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COLOUR DOPPLER IN DIZZINESS AND VERTIGO

Nitin Chaubal*, Monali Padwal**
*Consultant sonologist, Jaslok Hospital and Reasearch centre, Mumbai. **Consultant Radiologist-Thane Ultrasound Centre, Thane.

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Cerebrovascular disturbances form an important cause of fainting, syncope, dizziness and vertigo:

Colour Doppler of Carotid and Vertebral arteries forms an important part of evaluation of extra-cranial vascular insufficiency. It is also possible to study intra-cranial circulation with the help of transcranial probes, these are low frequency probes. The article describes the techniques, normal and abnormal findings in carotid, vertebro basilar and intra-cranial circulation.

Accurate diagnosis of haemodynamically significant stenosis is critical to identify patients who would benefit from surgical intervention. Duplex sonography, combining high resolution imaging and doppler spectrum analysis has proved to be popular, noninvasive, accurate and cost effective means of detecting and assessing carotid disease. Carotid sonography has largely replaced angiography for suspected extra-cranial carotid atherosclerosis.[4,5]

Besides estimating the degree of stenosis, the biggest advantage of sonography is its ability to characterize plaque, and identify plaques with higher risks of embolisation. With high resolution ultrasound, plaque can be characterised into relative risk groups for containing intraplaque haemorrhage which is thought by many to be the precursor of plaque ulceration.[6,7]

CAROTID DOPPLER - TECHNIQUE

The doppler examination usually takes 20-40 min; carotid arteries are examined with the patient in supine position, with the head mildly extended, and face turned away from the side of examination. Examination can be done facing the patient or sitting behind the patient. Careful scanning is done of extra-cranial carotids in longitudinal and transverse planes, examination being done on B-mode, followed by colour and followed by spectral trace. Exposure of the neck can be maximised by having the patient drop the ipsilateral shoulder. Occasionaly the vessels can be approached from posterior to sternocleidomastoid, rather than anterior; and both approaches may be necessary to evaluate the vessels.8

B-Mode : Transverse and longitudinal scan of both common carotid artery (CCA), internal carotid artery (ICA) and external carotid artery (ECA) are obtained from origin of both common carotids, bifurcation is identified. Depending on the level of bifurcation about 3.0 to 4.0 cm of ICA and ECA identified. Intima-media complex are identified. Plaques are identified, characterised and approximate diameter reduction is judged by calibers. Image directed grading of stenosis is reliable in less than 50% diameter reduction.

Colour : On colour identify any areas of flow channel narrowing, turbulance and high velocity especially in the region where a plaque has been identified. Make sure internal carotid is not occluded, differentiating occlusive from preocclusive lesion is best done by colour and power angio. Presence of colour in both vessels in transverse plane reassures that ICA is not occluded.

Spectral waveforms : Spectral waveforms are obtained at various points in common and internal carotids and at origin of external carotid. Areas which are suspicious on colour are examined more carefully. Spectral waveform forms a very important part of examination because, grading of stenosis is done mainly on velocity changes. One should make sure of angle correction as best as one can. ICA/CCA velocity ratio is important when CCA velocities are likely to be altered.

Normal anatomy

The right common carotid artery arises in most patients from the right innominate artery and the left arises mostly from the aortic arch. In 10-20% of patients there may be anomalous origins however, these have little effect on the duplex carotid examination. In most patients, the carotid artery is medial to the jugular vein. The bifurcation is typically at the C4 level, but it can sometimes be as low T2 or as high as Cavernous segment of the Internal carotid artery (C1). High bifurcations are difficult to image well due to the angle of mandible. Depending on the level of the bifurcation, usually several centimeters of the internal and external carotid arteries can be visualised. Common carotid widens some what as it bifurcates, this widened area is called the carotid bulb.

Differentiation of ICA and ECA
Feature	ECA	ICA
Size	Usually smaller	Usually larger
Orientation	oriented antero-medially, towards the face. towards the mastoid process.	oriented postero-laterally postero-laterally
Branches	present	absent
Doppler feature	high resistance type flow pattern	low resistance type flow pattern
Temporal tap noted	reflected pulsations seen	no pulsations

Normal Haemodynamics and Waveforms

Carotids reveal laminar flow. In the region of the carotid bulb, the vessel lumen expands, resulting in a decrease in the average flow and thus kinetic energy. However, to maintain total energy, pressure in this region increases and this increased pressure acts on the blood at the peripheral aspect of the vessel lumen resulting in localised flow reversal in this region. It has been observed that atherosclerotic plaques originate mainly in the region of flow reversal.[30]

The internal carotid artery supplies the brain parenchyma and reveals low resistance type of flow, with a wide systolic peak and relatively high levels of diastolic flow throughout the cardiac cycle.

