DOI: 10.1148/rg.275065737
RadioGraphics 2007;27:1311-1321
© RSNA, 2007
Three-dimensional Contrast-enhanced MR Angiography of Aortic Dissection: A Pictorial Essay1
Qi Liu, MD,
Jian Ping Lu, MD,
Fei Wang, MD,
Li Wang, MD, and
Jian Min Tian, MD
1 From the Department of Radiology, Changhai Hospital/2nd Military Med University, 174 Changhai Rd, Shanghai, Shanghai 200433, China. Received October 18, 2006; revision requested February 2, 2007; revision received March 8; accepted March 12. All authors have no financial relationships to disclose.
Address correspondence to Q.L. (e-mail: zzliuk{at}online.sh.cn).
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Abstract
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Aortic dissection is a catastrophic aortic disorder with high morbidity and mortality rates. Prognosis and treatment vary with different types of aortic dissection; therefore, prompt and accurate diagnosis is essential. Ultrasonography is widely available and can be used even in relatively unstable patients. However, it has limited diagnostic accuracy and cannot provide three-dimensional (3D) display images for treatment planning. Both computed tomographic (CT) angiography and 3D contrast material–enhanced magnetic resonance (MR) angiography can accurately demonstrate aortic dissection, with CT having the advantages of wider availability and shorter imaging times. However, contrast-enhanced MR angiography is more suitable in medically stable patients, does not involve nephrotoxic contrast agent or ionizing radiation, and offers greater ease and speed of postprocessing. In clinical practice, contrast-enhanced MR angiography can provide high-quality imaging data suitable for 3D reconstructions. It also has excellent spatial and contrast resolution and allows studies to be performed in multiple vascular phases, making it valuable for the diagnosis and classification of aortic dissection and in providing information that is helpful for treatment planning. Three-dimensional contrast-enhanced MR angiography with postprocessing is a fast, accurate, and noninvasive technique that may prove to be the optimal imaging modality in medically stable patients with aortic dissection.
© RSNA, 2007
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LEARNING OBJECTIVES
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After reading this article and taking the test, the reader will be able to:- Identify different types of aortic dissection.
- Discuss various applications of 3D contrast-enhanced MR angiography for the evaluation of aortic dissection.
- Describe the 3D contrast-enhanced MR angiographic features of aortic dissection.
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Introduction
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Aortic dissection is a catastrophic aortic disorder with high morbidity and mortality rates. Timely diagnosis and appropriate treatment are crucial for saving the lives of these patients (1–3). However, the diversity and complexity of the clinical presentation often predispose to misdiagnosis or a missed diagnosis (4). Furthermore, patients with different types of aortic dissection require different treatment (5–7). According to the Stanford classification scheme, dissection involving the ascending aorta or the aortic arch is classified as type A, whereas that involving the aorta distal to the origin of the left subclavian artery is classified as type B. Patients with Stanford type A dissection traditionally undergo surgical replacement of the ascending aorta. Patients with Stanford type B dissection are increasingly undergoing endovascular stent-graft treatment (1,8). The two most important steps in the evaluation of suspected aortic dissection are to confirm or rule out dissection and to identify the type of dissection. The strategy for therapy depends on the type of dissection, the site of entry, the extent of dissection, involvement of aortic branches, patency of the false lumen, and the presence of thrombus in the false lumen (9,10). Thus, proper imaging is critical for clinical treatment, with delineation of these features being an important part of the diagnostic work-up (11).
Conventional angiography or digital subtraction angiography (DSA), once the classic method for evaluating aortic diseases, is invasive and involves nephrotoxic iodinated contrast agent and ionizing radiation (1). In addition, it has a limited field of view, a limited view angle, and single-lumen opacification and can be quite difficult to interpret, especially in complex dissections (12). Currently, the noninvasive modalities that are most frequently used to identify aortic dissections are ultrasonography (US), computed tomography (CT), and magnetic resonance (MR) imaging. US, including transthoracic echocardiography and transesophageal echocardiography, is widely available and can be performed quickly and easily at the bedside. Thus, US can be used in most patients with aortic dissections, even in relatively unstable patients. However, US also has major shortcomings, namely, a limited field of view and a diagnostic accuracy that largely depends on the investigator’s experience (1,3). Furthermore, US does not provide images that can be used to plan therapy (10). CT, including helical CT and multisection CT, has the advantages of shorter scanning times, wide availability, and high diagnostic accuracy and has, therefore, classically been the modality of choice for the evaluation of aortic dissection. Like angiography, however, CT requires the use of iodinated contrast material and involves ionizing radiation. Furthermore, CT is not suitable for patients with renal insufficiency, which may be one of the complications of aortic dissection with renal artery involvement (13,14).
