@geocirese
I worked in cardiac echo and cath labs for over a decade. I ran an imaging training program. Measurements are somewhat in the eye of the beholder. First, consistency of imaging mode is important. Compare echo to echo or CT to CT for growth rate. Second, I go to the same place with same machine if possible. Third, no blood vessel is absolutely uniform in size and contour. So if you measure a little bit higher up or lower down on the vessel you may get some variation. Also if you have 3 people measure the same image you may get slight differences in each measurement. If you measure at the peak flow of a ventricular contraction it may be a mm more than in diastole (hence gating to the ekg helps comparative accuracy). I personally trust echo with a good tech more because they can move around and make sure they are getting the best perpendicular slice when they measure and can scan up and down the entire length so you see a more complete set of pictures - not just a single shot.
Bottom line - all measurements have a range of accuracy and error. So consistency - if one echo shows 4.2 and six months later shows 4.1 or 4.3 - in my mind no significant difference. For me - measure in the same modality for best comparisons. Also of note - dye studies light up inner diameter and echo and non-contrast show walls, inner and outer diameter - therefore another source of difference - again, compare only same modality in my world.