Longmore, FRCS Background. Helical and retrograde secondary flows have been recorded in the aorta, but their origins and movements in relation to the arch have not been clarified. We set out to do this using magnetic resonance velocity mapping. Methods and Results. Three-directional phase contrast cine magnetic resonance velocity mapping was used to map multidirectional flow velocities in the aortas of 10 healthy volunteers. Computer processing was used to visualize flow vector patterns in

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... ted planes. Right-handed helical flows predominated in the upper aortic arch in late systole, being clearly recognizable in 9 of the 10 subjects. Nonaxial components of velocity in this region reached 0.29 m/s (±0.05 m/s) as axial velocities declined from a peak of 1.0 m/s (±0.1 m/s). Helical flow patterns in the upper descending aorta varied between subjects, apparently depending on arch curvature. End-systolic retrograde flow originated from regions of blood with low momentum, usually along inner wall curvatures. Flow studies in a curved tubular phantom showed right-handed helical flow in the upper "arch" when the inflow section was positioned to simulate ascending aortic curvature, and retrograde flow occurred along the inner wall at end systole during pulsatile flow. Conclusions. Helical and retrograde streams are consistent features of intra-aortic flow in healthy subjects that result, at least in part, from the curvature of the arch and the pulsatility of flow in it. They may have significance in relation to circulatory dynamics and the pathogenesis of atheroma in the arch. nents running oblique or counter to the main forward stream, are almost unavoidable. Twisting streamlines occur frequently,17 and symmetrically paired, counterrotating helices tend to develop at simple pipe bends.18 Transparent models have been made that simulate, as far as possible, the morphology of curved arteries so that patterns of flow could be visualized in vitro.17'9 Even when helical flows can be seen, however, accurate mapping of velocity distributions in pulsatile, twisting streams presents major difficulties. The problem is not only three-dimensional or even four-dimensional (space and time) but effectively seven-dimensional, with three separate velocity components required to specify the flow vector for each point in space-time.6,79 This complexity helps to explain the relative lack of data on helical flow patterns in the aortic arch in humans. Some evidence of a rotational component to flow at certain locations in the aorta has been recorded.12'20 One study used transesophageal color Doppler ultrasonography to image nonaxial blood movements in the descending aorta in humans.16'21 The authors found evidence of helical flows in the majority of subjects, with variability between individuals. The method did not allow in vivo measurement flow in the arch, but part of the study recorded right-handed helical flow in the upper arch of an aortic model. Retrograde aortic flows, meaning blood movements running counter to the main forward stream, have been by guest on July 6, 2016 http://circ.ahajournals.org/ Downloaded from 33. Mohiaddin RH, Sampson C, Firmin DN, Longmore DB. Magnetic resonance morphological, chemical shift and flow imaging in peripheral vascular disease. Eur