If I understand your question correctly, you want to determine equivalent diameters of aortas that are not circular in cross-section?  If so,  measure the perimeter and divide by π.  regards. Deon

The research question you are trying to answer should be considered when trying to answer your question as stated: there are a number of ways of expressing an "equivalent diameter" for any given non-circular shape including approximations to x sectional area, perimeter, equivalent ellipse (major + minor diameters) etc.  This problem has been studied extensively in the work done on particle diameters (of fly ashes from combustion process eg).

Are you measuring aortic diameter in relation to aortic aneurysm? If so there are a number of things to consider such as the modality used to image the aorta (if in vivo) e.g. ultrasound or CT, the plane of imaging, the involvement of the wall structure in the measurements (e.g. inner to inner measurement or outer to outer measurement) amongst other aspects. Typically measurements are made perpendicular to the plane the artery is orientated to i.e. perpendicular to a centre line or in an orthogonal plane. The reproducibility of measurements should be assessed.

You might find the attached publication helpful. Good luck,

Geoff Rubin

Article Measurement of the aorta and its branches with helical CT

Thanks for your answer Mr. Deon Bezuidenhout , Mr. Tal Golesworthy, Mr Jonathan Golledge, Mr Geoffrey D Rubin.

Thanks Mr Geoffrey D Rubin, i want to measure in histological preparation in rat aorta

If you have a focal outpouching of the vessel then it should be measured in long and short axis.  However, if you consider the item to be a cylinder, then the short axis is the diameter.  Consider a cylinder.  If you cut it at any angle not perpendicular to the long axis, the resulting surface will be an oval and the short axis is the diameter.  You frequently see this on CT scan with a tortuous aorta

Ricky, While I have not measured histologic sections myself, if you can capture a size-calibrated image of the sections, then you can use an open source program like ImageJ to define and measure the circumference of the vessel, and then analyze similar to the paper that I uploaded. The reliability of the measurement will be predicated by the extent to which the sections are made orthogonal to the median axis of the aorta.

maybe this reference also can help you:

Importance of Measurement of the Diameter of the Aorta during Peroperative Blood Flow Monitoring; 14th Annual International Conference of the IEEE Engineering in Medicine and Biology Society; p.2402 - 2403; DOI: 10.1109/IEMBS.1992.5761425.

Found geometrical centr of lumen and calculate with macro distance from this center to all points at the lumen selection, mean of this distances will be radius