Simultaneous quantification of hyperpolarized xenon-129 ventilation and gas exchange with multi-breath xenon-polarization transfer contrast (XTC) MRI

PURPOSE

To demonstrate the feasibility of a multi-breath xenon-polarization transfer contrast (XTC) MR imaging approach for simultaneously evaluating regional ventilation and gas exchange parameters.

METHODS

Imaging was performed in five healthy volunteers and six chronic obstructive pulmonary disease (COPD) patients. The multi-breath XTC protocol consisted of three repeated schemes of six wash-in breaths of a xenon mixture and four normoxic wash-out breaths, with and without selective saturation of either the tissue membrane or red blood cell (RBC) resonances. Acquisitions were performed at end-exhalation while subjects maintained tidal breathing throughout the session. The no-saturation, membrane-saturation, and RBC-saturation images were fit to a per-breath gas replacement model for extracting voxelwise tidal volume (TV), functional residual capacity (FRC), and fractional ventilation (FV), as well as tissue- and RBC-gas exchange (fMem and fRBC , respectively). The sensitivity of the derived model was also evaluated via simulations.

RESULTS

With the exception of FRC, whole-lung averages for all metrics were decreased in the COPD subjects compared to the healthy cohort, significantly so for FV, fRBC , and fMem . Heterogeneity was higher overall in the COPD subjects, particularly for fRBC , fMem , and fRBC:Mem . The anterior-to-posterior gradient associated with the gravity-dependence of lung function in supine imaging was also evident for FV, fRBC , and fMem values in the healthy subjects, but noticeably absent in the COPD cohort.

CONCLUSION

Multi-breath XTC imaging generated high-resolution, co-registered maps of ventilation and gas exchange parameters acquired during tidal breathing and with low per-breath xenon doses. Clear differences between healthy and COPD subjects were apparent and consistent with spirometry.

Investigating the impact of RF saturation-pulse parameters on compartment-selective gas-phase depolarization with xenon polarization transfer contrast MRI

PURPOSE

To demonstrate the utility of continuous-wave (CW) saturation pulses in xenon-polarization transfer contrast (XTC) MRI and MRS, to investigate the selectivity of CW pulses applied to dissolved-phase resonances, and to develop a correction method for measurement biases from saturation of the nontargeted dissolved-phase compartment.

METHODS

Studies were performed in six healthy Sprague-Dawley rats over a series of end-exhale breath holds. Discrete saturation schemes included a series of 30 Gaussian pulses (8 ms FWHM), spaced 25 ms apart; CW saturation schemes included single block pulses, with variable flip angle and duration. In XTC imaging, saturation pulses were applied on both dissolved-phase resonance frequencies and off-resonance, to correct for other sources of signal loss and compromised selectivity. In spectroscopy experiments, saturation pulses were applied at a set of 19 frequencies spread out between 185 and 200 ppm to map out modified z-spectra.

RESULTS

Both modified z-spectra and imaging results showed that CW RF pulses offer sufficient depolarization and improved selectivity for generating contrast between presaturation and postsaturation acquisitions. A comparison of results obtained using a variety of saturation parameters confirms that saturation pulses applied at higher powers exhibit increased cross-contamination between dissolved-phase resonances.

CONCLUSION

Using CW RF saturation pulses in XTC contrast preparation, with the proposed correction method, offers a potentially more selective alternative to traditional discrete saturation. The suppression of the red blood cell contribution to the gas-phase depolarization opens the door to a novel way of quantifying exchange time between alveolar volume and hemoglobin.

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

PURPOSE

To demonstrate the feasibility of generating red blood cell (RBC) and tissue/plasma (TP)-specific gas-phase (GP) depolarization maps using xenon-polarization 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.

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

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 T 2 ∗ . 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.

Assessment of flip angle-TR equivalence for standardized dissolved-phase imaging of the lung with hyperpolarized 129Xe MRI

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, TR_90°,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, TR_90°,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.