(Radiographics. 1999;19:1535-1554.)
© RSNA, 1999
Diagnosis of Renal Vascular Disease with MR Angiography1
Qian Dong, MD ,
Stefan O. Schoenberg, MD ,
Ruth C. Carlos, MD ,
Mohammed Neimatallah, MD ,
Kyung J. Cho, MD ,
David M. Williams, MD ,
Sahira N. Kazanjian, MD and
Martin R. Prince, MD, PhD
1 From the Department of Radiology, Weill Medical College, Cornell University, 1300 York Ave, New York, NY 10021 (Q.D., M.R.P.); and the Department of Radiology, University of Michigan, Ann Arbor (Q.D., S.O.S., R.C.C., M.N., K.J.C., D.M.W., S.N.K.). Recipient of a Certificate of Merit award for a scientific exhibit at the 1998 RSNA scientific assembly. Received April 2, 1999; revision requested May 7 and received June 15; accepted June 21. Supported in part by the Whitaker Foundation, the Deutsche Forschungsgemeinschaft, and the Verein zur Foerderung der Krebserkennung und Krebsvorsorge. Address reprint requests to Q.D.
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Abstract
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Renal magnetic resonance (MR) angiography allows accurate evaluation of patients suspected to have renal artery stenosis without the risks associated with nephrotoxic contrast agents, ionizing radiation, or arterial catheterization. Other applications of renal MR angiography are mapping the vascular anatomy for planning renal revascularization, planning repair of abdominal aortic aneurysms, assessing renal bypass grafts and renal transplant anastomoses, and evaluating vascular involvement by renal tumors. A variety of pulse sequences provide complementary information about kidney morphology, arterial anatomy, blood flow, and renal function and excretion. Three-dimensional gadolinium-enhanced MR angiography can be combined with several other sequences to produce a comprehensive approach to renal MR angiography. This comprehensive approach is designed to allow hemodynamic characterization of renal artery stenosis with a single MR imaging examination that can be easily completed in 1 hour. Three-dimensional gadolinium-enhanced MR angiography demonstrates the renal arteries along with the abdominal aorta, iliac arteries, and mesenteric arteries in a 20–30-second acquisition that can be performed during breath holding. Numerous projections are reconstructed from a single three-dimensional volume of data acquired with a single injection of contrast material to obtain perpendicular and optimized views of each renal artery.
Index Terms: Magnetic resonance (MR), three-dimensional, 961.12942 • Magnetic resonance (MR), vascular studies, 961.12942 • Renal arteries, MR, 961.12942 • Renal arteries, stenosis or obstruction, 961.72
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INTRODUCTION
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Renal artery stenosis of sufficient severity to cause renal ischemia leads to an elevation of blood pressure that is often difficult to control with medical therapy. Over time, the stenosis progresses in severity and leads to occlusion and a permanent reduction in renal function. The importance of hypertension as a public health concern (1) and the devastating consequence of renal failure have driven the search for a safe, noninvasive, and relatively inexpensive method of diagnosing renal artery stenosis. Ultrasonography (US) has been considered promising because it is noninvasive and inexpensive and provides flow information from which hemodynamic effects can be inferred. However, the use of US as a screening tool for renal artery stenosis has been limited by the need for experienced operators, a variable failure rate, and controversy over the usefulness of the flow information. Positive US results must be confirmed with conventional angiography. Spiral computed tomography (CT) is also promising but is limited by the risks associated with iodinated contrast material and ionizing radiation. Magnetic resonance (MR) angiography is now established as an accurate and safe technique for evaluating the main renal artery and vein. Because most renal vascular diseases involve the main renal artery, it is possible to evaluate patients with renal failure, allergy to iodinated contrast material, or difficult arterial access and patients in whom conventional arteriography cannot be performed because of its risks, inconvenience, or expense. In patients with poor renal function, MR angiography is the preferred technique even at centers with expertise in US and CT.
In the past, the risks of iodinated contrast material and arterial catheterization limited definitive diagnosis of renal artery stenosis with conventional renal arteriography to patients in whom there was a high degree of clinical suspicion. Such patients included children and young adults with hypertension, patients with poorly controlled hypertension, patients with renal failure induced by angiotensin-converting enzyme inhibitors, and those with a family history of renal vascular hypertension. With the availability of simpler, safer, less expensive, but still accurate contrast material–enhanced MR arteriography, screening for renal artery stenosis can be expanded to a broader spectrum of patients.
Recent studies of renal MR angiography performed with high-dose gadolinium contrast material report sensitivities and specificities of more than 90% for detection of renal artery stenosis (>50%) when conventional angiography was used as the standard of reference (2–6). However, these studies were based on a highly simplified grading scheme that usually classified renal artery stenosis as 0%, 
50%, >50%–75%, or >75% stenosis. Even with state-of-the-art systems, the maximum spatial resolution of MR imaging is about 1 mm3. Conventional angiography still has a much higher spatial resolution. For this reason, overestimation of the morphologic degree of stenosis on three-dimensional (3D) gadolinium-enhanced MR angiograms is still a common finding, particularly in higher-grade stenoses.
Because of the limited spatial resolution of MR angiography, many researchers believe that the hemodynamic and functional significance of the stenosis should also be assessed instead of just the morphologic degree of stenosis (7–10). Clinicians want to know if a renal artery stenosis is hemodynamically significant because patients with such stenoses are likely to benefit from a renal revascularization procedure. MR angiography can help identify hemodynamically significant stenoses because these stenoses demonstrate spin dephasing at the site of stenosis at 3D phase-contrast (PC) imaging and altered flow-velocity curves at cine PC imaging. In addition, functional consequences can be identified, including asymmetry in kidney size, enhancement, and excretion and loss of corticomedullary differentiation. A variety of pulse sequences allow one to obtain functional data, which can be integrated into the work-up of a renal artery stenosis to help with the determination of significance.