The external carotid artery on the other hand supplies the facial muscles and scalp, hence reveals a high resistance type of flow. Its doppler waveform has narrow systolic peaks and low levels of diastolic flow or complete absence of diastolic flow. The common carotid shows features of both, usually with a good diastolic flow.

CAROTID WALL THICKNESS AND PLAQUE CHARACTERISATION

Carotid wall shows two parallel echogenic lines separated by a hypoechoic region (media), the inner is the lumen-intima interface and outer line is the media-adventitia interface. The distance between the lines is the combined thickness of intima and media (I-M complex). Thickness of more than 0.8 mm is suggestive of atherosclerotic disease and could be the first indication of the same. Thickening of I-M complex or focal plaque corelates with an increased risk for development of cardio-vascular symptoms in asymptomatic patients.10,11

Evaluation of plaque primarily consists of noting its extent, location, surface contours and resulting luminal stenosis. Less than half of patients with documented TIA have haemodynamically significant stenosis. In the remaining patients, embolisation is the common cause of TIA rather than flow limiting stenosis. It is therefore important to identify low grade atherosclerotic lesions that may contain haemorrhage or ulcerations that can serve as nidus for emboli that cause both TIA and stroke. Plaque analysis of carotid endarterectomy specimens has implicated multiple intraplaque haemorrhages as an important factor in evaluation of neurologic symptoms.[12],[7]

Plaques are generally classified as being homogeneous or heterogeneous. Homogeneous plaque has a uniform echopattern and smooth surface, these are made up of dense, fibrous, connective tissue. Heterogeneous plaque has more complex echopattern and contains at least one or more focal sonolucent areas. Heterogeneous plaque is characterised pathologically as containing intraplaque haemorrhage and/or deposits of lipid, cholesterol and proteinaceous material. Calcifications can be seen in both.[13,14]

The intimal surface of plaque can be smooth or irregular, when surface is irregular, plaque is always heterogeneous. When the surface is smooth, it can be either homogeneous or heterogeneous depending on presence or absence of a focal sonolucent area. Athough all ulcerative plaques appear to be heterogeneous, all heterogeneous plaques do not have ulcerations.

Ulcerations are characterised by focal depression or break in plaque surface, anechoic region within plaque extending to vessel lumen and eddies of colour within plaque. Though the diagnosis of ulceration is controversial, the ability to reliably predict intraplaque haemorrhage with its associated clinical implications underscores the importance of ultrasound plaque characterisation.

Plaques are also classified as,[7]

Type I : Predominantly echolucent plaque with a thin echogenic cap.

Type II : Substantially echolucent with small areas of echogenicity.

Type I and II : Are typically seen in symptomatic pts. with 70% stenosis.

Type III : Predominantly echogenic with small areas of echolucency.

Type IV : Uniformly echogenic.

Type III and IV are generally composed of fibrous tissue and/or calcification. They are more benign, stable plaques. Common in asymptomatic older individuals.

The presence of heterogenic, irregular plaques should be noted because haemorrhage within plaque in a stenosis of less than 50% may be considered as a surgical lesion in the appropriate clinical setting.

Unstable heterogeneous plaques require monitoring. Most of the studies gave importance to grading stenosis, however plaque characterisation is a very important part of the examination.

Grading stenosis

Grading of stenosis is done by image and doppler assessment, usually they agree. When there is disagreement, image assessment is more reliable, when degree of stenosis is less than 50-60%, doppler assessment is more reliable for stenosis above this. When there is disagreement between image and doppler assessment, correct angle correction should be ensured. It should be made sure that, plaque is not being underestimated because of acute haemorrhage. Angle correction is difficult in tortuous or calcified vessels.

