In hyperpolarized noble gas (HNG) magnetic resonance (MR) imaging, the available polarization is independent of magnetic field strength and for large radiofrequency (rf) coils, such as those used for chest imaging, the body noise becomes the primary noise source making signal-to-noise ratio (SNR) largely frequency independent at intermediate field strengths (0.1-0.5 T). Furthermore, the reduction in the transverse relaxation time, T2, of HNG in lungs with increasing field strength, results in a decrease in the achievable SNR at higher fields. In this work, the optimum field strength for HNG MR imaging was theoretically calculated in terms of both SNR and spatial resolution. SNR calculations used the principle of reciprocity and included contributions to the noise arising from both coil and sample losses in a chest-sized coil for lung imaging. The effects of susceptibility differences, transverse relaxation time, and diffusion were considered in the resolution calculations. The calculations show that the optimum field strength for HNG MR imaging of human lungs is between 0.1 and 0.6 T depending on gas type (helium or xenon) and sample size. At the field strengths currently used by conventional clinical proton MR imaging systems (1-3 T), the predicted SNR are 10%-50% lower than at the optimum field with only slightly worse spatial resolution (10%-20%). At higher fields (>3 T), however, the SNR degrades considerably reducing the achievable spatial resolution. Although HNG of the lung is still feasible at very low field strengths (<50 mT), the available SNR is much lower than at optimum fields and this reduces the achievable spatial resolution. These findings suggest that HNG imaging may be optimally performed at much lower field strengths (0.1-0.6 T) than conventional clinical proton MR imaging systems. This could considerably decrease cost, improve patient access, and reduce chemical shift and susceptibility artifacts and rf heating.

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We used the dual capability of hyperpolarized 129 Xe for spectroscopy and imaging to develop new measures of xenon diffusing capacity in the rat lung that (analogously to the diffusing capacity of carbon monoxide or D LCO ) are calculated as a product of total lung volume and gas transfer rate constants divided by the pressure gradient. Under conditions of known constant pressure breath-hold, the volume is measured by hyperpolarized 129 Xe MRI, and the transfer rate is measured by dynamic spectroscopy. The new quantities (xenon diffusing capacity in lung parenchyma (D LXeLP )), xenon diffusing capacity in RBCs (D LXeRBC ), and total lung xenon diffusing capacity (D LXe )) were measured in six normal rats and six rats with lung inflammation induced by instillation of fungal spores of Stachybotrys chartarum. D LXeLP , D LXeRBC , and D LXe were 56 ؎ 10 ml/min/mmHg, 64 ؎ 35 ml/min/mmHg, and 29 ؎ 9 ml/min/mmHg, respectively, for normal rats, and 27 ؎ 9 ml/min/mmHg, 42 ؎ 27 ml/min/ mmHg, and 16 ؎ 7 ml/min/mmHg, respectively, for diseased rats. Lung volumes and gas transfer times for LP (T trLP ) were 16 ؎ 2 ml and 22 ؎ 3 ms, respectively, for normal rats and 12 ؎ 2 ml and 35 ؎ 8 ms, respectively, for diseased rats. Xenon diffusing capacities may be useful for measuring changes in gas exchange associated with inflammation and other lung diseases. Anatomical and functional imaging of small animals has become important in the study of lung biology (1). The use of genetically altered mice has also increased the demand for methods to establish physiological phenotypes, including those corresponding to lung disease (2). Xe) for lung applications has been illustrated by the use of 3 He in rats and guinea pigs, including in vivo determination of regional ventilation (3-5), ventilation/perfusion ratio (6), and diffusion (7). The production of high-quality images in human subjects has also been demonstrated (8). Although hyperpolarized 129 Xe (hereafter referred to simply as "xenon") suffers from lower polarizations and gyromagnetic ratios relative to 3 He, it has several advantages due to its high lipid solubility and sensitivity of chemical shift to the molecular environment. In particular, dynamic xenon spectroscopy is capable of identifying the gas exchange properties of the lung. Wagshul et al. (9) were the first to report xenon spectra and images of the mouse lung consisting of three dissolved phase peaks at 190 ppm, 196 ppm, and 198 ppm with respect to the gas resonance, which were tentatively assigned to thoracic tissue, lung parenchyma (LP), and blood, respectively. Subsequent work identified three dissolved-phase resonances in tracheostomized Sprague Dawley rats at 191 ppm, 199 ppm, and 213 ppm, which were attributed to plasma/adipose tissue, LP, and red blood cells (RBCs), respectively (10). To confirm the compartmental assignment of these three peaks, Swanson et al. (11) correlated xenon spectroscopy with 1D chemical shift imaging in Sprague Dawley rats. They observed the in vivo dynamics of xenon in rats that breat...

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