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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3727_Библиотеки_им_академика_М_И_Перельмана
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154 Chapter 16 Computed tomography and MRI in venous disease
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16.9 Inferior vena cava (IVC) and external iliac vein thromboses. Contrast-enhanced axial computed tomography images with
low-attenuation thrombus in the infrarenal IVC
contrast, chronic occlusion presents with absence of the vein or nonopacied linear densities (arrowhead).
(a) and external iliac vein (b) consistent with acute bland thrombus (arrows). (c) In
16.10 Inferior vena cava (IVC) malignant thrombus. Con-
trast-enhanced coronal CT image with mixed density tumor
and bland thrombus lling the suprarenal and infrarenal IVC
(arrows).
of 96% for CT angiography.14 However, further research
using the latest CT technology may provide improved accuracy in small peripheral pulmonary arteries, with accurate
detection of pulmonary emboli of all sizes (Figure16.14).
In addition, the stratication of patients based on clinical
assessment provides the optimal strategy for the diagnosis
of pulmonary emboli.
Acute pulmonary emboli are diagnosed by lling defects
within the involved pulmonary arterial segments, which are
often slightly enlarged. The optimal administration of the
intravenous contrast material is the key factor in the acquisition of an accurate pulmonary arterial CT evaluation, and
this may be acquired in a single breath-hold by using a xed
acquisition delay of approximately 20–25 seconds after initiating the contrast injection or, more precisely, by using bolus
15
16.11 Venous branch thrombus. Contrast-enhanced axial CT
images with
and
(b) within the left common femoral vein (arrow).
(a) acute thrombus within the left renal vein (arrow)
tracking software that optimizes the CT acquisition based on
the peak enhancement of the central pulmonary arterial vasculature. Chronic pulmonary emboli are often represented by
recanalization of the thrombosed pulmonary arterial segment
(Figure16.15). Specic CT ndings for chronic pulmonary
16.2 Imaging technologies: CT of venous disease 155
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16.12 Traumatic disruption of the inferior vena cava (IVC). (a, b) Axial and coronal CT exams show extraluminal extravasation of
contrast material (arrows) consistent with infrahepatic disruption of the IVC. This nding was conrmed on subsequent catheterdirected cavagram (
16.13 Inferior vena cava (IVC) lter complications. (a) Volume-rendered coronal computed tomography image demonstrating migra-
tion of an IVC lter caudally near the common iliac vein bifurcation (arrows) rather than at the level of the renal veins (arrowhead). (b)
Curved planar reformat 3D reconstruction shows thrombus trapped within the IVC lter (arrow) as well as acute thrombus in the right
external iliac and common femoral veins (arrowhead).
emboli include peripheral eccentric thickening of the involved
pulmonary arterial vasculature, represented by soft tissue
rinds or linear webs (Figure16.15). On reconstructed arterial
segments, there may be abrupt caliber change of the involved
pulmonary arterial segments.
c, arrow).
16.2.1.4 Venous compression syndromes
Venous compression syndromes are a group of different
conditions in different parts of the body with the common denominator of extrinsic compression resulting in
increased venous pressure and/or occlusion. In this chapter
156 Chapter 16 Computed tomography and MRI in venous disease
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16.14 Acute pulmonary emboli in three patients. (a) Axial computed tomography imaging of pulmonary emboli in a small segmental
pulmonary artery (arrowhead), (b) bilateral pulmonary emboli (arrowheads), and (c) a large central pulmonary embolus (arrowheads)
in the main right pulmonary artery (“saddle” embolus).
we will focus on May–Thurner and nutcracker syndromes.
This group of diseases can be a challenging diagnosis due
to its underlying anatomical abnormality occurrence in
asymptomatic subjects. Imaging is crucial to demonstrate
the venous compression; however, it is not diagnostic and
needs to be correlated with the patient’s clinical presentation. May–Thurner syndrome is dened as isolated left
lower extremity swelling, pain, or thrombosis due to compression of the left iliac vein by the overlying right common
iliac artery. On axial CT evaluation, the maximum diameter of the left common iliac vein is decreased, measuring
3–4 mm in maximum diameter (Figure 16.16) versus a
normal-caliber iliac vein measuring 10–12mm in average
diameter.
16
Treatment for May–Thurner syndrome depends
on the severity of symptoms at presentation. Uncomplicated cases may be managed with medical therapy, but
severe cases typically require endovascular intervention. In
the acute setting, it may require thrombolysis followed by
angioplasty and stenting for relief of venous compression.
16.15 Chronic pulmonary emboli. Axial computed tomography
imaging with chronic pulmonary emboli characterized by recanalization and irregular wall thickening (a arrowheads) and thin
web (b arrow).
CT is a valuable modality to demonstrate post-treatment
complications such as stent occlusion (Figure16.17). The
success of primary and secondary endovascular techniques
approaches 90%.
