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34 J. A. Kaufman
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A B
C
FIGURE 5-6. Three-dimensional phase contrast (3-D PC)
magnetic resonance angiogram of the renal arteries of a
normal volunteer illustrating the directional information produced by PC imaging. Velocity encoding was 100 cm/sec for
this study, and flow in all three directions was imaged. A: Axial
maximum intensity projection (MIP) of the speed images (all
flow is white). Notice the anterior-to-posterior course of the
renal arteries (
(IVC) (
small straight arrow
The superior mesenteric artery (SMA) (
and the superior mesenteric vein (SMV) (
are anterior to the aorta. The splenic vein can be seen anterior
to the SMA. Note also the absence of any anatomic detail in
the background. B: Coronal MIP of the same images. Notice
the slightly caudal course of the right renal artery and the
E
right-to-left flow. The flow in the left renal artery is white (blood flows from the aorta to the left), whereas flow in the right renal
artery is black (blood flows from the aorta to the right). Notice how the aorta and SMA are not well visualized. Why is flow in
arrow
the splenic vein (
because flow is posterior toward the kidneys (refer to A). The SMV (
trajectory toward the portal vein. Again, notice how the aorta and SMA are not well seen. E: Axial image showing superior to
inferior flow. Using the preceding four images, explain why the aorta (
proximal right renal artery (
curved arrow
), and IVC (
) black? D: Axial image showing anterior-to-posterior flow. The renal arteries (
long straight arrow
open curved arrow
) are white, whereas the distal left renal artery (
) are black.
cranial course of the left renal artery. The tapered appearance
of the distal aorta is due to loss of signal from saturation of
spins within the 3-D imaging volume. C: Axial image showing
small arrow
short straight arrow
large straight arrows
) is anterior to the right renal artery.
) is also white because it has a slighly posterior
), SMA (
). The inferior vena cava
solid curved arrow
open curved arrow
large arrows
) are white
open straight arrow
curved arrow
), SMV (
D
)
)
), and
tightly
FIGURE 5-8. Aliasing artifact in a gated two-dimensional
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phase contrast magnetic resonance angiogram of the descending thoracic aorta. Velocity encoding was 40 cm/sec, flow
in the superior to inferior direction was imaged, and phase-difference signal processing was used. When flow in the superior
to inferior direction exceeded 40 cm/sec, it abruptly changed
color from white to black, with a sharp interface (
interface changed considerably throughout the cardiac cycle,
an important clue to the nature of the artifact.
arrows
). This
CT/CT angiography
Principles of CT
CT imaging revolutionized diagnostic and interventional
radiology because it allowed physicians to view the body in
a way that had been previously restricted to the gross
anatomy laboratory. Developed by Sir Gregory Hounsfield, the technique is based on computerized reconstruction of data acquired by rotating a narrow radiographic
source (mounted in a circular gantry) around a patient in
an axial plane. An axial image is created that looks as
though the body has been bisected at that point. This
technology has had a tremendous impact on diagnostic
visceral angiography, as pathology such as solid liver le-
MR and CT Angiography 35
FIGURE 5-7. Schematic diagram of phase differ-
ence signal processing in phase contrast magnetic
resonance angiography. Phase shifts are assigned
a pixel intensity that increases as the velocity increases. When the phase shift exceeds the maximum value of the velocity encoding (
color abruptly changes; this is termed
Choosing an appropriate
est will result in improved visualization of flow and
avoid aliasing.
V
enc
V
), the pixel
enc
aliasing.
for the vessel of inter-
sions now can be visualized and characterized with greater
accuracy and without the need for an invasive procedure.
The core principle of CT imaging is identical to that of
flouroscopy: variable absorption of ionizing radiation by
different tissues. Using sophisticated data processing, CT
is more sensitive to subtle differences in tissue density
than most conventional radiographic techniques. Conventional gastrointestinal and vascular contrast agents
can be used during scanning to enhance these differences
or aid in the identification or evaluation of structures.
The radiographic density of a structure can be measured
and is described in Hounsfield units (HU). Air, fat, muscle, fresh hematoma, bone, and contrast-enhanced blood
all have characteristic densities that permit confident
identification.
