(Radiographics. 2001;21:357-370.)
© RSNA, 2001
Three-dimensional Gadolinium-enhanced MR Angiography: Applications for Abdominal Imaging1
James F. Glockner, MD, PhD
1 From the Department of Radiology, St Louis University Hospital, 3635 Vista Ave at Grand Blvd, PO Box 15250, St Louis, MO 63110-0250. Presented as a scientific exhibit at the 1999 RSNA scientific assembly. Received April 3, 2000; revision requested June 20 and received August 8; accepted August 11. Address correspondence to J.F.G., Department of Radiology, Mayo Clinic, 200 First St SW, Rochester, MN 55971 (e-mail: glockner.james@mayo.edu).
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Abstract
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Three-dimensional (3D) gadolinium-enhanced magnetic resonance (MR) angiography is a versatile technique that combines speed, superb contrast, and relative simplicity. It has a wide range of applications, particularly in the abdomen and pelvis, where superb images of the abdominal aorta and renal arteries are routinely obtained. Aneurysms, atherosclerotic lesions, and occlusions of the major mesenteric arteries are also well depicted. In addition, 3D gadolinium-enhanced MR angiography is ideal for noninvasive evaluation of the systemic and mesenteric veins and can be used to demonstrate parenchymal lesions in the liver, pancreas, kidneys, and other organs. It is also useful in staging genitourinary neoplasms: Parenchymal lesions, venous extension, and adenopathy are all clearly depicted. Three-dimensional gadolinium-enhanced MR angiography can be useful in the preoperative evaluation of potential transplant donors and recipients and in the evaluation of vascular complications following transplantation. Delayed 3D acquisitions of the kidneys, ureters, and bladder can be performed routinely to generate gadolinium-enhanced urograms and demonstrate obstruction, delayed function, filling defects, and masses. A variety of methods for increasing the speed and improving the resolution of 3D acquisition are currently under investigation. These include novel imaging and reformatting techniques and the use of intravascular contrast agents with much longer vascular half-lives.
Index Terms: Abdomen, MR, 95.12942, 96.12942, 98.12942 • Angiography, 95.12942, 96.12942, 98.12942 • Magnetic resonance (MR), maximum intensity projection, 95.12942, 96.12942, 98.12942 • Magnetic resonance (MR), three-dimensional, 95.12942, 96.12942, 98.12942 • Magnetic resonance (MR), vascular studies, 95.12942, 96.12942, 98.12942
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Introduction
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Three-dimensional (3D) gadolinium-enhanced magnetic resonance (MR) angiography has become very popular in the few years since its inception. A technique that combines speed, superb contrast, and relative simplicity, 3D gadolinium-enhanced MR angiography has been applied to virtually all regions of the body from the extremities to the brain. The ability to cover large regions of interest within a single breath hold makes this technique particularly well suited for abdominal imaging, in which respiratory motion had previously been a major source of artifact.
Many of the initial studies describing this technique emphasized its application in arteriography of the aorta and renal arteries (1–5). It has become evident, however, that 3D gadolinium-enhanced MR angiography has many additional roles, particularly with regard to abdominal and pelvic imaging. Superb images of the systemic and mesenteric veins can be obtained routinely, often rivaling or surpassing the results of conventional angiography. In addition, source images or reformatted images can be obtained to help evaluate parenchymal lesions in the liver, pancreas, kidneys, and other organs. Acquisition of a delayed 3D data set allows depiction of the renal collecting system, ureters, and bladder. The combination of parenchymal and vascular information allows accurate staging of many abdominal neoplasms. In addition, vascular complications of hepatic and renal transplantation can be clearly demonstrated with 3D gadolinium-enhanced MR angiography.
In this article, we briefly discuss technical aspects of 3D gadolinium-enhanced MR angiography and methods of optimizing data acquisition and reformatting. We also illustrate various applications of this technique in abdominal imaging, including demonstrating the abdominopelvic vasculature, evaluating abdominal transplant donors and recipients, staging genitourinary neoplasms, and evaluating hepatic lesions and the renal collecting system.
