Intrapulmonary distribution of 99mtechnetium labelled ultrafine carbon aerosol (Technegas) in severe airflow obstruction.

Technegas (TG), an ultrafine dispersion of carbon aggregates labelled with 99mTechnetium (99mTc), has been recently introduced for clinical imaging of lung ventilation. In 12 selected subjects with severe chronic airflow limitation (FEV1 = 0.89 +/- 0.22; mean +/- SD, l) we have studied the regional intrapulmonary distribution of TG and compared it quantitatively with that of 133Xenon (133Xe). A 133Xe equilibration image was acquired for 10-15 s during a breathhold at total lung capacity (TLC). Six subjects (Group 1) inspired 100 ml boli of TG or 133Xe from functional residual capacity (FRC) and another 6 subjects (Group 2) inspired 1.0 l of labelled gas from FRC followed by air to TLC at a constant flow rate less than 0.5 l.s-1. Lung images were then acquired with the chest position rigorously controlled. From the equilibration image, upper, middle, lower, central and peripheral regions were defined. Relative regional fractional concentrations (RFC) were then calculated using the equilibration image to correct for ventilated lung volume. In addition, in four of the Group 2 subjects, each lung image was divided into multiple regions (12-17 per lung). The RFC were then calculated as above (RFCM). The highest and lowest RFC were not significantly different between 133Xe and TG in either Group 1 or Group 2 subjects. Similarly the RFCM analysis showed no systematic difference between 133Xe and TG. The ratio of peripheral to central RFC constitutes a penetration index which for TG was 0.99 +/- 0.23 that of 133Xe. Our results indicate that even in the presence of severe airflow limitation the radiolabelled tracer TG mimics the regional distribution of a real gas.