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FIGURE 18.9 Volume rendered computed tomography (CT) image of descending thoracic aortic aneurysm (arrow).
FIGURE 18.10 Axial computed tomography angiogram (CT) image shows mild, smooth thickening of the wall of aortic arch (arrow) in a 34-year-old
female patient, consistent with Takayasu arteritis.
Preoperative and Postoperative Evaluation of the Aorta
CTA is commonly used in the pre- and postoperative evaluation of patients. CT can evaluate complications such as fluid
collections, hematomas, infection, abscess (Fig. 18.13), graft dehiscence, rupture, leak (Fig. 18.14), aneurysm, pseudoaneurysm, and stenosis.
Acute Aortic Syndrome
Acute aortic syndromes including traumatic aortic injuries will be addressed in separate chapters.
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FIGURE 18.11 Coronal reformatted computed tomography (CT) image shows an irregular aneurysm of the arch (arrow) with surrounding soft tissue,
consistent with a mycotic aneurysm.
FIGURE 18.12 Axial computed tomography angiogram image shows an aortobiiliac graft, which is surrounded by soft tissue and locules of gas
(straight arrow) that are contiguous with a loop of duodenum (curved arrow), consistent with an aortoenteric fistula.
SUMMARY
CT angiography is an important imaging modality in the evaluation of various abnormalities of the aorta, both congenital and acquired. The high spatial resolution, multiplanar reconstruction capabilities, wide field of view, wide availability, and rapid turnaround make it ideal for evaluating aortic abnormalities including acute aortic syndromes. It is
also used in the presurgical/interventional evaluation as well as evaluation of complications following several surgeries/
interventions.
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FIGURE 18.13 Axial computed tomography angiogram at the level of aortic root shows pseudoaneuryms at the site of anastomosis with the aortic graft
(arrows).
FIGURE 18.14 Axial computed tomography angiogram shows a Type II endoleak of thoracic aorta (arrow) from intercostal artery.
REFERENCES
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2016;54(1):1–12. http://dx.doi.org/10.1016/j.rcl.2015.09.002.
[2] Hayter RG, Rhea JT, Small A, Tafazoli FS, Novelline RA. Suspected aortic dissection and other aortic disorders: multi-detector row CT in 373 cases
in the emergency setting. Radiology 2006;238(3):841–52. http://dx.doi.org/10.1148/radiol.2383041528.
[3] Tacelli N, Remy-Jardin M, Flohr T, Faivre JB, Delannoy V, Duhamel A, Remy J. Dual-source chest CT angiography with high temporal resolution
and high pitch modes: evaluation of image quality in 140 patients. Eur Radiol 2010;20(5):1188–96. http://dx.doi.org/10.1007/s00330-009-1638-5.
CT Angiography Chapter | 18 205
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[4] Goetti R, Baumuller S, Feuchtner G, Stolzmann P, Karlo C, Alkadhi H, Leschka S. High-pitch dual-source CT angiography of the thoracic and abdomi-
nal aorta: is simultaneous coronary artery assessment possible? AJR Am J Roentgenol 2010;194(4):938–44. http://dx.doi.org/10.2214/AJR.09.3482.
[5] Fleischmann D. CT angiography: injection and acquisition technique. Radiol Clin North Am 2010;48(2):237–47, vii. http://dx.doi.org/10.1016/
j.rcl.2010.02.002.
[6] Davenport MS, Cohan RH, Ellis JH. Contrast media controversies in 2015: imaging patients with renal impairment or risk of contrast reaction. AJR
Am J Roentgenol 2015;204(6):1174–81. http://dx.doi.org/10.2214/AJR.14.14259.
[7] ACR Committee on Drugs, contrast media. ACR manual on contrast media. 2016. version 10.2.
[8] Halpern EJ. Triple-rule-out CT angiography for evaluation of acute chest pain and possible acute coronary syndrome. Radiology 2009;252(2):332–45.
http://dx.doi.org/10.1148/radiol.2522082335.
[9] Raman SP, Johnson PT, Deshmukh S, Mahesh M, Grant KL, Fishman EK. CT dose reduction applications: available tools on the latest generation
of CT scanners. J Am Coll Radiol 2013;10(1):37–41. http://dx.doi.org/10.1016/j.jacr.2012.06.025.
