Radiol Oncol 2002; 36(2): 103-8.
review
Advances in contrast-enhanced MR-angiography: Indications and limitations
Manuela Adelinde Aschauer, Rudolf Stollberger, Franz Ebner
Magnetic Resonance Institute, Karl-Franzens-University Graz, Austria
Introduction. Gadolinium(Gd)-enhanced three-dimensional (3D) magnetic resonance angiography (MRA) is a newer technique that provides high resolution data rapidly for depiction of both arteries and veins throughout the body.
Conclusions. Contrast-enhanced 3D MRA represents a milestone for non-invasive vascular imaging. While its clinical utility has already been established in many vascular territories, the continuos development of hard- and software, as well as of new contrast agents, will likely result in a further widening in the spectrum of indications. Delayed 3D acquisitions of the kidney, ureters and bladder can be performed routinely to demonstrate and characterise obstruction, delayed function, filling defects and masses. Some patients are not candidates for MRA because of pacemakers, some kind of stents or immobilisation coils that cause considerable artifacts and obscure important structures. The resolution of CE-3D-MRA is lower compared with that of conventional angiography and visualisation of small peripheral arteries is limited.
Key words: magnetic resonance angiography – methods; contrast media
Introduction
Gadolinium(Gd)-enhanced three-dimensional (3D) magnetic resonance angiography (MRA) is a newer technique that provides high resolution data rapidly for depiction of both arteries and veins throughout the body.1
Received 27 May 2002 Accepted 6 June 202
Correspondence to: Manuela Aschauer, M.D., Magnetic Resonance Institute and Department of Radiology, Karl-Franzens-University Graz, Auenbruggerplatz 9, A-8036 Graz, Austria; E-mail: mannuela.aschauer@uni-graz.at
Improvements in gradient technology now allow an entire volume of 3D-Gd-enhanced MRA to be performed at 4 second intervals. It is now possible to acquire several volumes of high temporal resolution 3D data sets in a 10-20 second breath hold. Because it relies on T1 shortening effects of circulating Gd-chelate contrast media and not inherent flow characteristics, Gd-enhanced 3D-MRA can often depict pathologic vascular segments that are not adequately visualised using unenhanced flow based MRI-techniques. In addition, Gd-en-hanced 3D-MRA provides volumetric data that can be processed for multiplanar refor-
104
Aschauer M A et al. / Advances in contrast-enhanced MR-angiography
mation (MPR) and maximum intensity projection (MIP) viewing.
This article will review the basic techniques for MR-angiography with contrast media and the potential applications of MRA will be discussed with particular emphasis on the 3D-contrast-enhanced technique.
Clinical applications
Paramagnetic contrast agents shorten the T1 relaxation time of blood. Gadolinium has a high relaxivity and a favourable safety profile when bound to a chelate.2 During the short intravascular phase the intravenously injected T1-shortening contrast agent provides a signal in the arterial or venous system, thereby enhancing the vessel to background contrast to noise ratio and eliminating flow artifacts. Hence signal of flowing blood is no longer flow-dependent. Flow-induced artifacts seen with time of flight (TOF) or phase contrast MRA are therefore largely eliminated. This allows coverage of large vascular territories in short imaging times and generates images, which are similar in appearance to conventional contrast angiography.
Beyond the absence of ionising radiation, side effects of the paramagnetic contrast agents used for the examination are rare. Paramagnetic contrast agents are non-nephroto-xic and have a low incidence of allergic reactions. They are safe for use in patients with renal insufficiency as well as in patients with a history of allergic reactions to iodinated contrast media.
To achieve maximal T1-weighting, the sequence employed should be spoiled.3 Repetition and echotimes should be chosen as short as possible; a flip angle ranging between 20° and 45° provides adequate suppression of the surrounding tissues and has been shown to render excellent image quality.
Section thickness should be adjusted to between 1.5 and 2.5 mm in order to assure for
full coverage of the vascular system under consideration and still permit multiplanar reformations.
Breath-holding is crucial for achieving a good 3D-MRA image quality in the thorax and abdomen. The breath-hold interval can be increased by allowing the patient to practise before the exam and by providing some oxygen through a nasal canula.
It is essential to inject sufficient paramagnetic contrast agent to reduce arterial blood T1 to well under the T1 of surrounding tissues. Even though most extracellular agents available in European markets are approved up to a dose of 0.3 mmol per kg, cost considerations require careful dosing independent of the vascular territory under consideration. A dose ranging between 0.1 and 0.2 mmol per kg body weight is sufficient for most single-territory artery imaging protocols. Display of the run of arteries in multiple stations requires administration of the maximum allowable dose of 0.3 mmol per kg body-weight.4
If data collection is sufficiently fast for time resolved imaging, optimal image quality is virtually guaranteed in at least one of the collected data sets. Otherwise, timing of the contrast bolus is crucial (with a test bolus injection or automated bolus detection). The use of an automated injector facilitates contrast timing and delivery as it allows precise infusion of paramagnetic contrast using predefined weight adjusted rates and volumes. Maximal contrast concentration in the vessel of interest should be achieved during the acquisition of the central, contrast-determining portion of k-space.3
Image analysis
Analysis should not be limited to viewing maximum intensity projections but instead should include evaluation of reformatted images in all three planes and should include also the evaluation of the source images, viewed interactively on a work station.
