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head of the gastrocnemius, although a number of abnormal
relationships have been described. This syndrome usually
causes some degree of fi xed narrowing of the popliteal
artery, although there is commonly a dynamic component of
narrowing, usually during plantar fl exion or dorsifl exion.
Repetitive trauma to the artery as a result of the abnormal
relationship to the muscle may cause aneurysmal dilatation,
thrombosis, or thromboembolism. MRI is useful in demonstrating popliteal artery entrapment syndrome, where an
abnormal muscular slip courses medial to the popliteal
artery. In this case, the lower extremity may need to be
imaged in several positions including dorsifl exion and plantarfl exion [ 40 ]. This is also more easily performed with MR
angiography, since MR angiography is less sensitive to optimal vascular opacifi cation and images can be obtained at
different time-points. Also, as non-contrast means of performing MRA become more robust, some vascular pathology may be imaged without the administration of contrast.
Since the common femoral artery is a common site of vascular access, it is subject to iatrogenic complications including chiefl y pseudoaneurysm and arteriovenous fi stula
formation. Because of the focal nature of these complications, and because the portion of the artery involved is frequently very superfi cial, ultrasound with Doppler is usually
an adequate modality for the diagnosis and follow-up of iatrogenic femoral artery complications. On the other hand,
when a deep or retroperitoneal hematoma is suspected, CT
may be a more robust technique than ultrasound.
Other Modalities for Imaging the Peripheral
Arteries
Advancements in imaging of the peripheral arteries have
occurred in virtually every modality. As a result, the decision
between modalities is more complex. Physical exam and
ankle-brachial index measurement is an adequate means of
making an initial diagnosis of peripheral arterial disease [ 37 ].
Further evaluation with ultrasound is also useful in demonstrating and localizing atherosclerotic disease. Complete evaluation of the entire extremity with ultrasound is, however, very
time-intensive and detection of disease is technologist dependent. Detection and measurement of stenoses with ultrasound
is also dependent on technical factors, such as the angle of
insonation employed. Evaluation of the pelvic vasculature by
ultrasound is much more diffi cult, and portions of the vasculature may not be easily demonstrated with ultrasound due to
overlying bowel gas and osseous structures. In very obese
patients, ultrasound may be signifi cantly limited. Heavy or circumferential calcifi cation, such as that found within patients
with diabetes or renal insuffi ciency also signifi cantly limits
ultrasound. Determination of severity of disease by ultrasound
also has diffi culty in determining the severity of disease in
arterial segments distal to a high-grade stenosis.
MRI and MR angiography have several advantages in
patients, including the ability to perform imaging without
contrast. Non-contrast MR angiography techniques have
advanced dramatically, although there is still considerable
variability between institutions and MR technology. Clinically
useful imaging of tibial and pedal vessels using non-contrast
MR is generally not possible except in highly specialized
centers. Because calcium does not interfere with contrastenhanced MR angiography, evaluation of tibial vessels with
MRA, when a separate tibial imaging bolus is used instead of
a bolus-chase technique, is frequently superior to CTA in
patients with critical limb ischemia. In particular, the adequacy of MR sequences for imaging the arterial tree are
dependent on the scanner, sequences, and vendor-specifi c
techniques used. MR angiography has signifi cant limitations
in evaluating the post-surgical arterial tree, due to artifacts
such as failure of fat saturation and susceptibility artifacts due
to surgical clips, stents, or other foreign material. MR angiography is also contra-indicated in patients with non-MR conditional pacemakers or ICDs. Likewise, certain stents and
stent-grafts are MR conditional, such that patients with these
implants cannot undergo MR in 3 T machines. Patients with
claustrophobia or signifi cant back pain may not tolerate lying
still for the hour-long exam. Because of these limitations MR
is contraindicated in approximately 30 % of patients.
In the past, MR has been preferable in patients with renal
disease due to relatively lower nephrotoxicity of gadolinium,
compared to iodinated contrast media. However, the recent
recognition of nephrogenic systemic fi brosis as a complication of gadolinium administration has decreased the utility of
MR angiography in patients with chronic, severe renal disease [ 42 ]. Gadolinium should generally not be given to
patients with a creatinine clearance of 30 ccs per minute or
less. In patients who are already dialysis-dependent, iodinated contrast may be a better choice. CT has an advantage to
MR angiography in superior spatial resolution and depiction
of smaller vessels. Evaluation of the patency of circumferentially calcifi ed tibial vessels remains challenging for CTA,
however, and is one of the few circumstances where DSA
may be required.
