CT angiography of the renal arteries and veins: normal anatomy, variants, and pathology with emphasis on 3D volume rendering

Introduction

3D volume-rendered CT angiography (CTA) provides a fast, non-invasive modality for the evaluation of the renal vascular pedicle. CTA can reliably and accurately depict the renal arteries and veins and approaches conventional angiography in the assessment of most vascular abnormalities. The number, size, course and relationship of the renal vasculature are easily appreciated utilizing real-time interactive editing. This exhibit provides a CTA atlas of normal anatomy and common variants of the renal arteries and veins. Proper injection protocols and data acquisition techniques are reviewed. In addition, the spectrum of pathologic conditions are presented. Arterial disorders include renal artery stenosis, renal artery aneurysms, and dissection. Venous disorders include splenorenal shunts, thrombosis, and intravascular tumor extension. The role of CTA in the vascular evaluation of the renal transplant patient is also addressed.

Volume-rendered CT angiography: principles and technique

Of the many three-dimensional reconstruction algorithms available for performing CT angiography, volume rendering has emerged as the rendering technique of choice. With volume rendering, the user can actively interact with the dataset, editing and modifying the position, orientation, opacity and brightness of the image in real-time. For CT angiography, volume rendering is commonly preformed with a window/level transfer function that results in high density material (for example, enhanced vessels or vascular calcifications) to appear bright and opaque, while less-dense structures appear dim and translucent. Overlying structures are easily removed with an interactive clip-plane, and the vessels of interest are easily rotated into the best orientation for depiction of the region of interest. For evaluation of the renal hilum, axial, coronal, and sagittal views are often used in conjunction for optimal evaluation of the number, caliber, and course of the renal arteries and veins. Perspective rendering allow the user to view the dataset from ìwithinî the vessel, producing angioscopic views which are also helpful for identifying a vascular orifice and stenosis.

MIP represents the other common reconstruction algorithm commonly employed when evaluating the renal vasculature. The MIP technique evaluates each voxel from the viewerís eye through the dataset and selects the maximal voxel value as the value of the corresponding displayed pixel. The image produced lacks depth orientation, but a three-dimensional effect can be produced with rotational viewing of multiple projections. MIP images can provide useful information regarding atherosclerotic burden, vascular stents, and vascular stenoses, and are often reconstructed and interpreted in conjunction with volume-rendered images.

Normal renal vascular anatomy

Classically, each kidney is supplied by a single renal artery and a single renal vein, arising from the abdominal aorta and the inferior vena cava, respectively. These vessels typically originate off the aorta the level of L2 below the takeoff of the superior mesenteric artery, with the vein anterior to the artery. Both vessels then course anterior to the renal pelvis before entering the medial aspect of the renal hilum. The right renal artery typically demonstrates a long downward course to the relatively inferior right kidney, traversing behind the inferior vena cava. Conversely, the left renal artery, which arises below the right renal artery and has a more horizontal orientation, has a rather direct upward course to the more superiorly positioned left kidney. In addition, both renal arteries course in a slightly posterior direction because of the position of the kidneys.

The main renal artery divides into five segmental arteries near the renal hilum. The first division is typically the posterior branch, which arises just prior to the renal hilum and passes posterior to the renal pelvis to supply a large portion of blood flow to the posterior portion of the kidney. The main renal artery then continues before dividing into four anterior branches at the renal hilum: the apical, upper, middle, and lower anterior segmental arteries. The apical and inferior arteries supply the anterior and posterior surfaces of the upper and lower poles, respectively; the upper and middle arteries supply the remainder of the anterior surface. The segmental arteries then course through the renal sinus and branch into the lobar arteries, which supply one branch to each pyramid. Further divisions include the interlobar, arcuate, and interlobular arteries. Depiction of the relatively avascular plane between the anterior and posterior arterial divisions of the kidney is of importance to the surgeon, because it allows for a clean incision towards the renal pelvis at the time of surgery. This is usually located one third of the distance between the posterior and anterior surfaces of the kidney. A similar avascular transverse plane exists between the posterior renal segment and the polar renal segments.