Radiol Oncol 2002; 36(2): 103-8.
Aschauer M A et al. / Advances in contrast-enhanced MR-angiography
105
1.  Pulmonary arteries: Using a breath hold technique, the pulmonary artery tree can be visualized to its periphery (Figure 1). Contrast-enhanced 3D-MRA has also been shown to be a save and reliable technique for the detection of pulmonary embolism. Emboli are clearly identifiable on the reformatted images as filling defects.5
2.  Aorta: Contrast-enhanced 3D-MRA is emerging as the imaging modality of choice for assessing the whole aorta as an organ. Employing the described strategy all relevant disease processes of the aorta as well as the major neck and arm vessels can be fully depicted on the 3D-MR-angio-graphy images. The possible complex morphology of the diseased aorta and/or its branches may result in an inadvertent exclusion of important portions of the arterial anatomy from the 3D imaging volume. Therefore it is important to carefully con-
duct the localising process using breath-hold techniques. Stenoses are visualized as well as aneurysmal dilatations. Congenital lesions, such as coarctations, are particularly well suited for analysis with this technique. In addition, surgical grafts are well depicted. Furthermore MRA provides comprehensive analysis in suspected aortic dissection: The extent and relationship to branch vessels can be fully depicted and the true lumen can be separated from the false lumen (Figure 1).6 For evaluation of the aorta the advantages of cross sectional imaging over conventional catheter angiography is well established.
Furthermore it is possible to evaluate the renal arteries to the level of the renal hi-lum, as well as the superior mesenteric artery and even the celiac trunc and the inferior mesenteric artery.
Figure 1. MR angiography of the aorta showing
1. dissection (a,b) with exact delineation of true and false lumen and the dissection membrane, notice inferior me-senteric artery.
2. ascendens aneurysma in Marfan’s Syndrom (notice also pulmonary arteries).
Radiol Oncol 2002; 36(2): 103-8.
106
Aschauer M A et al. / Advances in contrast-enhanced MR-angiography
Figure 2. Renal angiograms showing
1. exact correlation between a) MRA and b) conventional angiography regarding the high grade renal artery stenosis with better delineation of pelvic artery stenoses in MRA.
2. discrete renal artery irregularities in fibromuscular dysplasia in d) MRA and c) conventional angiography.
With sensitivities and specificities up to 100 % it appears likely that the 3D-MRA techniques will replace conventional angio-graphy (Figure 2) as a primary means for the assessment of renal artery disease.7 3D-MRA is also the modality of choice for assessing complex arterial anatomy following renal transplantation.
3.  Peripheral vessels: Also in the peripheral va-sculature the contrast-enhanced approach offers several advantages over conventional MRA-techniques including rapid acquisition without arterial puncture. This technique uses several 3D-MRA-acquisistions in conjunction with movement of the MRA-scan-ning table to follow the bolus of contrast into the lower extremities (Figure 3).8,9
Radiol Oncol 2002; 36(2): 103-8.
Figure 3. MR angiography of the outflow-tract:
1. with Gadolinium (a,b,c): chronic long occlusion of the left femoral superficial artery and double popliteal artery
2. with blood pool agent (e,f): fresh thrombus visible in the superficial femoral vein according to conventional phlebography (d).
Three or even more 3D-data-sets, each consisting of 30 to 40 images are collected at set intervals over the lower extremities, resulting and a morphologic display of the entire run off system. To avoid venous overlap, which might mask patent arterial segments, imaging has to be completed before a too much contrast is present in the venous system. Therefore, for moving bed imaging a low infusion rate for the administration of contrast agent (0.3 to 0.6 ml per sec) is recommended to minimize venous enhancement. So the clinically relevant vascular pathologies are well displayed as confirmed in recent studies. Contrast-enhanced 3D-MRA achieves sensitivity and specificity values exceeding 90 % in differentiating non-significant from haemo-dynamically significant stenosis. 4. Whole body 3D-MRA: Since arteriosclero-tic disease effects entire arterial system, extended coverage allowing the concomi-
Aschauer M A et al. / Advances in contrast-enhanced MR-angiography
107
Figure 4. Abdominal angiograms in oncologic patients:
1. showing preembolization feeding vessels in a) MRA and b) conventional angiography (hepatocellular carcinoma).
