Ventilation (V) and perfusion (Q) transport oxygen and carbon in the lungs, and V/Q matching is essential for the efficiency of the gas exchange in the alveoli. However, most pulmonary diseases lead to heterogeneity in V/Q matching reducing the efficiency of the gas exchange. Accurate regional assessments of both ventilation and perfusion are needed to gain deeper insights into the patterns and causes of regional V/Q heterogeneity in different lung diseases.
Our group has developed and refined the 13NN-bolus injection method for ventilation-perfusion imaging using Positron Emission Tomography (PET). This method yields accurate values even under conditions where the distributions of ventilation and perfusion in the lungs are highly heterogeneous. Additionally, it allows the assessment of perfusion heterogeneity not biased by imaging noise.
For 13NN imaging, we monitor the tidal breathing of the subject. Perfusion imaging starts with a breath-hold at the mean lung volume between end-expiration and end-inspiration. Simultaneously with the beginning of the breath-hold, a bolus of 13-NN saline is injected intravenously and the acquisition of the dynamic PET scan is started.
When the 13NN reaches the pulmonary capillaries, it diffuses quickly into the alveoli reaching at the end of the breath-hold a plateau. PET images of the plateau phase are proportional to the relative perfusion showing the heterogeneity in regional perfusion in the lungs (Fig. 1).
After the breath-hold, the tracer kinetics of 13NN shows the washout by alveolar ventilation (Fig. 2). The time constant of the washout function is equal to the specific ventilation defines as alveolar ventilation divided by alveolar volume.
Very low ventilation and complete airway closure result in tracer retention at the end of the washout phase, e.g., a minor degree of retention in the example PET image (Fig. 2). During bronchoconstriction, severe airway narrowing and closure result in ventilation defects (VDefs) with very high tracer retention (Video 1).
The tracer-kinetics functions of the voxels of the PET image of different regions of interest (ROIs) can be dramatically different from each other if the ventilation is very heterogeneous, e.g., during bronchoconstriction in asthma. We developed for identifying the ventilation and perfusion parameters using four different models for curve fitting and a model selection based on the Akaike Information Criterion (AIC) for selecting the model with the best fit (Fig. 3). Remarkably, modeling of tracer kinetic that are the sum of two different functional compartments allows for the assessment of functional heterogeneity beyond the spatial resolution of the PET scanner.
We validated the estimated parameters in large animal studies with major V/Q heterogeneities. Our results show the blood gases predicted blood gases based on V/Q imaging were in excellent agreement with the measured blood gases.
Nature 2005;434(7034):777-82. doi: 10.1038/nature03490.
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