Regional measurement of alveolar oxygen partial pressure can be obtained from the relaxation rates of hyperpolarized noble gases, 3 He and 129 Xe, in the lungs. Recently, it has been demonstrated that measurements of alveolar oxygen partial pressure can be obtained using the spin-spin relaxation rate (R 2 ) of 3 He at low magnetic field strengths (<0.1 T) in vivo. R 2 measurements can be achieved efficiently using the Carr-Purcell-Meiboom-Gill pulse sequence. In this work, alveolar oxygen partial pressure measurements based on Carr-PurcellMeiboom-Gill R 2 values of hyperpolarized 3 He and 129 Xe in vitro and in vivo in the rat lung at low magnetic field strength (74 mT) are presented. In vitro spin-spin relaxivity constants for 3 He and 129 Xe were determined to be (5.2 6 0.6) 310 26 Pa 21 sec 21 and (7.3 6 0.4) 310 26 Pa 21 s 21 compared with spin-lattice relaxivity constants of (4.0 6 0.4) 310 26 Pa 21 s 21 and (4.3 6 1.3) 3 10 26 Pa 21 s 21 , respectively. In vivo experimental measurements of alveolar oxygen partial pressure using 3 He in whole rat lung show good agreement (r 2 5 0.973) with predictions based on lung volumes and ventilation parameters. For 129 Xe, multicomponent relaxation was observed with one component exhibiting an increase in R 2 with decreasing alveolar oxygen partial pressure. Magn Reson Med 64:1484-1490,

Hyperpolarized Helium‐3 (3He) Magnetic Resonance (MR) imaging is a novel technique used for investigating structural and functional properties of the lung in vivo. In particular, 3He diffusion measurements have shown remarkable sensitivity for diagnosing Chronic Obstructive Pulmonary Disease (COPD), particularly emphysema. Restricted diffusion reduces the free diffusion coefficient, thus resulting in a measured apparent diffusion coefficient (ADC) related to the size of the acinus and alveoli. ADC has been predicted to behave anisotropically, and can be described by two components: (i) a longitudinal component along the acinus and (ii) a transverse component perpendicular to the acinus which depend on the diffusion time (δ). We measured 3He ADC at both ultra short (< 1 ms) and long (δ=50 ms ‐ 1 s) diffusion times in both healthy rat lungs and rat lungs instilled with elastase to simulate emphysema. Ultra‐short domain ADC maps were measured using a 3.0 T MR imaging system, where the transverse ADC component was found to be most sensitive to elastase damage for diffusion times near 360 μs, consistent with theoretical models. Whole‐lung long‐range ADC measurements were acquired using a 74 mT MR imaging system, since the low magnetic field has the advantage of longer T2* from reduced susceptibility differences at the air‐tissue interfaces. Measured ADC correlated well with histology (i.e. mean linear intercept). Possible extension of this work to hyperpolarized Xenon‐129 (129Xe) anisotropic ADC measurements at low field is discussed, as an optimal diffusion time of 20.8 ms has been predicted, more favorable for clinical studies