Purpose To investigate the feasibility of describing the impact of any flip angle–TR combination on the resulting distribution of the hyperpolarized xenon‐129 (HXe) dissolved‐phase magnetization in the chest using a single virtual parameter, TR90°,equiv. Methods HXe MRI scans with simultaneous gas‐ (GP) and dissolved‐phase (DP) excitation were performed using 2D projection scans in mechanically ventilated rabbits. Measurements with DP flip angles ranging from 6–90° and TRs ranging from 8.3–500 ms were conducted. DP maps based on acquisitions of similar radio frequency pulse‐induced relaxation rates were compared. Results The observed distribution of the DP magnetization was strongly affected by acquisition flip angle and TR. However, for flip angles up to 60°, measurements with the same radio frequency pulse‐induced relaxation rates, resulted in very similar DP images despite the presence of significant macroscopic gas transport processes. For flip angles approaching 90°, the downstream signal component decreased noticeably relative to acquisitions with lower flip angles. Nevertheless, the total DP signal continued to follow an empirically verified conversion equation over the entire investigated parameter range, which yields the equivalent TR of a hypothetical 90° measurement for any experimental flip angle–TR combination. Conclusion We have introduced a method for converting the flip angle and TR of a given HXe DP measurement to a standardized metric based on the virtual quantity, TR90°,equiv, using their equivalent RF relaxation rates. This conversion permits the comparison of measurements obtained with different pulse sequence types or by different research groups using various acquisition parameters.

Purpose: To demonstrate the feasibility of generating red blood cell (RBC) and tissue/plasma (TP)-specific gas-phase (GP) depolarization maps using xenonpolarization transfer contrast (XTC) MR imaging. Methods: Imaging was performed in three healthy subjects, an asymptomatic smoker, and a chronic obstructive pulmonary disease (COPD) patient. Single-breath XTC data were acquired through a series of three GP images using a 2D multi-slice GRE during a 12 s breath-hold. A series of 8 ms Gaussian inversion pulses spaced 30 ms apart were applied in-between the images to quantify the exchange between the GP and dissolved-phase (DP) compartments. Inversion pulses were either centered on-resonance to generate contrast, or off-resonance to correct for other sources of signal loss. For an alternative scheme, inversions of both RBC and TP resonances were inserted in lieu of off-resonance pulses. Finally, this technique was extended to a multi-breath protocol consistent with tidal breathing, involving 30 consecutive acquisitions. Results: Inversion pulses shifted off-resonance by 20 ppm to mimic the distance between the RBC and TP resonances demonstrated selectivity, and initial GP depolarization maps illustrated stark magnitude and distribution differences between healthy and diseased subjects that were consistent with traditional approaches. Conclusion: The proposed DP-compartment selective XTC MRI technique provides information on gas exchange between all three detectable states of xenon in the lungs and is sufficiently sensitive to indicate differences in lung function between the study subjects. Investigated extensions of this approach to imaging schemes that either minimize breath-hold duration or the overall number of breath-holds open avenues for future research to improve measurement accuracy and patient comfort. K E Y W O R D S dissolved-phase imaging, hyperpolarized xenon-129, lung MRI, xenon-polarization transfer contrast, XTC

3D radial double golden-means acquisitions with variable flip angles provide a robust means for rapidly assessing lung function during a single breath hold, thereby constituting a particularly valuable tool for imaging uncooperative or pediatric patient populations.

Purpose To investigate biases in the measurement of apparent alveolar septal wall thickness (SWT) with hyperpolarized xenon‐129 (HXe) as a function of acquisition parameters. Methods The HXe MRI scans with simultaneous gas‐phase and dissolved‐phase excitation were performed using 1‐dimensional projection scans in mechanically ventilated rabbits. The dissolved‐phase magnetization was periodically saturated, and the dissolved‐phase xenon uptake dynamics were measured at end inspiration and end expiration with temporal resolutions up to 10 ms using a Look‐Locker‐type acquisition. The apparent alveolar septal wall thickness was extracted by fitting the signal to a theoretical model, and the findings were compared with those from the more commonly use chemical shift saturation recovery MRI spectroscopy technique with several different delay time arrangements. Results It was found that repeated application of RF saturation pulses in chemical shift saturation recovery acquisitions caused exchange‐dependent gas‐phase saturation that heavily biased the derived SWT value. When this bias was reduced by our proposed method, the SWT dependence on lung inflation disappeared due to an inherent insensitivity of HXe dissolved‐phase MRI to thin alveolar structures with very short T2∗. Furthermore, perfusion‐based macroscopic gas transport processes were demonstrated to cause increasing apparent SWTs with TE (2.5 μm/ms at end expiration) and a lung periphery‐to‐center SWT gradient. Conclusion The apparent SWT measured with HXe MRI was found to be heavily dependent on the acquisition parameters. A method is proposed that can minimize this measurement bias, add limited spatial resolution, and reduce measurement time to a degree that free‐breathing studies are feasible.

Rationale and Objective: In this study, we compared a newly developed multi-breath simultaneous alveolar oxygen tension and apparent diffusion coefficient (P A O 2-ADC) imaging sequence to a single-breath acquisition, with the aim of mitigating the compromising effects of inter-voxel flow and slow-filling regions on single-breath measurements, especially in COPD subjects. Materials and Methods: Both single-breath and multi-breath simultaneous P A O 2-ADC imaging schemes were performed on a total of ten human subjects (five asymptomatic smokers and five COPD subjects). Estimated P A O 2 and ADC values derived from the different sequences were compared both globally and regionally. The distribution of voxels with non-physiological values was also compared between the two schemes. Results: The multi-breath protocol decreased the ventilation defect volumes by an average of 12.9±6.6%. The multi-breath sequence generated non-physiological P A O 2 values in 11.0±8.5% fewer voxels than the single-breath sequence. Single-breath P A O 2 maps also showed more regions with gas-flow artifacts and general signal heterogeneity. On average, the standard deviation of the P A O 2 distribution was 16.5±7.0% lower using multi-breath P A O 2-ADC imaging, suggesting a more homogeneous gas distribution. Both mean and standard deviation of the ADC increased significantly from single-to multi-breath imaging (P=0.048 and p=0.070, respectively), suggesting more emphysematous regions in the slow-filling lung. Conclusion: Multi-breath P A O 2-ADC imaging provides superior accuracy and efficiency compared to previous imaging protocols. P A O 2 and ADC maps generated by multi-breath imaging allowed for the qualification of various regions as emphysematous or obstructed which singlebreath P A O 2 maps can only identify as defects. The simultaneous P A O 2 and ADC measurements