A. NONINVASIVE
TECHNIQUES
1. Hand held (Continuous
Wave) Doppler:
Relatively inexpensive and
uncomplicated. More recently the routine use of duplex ultrasound (including
color flow scanning) in the vascular laboratory has added a new and much
more sophisticated element to noninvasive vascular testing by supplying
more specific anatomic and physiological information.
The doppler transducer (probe)
consists of a transmission source that limits ultrasound waves at a fixed
frequency and a receiving crystal to collect the reflected sound waves.
The ultrasound waves are transmitted into the tissues by keeping the transducers
in close contact with skin using commercially available lubricating jelly.
These waves are reflected of moving targets, viz. red blood cells that are
circulating within a vessel. Since these cells are in a continuous motion
the "Doppler effect" comes into play. The examiner can appreciate this effect
by audible arterial flow signals (analogous to a stethoscope) which is a
representation of the basic parameter called 'Doppler Shift'. Making use
of the property to produce audible signals and a sphygmomanometer one can
measure perfusion pressures up to 20 to 30 mm Hg in distal arteries.
2. Duplex and color Doppler:
Duplex scanning currently represents
the most technologically sophisticated form of noninvasive vascular testing.
As the name suggests, it has two major components:
(1) B-Mode ultrasound imaging
and (2) pulsed doppler frequency spectral analysis. The B-Mode imaging is
identical to standard ultrasound imaging that has been used to evaluate
abdominal & other organs. The pulsed Doppler allows a focused sampling
of Doppler frequency shift information directly from vessels using the B-Mode
ultrasound image as a guide. The hand held Doppler instrument would readily
allow auscultation of an arterial flow signal but there would be no way
to determine the artery specifically, especially in the areas having complicated
vascular anatomy or in areas with close placements of important arterial
of branches. Duplex doppler overcomes this difficulty by first imaging the
vessels and recognizing the branches.
Fig 1: Duplex doppler
showing arterial image with spectral analysis.
Further sophistication and
clinical usefulness has been added to duplex ultrasound technology by the
development of color flow or color Doppler systems, which arbitrarily assign
colors to the in vivo Doppler derived flow velocities based on the direction
and absolute velocity of flow. In a typical vascular examination, flow towards
the ultrasound probe (most of the times arterial) would be assigned 'red'
and flow away from the probe (most of the times venous) would be assigned
'blue'. Variations in hue are produced by significant variations in flow
velocities, such as the marked increase in velocity produced by the turbulent
flow across a focal stenosis. Color-flow systems allow a more rapid identification
of standard vascular anatomy during B-mode imaging, which facilitates a
more efficient examination. They also allow more rapid identification of
areas of arterial pathology by visual identification of color changes or
absence of color. This modality can be very efficiently applied to neck
and extremity vascular examinations. Though the reliability and acceptability
of color Doppler decreases in the abdominal vessels, as compared to that
of peripheral vessels, it remains the best noninvasive tool to examine vasculature,
in an experienced examiner's hand.
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Fig 2:
Peripheral arterial CD.
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Fig 3:
CD of the neck vessels.
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Fig 4:
CD of a hemangioma showing mixed colour signals.
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3. Magnetic resonance angiography(MRA):
MRA has been intensively investigated as a noninvasive
technique for imaging the extra and intracranial cerebral circulation, the
heart, the aorta and its main branches, the vena cava and its major tributaries
and the arteries of the extremities. One of the most interesting phenomena
associated with magnetic resonance imaging is its ability to image flowing
blood without the need for contrast medium. Of the many sequences available
for imaging blood flow, two dimensional time-of -flight (2D TOF) has been
found to be the most useful for studying the extremities along with usage
of surface coils. Good images of abdominal aorta and iliac arteries can be
obtained by using intravenous contrast injection (Gadolinium) and a modified
protocol.
Fig 5:
MRA of an abdominal aorta and its bifurcation showing atherosclerotic disease.
MRA is displayed to resemble
a conventional angiogram by stacking axial images to create a 3D data set.
