Protocols and future directions in imaging of renal artery stenosis: CT angiography

Computed tomographic angiography (CTA) has become an established technique for minimally invasive imaging of renal artery stenosis. This article describes in detail the current scan protocols suggested for renal artery stenosis and explains the rationale behind the choice of such parameters as collimation, pitch, reconstruction interval, scan duration and scan length. It discusses the optimized use of contrast agents including bolus triggering and a saline flush to improve contrast agent utilization. Examples of optimized scan protocols are given for various scanner types including subsecond scanners and multislice computed tomography. With standard 1-s scanners, a slice collimation of 2 mm is recommended. A pitch factor of 1.5-2 is important for optimum volume coverage. Smoothing reconstruction kernels improve signal-to-noise ratio and allow for dose reduction. For optimum results, the plateau phase of contrast enhancement has to be matched as precisely as possible. Thus, a test bolus injection or an automated bolus triggering technique should be employed. Axial CT images remain the basis for making the diagnosis but require interactive viewing on a monitor display. Image processing (multiplanar reformats; maximum intensity projections; shaded surface displays) is employed to display the renal arteries. Given optimized examination protocols, high sensitivity and specificity can be achieved (90-98%) with negative predictive values of greater than 95%. Thus, a normal CT angiogram virtually rules out renal artery stenosis.