Hyperpolarized (HP) gas MRI offers unique ways to observe and quantify regional pulmonary function, where it is either impossible or impractical to do using other imaging and non-imaging techniques. The three primary pulmonary function measurements which can me probed using HP gas MRI are as follows:
Conventional pulmonary tests such as spirometry can only assess global lung ventilation, whereas hyperpolarized gas MRI provides a method to measure regional lung ventilation at a high resolution in a quantifiable, safe and non-invasive manner
Obstructive lung diseases such as emphysema, asthma, and cystic fibrosis adversely affect lung ventilation, and the effect can be regional (hence heterogeneous) and change over time. High-resolution, quantitative imaging of regional lung ventilation provides a more sensitive means to evaluate the location and extent of disease and its distribution in the lungs, without exposing the patient to harmful radiation from CT scans
Below is a 3D ventilation of a healthy human subject acquired with HP Xe MRI:
Ventilation defects can be accordingly imaged and quantified by combing the HP gas ventilation map and the standard 1H chest scan, as demonstrated here in a COPD subject:
With detailed regional ventilation information from HP Gas MRI, physicians can provide a more targeted and personalized therapeutic intervention, for example, in guiding the placement of a bronchial stent or to assess the efficacy of therapeutics on localized lung function improvement.
HP Gas MRI is unique in its ability to measure lung microstructure dimensions at the alveolar level.
Hyperpolarized gases in the pulmonary undergo a Brownian random motion in the airways. Their average traveled distance during a given timeframe can be measured using diffusion-weighted MRI techniques, and are referred to as Apparent Diffusion Coefficient, ADC. The variation and heterogeneity of gas ADC in lungs is a function of the heterogeneous lung microstructure and the size of small airways through which gas atoms navigate. Therefore ADC measurements provide a sensitive and reproducible marker for early detection and analysis of progression of lung diseases and other processes affecting the size of alveoli and small airways.
The example here shows the ADC map of a middle slice of a healthy subject, along with two COPD patients, one with and one without an apparent defect observed on the CT scan.
ADC has the potential to be used as a clinical endpoint in COPD and other pulmonary disease clinical trials, as it is quantifiable and can be performed without the risk of radiation.
Gas exchange is the most critical function of the lung. Many lung diseases, and especially interstitial pulmonary disorders like idiopathic pulmonary fibrosis, alter the normal pulmonary gas exchange function.
Hyperpolarized Gas MRI is well suited to measure gas exchange globally, regionally, and at the alveolar level. It is much more versatile and precise as compared to other available pulmonary function tests for this purpose.
CT scans can be very detailed in outlining lung morphology and for identifying lung regions with altered ventilation due to radiologically identifiable features. This imaging technique however exposes the patient to radiation, limiting its use in pediatric populations and longitudinal studies. HP Gas MRI offers better functional resolution, and with no radiation.
The pulmonary function test, DLCO, measures pulmonary gas exchange globally, but is easily confounded by the lungs’ regional compensatory mechanisms. HP Gas MRI provides a direct method to measure and quantify regional gas exchange at the alveolar level.
Regional gas exchange imaging is enabled by the unique NMR properties of HP 129Xe. Once inhaled, xenon diffuses through the alveolar membrane and dissolves in the pulmonary blood and tissues, while also shifting its resonant frequency, which can be detected by MRI. Dissolved 129Xe MRI is now possible, providing a spatially resolved, non-invasive equivalent of the DLCO test. This opens an entirely new avenue for studying lung function and diagnosing pulmonary disease, especially those which directly compromise membrane function such interstitial and fibrotic pulmonary diseases.
Below is an example of the gas exchange maps acquired using HP Xe MRI in a healthy, elderly and a COPD patient.