Reproducibility of densitometric measures of emphysema in computed tomography scans one week apart: the effect of breathhold and scanner calibration

RATIONALE: To examine the reproducibility of High-resolution Computed Tomography (HRCT) densitometric measures of emphysema in patients scanned twice one week apart, and to assess the effects of breathhold (measured by CT lung volume) and scanner calibration (measured by separate phantom scans) on reproducibility. METHODS: CT scans were obtained from our anonymized research database for 27 emphysema patients scanned at full inspiration on two separate visits (median 7 days, standard deviation 2.4 days). Patients were instructed on proper breathholding before scanning to maximize breathhold reproducibility. For each patient, the same scanner and protocol was used for both visits. An air and water phantom was scanned on the same scanner within 24 hours of each patient. For each scan, lung segmentation was performed including removal of gross airway structure. Lung volume (V), relative area below -950HU (RA950) and 15th percentile density (PD15) were computed, and mean Hounsfield Unit (HU) of water (Pwater) and air (Pair) were 15 obtained from the corresponding phantom scan. Difference between visits was computed for each quantity (?V, ?RA950, ?PD15, ?Pwater, ?Pair), and Bland-Altman analysis was used to assess reproducibility for V, RA950, and PD15. This analysis was repeated for a subgroup with |?V|<250mL (representing excellent breathhold reproducibility). Next, linear regression was performed for ?RA950 versus ?V, ?P water, and ?Pair, and also for ?PD15 versus ?V, ?Pwater, and ?Pair . RESULTS: Table 1 summarizes the quantities in this study. Table 2 displays the results of Bland-Altman analysis. Reproducibility of breathhold and RA950 were high, with limits of agreement within 500mL and 1.5 percentage points (p.p.), respectively. Reproducibility for PD15 was poorer overall but improved noticeably in the <250mL subgroup, from 7 to 3HU. This effect was not observed for RA950. For ?RA950, linear regression revealed an R^2 of 0.43, 0.00, and 0.00 versus ?V, ?Pwater , and ?Pair , respectively. ?PD15 exhibited an R^2 of 0.74, 0.03, and 0.01 versus ?V, ?Pwater, and ?Pair. Linear regression against ?V is displayed in Figures 1 and 2. CONCLUSION: This study consists of a population of patients with high breathhold reproducibility (within 500mL) imaged on scanners with good calibration. Under these conditions, our data suggests that the precision of RA950 is within 1.5p.p. PD15 demonstrated greater sensitivity to breathhold, and a precision of 3HU was achieved by requiring breathhold reproducibility of 250mL. Our analysis revealed no relationship between scanner calibration and density measures, although this finding may change for a wider range of calibrations.