Which calculi are radiopaque on CT?
On CT almost all stones are opaque, but vary considerably in density. Two radiolucent stones are 1) Indinavir stones : (antiretroviral) radiolucent and usually undetectable on CT, and 2) pure matrix stones...
In what position was this patient scanned? Why?
Prone. This is done in some centers to facilitate distinguishing a stone impacted at the vesico-ureteric juntion (VUJ) from a stone lying dependently in the bladder. The latter, when scanned prone, would fall to the front (dependent) part of the bladder. Note that the image has been flipped so that the anatomy is presented in the same way as a routine supine scan.
In many institutions patients with suspected renal colic are investigated with a CT KUB first rather than a plain film KUB. What is the rationale for this approach?
Plain film KUB is often insufficient to assess a suspected case of ureteric colic, as A) if no stone is seen, it does not exclude the diagnosis B) if a radiopacity is seen, it is often not definitely a ureteric stone C) complications cannot be assessed on KUB (e.g degree of hydronephrosis etc....) and D) other causes of pain are not easily identified (e.g appendicitis). As a result most patients go on to have a CT KUB anyway.
When a stone is identified on CT KUB, is there a benefit to performing a plain film KUB? Why?
Yes. If the stone is visible on plain film KUB, subsequent followup can be performed with plain films rather than the higher dose CT.
Non contrast CT KUB confirms the presence of a small left ureteric calculus at the level of L3. The collecting system proximal to it is normal in appearance. The stone measure 3 x 4 x 7mm and has a density of 480HU.