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The future of vascular ultrasound
RAD Magazine, 39, 456, 22
Professor N D Pugh
Consultant medical physicist
Dr R J Morris
Principal medical physicist
University Hospital of Wales, Cardiff
email: [email protected]
The advent of Duplex scanners in the late 1970s
and early 1980s enabled, for the first time, the
measurement of blood flow velocity from specific
areas within blood vessels. This in turn allowed
more accurate assessment of the degree of stenosis/occlusion. The further development of colour
flow scanners in the late 1980s and early 1990s
moved vascular ultrasound on to become the
front-line investigative technique for vascular
assessment.
Since that time there have been significant
advances in medical ultrasound hardware and
software that have improved quality and resolution of B-mode and colour flow to greatly enhance
their clinical usefulness. As computer speed and
power increase rapidly, it is probable that some
of the current emerging technologies will develop
to have as big a clinical impact as colour flow
and Duplex. This article gives a brief summary of
newer imaging techniques, and their potential for
development in vascular ultrasound.
Contrast enhanced ultrasound (CEUS) is one of the oldest of the ‘new’ technologies. It has not yet become a widespread or routine technique, but has undergone resurgence
in recent years. Microbubble contrast agents are used to
depict low volume flow and perfusion, on the principle of
increased backscatter and resonance from tiny encapsulated
bubbles of gas that are injected into the body. It is a safe
and easy technique, providing one has the appropriate
equipment, as most common techniques require contrastspecific software. It also has benefits over other perfusion
imaging methods, as it does not involve ionising radiation,
and has no nephrotoxicity. Several different imaging protocols are available, employing pulse inversion and harmonic
techniques. Contrast is currently used in echocardiography
(myocardial contrast echocardiography) and in general radiology for imaging the liver, prostate and breast. Limited use
of contrast agents in general vascular ultrasound imaging
has been reported,1,2 with further potential in areas such as
EVAR (endovascular aneurysm repair) surveillance3 (figure
1) and the identification of adventitial and intra-plaque
angiogenesis (vasa-vasorum).4,5
Another ultrasound development that has been available
for some time, but has not yet found an essential clinical
application, is 3D/4D scanning. First described in the mid
1980s, it became commercially available in the mid 1990s,
and mostly found use in obstetrics. Three-dimensional data
blocks are produced simply by sweeping a 2D beam through
a volume of tissue. The most significant recent development
has been the replacement of mechanical sweeping by electronic sweeping with 2D array transducers. These matrix
transducers, by imaging the three imaging planes simultaneously, also have the capability of displaying these orthogonal views together in real time. This should speed workflow
and reduce transducer manipulation to minimise repetitive
Figure 1
Patient with an expanding aneurysm sac following
EVAR. CEUS showing a small type III endoleak (hole
in graft material) that was not detected on CT.
Figure 2
3D ultrasound showing multi-slice views of an aortic
aneurysm.
strain injuries. The applications to vascular ultrasound are
not yet established, but acquisitions of volume data can
produce series of cross sectional B-mode images, similar
to CT and MR, which has been used in AAA screening6
(figure 2). Three-dimensional and 4D imaging could have a
role in carotid scanning, particularly in measuring lumen
diameters, and in allowing complete archiving and remote
reporting of scans. A weakness in using 3D in vascular
ultrasound to develop ‘ultrasound angiography’ is that colour
flow cannot be reproduced usefully within the greyscale data
set. At the moment there is a problem in the way the data
is collected due to the cardiac cycle. Each part of the 3D
volume is acquired during different parts of the cardiac cycle
so that there will be incomplete colour filling in arteries.
Solutions would involve some form of cardiac gating, or
selecting data at peak systole.
A general problem with colour flow is that conventionally it has poorer resolution than B-mode due to longer pulse
lengths. Broadband Doppler (such as Advanced Dynamic
Flow – Toshiba Medical Systems)7, and direct visualisation
of moving blood cells in B-mode (B-flow – GE Medical
Systems) overcome spatial resolution artefacts.8 These methods have great advantages in the assessment of stenoses
(figure 3) as they improve the display of surface character-
istics and flow channels, which allows more reliable measurement of the stenosis extent.