CT technology
The construction of conventional CT scanners permits
acquisition of only one image at a time, because the
gantry makes one rotation per slice. Regardless of how
quickly this rotation can occur, the gantry must return
(unwind) to a start position before the next image can be
acquired. Patients are imaged by lying on a table that is
fed incrementally through the gantry, with the table moving a predetermined distance between slice acquisitions.
Scanners with this type of gantry are well suited for body
interventions such as drainage or biopsy, in which individual slices are used for planning approaches and monitoring progress. Unlike ultrasound guided procedures,
however, there is limited capability for monitoring the
actual manipulation of needles or tubes, because most
CT scanners require too much time to acquire and process the necessary data to construct an image.
Regardless of how quickly an image can be acquired,
scanning on a slice-by-slice basis unavoidably results in a
time penalty related to the gantry and the table feed.
Certain dynamic processes, such as blood flow, cannot be
studied using this type of equipment. The application of
slip-ring technology to CT scan gantry construction allows
36
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J. A. Kaufman
continuous rotation of the tube, without the need to stop
and “unwind” (Fig. 5-9). Usually, each rotation requires 1
sec; therefore, the rate of acquisition is 1 slice/sec. Helical
(or spiral) CT scanners image continuously as the patient
is moved through the gantr y. In addition to the usual
parameters of thickness of the x-ray beam (collimation),
such as peak kilovolt (KvP) and milliampere second
(MaS), the operator must also choose a rate of table feed
(pitch, which is the ratio of the distance of table movement
to collimation assuming a 1 sec. tube rotation per slice).
Data are acquired as a volume rather than as individual
slices; so large anatomic areas can be scanned completely
in 20 to 30 sec. (a single breath-hold). In practical terms,
this means image acquisition time is limited by the heat
capacity of the equipment and the patient’s ability to suspend respiration. Because scanning is faster than image
reconstruction, image processing can be the longest portion of the study. The volumetrically acquired data can be
displayed in any plane, but most manufacturers display
the data as axial slices. The greatly reduced scan time of
helical scanners allows CT imaging of physiologic processes formerly beyond the capability of this technology,
such as tracking of a bolus of vascular contrast. The latter
process is CTA. This technique has been applied to a wide
range of vascular structures within the body, including the
carotid arteries, the aorta and its branches, the lower extremity arteries, the peripheral and central veins, and the
pulmonary arteries.
16–26
The basic principle of CTA is a carefully timed helical
acquisition of data during the rapid peripheral infusion
of large amounts of iodinated contrast. The amount of
anatomic coverage required and the size of the blood
vessel of interest determine the collimation, pitch, and
contrast injection. Typically, the collimation varies from 1
to 7 mm in thickness, and from 1 to 2, in pitch. For
example, a study of the iliac arteries might be performed
with a collimation of 5 mm and a pitch of 2, whereas a
study of the circle of Willis might use a collimation of 1
mm and a pitch of 1. In general, the thinner the collimation or greater the pitch, the lower the ratio of signal to
noise in the image.
Unlike MRA, CTA is entirely dependent on the imaging of an intravascular contrast agent; suppression of
information from background tissues is not possible.
Three factors regarding intravascular contrast agents in
CTA are important: rate of injection, volume of contrast,
and concentration of iodine. Visualization of vessels improves as the concentration of contrast in the blood increases. Therefore, rapid injection of a large amount of
contrast is necessary to maintain a density of intravascular
contrast that permits indentification of small vascular
structures. Most CTA studies require injection rates of 3
to 5 mL/sec for a total volume of 100 to 150 mL. The
concentration of iodine is as important as the rate and
volume of contrast injection, as dilute contrast will result
in inadequate opacification because of further dilution in
the bloodstream. For most applications, contrast containing 60% iodine (such as Renografin 60 or Ominpaque
300) provides the best results. Intravenous lines used for
CTA should be checked carefully for patency before contrast injection, because large subcutaneous extravasations
may result if the catheter is in a small vein or is partially
dislodged. Keep in mind that the volume of contrast used
for CTA is comparable to, and in some cases may actually
exceed, that used in conventional angiography.