[10] Apfaltrer P, Hanna EL, Schoepf UJ, Spears JR, Schoenberg SO, Fink C, Vliegenthart R. Radiation dose and image quality at high-pitch CT angiog-
raphy of the aorta: intraindividual and interindividual comparisons with conventional CT angiography. AJR Am J Roentgenol 2012;199(6):1402–9.
http://dx.doi.org/10.2214/AJR.12.8652.
[11] Beister M, Kolditz D, Kalender WA. Iterative reconstruction methods in X-ray CT. Phys Med 2012;28(2):94–108. http://dx.doi.org/10.1016/
j.ejmp.2012.01.003.
[12] Numburi UD, Schoenhagen P, Flamm SD, et al. Feasibility of dual-energy CT in the arterial phase: imaging after endovascular aortic repair. Am J
Roentgenol 2010;195(2):486–93.
[13] Stolzmann P, Frauenfelder T, Pfammatter T, et al. Endoleaks after endovascular abdominal aortic aneurysm repair: detection with dual-energy dual-
source CT. Radiology 2008;249(2):682–91.
[14] Sommer WH, Graser A, Becker CR, et al. Image quality of virtual non contrast derived images derived form dual energy CT angiography after
endovascular aneurysm repair. J Vasc Interv Radiol 2010;21:315–21.
[15] Mao SS, Ahmadi N, Shab B, et al. Normal thoracic aorta diameter on cardiac computed tomography in healthy asymptomatic adults: impact of age,
and gender. Acad Radiol 2008;15(7):827–34.
[16] Lu T, Huber CH, Rizzo E, et al. Ascending aorta measurements as assessed by ECG-gated multidetector computed tomography: a pilot study to
establish normative values for transcatheter therapies. Eur Radiol 2009;19(3):665–9.
[17] Lin FY, Devereux RB, Roman MJ, et al. Assessment of the thoracic aorta by multidetector computed tomography; age and sex-specific reference
values in adults without evidence cardiovascular disease. J Cardiovasc Comput Tomogr 2008;2(5):293–308.
[18] Tops LF, Wood DA, Delgado V, et al. Non invasive evaluation of aortic root with multislice computed tomography: implications for transcatheter
aortic valve replacement. J Am Coll Cardiol Imaging 2008;1(3):321–30.
[19] Ocak I, Lacomis JM, Deible CR, et al. The aortic root: comparison of measurements from ECG-gated CT angiography with transthoracic echocar-
diography. J Thorac Imaging 2009;24(3):223–6.
[20] Rogers IS, Massaro JM, Truong QA, et al. Distribution, determinants, and normal reference values of thoracic and abdominal aortic diameters by
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Chapter 19
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Magnetic Resonance Angiography
Carlos Capuñay, Patricia Carrascosa
Diagnóstico Maipú, Vicente López, Bs As, Argentina
Chapter Outline
Introduction 207
Patient Preparation, Imaging Technique,
and Data Postprocessing 207
Normal Aorta 208
Aortic Aneurysm 208
Acute Aortic Syndromes 210
INTRODUCTION
During the past two decades, considerable advances have been achieved in the field of noninvasive vascular imaging with
magnetic resonance imaging (MRI). With the use of high-field MRI scanners, reliable studies of the vessel can be performed with accuracy that equalizes other noninvasive techniques such as transesophageal echocardiography (TEE) and
multislice computed tomography (CT), and from a morphological point of view magnetic resonance angiography (MRA) is
sometimes preferable to digital subtraction angiography [1,2]. MRI is a multifaceted diagnostic tool for evaluating the aorta
that permits the determination of aortic pathology, the site and size of aneurysms, the extent of dissections, or the aortic
wall involvement. MRI can be performed as a preoperative and/or postoperative imaging examination and also can provide
analysis of the aortic valve and quantification of aortic flows.
When evaluating the thoracic aorta, MRA shows some advantages in comparison to TEE and CT angiography. Although
TEE is an alternative imaging procedure in cases of urgency and has the advantage of being a bedside exam, it is a minimally
invasive procedure usually not well tolerated by the patient and highly dependent on the operator’s experience. Compared
to TEE, MRA offers a larger field of view, more anatomical details, better evaluation of the ascending aorta and the aortic
arch, and also very important, MRA is a noninvasive technique. Compared to CT, MRI does not require the administration
of iodine-based contrast media and the use of ionizing radiation. On the other hand, MRI is a relatively complex study that
requires longer exploration times and is contraindicated in patients with metallic implants, noncompatible MR peacemakers, claustrophobia, and renal failure.
For the study of the abdominal aorta, both CT and MRI demonstrate excellent diagnostic accuracy and constitute the
noninvasive techniques of choice, depending on the equipment and expertise available [3,4].