17,18
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16.16 May–Thurner syndrome. Contrast-enhanced axial computed tomography shows marked compression of the left common iliac vein
by the right common iliac artery (arrow in
Nutcracker syndrome occurs when the left renal vein is
compressed between the SMA and the aorta, although other
uncommon anatomic variations may be responsible for the
compression. Patients typically present with left ank pain
and hematuria. Other symptoms include orthostatic hypotension, varicocele in males, and pelvic pain in females. The
computed tomography protocol to evaluate for nutcracker
syndrome includes an arterial and venous phase. The arterial
phase is specically obtained to demonstrate venous reux
into the left ovarian vein. Although the left renal vein opacies promptly and is commonly seen on the arterial phase,
it may not be homogeneously opacied, limiting the evaluation. Computed tomography demonstrates the left renal vein
compression between the aorta and SMA, as well as ancillary
ndings such as retroperitoneal collateral veins, dilated ovarian vein (>5mm), and pelvic varices (Figure16.18). Treatment depends on the severity of symptoms and varies from
conservative management to surgical venous decompression.
16.2.1.5 Tailored applications
State-of-the-art CT technology enables rapid acquisition
of sub-millimeter high-resolution CT scans that can be
interrogated and displayed in any imaging plane using 3D
volumetric imaging analysis for accurate characterization
of the sites of obstruction and for the patency of treated
a, c) and associated prominent left pelvic venous collaterals (arrowheads in b).
venous segments. Manipulation of the data sets allows
exquisite display of complex anatomical and pathological
relationships, including complex pulmonary arteriovenous
malformations (Figure 16.19), complex intra-abdominal
or pelvic venous malformations (Figure16.20), or direct
lower extremity venography (Figure16.21).
16.3 IMAGING TECHNOLOGIES: MRI
OF VENOUS DISEASE
16.3.1 Magnetic resonance venography
Magnetic resonance (MR) venography is usually not the rst
examination performed to evaluate the venous system, but
it has a wide range of applications, and is often successful
where other techniques yield ambiguous results. MRvenography comprises a number of different techniques, with different mechanisms of achieving vascular contrast. As such,
exibility is a major advantage of MR venography over
most competing technologies: where one technique may not
be particularly successful for a given application, it is usually
possible to apply a different method and achieve satisfactory
results. This exibility can also be something of a limitation,
however, since the range of choices can be somewhat daunting to those with limited experience.
158 Chapter 16 Computed tomography and MRI in venous disease
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16.17 Postprocedural stent occlusion with venous collaterals. (a, b) Contrast-enhanced axial computed tomography demonstrates
occlusion of the left iliac venous stent (arrows). (c) Volume-rendered 3D reconstructions demonstrate the extensive venous collaterals overlying the pubic symphysis (arrowheads).
Advantages of MR venography over CT venography include the ability to obtain diagnostic examinations
without intravenous contrast, superior contrast-to-noise
ratios of venous blood, and the ability to perform multiple
acquisitions while waiting for contrast to appear in veins
without incurring penalties in terms of additional radiation
dose. MRvenography is limited by generally lower spatial resolution in comparison to CT, by longer examination
times, and by the exclusion of patients who are unstable
or have pacemakers or automatic implantable cardioverter
debrillators and certain cerebral aneurysm clips.
16.3.2 Techniques
A large number of MRvenography techniques are available. We have arbitrarily divided these into black blood,
bright blood, and CE techniques.
Black blood MR venography pulse sequences are
employed relatively infrequently as a dedicated venography technique; however, black blood effects are commonly seen in spin-echo and fast spin-echo sequences as a
result of excited spins in blood owing out of the imaging
plane during acquisition (Figures16.22–16.24). Spin-echo
16.3 Imaging technologies: MRI of venous disease 159
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16.18 Nutcracker syndrome. (a) Contrast-enhanced axial computed tomography demonstrates signicant compression of the left
renal vein between the SMA and aorta (arrow). (b) Volume-rendered 3D reconstruction shows the left renal vein compression (arrow)
with reux of contrast into a dilated left ovarian vein (curved arrow) associated with retroperitoneal (curved arrow) and pelvic (thick
arrow) collaterals.
16
16.19 Computed tomography evaluation including (a) maximum intensity projection and (b) volume-rendered images with depiction
of a complex pulmonary arteriovenous malformation communicating with the thoracic aorta and pulmonary arteries (arrowheads)
and pulmonary veins (curved arrows).
160 Chapter 16 Computed tomography and MRI in venous disease
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16.20 Contrast-enhanced computed tomography of the abdomen and pelvis demonstrates a large arteriovenous malformation
surrounding the left hemipelvis on the
16.21 Lower extremity direct computed tomography (CT)
venography. (a) Volume-rendered images acquired via direct CT
venography with injection of contrast material into a dorsal vein
of the foot demonstrates a patent covered stent in the left iliac
vein (arrows) and a patent great saphenous vein (arrowheads).