Determining the significance of a stenosis requires a technically and clinically feasible, comprehensive approach to MR angiography in which data are obtained and analyzed in a standardized, reproducible fashion. This standardization must be applied to data acquisition, image analysis, image reconstruction, and interpretation of the results (11). In this process, numerous projections are reconstructed from a single 3D volume of data acquired with a single injection of contrast material to obtain perpendicular and optimized views of each renal artery (12–17). The ability to obtain such views is an inherent advantage of MR angiography over digital subtraction angiography.
Our cumulative experience with renal MR angiography in over 1,000 cases has been used to develop a comprehensive approach that allows evaluation of the aortorenal anatomy as well as the hemodynamic significance of any renal artery stenoses identified. In this article, we present our approach to renal MR angiography; discuss normal findings, pitfalls, and anatomic variations; and describe the appearances of renal vascular disease, which includes renal artery stenosis, extension of aortic dissection, renal artery aneurysm, stenosis of a transplanted renal artery, and renal vein and caval invasion by renal cell carcinoma.
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RENAL MR ANGIOGRAPHY
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Patient Selection
Initially, patients referred for renal MR angiography at our institution all had contraindications to conventional angiography. These included renal insufficiency (serum creatinine level 
2.0 mg/dL [177 µmol/L]) and allergy to iodinated contrast material. However, MR angiography has become the preferred primary technique for imaging the renal arteries. Now, all patients suspected to have renal artery stenosis undergo renal MR angiography routinely; conventional arteriography is performed only as part of an interventional procedure or in young patients suspected to have fibromuscular dysplasia when results of MR angiography are inadequate.
Technique
Several flow-based MR angiography techniques have been used to directly image the renal arteries and veins (18–34) (Tables 1, 2). However, there are limitations to these techniques, including turbulence-induced signal loss at stenoses (12,40,41); in-plane saturation (41); nonvisualization of small-caliber vessels such as distal renal arteries and accessory renal arteries; and poor quality due to slow flow in patients with cardiac disease or aortic aneurysm or older patients. Accordingly, contrast-enhanced MR arteriography performed with a high dose of gadolinium contrast material and a high-resolution 3D spoiled gradient-echo pulse sequence is preferred (36–44). Gadolinium contrast material can be used safely, even at high doses, in patients with renal failure (45). Three-dimensional gadolinium-enhanced MR angiography demonstrates the renal arteries along with the entire abdominal aorta, iliac arteries, and mesenteric arteries in a 20–30-second acquisition that can be performed during breath holding. The renal vein and inferior vena cava can be evaluated by repeating the examination during the venous and equilibrium phases.
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Table 1. Sensitivity and Specificity of Nonenhanced MR Angiography in Detection of Proximal Renal Artery Stenosis
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TABLE 2. Sensitivity and Specificity of Gadolinium-enhanced MR Angiography in Evaluating the Entire Renal Artery for Stenotic Disease
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However, morphologic assessment of the arterial lumen with 3D gadolinium-enhanced MR angiography is not sufficient for complete assessment in patients suspected to have renal artery stenosis. It is necessary to evaluate the hemodynamic significance of any stenosis identified to determine if the patient will benefit from a renal revascularization procedure. Many MR imaging–based techniques have been proposed for evaluating the hemodynamic significance of renal artery stenosis, including (a) measurement of renal blood flow with 2D cine PC imaging (8); (b) identification of the turbulence-induced spin dephasing at hemodynamically significant stenoses with 3D PC imaging (9,46); (c) examination of the temporal enhancement pattern (47); (d) identification of differential excretion of gadolinium (7); and (e) evaluation of the effect of angiotensin-converting enzyme inhibition on flow measurement with MR imaging or gadolinium clearance rates (Grist TM, oral communication, September 1998). Many of these approaches are difficult to implement because of the challenge of reliable electrocardiographic gating and the postprocessing required. However, 3D PC pulse sequences are widely available and easy to perform, allow easy postprocessing, and do not require electrocardiographic gating. In addition, the image quality of 3D PC MR angiography is substantially improved after administration of a high dose of gadolinium contrast material. These qualities make 3D PC MR angiography a highly complementary sequence for performance after 3D gadolinium-enhanced MR angiography.
Imaging Protocol
Our comprehensive imaging approach is designed to provide contrast-enhanced arteriograms and allow hemodynamic characterization of renal artery stenosis with a single MR imaging examination that can be easily completed in a 1-hour time slot (Table 3) by using a 1.5-T imaging system with high-performance gradients (LX Echospeed; GE Medical Systems, Milwaukee, Wis). Although the data are acquired by the technologist without monitoring, the radiologist performs the postprocessing. In about 50% of patients, the images obtained with one pulse sequence are of poor quality. However, the complementary information provided by this combination of sequences allows diagnostic results to be obtained in 95% of patients. Callbacks are rarely necessary. Our routine approach is as follows:
Patient Preparation.—Administration of oxygen helps patients who are short of breath to suspend breathing during the entire 3D gadolinium-enhanced MR angiography acquisition (48). It is acceptable and easiest to use a body coil because it provides a large field of view with homogeneous signal. Torso or body phased-array coils allow a higher signal-to-noise ratio to be achieved in thinner patients, although the bright, near-field artifact from tissue close to the coil can be a problem. A landmark is placed on the lower margin of the rib cage along the axillary line to center imaging on the kidneys.
