Edge artefacts are caused when sound strikes circular objects and is reflected. The reflections at the edge are mostly transmitted deep with little return to the scan head, therefore edges of round objects are generally poorly defined. Similarly looking at structures along their length, e.g. ventricular septum will give a poor return of echo and so poor edge definition (image, yellow arrow).
This can lead to lack of accuracy when measuring structures. Thus is the reason the aorta should be measured AP and not across its width.