(b) Patent stent in the upper femoral vein (curved arrows) with
chronic post-thrombotic changes in the femoral vein below the
stent (notched arrow).
(a) axial (arrows), (b) coronal (arrows), and (c, d) volume-rendered 3D reconstructions (arrows).
and fast spin-echo sequences employ a series of 90- to
180-degrees radio-frequency (RF) pulses prior to data acquisition. Stationary spins experience both pulses and contribute to the resulting signal in the expected manner. Moving
spins, on the other hand, may pass through the imaging slice
between the initial 90-degree pulse and data acquisition,
being replaced by nonexcited spins that never experienced
the initial pulse and therefore do not contribute signal to
the image. This leads to a signal void, or black blood effect.
More sophisticated black blood sequences employ electrocardiogram (ECG) gating as well as two inversion pulses
in order to improve the reliability of this effect. Most black
blood vascular sequences are designed for arterial imaging,
but are also effective for venography, or can be optimized
for venography by adjusting a few imaging parameters.
Black blood venography sequences can clearly demonstrate
lling defects in veins and often provide high-quality anatomic images. It should be noted, however, that these techniques are notoriously artifact prone—slow-owing blood,
for example, often yields an incomplete signal void and can
simulate venous thrombus. Likewise, in-plane rather than
through-plane ow can lead to positive signals within veins
that can be misinterpreted as thrombus. Diffusion-weighted
imaging (DWI) (Figure16.24) is an additional black blood
technique that is rarely employed as a dedicated venography
acquisition. DWI uses two or more gradient pulses applied
16.3 Imaging technologies: MRI of venous disease 161
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16.22 Black blood (a) and bright blood (b, c) noncontrast MRvenography in a patient with brosing mediastinitis and superior vena
cava (SVC) and right pulmonary artery occlusion. Occlusion of the SVC is demonstrated by the absence of ow void in the double
inversion recovery fast spin-echo image (arrow in a) and absence of bright blood signal in the fast gradient echo image (arrow in b).
Note also the lack of bright blood signal in the right pulmonary artery (arrowhead in c), consistent with thrombosis.
in opposite directions, separated by a short time interval.
The gradient pulses sensitize the acquisition to the effects of
microscopic diffusion, with signal loss occurring in proportion to small increments in microscopic diffusion. This technique is also exquisitely sensitive to bulk motion, such as
blood ow, and generates a very reliable black blood effect
in veins and arteries.
Most bright blood techniques rely on enhancing the signal of blood owing into the imaging plane. These methods
generally employ gradient echo or spoiled gradient echo
sequences with sequential acquisition, in which all of the
data for a single image are acquired before moving on to
the next slice. In sequential imaging, stationary spins in
a given slice are continually excited, and the magnetization does not have sufcient time to recover fully before
the next excitation. This phenomenon of spin saturation
results in a reduced signal in the stationary spins within the
imaging slice. Moving spins, on the other hand, may enter
the slice and contribute unsaturated signal to the image,
leading to a higher signal intensity, or bright blood effect
(Figure 16.22). Sequential gradient echo pulse sequences
represent the most common form of bright blood venography—these are also known as time-of-ight techniques
and have been used in both MRangiography and venog-
19–21
raphy.
Since blood owing into the slice carries a
bright signal from either direction, saturation pulses are
applied either above or below the imaging slice to eliminate the inow signal from arteries or veins. Time-of-ight
MR venography is generally performed as a contiguous
stack of thin 2D slices, ideally oriented perpendicular to the
veins of interest. The 2D data can then be used to obtain
3D reconstructions, applying standard algorithms such as
volume rendering or maximum intensity projection. Timeof-ight MRvenography is a more robust technique than
black blood methods but remains prone to ow-related
artifacts. Slow ow or in-plane ow may lead to pseudo-lling defects or poor vessel visualization.
A signicant limitation of time-of-ight MRvenography is the long acquisition times, typically on the order
of 5–15 minutes, depending on the in-plane spatial resolution, slice thickness, and anatomic coverage. This is most
problematic for imaging in the chest and abdomen, where
motion artifacts from breathing can severely limit image
quality. Images can be obtained during breath-holding—in
this case, thicker slices are usually acquired with lower spatial resolution, so that breath-holds and total acquisition
times are reasonable. This generally precludes 3D reconstructions of acceptable quality, however.
Steady-state free precession (SSFP) pulse sequences represent another bright blood technique (Figures16.23 and
162 Chapter 16 Computed tomography and MRI in venous disease
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16.23 Renal cell carcinoma with renal vein and inferior vena cava (IVC) tumor thrombus. (a) Arterial phase 3D fat-saturated spoiled
gradient echo image demonstrates extensive renal vein and IVC thrombus (arrows). Note linear enhancing thrombus in the left renal
vein. (b) Black blood single shot fast spin-echo image reveals a large lling defect in the IVC and right atrium (arrows) consistent with
tumor thrombus.
