Purpose: To compare mean lung density (MLD) of paired inspiratory and expiratory thin-section CT scans acquired after patient instruction or using spirometric gating.
Materials and methods: 21 patients (13 m, 8 f. median age 59 years, two with normal lung function, 15 with obstructive, 4 with restrictive impairment) underwent thin-section CT. Paired inspiratory and expiratory scans were performed in the upper, middle and lower lung fields. They were acquired after automatic patient instruction with constant intervals between instruction and scan. Spirometrically gated scans were acquired within 4 days at 80% and 20% of vital capacity (VC) which has been determined on the CT scanner in supine position directly prior to the examination using the built-in spirometric option. Semiautomatic segmentation of the lung was performed and MLD was calculated. VC of pulmonary function testing was available for correlation.
Results: Inspiratory MLD was -828 +/- 71 HU with spirometric gating and -818 +/- 87 HU without. Expiratory MLD was -750 +/- 119 HU (increase 78 +/- 58 HU) with spirometric gating and -748 +/- 119 HU (increase 70 +/- 46 HU) without. All differences were non-significant. Intraindividual differences were 10 +/- 21 HU for inspiration and 2 +/- 36 HU for expiration. VC under CT-conditions was significantly smaller than that known from pulmonary function tests (1.9 +/- 0.71 vs. 2.7 +/- 0.81, p < 0.005).
Conclusion: Spirometric gating does not lead to different results for MLD as compared with acquisition of paired inspiratory and expiratory thin-section CT after automatic patient instruction.