Lung MRI
Peder Larson1

1University of California - San Francisco, United States

Synopsis

The goal of this educational session is to provide understanding of the challenges and opportunities in lung MRI, state-of-the-art lung MRI methods, and new approaches in development. It will cover specialized pulse sequences and image reconstruction for 1H MRI, inhaled gas methods, and methods to provide functional measurements of lung perfusion, ventilation, and mechanics.

TARGET AUDIENCE

Imaging scientists and physicians with interests in lung MRI for research and clinical practice

OUTCOME/OBJECTIVES

The goal of this educational session is to provide understanding of the challenges and opportunities in lung MRI, state-of-the-art lung MRI methods, and new approaches in development.

PURPOSE

Pulmonary imaging is critical for evaluation of numerous lung pathologies, but the use of MRI is limited due to technical and physiologic challenges. These include low proton density and short T2* relaxation of lung parenchyma, high susceptibility differences, and motion. MRI for lung imaging would be advantageous compared to CT since it requires no ionizing radiation, making it especially appealing for radio-sensitive populations (e.g. pediatrics, pregnant women), for longitudinal studies, and for low-risk populations (e.g. asthma). In additional to morphological lung information, MRI can also provide functional measurements of lung perfusion, ventilation, and mechanics.

METHODS

1H MRI methods

Optimized MRI sequences for pulmonary imaging typically use short TEs to address the short-T2* and susceptibility differences, and either rapid scanning methods or motion management to minimize artifacts. Ultrashort and zero echo time (UTE and ZTE) pulse sequences have been especially successful for lung imaging[1–3], improving lung parenchyma signal and also providing motion management with center-out k-space trajectories. In addition to visualization of lung parenchyma, these sequences have also been used for resolving motion during respiration[4,5]. Rapid short TE pulse sequences with gradient-echoes and SSFP have also been successful for lung imaging[6,7]. With these methods, functional measurements of lung ventilation and perfusion can be assessed using several approaches. Observing the dynamics of respiration can be used to extract functional measurements[6,8,9]. Breathing 100% O2, so called Oxygen-enhanced MRI, also provides functional measurements[10–12].

Inhaled Gas methods

Lung MRI can also be performed using inhaled gases that are MR-visible. The most common approaches are hyperpolarized Helium-3, hyperpolarized Xenon-129, and Flourine-19 labeled gases. The hyperpolarization of 3He and 129Xe are created using spin-exchange optical pumping[13], and provide MR signal enhancements. With these methods, the airspace in the lung can be visualized to measure ventilation. 129Xe is also absorbed into the tissue and blood, so can provide additional measurements of gas exchange and perfusion[14].

RESULTS

1H MRI[15–20] and inhaled gas MRI has been successfully applied to a broad range of diseases, including cystic fibrosis, pulmonary nodules, pulmonary embolism, COPD, bronchopulmonary dysplasia, and asthma, in both adults and pediatrics[21,22].

DISCUSSION

1H MRI techniques have the advantage that they can be directly translated to any MRI scanner without modification, whereas inhaled gas MRI techniques require additional hardware (RF coils, enriched gasses, and/or hyperpolarizers) and the expertise to use these systems. Several 1H MRI techniques are available either through current pulse sequences or through research sequences. The major challenge remaining is the reliable management of motion, particularly because subjects with lung disease have difficulty with regular breathing. Pediatrics is also a very promising application area, but children also have difficulty staying still unless sedation is used. However, advanced reconstructions are rapidly mitigating these motion issues.

Inhaled gas MRI has the advantage that they provide direct images of functional lung characteristics such as ventilation, perfusion and gas exchange[23,24]. Currently Phase III clinical trials are being performed with Hyperpolarized 129Xe, and there is a clinical trial consortium now established for this technology. Hyperpolarized 3He is no longer commonly used due to supply shortages. 19F labelled gasses are in early stages of development, but have advantages that no hyperpolarizer is required and can use some existing 1H RF hardware because the gyromagnetic ratio is similar[25].

CONCLUSION

Structural lung MRI has become much more reliable with improved SNR due to pulse sequence and image reconstruction advances, particularly with UTE methods. Functional lung MRI, particularly with hyperpolarized 129Xe, is also progressing such that it can be applied to a great range of clinical studies.

Acknowledgements

No acknowledgement found.

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Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)