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Chapter 18
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CT Angiography
Asha Kandathil, Mina Hanna, Prabhakar Rajiah
UT Southwestern Medical Center, Dallas, TX, United States
Chapter Outline
Introduction 193
CT Angiography of the Aorta 193
CT Angiographic Technique 193
CT Protocols 195
Image Postprocessing 196
Radiation Dose Reduction Strategies 196
Normal Aortic Anatomy 196
Abnormalities of the Aorta 198
Congenital Anomalies 198
Atherosclerosis 198
Aortoiliac Occlusive Disease 198
Aneurysms 200
Inflammatory Diseases 200
Aortic Fistulas 201
Aortic Masses 201
Preoperative and Postoperative Evaluation of the Aorta 202
Acute Aortic Syndrome 202
Summary 203
References 204
INTRODUCTION
Computed tomography angiogram (CTA) is the most commonly used imaging modality in the evaluation of several aortic
disorders. According to the American College of Radiology appropriateness criteria, CTA provides the most clinically relevant information for diagnosis and treatment of patients with acute aortic syndromes and posttraumatic aortic injury. CTA
has been shown to have a sensitivity of 99% and specificity of 100% in the diagnosis of acute aortic disorders [1,2]. CTA is
routinely performed to determine aortic anatomy and measurements prior to endovascular therapy including transcatheter
aortic valve replacement (TAVR) and is the modality of choice for posttreatment follow-up.
Advantages of CT include widespread availability in most imaging centers on a 24-h basis and rapid turnaround time,
making it the primary imaging tool in trauma centers. The rapid acquisition also makes it valuable in the evaluation of critically ill and hemodynamically unstable patients. CTA has good submillimeter isotropic spatial resolution, which enables
multiplanar reconstruction in any plane. The wide field-of-view enables visualization of the aorta, other vascular systems
such as pulmonary arteries and veins and adjacent parenchymal organs, which facilitates diagnosis and characterization
of other pathologies, which may account for nonspecific symptoms. Disadvantages of CT include the use of ionizing
radiation, which has been associated with a risk of carcinogenesis; however, the radiation dose could be minimized using
several strategies. CT with intravenous contrast is contraindicated in patients with severe renal dysfunction due to the risk
of contrast nephropathy. Intravenous contrast is also contraindicated in patients with severe allergic reactions, unless these
patients are effectively premedicated.
In this chapter, we review the technical aspects and role of CTA in evaluation of aortic disorders.
CT ANGIOGRAPHY OF THE AORTA
CT Angiographic Technique
CTA relies on timed, rapid volumetric scanning, which enables continuous image acquisition during the first circulation of
intravenous contrast material through the vessels of interest. Modern CT scanners use the spiral technology, where a continuously rotating X-ray tube and detector set (gantry) acquires helical data set from the patient who is moved through it (table
motion). Pitch is defined as the table travel per gantry rotation divided by the collimation of the X-ray beam. For CTA, the
New Approaches to Aortic Diseases from Valve to Abdominal Bifurcation. http://dx.doi.org/10.1016/B978-0-12-809979-7.00018-3
Copyright © 2018 Elsevier Inc. All rights reserved.
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pitch is set to be <1.0 to obtain sufficient sampling/overlap. In addition, currently available scanners are all multidetector
CT (MDCT) scanners, which have multiple rows of detectors and a single X-ray tube. Over the last decade, the number
of the detector rows has increased from 1 to more than 320 (wide-array scanners), as a result of which the craniocaudal
z-coverage, i.e., the volume of patient that can be imaged in one X-ray tube rotation has significantly increased, which
reduces the overall scanning time. For example, the 320-row multidetector system can cover up to 16 cm of the patient
craniocaudally in one rotation of the X-ray tube. Modern scanners also have fast gantry rotation times, which enables rapid
scanning and high temporal resolution, which is useful in imaging moving structures. The latest scanners also have high
spatial resolution resulting in high-quality images with fine detail [3].
