Measurements and detection of abdominal aortic aneurysm growth: Accuracy and reproducibility of a segmentation software
Introduction
Abdominal aortic aneurysm (AAA) treatment, either by surgical or endovascular approach, currently rely on the maximal diameter (D-max) of the aneurysm [1], [2], [3], [4]. AAA D-max change over time, whether before or after intervention, also determine the need for further treatment [5], [6].
Several studies have advocated volume rather than diameter measurement for AAA follow-up, because volume changes reflect AAA growth in all directions irrespective of D-max changes whereas, by definition, D-max ignores cranio-caudal expansion [7], [8], [9], [10], [11].
In the setting of AAA follow-up of patients undergoing endovascular repair (EVAR), volumetric changes are also considered to be more sensitive than diameter measurement in the detection of changes in aneurysm size after EVAR [11], [12]. Volumetric analysis has also revealed size changes discordant with D-max in 14–19% of cases [9]. The variability of aortic volume measurement by manual segmentation correlates to level of experience of readers, being less than 2% for a well-trained technologist and up to 6% for readers with lesser experience [13]. A 2% volume increase has recently been suggested as a threshold for identification of endoleaks after EVAR, whereas no endoleak were observed when a volume decrease of more than 3% was observed [11]. Accurate AAA assessment will also become increasingly important to evaluate response to pharmacological therapies for small AAAs [14].
Despite theoretical advantages, AAA volumetry largely remains in the realm of research because segmentation methods are time-consuming and tedious post-processing may take up to 45 min [15]. Furthermore, current methods do not allow 3D-3D image registration between baseline and follow-up studies.
We have conceived and developed a semi-automated software enabling fast AAA segmentation and 3D modeling. This software was validated for the measurement of D-max measurement and showed an excellent accuracy and reproducibility [16].
To our knowledge, most of the studies investigating the potential of volumetric analysis for AAA follow-up used manual segmentation which is a tedious process. The aim of this study was to evaluate the ability of a semi-automated segmentation combined with 3D-3D registration between baseline and follow-up examinations to enable fast volumetric follow-up by operators with minimal training. While most previous studies on volumetric analysis of AAA have focused on changes after EVAR, we selected untreated patients to evaluate the software's ability to detect growth. More specifically, we evaluated the intra and inter-observer reproducibility and assessed the accuracy of this software to measure aneurysm diameter, volume and growth over time on CT angiography examinations.
Section snippets
Study design and patient selection
We performed a retrospective study on 28 consecutive patients with abdominal aortic aneurysm (AAA) followed by multi-detector computed tomography (MCDT) before any treatment. Patients were selected from the radiological information system, if they had an AAA equal or larger than 4.0 cm and at least 2 MDCT studies available on the local PACS with a minimum of 4 month interval between two studies between 2007 and 2009. Selected patients were then contacted by a research nurse and their approval
Patient demographics and aneurysm characteristics
Twenty-eight patients, 24 men, 4 women, with a mean age of 71 years, (range 49–83 years) were included in this study. The average interval between the baseline and follow-up MDCT was 17.5 ± 7.9 months (range 5–36 months). Considering all observers, the average D-max value was 49.6 ± 6.2 mm at baseline and 53.6 ± 7.9 mm at follow-up; the average volume was 117.2 ± 36.2 ml at baseline and 139.6 ± 56.3 ml at follow-up (Table 1).
D-max growth between baseline and follow-up examinations was 4.0 ± 3.8 mm in absolute
Discussion
Because of its volumetric acquisition mode suitable for multi-planar and three-dimensional reconstructions of complex anatomy, there is an inherent benefit to use CT rather than ultrasound for comparison between baseline and follow-up AAA examinations. While volumetric changes have been considered more sensitive than diameter changes in detection of AAA growth [12], [21], [22], [23], [24], [25], [26], volumetry has largely remained in the research domain because segmentation methods are
Conclusion
We conclude that AAA segmentation for volume follow-up is more sensitive than D-max follow-up, while providing an equivalent reproducibility. Clinical validation showed a high accuracy and reproducibility of D-max and volume measurements when operated by a reader with minimal training. Volume measurement can be used in addition to maximal diameter to refine AAA follow-up.
Acknowledgements
This work was supported by a clinical research scholarship (to GS) from Fonds de la recherche en santé du Québec (FRSQ) and an operating grant from the Ministère du développement économique, de l’innovation et de l’exportation du Québec (MDEIE) and the Canadian Head of Academic Radiologist (CHAR).
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