In the past, emergency MR imaging evaluation of aortic dissection has been impossible owing to prolonged examination times, but the advent of three-dimensional (3D) contrast material–enhanced MR angiography has changed this paradigm. Contrast-enhanced MR angiography has largely replaced unenhanced MR angiographic techniques and has dramatically shortened the total examination time required for confident diagnosis of aortic dissections (14); however, it is more suitable for stable patients. In principle, contrast-enhanced MR angiography is similar to CT angiography. Both modalities provide excellent evaluation of aortic dissections. In patients without contraindications for either modality, at most centers, the use of each modality is based on equipment availability, the personal preference of the radiologist or referring physician, and patient acceptance (15). Contrast-enhanced MR angiography has several advantages over CT angiography: It makes use of "safer" (ie, nonnephrotoxic) contrast material, does not involve ionizing radiation, and allows imaging in any plane desired, providing greater vessel coverage at high resolution with fewer sections. Furthermore, postprocessing is much faster with contrast-enhanced MR angiography than with CT angiography (13,16–18). However, few data are available on the use of 3D contrast-enhanced MR angiography for the evaluation of aortic dissection.
In this article, we describe 3D contrast-enhanced MR angiographic imaging technique. In addition, we discuss and illustrate the imaging features of aortic dissection at 3D contrast-enhanced MR angiography, along with the advantages and limitations of this modality.
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Imaging Technique
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Three-dimensional contrast-enhanced MR angiography was performed with a phase-array surface coil on a 1.5-T whole-body imaging unit (Magnetom Symphony or Avanto; Siemens Medical Systems, Malvern, Pa). Initially, coronal, axial, and sagittal two-dimensional true fast imaging with steady-state precession was performed to gain an overview of the major thoracoabdominal vessels and organs, and the resulting images were used as localizers. Next, contrast-enhanced MR angiography was performed in the coronal plane with a 3D fast low-angle shot with the following parameters: repetition time, 2.96 msec; echo time, 1.21 msec; flip angle, 25'; field of view, 30–40 x 48 cm; matrix, 256 x 512; one 3D slab; 80 sections with a section thickness of 1 mm; and acquisition time, 18–20 seconds. We administered 0.2 mmol/kg of gadolinium diethylenetriamine pentaacetic acid at a rate of 3 mL/sec via the antecubital vein with a power injector, followed by a 15-mL saline flush at the same flow rate. Scan delay time was determined on the basis of a test bolus. Imaging was performed prior to contrast material administration, followed by arterial and venous phase imaging with time allowed for patient respiration between phases. Finally, axial T1-weighted imaging of the thoracoabdominal aorta was performed. Imaging coverage extended from the starting segment of the three major branches above the aortic arch to the common iliac artery. Source images were subtracted and transferred to the workstation for postprocessing.
Three-dimensional postprocessing techniques consist mainly of volume rendering (VR), maximum intensity projection (MIP), multiplanar reconstruction (MPR), and virtual endoscopy (VE). As with CT angiography (19), a detailed review of the source images is mandatory prior to postprocessing. These source images contain all the information that is available from the data. Thrombus in the false lumen, especially intramural hematoma, will be missed if the source images are not reviewed meticulously.
