Acquisition and accuracy in rapid NMR imaging methods.

ABSTRACT

The echo-planar imaging method (EPI) and its variants can acquire data sufficient to form an image in a much shorter time than the Fourier or projection-reconstruction methods. Fundamental requirements on gradient waveforms and signal sampling are discussed for EPI and SEPI (a variant using sinusoidal gradient waveforms), based on the k-trajectory formulation of acquisition in NMR imaging. Using a new simulation method, image defects in EPI and SEPI are demonstrated which result from failure to meet these requirements. Faithful rendition of theoretically ideal gradient waveforms places severe demands on gradient system response in rapid imaging. It is shown, however, that nonideal gradient system response need not degrade images if appropriate reconstruction computations are used. Transfer functions and noise power spectra are derived for EPI and SEPI, and the ratio of these two functions is used to derive ideal observer signal-to-noise ratios for simple visual interpretive tasks. It is shown that ideal observer performance can differ significantly between EPI and SEPI for some visual tasks, using the same NMR imaging system.