Correction of Quantitative Emphysema Measures with Density Calibration Based on Measurements in the Trachea

Purpose: Emphysema quantification with computed tomography is known to be affected by variations in scanner type, scan protocol and proprietary reconstruction algorithms from manufacturers. We investigated if automatic calibration of density values based on per scan measurements in air outside the body or in the trachea reduced variability between emphysema scores from similar populations examined in a multi-center study. Method and materials: Three cohorts of heavy smokers, 500 subjects each, were randomly taken from lung cancer screening trials in two cities in The Netherlands and one in Denmark. Inclusion criteria, age, smoking history and lung function were comparable. Cohort A was scanned at full inspiration with 16 x 0.75 collimation with a Philips Brilliance scanner, low dose acquisition, reconstructed with a Philips B kernel to 1.0 mm section with 0.7 mm increment. Settings for the other cohorts were similar except that Cohort B was scanned with 1.0 mm increment and a Philips D kernel, and Cohort C was scanned with Siemens Sensation 16 and 64 scanners with a B30 kernel. All scans were processed with in-house developed software that segmented the lungs, excluded the airways, and computed the 15th percentile (PD15) emphysema measurement. The software also segmented the trachea and the region of air outside the body. Measurements were computed on the original data and on calibrated data. For calibration a Gaussian distribution was fitted to density values obtained from either air outside the body or from the trachea. HU values were shifted so that the peak of this Gaussian distribution coincided with -1000 HU. Results: On the original data, the mean(sd) of PD15 for the three cohorts was: -917(20), -942(21), -930(23) HU. Calibration using air outside the body was not effective: -908(20), -932(21), -921(23). Calibration using air in the trachea, however, almost completely removed the difference between the distributions of PD15: -938(18), -934(21), -932(22). Conclusion: The use of different scanners and reconstruction algorithms results in density variations in low attenuation areas within the body. A calibration procedure based on air measurements in the trachea can largely compensate for these effects. Clinical relevance/application: To obtain comparable emphysema measurements in multi-center studies, air density calibration based on measurements in air containing structures inside the body is essential.