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According to Thrush Hartshorne, Stroke is the 3rd most common cause of death in the UK, and 80 of these deaths are caused by ischemic strokes which occur in the area of the brain supplied by the carotid arteries. The North American Symptomatic Carotid Trial, (1999) proved that carotid endarterectomy is highly effective in patients that have carotid artery stenosis >70. As stroke is often the first clinical manifestation, it is important that it is identified and treated early. This report will focus on carotid duplex ultrasound, including its scientific basis, indications for use, reliability and value.
Carotid ultrasound can be used to assess carotid artery stenosis and the extent of occlusion, and therefore identify people who may be at risk of suffering from stroke or transient ischemic attack (TIA) (Kumar and Clarke, 2009).
Most laboratories will use duplex ultrasound scanning. This combines B-mode imaging, colour flow imaging and Pulsed Doppler ultrasound.
To understand colour flow imaging and pulsed Doppler ultrasound, it will be necessary to explain the Doppler Effect. The Doppler Effect is when the frequency of sound observed changes as a result of the source or observer being in motion. The change in frequency itself is known as the Doppler Shift. An example of this is the change in pitch that an ambulance's siren creates as it moves towards and then away from you. As it moves towards you, it will shorten the wavelengths as it moves between them, increasing the frequency and making the apparent sound higher. Conversely, as it moves away the sound waves are elongated, lowering the frequency and making the pitch lower. (Thrush Hartshorne)
B-mode ultrasound provides a two-dimensional real time black and white image of the anatomy of the vessels. The mechanism for this is specular reflection and back scattering from surfaces, boundaries and tissues. A beam is passed through tissues and the reflected echo signals are converted to a series of dots on the display. The location of the B-mode line on the display corresponds to its anatomical position. This creates an image which relates to the depth, direction and brightness of the individual reflected echo. (Zwiebel, 1992)
Colour flow imaging shows areas of blood flow which overlay the 2D image from the B-mode ultrasound. It also relies on the Doppler effect but unlike the pulsed Doppler the Doppler shift is displayed as a colour. This colour relates to the blood flow velocity, where the brighter the colour hue the greater the velocity.
Pulsed Doppler can either be viewed as a sonogram or spectral trace and shows the flow in a selected area of the vessel. It also relies on the Doppler Effect as a Doppler shift may be detected from moving red blood cells which scatter an ultrasound beam emitted from a stationary probe. Red blood cells have the ability to scatter ultrasound beams due to their small diameter of 7Î¼m, which is smaller than the wavelength of the ultrasound beam used (Thrush Hartshorne). The degree of scattering is picked up by a transducer and is proportional to blood flow velocity. Myers Clough (2004, p.6) states that, "The Doppler shift frequency (difference between the transmitted and received frequency) is given by: fo = 2fo v cosÎ¸
f = ultrasound frequency transmitted from the sound
v = velocity of the moving blood
Î¸ = angle between the direction of blood flow and ultrasound beam
c = velocity of ultrasound in tissue (1540 m/s)"
A localised area of increased velocity of blood flow, usually located at the carotid bifurcation, will indicate narrowing of the vessel and thus stenosis of the carotid artery, as indicated in the image below.
Fig 1. "Colour image showing a narrow channel of
low-velocity flow detected in a subtotal occlusion of the
internal carotid artery" (Thrush Hartshorne, 2010)
Indications for Use
The groups of indications of use of carotid ultrasound are mainly diagnostic and evaluative. According to Zweibel (1992 p.85), the indications for use can be summarized as:
General indications: where information about the state of the carotid arteries would be of diagnostic value
TIA - Lasting less than 24hours, or any patient with stroke suspected from the carotid territory
To rule out carotid artery disease when there are other possible causes, e.g atrial fibrillation
Amaurosis fugax - Monocular visual loss
Asymptomatic cervical bruit
Follow- up after carotid endarterectomy
Prior to surgery, including cardiac and aortic aneurismal surgeries
Clark et al and Llach et al, as cited by Zwiebel (1992, p. 85), states that it is seen to be somewhat controversial to perform carotid ultrasound on patients with an asymptomatic cervical bruit or those who are about to undergo surgery, as the small 3 perioperative stroke rate is too low to warrant more surgical interventions such as carotid endarterectomy. Benavente et al. (1998), confirmed that although there was a reduction in stroke in asymptomatic patients who underwent carotid endarterectomy, the modest benefit of surgery in unselected patients is too small for the intervention to be routinely recommended.
According to Norris et al. (1991), patients suffering from carotid artery stenosis of >70 have only a negligible risk of stroke. It is therefore imperative that any test is definitely indicated, completely necessary and has no contraindications.
Reliability of the Test
According to Miljkovic et al. (2009), sensitivity of carotid duplex ultrasound is 89.20. This is to say that if there is carotid artery disease, it will be detected in 89.20 of cases. The specificity was reported as 95.40, which tells us that in those patients where there is no carotid artery disease, 95.40 of these patients will be correctly told that they have no disease. The total accuracy is said to be 96. Wardlaw et al. (2006), state that sensitivity is 89 and specificity 84.
