What can computed tomography and magnetic resonance imaging tell us about ventilation?

Abstract

This review provides a summary of pulmonary functional imaging approaches for determining pulmonary ventilation, with a specific focus on multi-detector x-ray computed tomography and magnetic resonance imaging (MRI). We provide the important functional definitions of pulmonary ventilation typically used in medicine and physiology and discuss the fact that some of the imaging literature describes gas distribution abnormalities in pulmonary disease that may or may not be related to the physiological definition or clinical interpretation of ventilation. We also review the current state-of-the-field in terms of the key physiological questions yet unanswered related to ventilation and gas distribution in lung disease. Current and emerging imaging research methods are described, including their strengths and the challenges that remain to translate these methods to more wide-spread research and clinical use. We also examine how computed tomography and MRI might be used in the future to gain more insight into gas distribution and ventilation abnormalities in pulmonary disease.

Fig. 1.

Regional parenchymal deformation maps obtained using CT image registration in transverse (A) and coronal (B) sections for a supine dog at baseline and following acute lung injury (ALI) induced by the infusion of oleic acid into the pulmonary artery. Computed tomography (CT) images were obtained during constant breath holds at 5, 10, 15, and 20 cmH₂O transrespiratory pressure. Registrations were performed for 3 different inflation pressures pairs: 5–10 cmH₂O, 10–15 cmH₂O, and 15–20 cmH₂O. Volume changes are color coded such that yellow, orange, and red correspond to expansion; blue and purple to contraction; and green to no change in volume. Note that during baseline conditions, the lungs exhibited fairly uniform expansion throughout the transverse section for lower inflation pressures, although the registration algorithm does predict small regions of compression occurring near the mediastinum. For more moderate inflation pressures, expansion occurs preferentially in more dependent regions. Following ALI, expansion fields are more heterogeneous at all inflation pressures, with regions of relative compression interspersed throughout the transverse sections. [Figure modified from Kaczka et al. (36) with permission.]

Fig. 2.

Hyperpolarized ³He-MRI coregistered with ¹H-MRI as well as 4DCT acquired within 7 days in a single non-small cell lung cancer patient. Top: axial reconstructions; bottom: coronal slices. Magnetic resonance imaging (MRI) was obtained in the coronal plane during inspiration breath hold, after inhalation of a 1 liter gas mixture of hyperpolarized ³He and ultra-high purity N₂ gas from functional residual capacity. ¹H-MRI preceded ³He-MRI by ∼5 min and the ¹H- and ³He-MRI slices were rigidly coregistered using the carina for fiducial landmarks. To generate the 4DCT ventilation maps, first, thoracic CT images were acquired during a single tidal breathing maneuver (120 kV, 60–120 mA, rotation time of 0.5 s, 360° reconstruction, pitch <0.1). CT images were reconstructed at 10 different respiratory phases and tagged as a percent of full inspiration, with in plane resolution of 1.0 mm. Deformable image registration methods were used to generate a ventilation map. Coregistered ¹H (gray scale)- and ³He-MRI gas distribution images show gas in red, and focal ventilation defects are clearly shown where the ¹H thoracic cavity is exposed (in black) in the absence of gas. 4DCT ventilation maps are color-coded differently such that red, orange, and yellow correspond to regions of greater ventilation; and blue, purple, and black correspond to regions of lower or no ventilation with green representing average values. Note that there is a qualitative regional correspondence between 4DCT regions of decreased ventilation and MRI regions of ventilation defects.

Fig. 3.

Hyperpolarized ³He-MRI of gas distribution (in red) coregistered to ¹H-MRI of thoracic cavity (gray scale) of a healthy young never-smoker (A), older never-smoker (B), asthma (C), and COPD (D). MRI was obtained in the coronal plane during inspiration breath hold, after inhalation of a 1 liter gas mixture of hyperpolarized ³He and ultra-high purity N₂ gas from functional residual capacity. ¹H-MRI preceded ³He-MRI by ∼5 min and the ¹H- and ³He-MRI slices were coregistered using rigid registration methods using the carina for fiducial landmarks. Coregistered ¹H (gray scale)- and ³He-MRI gas distribution images show gas in red, and focal ventilation defects are clearly shown where the ¹H thoracic cavity is exposed (in black) in the absence of gas. Note the absence of gas distribution abnormalities in A for the healthy young never-smokers, but there are qualitative differences for the elderly never-smoker and much more obvious abnormalities in asthma (C) and COPD (D). Some qualitative differences can also be observed in the gas distribution obvious in the trachea, although for all subjects, the breath-hold maneuver was the same.

Fig. 4.

Conventional ¹H-, hyperpolarized ³He- and ¹²⁹Xe-MRI of a single subject each with COPD and asthma. ¹H-MRI of the thoracic cavity (gray scale) was obtained in the coronal plane during inspiration breath hold, after inhalation of 1 liter of ultra-high purity N₂ gas from functional residual capacity (FRC). Within 3 min, the same subject inhaled a 1 liter mixture of hyperpolarized ³He gas mixed with N₂ gas from FRC for static ventilation imaging of ³He gas distribution (shown in red) acquired in a 15 s breath hold. Approximately 3 min later, the subject inhaled a 1 liter mixture of hyperpolarized ¹²⁹Xe gas mixed with ⁴He from FRC for static ventilation imaging of ¹²⁹Xe gas distribution (shown in purple) acquired in a 15 s breath hold. Note the qualitative differences in gas distribution abnormalities that are obvious between ¹²⁹Xe- and ³He-MRI for both subjects. It is also important to note that the exact etiology of such gas distribution abnormalities has not yet been established. Such differences between ³He- and ¹²⁹Xe-MRI gas distribution may be attributable to differences in the gas properties such as diffusion, viscosity, or perhaps tissue solubility.