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We thank Dr Dalainas for his interesting comments. In their previous study to which he refers, Dalainas et al compared maximal ultrasound (US) scan diameter with maximal axial diameter on computed tomography (CT) (diameter of the maximal ellipse) only.1Dalainas I. Nano G. Bianchi P. Casana R. Lupattelli T. Stegher S. et al.Axial computed tomography and duplex scanning for the determination of maximal abdominal aortic diameter in patients with abdominal aortic aneurysm.European Surgery. 2006; 38: 312-314Crossref Google Scholar Differences between diameter measurement in different planes for the same aneurysm are of course related to angulation of the aneurysm, as the authors points out (though we prefer the term tortuosity), and also to noncylindrical cross-sectional diameter. In fact, the diameter of the maximal ellipse is the most sensitive to tortuosity, and so it is probably the least reliable measurement to take. It is for this reason that the diameter perpendicular to the centerline of flow (PCLF) has been advocated as the preferred modality by the Society for Vascular Surgery,2Chaikof E.L. Blankensteijn J.D. Harris P.L. White G.H. Zarins C.K. Bernhard V.M. et al.Reporting standards for endovascular aortic aneurysm repair.J Vasc Surg. 2002; 35: 1048-1060Abstract Full Text Full Text PDF PubMed Scopus (1331) Google Scholar as at least theoretically, this should minimize the effect of tortuosity on accurate diameter measurement. In our analysis, we did not use maximal US scan diameter in any axial direction, as used in the series of Dalainas et al, but rather maximal anterior-posterior (AP) diameter (as was used in the UK Small Aneurysm Trial).3Brown L.C. Epstein D. Manca A. Beard J.D. Powell J.T. Greenhalgh R.M. The UK Endovascular Aneurysm Repair (EVAR) trials: design, methodology and progress.Eur J Vasc Endovasc Surg. 2004; 27: 372-381Abstract Full Text Full Text PDF PubMed Scopus (113) Google Scholar Maximum US scan diameter in any axial direction will always be at least the same as the maximum AP diameter and usually bigger. We found the mean difference between US-AP and CT scan measured at the maximal ellipse was 9.6 ± 8 mm. While the difference was smaller for CT PCLF (7.3 ± 7 mm), it remains significant, suggesting that tortuosity is either not fully corrected by CLF models, or other factors such as US scan technique are of relevance, as we discuss. We fully accept that a more tortuous aorta will decrease the reliability of axial CT scan measurements (as has been previously reported).4Sprouse 2nd, L.R. Meier 3rd, G.H. Parent F.N. DeMasi R.J. Glickman M.H. Barber G.A. Is ultrasound more accurate than axial computed tomography for determination of maximal abdominal aortic aneurysm diameter?.Eur J Vasc Endovasc Surg. 2004; 28: 28-35Abstract Full Text Full Text PDF PubMed Scopus (69) Google Scholar While analysis of how more or less tortuosity might relate to inaccuracy of measurements made on axial CT scan might be of interest, it was never the aim of our study. Therefore, we did not attempt to quantify aortic tortuosity, nor did not perform any regression analysis, as is stated in the above letter. Rather we have analyzed the US scan and multiple CT scan measurements for the same aneurysm, in an attempt to quantify the differences that exist and to provide guidance on how to measure aneurysm diameter in the context of currently recommended reporting standards, and of previous trials on which decisions to intervene are based. Regarding “Abdominal aortic aneurysm diameter: A comparison of ultrasound measurements with those from standard and three-dimensional computed tomography reconstruction”Journal of Vascular SurgeryVol. 51Issue 1PreviewWe read with great interest the study by Manning et al1 comparing computed tomography (CT) scan and duplex ultrasound scan for the determination of the maximal aortic diameter in patients with abdominal aortic aneurysms (AAAs). In a similar study that we published in 20062 including 322 patients with AAA, a CT was found to overestimate the maximal aortic diameter compared with duplex scanning with a mean of 2.73 mm. The most likely explanation of the phenomenon is that the axial sections of the CT scan very often represent an oblique cut of the aneurysm if it is angulated, and most AAAs present a degree of angulation. Full-Text PDF Open Archive