To ensure correct classification of aortic dissection and provide additional and more detailed information, it is necessary to make full use of image postprocessing methods with contrast-enhanced MR angiography. VR images, especially clipped VR images, can provide direct and 3D information and display the entire morphologic character and extent of aortic dissections as well as clarify the relationships between entry sites and branches of the aorta for the vascular surgeon. As with DSA, MIP images can help differentiate between the true and false lumina and help clarify their anatomic relationship. MPR images can provide arbitrary sections to display entry sites, the true and false lumina, the intimal flap, the origin of the aortic branches, and thrombosis of the false lumen. VE images may provide a view of the initial entry tear and of the true and false lumina from inside the aortic lumen (20–22). The information needed for endovascular stent-graft treatment planning can be accurately obtained with MIP and MPR images. Although various 3D reconstruction techniques complement each other in displaying the external and internal structures of the aorta, it is difficult for any one technique to provide an overall picture of aortic dissection. Thus, the comprehensive use of various postprocessing methods is important in enhancing the value of 3D contrast-enhanced MR angiography in the diagnosis of aortic dissection.
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Imaging Features of Aortic Dissection
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Intimal Flap and True and False Lumina
Confident diagnosis and correct classification of aortic dissection are based on the detection of an intimal flap in the aorta that separates the true and false lumina (11). Such a flap manifests as a hypointense line with a linear, arc, or S shape in the axial plane (Fig 1).
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Figure 1a. Intimal flap in a Stan-ford type A aortic dissection. (a) Oblique sagittal sub-MIP image depicts an intimal flap (arrowheads) between the true lumen (1) and the false lumen (2) extending from the ascending aortic root to the abdominal aorta. (b) Axial MPR image at the level of the pulmonary trunk depicts the arc-shaped intimal flap (arrowheads), the oval true lumen (1), and the crescentic false lumen (2) in both the ascending and descending aorta.
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Figure 1b. Intimal flap in a Stan-ford type A aortic dissection. (a) Oblique sagittal sub-MIP image depicts an intimal flap (arrowheads) between the true lumen (1) and the false lumen (2) extending from the ascending aortic root to the abdominal aorta. (b) Axial MPR image at the level of the pulmonary trunk depicts the arc-shaped intimal flap (arrowheads), the oval true lumen (1), and the crescentic false lumen (2) in both the ascending and descending aorta.
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The true and false lumina can be differentiated on the basis of signal intensity, morphologic features, the relationship between the lumina, and the appearance of thrombosis.
- Signal intensity. Similar to CT angiography (12), contrast-enhanced MR angiography shows the true lumen with a higher signal intensity than the false lumen owing to a higher concentration of contrast material during the arterial phase, a finding that is clearly depicted on MIP images (Fig 2).
- Morphologic features. The true lumen is usually smaller than the false lumen and would be thin or flat from being pressed, appearing oval or semiround in the axial plane. The false lumen is expanded or very large, appearing crescentic (Fig 1b) or semiround or winding around the true lumen in the axial plane.
- Relationship between the lumina. The lumina may be parallel to each other, the false lumen may wind around the true lumen, or the true lumen may look like a ribbon floating in the false lumen (Fig 2).
- Appearance of thrombosis. The false lumen usually contains a degree of thrombus, especially at the retrograde end of the initial entry site, whereas the true lumen contains no thrombus in most cases.
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Figure 2a. Differing signal intensities and anatomic relationships of the true and false lumina. Coronal MIP images show a hyperintense true lumen (1) and a hypointense false lumen (2). The lumina are relatively parallel in a, the true lumen looks like a ribbon floating in the false lumen in b, and the false lumen winds around the true lumen in c.
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Figure 2b. Differing signal intensities and anatomic relationships of the true and false lumina. Coronal MIP images show a hyperintense true lumen (1) and a hypointense false lumen (2). The lumina are relatively parallel in a, the true lumen looks like a ribbon floating in the false lumen in b, and the false lumen winds around the true lumen in c.
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Figure 2c. Differing signal intensities and anatomic relationships of the true and false lumina. Coronal MIP images show a hyperintense true lumen (1) and a hypointense false lumen (2). The lumina are relatively parallel in a, the true lumen looks like a ribbon floating in the false lumen in b, and the false lumen winds around the true lumen in c.
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Entry Sites
The manifestation of entry sites, especially the initial entry tear, is the greatest concern in clinical application, especially for planning endovascular treatment (23). The initial entry can be clearly seen in the axial plane, but not its 3D relationship to the surrounding anatomic structures. In contrast, VR and MPR images can display both the initial entry and its 3D relationship to the neighboring arterial orifice. The intimal tear edge can be seen on VE images (Fig 3).
