Quantification of regional ventilation-perfusion ratios with PET.
The topographic matching of alveolar ventilation (V(A)) and perfusion (Q) is the main determinant of gas exchange efficiency of the lung. However, no pulmonary functional imaging technique has been shown to predict whole-lung gas exchange in health and disease. This study aims to present a PET-based method to estimate regional alveolar ventilation-to-perfusion ratios (V(A)/Q) predictive of arterial blood gases.


METHODS
The method is based on the regional tracer kinetics of (13)N-nitrogen ((13)NN) after an intravenous bolus injection during a breath-hold period and subsequent washout from the lungs with resumption of breathing. The method takes into account the presence of inter- and intraregional nonuniformities at length scales smaller than the imaging spatial resolution. An algorithm used regional tracer washout to classify regional V(A)Q/ uniformity. Intraregional V(A)/Q mismatch in nonuniform regions was described with a 2-compartment model. Regional V(A)/Q estimates were combined into a whole-lung distribution of V(A)/Q ratios and were used to compute global arterial blood gases. The method was applied to 3-dimensional PET data from anesthetized and mechanically ventilated sheep before and after methacholine bronchoconstriction (n = 3) and pulmonary embolism (n = 3) and after saline lung lavage (n = 3).


RESULTS
PET images revealed regional changes in ventilation and perfusion consistent with the different disease models. Quantification of the images using PET-derived V(A)Q/ distributions showed unimodal and narrow distributions in control conditions that became wider and unimodal after pulmonary embolism and saline lung lavage and bimodal after bronchoconstriction. Images of regional gas exchange allowed for visualization of regional gas exchange. Arterial blood gases estimated from the PET-based V(A)/Q distributions closely agreed with measured values (partial pressure of oxygen, arterial [PaO(2)]: r(2) = 0.97, P < 0.001; partial pressure of carbon dioxide, arterial [PaCO(2)]: r(2) = 0.96, P < 0.001).


CONCLUSIONS
Tracer kinetics analysis of PET images after an intravenous injection of (13)NN provides a quantitative assessment of regional V(A)/Q heterogeneity including that corresponding to length scales smaller than the spatial resolution of the imaging method. Quantification of V(A)/Q mismatch obtained with the presented technique is directly related to severity of gas exchange impairment as determined by arterial blood gases.
Vidal Melo
Marcos F
MF
Department of Anesthesia and Critical Care, Massachusetts General Hospital and Harvard Medical School, Boston, 02114, USA. mvidalmelo@partners.org
Layfield
Dominick
D
Harris
R Scott
RS
O'Neill
Kevin
K
Musch
Guido
G
Richter
Torsten
T
Winkler
Tilo
T
Fischman
Alan J
AJ
Venegas
Jose G
JG
D000465:Algorithms; D000818:Animals; D016084:Bronchoconstriction / Q000502:physiology; D003947:Diagnostic Techniques, Radioisotope; D008168:Lung / Q000000981:diagnostic imaging* / Q000503:physiopathology*; D008657:Metabolic Clearance Rate; D008954:Models, Biological; D009590:Nitrogen Radioisotopes / Q000493:pharmacokinetics; D011655:Pulmonary Embolism / Q000000981:diagnostic imaging* / Q000503:physiopathology*; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012756:Sheep; D013223:Statistics as Topic; D014055:Tomography, Emission-Computed / Q000379:methods*; D014692:Ventilation-Perfusion Ratio
D009590:Nitrogen Radioisotopes; D019275:Radiopharmaceuticals