Although these images are essentially a quick way of reviewing the study
in any projection desired, they can occasionally be misleading because small
vessels and filling defects may be observed if they are surrounded by high
signal intensity from flowing blood. One should always refer back to axial
images to evaluate problem areas. Due to relatively large pixel size of
the MRA digital system, the degree of stenosis in small vessels is commonly
underestimated by 5 to 15% when reading the individual axial images and
to an ever larger extent on the reconstructed projections.
Fig 6:
MRA - Abdominal aortic aneurysm
MRA can replace conventional
angiography in providing a road map for vascular reconstructive treatment,
especially involving the head, neck and extremities. If the MRA is not considered
satisfactory due to flow or metal artifacts a conventional but a 'directed'
angiogram can be performed to reassess the vascular segments that were not
adequately visualized.
Fig 7:
MRA- Circle of Willis
4. Spiral computed tomography
(spiral CT); CT angiography (CTA):
Helical or spiral CT scanning
is a modification of conventional CT scanning, whereby a continuous volume
of data is obtained rather than interrupted axial images. The x-ray tube
is continuously activated and rotated while the patient is simultaneously
advanced in a longitudinal direction through the scanner. Axial scans obtained
in this fashion are nearly identical to conventional CT scans. However,
with the addition of a properly timed bolus of intravenous contrast, spectacular
images of the vascular anatomy can be obtained. These can be re-figured
into detailed, three-dimensional images, so-called CT angiograms, which
can be rotated for viewing in any projection. The patient is required to
hold his breath for approximately 30 seconds, which can be accomplished
by approximately 90% patients after proper coaching and practice. For example,
the aorta, from the origin of the superior mesenteric artery to the iliac
bifurcation can be measured within these 30 seconds, with a single bolus
of contrast (approximately 120 to 150 ml). In future, helical CT or MRA
may replace both CT scans and conventional angiograms as most of the information
required by a surgeon or an interventionist can be obtained by this modality.
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Fig 8:
CTA - Abdominal aortic aneurysm
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Fig 9:
CTA - 3D reconstruction of an abdominal aortic aneurysm
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Fig 10:
CTA - Reconstructed image with addition of colors
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B. INVASIVE
TECHNIQUES
1. Digital subtraction angiography
(DSA):
As the name suggests DSA software
simply subtracts the unwanted shadows from an angiogram by digital processing
of the images. In conventional angiography of the regions like skull or
spine many small bones overlap the opacified arteries and cause confusing
shadows. Even during the angiography of other regions shadows cast by the
regional bones are unwanted. DSA technique nullifies all these unwanted
bony and soft tissue shadows to yield pure angiographic images.
The angiographic technique
in DSA is necessarily the same as that in conventional angiography. As one
starts fluoroscopy in digital subtraction mode a "mask" is acquired first
and stored in the computer's memory. This image of 'mask' is exactly congruent
to the regional image of bones and soft tissues (i.e., how it would appear
on the plain x-ray) but has totally opposite densities (i.e., similar to
'negative - positive'). Thus when the mask is superimposed upon the fluoroscopic
image during angiography the positive shadows of the 'mask' nullify the
negative images of the original image including soft tissues and bones.
It must be noted that during the acquisition of the mask the arteries are
not opacified. Hence after superimposition of the mask during angiography
the bones and soft tissues are subtracted but the arterial images are left
unaltered. The final presentation is a 'bone and soft tissue free' angiogram.
The DSA equipment, in addition,
eliminate the need of film-screen radiography and saves time of the procedure.
The amount of contrast medium needed is much lesser as compared to the conventional
angiography. Thus DSA techniques facilitate interventional procedures.
However, as the DSA images
are free of other tissue images (i.e., the usual landmarks are absent on
the images), they are inferior to conventional angiographic images in spatial
resolution.
2. Intravascular ultrasound
(IVUS):
In angiography, the plaque
and vessel wall are viewed as a "negative imprint" on contrast filled lumen.
Thus angiography does not allow for characterization of tissue elements
below the intimal surface. Visualization of a blood vessel in different
views (circumferentially), by this modality, is possible only at the expense
of additional injections of contrast agent. IVUS offers a potential solution
to many of these limitations inherent in conventional contrast angiography.