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Figure 3a. Initial entry in a Stanford type B aortic dissection. (a) Axial MPR image at the level of the aortic arch demonstrates the initial entry (arrowhead) but does not depict its 3D relationship to the left subclavian artery orifice. 1 = true lumen, 2 = false lumen. (b, c) Clipped oblique sagittal VR images clearly depict the morphologic features and size (2.35 cm) of the initial entry (arrowhead in b), which is about 1.85 cm from the left subclavian artery orifice. 1 = true lumen, 2 = false lumen. (d) VE image shows the free edge of the intimal flap (arrowhead) at the initial entry site from inside the aortic lumen. 1 = true lumen, 2 = false lumen. (e, f) On DSA images (more contrast material used to obtain f than e), the initial entry is not clearly depicted regardless of the amount of contrast agent used.
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Figure 3b. Initial entry in a Stanford type B aortic dissection. (a) Axial MPR image at the level of the aortic arch demonstrates the initial entry (arrowhead) but does not depict its 3D relationship to the left subclavian artery orifice. 1 = true lumen, 2 = false lumen. (b, c) Clipped oblique sagittal VR images clearly depict the morphologic features and size (2.35 cm) of the initial entry (arrowhead in b), which is about 1.85 cm from the left subclavian artery orifice. 1 = true lumen, 2 = false lumen. (d) VE image shows the free edge of the intimal flap (arrowhead) at the initial entry site from inside the aortic lumen. 1 = true lumen, 2 = false lumen. (e, f) On DSA images (more contrast material used to obtain f than e), the initial entry is not clearly depicted regardless of the amount of contrast agent used.
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Figure 3c. Initial entry in a Stanford type B aortic dissection. (a) Axial MPR image at the level of the aortic arch demonstrates the initial entry (arrowhead) but does not depict its 3D relationship to the left subclavian artery orifice. 1 = true lumen, 2 = false lumen. (b, c) Clipped oblique sagittal VR images clearly depict the morphologic features and size (2.35 cm) of the initial entry (arrowhead in b), which is about 1.85 cm from the left subclavian artery orifice. 1 = true lumen, 2 = false lumen. (d) VE image shows the free edge of the intimal flap (arrowhead) at the initial entry site from inside the aortic lumen. 1 = true lumen, 2 = false lumen. (e, f) On DSA images (more contrast material used to obtain f than e), the initial entry is not clearly depicted regardless of the amount of contrast agent used.
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Figure 3d. Initial entry in a Stanford type B aortic dissection. (a) Axial MPR image at the level of the aortic arch demonstrates the initial entry (arrowhead) but does not depict its 3D relationship to the left subclavian artery orifice. 1 = true lumen, 2 = false lumen. (b, c) Clipped oblique sagittal VR images clearly depict the morphologic features and size (2.35 cm) of the initial entry (arrowhead in b), which is about 1.85 cm from the left subclavian artery orifice. 1 = true lumen, 2 = false lumen. (d) VE image shows the free edge of the intimal flap (arrowhead) at the initial entry site from inside the aortic lumen. 1 = true lumen, 2 = false lumen. (e, f) On DSA images (more contrast material used to obtain f than e), the initial entry is not clearly depicted regardless of the amount of contrast agent used.
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Figure 3e. Initial entry in a Stanford type B aortic dissection. (a) Axial MPR image at the level of the aortic arch demonstrates the initial entry (arrowhead) but does not depict its 3D relationship to the left subclavian artery orifice. 1 = true lumen, 2 = false lumen. (b, c) Clipped oblique sagittal VR images clearly depict the morphologic features and size (2.35 cm) of the initial entry (arrowhead in b), which is about 1.85 cm from the left subclavian artery orifice. 1 = true lumen, 2 = false lumen. (d) VE image shows the free edge of the intimal flap (arrowhead) at the initial entry site from inside the aortic lumen. 1 = true lumen, 2 = false lumen. (e, f) On DSA images (more contrast material used to obtain f than e), the initial entry is not clearly depicted regardless of the amount of contrast agent used.
