Measurement of hyperpolarized gas diffusion at very short time scales

Abstract

We present a new pulse sequence for measuring very-short-time-scale restricted diffusion of hyperpolarized noble gases. The pulse sequence is based on concatenating a large number of bipolar diffusion-sensitizing gradients to increase the diffusion attenuation of the MR signal while maintaining a fundamentally short diffusion time. However, it differs in several respects from existing methods that use oscillating diffusion gradients for this purpose. First, a wait time is inserted between neighboring pairs of gradient pulses; second, consecutive pulse pairs may be applied along orthogonal axes; and finally, the diffusion-attenuated signal is not simply read out at the end of the gradient train but is periodically sampled during the wait times between neighboring pulse pairs. The first two features minimize systematic differences between the measured (apparent) diffusion coefficient and the actual time-dependent diffusivity, while the third feature optimizes the use of the available MR signal to improve the precision of the diffusivity measurement in the face of noise. The benefits of this technique are demonstrated using theoretical calculations, Monte-Carlo simulations of gas diffusion in simple geometries, and experimental phantom measurements in a glass sphere containing hyperpolarized 3 He gas. The advantages over the conventional single-bipolar approach were found to increase with decreasing diffusion time, and thus represent a significant step toward making accurate surface-to-volume measurements in the lung airspaces.

Quantitative characterization of tissue microstructure with temporal diffusion spectroscopy

“…By contrast, oscillating gradient spin echo (OGSE) methods at high frequencies can probe short diffusion times and, hence, have the potential to detect changes over much shorter length-scales [16]. The OGSE method has been successfully implemented experimentally to probe structural information including short length scales in packed beads [17], in vivo in ischemic rat brain [18], to study hyperpolarized gas diffusion [19] and to characterize tumors [20]. However, due to the relatively complicated gradient waveforms used in the OGSE method, analytical descriptions of DW signals with OGSE have not previously been reported.…”

Mapping of 3He apparent diffusion coefficient anisotropy at sub‐millisecond diffusion times in an elastase‐instilled rat model of emphysema

“…It has been shown that 3 He ADC obtained at millisecond diffusion times can distinguish normal from emphysematous tissue and correlates with mean linear intercept histology, an accepted measure of alveolar damage (12,13). 3 He ADC values obtained at sub-millisecond diffusion times (14) are expected to be even more sensitive to emphysema, especially in small animals. At longer diffusion times on the order of seconds, 3 He ADC is expected to reflect changes in lung anatomy over much larger length scales on the order of centimetres.…”

Resolution enhancement in MRI of laser polarized 3He by control of diffusion

“…The behavior of D in the presence of restrictions as a function of time is a well studied problem and generally used to obtain information on the surface to volume ratio [15,24,25]. Although, Eq.…”