to produce images of similar quality to 3he. in terms of detecting ventilation defects, 129xe has a lower signal-to-noise ratio but is more sensitive to defects due to its higher density and lower diffusivity. currently, using a larger volume of 129xe (up to 1 liter per scan) can compensate for its decreased signal-to-noise ratio compared to 3he (usually 0.1-0.3 liters per scan). diffusion weighted imaging diffusion-weighted mri has been proven effective and is commonly used with hyperpolarized gases to calculate the apparent diffusion coefficient (adc) of the gas. this is done by taking gas images with and without diffusion sensitizing gradients. the usefulness of this contrast comes from the fact that the diffusion of gases is limited by the structure of healthy lungs. in diseases like emphysema, where the airspaces are larger, the gases are free to diffuse more easily. this allows diffusion-weighting to differentiate normal airspaces from enlarged ones based on the degree of signal attenuation. the signal intensities in the weighted and non-weighted images are then used to calculate the adc on a voxel-by-voxel basis. adc maps show low values in healthy lung tissue, but in emphysematous lungs, elevated adc values are often seen. in addition to showing emphysema, 3he or 129xe adc values have been found to be sensitive to early changes in the lung tissue of smokers and people exposed to second-hand smoke. adc mri has also been shown to be sensitive to age-related changes in alveolar size in healthy individuals. comparisons to ct densitometry have shown that adc strongly correlates with dlco and may be able to detect early emphysema before it is visible on ct scans. while most adc imaging has used 3he mri, it has recently been shown that 129xe can also be used for this purpose. future direction 129xe dissolving imaging xenon has a lower gyromagnetic ratio and lower snr in images than helium. however, it has the useful property of being moderately soluble in lung tissue. this allows it to diffuse into the capillaries and blood stream, where it experiences shifts in frequency that provide information about gas exchange in the lungs. these shifts can be used to study ventilatory distribution and diffusive gas exchange. imaging the dissolved-phase of gases in the lungs can be difficult. the signal intensity in this phase is only 2% of the gas-phase, and its t2* is very fast at 2 ms. additionally, the dissolved-phase resonances are 200 ppm from the gas-phase on a 1.5t scanner, so rf excitation pulses must be carefully tuned to avoid exciting the gas-phase. early attempts at imaging the dissolved-phase used indirect methods like xenon polarization transfer contrast (xtc). this method used rf pulses applied to the dissolved-phase to slightly attenuate the gas-phase signal, allowing for the indirect mapping of the dissolved-phase distribution. as polarization and pulse sequence technology improved, direct imaging of dissolved 129xe became possible. by using frequency-selective rf pulses and a 3d radial pulse sequence, the first direct images of the dissolved-phase in humans were acquired in 2010. these images were lower resolution due to the small signal intensity of dissolved-phase 129xe, but still showed interesting aspects of lung function. soon after this technique was introduced, mugler et al. showed the value of acquiring the gas-phase distribution in the same breath, allowing for the quantification of the dissolved-phase distribution. this was later extended to a radial acquisition strategy, which allowed for the analysis of the effects of posture on gas transfer. it is important to be able to separately detect the transfer of 129xe to red blood cells (rbcs) because the pathway xenon follows to reach rbcs is the same as that of oxygen. recently, spectra of 129xe in the dissolved phase were acquired in subjects with idiopathic pulmonary fibrosis and showed greatly reduced 129xe transfer to rbcs compared to healthy volunteers. this work showed that separating the dissolved 129xe resonances is critical for detecting diffusion limitation caused by lung tissue thickening. 129xe measurements correlated strongly with dlco and showed that the frequency of the 129xe rbc resonance may be a sensitive measure of blood oxygenation at the capillary level. this work also emphasized the need for imaging to separately detect xenon uptake in barrier tissues and rbcs. separately imaging 129xe in barrier tissues and rbcs is similar to separating fat and water in 1h mri. the two resonances are similarly spaced, so fat-water separation algorithms can be used. qing et al. used the hierarchical ideal algorithm to image all three resonances of xenon in a single breath. the 1-point dixon strategy has also been successful and may be more robust against the short t2* of the dissolved-phase 129xe signal. this technique was also recently used to image all three resonances of xenon in a single breath.