From a practical standpoint, if the goal is to have a clear high resolution image capable of detecting even small abnormalities, the choice should have been an MRI. They are, however, relatively expensive as one would expect from
the gold standard of imaging ... yuk, yuk. A CT/CAT scan is cheaper but would have a reasonable degree of resolution. A sonogram is basically an ultrasound echo-based generated image, a distant far more sophisticated relative of a sonar trace. Compared to an MRI or CT/CAT scan the resolution is far inferior.
But there is another major difference. MRI and CT/CAT scans produce static still images. However, sonograms can be used to see motion and the images can be viewed as a "motion picture" and as such may show the fetus moving and changing position. The argument can be made, based on Perry's own personal publicly stated religious beliefs, that the goal of the legislation was not to assure the woman's health by possibly detecting any hidden abnormalities, otherwise a technique that generates a sharp image would have been required. Rather, it would seem that the goal of the requirement was to hopefully catch a bit of fetal movement, show the woman and perhaps influence or shame/guilt the woman into not aborting her fetus.
I am sure that a physician that suspected that the woman had specific medical risks would order some sort of imaging study to assure that they would be addressed. But if they were practicing defensive medicine to avoid a malpractice suit, it's not likely that their first choice would be to rely on a sonogram as an imaging study. From the standpoint of fiscal conservatism, it seems odd that a proponent of that view would want to require a
mandatory medical screening procedure in
all cases without regard for the physician's opinion on its medical necessity. It seems like an unfunded government mandate which would incrementally add to rising medical costs and expenditures for the health care system to absorb and pass on to everyone else.
I can't say I understand the reasoning. But then again, I still haven't had my waitress show up with my chips and queso.