Timing of imaging in relation to contrast injection is
critical in CTA (Fig. 5-10). A sufficient delay must be used
to allow contrast to reach the vascular bed of interest before commencing the scan. Furthermore, the duration of
contrast injection must be long enough to ensure adequate vascular opacification for the entire time image acquisition. For example, a delay of 20 to 30 sec. is typical for
most abdominal applications,whereas a delay of only 10 to
15 sec. may be necessary for the carotid arteries. The ideal
approach is to perform a test bolus of contrast injected at
the planned rate, with acquisition of images at a fixed
point at the top of the helical volume every 1 to 2 secs.
following a 5- to 10-sec. delay. A time/density curve then
can be plotted from a region of interest placed in the vessel
of interest. The peak of the curve then determines the
delay for the helical scan. The only disadvantage to this
approach is the tube heating, which may delay the helical
acquisition. Some scanners can automatically detect the
arrival of the bolus of contrast in the vessel of interest.
Suspended respiration is important for many abdominal and thoracic applications of CTA. Motion during the
scan will result in blurring and partial volume averaging.
The duration of the scan thus may be determined by the
patient’s ability to hold his or her breath; patients on
respirators or with severe tachypnia may not be suitable
candidates for this technique. Certain types of motion
cannot be avoided during a scan; in particular, cardiac
pulsation can distort the appearance of the adjacent aorta
and other structures (Fig. 5-11).
By convention, CTA images are displayed on the scanning console as sequential images (i.e., no overlap) in the
axial plane. To provide the smoothest angiographic images during subsequent image postprocessing, the degree of overlap of adjacent slices should be close to 50%.
For this reason, a second set of overlapping images is
reconstructed from the source data set from which angiographic images will be created. The degree of overlap
must be specified by the operator; for example, an acquisition of the renal arteries with a collimation of 3 mm and
a pitch of 2 might be reconstructed as 3-mm slices at
1.5-mm intervals. In this case, data from two sequential
source images in the center of the acquisition would be
included in five overlapping reconstructed slices. Obviously, reconstructed data image sets are usually much
larger than source image sets.
FIGURE 5-9. Schematic diagram of the difference between helical computed tomography (CT) and conventional CT. With helical acquisition of data, a spiral of
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contiguous data is acquired as the patient moves through the gantry. Slices then are reconstructed from this volume of data. With conventional scanners, one discrete
slice is obtained during one rotation of the gantry while the patient remains stationary. While the gantry unwinds, the patient is repositioned for the next slice.
IGD-ISD-(Diagram courtesy of GE Medical Systems, Milwaukee, WI.)
38 J. A. Kaufman
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FIGURE 5-10. Axial image from a computed tomographic
angiogram of a patient with abdominal aortic aneurysm and
severe tricuspid regurgitation. Although a 30-sec. delay between initiation of contrast injection and initiation of scanning
was used, poor aortic opacification is present (
Note the intense opacification of the inferior vena cava and
right hepatic vein (
from the right atrium. Contrast was injected in a peripheral vein
in the left upper extremity.
black arrows
) caused by reflux of contrast
white arrow
One of the major advantages of CTA is that the source
images are simply slices from a CT scan with intense vascular opacification. In other words, all the information that
one would normally expect regarding the background tissues on a CT scan is also present, allowing more comprehensive evaluation of the vessel wall and adjacent structures than with MRA. For example, CTA of the pulmonary
arteries performed to look for pulmonary emboli also
permits evaluation of the lung parenchyma, pleural cavity,
and mediastinum for coexistent pathology or alternative
diagnoses.
Limitations
CTA is a quick, readily available technique, but it has
certain important limitations. Patients who have contrast
allergies or renal failure (serum creatinine⬎ 1.5 mg/dL)
are not suitable, nor are those who have contraindications to exposure to ionizing radiation (e.g., patients in
the first trimester of pregnancy). Heavily calcified vessels
can be difficult to evaluate, as dense mural calcium can
obscure the vessel lumen. As with MRA, patients must be
able to cooperate during the study to avoid detrimental
motion or respiratory artifacts.
Image postprocessing
To view the data from MRA and CTA studies as angiograms, postprocessing of source images is usually necessary. This crucial step occurs after the study has been
).
FIGURE 5-11. Pulsatility artifact due to cardiac motion in computed tomographic angiography of the thoracic aorta in a patient with a type I aortic dissection. This image is an oblique
sagittal slice created retrospectively from a contrast-enhanced
helical scan. Note the serrated appearance of the mobile intracardiac portion of the aortic root (
parison to the smooth contour of the fixed descending thoracic
aorta (
open white arrows
tially to motion of the aortic root with cardiac activity during
imaging as well as to slice thickness (7 mm) and the curved
contour of the vessel wall. The dissection flap (
seen as a dark line within the vessel lumen. The serrated
appearance of the flap is due to movement of the flap during
the cardiac cycle.
