Measuring diffusion in inhomogeneous systems in imaging mode using antisymmetric sensitizing gradients

Abstract

A principle for self-diffusion pulse-sequence design which cancels harmful cross terms involving both imaging and sample susceptibility-induced gradients by using antisymmetric diffusion-sensitizing-gradient pulses is proposed. A water diffusion coefficient of 1.94 × 10⁻⁵ cm² s⁻¹ was obtained by the cross-term-free sequence, a bipolar sensitizing-gradient pulse (BGP) scheme, while the conventional method gave 2.22 and 1.60 × 10⁻⁵ cm² s⁻¹ with sensitizing gradient running parallel and antiparallel to the imaging gradient, respectively. A diffusion coefficient of 1.38 × 10⁻⁵ cm² s⁻¹ was obtained from an apple, in fair agreement with 1.56 × 10⁻⁵ cm² s⁻¹ for its juice, using the BGP sequence. Although the conventional technique gave similar values for apple juice, it gave only about half as large a value for the whole apple, presumably because of internal gradients from inhomogeneous susceptibility. The rms value of these local gradients in the apple was estimated as 6.0 kHz/cm in our 200 MHz instrument.