To minimize motion artifacts from the aortic root and ascending aorta, CTA image acquisition is synchronized with
electrocardiogram (ECG) tracings. This can either be retrospective ECG gating or prospective ECG triggering. In retrospective ECG gating, imaging data from the entire cardiac cycle (R-R interval) are utilized in image reconstruction, which
is useful when dynamic information is needed throughout the cardiac cycle, evaluation of cardiac function is required, or
when high heart rates or arrhythmias are encountered. However, for evaluating the thoracic aorta, ECG synchronization is
done through prospective ECG triggering, where data are acquired in only one specific phase of the cardiac cycle, typically
the late diastole (75% R-R interval), with the scanning performed in the axial mode (“Step-and-shoot”) where the scanner
acquires images from a portion of the body and then moves on to the next location without overlap between the slices.
Dual-source CT scanners have two X-ray tubes, which are oriented at 90 degrees to each other, with two sets of detectors. If these tubes are operated at the same energies, the scanner has a high temporal resolution and can obtain motion-free
images of aorta or coronary arteries, even at higher heart rates. The latest generations of these dual-source CT scanners
also have a high-pitch helical mode, where a high pitch, i.e., up to 3.4 can be utilized, but without the expected gaps in data
because these gaps are filled by data from the second X-ray tube. This can be performed either with or without prospective
ECG triggering and is another mode for rapidly imaging the patient, with the coverage of the chest and abdomen in few
seconds [4].
Dual-energy scanners are those which use attenuation data from X-rays at different energy levels to separate different
tissues or materials. With the dual-source scanners, this can be obtained by operating the tubes at two different energy levels
(80/100 kVp and 120/140 kVp). There are also other technologies of dual energy such as rapid kVp switching, where there
is a single X-ray tube, but for each projection, data are obtained at two energies by rapidly switching the kVp; dual-spin
technology, where the same volume of the patient is scanned twice consecutively at different energies using a volume scanner; and the dual-layer detector-based spectral technology, where there is a single X-ray tube, but two layers of detectors
with the top layer absorbing high-energy photons and bottom layer absorbing low-energy photons. Although not widely
available, dual-energy CT scanners have several uses in the evaluation of the aorta, mainly in reducing the dose of intravenous contrast, salvaging suboptimal vascular enhancement studies, using calcium separation to optimize visualization of
lumen, and virtual noncontrast images to avoid the need for a true noncontrast image.
CTA of the aorta in adults does not require any sedation or anesthesia, but in children sedation/general anesthesia
may be required, although it is usually not required with modern scanners, particularly when only the aorta is evaluated.
CTA is performed by intravenously injecting iodinated contrast material, using the largest possible cannula feasible for
the patient size, with 18G preferred in adults. Arterial contrast enhancement depends not only on the rate and duration
of contrast injection, but also on the cardiovascular status of the patient [5]. Contrast volume, concentration, and flow
should be optimized to obtain at least 250–300 Hounsfield unit attenuation in aorta. Low or isoosmolar contrast agents are
used (300 mg I/mL or greater, typically 350 or 370) to minimize adverse reactions and extravasations. Contrast volume in
adults is typically 60–140 mL and in children is usually 1–2 mL/kg body weight. The contrast volume is calculated by the
injection rate (mL/s), multiplied by the scan duration (depends on patient length and scanner) plus 5–10 s for the time it
takes for the scanner to move from tracking position to the top of chest. Contrast injection is followed by saline flush to
increase the arterial opacification and decrease contrast and streak artifacts from veins. Power injector is used to achieve
homogeneous opacification, with flow rates from 0.5 mL/s for pediatric cannulas to 6 mL/s for 18 G cannulas and power
ranging from 50 to 300 psi. Scanning can be initiated using either real-time bolus tracking or timing bolus. Real-time
bolus tracking is performed with a region of interest placed in the aorta and dynamic low-dose scan performed at this level
every 1–3 s. When the attenuation in the aorta reaches a predetermined threshold, usually 100 HU above baseline value,
the scanning is initiated. In timing bolus technique, a smaller volume of contrast is injected first (15–20 mL) and multiple,
dynamic low-dose CT images are obtained at the same level of aorta, using which the contrast transit time is calculated.