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Discussion
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Technique
Technical aspects of 3D gadolinium-enhanced MR angiography are explained in greater detail elsewhere (6–8). Briefly, a fast 3D spoiled gradient-echo MR imaging sequence with minimum repetition time and echo time is used, with flip angles ranging from 25° to 50°. Intrinsic tissue contrast is low; vascular contrast is achieved with high intravascular concentrations of gadopentetate dimeglumine, which is injected intravenously as a bolus at rates ranging from 0.5 to 4 mL/sec in concentrations ranging from 0.05 to 0.3 mmol/kg, preferably with an automatic injector. The first pass of the contrast material bolus provides a brief temporal window of high intravascular concentration, which in turn generates high signal intensity by means of T1 shortening of blood. As can be inferred from the preceding discussion, timing is important; the best images are obtained when acquisition of the central portion of k space corresponds with the maximum concentration of contrast material in the vessels of interest. There should also be a good match between the duration of the contrast material bolus and the length of the 3D sequence: Rapid variation in the vascular concentration of contrast material during acquisition of the central portion of k space can generate significant artifacts.
A variety of image acquisition and injection strategies have been described to maximize vascular contrast. The simplest method with regard to bolus timing is the "best guess" method, in which a standard travel time from peripheral vein to central arteries is assumed and the 3D sequence is timed so that the central portion of k space is acquired at the presumed peak vascular contrast concentration (8,9). This has the virtue of simplicity; in practice, however, the range of travel times is broad and depends on patient age, hydration state, and cardiovascular status, so that achieving optimal timing is often a matter of luck.
An alternative method is the timing bolus, in which 1–2 mL of gadopentetate dimeglumine is injected along with saline solution for flushing and the vessel of interest is scanned approximately once per second. The travel time can be directly observed and then used to calculate the correct scan delay (1,10). This method almost always yields MR angiograms with excellent contrast. Disadvantages of this technique include residual contrast material in the renal collecting systems from the test bolus, which can obscure visualization of branch vessels of the renal arteries during MR angiography, as well as the additional time required for the test bolus sequence.
Other recent developments include fluoroscopic triggering and automated triggering, in which the vessel of interest is continuously scanned until the contrast material bolus is directly observed or the signal intensity reaches a specified level, at which time the 3D sequence is begun (4,11,12). Centric phase encoding is used in conjunction with these techniques, so that central k space is acquired at the beginning of the scan when vascular contrast is maximal.
These timing strategies are generally used to optimize the arterial-phase imaging. We have found that acquiring two additional 3D volumes after the optimized arterial-phase imaging almost always yields good visualization of the portal and systemic veins with one or both of the later sequences. However, if the clinical question primarily involves the venous system, it is possible to use any of the timing strategies described earlier to optimize the venous-phase imaging, performing this initially instead of the arterial-phase imaging. This might be particularly useful in patients with limited breath-holding ability because there is likely to be considerable motion artifact with the second, third, or fourth long breath hold.
Our typical abdominal MR angiographic examination consists of axial fast spin-echo MR imaging through the region of interest. This is useful for the evaluation of abdominal organs and can help determine landmarks to be included in the 3D volume. A timing bolus is then injected and an appropriate scan delay determined. We acquire a precontrast 3D data set, which serves as a final check that the volume has been positioned correctly and also tests the patient’s breath-holding ability. Contrast material is administered (0.15–0.2 mmol/kg at 2 mL/sec), and the arterial-phase breath-hold sequence is performed, followed immediately by two additional acquisitions with minimal intersequence delay. A final post–MR angiography axial fat-saturated spoiled gradient-echo sequence is performed to help visualize infarcts and parenchymal lesions. Delayed 3D acquisitions centered over the kidneys and ureters can be performed to generate an MR urogram.
Data Processing
Data processing is an important aspect of 3D gadolinium-enhanced MR angiography, particularly in the abdomen, where the appearance of nonvascular structures may also be of great interest. A variety of reformatting techniques are now available to the radiologist, and it is important to be well versed in as many of these as possible. Each technique has its own strengths and weaknesses, which can lead to pitfalls and artifacts in inexperienced hands.
