, diffusion coefficients, and relaxation properties. The
results provide a practical guide on how to choose between phase and frequency
encoding. Images of excised rat spinal cord at 10 x 10 microns in-plane
demonstrate benefits of phase encoding in the form of higher measured
resolution and SNR vs the same image acquired with a conventional readout. We
demonstrate the extent to which phase encoding outperforms readout gradients in
SNR and resolution over a wide range of voxel sizes, sample, and hardware
properties.