This technique is useful in patients with variable circulation times, although it requires additional dose of contrast and
time.
Iodinated contrast should not be administered to patients with acute renal failure or severe chronic renal dysfunction
(estimated Glomerular Filtration Rate < 30 mL/min), unless they are already being dialyzed. In patients with chronic renal
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failure with eGFR between 30 and 44 mL/min, iodinated contrast can be administered with pre- and postprocedural hydration [6]. The contrast dose and iodine concentration should be reduced in these patients. Techniques for improving contrast
visualization with low-contrast doses include using lower kVp or using dual-energy CT to obtain virtual monoenergetic
images at lower energy, which will result in higher contrast attenuation because iodine appears brighter at lower energies.
Iterative reconstruction techniques may also be useful to improve contrast attenuation. Patients with contrast allergies
should be premedicated using a regimen of corticosteroids and antihistamines, at least for 12 h prior to study. Metformin is
withheld for 48 h prior to administration of contrast to prevent lactic acidosis [7].
CT Protocols
Depending on the clinical indication, dedicated and standardized protocols can be established to ensure data acquisition at
peak enhancement without missing the contrast bolus. The following are some examples of standard protocols for aorta.
1. Standard aortic protocol: This protocol is used for measurement of the aorta and for the most nonacute conditions. If
only the thoracic aorta is to be imaged, the scan is performed from the base of the neck to the upper abdomen in prospec-
tive ECG triggered axial mode. For imaging abdominal aorta, the scan is performed from the dome of the diaphragm to
the femoral neck, in a helical mode, without ECG gating. For imaging the entire aorta, the thoracic component of CTA
is performed in axial mode with prospective ECG triggering followed by the abdominal component, which is performed
in helical non-ECG-gated mode.
2. Vascular ring and congenital disease protocol: This protocol is the same as the standard aortic protocol, but ECG gating
is not required because the abnormality is usually in the aortic arch and children are the most commonly affected population.
Timing bolus may be used but in young children an empirical delay may also be used. For example, 12–15 s for <10 kg and
20–25 s for >10 kg. Another option that may be useful, especially when hand injection is performed in neonates/young infants
is to initiate the acquisition, when three-fourths or the entire contrast bolus has been injected into the body.
3. Acute aortic syndrome protocol: In this protocol, there is a noncontrast phase to evaluate for intramural hematoma,
followed by an arterial-enhanced phase, which can be either the thorax alone or the entire aorta, with parameters similar
to that of the standard aortic protocol. In some centers, a delayed phase (1–2 min) is also obtained to assess for late fill-
ing of false lumen in aortic dissection.
4. Trauma protocol: In patients with chest trauma and suspected vascular injury, an arterial and delayed phase (1–2 min)
is obtained. The delayed phase is useful for evaluation of contrast extravasation from aortic rupture and venous leaks.
ECG gating is not required in this protocol. This protocol may also be used for evaluation of inflammatory tissue in
infection and vasculitis.
5. Clot protocol: This protocol is used for evaluation of clots and is similar to the trauma protocol, with arterial and
delayed phases. A clot is seen in both the arterial and venous phase, but slow flow, a commonly encountered confounder
is seen only in the arterial phase but not in the delayed phase. Alternatively a split bolus of contrast can be utilized to
eliminate artifactual filling defects caused by contrast admixture.
6. Thoracic Endovascular Aortic Repair protocol: This protocol is used in the evaluation of patients with endovascular
stent grafts. This consists of non-ECG-gated noncontrast, arterial and delayed phases. Noncontrast phase is used for
evaluation of calcifications and surgical material, which can be confused with endoleaks. Delayed phase is required
for evaluation of slow endoleaks.