Radiation Dose in Peripheral CT
Angiography
The radiation dose in CT angiography remains high and is
increasingly a consideration in most CT applications.
Concerns of radiation are somewhat mitigated by the fact
that the extremities contain less radiosensitive tissues. When
imaging the extremities, breast and abdominal shielding can
easily be employed with no compromise to image quality.
Shielding signifi cantly decreases scatter and is under-utilized
in patients undergoing CT in general, including peripheral
CT angiography.
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The radiation dose for conventional angiography is, however, much higher than for CT angiography [ 43 ]. This is in
contradistinction to radiation doses in the heart, where catheterization results in lower radiation doses compared to
CT. One study found that for a 16-slice CT scanner, the average radiation dose for a peripheral CT angiogram was
3.0 mSv in men, whereas the radiation dose for a conventional angiogram had an average of 11.0 mSv. Other studies
have shown similar results, with CT angiography generally
found to have a fourfold lower radiation dose in comparison
with peripheral angiography [ 44 ]. Although peripheral CT
angiography has a relatively low radiation dose and relatively less radiosensitive tissues are exposed, the risks of
radiation should not be taken lightly.
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© Springer International Publishing 2016
M.J. Budoff, J.S. Shinbane (eds.), Cardiac CT Imaging: Diagnosis of Cardiovascular Disease,
DOI 10.1007/978-3-319-28219-0_18
Aortic, Renal, Mesenteric and Carotid CT
Angiography
Anas Alani and Matthew J. Budoff
Abstract
Computed tomography angiography has an increasing role in vascular imaging of the aorta,
renal, mesenteric, and carotid arteries. There has been tremendous improvement in computed tomography technology that has made such images the preferred choice for diagnosing various acute and chronic vascular diseases and replacing non-invasive and invasive
tests.
Keywords
Computed Tomography Angiography • Aorta CT Angiography • Renal CT Angiography •
Mesenteric CT Angiography • Carotid CT Angiography • Vascular CT Angiography •
Aortic Dissection • CT Angiography Acquisition and Protocol
Introduction
Computed tomographic angiography (CTA) of vascular
beds is signifi cantly easier to perform and interpret than
coronary studies. There is no cardiac motion to contend
with, so gating is most often not necessary. The exception is
the ascending aorta, where pseudodissections (an appearance of a dissection caused by motion of the aortic root –
Fig. 18.1 ) have plagued earlier studies with single-slice
computed tomography (CT) due to motion artifacts [ 1 ].
Most of the large vessels of interest (the carotid, renal, and
mesenteric arteries) have signifi cantly larger diameters than
coronary arteries, as well as less tortuous courses. The renal
and carotid arteries are usually straight structures, so reconstructions are signifi cantly less complicated than coronary
imaging. Also, due to the increased speed of newer systems
(electron beam tomography (EBT) and 16+ row multidetector computed tomography (MDCT)), venous enhancement is less common, so it is easier to see the arteries
without superimposed contrast- fi lled structures (venous
contamination). This is another reason why CT is most often
superior to magnetic resonance imaging (MRI) in these vascular beds.
In regard to the aorta, CTA can diagnose aneurysm, dissection, and wall abnormalities such as ulceration, calcifi cation, or thrombus throughout the full length of the aorta, as
well as the involvement of branch vessels. Disease of the
aorta or great vessels can present with a broad clinical spectrum of symptoms and signs. The accepted diagnostic gold
standard, selective digital subtraction angiography, is now
being challenged by state-of-the-art CTA and magnetic resonance (MR) angiography. Currently, in many centers,
cross-sectional imaging modalities are being used as the
fi rst line of diagnosis to evaluate the vascular system, and
conventional angiography is reserved for therapeutic
intervention.
