Lung functional imaging

Abstract

Pulmonary functional imaging modalities such as computed tomography, magnetic resonance imaging and nuclear imaging can quantitatively assess regional lung functional parameters and their distributions. These include ventilation, perfusion, gas exchange at the microvascular level and biomechanical properties, among other variables. This review describes the rationale, strengths and limitations of the various imaging modalities employed for lung functional imaging. It also aims to explain some of the most commonly measured parameters of regional lung function. A brief review of evidence on the role and utility of lung functional imaging in early diagnosis, accurate lung functional characterisation, disease phenotyping and advancing the understanding of disease mechanisms in major respiratory disorders is provided.

FIGURE 1

Successive steps involved in the acquisition and computation of a parametric map of a local functional parameter, such as lung ventilation measured by dual energy computed tomography imaging. a) A source of contrast is quantified within the image. b) Image acquisition is repeated over short periods of time. c) A mathematical model is usually used to fit the temporal dynamics of contrast as a function of time. d) The set of images is converted to a parametric map. e) Imaging of tidal expansion and alveolar ventilation assesses different lung function parameters. Changes in lung volume are related to the airflow at the airway opening, tidal volume, ventilation mechanics, and relative volume expansion, commonly referred to as lung strain. By contrast, alveolar ventilation is related to gas exchange and transport, such as oxygen and carbon dioxide.

FIGURE 2

a) Registration-based computation of local volume change and deformation. Computed tomography images acquired at total lung capacity (TLC) and residual volume (RV) are first segmented. The segmented images are superimposed using rigid image registration. Greyscale inverted to highlight the deformation between the two states. Non-rigid registration algorithms are then applied to warp the RV to fit the TLC image. b, c) lung image attenuation, specific volume change (sΔV) and Jacobian determinant (J) in b) a Global Initiative for Chronic Obstructive Lung Disease (GOLD) stage 4 and c) a stage 1 COPD patient. HU: Hounsfield units.

FIGURE 3

Sample hyperpolarised helium-3 (³He) images in a) a normal subject and b) patients with COPD. Note the inhomogeneous distribution of the inhaled ³He gas in the subjects with COPD.

FIGURE 4

Schematic of the positron emission tomography ¹³NN-saline bolus-injection method for assessment of regional perfusion (Q̇) and ventilation (V̇). The ¹³NN tracer reaching the alveoli is proportional to perfusion and diffuses due to its low solubility from the blood into the alveoli. The tracer kinetics during washout using four models allows the assessment of sub-resolution functional compartments. Combining the parameters of ¹³NN kinetics with gas fraction measurements from corresponding computed tomography images yields a substantial number of lung function parameters and their regional differences. ¹³NN: N₂ containing ¹³N; units: MBq.

FIGURE 5

Example of positron emission tomography imaging of ventilation defects (VDefs) during bronchoconstriction in asthma using ¹³NN. The severe airway constriction or closure of airways within VDefs results in the retention of the ¹³NN, delivered by perfusion, inside VDefs, which are affected by body position. The selected slices using a “hot” colour scale show the magnitude of the tracer activity from low (black) to high (white). The green outlines illustrate the identified boundaries of VDefs visualised in three-dimensional renderings as VDef volumes inside the lungs (transparent blue). Images are from the same subject studied on two different days after receiving the methacholine challenge in the prone position on both occasions. Note the tendency of the VDefs location to be in a dependent part of the lungs suggesting an effect of regional parenchymal forces in contrast to aerosol deposition during prone inhalation. Reproduced from [135] with permission. ¹³NN: N₂ containing ¹³N.