GENERAL NUCLEAR MEDICINE Pulmonary Perfusion and Density Gradients in Healthy Volunteers

The goal of this study was to measure regional pulmonary perfusion using SPECT and transmission tomography for attenuation correc tion and density measurements. Methods: Regional pulmonary perfusion was studied after intravenous injection of radiolabeled particles in 10 supine healthy volunteers using SPECT. Transmis sion tomography was used to correct for attenuation, measure lung density and delineate the lungs. The effects of attenuation correction on pulmonary perfusion gradients were investigated. Results: In perfusion measurements not corrected for attenuation, we found significant perfusion gradients in the direction of gravity but also significant gradients at isogravitational level. After correction for attenuation, the gravitational gradient was significantly greater than before correction, and gradients at isogravitational level were no longer observed. Perfusion in the ventral lung zone was half of that in the dorsal lung zone. Mean lung density was 0.28 ±0.03 g/ml, and density showed a significant increase in the direction of gravity and at isogravitational level. Conclusion: We found that SPECT perfusion studies of the lung not corrected for attenuation gave a false impression of nongravitational gradients and underestimate the gradient that is gravity-dependent. Transmission tomography, used for attenuation correction, also quantifies lung density and shows gravity dependent and nondependent density gradients. Key Words: pulmonary circulation, attenuation correction, trans mission tomography, SPEC