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Using the terms sensitivity and specificity as defined previously, the sensitivity of ultrasonic arteriography in detecting abnormalities in vessels with stenosis or occlusion was 96%. The specificity of ultrasonic arteriography was only 44%, because 20 out of 36 normal carotids showed segments that could not be visualised because of arterial wall calcification. The ability of ultrasonic arteriography to predict or exclude the need for carotid endarterectomy in the presence of a normal ultrasonic image or an occluded internal carotid artery, correlated with the lack of need for endarterectomy in 90% of cases. However, if the ultrasonic image demonstrated a stenosis, this was suitable for endarterectomy in only 60%. of vessels because of the difficulty in visualizing the carotids due to calcification. Barnes goes on to show that if the Doppler imaging system is combined with spectral analysis, the combined validity (overall accuracy) is 89% compared with 71%. for ultrasonic arteriography alone, sensitivity of 90%, compared with 96%, and a specificity of 92%., compared with 42%. The predictive value of the combined technique corresponded with the lack of need for carotid endarterectomy in 90%. of cases when a normal ultrasonic appearance was shown. If a stenosis was demonstrated the vessel was amenable to endarterectomy in 86%. of cases.
Blackshear et al. (1979) also compared pulsed Doppler imaging with contrast arteriography. The carotid arteries were graded by percentage reduction in lumenal diameter, into five categories, less than 10%. stenosis (wall irregularity), 10-49% stenosis, 50-99% stenosis, and complete occlusion. The contrast angiograms were reported independently of the ultrasonic arteriograms. In this study, contrast angiograms were performed on 66 carotids, the results on a comparison of the two techniques are shown in Table 5.3b. Blackshear et al. do not produce figures for Doppler scanners alone but are the results of the combined scan and audible assessment of the Doppler signals. With the ultrasonic techniques, there was one false positive in eight angiographically normal vessels. In the range 10-49% stenosis, audible changes were detected in 59% of vessels; in the range 50-99%, audible changes were detected in 91%. of vessels. Three out of four internal carotid occlusions were diagnosed correctly. The one error was caused by an uncooperative patient, when a major stenosis rather than a complete occlusion was diagnosed. Blackshear et al. conclude that ultrasound angiography plus the audio quality was correct in diagnosing a lesion sufficient to reduce flow, in 88% of cases, and in 75% of cases where there was any disease at all.
Sumner et al. (1979) have also compared pulsed Doppler imaging with contrast arteriography and their findings are summarised in Table 5.3c.
Lusby et al. (1980) in an investigation using the MAVIS multichannel Doppler imaging system showed a sensitivity of 88% in detecting all grades of disease. and a specificity of 100%. For a detailed analysis of pulsed Doppler imaging, see Tables 5.3a-d.
Several examples of the multichannel Doppler imaging system are shown in Figs 5.6 to 5.l4.
(ii) Continuous Wave Doppler Imaging Systems
Continuous wave Doppler scanning systems are similar to the pulsed systems in operation, but cannot produce depth information. This. in general, limits the image to the anterior-posterior projection. However, it is possible sometimes to rotate the patient through 90° about the body axis, to produce an orthogonal scan. Spencer et al. (l977) have reported their results on the carotid arteries, using such a system combined with an assessment of the audio-signal, and spectral analysis. In 65 internal carotid arteries which were greater than 50% stenosed, or occluded the Doppler scan diagnosis was correct in 92.9% of cases. In 35 external carotid arteries the diagnosis was correct in 91.2%. of cases.
Spencer et al. discuss the problem of the detection of plaques without stenosis. Barnes et al. found calcification occurred in 56%. of "normal" carotid arteries in their study.
