Quantitative assessment of the spatial distribution of low attenuation areas on X-ray CT using texture analysis in patients with chronic pulmonary emphysema

An automated and quantitative assessment of the spatial distribution of low attenuation areas (LAA) on X-ray CT was performed using texture analysis in chronic pulmonary emphysema (CPE) patients. LAA was defined as those areas having a density less than the mean minus one SD of the control (-960 HU). The probability of change from non-LAA to LAA between a given pair of pixels with horizontal intervals of i pixels (RNi) was evaluated, because this reflects the interaction between LAA and non-LAA regions with different resolutions. The relationship between the percentage area of the LAA over the total area of the entire lung field (LAA%) was subsequently estimated. The RNi increased sharply as the i value increased from 1 to 5, but then almost became a plateau for i values larger than 5. This suggests that the fundamental structures in the LAA areas ranged from 1 x 1 to 5 x 5 pixels in size. RN1-LAA% and RN5-LAA% plots produced curves which were convex, with peak values at approximately 50 LAA% of 0.09 and 0.18, respectively. In the RN5/RN1-LAA% plot, the RN5/RN1 ratio remained constant at 2.0 regardless of the LAA%. A random process simulation was performed to determine the patterns of LAA proliferation if the spatial distribution of the LAA units was random. When the unit size was kept constant, the results of the simulation did not fit the empirical relationship between the LAA% and the three parameters (RN1, RN5 and RN5/RN1). The simulation provided the best-fitting curves when the unit size of the LAA increased in proportion with the LAA%, starting from a 1 x 1 pixel size increasing at a ratio 1 x 1/(5 LAA%). This suggested that the LAA units do not proliferate randomly in spatial orientation at a fixed unit size, but rather spread throughout the whole lung field in a congregated form whilst increasing their unit size. Thus, it may be concluded that healthy lung tissues near emphysematous lesions have a high probability of suffering from emphysema in the future. This may be due to a direct effect of the neighboring emphysematous lesion or due to a pathologic change in the larger bronchii which dominate both the healthy tissues and the emphysematous lesions.