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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3656_Библиотеки_им_академика_М_И_Перельмана
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180 Computed tomography and magnetic resonance imaging in venous disease
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(a)
(b)
(c)
Figure 16.4 Contrast-enhanced axial computed tomography demonstrates (a) occlusion of the SVC (arrow) with (b) dilata-
tion of the azygous and hemiazygous systems (arrowheads).
occlusion of the brachiocephalic veins bilaterally (arrows) with extensive chest wall collaterals.
contrast material through the kidneys. Anatomic variants
of the IVC are due to persistent embryologic remnants. e
prevalence of the most common anatomic variants include
persistence of a solitary le-sided IVC (<1%) and duplica-
tion of the infrarenal IVC segment (1%–3%), retroaortic
le renal vein (2%–3%), and circumaortic le renal vein
(2% –9%).
7
Optimal opacication of the IVC typically requires
delayed CT imaging at 90–110 seconds following the administration of iodinated contrast material, to allow homogeneous opacication of the entire infrarenal cava. As with
the SVC, current CT evaluation provides accurate o-axis
display of the entire caval segment in any orientation; however, the coronal display most commonly provides the most
complete depiction because of the longitudinal orientation
of the IVC within the abdominal cavity. e most common
(c) Volume-rendered 3D image of the chest demonstrates
pathology depicted within the IVC is bland thrombus due
to thrombotic disease, and may be visualized within the
central cava (Figure 16.8), or due to extension of thrombus
from malignant occlusive changes (Figure 16.9). rombus
may also be visualized within venous branches such as the
renal veins or common femoral veins (Figure16.10). Tumor
thrombus within the IVC is most commonly due to local
extension from adjacent organs such as the kidneys (renal
cell carcinoma), liver (hepatocellular carcinoma), or adrenal glands (adrenal cortical carcinoma). An uncommon
cause of a lling defect within the IVC is due to a primary
tumor arising in the smooth muscle of the IVC, as seen with
10,11
leiomyosarcoma.
Rarely, the IVC may be traumatically
disrupted (Figure 16.11). e ability of the CT evaluation to
display the orientation of an IVC lter can be helpful for
depicting associated thrombus or migration (Figure 16.12).
16.2 Imaging technologies: CTofvenous disease 181
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(b)
(a)
Figure 16.5 (a, b) Contrast-enhanced axial computed tomography demonstrates flow artifact (arrow) from unopacified
blood from the infrarenal inferior vena cava streaming into the juxtrarenal (IVC) with admixture of opacified blood from the
renal veins (arrowheads).
(a) (b)
Figure 16.6 (a, b) Contrast-enhanced computed tomography with axial and coronal volume-rendered images demon-
strates the anatomic relationships of a single retroaortic left renal vein (arrow).
(a)
Figure 16.7 (a, b) Contrast-enhanced axial computed tomography demonstrates duplication of the infrarenal inferior vena
cava (IVC) with a right-sided (arrows) and left-sided infrarenal IVC (arrowheads).
(b)
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(a) (b)
Figure 16.8 Inferior vena cava (IVC) bland thrombus. (a) Contrast-enhanced axial and (b) coronal computed tomography
images with low attenuation thrombus in the infrarenal IVC consistent with bland thrombus (arrows).
Figure 16.9 Inferior vena cava (IVC) malignant thrombus.
Contrast-enhanced coronal CT image with mixed density
tumor and bland thrombus filling the suprarenal and infrarenal IVC (arrows).
16.2.1.3 PULMONARY ARTERIES
CT of the pulmonary arteries has largely replaced catheterdirected pulmonary angiography and ventilation and perfusion scintigraphy (VQ scans) of the pulmonary arteries
because of the rapidity of the scan’s acquisition and the high
sensitivity and specicity (approaching 100%) for central
pulmonary emboli.
12,13
In other studies, the detection of
small sub-segmental pulmonary emboli has been shown to
96% for CT angiography.
14
However, further research using
the latest CT technology may provide improved accuracy
(a)
(b)
Figure 16.10 Venous branch thrombus. Contrast-
enhanced axial CT images with (a) acute thrombus within
the left renal vein (arrow) and (b) within the left common
femoral vein (arrow).
(a)
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16.2 Imaging technologies: CTofvenous disease 183
(b)
(c)
Figure 16.11 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 finding was confirmed on
subsequent catheter directed cavagram (
(a)
c, arrow).