Sagittal Localization.—A T1-weighted spin-echo technique, preferably performed with interleaved acquisition and an echo time of approximately 15–20 seconds, provides a "black blood" effect for locating arteries (Fig 1a). Alternatively, spoiled gradient-echo imaging can be used with one or two breath holds. Single-shot fast spin-echo imaging can be used even without breath holding, although breath holding is preferred (Fig 1b).
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Figure 1a. Sagittal T1-weighted spin-echo (a) and single-shot fast spin-echo (b) MR images obtained for localization show the abdominal aorta and the origins of the celiac artery (open arrow) and superior mesenteric artery (solid arrow). The position of the 3D volume for gadolinium-enhanced MR angiography is represented by the rectangular black outline in a, which includes the abdominal aorta and most of the kidneys (curved black outlines). The tracker for automatic triggering is placed on the aorta at the level of the superior mesenteric artery.
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Figure 1b. Sagittal T1-weighted spin-echo (a) and single-shot fast spin-echo (b) MR images obtained for localization show the abdominal aorta and the origins of the celiac artery (open arrow) and superior mesenteric artery (solid arrow). The position of the 3D volume for gadolinium-enhanced MR angiography is represented by the rectangular black outline in a, which includes the abdominal aorta and most of the kidneys (curved black outlines). The tracker for automatic triggering is placed on the aorta at the level of the superior mesenteric artery.
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Axial 2D T2-weighted Imaging.—Axial T2-weighted images obtained with fat saturation are useful for characterizing any masses that are present. On T2-weighted images, simple renal cysts can be distinguished from more complex lesions that are suspicious for malignancy. The entire length of the kidneys is covered in this sequence. By using a sufficient number of signals and repetition time to make this sequence at least 4–5 minutes long, there will be sufficient time to set up the following more complex 3D gadolinium-enhanced MR angiography sequence.
Three-dimensional Gadolinium-enhanced MR Angiography.—A 3D spoiled gradient-echo volume that includes the entire abdominal aorta, renal arteries, and iliac arteries is used. In selecting the specific parameters for the 3D spoiled gradient-echo sequence, one can take advantage of several interesting aspects of MR physics to obtain the highest-quality images. In general, faster is better for data acquisition with 3D gadolinium-enhanced MR angiography. Faster data acquisitions allow the same dose of gadolinium contrast material to be injected over a shorter data acquisition time with a faster injection rate to produce a higher arterial gadolinium concentration. This higher arterial gadolinium concentration may then make up for the reduced time for T1 relaxation and signal averaging. Fast data acquisitions also result in less motion artifact and make it easier for patients to suspend breathing. To make the data acquisition fast, use the shortest possible repetition time, a short echo time, and the smallest number of sections sufficient to cover the arterial anatomy with adequate resolution. However, avoid using too wide of a bandwidth. Widening the bandwidth makes the data acquisition faster but at a substantial sacrifice in signal-to-noise ratio. Also take into consideration that the timing of bolus injection is difficult with data acquisitions of 30 seconds or shorter but relatively easy with data acquisitions of 40 seconds or longer.
It is important to make the echo time less than 3 msec to avoid excessive spin dephasing, especially from the swirling jet flow distal to stenoses. Selecting an echo time at which fat and water are out of phase (~2–2.5 msec) will help suppress fat. Fat suppression is important because fat is the brightest background tissue. The signal of background tissue can also be reduced by obtaining a 3D data set before contrast material administration to use for digital subtraction.
In theory, the flip angle should be tuned for optimum T1 contrast on the basis of the repetition time and expected blood gadolinium concentration. However, we have found that a flip angle of 45° works well in nearly all cases. The flip angle could be larger for higher doses of contrast material and longer repetition time or smaller for lower doses of contrast material and shorter repetition time.
Correctly estimating the breath-hold capacity of the patient is essential because breath holding is required during image acquisition (49). Usually, older patients, patients who smoke, and patients with cardiopulmonary disease can suspend breathing for only 20–25 seconds or less. Younger, nonsmoking patients without cardiopulmonary disease can easily hold their breath for 30–40 seconds or longer. The section thickness, number of sections, and number of phase-encoding steps should be adjusted to ensure that the acquisition time is short enough for the patient to suspend breathing for the entire data acquisition.
When prescribing the 3D spoiled gradient-echo image volume, one should first estimate how long the patient can suspend breathing. Then, adjust the coverage, section thickness, and number of phase-encoding steps so that the data acquisition covers the aorta and renal arteries and can still be completed within the patient's breath-hold capacity. Prescribe the image volume from anterior to the abdominal aorta to posterior to the midkidney by using 2–3-mm-thick sections (Fig 1a). Zero padding or zero filling by a factor of 2 in the section direction is useful because it doubles the number of sections without increasing imaging time. Set the top of the imaging volume 2–3 cm above the celiac artery. Use a field of view that is about as large as the patient is wide to avoid wraparound artifact. Typically, 30–36 cm is sufficient and will also ensure that the iliac arteries are included inferiorly. It is also essential to elevate the arms with cushions, exclude them with Faraday shields, or elevate them over the chest or head to prevent wraparound of the arms into the imaging volume.
Correct timing of the bolus injection is critical to synchronize the moment of peak renal artery enhancement with acquisition of central k-space data. Correct timing of the injection can be accomplished with a gadolinium detection pulse sequence (SmartPrep [GE Medical Systems]; Bolustrack [Philips Medical Systems, Shelton, Conn]; Care Bolus [Siemens Medical Systems, Iselin, NJ]; or a test bolus injection with any imager) (50–52) or empirically on the basis of the patient's age and cardiovascular status. Typically, a delay of 10 seconds between the start of injection and the start of imaging works in the majority of patients when sequential phase encoding and a 40–50-second data acquisition are used. Shorter data acquisitions are more difficult to time consistently without performing a test bolus injection or gadolinium detection pulse sequence.
