From the Guidelines
A CT scan without contrast is the preferred imaging method to rule out renal stones. Non-contrast CT is highly sensitive and specific for detecting kidney stones, as the stones appear as high-density objects that stand out clearly against the surrounding tissue, with a reported sensitivity as high as 97% 1. Contrast is not needed because it can actually obscure small stones and make them harder to detect. Additionally, using contrast adds unnecessary risk of allergic reactions and potential kidney injury, especially in patients who may already have compromised renal function due to obstruction from stones. Non-contrast CT also allows for assessment of stone size, location, and degree of obstruction, which are important factors in determining management, and has been shown to yield equivalent stone measurements compared with standard-dose CT 1.
Some key points to consider when using non-contrast CT for renal stone evaluation include:
- The sensitivity for stone detection decreases with smaller stone size, and the sensitivity for small stones can be further hampered with increasing dose reduction 1
- Low-dose CT has been shown to have a pooled sensitivity of 97% and a pooled specificity of 95% for detecting urolithiasis, and is considered a suitable alternative to standard-dose CT 1
- Stone location and size can be accurately depicted at non-contrast CT, and have been associated with spontaneous stone passage rates, with more proximal as well as larger stones having a higher need for intervention 1
- CT techniques such as use of coronal reformations, viewing on bone window, and use of magnified views can improve the accuracy of stone measurements 1
If there is concern for other pathologies beyond stones, such as infection, tumor, or vascular abnormalities, a contrast study might be considered as a follow-up, but the initial evaluation for suspected renal stones should be with a non-contrast CT scan. It's also worth noting that low-dose regimens should be used in place of conventional dosing when evaluating for renal or ureteral stones to minimize radiation exposure 1.
From the Research
CT Scan Options for Renal Stone Diagnosis
- Non-contrast CT scans are commonly performed for patients with suspected renal calculus, among other conditions, as they can help identify vascular structures and other abnormalities without the need for contrast agents 2.
- Non-contrast computerized tomography (NCCT) is a popular modality for diagnosing kidney stones, particularly in the acute setting, due to its high sensitivity and specificity 3.
Effectiveness of Non-Contrast CT for Renal Calculi
- Studies have shown that NCCT has a high sensitivity for detecting renal calculi, with reported sensitivities ranging from 95-100% 4.
- However, NCCT may underreport the number of renal calculi, particularly small stones, and may not be able to distinguish stones lying in close proximity to one another 4.
Alternative CT Scan Protocols
- A diuresis enhanced non-contrast computed tomography (NCCT) protocol, which includes oral hydration and an oral diuretic, can help maximize collecting system distention and improve the visibility of the renal collecting system 5.
- This protocol has been shown to increase the visible collecting system volume and surface area, and can be useful for preoperative planning and stone assessment.
Use of Contrast Agents in CT Scans
- The use of contrast agents in CT scans can be beneficial for diagnosing certain conditions, but it also carries a risk of contrast-induced nephropathy (CIN) 6.
- However, the risk of CIN is negligible in patients with normal renal function, and the benefits of diagnostic information gained from contrast-enhanced CT scans can outweigh the risks in certain clinical scenarios 6.