ERS 2022: Hyperpolarized Gas MRI: A New Frontier in Functional Pulmonary Imaging
In this session, the panel of experts discussed the recent advancements in lung imaging and the role of hyperpolarised gas magnetic resonance imaging (MRI) for lung imaging in patients with various respiratory diseases.
The first talk focused on the evolution of hyperpolarised gas magnetic resonance imaging (MRI) for lung imaging patients with various respiratory diseases. 1H MRI (and CT) cannot resolve small airways in vivo. Due to the low image signal-to-noise and short T2* caused by magnetic susceptibility differences between air and lung parenchyma, pulmonary MRI has historically had a limited clinical impact. While CT remains the gold standard for structural lung imaging in many clinical indications, the development of ultrashort and zero echo time (UTE/ZTE) acquisition techniques has allowed 1H pulmonary MRI to advance to the point where it is now clinically recommended for certain disorders. As inhaled MRI contrast agents, hyperpolarised (HP) gases, 3HE and 129Xe, provide a wealth of complementary information about the function and microstructure of the lung. They have great potential as a clinical tool for early detection and understanding of certain lung diseases' pathophysiology. Evidence suggests that when performed as a part of a multidisciplinary team (MDT) evaluation of severe disease, 129Xe MRI can offer additional unique and valuable information in the evaluation of clinical presentations of asthma.
The symptoms of long COVID are worse and can keep fluctuating. The talk discussed the role of hyperpolarised gas MRI in evaluating persistent symptoms in patients with long COVID. Long COVID is very similar to another post-viral syndrome. About 1.8 million people in the UK have self-reported long COVID in 2022. Long COVID affects the younger working population.
In some cases of long COVID, the reason for breathlessness is quite clear, and they require hospital admission. However, in most cases, the CT scans or other investigations are unremarkable. The main cause of breathlessness in long COVID can include lung damage, pulmonary vasculature abnormalities, breathing pattern disorders, post-COVID airways disease, deconditioning, pain, and fatigue. Hyperpolarised 129Xe MRI provides information about lung structure and function. In addition, it can evaluate regional lung abnormalities not necessarily apparent using global measures.
Studies indicate that it can identify abnormalities below the resolution of conventional investigations. In a prospective study of 11 participants, non-hospitalized long COVID patients had near-normal CT scores. However, the diffusing capacity of the lung for carbon monoxide was significantly lower between non-hospitalized long COVID and post-hospitalized COVID patients, potentially indicating a decrease in lung function but not structure. A study by Matheson AM et al. showed that 129Xe MRI measurements were lower in hospitalized participants with post- acute COVID-19-syndrome compared to controls. Lower 129Xe MRI measurements were associated with CT pulmonary vascular density, DLCO, exercise capacity, and dyspnea.
The role of hyperpolarised gas MRI in guiding patient management in severe asthma and COPD has been discussed in the next talk. Ventilation defects are regionally heterogeneous and patient-specific. Svenningsen S et al. showed that patients with asthma with 3He ventilation defects were older, had significantly worse airway hyper-responsiveness, inflammation and airway remodelling but had similar FEV1 as patients with asthma without defects; hyperpolarised 3He ventilation abnormalities were spatially and quantitatively associated with abnormally remodelled airways. Kirby M et al. demonstrated that ventilation defect percentages (VDP) obtained with hyperpolarised 129Xe MR imaging were significantly greater than that with 3He MR imaging in patients with COPD, indicating incomplete or delayed filling of lung regions that may be related to the different properties of 129Xe gas and physiologic and/or anatomic abnormalities in COPD.
COPD biomarkers are urgently required for clinical trials of new therapies. Another study by Kirby M et al. assessed the longitudinal change and relationship of MRI and CT biomarkers of COPD with St. George's
Respiratory Questionnaire (SGRO) and FEV1 worsening over 30 months. Among imaging biomarkers, only the longitudinal change in MRI VDP was more significant in the ever-smoker and COPD subgroups compared to a never-smoker. Only the longitudinal change in VDP was related to the change in SGRQ. Only baseline VDP predicted the longitudinal change in SGRQ>minimum clinically significant difference in mild-to-moderate COPD.
MRI VDP may be more sensitive to treatment in asthma and COPD than the gold-standard spirometry measurements of airflow obstruction. Therefore, similar asthma quality of life and control improvements can be achieved with one bronchial thermoplasty (BT) guided by HP gas MRI compared to standard three-treatment-session BT.
HP gas MRI is now positioned to impact clinical management by providing early detection, safe and sensitive monitoring of disease progression and response to therapy. The method has much potential in drug development studies and is already in pharmaceutical research and development pipelines.
European Respiratory Society (ERS) International Congress 2022, 3rd-6th Sept. 2022, Barcelona