Maximum-intensity-projection (MIP) imaging is the most common means of displaying data. With this technique, a ray is projected along the data set in the desired direction, and the highest voxel value along the ray becomes the pixel value of the two-dimensional MIP image. This method is well suited to gadolinium-enhanced MR angiography, particularly arterial-phase imaging, in which background signal is low and arterial contrast is high. Nevertheless, MIP images obtained from the entire data set are almost always contaminated somewhat by wraparound or edge artifacts, which can limit the visibility of vessels. Image quality can almost always be improved by obtaining subvolume MIP images or by manually editing the entire data set. We routinely edit the data set in three orthogonal projections to remove obvious artifact and then generate a series of MIP images which rotate through 180° or 360°. These views are often sufficient for diagnosis; if not, additional subvolumes and projections can easily be obtained.
MIP images are useful and are generally preferred by clinicians. However, source images or thin-section reformatted images should be examined routinely because arterial dissection and nonocclusive thrombus can easily be missed on MIP images.
Volume rendering, surface rendering, and virtual endoscopy are alternative techniques that may be useful in certain applications. Volume rendering in particular is now widely available on most new MR and computed tomography (CT) workstations and will likely be used much more frequently in the near future (13). Again, familiarity with this technique and its variables is important so that critical details are revealed rather than obscured.
Subtraction of a precontrast data set from the arterial- phase data is widely used in 3D gadolinium-enhanced MR angiography of the lower extremities to eliminate background noise and provide better visualization of smaller vessels (14, 15). However, this technique is more problematic in the abdomen, where any discrepancy in breath holding between acquisitions can result in misregistration artifact. In practice, arterial-phase images of the abdomen are almost always diagnostic without subtraction as long as good bolus timing has been achieved. We have found subtraction much more useful in the venous phase. Arterial-phase data are subtracted from venous-phase data, a procedure that often generates a pure venous image with little or no arterial contamination. As discussed earlier, this technique is fraught with artifacts and is generally most useful in displaying complex venous anatomy for the clinician’s benefit, with findings verified on source images or thin-section reformatted images obtained from unsubtracted data.
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Clinical Applications
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Arterial System
Three-dimensional gadolinium-enhanced MR angiography has been studied extensively as a technique for assessing the abdominal aorta (1,2,16–19). The entire aorta can often be visualized with a single acquisition, allowing accurate determination of the extent of aneurysms, dissections, and other abnormalities (Figs 1, 2). Vascular contrast is generally excellent, and data can be viewed in any projection, which facilitates surgical planning. No iodinated contrast material or ionizing radiation is necessary.
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Figure 1a. Abdominal aortic aneurysm. Sagittal (a) and coronal (b) arterial-phase MIP images from a 3D gadolinium-enhanced MR angiographic examination reveal a small infrarenal aneurysm, bilateral stenosis of the common iliac arteries, and stenosis of the right renal artery origin and celiac axis.
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Figure 1b. Abdominal aortic aneurysm. Sagittal (a) and coronal (b) arterial-phase MIP images from a 3D gadolinium-enhanced MR angiographic examination reveal a small infrarenal aneurysm, bilateral stenosis of the common iliac arteries, and stenosis of the right renal artery origin and celiac axis.
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Figure 2a. Multifocal vascular disease involving the aorta, mesenteric arteries, and renal arteries. (a, b) Coronal (a) and sagittal (b) oblique MIP images from an arterial-phase gadolinium-enhanced MR angiographic examination reveal occlusion of the abdominal aorta below the right renal artery origin. The left renal artery is not visualized, nor is the inferior mesenteric artery. Note the fusiform aneurysm of the proximal right renal artery (arrow) and the prominent collateral vessel supplying the left side of the colon (arrowheads in a). (c) Axial fat-saturated spoiled gradient-echo MR image obtained following MR angiography more clearly demonstrates the thrombosed aorta and left renal artery (arrowheads).