) at the same level. This is due par-
solid white arrows
black arrows
completed, usually after the patient has been removed
from the scanner. Thus, a thorough understanding and
familiarity with this technique is important to ensure
proper interpretation of the images. A variety of postprocessing options are available, ranging from simple
reformating of data into different planes (i.e., coronal
slices from axially acquired data) to 3-D renderings that
permit an endoscopic viewpoint of the vascular lumen.
Excellent postprocessed images can be created only from
excellent source data. Even the most sophisticated postprocessing techniques cannot recover useful information
from a poor study (the old computer science dictum
“garbage in, garbage out”).
The source data for MRA and CTA are composed of
discrete elements termed voxels. Each element has only
one value. This concept is important because postprocessing is essentially manipulation of these elements. For
example, in 3-D imaging, if a voxel contains two structures with similar but differing values, it will be assigned
) in com-
)is
MR and CT Angiography 39
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a value that is an average of both. There is no way to
separate these two structures retrospectively using current postprocessing techniques.
A major determinant of the value assigned to a voxel
are its dimensions. If an image is constructed of voxels
that are too large, small objects may get “lost” (partial
volume averaging). The dimensions of voxels are determined by the field of view of an image, the slice thickness,
and the matrix size. For example, in a 3-D MRA with a
20-cm field of view, a slice thickness of 2 mm, and a 256
⫻ 256 matrix, the effective voxel size will be 0.8 ⫻ 0.8 ⫻
2.0 mm. Because these voxels are rectangular, the highest
resolution images will be obtained when viewing the data
set from the angle that presents the voxels on end. When
voxels are true cubes, they are considered isotropic and
can be viewed with equal resolution from any angle. One
of the objectives in MRA and CTA is to create source
images that are composed of the smallest pixels and
voxels possible while maintaining adequate signal-tonoise ratios (the smaller the voxel, the greater the impact
of background noise). Matrices of 512 ⫻ 512 are routinely available with CTA, whereas 256 ⫻ 256 is more
typical for MRA.
The most common technique for image postprocessing is maximum intensity projection (MIP), in which the
highest value voxels within a stack of 2-D images or a 3-D
volume are projected as a single image in any plane (Fig.
5-12). Multiple images can be created with incremental
degrees of rotation around any axis. Restricted MIPs can
be created easily by limiting the amount of the source
data set to be projected. The chief advantage of this
technique is that the low-intensity background structures
are not projected, which results in images that are pleasing to view because of a very high signal-to-noise ratio.
This is also the chief disadvantage of the technique, because subtle abnormalities present on source images may
not appear on the final projection.
27
A simple postprocessing technique that retains all the
data present in source images is reformatting, a technique
that essentially reslices the source data in different planes
from the original acquisition. For example, a stack of
axial images can be reformatted into images in the coronal or sagittal plane, or a curved reformat can be used to
trace the path of a renal artery as it arises from the aorta
and travels through the retroperitoneum (Fig. 5-13). The
integrity of the source data is preserved while the structure of interest is depicted from the most advantageous
angle. A major limitation of this technique is the extreme
dependence on operator input to maintain accuracy: Appropriate selection of viewing planes or curves is essential
for accurate diagnosis. Furthermore, reformatted images
can only be one pixel or voxel thick, thus limiting the
amount of data that can be displayed.
Segmentation techniques divide the image data sets
into flow and nonflow groups, or “segments,” using a
number of different strategies. In thresholding, one of the
most widely available methods of segmentation, an operator can set a threshold value below which all data are
discarded. Other techniques include region-growing
(seed) algorithms, in which the user defines a starting
point within a vessel, and adjacent voxels are tested automatically for inclusion or exclusion. Segmentation techniques permit modification of large data sets to include
only pertinent information. As with MIPs, insensitivity to
subtle abnormalities is a major limitation unless these
techniques are performed carefully.