(arrow).
16.24). These sequences maintain a steady state of both
longitudinal and transverse magnetization by application
of a series of balanced RF pulses. 2D SSFP sequences are
generally faster than gradient echo or spoiled gradient echo
acquisitions and have higher SNRs. The most important
difference, however, is that the bright blood appearance in
SSFP images is primarily the result of the intrinsic magnetic
relaxation properties of blood, rather than an inow effect.
This in turn means that there are fewer artifacts related
to slow ow or in-plane ow. Acquisition times (particularly if used in conjunction with parallel imaging) are fast
enough that two to four images can be obtained per second,
and image quality is usually acceptable, even in patients
who are unable to suspend respiration.
sequences can be acquired at a longer breath-held acquisition of 15–20 seconds or can be obtained with respiratory triggering. Respiratory-triggered 3D SSFP magnetic
(c) Axial steady-state free precession image reveals a left renal mass (arrowhead) and renal vein tumor thrombus
resonance angiography (MRA) or magnetic resonance
venography (MRV) pulse sequences often have additional
modications for improving background suppression and
improving the SNR and contrast-to-noise ratio (CNR) of
blood in order to enable 3D reconstructions—this generally involves a form of spin labeling, where a slab-shaped
inversion pulse suppresses the signal of stationary spins
within the imaging volume, while blood owing into the
slab is bright. These techniques are almost all designed
with MRarteriography in mind, rather than venography,
but can generally be modied fairly easily to accommodate
venous imaging.
22,23
3D SSFP pulse
The limitations of SSFP sequences include fairly high background signals, even with fat suppression, so that 3D reconstructions are usually not practical. Banding artifacts near
the edge of the eld of view and adjacent to gas-containing
structures are occasionally problematic. Optimal SSFP
16.3 Imaging technologies: MRI of venous disease 163
(a)
(b)
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(d)
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16.24 Inferior vena cava (IVC) sarcoma imaged with noncontrast black and bright blood techniques. Axial fast spin-echo (FSE) black
blood image (a) reveals a large heterogeneous mass expanding the intrahepatic IVC (large arrow). Note the absence of ow voids in
the left (small arrow) and right hepatic veins due to the presence of slow in-plane ow. Aow void is present in the middle hepatic
vein (arrowhead). Diffusion-weighted image
to the FSE image. Diffusion-weighted images show a more robust black blood effect, with dark ow voids in all hepatic veins. Bright
blood axial 2D steady-state free precession (SSFP) image (c) again demonstrates an IVC mass, with bright signal intensity in the
hepatic veins. Coronal 3D SSFP image (d) reveals a small amount of tumor thrombus in the orice of the middle hepatic vein (arrowhead). Note also the bland thrombus with darker, more uniform signal intensity along the inferior margin of the IVC mass (arrow).
(b) at a similar location again shows the IVC mass, with greater contrast in comparison
images with minimal artifacts require high-performance
gradients, which are not universally available.
Phase-contrast pulse sequences are relatively uncommon in MRV. In this technique, additional positive and
negative gradient pulses are applied to a standard gradient recalled echo or spoiled gradient recalled echo (SPGR)
sequence. Stationary spins experience no net accumulation of phase, whereas spins moving across the gradient
accumulate a phase proportional to velocity. By adjusting the strength of these velocity-encoding gradients, a
range of velocities can be detected and measured. The
major advantage of phase-contrast venography is that it
generates images in which the velocity of each pixel can
be determined. By incorporating ECG triggering, venous
ow can be measured with high accuracy. This can be
useful in the setting of chronic mesenteric ischemia and
in evaluating the signicance of a venous stenosis. The
major limitation of phase-contrast techniques is that the
acquisition times are longer than for time-of-ight and
SSFP sequences.
CE MRV is probably the most widely used technique
currently. This technique is essentially identical to 3D CE
MRA, employing a 3D spoiled gradient echo sequence,
with or without fat saturation, in conjunction with a bolus
of gadolinium-based contrast (Figures 16.23 and 16.25–
16.27). Vascular contrast is the result of the T1-shortening
effects of gadolinium on adjacent water protons and has relatively little dependence on inow effects. The T1-weighted
3D SPGR sequence provides a moderate amount of background suppression.
24–26
The simplest 3D CE MRV techniques involve one or
more additional acquisitions after performing MRA: the
contrast bolus is injected and MRA is performed when the
concentration of the gadolinium contrast agent is maximal
in the arteries. Additional phases are then acquired until
venous contrast is maximal. Alternatively, a test bolus or
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