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Figure 3f. Initial entry in a Stanford type B aortic dissection. (a) Axial MPR image at the level of the aortic arch demonstrates the initial entry (arrowhead) but does not depict its 3D relationship to the left subclavian artery orifice. 1 = true lumen, 2 = false lumen. (b, c) Clipped oblique sagittal VR images clearly depict the morphologic features and size (2.35 cm) of the initial entry (arrowhead in b), which is about 1.85 cm from the left subclavian artery orifice. 1 = true lumen, 2 = false lumen. (d) VE image shows the free edge of the intimal flap (arrowhead) at the initial entry site from inside the aortic lumen. 1 = true lumen, 2 = false lumen. (e, f) On DSA images (more contrast material used to obtain f than e), the initial entry is not clearly depicted regardless of the amount of contrast agent used.
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The initial entry in Stanford type A dissection is usually located above the root of the ascending aorta (Fig 4), whereas that in type B dissection is usually located at the descending junction of the aortic arch (1). In the latter cases, attention should be paid to the relationship between the entry and the left subclavian artery (Fig 3c), because this relationship directly affects the design of the therapy. If the entry is far enough from the subclavian artery (usually > 1 cm), endovascular graft exclusion can be considered, but if it is too close to the subclavian artery, endovascular treatment may be much more difficult. Depiction of the initial entry and its relationship with the neighboring arterial orifice may be clearer on VR or MPR images than on DSA images (Figs 3, 5), which greatly facilitates endovascular treatment by helping determine the correct projection direction for DSA. MIP or MPR images can help accurately measure the length and diameter of the proximal neck, the size of the initial entry and the diameters of various parts of the descending aorta, and the maximum diameter of the false lumen, measurements that are helpful in preparing the stent-graft prior to endovascular treatment. In rare cases, the initial entry may be at the middle or lower segment of the descending aorta, the abdominal aorta, or even the renal artery.
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Figure 4. Initial entry in a Stanford type A aortic dissection. Clipped coronal VR images reveal the initial entry (arrowhead) at the right wall of the ascending aortic root, about 2.23 cm above the aortic valvular ring. 1 = true lumen, 2 = false lumen.
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Figure 5a. Initial entry in a Stanford type B aortic dissection. (a, b) Clipped oblique posteroanterior VR images show the initial entry (arrowhead) at the descending junction of the aortic arch, about 9.92 mm from the origin of the left subclavian artery. 1 = true lumen, 2 = false lumen. (c) On a DSA image, the relationship of the initial entry to the left subclavian artery is not as clearly depicted.
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Figure 5b. Initial entry in a Stanford type B aortic dissection. (a, b) Clipped oblique posteroanterior VR images show the initial entry (arrowhead) at the descending junction of the aortic arch, about 9.92 mm from the origin of the left subclavian artery. 1 = true lumen, 2 = false lumen. (c) On a DSA image, the relationship of the initial entry to the left subclavian artery is not as clearly depicted.
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Figure 5c. Initial entry in a Stanford type B aortic dissection. (a, b) Clipped oblique posteroanterior VR images show the initial entry (arrowhead) at the descending junction of the aortic arch, about 9.92 mm from the origin of the left subclavian artery. 1 = true lumen, 2 = false lumen. (c) On a DSA image, the relationship of the initial entry to the left subclavian artery is not as clearly depicted.
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In most cases, the reentry site is at the iliac artery, either on the left side, the right side, or both (Fig 6). Occasionally, the reentry site can be at the abdominal aorta, descending aorta, renal artery, or celiac artery (Fig 7). Unlike the initial entry site, the reentry site is sometimes difficult to visualize clearly during the arterial phase because there is less contrast material in the false lumen, although it can be clearly seen during the venous phase.
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Figure 6a. Differing locations of the reentry site. Clipped coronal VR images show the reentry site at the left iliac artery (arrowhead in a), the right external iliac artery (arrowhead in b), and the common iliac arteries (arrowheads in c). 1 = true lumen, 2 = false lumen.
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Figure 6b. Differing locations of the reentry site. Clipped coronal VR images show the reentry site at the left iliac artery (arrowhead in a), the right external iliac artery (arrowhead in b), and the common iliac arteries (arrowheads in c). 1 = true lumen, 2 = false lumen.