The dose of gadolinium contrast material is one of the most important determinants of image quality. A dose of 40 mL is given in most cases to simplify a standard pattern of manual injection with an injection rate of 2 mL/sec. It is necessary to flush the contrast material through the intravenous tubing and veins with at least 20 mL of normal saline solution to ensure delivery of the entire dose and rapid venous return. It is helpful to use a standardized intravenous tubing set with an automatic mechanism for switching between contrast material infusion and flushing with no delay (SmartSet; Topspins, Ann Arbor, Mich).
For further characterization of renal function, the rate of contrast material transit in the kidney should be assessed. Such assessment can be accomplished by obtaining repeat data sets during the arterial, venous, and equilibrium phases with three separate breath holds. Alternatively, several 3D data sets can be acquired in a single breath hold with fast multiphase 3D gadolinium-enhanced MR angiography (38). With this technique, the acquisition time for a single 3D data set is reduced to just a few seconds, thus allowing demonstration of minor changes in the temporal evolution of renal enhancement (Fig 2).
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Figure 2a. Coronal images obtained with multiphase 3D gadolinium-enhanced MR angiography (repetition time msec/echo time msec = 3.2/1.1, field of view = 27 x 36 cm, slab thickness = 8 cm, number of reconstructed sections = 44, acquisition time per phase = 6.3 seconds) show the evolution of renal enhancement. (a) Early arterial-phase image shows that the renal arteries are completely enhanced. There is a proximal high-grade stenosis of the left renal artery (arrow) and a normal right renal artery. No parenchymal enhancement is present. (b) Late arterial-phase image shows the stenosis (solid arrow) and delayed enhancement of the shrunken left kidney (open arrows). (c) Early venous-phase image shows that the enhancement of the left kidney (arrows) is equal to that of the right kidney. (Reprinted, with permission, from reference 53.)
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Figure 2b. Coronal images obtained with multiphase 3D gadolinium-enhanced MR angiography (repetition time msec/echo time msec = 3.2/1.1, field of view = 27 x 36 cm, slab thickness = 8 cm, number of reconstructed sections = 44, acquisition time per phase = 6.3 seconds) show the evolution of renal enhancement. (a) Early arterial-phase image shows that the renal arteries are completely enhanced. There is a proximal high-grade stenosis of the left renal artery (arrow) and a normal right renal artery. No parenchymal enhancement is present. (b) Late arterial-phase image shows the stenosis (solid arrow) and delayed enhancement of the shrunken left kidney (open arrows). (c) Early venous-phase image shows that the enhancement of the left kidney (arrows) is equal to that of the right kidney. (Reprinted, with permission, from reference 53.)
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Figure 2c. Coronal images obtained with multiphase 3D gadolinium-enhanced MR angiography (repetition time msec/echo time msec = 3.2/1.1, field of view = 27 x 36 cm, slab thickness = 8 cm, number of reconstructed sections = 44, acquisition time per phase = 6.3 seconds) show the evolution of renal enhancement. (a) Early arterial-phase image shows that the renal arteries are completely enhanced. There is a proximal high-grade stenosis of the left renal artery (arrow) and a normal right renal artery. No parenchymal enhancement is present. (b) Late arterial-phase image shows the stenosis (solid arrow) and delayed enhancement of the shrunken left kidney (open arrows). (c) Early venous-phase image shows that the enhancement of the left kidney (arrows) is equal to that of the right kidney. (Reprinted, with permission, from reference 53.)
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Three-dimensional PC Imaging.—Finally, axial 3D PC images can be acquired immediately after the dynamic gadolinium-enhanced acquisition to further characterize the functional significance of renal artery stenoses and to improve evaluation of the distal renal artery. The velocity encoding should be set at 50 cm/sec for patients with normal renal blood flow. It can be reduced to 30 cm/sec for patients with heart failure, renal failure (creatinine level >2.0 mg/dL [177 µmol/L]), or aortic aneurysm or those aged 70 years or older. Young (<30 years of age) or athletic patients may require a velocity encoding of 60 cm/sec. Precise quantification of renal blood flow can be performed with an electrocardiographically gated 2D cine PC flow measurement technique. With this technique, multiple 2D images are obtained at a single location perpendicular to the long axis of the renal artery to show the cross-sectional renal blood flow at high temporal resolution over the cardiac cycle (Fig 3).
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Figure 3. Diagram shows renal blood flow measurements obtained with cine PC MR imaging (repetition time/echo time = 26/6) performed bilaterally perpendicular to the vessel axis. The right renal artery has a normal flow profile with an early systolic peak. The mean flow rate is 310 mL/min. The left renal artery has a flattened flow profile with loss of the systolic velocity components. The mean flow rate is only 93 mL/min. The diagnosis of a hemodynamically and functionally significant stenosis of the left renal artery was made. (Reprinted, with permission, from reference 53.)
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Image Analysis
Measurement of Kidney Length and Parenchymal Thickness.—Kidney length and parenchymal thickness can be measured and corticomedullary differentiation can be demonstrated on the sagittal localization images (Fig 4a). Alternatively, 3D gadolinium-enhanced MR angiograms can be used, especially if the renal axis is unusual (Fig 4b, 4c). Kidney length and parenchymal thickness are reduced in patients with long-standing renal artery stenosis.
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Figure 4a. (a) Sagittal T1-weighted spin-echo MR image shows a right kidney of normal length and parenchymal thickness. (b, c) Oblique reformation images from arterial-phase 3D gadolinium-enhanced MR angiography also show measurement of kidney length.
