Computed Tomography Studies of Lung Ventilation and Perfusion

With the emergence of multidetector-row computed tomography (CT) it is now possible to image both structure and function via use of a single imaging modality. Breath-hold spiral CT provides detail of the airway and vascular trees along with texture reflective of the state of the lung parenchyma. Use of stable xenon gas wash-in and/or wash-out methods using an axial mode of the CT scanner whereby images are acquired through gating to the respiratory cycle provide detailed images of regional ventilation with isotropic voxel dimensions now on the order of 0.4 mm. Axial scanning during a breath hold and gating to the electrocardiogram during the passage of a sharp bolus injection of iodinated contrast agent provide detailed images of regional pulmonary perfusion. These dynamic CT methods for the study of regional lung function are discussed in the context of other methods that have been used to study heterogeneity of lung function

Differences in regional wash-in and wash-out time constants for xenon-CT ventilation studies

Xenon-enhanced computed tomography (Xe-CT) has been used to measure regional ventilation by determining the wash-in (WI) and wash-out (WO) rates of stable Xe. We tested the common assumption that WI and WO rates are equal by measuring WO–WI in different anatomic lung regions of six anesthetized, supine sheep scanned using multi-detector-row computed tomography (MDCT). We further investigated the effect of tidal volume, image gating (end-expiratory EE versus end-inspiratory EI), local perfusion, and inspired Xe concentration on this phenomenon. Results: WO time constant was greater than WI in all lung regions, with the greatest differences observed in dependent base regions. WO–WI time constant difference was greater during EE imaging, smaller tidal volumes, and with higher Xe concentrations. Regional perfusion did not correlate with WI–WO. We conclude that Xe-WI rate can be significantly different from the WO rate, and the data suggest that this effect may be due to a combination of anatomic and fluid mechanical factors such as Rayleigh–Taylor instabilities set up at interfaces between two gases of different densities