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Figure 6c. Differing locations of the reentry site. Clipped coronal VR images show the reentry site at the left iliac artery (arrowhead in a), the right external iliac artery (arrowhead in b), and the common iliac arteries (arrowheads in c). 1 = true lumen, 2 = false lumen.
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Figure 7a. Differing locations of the reentry site. Clipped coronal VR images show the reentry site at the abdominal aorta (arrowhead in a), the origin of the right renal artery (arrowhead in b), and the origin of the celiac artery (CA) (arrowhead in c). SMA = superior mesenteric artery, 1 = true lumen, 2 = false lumen.
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Figure 7b. Differing locations of the reentry site. Clipped coronal VR images show the reentry site at the abdominal aorta (arrowhead in a), the origin of the right renal artery (arrowhead in b), and the origin of the celiac artery (CA) (arrowhead in c). SMA = superior mesenteric artery, 1 = true lumen, 2 = false lumen.
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Figure 7c. Differing locations of the reentry site. Clipped coronal VR images show the reentry site at the abdominal aorta (arrowhead in a), the origin of the right renal artery (arrowhead in b), and the origin of the celiac artery (CA) (arrowhead in c). SMA = superior mesenteric artery, 1 = true lumen, 2 = false lumen.
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Involvement of the Aortic Branches
Involvement of the major aortic branches and their relationships with aortic dissection are also of concern in clinical treatment (24). Special attention should be paid to displaying the coronary arteries and arch branches. Type A dissection may involve the coronary arteries and the vessels above the aortic arch, whereas type B dissection may affect the orifice of the left subclavian artery.
In evaluating the involvement of branches of the abdominal aorta, attention should be paid to the origin of the involved branches, which may originate from the false lumen or from both the true and false lumina (12). VR and MPR images are helpful in that they can accurately display the origin of the branches (Fig 8).
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Figure 8a. Involvement of the abdominal aortic branches in a Stanford type A aortic dissection. Clipped coronal VR image (a) and axial MPR images (b–d) at the level of the orifice of the abdominal aortic branches show that the celiac artery (CA) originates from both the true (1) and false (2) lumina, the left renal artery (LRA) originates from the false lumen, and the superior mesenteric artery (SMA) and right renal artery (RRA) originate from the true lumen. Arrowheads in a indicate the reentry site.
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Figure 8b. Involvement of the abdominal aortic branches in a Stanford type A aortic dissection. Clipped coronal VR image (a) and axial MPR images (b–d) at the level of the orifice of the abdominal aortic branches show that the celiac artery (CA) originates from both the true (1) and false (2) lumina, the left renal artery (LRA) originates from the false lumen, and the superior mesenteric artery (SMA) and right renal artery (RRA) originate from the true lumen. Arrowheads in a indicate the reentry site.
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Figure 8c. Involvement of the abdominal aortic branches in a Stanford type A aortic dissection. Clipped coronal VR image (a) and axial MPR images (b–d) at the level of the orifice of the abdominal aortic branches show that the celiac artery (CA) originates from both the true (1) and false (2) lumina, the left renal artery (LRA) originates from the false lumen, and the superior mesenteric artery (SMA) and right renal artery (RRA) originate from the true lumen. Arrowheads in a indicate the reentry site.
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Figure 8d. Involvement of the abdominal aortic branches in a Stanford type A aortic dissection. Clipped coronal VR image (a) and axial MPR images (b–d) at the level of the orifice of the abdominal aortic branches show that the celiac artery (CA) originates from both the true (1) and false (2) lumina, the left renal artery (LRA) originates from the false lumen, and the superior mesenteric artery (SMA) and right renal artery (RRA) originate from the true lumen. Arrowheads in a indicate the reentry site.
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Thrombosis of the False Lumen
Thrombosis usually occurs in the false lumen, most often in the retrograde end or in the peripheral wall of a markedly expanded false lumen. The thrombus is usually difficult to visualize at contrast-enhanced MR angiography during the arterial phase, but during the venous phase, the hypointensity of thrombus can be clearly distinguished from the enhanced outer wall of the false lumen and the contrast material in the false lumen, especially on MPR, source, and axial contrast-enhanced T1-weighted MR images (Fig 9).