7. TAVR protocol: Patients being considered for TAVR need evaluation of the aortic root for sizing and measurement of
access vessels. In our institute, we initially scan only the heart in prospective ECG-triggered mode (in systolic phase)
for measuring the aortic annulus and follow it with a non-ECG-gated helical acquisition of the entire thoracoabdominal
aorta using an additional contrast bolus injection. Another option is to scan the entire chest with ECG gating (prospec-
tive or retrospective) and entire abdomen with non-ECG triggering, utilizing either one or two separate contrast boluses.
Some sites prefer to scan the heart in retrospective ECG gating mode.
8. Retrospective ECG-gated protocol: This protocol may be required in the evaluation of the aortic root, particularly
where dynamic information of the aortic valve is required. This may also be used in the evaluation of valve dehiscence,
vegetations, paravalvular abscess, and pseudoaneurysm.
9. Triple rule-out protocol is used in patients with acute chest pain to evaluate coronary artery disease, pulmonary embo-
lism, and aortic dissection [8]. Triphasic injection (contrast followed by contrast-saline admixture followed by saline)
is utlized to provide simultaneous opacification of the aorta, coronary arteries, and pulmonary arteries in a single ECG-
gated scan. Disadvantages include increased contrast and radiation dose, lack of a noncontrast-enhanced scan to rule out
intramural hematoma, and lack of coverage of the abdominal aorta.
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Image Postprocessing
After acquisition of the data, the CT images are postprocessed and viewed in several different ways.
l Axial source images: The CT images are obtained in the axial plane, usually at 0.5–0.75 mm and are reconstructed at
1–5 mm slice thickness (Fig. 18.1A) with variable overlap. These images are used for initial interrogation of anatomy
and pathology. Thinner slices have higher spatial resolution and are useful for additional isotropic reconstructions.
Thicker slices have lower spatial resolution, but have lower noise as well. Spatial resolution can also be improved by
using small field-of-view. Reconstructions at higher thickness and full field-of-view are used for evaluating other paren-
chymal organs for incidental findings.
l Multiplanar reconstructions: Using submillimeter isotropic voxels, two-dimensional (2D) images are reconstructed
in multiple other planes, typically sagittal and coronal, at thickness of 2–5 mm (Fig. 18.1B). This provides excellent
anatomic detail of the aorta, its branches, collateral circulation, and adjacent structures.
l Curved multiplanar reconstructions: In the curved multiplanar reconstruction the software stretches (Fig. 18.1C) or
straightens out the entire aorta, as a result of which even a tortuous aorta can be visualized in a single plane (Fig. 18.1D).
This plane is also useful for accurate measurement of the aorta. Using centerline analysis, the aorta can be tracked along
an automated or semiautomated central axis (Fig. 18.1C).
l Double oblique reconstruction: A double oblique image is reconstructed from two orthogonal views (Fig. 18.1E). Due
to tortuous course of the aorta, these true axial images reformatted from coronal and sagittal images have to be used to
for accurate and reproducible measurements.
l Maximal intensity projection (MIP) images preferentially display high-density voxels, which can be of any plane or
slice thickness (Fig. 18.1F). This is useful for evaluation of small vessels and high attenuation structures such as contrast,
calcification, and grafts. Stenosis will be underestimated in MIP images and hence should be evaluated in source images.
l 3D reconstructions: 3D renderings of the aorta can be obtained using either volume rendering (Fig. 18.1G) or shaded
surface display. These renderings provide exquisite images of anatomy, which can occasionally be complex in congeni-
tal disorders or following surgeries. Volume rendering is useful in providing a map for surgeries and interventions. In
addition, it is also useful for educating patients.
l Endoscopic view: These images mimic the appearance of vessel if it is looked from inside. This can be utilized for
evaluating the origins of branch vessels from aorta.
