Current management of abdominal aortic aneurysm (AAA) disease is based on the maximal diameter of the aneurysm, and intervention is recommended when the AAA has a diameter larger than 5.5 cm. Computed Tomography angiography (CTA) is the gold standard for AAA imaging, but requires radiation and iodinated contrast, and is therefore not the preferred modality for serial monitoring. Black blood MRI is a promising tool for AAA monitoring as it does not require radiation or iodinated contrast agent and has excellent soft tissue contrast that allows characterization of the aortic wall and intra-luminal thrombus (ILT). However the accuracy and reproducibility of BB-MRI has not been established. A previous study suggested AAAs with fresh ILT (hyper-intense on T1-weighted MRI) were likely to progress two times faster compared with those without fresh ILT ¹. We have previously developed an isotropic 3D black blood technique for AAA imaging which showed good image quality and the potential to identify ILT age ². This study aims to 1) validate 3D MRI for quantifying AAA dimensions using CTA as a reference standard, and evaluate its inter-observer reproducibility; 2) compare MRI and CTA for evaluating ILT composition.Study population: 26 patients (22 male, age 73±9) with AAA disease were recruited in two centres (UCSF and Shanghai). All patients underwent high resolution MRI and CTA scans within an average interval of 11 days. Scanning protocols: MRI scans were undertaken in Siemens Skyra 3T scanners. Two T1-weighted 3D sequences were acquired. 1) Blood suppressed fast-spin echo with variable flip angle train (DANTE-SPACE) 2: TR/TE: 800ms/20ms, 1.3mm isotropic resolution 2) 3D GRE (VIBE): TR/TE 4.2ms/1.2ms, 1.3x1.3x2.6mm. CTAs were acquired using clinical protocols with ~0.8mm isotropic resolution. Image analysis: Two experienced radiologists measured the maximal diameter of the AAAs using multi-planar reconstruction (MPR) on both the black blood MRI images and the CTA images. ILT and psoas muscle signal intensities were recorded at the location of maximal diameter of the AAA on MRI images. ILT signal ratio (ILTr) was calculated as SignalILT/SignalMuscle. ILTs were characterized as fresh (hyper-intense) or old (iso-intense). Another reviewer manually segmented the lumen and outer wall boundaries of the AAAs using both MRI and CT images, and the overall AAA volume and lumen volume were quantified. Statistics: Bland Altman plots, coefficient of variance (CV) and intra-class coefficient (ICC) were used to compare between methods.Good quality MRI and CTA images were successfully acquired in all patients. Quantitative comparison results are shown in Figure 1 and Table 1, and representative patients images are shown in Figure 2-4. There is an excellent agreement between black blood MRI and CTA for diameter and volume measurements (ICC>0.99, CV<5%). Both MRI and CTA are highly reproducible for diameter quantification (ICC>0.99, CV<2.5%). ILT shows homogeneous signal at CTA, but heterogeneous signal on MRI. Three types of ILT composition were classified: (I: predominantly fresh ILT; II: predominantly old ILT; III: mixture of both.) There was good agreement for identifying fresh ILT using DANTE-SPACE and VIBE, however, only moderate agreement was found for the ILTr quantification (Pearson r=0.31). To our knowledge, this is the first study comparing 3D black blood MRI against CTA for AAA imaging. DANTE-SPACE is shown to be accurate and reproducible for AAA dimension measurements. The advantage of the proposed technique over CTA is that it is totally non-invasive and can differentiate between different thrombus components, which can possibly be used to improve the risk stratification of patients ¹. A previous study used a T1-weighted GRE sequence to identify fresh ILT (hyper-intense signal) ¹. We validated the T1 weighting of SPACE against a GRE sequence (VIBE) for fresh ILT characterization (Figure 2). In addition, the higher resolution of SPACE improves the visualization of ILT composition (Figure 2-4). Sub-fresh/old ILT is hypointense on SPACE compared to VIBE (Figure 4), given its T2/T2* weighting induced by the long echo train. Detailed composition and morphology of ILT was clearly shown on SPACE images. However, the relationship of this information to AAA progression needs to be investigated in larger scale longitudinal studies. 3D black blood provides accurate and reproducible AAA dimension measurements as validated by CTA. In addition, it provides additional information on the ILT age and composition that is closely related to progressive AAA disease. 3D black blood MRI can be used as a non-invasive tool for AAA serial monitoring and ILT evaluation, and has the potential to improve patient risk stratification.