3-D rendering techniques provide the most dramatic
displays of MRA and CTA data. This form of postprocessing imparts a “real” appearance to the vessels in that they
seem to have both volume and depth (Fig. 5-14). Spatial
relationships are self-evident, rather than deduced. Colorization schemes can be superimposed on the images to
emphasize various structures. The two most common displays are surface or volume rendering. The former is a representation of the vascular data as a shell, and the latter
presents the data as a solid object. These post-processing
techniques usually are performed on independent workstations equipped with fast processors using proprietary
software. Data sets must be edited heavily when using segmentation techniques and excision of unwanted portions
28
of the images to produce satisfactory 3-D renderings.
In
addition, the actual source images must have good contrast between vascular structures and background tissues.
For these reasons, all 3-D renderings must be viewed with
care before rendering a diagnosis, because important information can be omitted at several stages during creation
of the final images. A fundamental rule of MRA and CTA
is that the source images always must be reviewed whenever a questionable finding is present on any type of postprocessed image.
■ Specific Applications
Lower-extremity arteries
Peripheral arterial occlusive disease is a common problem in developed countries. The evaluation of patients
with claudication, rest pain, or tissue loss begins with an
accurate history, a careful physical examination, and noninvasive tests, such as pulse-volume recordings. Before any
intervention, accurate morphologic information about
the lower-extremity arteries is required. Conventional
angiography is the “gold standard” method for acquiring
these data, despite its invasive nature. Among the noninvasive techniques, MRA is particularly suited to evaluation
of lower-extremity occlusive disease. In some centers,
MRA has become the primary means of imaging these
patients.
5
40 J. A. Kaufman
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A B
C
FIGURE 5-12. Maximum intensity projection (MIP) display of a two-dimensional time-of-flight magnetic resonance (MR) veno-
gram of the pelvis. A: Axial slice from the source MR venogram shows the external iliac veins (
the external iliac veins (
stack of axial slices. The vague area of increased signal in the upper right of the image is due to bowel motion. C: Coronal
projection of a MIP of the entire stack of slices creates a display of the data that looks like a venogram. D: Right posterior oblique
projection of the MIP of the entire stack of slices shows the 3-dimensional relationships that can be demonstrated with this
technique. (
Continued
MRA
Flow in the normal peripheral muscular arteries is pulsatile, with a triphasic wave form (rapid for ward flow with
systole, a brief period of retrograde flow as the aortic
valve closes, and then slow forward flow during diastole).
In the presence of occlusive disease, flow becomes slow
and nonpulsatile. This type of flow is ideal for imaging
small arrows
)
large arrows
). Note the excellent background signal suppression. B: Axial projection of a MIP of the entire
) and branches of
with 2-D TOF sequences. Most lower-extremity MRA protocols are based in part on 2-D TOF techniques, with
image acquisition in the axial plane, and an inferior saturation band to mask venous flow. Imaging acquisition in
the axial plane is used because flow in most of the peripheral arterial segments will be perpendicular to the slice
and thus have maximal flow-related enhancement. Two
D
MR and CT Angiography 41
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A
E
FIGURE 5-12. (
central 4 cm of each slice improves the appearance of the final
image by eliminating much of the background noise. Note that
the internal iliac veins are truncated because they are excluded
from the MIP.
Continued
)E:Restricted coronal MIP of the
areas where this is not true, and thus where in-plane
saturation is difficult, are tortuous iliac arteries in the
pelvis and the origin of the anterior tibial artery. Typical
slice thicknesses are 1.5 to 3.0 mm, with the highest-resolution images obtained when imaging with thin slices,
high matrices, and small fields of view. Recently, gadolinium-enhanced 3-D acquisitions of the aorta, pelvis, and
thighs have replaced 2-D TOF as the primary imaging
technique. 2-D TOF remains important for imaging the
tibial and pedal vessels. The pelvis and thigh usually can
be imaged in the body coil, but the lower extremities
must be imaged in either the extremity or the head coil
to achieve consistent diagnostic results (Fig. 5-15). The
average study requires 1 to 2 hours to image from the toes
to the aortic bifurcation.
One of the chief advantages of MRA in imaging distal
vessels is that extremely slow flow beyond occlusions is
detected easily using this technique, but it is sometimes
difficult to demonstrate with conventional angiography if
collateral formation is poor (Fig. 5-16). Although results
FIGURE 5-13. Curved coronal reformation of the renal arteries from a computed tomographic angiogram of a patient with
an aortic dissection. A: This coronal image, which is 1 pixel
thick, was created by tracing the course of the renal arteries
through the retroperitoneum from the axial source images. This
explains why the aorta appears truncated at the top of the
image. B: Schematic showing how the curved coronal reformat
is proscribed from the axial source image.
vary among institutions, depending on the specifics of
both the MR and the conventional angiographic techniques, thereis no doubt that MRA is an excellent method
for imaging the pedal vessels in the presence of extensive
proximal occlusive disease.