Radiation Dose Reduction Strategies
Several radiation dose reduction strategies are employed. CTA of the aorta should be performed only if the clinical indication is appropriate and the scan should be limited to the area of interest. ECG-gating increases the radiation dose and should
be limited only to the evaluation of aortic root and ascending aorta such as in the pre-TAVR evaluation. If ECG gating is
done, prospective ECG triggering should be used, because it reduces radiation dose by 60% and there is no need to acquire
data throughout the cardiac cycle [9]. If retrospective ECG gating is employed, ECG-based tube current modulation is
employed, i.e., maximum tube current is applied in only one particular cardiac phase and the tube current is decreased to
30% for the other cardiac phases. This reduces radiation dose by 40%–50% [9]. The tube current should be kept to the
minimum possible, which is determined by body habitus. Most of the modern scanners also have automatic tube current
modulation, where the tube current is adjusted according to the body region, with lower doses utilized for thinner areas and
higher doses for thicker areas. The tube voltage should also be kept to the least possible for a particular patient size. In children, 80 kVp is used, whereas for adults 100 kVp is used if the body mass index (BMI) <30 and 120 kVp is used if the BMI
>30. The use of the dual source, high-pitch scanner for CTA substantially reduces radiation dose in comparison to standard
helical CT [10]. Newer reconstruction algorithms, such as iterative reconstruction, decrease noise in the scanned image,
thus enabling the use of lower radiation doses techniques, including low tube current and tube voltages [11]. Also, the use
of virtual noncontrast images from dual-energy scanners eliminates the need for true noncontrast images in multiphasic
studies such as TEVAR protocol and can result in radiation dose savings up to 60% [12–14].
Normal Aortic Anatomy
The aorta can be divided into six segments: aortic root—between the aortic annulus and sinotubular junction; ascending
aorta—between the sinotubular junction and the brachiocephalic trunk; aortic arch—between the origin of brachioce-
phalic trunk and origin of left subclavian artery; isthmus—between the left subclavian artery and ligamentum arteriosum;
descending thoracic aorta—between the ligamentum arteriosum and diaphragmatic hiatus (T12); abdominal aorta—
between the diaphragmatic hiatus and aortic bifurcation (L4).
CT Angiography Chapter | 18 197
(A)
(B)
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(C)
(D)
(E) (F) (G)
FIGURE 18.1 Types of computed tomography (CT) images used in the evaluation of aorta. (A) Axial source images that are used in the initial analysis.
(B) Multiplanar reformats (MPR) in the coronal and sagittal planes. (C) Curved MPR with the demonstration of centerline. The software generates crosssectional images of the aorta along the centerline, which is used for measurement. (D) Straight MPR, where the aorta has been stretched into a straight
structure. (E) Double oblique axial reconstruction at the level of aortic valve using coronal and sagittal images. (F) Maximum intensity projection (MIP)
image of the aorta in sagittal plane. (G) Volume rendered 3D image of the aorta.
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TABLE 18.1 Normal Measurements of the Aorta [15–20]
Anatomy Normal Values
Aortic annulus 26.3 ± 2.8 mm (coronal)
23.5 ± 2.7 mm (sagittal)
25–37 mm [95% confidence interval (CI)] (end diastolic)
Sinus of Valsalva 34.2 ± 4.1 mm [2 standard deviation (SD)]
36.9 ± 3.8 mm (2 SD) (end diastolic)
Sinotubular junction 29.7 ± 3.4 mm (2 SD)
Ascending aorta 32.7 ± 3.8 mm (2 SD)
33.6 ± 4.1 mm (2 SD) (male/end systolic)
31.1 ± 3.9 mm (2 SD) (female, end systolic)
21–35 mm (95% CI) (end diastolic)
Descending aorta 17–26 mm (95% CI) (end diastolic)
Abdominal aorta, infrarenal 19.3 mm (SD 2.9 mm) men
16.7 mm (SD 1.8 mm) women
Abdominal aorta, lower 18.7 mm (SD 2.7 mm) men
16.0 mm (SD 1.7 mm) women
Branches of thoracic aorta are coronary arteries from aortic root; right brachiocephalic artery, left common carotid
and left subclavian artery from the aortic arch; intercostal, bronchial, spinal, superior phrenic and numerous mediastinal
branches from the descending thoracic segment. Branches of the abdominal aorta are unpaired visceral branches (celiac,
superior mesenteric, and inferior mesenteric); paired renal, adrenal, gonadal, inferior phrenic, and lumbar arteries; artery of
Adamkiewicz; and midline median sacral artery.
