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.

Many forms of lung disease manifest themselves as pathological changes in the transport of gas to the circulatory system, yet the difficulty of imaging this process remains a central obstacle to the comprehensive diagnosis of lung disorders. Using hyperpolarized xenon-129 as a surrogate marker for oxygen, we derived the temporal dynamics of gas transport from the ratio of two lung images obtained with different timing parameters. Additionally, by monitoring changes in the total hyperpolarized xenon signal intensity in the left side of the heart induced by depletion of xenon signal in the alveolar airspaces of interest, we quantified the contributions of selected lung volumes to the total pulmonary gas transport. In a rabbit model, we found that it takes at least 200 ms for xenon gas to enter the lung tissue and travel the distance from the airspaces to the heart. Additionally, our method shows that both lungs contribute fairly equally to the gas transport in healthy rabbits, but that this ratio changes in a rabbit model of acid aspiration. These results suggest that hyperpolarized xenon-129 MRI may improve our ability to measure pulmonary gas transport and detect associated pathological changes.

Background Prone ventilation redistributes lung inflation along the gravitational axis; however, localized, nongravitational effects of body position are less well characterized. The authors hypothesize that positional inflation improvements follow both gravitational and nongravitational distributions. This study is a nonoverlapping reanalysis of previously published large animal data. Methods Five intubated, mechanically ventilated pigs were imaged before and after lung injury by tracheal injection of hydrochloric acid (2 ml/kg). Computed tomography scans were performed at 5 and 10 cm H2O positive end-expiratory pressure (PEEP) in both prone and supine positions. All paired prone–supine images were digitally aligned to each other. Each unit of lung tissue was assigned to three clusters (K-means) according to positional changes of its density and dimensions. The regional cluster distribution was analyzed. Units of tissue displaying lung recruitment were mapped. Results We characterized three tissue clusters on computed tomography: deflation (increased tissue density and contraction), limited response (stable density and volume), and reinflation (decreased density and expansion). The respective clusters occupied (mean ± SD including all studied conditions) 29.3 ± 12.9%, 47.6 ± 11.4%, and 23.1 ± 8.3% of total lung mass, with similar distributions before and after lung injury. Reinflation was slightly greater at higher PEEP after injury. Larger proportions of the reinflation cluster were contained in the dorsal versus ventral (86.4 ± 8.5% vs. 13.6 ± 8.5%, P < 0.001) and in the caudal versus cranial (63.4 ± 11.2% vs. 36.6 ± 11.2%, P < 0.001) regions of the lung. After injury, prone positioning recruited 64.5 ± 36.7 g of tissue (11.4 ± 6.7% of total lung mass) at lower PEEP, and 49.9 ± 12.9 g (8.9 ± 2.8% of total mass) at higher PEEP; more than 59.0% of this recruitment was caudal. Conclusions During mechanical ventilation, lung reinflation and recruitment by the prone positioning were primarily localized in the dorso-caudal lung. The local effects of positioning in this lung region may determine its clinical efficacy. Editor’s Perspective What We Already Know about This Topic What This Article Tells Us That Is New

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.

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