Which imaging study is best to diagnose renal calculi?

The best imaging study to diagnose renal calculi is a non-contrast CT abdomen. This choice is favored because non-contrast CT scans are highly sensitive and specific for detecting renal stones of all sizes and types. The CT scan is able to provide detailed images of the urinary tract without the interference of contrast material, which can obscure small stones or complicate the diagnostic process.

Non-contrast CT is particularly effective because it can identify not just the presence of stones but also their location, size, and any potential complications like ureteral obstruction. Additionally, it is a rapid procedure, making it suitable for the evaluation of acute flank pain in suspected cases of kidney stones.

While ultrasound is useful in certain situations, such as in pregnant patients or when evaluating for hydronephrosis, it has limitations in detecting all types of stones, particularly if they are small or if there is overlying bowel gas. An X-ray of the abdomen, although sometimes used, is less reliable because not all stones are radiopaque; some types may not be seen on an X-ray. Contrast-enhanced CT is not preferred in this scenario as the introduction of contrast can obscure stones and is unnecessary when the non-contrast approach is so effective for identifying calculi.