Careful contrast timing and sophisticated acquisition techniques address new applications ranging from transplants to aneurysms
One of the early applications of CT angiography in the 1990s was assessment of the renal arteries.1-2 Using single-detector-row technology, CTA was well suited for this use. Moderately thin sections (~3 mm) could be acquired through the renal vasculature in a relatively short duration. Researchers demonstrated reliable performance in evaluating suspected renal artery stenosis (RAS)2 and in screening potential living related kidney donors.3 Renal CTA was quickly adopted by many clinicians and imagers as an alternative to invasive catheter angiography.
Introduced in 1998, multidetector-row CT technology presented key advantages over single-detector-row scanners to improve the use of renal CTA. Thinner sections (0.63 to 1.5 mm) could be acquired through similar volume coverage with faster speed. The result has been improved spatial resolution along the z-axis, reduced breath-hold durations, and improved quality of 2D and 3D images. Multislice CT technology has afforded multiphase acquisitions through different physiologic renal states and multi-data set reconstructions from the original scan.
Since MSCT technology was introduced, assessment of RAS (Figure 1) and scanning of living related kidney donors remain core applications. Other clinical applications include evaluation of renal transplants, suspected crossed ureteropelvic junction vessels, renal aneurysms (Figure 2), arteriovenous fistulae (congenital and acquired), vasculitis, dissection, thromboembolism, suspected trauma, and renal masses. Early work also suggests that MSCTA may be a useful modality to quantify the glomerular filtration rate.4
RENAL MSCTA TECHNIQUE
Renal CTA requires synchronizing a high-flow intravenous contrast medium injection with a thin-section helical acquisition through the kidneys and their vascular supply. Images are interpreted using a combination of 2D and 3D displays. The ability to render high-quality images and interpret the exam is dependent on the quality of the CT acquisition and contrast medium administration.
To image the kidney, main renal arteries and veins, potential accessory arteries and veins, and normal variant anatomy, scan coverage should include from the supraceliac aorta through to the iliac bifurcation. If the exam is meant to assess transplant kidney vasculature, coverage is through the transplant kidney, which is typically in the right or left iliac fossa.
In general, MSCTA slice thickness should not exceed 1.5 mm. Depending on the number of MSCT channels (four to 10 versus 16 to 64), scan distance, body habitus, and noise, submillimeter slice thickness is recommended to generate isotropic data sets and improve small vessel depiction. Additional potential acquisitions include an initial unenhanced scan, a delayed nephrographic scan, and a delayed urographic scan.
For contrast medium administration, high-concentration material, high iodine flux (injection rate of 4 to 5 cc/sec), and injection duration of at least 20 seconds are essential. Timing is synchronized to enhancement of the midabdominal aorta, using either automated bolus-triggering software or a test bolus. For acquisitions greater than 20 seconds, the injection duration should be prescribed to equal the scan duration. If automated bolus triggering is used, the injection duration is extended to account for the inherent delay when using this software. For acquisitions less than 20 seconds, either slowing the table speed or increasing the scan delay should ensure that the acquisition and injection durations are equal, approaching 20 seconds.
I interpret the vasculature from a volumetric approach on a 3D workstation. I first use interactive volume-rendered images to form a structural overview of the aorta, iliac arteries, mesenteric arteries, and renal vasculature. Next, I interrogate the vessels of interest using either an interactive thin-slab maximum intensity projection, thin-slab volume-rendered image, or multiplanar reformation. To characterize segments of vessels, I use targeted orthogonal MPR projections.
Renal MSCTA can be used to investigate a broad spectrum of renal vascular pathology. Combined with visceral imaging, a comprehensive kidney and urologic evaluation can also be performed.