A. Alani , MD
Department of Medicine , University of Florida – Gainesville ,
Gainsville , FL , USA
Los Angeles Biomedical Research Institute at Harbor-UCLA ,
1124 W Carson Street , Torrance , CA 90502 , USA
e-mail: aaj.alani@gmail.com
M. J. Budoff , MD (
*)
David Geffen School of Medicine at UCLA , Los Angeles
Biomedical Research Institute , Torrance , CA USA
e-mail: mbudoff@labiomed.org
1 8
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Principles of Imaging
In aortic imaging, the volume coverage capabilities of
MDCT have come to full use without having to compromise
on resolution or detail [ 2 , 3 ]. With the current confi guration
of 64-row (or greater) CT scanners, the entire abdominal
aorta and the iliac arteries can be covered within seconds and
with isotropic resolution (Chap. 1 ). Investigation of the
dataset can now be done on the anteroposterior (coronal) and
lateral (sagittal) planes, which has been the convention with
invasive angiography. A few important technical advances
have further improved aorta imaging using CTA. First is the
increased number of detector rows for the acquisition of
images over greater z-axis lengths with one gantry rotation.
With up to 320 detectors, volume coverage per rotation is as
much as 160 mm. The typical distance needed for the
abdominal aorta is on the order of 400 mm, so two to three
rotations would cover the entire abdomen. Using a rotation
speed of <500 ms, this could be accomplished in 1–2 s.
Second, the use of dual-source technology allows for further
reduction of acquisition times, since the entire aorta can be
scanned in one breath hold. This technology has considerably
improved temporal resolution compared to single-source
acquisition (for which resolution approaches 0.4 mm) [ 4 ].
CT Technique
Understanding the principles of CTA techniques is essential to
acquire diagnostic images consistently. This section reviews
current CTA methods used in the evaluation of great vessels.
The following broad approach is a guide to CT scan acquisition for various scanners. For peripheral imaging, where electrocardiogram (ECG) gating is not required, 16–320-slice
scanners are more than adequate to image the entire volume.
In addition, there is no need for the speed that is required for
cardiac work (temporal resolution or rotation speed).
1. Intravenous injection of 35–70 mL of a nonionic contrast
agent (300–370 mg I/mL), decreasing with scanners with
higher numbers of detectors.
2. Monophasic or biphasic injection rate: most commonly a
monophasic injection at 4 mL/s (followed by a saline
bolus). Three phase injections (pure contrast, followed by
mixed contrast saline and pure saline) are more important
and common with cardiac applications.
3. Scan delay is determined by test injection (10 mL at
4 mL/s) or by automated triggering (to achieve imaging to
coincide with contrast arrival in the aortic root close to the
area of interest). The scan delay should be determined
near the start of the section being imaged (transverse
aorta for carotids, abdominal aorta for renals or runoffs).
4. Pitch:
• For 16-detector MDCT: 16 × 0.625 mm detector
confi guration with 1.25-mm reconstruction thickness
and pitch = 1.7 (table speed 17.5 mm/rotation divided
by 10-mm detector coverage (16 × 0.625 = 10 mm)),
reconstructed retrospectively with 0.37-mm intervals
for 3D and multi-planar reconstruction (MPR).
• For 64-detector MDCT: 64 × 0.625 mm detector confi guration with 0.625–1.25-mm reconstruction thickness and pitch = 1 (moving the table 40 mm and covering
40 mm with each rotation) up to a pitch of 1.375 (table
speed 55 mm/rotation divided by 40-mm detector
width), reconstructed retrospectively with 0.3-mm
intervals for 3D and MPR (the 40-mm detector width
coverage per rotation used is currently available in the
GE and Phillips 64 systems. The Siemens single- or
dual-source has a collimation of 19.2–38.4 mm, increasing with the Philips 256 (128 detectors of 0.625 mm
allowing 80 mm of coverage per rotation; the Toshiba
320 allows 160 mm of coverage per rotation)).
Aortic CTA
The speed and ease of modern CTA make it the technique of
choice for diagnosing chronic and acute aortic pathologic
fi ndings, such as intramural hematoma, aneurysm, traumatic
injuries, atherosclerosis, and dissection (Fig. 18.2 ). With the
current confi guration of 64-row CT scanners, the entire
abdominal aorta and the iliac arteries can be covered with
isotropic resolution. Moreover, the high scan speed allows
substantial reduction of the amount of contrast material used
in earlier studies, hence reducing the adverse effects.