5
A major disadvantage of
MRA is that it is sometimes difficult to determine the
quality of the vessel, as calcium is not seen on these images. Retrograde flow below an occlusion is frequently
saturated by the inferior saturation band in 2-D TOF imaging, thus making an occlusion appear longer than it
actually is.
The pelvic arteries can be difficult to image accurately
by using 2-D TOF techniques if the vessels are tortuous or
aneurysmal. Contrast-enhanced MRA using rapid injection of gadolinium chelates during acquisition of a 3-D
TOF sequence has largely resolved this issue (Fig. 5-17).
Pitfalls of this technique include difficulty with timing of
contrast injections when ultrafast acquisitions are used.
B
42 J. A. Kaufman
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FIGURE 5-14. Shaded surface display of the two-dimensional
time-of-flight pelvic magnetic resonance venogram from Figure
5-12. Note that the three-dimensional display allows appreciation of such details as the impression on the inferior vena cava
and iliac veins by the common iliac arteries (
arrows
).
In addition to occlusive disease, other uses of lower-extremity MRA include evaluation of tibial vessels before
plastic reconstructive procedures, staging of vascular malformations and tumors, and diagnosis of popliteal artery
entrapment by anomalous muscular or tendonous structures.
Limitations
There are many limitations of MRA of the lower extremities; careful patient selection is therefore important to
avoid unnecessary examinations. Patients who are uncooperative, subject to frequent involuntary movements, or
claustrophobic, or who have metallic joint prostheses in
the extremity of interest should not undergo MRA because the studies undoubtedly will be inadequate or incomplete. MRA does not have the image resolution to
detect subtle intimal irregularities or to distinguish between atherosclerosis and other types of occlusive disease
such as arteritis. Patients who will clearly require a cathe-
FIGURE 5-15. Coronal maximum intensity projection of a twodimensional time-of-flight magnetic resonance angiogram of
the tibial arteries in a patient with occlusion of the external iliac,
femoral, and popliteal arteries bilaterally. Performed in the
head coil, both legs were imaged at the same time. On the
patient’s right, the peroneal (
(
open arrow
peroneal (
ies are patent. Multiple small collateral vessels are present in
the medial calf bilaterally.
) arteries are patent. On the patient’s left, the
solid arrow
) and anterior tibial (
solid arrow
) and posterior tibial
curved arrow
) arter-
ter-based intervention based on history, physical examination, and noninvasive studies, should proceed directly
to angiography if access to MR scanners is limited. Lastly,
patients with traumatized or acutely ischemic, threatened
limbs should not be studied with MRA because valuable
time may be lost if the study is inadequate or a catheterbased intervention is needed.
CTA
The role of CTA in the evaluation of peripheral vascular
occlusive disease is dependent on the anatomic area that
must be imaged. CTA of the infrainguinal runoff is subject to the same limitations as conventional contrast angiography: Vessels distal to severe proximal occlusive disease are difficult to opacify with contrast. In general, this
is not a good modality for studying diseased infrapopliteal and pedal arteries. CTA is an excellent modality for
evaluation of the pelvic and femoropopliteal arteries, because there is no degradation of the images as a result of
tortuousity or the presence of aneur ysms. In addition,
there is little movement of retroperitoneal structures in
the pelvis with respiration compared with the abdomen,
so breath-holding is less of an issue. In many cases, pelvic
CTA is combined with a study of the abdominal aorta.
MR and CT Angiography 43
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High-quality pelvic CT angiograms can be obtained using injection of contrast (60% iodine) at 3 to 5 mL/sec,
5 mm collimation, and a pitch of 1 to 2. An appropriate
delay should be used, preferably calculated from a test
bolus of 20 mL and sequential acquisition of slices at the
aortic bifurcation over20 to30 secs.Bowel contrast should
be avoided, as high density barium or gastrografin interferes with subsequent image postprocessing. Diagnostic
lower-extremity studies from the inguinal ligament to the
midcalf can be acquired using a 5-mm collimation, a pitch
of 1,200 mL of contrast injected at 2 to 3 mL/sec, and two
60-sec. spirals.