The renal cortex is drained sequentially by the interlobular veins, arcuate veins, interlobar veins, and lobar veins. The lobar veins join to form the main renal vein. The renal vein usually lies anterior to the renal artery at the renal hilum. The left renal vein is longer than the right renal vein. The left renal vein averages 6 to 10 cm in length and will normally course anteriorly between the SMA and the aorta before emptying into the medial aspect of the inferior vena cava. The right renal artery averages 2 to 4 cm in length and joins the lateral aspect of the inferior vena cava. Unlike the right renal vein, the left renal vein receives several tributaries before joining the inferior vena cava. It receives the left adrenal vein superiorly, the left gonadal vein inferiorly, and a lumbar vein posteriorly.

Volume rendered CT angiography can very quickly and accurately determine the location and course of the renal vascular anatomy. Angioscopic and MIP views provide additional information on the renal vascular anatomy and compliment conventional volume-rendered images. Typically, arterial branches can be confidently identified to at least the segmental level. Limitation for detection occurs with vessels smaller than 2 mm in size. Sensitivity for the demonstration and location of main renal arteries, however, approaches 100%. Surgical and CT findings correlate in over 95% of cases. The renal venous anatomy is also well demonstrated with CT angiography, and is especially important to document for patients undergoing evaluation for laparoscopic donor nephrectomy. The left renal anatomy is especially critical, and this is the preferred side for donation. Tributaries into the left renal vein, especially posterior lumbar branches, are confidently displayed and are of potential surgical importance if noted to be enlarged.

Normal variants

Aberrant or accessory renal arteries arise off the aorta or iliac arteries anywhere from the level of T-11 to the level of L-4. They are seen in up to 25 percent of patients. Usually the accessory artery will be seen coursing into the renal hilum to perfuse the upper of lower polar regions. Prehilar arterial branching is a common variant necessary for detection for patients undergoing evaluation for donor nephrectomy

The most common anomaly of the left renal venous system is the circumaortic renal vein, seen in up to 15% of patients. In this anomaly, the left renal vein bifurcates into ventral and dorsal limb which encircle the abdominal aorta. Less common is the retroaortic renal vein, seen in up to 4% of patients. Here, the single left renal vein courses posterior to the aorta and drains into the lower lumbar portion of the IVC. In addition, multiple renal veins are seen in approximately 15 percent of the patients.

Renal transplant evaluation

CT angiography is also well suited for evaluation of the post-transplant kidney. One relatively common complication after renal transplantation is graft renal artery stenosis. This has been reported in 3-15% of patients, usually within the first 3 years after transplantation. CT angiography can non-invasively image the transplant pedicle and document the presence of stenosis. Occasionally, surgical clips can result in artifacts near the transplanted artery and limit evaluation. If so, they can usually be edited from the data set.

Renal artery stenosis

Renal artery stenosis is important to detect as it represents a potentially reversible etiology of hypertension. It accounts for fewer than 5% of adult patients with hypertension. CT angiography represents a reliable, non-invasive screening exam for the detection of renal artery stenosis, with reported accuracy of up to 96%. CT angiography is also very sensitive and specific for the demonstration of renal artery occlusion. Both MIP and volume rendering are useful and complimentary in the evaluation of renal artery stenosis.

Axial images alone are not sufficient for the evaluation renal artery stenosis because the renal arteries often have a tortuous and variable course. Additional views provided by CT angiography allow for display of the renal arteries in multiple planes and projections, often necessary for depiction of stenosis. In cases with extensive calcification, stenosis can be obscured by MIP rendering techniques and requires careful evaluation with the volume rendered images. Angioscopic views often provide the best analysis. CT angiography can also depict secondary signs of renal artery stenosis, including poststenotic dilatation and renal parenchymal changes of atrophy and decreased cortical enhancement.

CT angiography is also very helpful in the post treatment evaluation of renal stent grafts, and can usually delineate between the highly attenuating graft material and the intraluminal contrast material.