(b)
Figure 16.12 Inferior vena cava (IVC) filter migration. (a) Volume-rendered coronal and (b) sagittal computed tomography
images demonstrate migration of an IVC filter caudally near the common iliac vein bifurcation (arrows) rather than at the
level of the renal veins (arrowhead).
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(a)
(b)
(c)
Figure 16.13 Acute pulmonary emboli in three patients. (a) Axial computed tomography imaging of pulmonary emboli in a
small segmental pulmonary artery (arrowhead),
nary embolus (arrowheads) in the main right pulmonary artery (“saddle” embolus).
in small peripheral pulmonary arteries, with accurate
detection of pulmonary emboli of all sizes (Figure 16.13).
In addition, the stratication of patients based on clinical
(b) bilateral pulmonary emboli (arrowheads) and (c) a large central pulmo-
rinds or linear webs. On reconstructed arterial segments,
there may be abrupt caliber change of the involved pulmonary arterial segments.
assessment provides the optimal strategy for the diagnosis
of pulmonary emboli.
15
Acute pulmonary emboli are diagnosed by lling
defects within the involved pulmonary arterial segments,
which are oen slightly enlarged. e optimum 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 aer initiating the contrast injection, or by using bolus tracking soware that optimizes
the CT acquisition based on the peak enhancement of the
central pulmonary arterial vasculature. Chronic pulmonary emboli are oen represented by recanalization of the
thrombosed pulmonary arterial segment (Figure 16.14).
Specic CT ndings for chronic pulmonary emboli
include peripheral eccentric thickening of the involved
pulmonary arterial vasculature, represented by so tissue
16.2.1.4 MAY–THURNER SYNDROME
Obstruction of the common iliac and external veins may be
due to bland tumor thrombus or malignancy, as has been
described for the IVC. One unique diagnosis seen with
the iliac veins is that of May–urner syndrome, which
is dened as isolated le lower extremity swelling due
to compression of the le iliac vein by the right common
iliac artery. On axial CT evaluation, the maximum diameter of the le common iliac vein is decreased, measuring
3–4 mm in maximum diameter (Figure 16.15) versus a
normal caliber iliac vein measuring 10–12 mm in average
diameter.16 Treatment for May–urner syndrome has historically involved anticoagulant therapy, but advances in
interventional therapy have enabled relief of the associated
mechanical compression by open surgical or endovascular
repair (Figure 16.16). e success of primary and secondary
endovascular techniques approaches 90%.
17,18
Figure 16.14 Chronic pulmonary emboli. Axial computed
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tomography imaging with chronic pulmonary emboli characterized by recanalization and irregular wall thickening
(arrowheads).
16.3 Imaging technologies: MRI of venous disease 185
16.2.1.5 TAILORED APPLICATIONS
State-of-the-art CT technology enables rapid acquisition of
submillimeter 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 venous
segments. Manipulation of the datasets allows exquisite
display of complex anatomical and pathological relationships, including complex pulmonary arteriovenous malformations (Figure 16.17), complex intra-abdominal or
pelvic venous malformations (Figure 16.18), or direct lower
extremity venography (Figure 16.19).
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 oen successful
(a)
(b)
(c)
Figure 16.15 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 a, c) and associated prominent left pelvic venous collaterals (arrowheads in b).
186 Computed tomography and magnetic resonance imaging in venous disease
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(a)
(b)
(c)
Figure 16.16 Post-procedural stent occlusion with venous collaterals. (a, b) Contrast-enhanced axial computed tomogra-
phy demonstrates occlusion of the left iliac venous stent (arrows). (c) Volume-rendered 3D reconstructions demonstrate
the extensive venous collaterals overlying the pubic symphysis (arrowheads).
where other techniques yield ambiguous results. MR venography comprises a number of dierent 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 dierent method and achieve satisfactory
results. is exibility can also be something of a limitation,
however, since the range of choices can be somewhat daunting to those with limited experience.
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.