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Figure 2b. Multifocal vascular disease involving the aorta, mesenteric arteries, and renal arteries. (a, b) Coronal (a) and sagittal (b) oblique MIP images from an arterial-phase gadolinium-enhanced MR angiographic examination reveal occlusion of the abdominal aorta below the right renal artery origin. The left renal artery is not visualized, nor is the inferior mesenteric artery. Note the fusiform aneurysm of the proximal right renal artery (arrow) and the prominent collateral vessel supplying the left side of the colon (arrowheads in a). (c) Axial fat-saturated spoiled gradient-echo MR image obtained following MR angiography more clearly demonstrates the thrombosed aorta and left renal artery (arrowheads).
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Figure 2c. Multifocal vascular disease involving the aorta, mesenteric arteries, and renal arteries. (a, b) Coronal (a) and sagittal (b) oblique MIP images from an arterial-phase gadolinium-enhanced MR angiographic examination reveal occlusion of the abdominal aorta below the right renal artery origin. The left renal artery is not visualized, nor is the inferior mesenteric artery. Note the fusiform aneurysm of the proximal right renal artery (arrow) and the prominent collateral vessel supplying the left side of the colon (arrowheads in a). (c) Axial fat-saturated spoiled gradient-echo MR image obtained following MR angiography more clearly demonstrates the thrombosed aorta and left renal artery (arrowheads).
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It is important to remember that arterial-phase MIP images demonstrate contrast material within the patent lumen and therefore may not accurately depict the true diameter of an aneurysm. The walls of vessels can usually be identified on source images or axial reformatted images. Visualization is often improved on venous-phase images. In addition, we always perform axial T1-weighted spoiled gradient-echo or spin-echo MR imaging following angiography, which is very useful in determining the diameter of aneurysms and the extent of intraarterial plaque or thrombus. Likewise, it is important to scrutinize source images or thin-section reformatted images in patients who may have a dissection; an intimal flap can easily be missed if only MIP images are viewed.
Three-dimensional gadolinium-enhanced MR angiography is an excellent tool for evaluation of the renal arteries (Figs 1, 2). In most studies, 3D gadolinium-enhanced MR angiography demonstrated a sensitivity and specificity for significant renal artery stenosis similar to those of conventional angiography (17,20–28). Accessory renal arteries are well seen with MR angiography; however, stenosis in small accessory arteries may be difficult to detect given the relatively low resolution of 3D gadolinium-enhanced MR angiogra-phy compared with conventional angiography. Resolution is also an issue in patients with fibromuscular dysplasia: The accuracy of gadolinium-enhanced MR angiography in detecting these lesions is not clear, and therefore this technique may not be as successful in young patients with suspected renal vascular hypertension. Several authors have advocated combining 3D gadolinium-enhanced MR angiography with 3D phase-contrast MR angiography to increase the specificity of the examination for renal artery stenosis. The former provides more reliable anatomic information, whereas the latter may demonstrate signal loss from intravoxel dephasing in patientswith functionally significant stenosis (29,30). Renal size and cortical thickness have been correlated with the success of revascularization in patients with renal artery stenosis. This information can easily be obtained from 3D data sets (31).
Aneurysms, atherosclerotic lesions, and occlusions of the major mesenteric arteries are also well seen (Figs 1–3). Gadolinium-enhanced MR angiography has also been advocated as a method of evaluating patients with suspected chronic mesenteric ischemia (32–36). In general, at least two of three major mesenteric vessels need to be occluded or stenotic to account for ischemic symptoms. However, many patients with severe lesions are asymptomatic, so that additional information is needed to make a confident diagnosis. The combination of functional techniques including pre- and postprandial measurements of mesenteric venous oxygenation, arterial blood flow, and venous blood flow with the anatomic information provided by 3D gadolinium-enhanced MR angiography has been investigated, with promising results (32,37,38).
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Figure 3. Splenic artery aneurysm in a patient with portal hypertension. Axial oblique arterial-phase MIP image from a 3D gadolinium-enhanced MR angiographic examination reveals a large aneurysm in the splenic artery.