Aortic measurements are made from true axial images from one blood-wall boundary to the other. Aortic diameter varies
with age and sex of the patients. Normal measurements are listed in Table 18.1 [15–20]. The ascending aorta is considered
dilated when it measures more than 4 cm and aneurysmal when it measures more than 5 cm. The descending thoracic is
considered dilated when it measures more than 3 cm and aneurysmal when it measures more than 6 cm. Abdominal aorta is
considered aneurysmal when it is more than 3 cm.
ABNORMALITIES OF THE AORTA
Congenital Anomalies
CTA is useful in mapping the complex morphology of various congenital aortic disorders such as bicuspid aortic valve
(Fig. 18.2), transposition of great arteries (Fig. 18.3), truncus arteriosus, patent ductus arteriosus (Fig. 18.4), and
vascular rings (Fig. 18.5). In aortic coarctation, MDCT displays the focal aortic narrowing with preductal dilatation and
collateral circulation (Fig. 18.6).
Atherosclerosis
Calcified and noncalcified plaques are well depicted on CT (Fig. 18.7). Complex plaque, plaque >4 mm, ulcer >2 mm, and
mobile thrombi have higher association with coronary artery disease.
Aortoiliac Occlusive Disease
Aortoiliac occlusive disease is a sequel of chronic atherosclerosis, common in the infrarenal abdominal aorta and common
iliac arteries (Fig. 18.8).
FIGURE 18.2 Short axis computed tomography (CT) view of the aortic valve shows bicuspid aortic valve.
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FIGURE 18.3 Reformatted computed tomography (CT) image in sagittal plane shows the aorta (Ao) and the pulmonary artery (MPA) in the same
image, which is consistent with transposition, in this case, Dextro-Transposition of the Great Arteries.
FIGURE 18.4 Axial computed tomography (CT) scan shows a patent ductus arteriosus (arrow) between the aortic arch (A) and the pulmonary artery (P).
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FIGURE 18.5 Coronal reformatted computed tomography (CT) images shows double aortic arch (arrows), with the right and left aortic arches encir-
cling the trachea and esophagus.
FIGURE 18.6 Sagittal volume rendered computed tomography (CT) image in a patient who had a repaired coarctation shows narrowing (arrow), which
is indicative of recoarctation.
Aneurysms
True aneurysms (Fig. 18.9) involve all vessel layers, whereas in pseudoaneuryms, there is focal disruption of the intima
and media, which is contained by an intact adventitia. Based on morphology aortic aneurysms are classified into fusiform
(diffuse) or saccular (focal outpouching) types.
Inflammatory Diseases
Takayasu arteritis (Fig. 18.10) and giant cell arteritis are the most common large vessel idiopathic arteritis, which involve
the aorta. Bacterial aortitis results in the formation of a mycotic aneurysm that frequently has an irregular, saccular appearance (Fig. 18.11).
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FIGURE 18.7 Coronal computed tomography angiogram shows extensive nonatherosclerotic plaques throughout the descending aorta causing contour
irregularity (arrows).
FIGURE 18.8 Axial computed tomography (CT) scan shows occlusion of the left common iliac artery (arrow), in a patient with severe aortoocclusive
disease.
Aortic Fistulas
Fistulous connection can be seen between aorta and various structures, either directly from an aortic aneurysm or secondary
to complication of aortic surgery or endovascular stent-graft repair (Fig. 18.12).
Aortic Masses
Primary tumors of the aorta seen as irregular contrast-enhancing masses are extremely rare, with the majority of them being
unclassified sarcomas followed by malignant fibrous histiocytoma.
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