Investigating biases in the measurement of apparent alveolar septal wall thickness with hyperpolarized 129Xe MRI

Abstract

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.

Measuring pulmonary gas exchange using compartment‐selective xenon‐polarization transfer contrast (XTC) MRI

“…Furthermore, acquiring all data at end expiration greatly reduces motion artifacts due to involuntary exhalation and, because of the higher lung tissue density, increases the GP depolarization per contrast‐generating RF pulse. Although the observed GP depolarization increase of almost a factor 2.7 relative to an inspiratory measurement looks high, it seems to be fairly consistent with the increase in the DP:GP ratio in rabbits when lung inflation is changed from end inspiration to end expiration 38 . It is conceivable that the repeated inhalation of xenon gas is filling poorly ventilated regions with high tissue densities, which might pull up the average GP depolarization in the process.…”

“…The XTC MRI method, on the contrary, only detects xenon that actually returns from the tissue to the GP; because the XTC MRI maps lack a transport component, their interpretation is more straightforward. 38 The gas volumes quantified by these two techniques can, therefore, be quite different. Furthermore, the depicted signal intensities themselves are affected by the MRI acquisition parameters to a different degree.…”

“…Although the observed GP depolarization increase of almost a factor 2.7 relative to an inspiratory measurement looks high, it seems to be fairly consistent with the increase in the DP:GP ratio in rabbits when lung inflation is changed from end inspiration to end expiration. 38 It is conceivable that the repeated inhalation of xenon gas is filling poorly ventilated regions with high tissue densities, which might pull up the average GP depolarization in the process. While no substantive conclusions can be drawn from a single data point, high GP depolarization rates per RF pulse, if confirmed, are highly desirable, as they increase the sensitivity of the technique: less than half as many RF pulses are required for optimal contrast-to-noise ratios, which shortens the contrast generation period and minimizes signal loss due to inherent GP T1 decay.…”

“…Consequently, the DP signal of the direct imaging technique observes the xenon accumulation in the lung tissue, including those volumes that are being transported away by the blood stream. The XTC MRI method, on the contrary, only detects xenon that actually returns from the tissue to the GP; because the XTC MRI maps lack a transport component, their interpretation is more straightforward 38 . The gas volumes quantified by these two techniques can, therefore, be quite different.…”

In vivo methods and applications of xenon-129 magnetic resonance

“…In IPF patients, pronounced septal thickening has been observed, consistent with pulmonary fibrosis [106] , [227] . Model-derived lung function metrics are sensitive to the lung inflation level [231] , [232] , [317] , and the septal thickness correlates with the clinical standard pulmonary function test for gas exchange (T LCO ) [106] , [232] and was found to be repeatable in patients with COPD [231] .…”