Measurement of Apparent Long-Range Diffusion of He using Pulsed Gradient Spin Echo T2 in the Rat Lung at Low Magnetic Field Strength

Introduction: Measurement of Apparent Diffusion Coefficients (ADC) of hyperpolarized He gas in the lungs, can provide information on the micro-anatomical changes associated with disease over length scales permitted by the diffusion time. Improved sensitivity of ADC to chronic obstructive pulmonary disease (COPD) has recently been demonstrated using diffusion times on the order of seconds. These long diffusion times are generally not possible using pulsed-gradient echo methods at clinical field strengths due to short transverse relaxation times in the lung, thereby requiring alternative approaches (e.g. stimulated echoes) [1]. However, pulsed-gradient echo methods may be extended to longer diffusion times at low magnetic field strengths, taking advantage of the long transverse relaxation time (i.e. reduced susceptibility gradients) and independence of the available magnetization on field strength [2]. Low fields have previously been used to reduce susceptibility gradients in the lungs for measurement of T2 relaxation of hyperpolarized gases in the presence of oxygen [3]. The purpose of this work was to explore, theoretically and experimentally, the measurement of long range He ADC using a Pulsed Gradient Spin Echo (PGSE) approach at low magnetic field strength.