In vascular imaging, the counterpart to improvements in
the visualisation of flow are improvements to the imaging of
surrounding tissue. The quality and resolution of B-mode
has developed with upgrades in transducer and scanner
technologies. Nevertheless, it is still generally true in ultrasound that, however good the quality of the image, the ability to distinguish different tissue types will rely on the
experience and training of the operator. Several attempts
have been made over many years to produce automated systems that can distinguish between cyst, haematoma, calcification, lipid, etc, and code those differences on an image.
Ideally, this would allow assessments to be made of the
likely presence of disease within tissue during a scan, or to
determine the composition of a specific lesion.
Elastography is an established tissue characterisation
technique based on the measurement of local tissue deformation in response to some sort of applied mechanical
stress. Although requiring some assumptions about the
physical properties of the tissue, it gives a more quantitative
measure of stiffness than manual palpation, together with a
strain map. Current clinical applications are in the assessment of the breast, liver and prostate. From a vascular perspective, intravascular elastography can be used in plaque
characterisation, using changes in intra-luminal pressure to
produce the stress.5 The strain image shows areas of high
strain in soft areas and low strain in calcified regions. A
future development in vascular imaging may be to perform
venous elastography to assess the age of venous thrombus.
Acoustic Structure Quantification is a tissue characterisation package developed by Toshiba Medical Systems for
use in liver assessment. Changes in the probability density
function of the echo signal between normal and fibrotic tissue are used to produce quantitative data and to colour-code
B-mode images. This approach may again be applicable in
vascular ultrasound to investigate plaque morphology and
thrombus age.
Characterisation of tissue would automate the assessment of disease currently confined to the subjective assessments of experienced users. A further approach is to make
vascular imaging more accessible to interpretation by nonspecialists. Perspective volume rendering (Fly Thru –
Toshiba Medical Systems) displays a 3D volume data set in
a way that allows interrogation of the interior surface of
cavities, ducts and blood vessels. Cross-sectional ultrasound
data is added to the plane surface data to enable the user to
‘fly through’ a cavity or vessel, viewing the interior surface
of the cavity. This has the potential, with refinement, to aid
in the visualisation of the surface characteristics of plaque
and the friability of thrombus (figure 4).
The future of vascular ultrasound indicated by these
emerging techniques is both to enhance the quality of information available to the operator, and to provide new diagnostic information for clinicians. None of these will move
from limited or experimental use to become normal and routine parts of a scan until they can be shown to improve the
quantification of vascular disease, or to reduce current subjective limitations. However, as these methods clearly show
potential in this direction, they will only be adopted more
widely if we make the effort to put them into practice and
report on their usefulness.
References
Figure 3
Conventional colour flow (left) and Toshiba Advanced
Dynamic Flow images (right) of a significant carotid
stenosis. The improved spatial resolution of the
Advanced Dynamic Flow overcomes the ‘bleeding
artefact’ associated with conventional colour flow
imaging and allows for better measurement of degree
of stenosis and assessment of plaque morphology.
Figure 4
1, Duerschmied D, Maletzki P, Freund G, Olschewski M, Bode C, Hehrlein C.
Success of arterial revascularization determined by contrast ultrasound
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2011;80:68-76.
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Roentgenol 2007;188:W262-68.
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6, Nyhsen C M, Elliott S T. Rapid assessment of abdominal aortic aneurysms
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method of vascular imaging in prenatal medicine. A pilot study of its
applicability. Ultraschall Med 2004;25:280-84.
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Toshiba Fly Thru images of a normal carotid artery (right) and a carotid artery with significant intraluminal plaque (left). Fly Thru allows surface rendering of the plaque and the potential for assessing plaque
morphology.