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Figure 4b. (a) Sagittal T1-weighted spin-echo MR image shows a right kidney of normal length and parenchymal thickness. (b, c) Oblique reformation images from arterial-phase 3D gadolinium-enhanced MR angiography also show measurement of kidney length.
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Figure 4c. (a) Sagittal T1-weighted spin-echo MR image shows a right kidney of normal length and parenchymal thickness. (b, c) Oblique reformation images from arterial-phase 3D gadolinium-enhanced MR angiography also show measurement of kidney length.
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Characterization of Renal Masses.—The axial T2-weighted images allow evaluation and characterization of any renal masses or other abdominal pathologic conditions. Any suspicious lesion can also be evaluated by examining the 3D gadolinium-enhanced MR angiography source images, which are somewhat analogous to contrast-enhanced CT scans. Do not analyze masses on maximum-intensity projection (MIP) images because important details and sometimes the entire mass may be obscured.
Reconstruction and Reformatting.—After acquisition of the 3D MR angiographic data, postprocessing is performed, which requires an additional 10–20 minutes of work on a computer workstation (Advantage Windows; GE Medical Systems) to obtain MIP and reformation images of the abdominal aorta, each renal artery, the celiac and superior mesenteric arteries (Fig 5), and the common iliac arteries. Subvolume MIP images encompassing each renal artery in coronal oblique and axial oblique planes are produced so that each renal artery origin is evaluated with perpendicular views to help identify eccentric atherosclerotic plaque (Fig 6).
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Figure 5a. (a, b) Axial reformation images show the origins of the celiac artery (a) and superior mesenteric artery (SMA) (b). (c) Sagittal subvolume MIP image reconstructed from axial reformation images shows normal celiac, superior mesenteric (SMA), and inferior mesenteric (IMA) arteries.
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Figure 5b. (a, b) Axial reformation images show the origins of the celiac artery (a) and superior mesenteric artery (SMA) (b). (c) Sagittal subvolume MIP image reconstructed from axial reformation images shows normal celiac, superior mesenteric (SMA), and inferior mesenteric (IMA) arteries.
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Figure 5c. (a, b) Axial reformation images show the origins of the celiac artery (a) and superior mesenteric artery (SMA) (b). (c) Sagittal subvolume MIP image reconstructed from axial reformation images shows normal celiac, superior mesenteric (SMA), and inferior mesenteric (IMA) arteries.
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Figure 6a. (a) Axial reformation image shows the renal arteries. (b) Coronal oblique subvolume MIP image reconstructed from axial reformation images shows the renal arteries. (c) Axial oblique subvolume MIP image reconstructed from a coronal oblique subvolume MIP shows the entire renal arteries.
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Figure 6b. (a) Axial reformation image shows the renal arteries. (b) Coronal oblique subvolume MIP image reconstructed from axial reformation images shows the renal arteries. (c) Axial oblique subvolume MIP image reconstructed from a coronal oblique subvolume MIP shows the entire renal arteries.
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Figure 6c. (a) Axial reformation image shows the renal arteries. (b) Coronal oblique subvolume MIP image reconstructed from axial reformation images shows the renal arteries. (c) Axial oblique subvolume MIP image reconstructed from a coronal oblique subvolume MIP shows the entire renal arteries.
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Assessing the Severity of Renal Artery Stenosis.—Initial grading of any renal artery stenosis can be performed by evaluating 3D gadolinium-enhanced MR angiograms and 3D PC images for spin dephasing (Table 4) (Fig 7). The determination of hemodynamic significance can be further refined by considering additional factors. Poststenotic dilatation is also associated with hemodynamically significant stenoses. Semiquantitative assessment of the hemodynamic significance can be accomplished by looking at changes in the flow profile produced with cine PC imaging, particularly for delay or loss of the early systolic peak (8). In addition, functional changes in the renal parenchyma can be appreciated by looking for loss of corticomedullary differentiation, delayed renal enhancement, and asymmetric concentration of gadolinium in the collecting systems as well as reduced kidney length and parenchymal thickness.
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Figure 7a. Coronal (top) and axial (middle) 3D gadolinium-enhanced MR angiograms and axial 3D PC MR images (bottom) show normal renal arteries (a), mild right renal artery stenosis (<50%) (b), and moderate left renal artery stenosis and severe right renal artery stenosis (c). The 3D PC image shows a normal artery when there is only mild stenosis (solid arrow) but shows spin dephasing in the region of the severe stenosis (arrowhead). Therefore, in mild stenosis, lesion severity is underestimated with 3D PC imaging and the artery appears normal. In severe stenosis, lesion severity is overestimated with 3D PC imaging, which shows focal occlusion. In moderate stenosis (open arrow), the 3D gadolinium-enhanced MR angiograms and 3D PC images have a similar appearance.
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Figure 7b. Coronal (top) and axial (middle) 3D gadolinium-enhanced MR angiograms and axial 3D PC MR images (bottom) show normal renal arteries (a), mild right renal artery stenosis (<50%) (b), and moderate left renal artery stenosis and severe right renal artery stenosis (c). The 3D PC image shows a normal artery when there is only mild stenosis (solid arrow) but shows spin dephasing in the region of the severe stenosis (arrowhead). Therefore, in mild stenosis, lesion severity is underestimated with 3D PC imaging and the artery appears normal. In severe stenosis, lesion severity is overestimated with 3D PC imaging, which shows focal occlusion. In moderate stenosis (open arrow), the 3D gadolinium-enhanced MR angiograms and 3D PC images have a similar appearance.