Renal vein pathology

Involvement of the renal vein with either bland thrombus or tumor thrombus is a common indication for evaluation of the renal pedicle. Renal vein involvement by tumor is crucial in the determination of surgical options for removing a renal tumor. The renal veins are well depicted on the CT angiogram during the corticomedullary or ìarterialî phase of enhancement. Complete IVC opacification usually requires a second helical acquisition 90-120 seconds after injection.

Left-sided venous enlargement from spontaneous spleno-renal shunts is sometimes demonstrated in patients with portal hypertension. Rarely, venous enlargement can also be demonstrated in patients with high flow states resulting from tumor shunting.

Renal artery pathology

CT angiography can be used to evaluate the abdominal aorta and diseases that involve the renal arteries. This includes the evaluation of aortic aneurysms and dissection. CT angiography can accurately define the extent and location of aortic pathology as it relates to the renal arteries and is very helpful for preoperative assessment. CT angiography can not only define the renal vascular anatomy but depicts the secondary parenchymal changes as well, including infarcts and atrophy.

Aneurysms of the renal arteries can be seen in isolation or as a manifestation of a systemic vasculitis. Calcification can obscure depiction of some aneurysms on MIP images and are best seen with volume rendering.

Conclusion

CT angiography has many documented applications in the renal pedicle, including the evaluation of renal artery stenosis, renal arterial disease related to aortic diseases, preoperative evaluation of renal donors, and preoperative evaluation of renal anatomy prior to surgery. CT angiography provides an accurate assessment of the renal vasculature in a fast and efficient manner without the risks of conventional angiography. This exhibit has demonstrated the normal and variant anatomy of the renal pedicle, and well as the spectrum of vascular conditions which can be evaluated with CT angiography.

References

• 1.Kattee R, Beek F, de Lange E, et al. Renal artery stenosis: detection and quantification with spiral CT angiography and optimized digital subtraction angiography. Radiology 1997;205:121-127.
• 2.Platt J, Ellis J, Korobkin M, Reige K. Helical CT evaluation of potential kidney donors: findings in 154 subjects. AJR 1997;169:1325-1330.
• 3.Rubin GD, Alfrey EJ, Dake MD, et al. Assessment of living renal donors with spiral CT. Radiology 1995;195:457-462.
• 4.Rubin GD. Spiral (helical) CT of the renal vasculature. Semin US CT MR 1996; 17:374-397.
• 5.Smith PA, Fishman EK. RSNA catagorical course in vascular imaging 1998; 35-45.
• 6.Smith PA, Marshall FF, Urban BA, Heath DG, Fishman EK. Three-dimensional CT stereotopic visualization of renal masses: impact on diagnosis and management. AJR 1997;169:1331-1334.
• 7.Smith PA, Ratner LE, Lynch FC, Corl FM, Fishman EK. Role of CT angiography in the preoperative evaluation for laparoscopic nephrectomy. RadioGraphics 1998; 18:589-601.

Table 1: Renal CT angiography: helical CT technique

• A large bore IV (18 gauge) is placed in the antecubital fossa.
• The patient is instructed in breath-hold technique. Most scanners require a 30-40 second breath hold for evaluation of the renal hilum.
• The region of interest (renal hilum) is localized accurately with a few non-contrast images. The appropriate table position is calculated for evaluation of the renal hilum.
• Helical CT scanning parameters are entered. Narrow collimation is crucial and collimation of 2-3 mm is ideal. A pitch of 2 is used to ensure adequate coverage, and will not significantly decrease image quality.
• Contrast is injected at 3-4 ml/sec for a volume of 120-150 cc.
• The helical scan is initiated after a preset delay of 20-25 seconds after the start of injection. Alternatively, bolus-triggering devices can be used to initiate the scan.
• Images are reconstructed equally throughout the data set. 1-mm interscan spacing is ideal for evaluation of the renal hilum and especially for renal artery stenosis. 3-mm interscan spacing is suitable for routine anatomical evaluation.
• Data is transferred over the network to an imaging workstation. Free-standing workstations (GE Advantage Windows, Siemens 3D Virtuoso) provide the most flexibility and have more capabilities.