The table below shows the sensitivity and specificity of carotid ultrasound compared with MR angiography. We can see that the sensitivity of ultrasound is the same as MR angiography, however the specificity and overall accuracy are stated to be lower. This may lead one to think that MR angiography would always be preferred, however when we go on to look at other factors such as cost effectiveness we will see that this is not necessarily the case.
Table 1. Diagnostic performance of Doppler ultrasound compared with MR angiography. (American Journal of neuroradiology, 1995)
Despite its relatively high degree of sensitivity and specificity, carotid ultrasound is susceptible to a number of limitations. These can be divided into limitations of the people carrying out the test and limitations of the test itself.
According to Grant et al. (2003, p.342), "Overall, carotid US is often performed inconsistently within a given laboratory, and there is nonuniformity in practise between one laboratory and the next". The paper also stated that, "The interpretative criteria for carotid stenosis are either indiscriminately applied or the interpreters are uncertain about exactly how to make the diagnosis of carotid artery stenosis." These findings resulted in concern for patient safety and care. It called for standardisation of the protocol for both carrying out and interpreting carotid duplex ultrasound data.
The table below represents the diagnostic criteria recommended for reporting carotid ultrasound scans in the UK.
Table 2. Joint recommendations for reporting carotid ultrasound investigations in the UK: diagnostic
Criteria ( Oates et al. 2008 as cited by Thrush Hartshorne 2010 )
It is important that there are agreed acceptable guidelines that can be used nationally to set standards in individual laboratories. This is because it is mainly patients suffering >70 stenosis who will benefit most from surgery and is therefore important that the test can be used to grade disease into discrete bands.
The Department of Health (2008, p.24) also states that duplex ultrasound is highly operator dependent and it requires significant skill in the obtaining and interpreting of data. There is some worry that ultrasound is being described as the 'new stethoscope'. This presumes that it is cheap, portable, safe and easy to use, the latter of which is certainly not true. There is therefore worry that this is giving people a false sense of confidence and discouraging them from seeking proper training (Edwards, 2010). This shows that the reliability of carotid ultrasound may be compromised by the quality and degree of training of a sonographer and their subsequent skill.
Whilst carrying out the scan itself, there are several things which may affect the accuracy of the test :
The operator may use the wrong angle of isonation, which alters the detected Doppler shift frequency and may lead to over or underestimation of the disease.
Excessive transducer pressure may compress arteries, causing an apparent rise in blood flow velocity.
The ultrasound may need to be performed with the patient upright, for example in a patient who is in a wheelchair. The patient's position may affect the observed velocity values. Fig 2. As the angle of isonation changes, so too
does the detected Doppler shift frequency.
(Thrush Hartshorne, 2010)
The higher the frequency of the beam, the better the resolution of the blood vessel anatomy. However in some patients it may be necessary to use a lower frequency as the blood vessels may lie much deeper in the neck.
The presence of a calcified plaque may prevent colour flow imaging, B-mode imaging and spectral Doppler imaging causing the sonographer to believe that the artery is completely occluded.
Tortuosity of a vessel may cause a poor quality of image due the fact that the vessel may run in parallel to the ultrasound beam.
Side effects and Complications
Ultrasound is a very accurate, non invasive technique which does not use ionizing radiation and doesn't require the use of contrast agents which have the potential to cause allergic reactions. The intensities of ultrasound used as guided by the World Federation of Ultrasound in Medicine and Biology are too low to cause any side effects.
However, carotid ultrasound is not very tightly regulated and it is important that sonographers are adequately trained to understand the power output in all ultrasound exams and use the "as low as reasonably achievable" principle of power output.
One of the biggest complications of ultrasound is misdiagnosis due to errors which I have highlighted above including inadequate training and poor technique. This may lead to a patient having to undergo relatively major surgery which is not necessary.
Cost of the Test
This tale indicates that colour Doppler ultrasound costs $130. This is remarkably less than both MR angiography and digital subtraction angiography and roughly half the cost of a CT scan.
Another benefit of ultrasound with regard to cost is that it only requires one operator whereas a CT scan may require a number of staff, bringing up the associated costs of the investigation.
Table 3. Costs of some radiologic investigations. (American Journal of Neuroradiology)
Value of the Test
The department of health (2008, p.8) states that 20 of patients who suffer a TIA will go on to have a full stroke within four weeks. If those patients suffering from a TIA are assessed and treated within 24 hours then the number of people who will go on to suffer from a stroke will be reduced by 80.
Now many centres also use ultrasound to select patients directly for surgery without carrying out pre operative angiography. This is because ultrasound is non-invasive and safe whereas angiography carries a risk of transient and permanent neurological damage.
There is currently no national screening programme for carotid artery stenosis, although there isscreening for aortic aneurysms. Screening programmes must be considered to be safe, simple, valid, acceptable to the population and cost effective. At this time carotid ultrasound must not fulfil one or more of these requirements.
This shows the value of carotid ultrasound in the safe and effective management of patients suffering carotid artery stenosis.