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Figure 9a. Thrombosis of the false lumen in a Stanford type B aortic dissection. (a) Clipped sagittal VR image shows an apparent thrombus (arrowheads) in the proximal end of the false lumen (2). LSCA = left subclavian artery. (b, c) Coronal (b) and axial (c) MPR images more clearly depict the thrombus (arrowheads). 1 = true lumen, 2 = false lumen.
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Figure 9b. Thrombosis of the false lumen in a Stanford type B aortic dissection. (a) Clipped sagittal VR image shows an apparent thrombus (arrowheads) in the proximal end of the false lumen (2). LSCA = left subclavian artery. (b, c) Coronal (b) and axial (c) MPR images more clearly depict the thrombus (arrowheads). 1 = true lumen, 2 = false lumen.
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Figure 9c. Thrombosis of the false lumen in a Stanford type B aortic dissection. (a) Clipped sagittal VR image shows an apparent thrombus (arrowheads) in the proximal end of the false lumen (2). LSCA = left subclavian artery. (b, c) Coronal (b) and axial (c) MPR images more clearly depict the thrombus (arrowheads). 1 = true lumen, 2 = false lumen.
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Intramural Hematoma
Intramural hematoma is atypical dissection that is caused by spontaneous hemorrhage of the vasa vasorum of the medial layer without an intimal tear (25). In cases of intramural hematoma, the morphologic features and size of the aorta seem to be normal on VR or MIP images, whereas unenhanced hematoma in the medial layer can be clearly seen on MPR images (Fig 10). Thus, the radiologist should always review source and MPR images in addition to VR or MIP images to avoid overlooking the presence of intramural hematoma.
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Figure 10a. Intramural hematoma in a Stanford type A aortic dissection. (a) On a clipped sagittal VR image, the aorta appears to be normal. (b, c) Sagittal (b) and axial (c) MPR images clearly delineate a thick, unenhanced hematoma (arrowheads) in the medial layer of the ascending and descending aorta.
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Figure 10b. Intramural hematoma in a Stanford type A aortic dissection. (a) On a clipped sagittal VR image, the aorta appears to be normal. (b, c) Sagittal (b) and axial (c) MPR images clearly delineate a thick, unenhanced hematoma (arrowheads) in the medial layer of the ascending and descending aorta.
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Figure 10c. Intramural hematoma in a Stanford type A aortic dissection. (a) On a clipped sagittal VR image, the aorta appears to be normal. (b, c) Sagittal (b) and axial (c) MPR images clearly delineate a thick, unenhanced hematoma (arrowheads) in the medial layer of the ascending and descending aorta.
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Advantages and Limitations of Contrast-enhanced MR Angiography
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Three-dimensional contrast-enhanced MR angiography can provide complete and dynamic display of aortic dissection (Fig 11); help determine the type of dissection; and display the true and false lumina, the intimal flap, the location and size of the initial entry and its relationship with the neighboring arterial orifice, the origin of the aortic branches, and the presence and amount of thrombus in the false lumen. These data are important for the planning of surgical and endovascular therapy, as well as for proper pre- and postoperative assessment (Fig 12) and follow-up (26). Furthermore, contrast-enhanced MR angiography helps avoid the risks inherent in arterial catheterization, does not require iodinated contrast material, is radiation free, and has low morbidity. It combines the advantages of arterial angiography with those of MR imaging and, unlike CT angiography, does not require the removal of bone structures for adequate visualization of the vasculature (15,17,22).
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Figure 11a. Complete and dynamic display of a Stanford type B aortic dissection. (a, b) Clipped coronal arterial phase (a) and oblique sagittal venous phase (b) VR images demonstrate the initial entry (black arrowhead) at the middle segment of the descending aorta and the reentry site (white arrowhead) at the orifice of the left renal artery. (c, d) DSA images demonstrate the initial entry (c) and the reentry site (d).
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Figure 11b. Complete and dynamic display of a Stanford type B aortic dissection. (a, b) Clipped coronal arterial phase (a) and oblique sagittal venous phase (b) VR images demonstrate the initial entry (black arrowhead) at the middle segment of the descending aorta and the reentry site (white arrowhead) at the orifice of the left renal artery. (c, d) DSA images demonstrate the initial entry (c) and the reentry site (d).