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Systemic Veins
Evaluation of the systemic veins of the abdomen with 3D gadolinium-enhanced MR angiography is less frequently cited in the literature than the various arterial applications (39); however, results are almost always diagnostic. Data analysis is complicated by arterial and mesenteric venous contamination, which limits the usefulness of full-volume MIP images. In our experience, 3- to 5-mm-thick reformatted images in multiple planes are generally most useful in revealing the presence or absence of venous disease. Subvolume MIP images and MIP images generated from a subtracted data set can also be useful and may in fact be preferred by clinicians. Limitations of this technique include reduced signal-to-noise ratios compared with arterial-phase imaging as well as problems with breath holding: Venous contrast is usually optimal on the second or third postcontrast acquisition, and sustaining multiple long breath holds becomes increasingly difficult for patients with respiratory insufficiency. Administration of oxygen may improve breath-hold ability; in addition, if the clinical question specifically involves the systemic veins, the timing bolus can be used to optimize imaging during this phase of the examination.
Applications include evaluation of patients with suspected inferior vena cava (IVC) thrombosis or Budd-Chiari syndrome (Fig 4), vascular extension by renal, adrenal, or hepatic tumors (Fig 5), and evaluation of anatomic anomalies of the IVC and systemic veins.
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Figure 4. Normal venacavographic findings in a patient with suspected Budd-Chiari syndrome. Coronal MIP image from a 3D gadolinium-enhanced MR angiographic examination (arterial-phase data subtracted from venous-phase data) reveals a normal IVC and hepatic veins. The renal veins are filled with contrast material on the arterial-phase image and are therefore not visualized on the subtracted MIP image.
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Figure 5a. Hepatoma with venous thrombosis. (a) Coronal venous-phase reformatted image from a 3D gadolinium-enhanced MR angiographic examination reveals a heterogeneously enhancing mass in the right hepatic lobe (arrows) as well as thrombus in the IVC. (b) Venous-phase reformatted image obtained slightly anterior to a reveals extensive thrombus in the hepatic veins and IVC.
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Figure 5b. Hepatoma with venous thrombosis. (a) Coronal venous-phase reformatted image from a 3D gadolinium-enhanced MR angiographic examination reveals a heterogeneously enhancing mass in the right hepatic lobe (arrows) as well as thrombus in the IVC. (b) Venous-phase reformatted image obtained slightly anterior to a reveals extensive thrombus in the hepatic veins and IVC.
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Mesenteric Veins
Much of the preceding discussion also applies to the evaluation of mesenteric veins. Excellent image quality is almost always achieved with imaging performed immediately following the arterial phase, and reformatted images or subvolume MIP images are generally the most useful for evaluating the data. MIP images generated from a subtracted data set can be useful in complex cases; however, they are prone to artifacts and should never be interpreted without reference to source images or thin-section reformatted images. Gadolinium-enhanced MR angiography is useful in evaluating patients with suspected portal or mesenteric vein thrombosis or portal hypertension (Fig 6) (34,35,40–43). Portosystemic shunts are clearly depicted, and their anatomic relationships can be viewed in any desired projection. Gadolinium-enhanced MR angiography will not help determine the direction of portal vein flow in patients with severe portal hypertension, but the superb depiction of mesenteric vessels and varices can be combined with a limited phase-contrast examination to determine flow direction.
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Figure 6. Portal hypertension and varices in a patient with cirrhosis. Coronal venous-phase reformatted image from a 3D gadolinium-enhanced MR angiographic examination (arterial-phase data subtracted from venous-phase data) reveals marked splenomegaly and extensive varices.