Aortitis 210
Aortic Stenosis 210
Aortic Coarctation 211
Follow-up Examinations 213
Conclusions 214
References 214
PATIENT PREPARATION, IMAGING TECHNIQUE, AND DATA POSTPROCESSING
To ensure the best signal-to-noise ratio, the use of phased array surface coils is recommended in the scanning region. When
the complete thoracoabdominal aorta must be examined, the body coil is employed using the surface coil in the area that
requires better image quality.
Both static and dynamic sequences are essential in the scan protocol. Noncontrast electrocardiography-gated T1-weighted
double inversion recovery and balanced steady-state-free precession images offer excellent imaging of the aorta, with accurate discrimination between the blood pool and the vascular wall, helpful in the assessment of aortic morphology and size,
as well as for the detection of acute aortic syndrome features [5,6]. T2-weighted images are useful in identifying aortic
wall edema in inflammatory conditions. These image sequences also provide meticulous information of adjacent structures.
Contrast-enhanced three-dimensional (3D) gradient echo breath-hold MRA techniques have significantly improved the
New Approaches to Aortic Diseases from Valve to Abdominal Bifurcation. http://dx.doi.org/10.1016/B978-0-12-809979-7.00019-5
Copyright © 2018 Elsevier Inc. All rights reserved.
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image quality in the last years, mainly with the possibility to accelerate dynamic image data acquisition using the k-space
substitution technique. Using MR fluoroscopic triggering, a real-time monitoring of the arrival of the contrast material
in the targeted vessel allows the operator to start the scan sequence at the most appropriate time. Delayed postcontrast
T1-weighted images are recommended when enhancement of the aortic wall is suspected, such as in cases of infectious or
noninfectious aortic wall inflammation [7].
Although it is preferable to have a standardized study protocol, MRI sequences must be optimized for each individual
patient to achieve the best image quality and diagnostic result according to the clinical suspicion. Scan protocol includes
parasagittal planes parallel to the course of the aortic arch, as well as images orthogonal to the longitudinal axis of the
examined vessel. Phase-contrast cine images quantitatively evaluate vascular flows, useful in several entities such as aortic
coarctation, aortic dissection, or aortic valvular regurgitation.
3D contrast-enhanced MRA raw data are then postprocessed in dedicated off-line workstations using advanced
postprocessing software. Both maximum intensity projection (MIP) and multiplanar reconstructions increase the value
of the examination, allowing visualization of anatomical details, which sometimes are difficult to analyze using axial
reconstructions alone.
NORMAL AORTA
The aorta extends from the aortic valve to the level of the bifurcation into the right and left common iliac arteries. The
aortic root starts at the level of the aortic valve and ends at the sinotubular junction, where the ascending aorta begins.
Coronary arteries arise from this segment, in the coronary sinuses. The ascending aorta extends up to the origin of the
brachiocephalic artery. The next section is the aortic arch, where the arteries to the neck and upper limbs originate. In
70% of the population, three great arteries arise sequentially as follows: innominate artery; left common carotid artery;
and left subclavian artery. In 20%–30% of people, a combined origin of the innominate artery and the left common
carotid artery can be found (bovine arch variant), whereas in 5% of individuals, the left vertebral artery arises from the
aortic arch [8,9]. The aortic arch ends at the level of the ligamentum arteriosum. The next aortic segment is the descending thoracic aorta, extending between the last artery arising from the aortic arch and the celiac trunk. At this level starts
the abdominal aorta.
AORTIC ANEURYSM
Normal aortic diameters are strongly related to age, gender, and body size. Being larger in men than in women, the
ascending aorta in healthy individuals does not exceed 40 mm and becomes gradually narrower from the aortic root to
the iliac bifurcation. Aortic aneurysm is defined as a permanent abnormal focal dilatation of the aorta that involves the
three layers of the aortic wall and the diameter of the artery is at least 50% greater than the normal size of the vessel [10].