21
It should be kept in mind that the average
conventional bilateral lower-extremity angiogram requires 70 to 100 mL of contrast.
An advantage of CTA over MRA in the peripheral vessels is that the study is always performed with contrast, so
that pulsatility, turbulence, and retrograde or in-plane
flow do not degrade image quality. For example, crossfemoral grafts are imaged easily by CTA.
Limitations
Major limitations of CTA are the large amount of iodinated contrast required, and the difficulty of grading the
degree of stenosis in the presence of heavy vascular calcification.
Lower-extremity veins and the inferior
vena cava
MR venography
Blood flow in veins is, as a rule, slower and less pulsatile
than flow in adjacent arteries. Most MRA techniques favor blood flow with these exact characteristics. Indeed,
MR venography has proved a highly accurate means of
evaluation of the lower-extremity and pelvic veins and the
inferior vena cava. In most studies, the sensitivity and
specificity for detection of lower-extremity deep venous
thrombosis with MR venography exceed 90%.
The indications for MR venography include detection
of thrombus; evaluation of anomalous lower-extremity
venous drainage, such as Klippel-Trenauny disease; and
presurgical evaluation of venous anatomy in patients
with suspected intracaval extension of renal cell carcinoma. In most institutions, lower-extremity ultrasound
is the study of choice (on both the basis of cost and
availability) for detection of infrainguinal deep venous
thrombosis, and MR venography is useful for documenting pelvic extent or origin of thrombus, as this anatomic
region is an important “blind spot” for ultrasound (Fig.
5-18).
The most commonly applied technique for MR venography is 2-D TOF, because of its sensitivity to slow flow and
its ability to cover large anatomic areas.
flow, the saturation band is placed over the arterial inflow
(superior if axial slices are used). Thicker slices are used
8
8
To select venous
with MR venography than with MRA, because the detection of thrombus does not require the same resolution in
all planes as grading of stenoses. The thicker slices also
permit more anatomic coverage in less time. Other than
increased slice thickness and repositioning the saturation
band, little modification is required of arterial 2-D TOF
sequences; however, anatomic T1-weighted images are an
important part of pelvic and abdominal MR venography
with which to exclude the presence of extrinsic compression of venous structures by masses.
CT venography
The indications for CT evaluation of the lower extremity
and pelvic veins, and the inferior vena cava are the same
as MR venography. CT is a highly accurate means of
evaluating the pelvic veins and inferior vena cava for
patency and involvement by masses. An advantage of CT
of the veins in the pelvis and abdomen is the acquisition
of useful cross-sectional anatomic information about extravascular structures. A major limitation is the demand
placed on the tube by the large area (ankles to inferior
vena cava) that must be covered in patients with suspected thromboembolic disease.
Lower-extremity CT venography can be performed by
direct injection of contrast into the extremity under evaluation. Dilute contrast injected at a low rate (Omni 300 or
equivalent diluted 1:5 with normal saline and injected at 2
mL/sec), with a 35-sec. delay, 10-mm collimation, and a
pitch of 2 will permit examination of the lower extremity
from the ankle to the inferior vena cava (17). Although
initial results are promising, the indications for extremity
CT venography have yet to be determined. In certain
situations, the cross-sectional images obtained with CT
venography may be essential to resolve complex anatomic
relationships, an extrinsic lesion with mass effect on a
vein, or to characterize an intravascular process.
The pelvic veins and inferior vena cava also can be
imaged by infusion of contrast into a peripheral upper
extremity vein. Contrast is injected at a lower rate than
for CTA (2–3 mL/sec), and a delay sufficient to allow
venous enhancement is used. Scanning too soon will result in differential opacification of the vena cava at the
level of the renal veins, which may lead to interpretive
errors. The thickness of the collimation will vary with the
desired degree of coverage, but 5 mm should be adequate for most cases. Similarly, the pitch will vary from 1
to 2 based on the amount of coverage required. Breathholding is necessary for optimal helical CT studies of the
inferior vena cava.
Abdominal aorta and visceral branches
The primary indications for MRA and CTA of the abdominal aorta and its visceral branches are preoperative
staging of aortic aneurysms or occlusions, diagnosis of
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