CT-based assessment of regional pulmonary microvascular blood flow parameters.

To determine regional pulmonary microvascular mean transit times (MTTs), we used electrocardiogram-gated X-ray computed tomographic imaging to follow bolus radiopaque contrast material through the lungs in anesthetized animals (7 dogs and 1 pig, prone and supine). By deconvolution/reconvolution of regional time-attenuation curves obtained from parenchyma and large lobar arteries, we estimated the microvascular residue function and reconstituted the regional microvascular time-attenuation curves and, thus, regional microvascular MTTs. The mean microvascular MTTs in the supine and prone postures were 3.94 +/- 1.0 and 3.40 +/- 0.84 (mean +/- SD), respectively. The dependent-nondependent vertical gradient of MTT was greater in the supine [slope = 0.25 +/- 0.10 (SD), P < 0.001 by t-test] than in the prone (-0.03 +/- 0.06 in 6 of 8 animals; 2 outliers had positive slopes) posture. In both postures, there was a trend toward faster transit times in the dorsal-basal lung region in six of the eight animals, suggesting gravity-independent higher vascular conductance dorsocaudally. We conclude that deconvolution methods, in association with electrocardiogram-gated high-speed X-ray computed tomography, can provide insights into regional heterogeneity of pulmonary microvascular MTT in vivo.

Subsecond multisection CT of regional pulmonary ventilation.

RATIONALE AND OBJECTIVES: To evaluate the adequacy of multibreath and single-breath stable xenon gas techniques to measure regional ventilation during cardiac-gated, high-speed, multisection imaging, the authors carried out a series of studies using electron-beam computed tomography (CT) and a recently introduced subsecond multisection spiral CT scanner. MATERIALS AND METHODS: In four anesthetized pigs, the authors implemented single-breath and/or dynamic multibreath wash-in and washout protocols with respiratory-- and cardiac-gated image acquisition. The effects of varying tidal volume and inspiratory flow rate were evaluated independently. Scanning was done at end expiration to avoid artifacts from partial volumed conducting airways, which are filled with inspired gas concentration during inspiration. RESULTS: A single breath of 100% xenon provides adequate enhancement in the lung parenchyma (mean, 32 HU +/- 1.85 [standard error]) and should not cause unwanted side effects (mean xenon concentration in lung periphery, 21%). The single-breath method is suitable for studies requiring only short periods of apnea. Using the multibreath method, in dependent portions of the lung, there was close agreement between measured changes and predictions based on the xenon calibration data. More than 10 breaths were needed to clear tracer from poorly ventilated areas, and some nondependent regions demonstrated apparently "linear" rather than exponential clearance curves, possibly reflecting longer washout times. Analysis of wash-in and washout curves revealed vertical ventilation gradients and, at higher inspiratory flow rates, redistribution of ventilation to areas of the lung with greater pathway conductance. CONCLUSION: With careful attention to lung volume and use of cardiac gating, it is now possible to correlate lung structure with function to a degree heretofore not possible.