White and Curry (1978) have reported on the use of a colour coded CW scanner in 424 carotid bifurcations. In this system the peak blood velocity over the cardiac cycle is colour
coded electronically, so that normal velocities appear as red, slightly increased peak velocities
as yellow, and markedly increased peak velocities as blue. This colour coding presents the same information that is assessed audibly in Spencer et al.'s paper, but is more objective in
that the image is displayed in the appropriate colour on a TV monitor. In this study all carotid angiograms were scored by a neuroradiologist without knowledge of the Doppler scan results. The contrast angiograms were divided into classes showing no stenosis, mild
stenosis (< 24%), moderate stenosis (25-75%), marked stenosis (>75%) and complete
occlusion. These results were then compared with the colour display, red relating to a normal vessel, yellow to a moderate stenosis and blue to a severe stenosis. White and Curry found that of 956 arteries reported as normal angiographically the Doppler scan was normal in 893 arteries (93%.) while in a further 54 arteries the Doppler scan showed a moderate increase in velocity. This was found to be an acceptable result by White and Curry who quote a correlation of 99%, between a normal Doppler scan and a normal angiogram. Of 58 angiograms demonstrating occlusions the carotid scans correctly identified these in 53 cases (91%). In 82 cases of severe stenosis ( > 75%) the image was coded blue in 71 (86%), while a further seven cases were displayed as yellow. This results in an overall success rate of 95%. White and Curry noted four cases in which the blood flow through a high grade stenosis was not sufficient to record an image. For moderate stenosis, these authors quote an overall correlation between Doppler scanning and arteriography of 62%. White and Curry's results have been assessed separately by the present authors, so that they can be compared and contrasted with other authors. All the results are shown in Table 5.4.
Shoumaker and Bloch (1978) have used a CW Doppler scanner in the study of carotid artery disease, and their results are also shown in Table 5.4.
(iii) Comparison of Doppler Imaging Systems
As mentioned in the previous section the two major questions to be answered concerning Doppler imaging systems is, how good are they when compared with contrast arteriography, and, are pulsed and continuous wave systems equally reliable in detecting arterial disease? The results from various authors are summarized in Table 5.4. It is difficult to compare the various studies because the scoring of the contrast arteriograms is different in each case. Some radiologists have scored a category of < 10% stenosis, others have only two classes, > 50% stenosis and occlusion. However. some conclusions can be drawn from the combined results. Firstly for over 50% diameter stenosis there is excellent agreement between both types of Doppler scanner and arteriography in visualizing internal carotid artery stenoses. Below 50% diameter stenosis the pulsed systems are better than the continuous wave systems. The probable reason for this is that with the three dimensional capability of the pulsed system, the Doppler shift waveform can be sampled at a precise location in the vessel. This allows the investigation of the Doppler shift signal from the vicinity of suspect areas of minor stenosis. The pooled results from the various authors detailed in Table 5.4 are shown in Table 5.5. Because of the difficulties in comparing results from various authors, this table should be taken only as an approximate guide to performance.
(iv) Duplex Scanning
Duplex scanners* consist of a real-time ultrasonic scanner and a pulsed Doppler flowmeter to examine blood flow characteristics at precise locations in the blood vessel. A typical system for investigation of peripheral blood vessels usually operates at a frequency of 5 MHz. The probe consists of three single element transducers mounted on a wheel which rotates continuously. As each transducer comes into contact with the patient it sweeps out a new image frame at a rate of 30 per second. The image can be stored, and one of the transducers used as a gated pulsed Doppler transducer to sample the blood velocity at a particular point in the blood vessel. The position of the Doppler beam is indicated by a white line on the display screen, the position of the sample volume is shown as a white spot on this line. The position of the sample volume and the direction of the Doppler ultrasound beam are adjusted using an arm which is connected by means of a servomechanism to the transducer. Video tape recordings can be made of both the B-scan display and the Doppler signals. Several clinical examples are illustrated below. Figure 5.15 shows a major stenosis of the common carotid artery. The blood-velocity/time waveforms in the proximal common carotid, and within the stenosis are shown. The velocity within the stenosis is so high that the zero-crossing frequency detector is unable to follow the peak forward flow. This is illustrated by the apparent large reverse flow component in systole. There is no detectable flow signal in the distal internal carotid. The diagnosis later confirmed at arteriography was of an 80% stenosis of the common carotid with an occluded internal carotid. Figure 5.16 shows a post-operative evaluation of this patient. A by-pass graft was implanted into the subclavian artery and into the bifurcation of the carotid. The distal anastomosis is clearly seen on the B-scan image. The external carotid blood velocity waveform is of the correct shape, and although the internal carotid remains occluded, the brain hemisphere is supplied by collaterals from the external carotid.
Figure 5.17 shows the B-scan image pre- and post-operatively with the angiogram for comparison. There is evidence of atheroma around the bifurcation, also involving the proximal Internal carotid. After carotid endarterectomy the lumen appears smoother and is of a larger diameter. Figure 5.18 is an interesting example of a complete loop in the internal carotid artery. The B-scan image clearly shows the kink at the origin but not the loop.
Further examples will be shown in the section comparing Duplex systems with Doppler imaging systems, in the investigation of the carotid circulation.
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