Variability of vascular CT measurement techniques used in the assessment abdominal aortic aneurysms

Abstract 

Purpose

The aim of this project is to assess the variability of six CT measurement techniques for sizing abdominal aortic aneurysms (AAAs).

Method

37 CT scans with known AAAs were loaded on to a departmental picture archiving and communication system (PACS). A team of three observers, with experience in aortic CT measurements and the PACS performed a series of 2D and 3D measurements on the abdominal aorta. Each observer was asked to measure 3 quantities; anterior–posterior AAA diameter, maximum oblique AAA diameter, maximum aneurysm area using both 2D and 3D techniques. In order to test intra-observer variability each observer was asked to repeat their measurements. All measurements were taken using electronic callipers, under standardised viewing conditions using previously calibrated equipment. 3D measurements were conducted using a computer generated central luminal line (CLL). All measurements for this group were taken perpendicular to the CLL.

Results

A total of 972 independent measurements were recorded by three observers. Mean intra-observer variability was lower for 2D diameter measurements (AP 1.3 ± 1.6 mm; 2D Oblique 1.2 ± 1.3 mm) and 2D areas (0.7 ± 1.3 cm²) when compared to inter-observer variability (AP 1.7 ± 1.9 mm; Oblique 1.6 ± 1.7 mm; area 1.1 ± 1.5 cm²). When comparing 2D with 3D measurements, differences were comparable except for 3D AP diameter and area which had lower inter-observer variability than their 2D counterparts (AP 2D 1.7 ± 1.9 mm, 3D 1.3 ± 1.3 mm; area 2D 1.1 ± 1.5 cm², 3D 0.7 ± 0.7 cm²). 3D area measurement was the only technique which had equal variability for intra- and inter-observer measurements. Overall observer variability for the study was good with 94–100% of all paired measurements within 5.00 mm/cm² or less. Using Pitman's test it can be confirmed that area measurements in the 3D plane have the least variability (r = 0.031) and 3D oblique measurements have the highest variability (r = 0.255).

Conclusion

3D cross-sectional area measurement techniques have the lowest variability and should be preferred for repeatable measurements of AAAs where possible. Results confirm that both inter- and intra-observer variability exists for all measurement techniques.

Keywords: Abdominal aortic aneurysm, Computed tomography, Radiological measurement, Observer variability, Vascular disease