2. differentiation between c) tumour thrombus (proximal) and clot (distal) in the VCI (renal cell carcinoma) and d) partial tumor ingrown with patent VCI (also renal cell carcinoma).
tant assessment of the arterial system from supraaortic arteries to the distal run of vessels appears desirable. Five 3D data sets are collected over 72 seconds during a continuous contrast infusion lasting 60 seconds. It allows displaying of the arterial vascular tree from supraaortic arteries to distal run of vessels in a single examination lasting nearly 72 seconds.10 Extended coverage including the entire arterial vascular tree, high diagnostic accuracy, non-invasiveness, lack of side effects and very short examination times combine to open interesting perspectives regarding the use of this technique for vascular disease screening. 5.   Systemic veins
Evaluation of the systemic veins not only of the abdomen with 3D-MRA is usually
optimal on the second or third postcon-trast acquisition. Applications include evaluation of patients with suspected venous thrombosis, vascular extension by renal, hepatic or other tumors and evaluation of anatomic anomalies of the inferior vena cava and systemic veins (Figure 4).11
Future developments
Blood pool or intravascular contrast agents are large enough that they do not leak out of the capillaries but stay within the intravascu-lar compartment for more than 1 hour.2 At this time it appears likely however, that these agents will enhance imaging of the venous system as well as of the coronary arteries (Figures 3 e,f).
Conclusions
Contrast-enhanced 3D MRA represents a milestone for non-invasive vascular imaging. While its clinical utility has already been established in many vascular territories, the con-tinuos development of hard- and software, as well as of new contrast agents, will likely result in a further widening in the spectrum of indications. Including MR-imaging MRA allows the one-stop shop examination of the most pathologies of the abdomen, neck and extremities as well as the thorax. Delayed 3D acquisitions of the kidney, ureters and bladder can be performed routinely to demonstrate and characterise obstruction, delayed function, filling defects and masses. CE-3D-MRA is not without limitations. Some patients are not candidates for MRA because of pacemakers, some kind of stents or immobilisation coils that cause considerable artifacts and obscure important structures. The resolution of CE-3D-MRA is lower compared with that of conventional angiography and visualization of small peripheral arteries is limited.
Radiol Oncol 2002; 36(2): 103-8.
108
Aschauer M A et al. / Advances in contrast-enhanced MR-angiography
References
1.   Prince MR, Yucel EK, Kaufman JA, Harrison DC, Geller SC. Dynamic gadolinium-enhanced three-dimensional abdominal MR arteriography. J Magn Reson Imaging 1993; 3: 877-81.
2.   Knopp MV, von Tengg-Kobligk H, Floemer F, Schoenberg SO. Contrast agents for MRA: future directions. J Magn Reson Imaging 1999; 10: 314-6.
3.   Stollberger R, Aschauer M, Hausegger KA, Tiesen-hausen K, Ebner F. Kontrastmittelverstšrkte MR-Angiographie in Atemanhaltung. Biomedizinische Technik 1998; 43: 15-7.
4.   Goyen M, Ruehm SG, Debatin JF. MR-angio-graphy: the role of contrast agents. Eur J Radiol 2000; 34: 247-56.
5.   Meaney JF, Weg JG, Chenevert TL, Stafford Johnson D, Hamilton BH, Prince MR. Diagnosis of pulmonary embolism with magnetic resonance angiography. N Engl J Med 1997; 336: 1422-7.
6.   Rofsky NM. MR angiography of the aortoiliac and femoropopliteal vessels. Magn Reson Imaging Clin N Am 1998; 6: 371-84.
7.   Hany TF, Debatin JF, Leung DA, Pfammatter T. Evaluation of the aortoiliac and renal arteries: comparison of breath-hold, contrast-enhanced, three-dimensional MR angiography with conventional catheter angiography. Radiology 1997; 204: 357-62.
8.   Ho KY, Leiner T, de Haan MW, Kessels AG, Kit-slaar PJ, Engelshoven JM. Peripheral vascular tree stenoses: Evaluation with moving bed infusion – tracking MR angiography. Radiology 1998; 206: 683-92.
9.   Ruehm SG, Hany TF, Pfammatter T, Schneider E. Ladd M, Debatin JF. Pelvic and lower extremity arterial imaging: diagnostic performance of 3D contrast-enhanced MRA. AJR 2000; 174: 1127-35.
10. Goyen M, Quick HH, Debatin JF, Ladd ME, Barkhausen J, Herborn CU, et al. Whole body 3D MR angiography using a rolling table platform: initial clinical experience. Radiology 2002; in press
11. Obernosterer A, Aschauer M, Schnedl W, Lipp RW. Anomalies of inferior vena cava in patients with iliac venous thrombosis. Ann Intern Med 2002; 136: 37-41
Radiol Oncol 2002; 36(2): 103-8.