Fig 11:
IVUS of a normal vessel showing all the layers of the artery
IVUS is unequivocally superior
to contrast angiography in its ability to demonstrate detailed characteristics
at the lumen-vessel wall interface, as well as to depict structures within
the plaque and vessel wall. Several investigators have demonstrated that
IVUS is exquisitely sensitive in detecting plaque and other details that
are angiographically "silent". Reconstructions obtained by computing the
cross sectional images, i.e., three dimensional images makes the comparison
of neighboring segments of an artery very easy. Images of the vessels derived
from IVUS typically demonstrate a layered appearance surrounding the probe.
The presentation of distinct layers is a consequence of the differing acoustic
reflectivity of different tissues. Dense hyperreflective tissues are represented
by bright echoes, and less dense tissues produce darker signals on gray
scale. "Acoustic discrepancy" (acoustic impedance) or "mismatch", i.e.,
change in sono-reflectivity between adjacent structures is the most important
factor in defining the border between structures. Each layer of the arterial
wall can be recognised by a typical ultrasound "signature". IVUS provides
a potent tool to decide the treatment options for the patient, provides
advanced means to study the mechanism of balloon angioplasty and, of course,
is one of the best research tools available to date.
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| Fig 12a: IVUS of a stenosed artery
- before balloon angioplasty |
Fig 12b: IVUS of the same vessel
- after balloon angioplasty |
3. Carbon dioxide (CO2)
angiography:
Although the newer nonionic
contrast agents have very low complication rates, they are still associated
with renal failure and severe allergic reactions and are very expensive.
CO2 as a contrast agent is not associated with renal failure
or allergic reactions. Combined with advances in digital subtraction angiography,
it is a viable alternative to iodinated contrast agents. It also occasionally
provides additional valuable information that cannot be obtained with iodinated
contrast agents, such as,
(a) demonstration of collateral
arteries
(b) arterio-venous shunting
in tumours
(c) opacification of tumours
that appear avascular with iodinated contrast
(d) detection of minute bleeding,
and
(e) demonstration of portal
system with wedged hepatic venography.
There are special mechanical
injectors and plastic bag-hand delivery systems.
The ability to totally displace
blood from the vascular system and the possible usage of unlimited quantities
of CO2 make it an ideal agent for clear angioscopic viewing.
The vascular imaging modalities
described here are evolving rapidly. Real time imaging and three dimensional
constructions of the images are easily interpretable and useful for most
physicians. The implications of these are widespread and encompass a number
of potential diagnostic and therapeutic applications. This, in future, will
significantly improve the longevity and quality of life of many patients.
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| Fig 13: CO2 DSA - abdominal
aorta |
Fig 14: CO DSA -showing left renal
artery stenosis |
4. Angioscopy:
Angioscopy or vascular endoscopy
utilizes fiber optic and chip camera technology to enable intraluminal visualization
through small diameter scopes. Current angioscopes range from 3-4 mm to
500 micrometer diameter. Larger diameter scopes have room in the shaft of
the catheter for accessory lumen which can be used for infusion of irrigation
and contrast medium or for passage of guide wires or other instruments.
Smaller scopes do not possess ancillary channels and hence they must be
passed through an introducer catheter or sheath, in order to use other instruments
coaxially or to accomplish a selective delivery.. Although an eye piece
may be attached to the end of angioscope to enable direct inspection through
the imaging elements, the majority of the endoscopic surgical devices are
connected to a television monitor for magnified display of images. High
intensity light sources, i.e., usually about 300 w are needed to have a
clear image.
The major advantage of this
technique is the ability to observe three dimensional intraluminal perspective
of normal vessels and pathologic lesions via direct vision. This information
can be very useful in determining the etiology of pathologic lesions, for
enhancing the choice of interventional methods and for gauging the adequacy
of therapy. This encompasses a spectrum of applications including angioscopy
assisted in situ vein bypass vavulotomy, thrombectomy, monitoring of bypass
graft function, removing intravascular foreign body and inspection of percutaneous
angioplasty procedures before, during and following the intervention.
Given below are few algorithms for worked up
of vascular disease in various regions in the body. These are as per the general
practice in our hospital taking into consideration local conditions and expertise
and hence may vary from institution to institution.
Click
here for Flow Charts
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