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Abdominal Transplants
Three-dimensional gadolinium-enhanced MR angiography can be useful in the preoperative evaluation of both potential transplant donors and recipients (44–46) as well as in the evaluation of vascular complications following transplantation (47–51). Evaluation of potential renal donors with gadolinium-enhanced MR angiography reveals accessory arteries and other variant vascular anatomy with superb sensitivity and specificity. Renal lesions are usually detectable on venous-phase images and can also be evaluated with additional conventional spin-echo or gradient-echo sequences. Limitations of gadolinium-enhanced MR angiography include its uncertain sensitivity in detecting lesions of fibromuscular dysplasia and its inability to demonstrate small renal calculi. Likewise, vascular anatomy in potential reduced-segment hepatic transplant donors can be accurately determined and parenchymal lesions identified. Potential transplant recipients can also be evaluated preoperatively with gadolinium-enhanced MR angiography. Iliac arteries and veins can be screened noninvasively in potential renal transplant recipients for potential inflow or outflow lesions. Gadolinium-enhanced MR angiography can be included in the preoperative evaluation of potential hepatic transplant recipients to evaluate for varices and portal or mesenteric vein thrombosis.
Postoperative patients are initially evaluated with ultrasonography (US), which is highly accurate in detecting vascular thrombosis and reasonably so in detecting stenotic vessels. US is occasionally limited or indeterminate and is always operator dependent. Gadolinium-enhanced MR angiography is a useful secondary technique in patients with indeterminate US findings or negative US findings with high clinical suspicion and in patients who are poor candidates for conventional angiography (Figs 7, 8). Limited data have shown gadolinium-enhanced MR angiography to be accurate in revealing vascular complications of abdominal transplantation. Limitations include susceptibility artifact from surgical clips or embolization coils, which can create pseudostenoses or obscure important vascular structures (Fig 9). Long breath holds in the immediate postoperative setting can also be problematic.
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Figure 7. Arterial stenosis following renal transplantation. Coronal oblique arterial-phase MIP image from a 3D gadolinium-enhanced MR angiographic examination reveals dual arterial supply to the transplanted kidney with severe stenosis of the artery supplying the lower pole (arrowhead). Irregularity of the distal external iliac artery represents susceptibility artifact from adjacent surgical clips. These findings were confirmed at conventional angiography.
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Figure 8. Arterial stenosis following hepatic transplantation in a patient with elevated liver enzyme levels, biopsy results that did not suggest rejection, and normal US findings. Axial oblique arterial-phase MIP image from a 3D gadolinium-enhanced MR angiographic examination reveals severe stenosis of the hepatic artery at the anastomosis (arrow). These findings were confirmed at conventional angiography.
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Figure 9a. Artifact from surgical clips in a renal transplant recipient. (a) Coronal arterial-phase MIP image from a 3D gadolinium-enhanced MR angiographic examination reveals multiple lesions in the left common and external iliac arteries. Note also the small pseudoaneurysm in the right common femoral artery (arrowhead). (b) Conventional radiograph shows multiple metallic surgical clips in the left side of the pelvis. Subsequent angiography revealed that the uppermost lesion in the left common iliac artery was real, whereas the remaining lesions were artifactual.
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Figure 9b. Artifact from surgical clips in a renal transplant recipient. (a) Coronal arterial-phase MIP image from a 3D gadolinium-enhanced MR angiographic examination reveals multiple lesions in the left common and external iliac arteries. Note also the small pseudoaneurysm in the right common femoral artery (arrowhead). (b) Conventional radiograph shows multiple metallic surgical clips in the left side of the pelvis. Subsequent angiography revealed that the uppermost lesion in the left common iliac artery was real, whereas the remaining lesions were artifactual.
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Genitourinary Neoplasms
We have incidentally detected several renal cell carcinomas in patients undergoing renal MR angiography and now routinely evaluate the renal parenchyma carefully in all patients examined for renal artery stenosis. Axial T1- and T2-weighted spin-echo MR images and postcontrast T1-weighted spoiled gradient-echo images are also helpful for lesion characterization. Parenchymal lesions, venous extension, and adenopathy are all clearly visualized at gadolinium-enhanced MR angiography (Figs 10–12) (52). Enlarged feeding arteries are demonstrated and may be embolized prior to surgery. Renal artery stenosis in the opposite kidney can be detected and addressed prior to nephrectomy.
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Figure 10a. Incidental renal cell carcinoma in a patient with hypertension. (a) Coronal venous-phase reformatted image from a 3D gadolinium-enhanced MR angiographic examination reveals a large, heterogeneous mass in the upper pole of the left kidney. (b) Reformatted image obtained slightly anterior to a demonstrates patency of the left renal vein and IVC.
