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.