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Figure 7c. Coronal (top) and axial (middle) 3D gadolinium-enhanced MR angiograms and axial 3D PC MR images (bottom) show normal renal arteries (a), mild right renal artery stenosis (<50%) (b), and moderate left renal artery stenosis and severe right renal artery stenosis (c). The 3D PC image shows a normal artery when there is only mild stenosis (solid arrow) but shows spin dephasing in the region of the severe stenosis (arrowhead). Therefore, in mild stenosis, lesion severity is underestimated with 3D PC imaging and the artery appears normal. In severe stenosis, lesion severity is overestimated with 3D PC imaging, which shows focal occlusion. In moderate stenosis (open arrow), the 3D gadolinium-enhanced MR angiograms and 3D PC images have a similar appearance.
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NORMAL FINDINGS, PITFALLS, AND ANATOMIC VARIATIONS
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The length of a normal kidney is typically 11–13 cm. The right kidney is typically a little shorter (by about 1 cm) than the left. One renal MR angiography study showed the renal parenchymal thickness to be 1.7 cm ± 0.3 for kidneys supplied by widely patent renal arteries (9). In patients with normal renal function, 3D PC imaging demonstrates intense cortical enhancement with corticomedullary differentiation and bright arteries. With the combination of 3D PC imaging and 3D gadolinium-enhanced MR angiography, the entire renal artery up to the first level of branching in the renal hilum can be evaluated (Fig 7a). When 3D PC imaging or 3D gadolinium-enhanced MR angiography shows a normal renal artery, it is considered to be normal.
The most common pitfall is poor-quality images caused by failure to suspend breathing, poor timing of the bolus injection, or inadequate dose of gadolinium contrast material. In these instances, 3D PC imaging may be the best sequence for evaluating the renal arteries. However, 3D PC imaging is limited in patients with slow flow or renal failure or when the velocity encoding does not closely match the renal flow velocity. In addition, 3D PC imaging commonly demonstrates artifactual spin dephasing at the renal artery origins. One should also watch out for surgical clip and stent artifacts, which may be identified on source images as an area of signal dropout adjacent to an extremely bright spot related to mismapping of signal. Finally, there are many pitfalls associated with image reconstruction. Eccentric disease may be identified on only one view; thus, it is important to look at renal arteries on multiple views. When the renal artery overlaps the enhancing cortex or renal vein on an MIP image, there may be an artifactual appearance of stenosis if the MIP is too thick. However, if the MIP is too thin, artifactual stenosis or occlusion may be caused by the artery being outside the MIP volume. One last pitfall to be aware of is ringing artifact from bolus timing errors, which can mimic dissection (54).
It is important to be aware of the possibility of accessory or aberrant renal arteries, which occur in an estimated 30% of patients (55). Most accessory renal arteries originate from the abdominal aorta (Fig 8). However, rarely, an accessory renal artery arises from a common iliac artery. Accessory renal arteries are especially common in patients with horseshoe kidney (Fig 9). The left renal vein usually passes anterior to the aorta just behind the superior mesenteric artery; however, the left renal vein may course behind the aorta, a variant known as a retroaortic renal vein (Fig 10). The presence of both a normal left renal vein and a retroaortic vein is known as a circumaortic renal vein. Duplication of the right renal vein is also common.
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Figure 8. Coronal subvolume MIP image from 3D gadolinium-enhanced MR angiography shows normal renal arteries bilaterally with an accessory left renal artery (arrow) arising from the aorta.
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Figure 9a. (a) Coronal subvolume MIP image from arterial-phase 3D gadolinium-enhanced MR angiography shows the main left and right renal arteries in their expected locations as well as bilateral accessory renal arteries (arrows). (b) Coronal subvolume MIP image from venous-phase 3D gadolinium-enhanced MR angiography shows both renal veins (arrows) and fusion of the lower renal poles (horseshoe kidney).
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Figure 9b. (a) Coronal subvolume MIP image from arterial-phase 3D gadolinium-enhanced MR angiography shows the main left and right renal arteries in their expected locations as well as bilateral accessory renal arteries (arrows). (b) Coronal subvolume MIP image from venous-phase 3D gadolinium-enhanced MR angiography shows both renal veins (arrows) and fusion of the lower renal poles (horseshoe kidney).
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Figure 10. Coronal subvolume MIP image from equilibrium-phase 3D gadolinium-enhanced MR angiography shows a retroaortic left renal vein (arrow) with its characteristic inferior insertion on the inferior vena cava.
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RENAL VASCULAR DISEASE
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Renal Artery Stenosis
Atherosclerotic stenosis is the most common pathologic condition of the renal arteries. Usually, atherosclerotic renal artery stenosis is a manifestation of generalized atherosclerosis that also involves coronary, cerebral, and peripheral vessels; however, in 15%–20% of cases, atherosclerotic renal artery stenosis is not associated with disease elsewhere. In patients with renovascular hypertension, atherosclerosis is usually present in the aorta and typically compromises the ostium or the proximal 1–2 cm of one or both renal arteries. In rare cases, atherosclerosis may be isolated to the distal renal artery or renal artery branches. When left untreated, atherosclerotic stenosis progresses to renal artery occlusion and permanent loss of the renal parenchyma (Fig 11). For this reason, in patients with renovascular hypertension or renal insufficiency, it is important to detect and treat renal artery stenosis early. The sensitivity and specificity data in Table 2 show the accuracy of 3D gadolinium-enhanced MR angiography in the detection of renal artery stenosis since 1995. Furthermore, several studies have identified functional changes that indicate the severity of stenosis, including significant differences in parenchymal enhancement and cortical thickness (9), signal dropout at the region of stenosis on 3D PC angiograms (9,10), reduction of mean flow and the early systolic peak in flow profiles
produced with cine PC imaging (8), changes in the gadolinium extraction fraction and glomerular filtration rate at MR imaging (56), and changes at captopril-sensitized dynamic MR imaging in patients with renovascular hypertension (57).