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Figure 10b. Incidental renal cell carcinoma in a patient with hypertension. (a) Coronal venous-phase reformatted image from a 3D gadolinium-enhanced MR angiographic examination reveals a large, heterogeneous mass in the upper pole of the left kidney. (b) Reformatted image obtained slightly anterior to a demonstrates patency of the left renal vein and IVC.
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Figure 11a. Renal cell carcinoma with vascular invasion. (a) Coronal arterial-phase reformatted image from a 3D gadolinium-enhanced MR angiographic examination reveals streaky linear enhancement in the left renal vein and IVC, a finding that is compatible with tumor thrombus. (b) Coronal venous-phase reformatted image more clearly demonstrates a large left renal mass (arrows) and the extent of thrombus within the intrahepatic IVC.
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Figure 11b. Renal cell carcinoma with vascular invasion. (a) Coronal arterial-phase reformatted image from a 3D gadolinium-enhanced MR angiographic examination reveals streaky linear enhancement in the left renal vein and IVC, a finding that is compatible with tumor thrombus. (b) Coronal venous-phase reformatted image more clearly demonstrates a large left renal mass (arrows) and the extent of thrombus within the intrahepatic IVC.
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Figure 12. Renal cell carcinoma with adenopathy. Coronal venous-phase reformatted image from a 3D gadolinium-enhanced MR angiographic examination reveals a patent left renal vein and IVC with extensive aortocaval adenopathy (arrowheads). Note the large mass in the lower pole of the left kidney.
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Hepatic Lesions
A few investigators have begun to evaluate the potential of 3D gadolinium-enhanced MR angiography in detecting and characterizing parenchymal lesions in the liver and other organs (53,54). The 3D acquisition is often performed in an axial plane, and spectral fat saturation can be added to the pulse sequence at the cost of a few extra seconds. This technique has several inherent advantages. Multiphase imaging is possible, and therefore the enhancement characteristics of vascular lesions such as hemangiomas, focal nodular hyperplasia, and hepatomas can be completely characterized (Fig 13). Vascular invasion by malignancies such as hepatoma and cholangiocarcinoma can easily be demonstrated (Fig 5). Volumetric acquisitions are performed with contiguous thin sections, providing more complete anatomic coverage than is typically achieved with conventional techniques. It is often possible to obtain isotropic voxels, so that the 3D data can be reformatted and viewed in any desired projection without loss of resolution, and small or subtle lesions are less likely to be missed. However, it is uncertain whether these advantages will compensate for the intrinsically low parenchymal contrast of the 3D gradient-echo sequence, and sensitivity and specificity relative to other techniques have not yet been determined.
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Figure 13a. Hepatic hemangioma. (a) Coronal oblique early-venous-phase reformatted image from a 3D gadolinium-enhanced MR angiographic examination reveals a large lesion adjacent to the IVC with peripheral nodular enhancement. (b, c) On delayed images (c obtained slightly after b), the lesion demonstrates progressive filling.
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Figure 13b. Hepatic hemangioma. (a) Coronal oblique early-venous-phase reformatted image from a 3D gadolinium-enhanced MR angiographic examination reveals a large lesion adjacent to the IVC with peripheral nodular enhancement. (b, c) On delayed images (c obtained slightly after b), the lesion demonstrates progressive filling.
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Figure 13c. Hepatic hemangioma. (a) Coronal oblique early-venous-phase reformatted image from a 3D gadolinium-enhanced MR angiographic examination reveals a large lesion adjacent to the IVC with peripheral nodular enhancement. (b, c) On delayed images (c obtained slightly after b), the lesion demonstrates progressive filling.
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Renal Collecting System
Delayed 3D acquisitions of the kidneys, ureters, and bladder can be performed routinely to demonstrate obstruction, delayed function, filling defects, and masses (Figs 14, 15) (55). It is unlikely that gadolinium-enhanced MR urography will ever become a stand-alone examination; there are many less expensive and less invasive alternatives. However, an additional 20-second sequence performed in a patient who has already received an intravenous injection of contrast material is a fast and easy means of evaluating the renal collecting system.
