Letter by Frydrychowicz and Markl Regarding Article, “Association Between Aneurysm Shoulder Stress and Abdominal Aortic Aneurysm Expansion: A Longitudinal Follow-Up Study”
To the Editor:
We read with great interest the article by Li et al,1 who present convincing data on the calculation of wall shear stress (WSS) in abdominal aneurysms based on computed tomography–derived vascular geometries and numeric simulations (computational fluid dynamics). They conclude that increased WSS at the aneurysm shoulder is associated with aneurysm expansion, which in turn, demographically, is associated with a higher risk of rupture. We believe that the authors have to be congratulated on these results, which point toward a more individualized patient-specific clinical management of aneurysms. Such modeled geometries and numeric simulation could help to understand the contribution of the angle at the shoulder and the impact of thrombus in this region on further aneurysm development.
In their work, there is also the notion of accurate WSS not being available in vivo at present. Although we agree that it is difficult to quantify WSS in vivo, a discussion of alternative techniques and current developments relative to in vivo WSS estimation pursued by various research groups is missing. Recent methodological improvements in magnetic resonance (MR) imaging, and particularly the increased availability of quantitative MR velocity mapping, have led to the ability to measure time-resolved 3-dimensional blood flow velocities in vivo with isotropic spatial resolutions up to 1 mm. A number of groups have shown that these data can be used to analyze regional blood flow velocity profiles and to reproducibly estimate in vivo WSS. The technique has already been applied to various vascular territories, healthy individuals, and patients with various vascular pathologies.2–5
Although WSS data derived from MR are still limited by suboptimal spatial resolution, these shortcomings are already being addressed, and higher-resolution imaging is becoming available. There are further advantages of MR-based estimates of WSS over computational fluid dynamics that make MR an interesting approach to comprehensive aneurysm evaluation: In contrast to computational fluid dynamics, no assumptions about boundary conditions (blood rheology, vessel wall elasticity, in-flow profiles) that do not fully reflect the nature of blood and vessel wall properties have to be made. In addition, MR-based WSS analysis can be achieved within minutes after a scan on a standard personal computer. We therefore believe that MR, with its lack of ionizing radiation, and hence ideal suitability for follow-up studies, may provide a valid alternative for individualized WSS analysis.
In essence, we believe that the data of Li et al are vital for the understanding of aneurysm development and improved therapeutic decision making. In such preclinical work, however, we strongly believe that new developments in MR imaging techniques should not be overlooked.
Alex Frydrychowicz, MD Department of Radiology
University of Wisconsin–Madison
Michael Markl, PhD Departments of Radiology and Biomedical Engineering
Sources of Funding
Dr Markl receives funding from the Bundesministerium für Bildung und Forschung (grant 01EV0706).
- © 2011 American Heart Association, Inc.
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