performing hyperpolarized xenon-129 inhalation for chemical shift saturation recovery (cssr) measurement

Assessing Lung Function with Hyperpolarized Xenon-129 MRI

Hyperpolarized Xenon-129 (HXe) magnetic resonance imaging (MRI)1 is a technique that offers unique insights into lung structure, function, and gas exchange processes. By dramatically amplifying the magnetization of Xenon gas through spin-exchange optical pumping, HXe MRI achieves an order-of-magnitude improvement in signal-to-noise ratio compared to thermally polarized Xenon MRI2,3,4,5,6. This hyperpolarization enables the direct visualization and quantification of Xenon gas uptake into lung tissue and blood, which would otherwise be undetectable with conventional thermally polarized MRI7.
Chemical shift saturation recovery (CSSR) MR spectroscopy8,9,10,11,12,13 has proven to be one of the most valuable HXe MRI techniques. CSSR involves selectively saturating the magnetization of Xenon dissolved in lung tissue and blood using frequency-specific radiofrequency (RF) pulses. The subsequent recovery of the dissolved-phase (DP) signal as it exchanges with fresh hyperpolarized Xenon gas in the airspaces on a timescale of ms offers important functional information about the lung parenchyma.
Since its development in the early 2000s, the techniques behind CSSR spectroscopy have been progressively refined14,15,16,17,18,19,20,21,22,23. Further, advances in modeling Xenon uptake curves have enabled the extraction of specific physiological parameters, such as alveolar wall thickness and pulmonary transit times10,24,25,26. Studies have shown CSSR&#39;s sensitivity to subtle changes in lung microstructure and gas exchange efficiency in the form of pulmonary abnormalities found in clinically healthy smokers27, as well as in a range of lung diseases, including chronic obstructive pulmonary disease (COPD)18,27,28, fibrosis29, and radiation-induced lung injury30,31. CSSR spectroscopy has also been demonstrated to be sensitive to detect oscillations in the DP signal corresponding to pulsatile blood flow during the cardiac cycle32.
While significant progress has been made, practical challenges remain in implementing CSSR spectroscopy on clinical MRI systems. Scan times requiring single-dose breath holds approaching 10 s may be too long for pediatric subjects33,34 or patients with severe lung disease35,36. Additionally, the technique is susceptible to measurement biases if acquisition parameters such as the order of the saturation delay times or the efficacy of the dissolved-phase saturation are not properly optimized21. To address these limitations and make CSSR more accessible to the broader research community, clear, step-by-step protocols for both conventional breath hold and free-breathing acquisitions, currently under development, are needed.
The objective of this paper is to present a detailed methodology for performing optimized CSSR MR spectroscopy using HXe gas. The protocol will cover polarization and delivery of the Xenon gas, RF pulse calibration, sequence parameter selection, subject preparation, data acquisition, and key steps in data analysis. Examples of experimental results will be provided. It is hoped that this comprehensive guide will serve as a foundation for CSSR implementations across sites and help realize the full potential of this technique for quantifying lung microstructural changes in a range of pulmonary diseases.

Author Spotlight: Advancing Lung Disease Research with Free-Breathing Hyperpolarized Xenon-129 MRI

Quantitative Measure of Lung Structure and Function Obtained from Hyperpolarized Xenon Spectroscopy

Our research focuses on using hyperpolarizing on 129 MRI technology to assess pulmonary function and to investigate lung diseases. The main objective is to see how this imaging technique can lead to better detection, understanding, and tracking of different lung conditions. Most hyperpolarized lung MR imaging is performed during a breath hold.We believe that pulmonary function quantified under such an artificial condition, may not fully reflect gas exchange during regular respiration, and we are testing new free breathing measurements that do not require any breath hold. Current challenges include the optimization of the CSSR acquisition technique for consistency and accuracy in the face of physiological variation and patient compliance. Severely ill or pediatric patients struggle with maintaining lengthy breath holds.The CSSR heart technique appears particularly sensitive to changes in alveolar septal wall thickness. As we and others in the field of demonstrated, alveolar wall thickening can indicate the presence of interstitial edema, inflammation, or pulmonary fibrosis. All metrics derived from CSSR spectroscopy acquisitions are currently based on the temporal dynamics of the signal amplitudes.In the future though, we hope to include changes in peak shape and center frequency in the analysis to obtain a more complete picture of the ongoing gas exchange processes.

Performing Hyperpolarized Xenon-129 Inhalation for Chemical Shift Saturation Recovery (CSSR) Measurement

To perform xenon inhalation using the breath hold method, place a nose clip on the nose of the subject to facilitate breathing through the mouth. At the end of a normal exhalation to functional residual capacity, insert the mouthpiece of the xenon bag into the subject's mouth. Allow the subject to inhale 500 milliliters of xenon gas from the xenon bag.Then remove the mouthpiece and instruct the subject to continue inhaling room air until the lungs are full. At the end of inhalation, ask the subject to raise their thumb. Request the nurse coordinator to verbally convey this information to the scanner operator to start the pulse sequence.After the data acquisition period, instruct the subject to return to normal breathing. Move the subject out of the MRI scanner. Place a face mask over their nose and mouth.Connect the hoses to the mask, and then connect the pre-fitted straps from behind the head to the mask and secure it in place. Move the subject to its original position inside the MRI scanner. Once the subject follows the breathing protocol, ask the nurse coordinator to inform the MRI operator to initiate data acquisition.The nurse coordinator then opens the valves on the gas delivery system and the subject inhales 50 milliliters of hyperpolarized xenon 1-29 mixed with the airflow inside the breathing mask. Instruct the subject to continue breathing for approximately 10 breaths until the volume of xenon gas has been used up for the imaging protocol. Load the CSSR pulse sequence for free breathing.Set the acquisition frequency according to the hyperpolarized xenon gas phase frequency determined during the calibration scan. Adjust the reference voltage to match the value obtained from the calibration scan. Select the wait for user option, or its equivalent, for sequence execution, following the system vendor's operating instructions.Start the MRI scanner sequence. The scanner will pause for user input before data acquisition. Once the nurse coordinator switches the hyperpolarized xenon gas or air mixture, start data acquisition.Ensure the sequence is already running before the subject inhales the first dose of xenon gas. After the three minute data acquisition, or exhaustion of hyperpolarized xenon gas, remove the subject from the MRI scanner. In the young, healthy female, the membrane signal increases rapidly due to the quick filling of thin walls.In the older, lung transplant recipient, the membrane signal rises more gradually reflecting the longer time required to fill the thicker septal walls.

Research

JoVE Journal

Medicine

Hyperpolarized Xenon-129 (HXe) magnetic resonance imaging (MRI) provides tools for obtaining 2- or 3-dimensional maps of lung ventilation patterns, gas ...