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Figure 14. Normal findings at gadolinium-enhanced MR urography. Coronal MIP image from a 3D acquisition performed approximately 10 minutes after contrast material injection reveals a normal renal collecting system and ureters.
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Figure 15. Hydronephrosis in a renal transplant recipient. Coronal oblique gadolinium-enhanced MR urogram reveals marked hydronephrosis superior to a sharp bend in the ureter (arrow).
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Limitations and Alternative Techniques
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Three-dimensional gadolinium-enhanced MR angiography is not without limitations. Many patients are not candidates for MR angiography because of pacemakers, aneurysm clips, or claustro-phobia. Others may not be capable of performing breath holds sufficient to obtain a diagnostic angiogram. Metallic clips, stents, and embolization coils can cause considerable artifact and obscure important structures. Even when the study is optimal, the resolution of gadolinium-enhanced MR angiography is relatively low compared with that of conventional angiography, and visualization of small peripheral arteries is very limited. Gadolinium-enhanced MR angiography, although less expensive than conventional angiography, is still an expensive examination.
Noninvasive alternatives to gadolinium-enhanced MR angiography include US and CT angiography. US is inexpensive and portable and can be attempted in all patients. Velocity and flow can be measured, providing additional physiologic information not available with other techniques. However, US is often limited by inconsistent visualization of vessels deep in the abdomen in large patients and in patients with extensive bowel gas. One recent study comparing color Doppler US with 3D gadolinium-enhanced MR angiography in visualizing renal arteries and detecting renal artery stenosis found that gadolinium-enhanced MR angiography was superior in detecting accessory renal arteries and had better sensitivity and negative predictive value in depicting renal artery stenosis (26). CT angiography is another attractive alternative to conventional angiography and 3D gadolinium-enhanced MR angiography. Resolution is generally higher than that achieved with gadolinium-enhanced MR angiography, and the speed and quality of studies will improve with widespread availability of multidetector technology. Although CT angiography can be performed in many patients who are not candidates for gadolinium-enhanced MR angiography, a significant percentage of patients have contraindications for CT angiography, including compromised renal function and contrast material allergy. Very few direct comparisons of 3D gadolinium-enhanced MR angiography and CT angiography have been made. One recent study evaluated both techniques in screening living renal donors and found that the ability to demonstrate accessory vessels was similar, with interobserver intramodality variation as important as variation related to modality (56).
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Future Directions
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A variety of methods to increase the speed of 3D acquisition are currently under investigation. Time-resolved MR angiographic techniques allow repeated acquisition of a volume of interest during the passage of the contrast material bolus. Strategies include keyhole imaging and time-resolved imaging of contrast kinetics (TRICKS), in which low spatial frequencies in k space are rapidly sampled and combined with peripheral k-space data, which is sampled less frequently (57,58). Novel imaging and reformatting techniques may allow faster acquisition or improved resolution (59). Techniques that use the spatial information of phased-array coil elements to reduce the number of phase-encoding steps required to produce an image may allow substantial reduction in acquisition times (60,61). Echoplanar imaging may eventually become a viable alternative to the standard gradient-echo acquisition.
Several intravascular contrast agents are currently undergoing clinical trials. These agents have much longer vascular half-lives and may allow high-resolution imaging of the arterial and venous system with appropriate respiratory gating (62–64).
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Conclusions
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Three-dimensional gadolinium-enhanced MR angiography is a versatile technique with a wide range of applications beyond arteriography. Superb visualization of systemic and mesenteric veins is almost always achieved, and parenchymal lesions in abdominal organs can often be characterized. Gadolinium-enhanced MR angiography is an effective tool for staging neoplasms and is frequently useful in both pre- and postoperative imaging of abdominal transplant recipients.
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Footnotes
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Abbreviations: IVC = inferior vena cava, MIP = maximum-intensity-projection, 3D = three-dimensional
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Abstract
Introduction
