Table 1. Comparisons and Correlations between Percentages of CT Right Pulmonary Vascular Volumes Quantified with Fixed Attenuation Thresholds and Nuclear Medicine Right Lung Perfusion Percentages.

Fixed Threshold	80	120	160	200	240	280	320*	360*	400*	440	480	520	560	600
CT right pulmonary vascular volume percentage (%)	65.8 ± 13.8	66.1 ± 14.1	66.5 ± 14.5	67.1 ± 14.9	67.6 ± 15.4	67.9 ± 15.9	68.4 ± 16.5	69.3 ± 17.0	70.0 ± 17.2	70.4 ± 16.9	69.6 ± 18.9	68.7 ± 21.0	69.3 ± 24.6	69.8 ± 26.0
p-value of paired t test	< 0.001	< 0.001	< 0.003	< 0.02	> 0.06	> 0.1	> 0.4	> 0.7	> 0.2	> 0.2	> 0.5	> 0.8	> 0.6	> 0.6
Pearson correlation coefficient	0.93	0.93	0.93	0.93	0.93	0.93	0.93	0.93	0.93	0.90	0.80	0.69	0.63	0.48
Mean absolute difference (%)	5.4	5.3	5.0	4.8	4.6	4.6	4.7	4.6	4.9	5.6	9.1	12.7	17.9	22.1

^*Optimal thresholds for CT pulmonary vascular volume ratio showing smallest mean absolute difference (≤ 5.0%), lowest level of statistically significant difference compared to reference values measured by lung perfusion scintigraphy, and highest Pearson correlation coefficient. HU = Hounsfield units