Lung tissue density measured by low-dose CT as a part of pulmonary perfusion SPECT/CT could differentiate pulmonary embolism from chronic obstructive pulmonary disease

Objectives Interpretation of lung perfusion scintigraphy in patients with suspected pulmonary embolism (PE) is difficult, especially in the presence of chronic obstructive pulmonary disease (COPD) and most often has to be combined with pulmonary ventilation scintigraphy. We investigated the data from the CT portion of pulmonary perfusion SPECT/CT to try to solve this problem.

Methods We retrospectively assessed data from 12 patients (4 m, 8 f, mean age 68y), 6 with PE, 6 with COPD. The final diagnosis was based on ventilation/perfusion scintigraphy. Perfusion scintigraphy was performed with a hybrid gamma camera with non-diagnostic CT (GE Infinia Hawkeye). Lung tissue density in the areas of perfusion defects on SPECT was evaluated from the CT data in Hounsfield units (HU), radioactivity in counts. We analyzed 32 areas with a perfusion defect in patients with PE and 25 areas in patients with COPD. Data from both groups were compared with non-parametric Kolmogorov-Smirnov test (not-normal distribution) for two independent samples, p<0.05 was considered statistically significant.

Results The mean lung tissue density in perfusion defects caused by PE was -695 HU (range from -829 to -516), in defects caused by COPD -900 HU (range from -973 to -802). The mean radioactivity in perfusion defects caused by PE was 51 counts (range from 13 to 127), in defects caused by COPD 32 counts (range from 10 to 80). Both the density and radioactivity differed significantly (p for density = 0.000, p for radioactivity = 0.041).

Conclusions Lung tissue density measured by CT is significantly lower in perfusion defects caused by COPD than in those caused by PE and the overlap of values is minimal. We believe this data could be used as an adjunct in interpretation of pulmonary perfusion studies and potentially help avoid the performance of ventilation scintigraphy in patients with COPD