Fig. 18.1 Axial view of pseudo-dissection of aorta caused by motion
artifact in an ungated computed tomography (CT) scan of the chest
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Gated vs. Non-ECG Gated CTA
Non-gated CTA allows for very fast acquisition, and interpretation is signifi cantly less complicated. Without ECG gating or breath-holding, artifacts such as misregistration do not
occur, which improves image quality compared to cardiac
studies. Non-ECG-gated CT is performed in parts of the
aorta without much aortic dynamics, such as the abdominal
aorta. ECG gating means that the scan is synchronized to the
cardiac beat. This will lead to decreased motion artifacts
from cardiac movement and aortic dynamics [ 5 ] (Fig. 18.1 ).
Such motion artifacts may mimic the appearance of a dissection in the ascending aorta and lead to misdiagnosis [ 6 ]. This
ECG triggering should be considered in the ascending aorta,
the aortic arch, and the descending thoracic aorta [ 7 ]. ECG-
gated CTA requires longer acquisition to obtain the specifi c
phase of the ECG cycle, which will result in more contrast
media [ 8 ]. The possibility of applying ECG-controlled
X-ray-tube dose modulation is another step forward for
reducing radiation exposure rates.
Challenges
CTA application can be limited due to radiation doses and
nephrotoxicity. Of course, the requirements of radiation
(which are more signifi cant for carotid imaging due to
radiation-sensitive organs such as the thyroid and orbits) and
contrast (which is more signifi cant for renal artery imaging
due to the frequent coexistence of renal insuffi ciency and
renal artery stenosis) make MR more attractive for selective
cases. New scanners with more detectors reduce contrast,
since the imaging territory is covered in a shorter period. If
there is no ECG gating, contrast requirements are minimal
(30–40 cc per study). Another technique to minimize contrast
is using saline to fl ush the contrast through the system (Chap.
2 ). The saline chaser offers two signifi cant benefi ts with CTA
imaging. One is that the contrast is forced from the tubing
and extremity veins into the central circulation, allowing for
a reduction in the total contrast dose. A second benefi t is that
the contrast sitting in the vein during imaging can cause
partial volume (beam hardening) artifacts. Moving the
contrast out of the venous system is important for cardiac
imaging (where the scatter from the superior vena cava can
cause artifacts in the right atrium and right coronary artery),
carotid imaging (obscuring the proximal brachiocephalic
artery or carotid base), and pulmonary imaging.
With fast imaging, the venous circulation is not fi lled;
reducing venous contamination (large veins obscuring
smaller arteries). This could be problematic in renal beds and
runoff studies, as is often seen with MRI. Thus, using new
scanners, large areas can be scanned with minimal contrast
use. The most common protocols employed increase the
image acquisition time from 100 ms per image to 200–
300 ms per image to improve tissue penetration and reduce
image noise. Still, 50–60 mL of contrast at most is all that is
necessary to complete a thoracic and abdominal aortic study.
The radiation dose of CTA has improved dramatically
over the last few years [ 9 ]. Using a low and reasonably
achievable dose to obtain a diagnostic image can be done by
decreasing tube current, tube voltage, scan coverage, and
other dose-saving strategies. Newer-generation scanners
with a large detector array can cover a larger area of the aorta
with each gantry rotation and provide prospective triggering
over a larger span of the aorta, resulting in less radiation.
Aortic Dissection
The superior temporal resolution of current MDCT systems
signifi cantly improves imaging of the aorta, because motion
artifacts are eliminated in the ascending aorta. CT is often
considered a superior method over other imaging methods
for the identifi cation of aortic dissection, as the intimal fl ap
is usually well delineated, even in branches of the aorta. The
3D nature and the ability to see the outer wall, false lumens,
and the presence of a clot make this technique superior to
even invasive angiography for the evaluation of dissection
(Fig. 18.3 ). The extent of the dissection, including the
proximal entry and distal re-entry sites, the involvement of
Fig. 18.2 A volume-rendered image depicting an aortic dissection
involving the abdominal aorta ( arrow ), starting below the renal arteries
and ending prior to the iliac arteries
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adjacent branch vessels, and the potential comprise of the
true lumens are thoroughly evaluated.
The ability to visualize the great vessels in the transverse
aorta, neck, and arms makes CT signifi cantly more robust than
transthoracic and transesophageal echocardiographic imaging
and tolerance by patients is signifi cantly better. Transthoracic
echocardiography visualizes the aortic root well but is poor at
imaging the mid-ascending and descending thoracic aorta.