Although atherosclerosis is the most common cause of aortic aneurysm, the evidence suggests a multifactorial etiology
including congenital and genetic factors. The aneurysms may involve one or more of the aortic segments in the thoracic
aorta and usually are located distal to the origin of the renal arteries when the abdominal aorta is affected. Because the
aortic diameter is the most important predictor of aortic rupture or dissection, noninvasive imaging techniques must
accurately measure the aortic aneurysm in the three axes, giving anterior–posterior, transverse and longitudinal dimensions [11]. MRI is the preferable imaging modality for comparative follow-up studies, especially in stable and young
patients. It precisely establishes the diameter of the aorta, the aneurysm extent, and the relationship with the arterial
branches. The implementation of postprocessing techniques such as MIP and volume rendering images are valuable
tools for treatment planning. It is important to highlight that measurements should be carried out in the plane perpendicular to the aorta’s major axis, combining both the measures done on contrast-enhanced 3D MRA images and those of
the T1- and T2-weighted sequences that provide more detail of the outer edges of the aorta. This combination prevents
the mistake of underestimating the diameter of the aorta that can be generated by using only MIP images that represent a
mold of the vascular lumen (see Figs. 19.1 and 19.2). Conventional images also give information for the characterization
of the aortic lumen, extent, and age of the thrombus based on its signal intensity due to differences in thrombi composition and the presence of wall calcifications [12,13]. MRI is also very useful in determining the relationship to adjacent
structures and the involvement of peri- and para-aortic tissues. This is particularly important in the identification of
inflammatory aortic aneurysm, a different clinical entity from the atherosclerotic ones. They represent between the 5%
and 10% of all abdominal aortic aneurysms, being extremely rare in the thoracic aorta. They exhibit several distinctive
features including uncommon aortic wall thickening surrounded by fibrous tissue that can extend to adjoining visceral
organs and retroperitoneal structures [14].
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FIGURE 19.1 Large abdominal aortic aneurysm with eccentric thrombus. Note that the thrombus is not clearly delineated on maximum intensity projection (MIP) images. (A) Coronal contrast-enhanced magnetic resonance angiography (MRA) MIP image. (B) Sagittal contrast-enhanced MRA MIP image.
(C) Axial three-dimensional (3D) gradient echo T1-weighted fat-suppressed image. (D) Coronal 3D gradient echo T1-weighted fat-suppressed image.
FIGURE 19.2 Abdominal aortic and left common iliac arteries aneurysms. (A) Coronal contrast-enhanced magnetic resonance angiography (MRA)
maximum intensity projection (MIP) image. (B) Sagittal contrast-enhanced MRA MIP image.
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ACUTE AORTIC SYNDROMES
MRI plays an important role in the detection and follow-up of potentially life-threatening pathologies. Determination of
involvement or noninvolvement of the ascending aorta in these acute aortic diseases is critical because its affection worses
the prognosis and requires surgical intervention.
Aortic dissection is the most common cause of acute aortic syndrome, with the highest mortality rate [8]. Dissection is
characterized by an intimal tear in the aortic wall, leading to leak of the blood in the tunica media that propagates proximal
and distal, displacing the intima inward and forming a false lumen. Aortic dissection classifications are based on the origin
of the tear (DeBakey system) and extent of the aortic involvement (Stanford system). MRI is considered a very confident
diagnostic technique for the diagnosis of aortic dissection, with sensitivity and specificity of 98%. Double-inversion recovery T1-weighted imaging and balanced steady-state-free precession sequences allow noncontrast evaluation of the aorta
[5]. These ECG-gated sequences depict the intimal flap as a linear structure, showing the signal void within the true lumen
and higher signal intensity in the false lumen, secondary to turbulent flow. Gradient echo sequences and steady-state cine
MRI are alternative techniques to differentiate thrombus from slow flow in the false lumen, as well as they are useful in the
assessment of changes in the intimal flap configuration during the cardiac cycle for evidence of collapse of the true lumen
and potential ischemia in the distal organs. MRI is considered to be superior to CT in the differentiation of acute intramural
hematoma from atherosclerotic plaque, assessment the chronicity of the hematoma, evaluating the periaortic tissues, and
for aortic regurgitation. Contrast-enhanced MRA provides greater image details of the aorta, being highly accurate in the
diagnosis of dissection, localization of intimal tears, determination of branch vessel involvement, and resulting damage
to supplied organs (see Fig. 19.3). Current gadolinium-enhanced 3D MRA with shorter acquisition times facilitate the
evaluation of unstable patients [15–17]. Nevertheless, the combination of advances in CT, its worldwide availability, and
simplicity of monitoring unstable patients have limited the role of MRI to the surgical follow-up and monitoring of patients
with conservative treatment.
Image evaluation should include the analysis of both source data and MIP images in combination to avoid misdiagnosis. In certain situations, MIP images fail to identify the intimal flap, especially when calcifications of the intima layer are
absent.
