Intra-Luminal Thrombus Volume in Small Abdominal Aortic Aneurysms Assessed with Contrast Enhanced Three-Dimensional Ultrasound

Abstract

Introduction - The intra-luminal thrombus (ILT) in abdominal aortic aneurysms (AAA) is suggested to weaken the aortic wall and contribute to aneurysm growth and eventually rupture (1–4). Thus, investigation of ILT has the potential to improve rupture risk prediction which today is predominantly based on diameter alone. Until recently, the ILT volume could only be determined by means of three-dimensional computed tomography angiography (3D-CTA) and ultrasound has not had a place in this context, although attractive due to its noninvasive nature. The primary aim of this study was to assess ILT volume using a novel ultrasound technology which combines three-dimensional ultrasound with contrast enhanced ultrasound (3D-CEUS) and compare these measures with 3D-CTA, using the latter as reference method. The second aim was to determine the reproducibility and repeatability of 3D-CEUS in ILT volume measurements. Methods - Between 1 January 2015 and 31 October 2016 (22 months), 143 patients with small asymptomatic AAA (US-diameter: 30-55 mm) were consecutively recruited. All patients underwent 3D-CEUS (Figure 1) and 3D-CTA the same day, in a mutually blinded setup. In total 15 patients were excluded due to CTA protocol deviation (n=2), inferior CEUS image quality (n=8) and AAA diameter > 55mm on US (n=5). Therefore, 128 patients (F/M; 22/106) with a mean AAA diameter of 45.1mm (SD ±6.2) were eligible for further analysis. Reproducibility assessment was performed on 30 patients who had an extra 3D-CEUS performed by a second physician. The ILT volume and maximum ILT thickness perpendicular to the AAA centerline was quantified off-line on a semi-automated quantification software (AAA-prototype v.2.0, Philips Research, Suresnes, France). Results - In 25 out of 128 patients, no ILT was found on 3D-CEUS, and confirmed on 3D-CTA analysis in all but one case. The mean ILT volume difference between 3D-CEUS and 3D-CT was 2.2ml (p=0.0001) equivalent to a 6% difference. The range of variability (ROV) was ±10.5ml. Likewise, the mean ILT thickness difference was 0.7mm (p= 0.002), with a ROV of ±4.5mm. The inter-observer variation of both ILT volume and ILT thickness measurement with 3D-CEUS was low, with reproducibility coefficients of 8.9ml and 3.0mm. A similar corresponding intra-observer variation of 7.5ml and 3.4mm, respectively, was found. Conclusion - Three-dimensional contrast enhanced ultrasound has demonstrated both good agreement and reproducibility in the assessment of ILT volume. It is expected that the method in the future can be implemented as a clinical tool in line with other advanced CEUS investigations. We foresee that ILT assessment with this novel ultrasound technique could be a valuable supplement in predicting rupture risk without radiation and nephrotoxic contrast agents.