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Figure 11c. Complete and dynamic display of a Stanford type B aortic dissection. (a, b) Clipped coronal arterial phase (a) and oblique sagittal venous phase (b) VR images demonstrate the initial entry (black arrowhead) at the middle segment of the descending aorta and the reentry site (white arrowhead) at the orifice of the left renal artery. (c, d) DSA images demonstrate the initial entry (c) and the reentry site (d).
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Figure 11d. Complete and dynamic display of a Stanford type B aortic dissection. (a, b) Clipped coronal arterial phase (a) and oblique sagittal venous phase (b) VR images demonstrate the initial entry (black arrowhead) at the middle segment of the descending aorta and the reentry site (white arrowhead) at the orifice of the left renal artery. (c, d) DSA images demonstrate the initial entry (c) and the reentry site (d).
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Figure 12a. Pre- and postoperative assessment of a Stanford type B aortic dissection in a 46-year-old man. (a, b) Clipped oblique sagittal (a) and coronal (b) VR images depict a type B aortic dissection of the descending aorta involving the upper segment of the abdominal aorta and the orifice of the celiac artery. 1 = true lumen, 2 = false lumen. (c, d) Clipped oblique sagittal (c) and coronal (d) VR images obtained 1 month after surgery show elimination of the false lumen, with only a small cavity around the reentry site (arrowhead).
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Figure 12b. Pre- and postoperative assessment of a Stanford type B aortic dissection in a 46-year-old man. (a, b) Clipped oblique sagittal (a) and coronal (b) VR images depict a type B aortic dissection of the descending aorta involving the upper segment of the abdominal aorta and the orifice of the celiac artery. 1 = true lumen, 2 = false lumen. (c, d) Clipped oblique sagittal (c) and coronal (d) VR images obtained 1 month after surgery show elimination of the false lumen, with only a small cavity around the reentry site (arrowhead).
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Figure 12c. Pre- and postoperative assessment of a Stanford type B aortic dissection in a 46-year-old man. (a, b) Clipped oblique sagittal (a) and coronal (b) VR images depict a type B aortic dissection of the descending aorta involving the upper segment of the abdominal aorta and the orifice of the celiac artery. 1 = true lumen, 2 = false lumen. (c, d) Clipped oblique sagittal (c) and coronal (d) VR images obtained 1 month after surgery show elimination of the false lumen, with only a small cavity around the reentry site (arrowhead).
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Figure 12d. Pre- and postoperative assessment of a Stanford type B aortic dissection in a 46-year-old man. (a, b) Clipped oblique sagittal (a) and coronal (b) VR images depict a type B aortic dissection of the descending aorta involving the upper segment of the abdominal aorta and the orifice of the celiac artery. 1 = true lumen, 2 = false lumen. (c, d) Clipped oblique sagittal (c) and coronal (d) VR images obtained 1 month after surgery show elimination of the false lumen, with only a small cavity around the reentry site (arrowhead).
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Nevertheless, 3D contrast-enhanced MR angiography has its limitations. It cannot be performed if the patient is unstable and is not appropriate for patients with implanted electronic devices. The accuracy of display is operator dependent. It cannot depict calcification of the arterial wall or intimal flap. In addition, contrast-enhanced MR angiography may be of limited value in follow-up studies performed after endovascular treatment because of artifacts related to the metallic composition of the stent (13).
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Conclusions
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Three-dimensional contrast-enhanced MR angiography with postprocessing is a fast, accurate, and noninvasive technique for the diagnosis and classification of aortic dissection. It is helpful in designing therapeutic protocols for aortic dissection and may provide accurate 3D anatomic information for surgical and endovascular treatment. Three-dimensional contrast-enhanced MR angiography may prove to be the optimal imaging modality in medically stable patients with aortic dissection.
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Footnotes
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Abbreviations: DSA = digital subtraction angiography, MIP = maximum intensity projection, MPR = multiplanar reconstruction, VE = virtual endoscopy, VR = volume rendering, 3D = three-dimensional
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Abstract
LEARNING OBJECTIVES