Transesophageal echocardiography is minimally invasive but
does not image the distal ascending thoracic aorta or arch well.
Because imaging protocols for MDCT can be performed in
less than 10 min (signifi cantly shorter than MR or transesophageal echocardiography), even unstable patients can be evaluated and triaged quickly. With the use of fl ow modes (usually
used for timing of contrast), assessing luminal fl ow in the true
and false lumens is possible.
Thoracic Imaging
Diseases of the thoracic aorta present a diagnostic challenge. Many aortic conditions such as aneurysms typically
cause no symptoms and often go clinically unrecognized
until a life- threatening complication occurs. CT is the primary means of imaging the lung, thoracic trauma (blunt
and penetrating), aneurysms, and aortic dissections [ 10 ].
CT is playing an increasingly important role in the diagnosis and management of thoracic aortic pathology [ 11 , 12 ].
Once aortic disease is detected, a comprehensive evaluation
of the entire thoracic aorta is indicated to demonstrate the
maximal aortic diameter and to detect associated disease in
other segments of the aorta. In the situation of an acute lifethreatening event, CT can provide extensive information
concerning the heart, aorta, and great vessels with a single
scan protocol (Fig. 18.4 ). In addition, during the same
examination, the brain and spinal canal can be evaluated if
necessary. The entire global CT examination (head, cervical spine, chest, abdomen, and pelvis) can be completed on
modern MDCT systems with scan times of 20 s and exam
times of <15 min [ 13 ].
Comparison to Other Methods
Although MRI and transesophageal echocardiography can
provide excellent and unique information, the robust nature
of CT often makes it preferable. Advantages are the ability
to image the entire aorta and beyond, the demonstration of
surrounding structures and organs, quantitative measures of
aneurysm size and location, and a rapid examination time.
Limitations are the negative effects of iodinated contrast on
renal function, the rare adverse reactions to iodinated contrast, and the inability to directly measure blood fl ow (which
is useful for determining true and false lumens). A current
MDCT protocol for CTA provides high-resolution arterial
phase images from the thoracic inlet to the femoral arteries.
This coverage incorporates the entire aorta, as well as the
organs of the chest, abdomen, and pelvis. Beyond classifying dissections as involving the ascending (Stanford type A)
or descending (type B), CT can demonstrate associated fi ndings that are critical to patient care, such as mediastinal
hematoma, pericardial effusions, pseudoaneurysm formation, and active extravasation of contrast from the aorta.
Quantitative measurement of aneurysm size, location, and
relation to branch vessels can be used for planning operative
or intravascular repair and for monitoring post-procedure
anatomy.
The need for precise and quantitative measurements with
CT has become more critical with continued advancements
in endovascular repair with stent grafts [ 14 , 15 ]. CTA is less
operator-dependent than transesophageal echocardiography,
it allows for complete organ visualization, and it is faster and
more convenient for patients than MRI and digital subtraction angiography. The latter issues are especially important
with severely ill patients. In the setting of blunt and penetrating trauma, CT of the chest can be extremely useful in diagnosis and as an aid to surgical management [ 16 ]. Another
major advantage over MR is that these examinations are performed in critically ill patients who may require mechanical
ventilation, invasive monitoring, intravenous infusion
pumps, and cardiac pacing.
Fig. 18.3 Spiral aortic dissection seen on a sagittal view of a gated
64-multidetector computed tomography (MDCT) cardiac scan
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Abdominal Aorta
Aortic aneurysm is associated with risk of sudden death due
to aortic dissection or ruptures. It can occur in association
with connective tissue disorders or acquired cardiovascular
disease [ 17 ]. The ability to measure the diameter, wall throm-
bus, and calcifi cation makes CT an ideal modality for
sequentially following patients and making accurate assessments for surgical planning or medical therapy (Fig. 18.5 ).
Aortic endovascular stenting is gaining acceptance as an
alternative to traditional open surgical repair for abdominal
aortic aneurysms. CT imaging is the predominant method
used for preoperative planning to assess the feasibility of
endovascular aortic stenting and to select the appropriate
aortic stent graft. The abdominal aorta is usually scanned
before and following intravenous contrast enhancement,
which enables detection of calcifi cation of the arterial wall
that will be partly obscured following contrast enhancement.