Intramural hematoma represents a hematic collection within the medial layer of the aortic wall without communication with the vascular lumen. As mentioned earlier, MRI easily contributes to the differential diagnosis between thrombus,
thrombosed false lumen in an aortic dissection, or aortic wall thickening secondary to atherosclerotic disease in cases where
CT failed. The age of the aortic hematoma can be determined based on T1- and T2-weighted signal intensity, according to
the presence of oxyhemoglobin (acute phase), methemoglobin (subacute phase), or hemosiderin (chronic phase). Based
on that in the acute stage the hematoma shows intermediate signal intensity on T1-weighted spin-echo images and a high
signal intensity in the subacute stage [8].
Penetrating aortic ulcer is characterized by the identification of a localized ulceration deep in the aortic wall, usually
associated with a focal intramural hematoma.
AORTITIS
The term aortitis refers to the inflammation of the aortic wall. The majority of the cases are noninfectious inflammatory vasculitis, including giant cell and Takayasu arteritis, among others. Less common, infective aortic disease due to
Staphylococcus or mycobacteria can be present. MRI plays a critical role in the diagnosis and follow-up of these pathological entities, showing thickening of the aortic wall, as well as aortic wall edema on T2-weighted images, an indicator of
active disease and a helpful sign for monitoring treatment response or relapse [18–20]. Contrast-enhanced 3D MRA images
are useful for the visualization of associated stenosis (see Fig. 19.4).
AORTIC STENOSIS
Atherosclerosis is the most common cause of steno-occlusive pathology of the aorta, being the inflammatory and congenital
conditions rare. The abdominal aorta, in the majority of cases below the renal arteries, is the classical location of the atherosclerotic stenosis, and usually extends distal to the iliac arteries. MRI clearly depicts aortic wall abnormalities, including
atherosclerotic plaques and thrombus, irregularities of the vascular lumen, and the degree and extension of the vascular lumen
stenosis; on the other hand, the MRI has scarce visibility of calcifications (see Figs. 19.5 and 19.6). Contrast-enhanced MRA
technique has been reported superior than time-of-flight MR technique for the diagnosis of vascular stenosis greater than 50%.
Although there is a good correlation with conventional angiography, MRA tends to overestimate the degree of stenosis.
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FIGURE 19.3 Type A aortic dissection that involves the aortic root, the ascending thoracic aorta, the aortic arch, the descending thoracic aorta, and
the abdominal aorta. (A) Axial view of the proximal ascending aorta showing thrombosis at the false lumen. In the descending thoracic aorta, both true
and false lumens are patent. (B) Axial view at the aortic arch showing a large intimal tear. (C) Axial view at the abdominal aorta showing different signal
intensity in the true (hyperintensity) and false (hypointensity) lumens. (D) Coronal view at the descending thoracic and abdominal aorta.
AORTIC COARCTATION
Aortic coarctation is defined as a narrowing of the aorta usually located distal to the origin of the left subclavian artery. This
narrowing in aortic lumen is associated with the reduction of blood flow to the trunk and lower limbs and clinical presentation varies accordingly to the degree of stenosis, with different signs and symptoms ranging from asymptomatic patients to
those with hypertension and congestive heart failure.
Diagnosis of aortic coarctation usually is made using TTE with Doppler imaging. Nevertheless, MRI is extremely useful to confirm the exact location, length, degree of aortic obstruction, and presence and extent of collateral vessels. This
anatomic delineation using spin-echo MRI images is complemented with phase-contrast cine MRI images that provide
quantitatively evaluation of vascular flows, determining flow direction, magnitude of collateral circulation, and pressure
gradients across the segment of aortic stenosis. 3D contrast-enhanced MRA allows the detection of small collateral arteries
not visible at anatomic spin-echo MRI images (Fig. 19.7).
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FIGURE 19.4 Takayasu arteritis with narrowing of the thoracic and abdominal aorta. (A) Oblique sagittal contrast-enhanced magnetic resonance angiog-
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raphy (MRA) maximum intensity projection (MIP) image of the thoracic aorta. (B) Coronal contrast-enhanced MRA MIP image of the abdominal aorta.
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FIGURE 19.5 Abdominal aortic wall calcifications are clearly seen on computed tomography (CT) images, whereas they are visible on magnetic
resonance (MR) images. (A) Axial view CT image. (B) Axial view contrast-enhanced magnetic resonance angiography (MRA) image. (C) Coronal CT
maximum intensity projection (MIP) image. (D). Coronal MRA (MIP) image.
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