In addition, a blood pool gadolinium-based MRI contrast
agent (gadofosveset tridsodium) has recently been introduced, with an intravascular half-life of approximately 30
minutes. is agent provides a sustained high intravenous
signal-to-noise ratio (SNR) in comparison to traditional
extracellular gadolinium contrast agents, and enables
improved exibility in acquiring post-contrast MR venography data. MR venography is limited by generally lower
spatial resolution in comparison to CT, by longer examination times, and by the exclusion of patients who are unstable
16.3 Imaging technologies: MRI of venous disease 187
(a) (b)
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Figure 16.17 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
(arrows) and pulmonary arteries (arrowheads) and pulmonary veins (curved arrows).
(a)
(b)
(c) (d)
Figure 16.18 Contrast-enhanced computed tomography of the abdomen and pelvis demonstrates a large arteriovenous
malformation surrounding the left hemipelvis on the (a) axial (arrows), (b) coronal (arrows), and (c, d) volume-rendered 3D
reconstructions (arrows).
188 Computed tomography and magnetic resonance imaging in venous disease
(a) (b)
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Figure 16.19 Lower extremity direct computed tomogra-
phy (CT) venography.
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 greater
saphenous vein (arrowheads).
mal superficial femoral vein (curved arrows) with chronic
changes due to deep venous thrombosis in the superficial
femoral vein adjacent to the stent (notched arrow).
or have pacemakers or automatic implantable cardioverter
debrillators, and certain cerebral aneurysm clips.
16.3.2 Techniques
A large number of MR venography 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 eects 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 (Figures 16.20 through 16.22). Spin-echo and
fast spin-echo sequences employ a series of 90–180° radiofrequency (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° pulse and data acquisition, being replaced by nonexcited spins that never experienced the initial pulse and
therefore do not contribute signal to the image. is leads to
a signal void, or black blood eect. More sophisticated black
blood sequences employ electrocardiogram (ECG) gating as
well as two inversion pulses in order to improve the reliability of this eect. Most black blood vascular sequences
are designed for arterial imaging, but are also eective for
venography, or can be optimized for venography by adjusting a few imaging parameters. Black blood venography
(a) Volume-rendered images acquired
(b) Patent stent in the proxi-
sequences can clearly demonstrate lling defects in veins,
and oen provide high-quality anatomic images. It should
be noted, however, that these techniques are notoriously artifact prone—slow-owing blood, for example, oen 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. Diusion-weighted imaging (DWI) (Figure
16.22) is an additional black blood technique that is rarely
employed as a dedicated venography acquisition. DWI uses
two or more gradient pulses applied in opposite directions,
separated by a short time interval. e gradient pulses sensitize the acquisition to eects of microscopic diusion, with
signal loss occurring in proportion to small increments in
microscopic diusion. is technique is also exquisitely
sensitive to bulk motion, such as blood ow, and generates a
very reliable black blood eect in veins and arteries.
Most bright blood techniques rely on enhancing the signal of blood owing into the imaging plane. ese 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 sucient time to recover fully before the next
excitation. is 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 eect (Figure16.20).
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
19–21
both MR angiography and venography.
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. e 2D data can then be
used to obtain 3D reconstructions, applying standard algorithms such as volume rendering or maximum intensity
projection. Time-of-ight MR venography 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 MR venography
is the long acquisition times, typically in the order of 5–15
minutes, depending on the in-plane spatial resolution, slice
thickness, and anatomic coverage. is 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. is generally precludes 3D reconstructions
of acceptable quality, however.
16.3 Imaging technologies: MRI of venous disease 189
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(a
(c)
b)
Figure 16.20 Black blood (a) and bright blood (b, c) non-contrast MR venography in a patient with fibrosing mediastinitis
and superior vena cava (SVC) and right pulmonary artery occlusion. Occlusion of the SVC is demonstrated by the absence
of flow 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
c), consistent with thrombosis.
Steady-state free precession (SSFP) pulse sequences
represent another bright blood technique (Figures 16.21
and16.22). ese 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. e most important
dierence, 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 eect.
is 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.
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 resonance angiography (MRA) or magnetic resonance venography (MRV)
pulse sequences oen have additional modications for
improving background suppression and improving the SNR
b). Note also the lack of bright blood signal in the right pulmonary artery (arrowhead in
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. ese techniques are
almost all designed with MR arteriography in mind, rather
than venography, but can generally be modied fairly easily
to accommodate venous imaging.
e 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 images with minimal artifacts require highperformance gradients, which are not universally available.
22,23
3D SSFP pulse sequences
Phase-contrast pulse sequences are relatively uncommon in MR venography. 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
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