- Renal artery stenosis. RAS accounts for up to 5% of patients with hypertension, most commonly from atherosclerosis (90%, Figure 1) or fibromuscular dysplasia ( < 10%). The objective of CTA is to determine renal artery patency and characterize the vessel wall, including ostial, postostial, and segmental regions. Equally important is the assessment of parenchymal findings that suggest the presence of hemodynamically significant stenoses. These include renal atrophy, cortical thinning, and decreased cortical enhancement. In both instances, initial unenhanced images can aid diagnoses by detecting calcium, stents, and surgical clips. Evaluating suspected RAS with four-channel MSCT, authors report a sensitivity of 86% to 100%, specificity of 98.6% to 100%, and accuracy of 96.9% to 99%.5,6 Evaluating renal stent patency can also be achieved with reliable confidence.7
- Living related kidney donor. Determining which kidney may be more suitable for transplantation requires a more comprehensive exam. In addition to noncontrast and first-pass phases, delayed nephrographic and urographic phases are essential, as nonvascular findings such as nephrolithiasis, cysts, solid masses, cortical scarring, hydronephrosis, and ureteral duplications may preclude donation. Interpretation of a living related kidney donor CTA begins with the noncontrast images to identify any calculi or vascular calcifications. Vascular assessment addresses the number, location, patency, and branching pattern of renal arteries and veins, including accessory vessels and normal variants. Parenchymal assessment addresses the presence of masses and scarring in addition to calculating kidney dimensions such as length and volume. Finally, thorough evaluation of the upper and lower collecting systems addresses the draining pattern and patency.
- Renal transplants. Many post-renal transplant patients have multiple surgical clips in the pelvis, which can degrade MR image quality. CTA is therefore an excellent alternative to catheter angiography to assess renal transplant arteries and veins. The imaging objectives and protocol are similar to RAS evaluation, with two exceptions: Coverage is limited to the pelvis, and acquisition occurs in the angionephrographic phase so that arteries and veins can be evaluated.
- Renal artery aneurysms. Renal artery aneurysms (RAA) are uncommon, occurring in approximately 0.09% of patients.9 RAA have a slight female predominance and are typically unilateral and solitary, occurring slightly more frequently in the right kidney. Most patients with RAA are asymptomatic (Figure 2). However, RAA are associated with hypertension and can cause flank pain, hematuria, and renal function demise, depending on their size, the presence of microemboli, and occurrence of rupture. CTA is a reliable means to define the location, type, and size of RAA as well as the presence of calcification, thrombus, and other renal arterial pathology.
In these and other clinical applications of renal MSCTA, it is important to remember that success depends on a fundamental understanding of scan acquisition, contrast medium administration, and image display principles.
Dr. Hellinger is an assistant professor of radiology and cardiology at The Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, and director of cardiovascular imaging and of the 3D laboratory.
1. Rubin GD, Dake MD, Napel SA, et al. Three-dimensional spiral CT angiography of the abdomen: initial clinical experience. Radiology 1993;186(1):147-152.
2. Rubin GD, Dake MD, Napel SA, et al. Spiral CT of renal artery stenosis: comparison of three-dimensional rendering techniques. Radiology 1994;190(1):181-189.
3. Rubin GD, Alfrey EJ, Dake MD, et al. Assessment of living renal donors with spiral CT. Radiology 1995;195(2):457-462.
4. Sommer G, Olcott EW, Chow LC, et al. Measurement of renal extraction fraction with contrast-enhanced CT. Radiology 2005;236:1029-1033.
5. Willmann JK, Wildermuth S, Pfammatter T, et al. Aortoiliac and renal arteries: prospective intraindividual comparison of contrast-enhanced three-dimensional MR angiography and multi-detector row CT angiography. Radiology 2003;226(3):798-811.
6. Fraioli F, Catalano C, Bertoletti L, et al. Multidetector-row CT angiography of renal artery stenosis in 50 consecutive patients: prospective interobserver comparison with DSA. Radiol Med (Torino) 2006;111(3):459-468.
7. Raza SA, Chughtai AR, Wahba M, et al Multislice CT angiography in renal artery stent evaluation: prospective comparison with intra-arterial digital subtraction angiography. Cardiovasc Intervent Radiol 2004;27(1):9-15.
8. Frauscher F, Janetschek G, Helweg G, et al. Crossing vessels at the ureteropelvic junction: detection with contrast-enhanced color Doppler imaging. Radiology 1999;210(3):727-731.
9. Henke PK, Cardneau JD, Welling TH, et al. Renal artery aneurysms: a 35-year clinical experience with 252 aneurysms in 168 patients. Ann Surg 2001;234(4):454-462.