To determine the relationship between the pulmonary function test (PFT) and the optimal threshold value for the emphysema index at volumetric CT in chronic obstructive lung disease (COPD).

Volumetric CT scans were performed in 53 patients with COPD at inspiration and expiration using the same CT scanner. By using the in-house software, the emphysema index, which included the threshold value between -700 and -990 HU, was calculated automatically. The data were analyzed by calculating the Pearson correlation coefficient between each set of CT data and the forced expiratory volume occurring over 1 second (FEV1), the forced expiratory volume occurring over 1 second over the vital capacity ratio (FEV1/FVC), and the diffusion capacity of carbon monoxide (DLco).

The strongest correlation between FEV1 and FEV1/FVC was found to have a threshold of -940 HU at inspiration and -910 HU during an expiration CT scan. The strongest correlation with DLco was observed with a threshold of -970 HU at inspiration and -960 HU at expiration.

The threshold showing the best correlation with the PFT parameters was variable. As for the inspiration CT scan, the threshold area, showing a relatively good correlation with PFT, occurred over -940 to -970 HU. Hence, it is pertinent that -950 HU or -960 HU was a common benchmark used for threshold of the emphysema index.