It also provides a baseline for evaluating any vascular injury
with hemorrhage or thrombus that will be seen in the postcontrast acquisition. 3D sagittal and coronal reconstructions
are routinely performed (Figs. 18.6 and 18.7 ). Maximum
intensity projection (MIP) provides images similar to conventional angiography and is useful to visualize calcifi cation
and the relationship of the aneurysm to adjacent vessels.
Fig. 18.4 Thoracic aortic dissection extending into the transverse aorta ( left ) and descending thoracic aorta ( right ). The intramural thrombus is
easily identifi ed by the arrow
Fig. 18.5 Aortic wall calcifi cation ( arrow ) and aneurysm on an axial
image at the level of abdominal aorta
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Fig. 18.6 A representation of the 2D axial images ( top left ), curved
multiplanar reformat ( top right ), and volume-rendered images ( bottom )
of a patient with an abdominal aortic aneurysm. The iliacs and femoral
bifurcations can be seen best in their true anatomic 3D orientation with
the volume-rendered image. The thrombus, however, is only visible on
the 2D images and curved MIP image ( green arrows ). The white arrow
demonstrates the iliac aneurysm
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Common Indications for CTA
of Abdominal Aorta
1. Detection and depiction of atherosclerotic occlusive
disease or aneurysmal dilatation of the abdominal aorta
and iliac arteries.
2. Preoperative assessment of aortoiliac aneurysms to
determine whether open repair or stent grafting is
indicated.
3. Preoperative measurement of the aneurysm for selecting
the appropriate stent graft.
4. Follow-up for the size and progression or regression of
abdominal aortic aneurysms.
5. Diagnosis of the presence and severity of complications
following aortic stent-graft placement, including
endoleaks, aneurysm expansion, rupture, and pseudoaneurysm, thrombus, and graft migration [ 18 ].
6. Detection and depiction of aortic dissection.
7. Detection of the presence of aortic aneurysm rupture.
Accurate measurements of the aortic root diameter can be
made easily, and the extent of the aneurysm can be defi ned.
Luminal thrombus is easily identifi ed by differences in tissue
density during contrast enhancement. The tomographic
format of CT provides excellent defi nition of the relationship
of aortic aneurysms to adjacent structures. Blood leakage
from the aneurysm or stent may be recognizable with contrast
enhancement of surrounding tissues.
The 2D images (axial data), MIP, and multiplanar imaging
allow accurate measurement of the length, location, and
diameter of aneurysms. The involvement of branch vessels
(renals, mesenterics, iliacs, etc.) is also easily assessed with
minimal contrast requirements. CTA has become the fi rst- line
modality for evaluation to plan stent-graft deployment
(Fig. 18.7 ) and post-procedural assessment (Fig. 18.8 ).
Cephalocaudal coverage from the celiac trunk to the proximal
thighs provides a suitable study volume to detect aortic disease. Although the preoperative assessment requires a true
early arterial phase to investigate all preoperative necessities
(e.g., aortic neck diameters, angle and distance from the renal
arteries), postoperative study requires a biphasic scan protocol for more detailed inspection of the perigraft space to rule
out possible endoleaks. High- resolution thin-slice protocols
are preferable, especially for the post-processing task.
Comparison to Other Modalities
Like CT, magnetic resonance angiography (MRA) of the
abdomen is always acquired as part of a routine lowerextremity runoff procedure most commonly performed for
symptoms of claudication. With CT, the renals can be
routinely evaluated during an abdominal aorta study. For
MR, the evaluation of the renal arteries for characterizing
potential renal artery stenosis in patients with hypertension
must be done as a separate procedure with different imaging
protocols. This is also true for the evaluation of a potential
renal donor. In these patients, dedicated abdominal MRA
acquisition is required with greater contrast enhancement,
which is not feasible when the legs and feet must also be
imaged at the same time. This is because there is a limit on
the total volume of gadolinium, which is usually 30–45 mL
for an adult. An abdominal MRA performed for the
indications listed is often scanned as part of the same procedure as a thoracic MRA, as it is for CT.
Fig. 18.7 Abdominal aortic aneurysm with large intramural thrombus,
seen on maximal intensity projection image ( top, green arrow ) and
volume-rendered ( bottom, blue structure )
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