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Figure 11a. (a) Sagittal T1-weighted MR image shows that the left kidney is reduced in length with parenchymal thinning. (b) Sagittal T1-weighted MR image shows that the right kidney is of normal size. (c) Coronal subvolume MIP image from 3D gadolinium-enhanced MR angiography shows atherosclerotic changes in the abdominal aorta, stenosis of the right renal artery (solid arrow), and occlusion of the left renal artery (open arrow). (d) Axial MIP image from 3D PC imaging shows spin dephasing in the region of stenosis (solid arrowhead). The occluded left renal artery is not visible (open arrowhead). (e) Conventional arteriogram shows the same findings seen on the MR angiograms (c, d). There was a pressure gradient of 60 mm Hg across the right renal artery stenosis.
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Figure 11b. (a) Sagittal T1-weighted MR image shows that the left kidney is reduced in length with parenchymal thinning. (b) Sagittal T1-weighted MR image shows that the right kidney is of normal size. (c) Coronal subvolume MIP image from 3D gadolinium-enhanced MR angiography shows atherosclerotic changes in the abdominal aorta, stenosis of the right renal artery (solid arrow), and occlusion of the left renal artery (open arrow). (d) Axial MIP image from 3D PC imaging shows spin dephasing in the region of stenosis (solid arrowhead). The occluded left renal artery is not visible (open arrowhead). (e) Conventional arteriogram shows the same findings seen on the MR angiograms (c, d). There was a pressure gradient of 60 mm Hg across the right renal artery stenosis.
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Figure 11c. (a) Sagittal T1-weighted MR image shows that the left kidney is reduced in length with parenchymal thinning. (b) Sagittal T1-weighted MR image shows that the right kidney is of normal size. (c) Coronal subvolume MIP image from 3D gadolinium-enhanced MR angiography shows atherosclerotic changes in the abdominal aorta, stenosis of the right renal artery (solid arrow), and occlusion of the left renal artery (open arrow). (d) Axial MIP image from 3D PC imaging shows spin dephasing in the region of stenosis (solid arrowhead). The occluded left renal artery is not visible (open arrowhead). (e) Conventional arteriogram shows the same findings seen on the MR angiograms (c, d). There was a pressure gradient of 60 mm Hg across the right renal artery stenosis.
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Figure 11d. (a) Sagittal T1-weighted MR image shows that the left kidney is reduced in length with parenchymal thinning. (b) Sagittal T1-weighted MR image shows that the right kidney is of normal size. (c) Coronal subvolume MIP image from 3D gadolinium-enhanced MR angiography shows atherosclerotic changes in the abdominal aorta, stenosis of the right renal artery (solid arrow), and occlusion of the left renal artery (open arrow). (d) Axial MIP image from 3D PC imaging shows spin dephasing in the region of stenosis (solid arrowhead). The occluded left renal artery is not visible (open arrowhead). (e) Conventional arteriogram shows the same findings seen on the MR angiograms (c, d). There was a pressure gradient of 60 mm Hg across the right renal artery stenosis.
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Figure 11e. (a) Sagittal T1-weighted MR image shows that the left kidney is reduced in length with parenchymal thinning. (b) Sagittal T1-weighted MR image shows that the right kidney is of normal size. (c) Coronal subvolume MIP image from 3D gadolinium-enhanced MR angiography shows atherosclerotic changes in the abdominal aorta, stenosis of the right renal artery (solid arrow), and occlusion of the left renal artery (open arrow). (d) Axial MIP image from 3D PC imaging shows spin dephasing in the region of stenosis (solid arrowhead). The occluded left renal artery is not visible (open arrowhead). (e) Conventional arteriogram shows the same findings seen on the MR angiograms (c, d). There was a pressure gradient of 60 mm Hg across the right renal artery stenosis.
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Fibromuscular dysplasia is the second most common cause of renal artery stenosis. Fibromuscular dysplasia manifests as a nonatheromatous vascular lesion in medium-sized and small arteries (58). The lesion most commonly involves the renal, carotid, and intracerebral arteries, although it has been reported in other arterial beds, including the subclavian, axillary, mesenteric, hepatic, splenic, and iliac arteries. The majority of patients are female, and fibromuscular dysplasia almost always manifests at a young age (<40 years). The histologic classification of fibromuscular dysplasia is based on the angiographic appearance and the layer of arterial wall that is primarily affected. Therefore, lesions can be classified as intimal fibroplasia, medial fibromuscular dysplasia, or perimedial (adventitial) fibroplasia. Medial fibromuscular dysplasia is the most common type and is further subdivided into medial fibroplasia (common), medial hyperplasia (rare), and perimedial dysplasia (rare). The angiographic findings of medial fibroplasia are a "string-of-beads" appearance with weblike stenoses alternating with small fusiform or saccular aneurysms. Usually, the distal two-thirds of the main renal artery is involved, sometimes with extension into segmental vessels. Bilateral involvement is common. MR angiography may not always demonstrate subtle irregularities of the distal main renal arteries associated with fibromuscular dysplasia because maximum spatial resolution is required (Fig 12). Therefore, the accuracy of MR angiography in diagnosis of fibromuscular dysplasia is not established.
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Figure 12a. (a) Coronal subvolume MIP image from 3D gadolinium-enhanced MR angiography optimized for the right renal artery shows severe stenosis (arrow). The stenosis was thought to be atherosclerotic on the basis of MR angiographic findings. (b) Digital subtraction angiogram shows fibromuscular dysplasia with much more extensive involvement of the artery (arrow).
