The future of vascular ultrasoundRAD Magazine, 39, 456, 22

Professor N D PughConsultant medical physicist

Dr R J MorrisPrincipal medical physicist

University Hospital of Wales, Cardiffemail: pughnd@cardiff.ac.uk

The advent of Duplex scanners in the late 1970sand early 1980s enabled, for the first time, themeasurement of blood flow velocity from specificareas within blood vessels. This in turn allowedmore accurate assessment of the degree of steno-sis/occlusion. The further development of colourflow scanners in the late 1980s and early 1990smoved vascular ultrasound on to become thefront-line investigative technique for vascularassessment.

Since that time there have been significantadvances in medical ultrasound hardware andsoftware that have improved quality and resolu-tion of B-mode and colour flow to greatly enhancetheir clinical usefulness. As computer speed andpower increase rapidly, it is probable that someof the current emerging technologies will developto have as big a clinical impact as colour flowand Duplex. This article gives a brief summary ofnewer imaging techniques, and their potential fordevelopment in vascular ultrasound.

Contrast enhanced ultrasound (CEUS) is one of the old-est of the ‘new’ technologies. It has not yet become a wide-spread or routine technique, but has undergone resurgencein recent years. Microbubble contrast agents are used todepict low volume flow and perfusion, on the principle ofincreased backscatter and resonance from tiny encapsulatedbubbles of gas that are injected into the body. It is a safeand easy technique, providing one has the appropriateequipment, as most common techniques require contrast-specific software. It also has benefits over other perfusionimaging methods, as it does not involve ionising radiation,and has no nephrotoxicity. Several different imaging proto-cols are available, employing pulse inversion and harmonictechniques. Contrast is currently used in echocardiography(myocardial contrast echocardiography) and in general radi-ology for imaging the liver, prostate and breast. Limited useof contrast agents in general vascular ultrasound imaginghas been reported,1,2 with further potential in areas such asEVAR (endovascular aneurysm repair) surveillance3 (figure1) and the identification of adventitial and intra-plaqueangiogenesis (vasa-vasorum).4,5

Another ultrasound development that has been availablefor some time, but has not yet found an essential clinicalapplication, is 3D/4D scanning. First described in the mid1980s, it became commercially available in the mid 1990s,and mostly found use in obstetrics. Three-dimensional datablocks are produced simply by sweeping a 2D beam througha volume of tissue. The most significant recent developmenthas been the replacement of mechanical sweeping by elec-tronic sweeping with 2D array transducers. These matrixtransducers, by imaging the three imaging planes simulta-neously, also have the capability of displaying these orthog-onal views together in real time. This should speed workflowand reduce transducer manipulation to minimise repetitive

strain injuries. The applications to vascular ultrasound arenot 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 arole in carotid scanning, particularly in measuring lumendiameters, and in allowing complete archiving and remotereporting of scans. A weakness in using 3D in vascularultrasound to develop ‘ultrasound angiography’ is that colourflow cannot be reproduced usefully within the greyscale dataset. At the moment there is a problem in the way the datais collected due to the cardiac cycle. Each part of the 3Dvolume is acquired during different parts of the cardiac cycleso that there will be incomplete colour filling in arteries.Solutions would involve some form of cardiac gating, orselecting data at peak systole.

A general problem with colour flow is that convention-ally it has poorer resolution than B-mode due to longer pulselengths. Broadband Doppler (such as Advanced DynamicFlow – Toshiba Medical Systems)7, and direct visualisationof moving blood cells in B-mode (B-flow – GE MedicalSystems) overcome spatial resolution artefacts.8 These meth-ods have great advantages in the assessment of stenoses(figure 3) as they improve the display of surface character-

Figure 1 Patient with an expanding aneurysm sac followingEVAR. CEUS showing a small type III endoleak (holein graft material) that was not detected on CT.

Figure 2 3D ultrasound showing multi-slice views of an aorticaneurysm.

istics and flow channels, which allows more reliable mea-surement of the stenosis extent.

In vascular imaging, the counterpart to improvements inthe visualisation of flow are improvements to the imaging ofsurrounding tissue. The quality and resolution of B-modehas developed with upgrades in transducer and scannertechnologies. Nevertheless, it is still generally true in ultra-sound that, however good the quality of the image, the abil-ity to distinguish different tissue types will rely on theexperience and training of the operator. Several attemptshave been made over many years to produce automated sys-tems that can distinguish between cyst, haematoma, calci-fication, lipid, etc, and code those differences on an image.Ideally, this would allow assessments to be made of thelikely presence of disease within tissue during a scan, or todetermine the composition of a specific lesion.

Elastography is an established tissue characterisationtechnique based on the measurement of local tissue defor-mation in response to some sort of applied mechanicalstress. Although requiring some assumptions about thephysical properties of the tissue, it gives a more quantitativemeasure of stiffness than manual palpation, together with astrain map. Current clinical applications are in the assess-ment of the breast, liver and prostate. From a vascular per-spective, intravascular elastography can be used in plaquecharacterisation, using changes in intra-luminal pressure toproduce the stress.5 The strain image shows areas of highstrain in soft areas and low strain in calcified regions. Afuture development in vascular imaging may be to performvenous elastography to assess the age of venous thrombus.

Figure 3 Conventional colour flow (left) and Toshiba AdvancedDynamic Flow images (right) of a significant carotidstenosis. The improved spatial resolution of theAdvanced Dynamic Flow overcomes the ‘bleedingartefact’ associated with conventional colour flowimaging and allows for better measurement of degreeof stenosis and assessment of plaque morphology.

Figure 4 Toshiba Fly Thru images of a normal carotid artery (right) and a carotid artery with significant intra-luminal plaque (left). Fly Thru allows surface rendering of the plaque and the potential for assessing plaquemorphology.

Acoustic Structure Quantification is a tissue characteri-sation package developed by Toshiba Medical Systems foruse in liver assessment. Changes in the probability densityfunction of the echo signal between normal and fibrotic tis-sue are used to produce quantitative data and to colour-codeB-mode images. This approach may again be applicable invascular ultrasound to investigate plaque morphology andthrombus age.

Characterisation of tissue would automate the assess-ment of disease currently confined to the subjective assess-ments of experienced users. A further approach is to makevascular imaging more accessible to interpretation by non-specialists. Perspective volume rendering (Fly Thru –Toshiba Medical Systems) displays a 3D volume data set ina way that allows interrogation of the interior surface ofcavities, ducts and blood vessels. Cross-sectional ultrasounddata is added to the plane surface data to enable the user to‘fly through’ a cavity or vessel, viewing the interior surfaceof the cavity. This has the potential, with refinement, to aidin the visualisation of the surface characteristics of plaqueand the friability of thrombus (figure 4).

The future of vascular ultrasound indicated by theseemerging techniques is both to enhance the quality of infor-mation available to the operator, and to provide new diag-nostic information for clinicians. None of these will movefrom limited or experimental use to become normal and rou-tine parts of a scan until they can be shown to improve thequantification of vascular disease, or to reduce current sub-jective limitations. However, as these methods clearly showpotential in this direction, they will only be adopted morewidely if we make the effort to put them into practice andreport on their usefulness.

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