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Figure 12b. (a) Coronal subvolume MIP image from 3D gadolinium-enhanced MR angiography optimized for the right renal artery shows severe stenosis (arrow). The stenosis was thought to be atherosclerotic on the basis of MR angiographic findings. (b) Digital subtraction angiogram shows fibromuscular dysplasia with much more extensive involvement of the artery (arrow).
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Extension of Aortic Dissection
Aortic dissection occurs when blood dissects into the media of the aortic wall through an intimal tear. Such dissection is related to degeneration of an aging aorta and may be accelerated by hypertension or in patients with underlying mural defects, including those with Marfan syndrome, Ehlers-Danlos syndrome, relapsing polychondritis, coarctation, Turner syndrome, or a replaced aortic valve. Renal artery flow can be compromised in patients with aortic dissections that extend down into the abdominal aorta (Fig 13). The dissection may extend into a renal artery, or a normal renal artery arising from the true lumen may have reduced flow due to collapse of the true lumen (59). The intimal flap may intermittently cover the renal arterial origin or may extend into the main and segmental renal arteries, thus interrupting renal blood flow. Reduced perfusion pressure in the true lumen may also reduce renal blood flow despite patency of the renal artery.
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Figure 13a. Coronal (a) and axial (b) subvolume MIP images from 3D gadolinium-enhanced MR angiography show an aortic dissection that extends distally to the aortic bifurcation. The superior mesenteric artery and right renal artery arise from the true lumen (solid arrow); the left renal artery arises from the false lumen (open arrow).
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Figure 13b. Coronal (a) and axial (b) subvolume MIP images from 3D gadolinium-enhanced MR angiography show an aortic dissection that extends distally to the aortic bifurcation. The superior mesenteric artery and right renal artery arise from the true lumen (solid arrow); the left renal artery arises from the false lumen (open arrow).
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Renal Artery Aneurysm
Aneurysms due to atherosclerosis usually occur in the infrarenal aorta and common iliac arteries. However, they can also be found in the renal arteries (Fig 14). Most renal artery aneurysms have been found in persons 50–70 years of age (60). Renal artery aneurysms are subject to the same complications as aneurysms elsewhere, including rupture, thrombosis, embolization, and dissection. Owing to its 3D nature, MR angiography is helpful in evaluating the anatomic characteristics of an aneurysm, its relationship to other vascular structures, its diameter in all orientations, and the aneurysm type, such as saccular or fusiform.
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Figure 14. Coronal subvolume MIP image from 3D gadolinium-enhanced MR angiography shows a saccular aneurysm of the right renal artery (solid arrow) and a fusiform aneurysm of the left renal artery (open arrow).
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Stenosis of a Transplanted Renal Artery
Whenever the serum creatinine level rises in a renal transplant recipient, the possibility of stenosis of a transplanted renal artery must be considered. Typically, stenoses occur at the surgical anastomosis connecting the transplanted artery to the iliac artery. There may also be atherosclerotic narrowing of the iliac artery proximal to the transplant arterial anastomosis (Fig 15). Sometimes, such narrowing occurs at the site where a surgical clamp was placed on the iliac artery at the time of transplantation. If the transplanted renal artery is widely patent but there is minimal renal enhancement and no excretion of gadolinium, then the kidney has probably been rejected.
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Figure 15. Coronal subvolume MIP image from 3D gadolinium-enhanced MR angiography shows a stenosis of the left external iliac artery (solid arrow) and a normal transplanted renal artery (open arrow). The stenosis compromised flow to the transplanted kidney, resulting in hypertension and elevation of serum creatinine level, which improved after balloon angioplasty.
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Renal Vein and Caval Invasion by Renal Cell Carcinoma
Renal cell carcinoma tends to involve the renal vein and extend into the inferior vena cava. The extent of tumor invasion of the inferior vena cava may be difficult to assess with US and even with contrast-enhanced CT. Kallman et al (61) and Choyke et al (62) have shown that MR imaging has 100% sensitivity for detection of tumor thrombus beyond the distal renal vein. T1- and T2-weighted spin-echo sequences combined with axial time-of-flight imaging allow comprehensive assessment of patients with solid renal masses. Tumor enhancement and renal vein extension can be demonstrated during the arterial and venous phases of 3D gadolinium-enhanced MR angiography (Fig 16).
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Figure 16a. Axial T2-weighted MR image (a), coronal subvolume MIP image from arterial-phase 3D gadolinium-enhanced MR angiography (b), and coronal reformation image from venous-phase 3D gadolinium-enhanced MR angiography (c) of a patient with a large renal cell carcinoma show a heterogeneously enhancing mass in the left kidney (solid arrows) with tumor extension into the left renal vein (open arrow). Two right renal veins are also seen (arrowheads).
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Figure 16b. Axial T2-weighted MR image (a), coronal subvolume MIP image from arterial-phase 3D gadolinium-enhanced MR angiography (b), and coronal reformation image from venous-phase 3D gadolinium-enhanced MR angiography (c) of a patient with a large renal cell carcinoma show a heterogeneously enhancing mass in the left kidney (solid arrows) with tumor extension into the left renal vein (open arrow). Two right renal veins are also seen (arrowheads).
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Figure 16c. Axial T2-weighted MR image (a), coronal subvolume MIP image from arterial-phase 3D gadolinium-enhanced MR angiography (b), and coronal reformation image from venous-phase 3D gadolinium-enhanced MR angiography (c) of a patient with a large renal cell carcinoma show a heterogeneously enhancing mass in the left kidney (solid arrows) with tumor extension into the left renal vein (open arrow). Two right renal veins are also seen (arrowheads).
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CONCLUSIONS
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Abstract
INTRODUCTION