Image guided grading of stenosis

This is done in transverse plane perpendicular to long axis of vessel at the site of plaque. Usually machines have preset programmes. The outer measurement is taken from the media to media and residual diameter excluding the plaque. Generally diameter reduction is taken rather than area reduction. Attempts have also been made to grade diameter reduction by comparing distal ICA in a longitudinal plane as in an angiogram. Measurements made on longitudinal scans may overestimate the severity of stenosis by partial voluming through an eccenteric plaque. Image directed grading is generally accurate for diameter reduction less than 50-60%.

Doppler

Doppler is complementary to image guided grading since it gives physiological information. It is more accurate in knowing flow restriction. Changes in the appearance of spectrum, systolic and diastolic velocities are observed. Stenosis causing less than 50% diameter reduction does not alter spectrum much. Between 30-60% there is filling of spectral window and spectral broadening. Above 60% diameter reduction, the peak systolic velocities increase, the diastolic velocities increase when stenosis becomes critical i.e. 80% diameter reduction. Systolic velocities are eleveted in moderate and severe stenosis. Diastolic velocities are elevated in critical stenosis.

The most commonly used classification to grade stenosis is as follows.

Table 1[4]
Dia. steno (%)	Peak syst velo ratio	End diast. vel ratio	Systo vel ratio (VICA:VCCA)	Diasto vel ratio	Spectral boardening
0 (normal)	<110	<40	<1.8	<2.4	<30
1-39 (mild)	<110	<40	<1.8	<2.4	<40
40-59 (moderate)	<130	<40	<1.8	<2.4	<40
60-79 (severe)	>250	>10	>3.7	>5.5	>80
100 (occlusion)	--	--	--	--	--

Table 2[7]
Diamater stenosis ratio	ICA/CCA peak systolic velocity ratio	ICA/CCA diastolic velocity ratio	Peak systolic velocity	peak end diastolic velocity
0-40%	< 1.5	< 2.6	< 110 ncm/sec	< 40 cm/sec
41%-59%	< 1.8	< 2.6	> 120 cm/sec	< 40 cm/sec
60%-69%	> 1.8	> 2.6	> 150 cm/sec	> 40 cm/sec
70%-79%	> 3.0	> 3.3	> 210 cm/sec	> 70 cm/sec
80%-99%	> 3.7	> 5.5	> 280 cm/sec	> 100 cm/sec

*VICA-velocity of the internal carotid artery. * VCCA-velocity of the common carotid artery.

The internal carotid to common carotid ratios become important when velocities in common carotid are themselves altered. In this case we cannot rely on the peak systolic or diastolic velocities. Situations where this is likely to occur are, low cardiac output status, aortic stenotic valve disease or high stenosis at origin of common carotid. Similarly, when there are “Tandom” lesions, velocities are likely to be altered due to proximal lesions and can not be solely depended upon.

Other carotid arterial problems

In addition to atherosclerotic disease, colour doppler can be useful in the diagnosis of carotid artery dissection, pseudoaneurysms, aortoarteritis etc. A combination of high resolution B-mode and specific flow characteristics clearly define the anatomy and morphology of these conditions.

A thin linear echogenic flap within the vessel lumen is seen in case of a dissection, while generalised uniform vessel wall thickening with marked reduction in the size of lumen is seen in aortoarteritis.

Colour flow imaging is particularly useful in identifying unusual abnormalities such as carotid body tumours, where a vascular mass is visualised at the bifurcation of ICA and ECA.

Distinguishing between a pseudoaneurysm and a highly tortuous carotid artery is also possible. Other neck masses like lymph nodes and thyroid nodules are often detected incidentally during carotid doppler.

Fig. 1 Internal carotid stenosis on colour

Fig. 2 Large plaques ICA causing flow channel narrowing.

Fig. 3 Normal vertebral artery and vein seen between transverse processes.

Fig. 4 Subclavian steal with reversal of flow in left vertebral artery

Fig. 5 Transtemporal window showing MCA and ACA

Fig. 6 Carotid cavernous fistula periocular venous channels showing pulsatile flow

Post surgical or interventional study

Duplex doppler is an excellent non invasive technique to study carotids after endarterectomy post angioplasty and after stenting. Restenosis can be picked up without using any invasive technique.

Vertebral Artery Doppler

Technique : The patient is positioned in the same manner as for the carotid study.

To locate the vertebral artery, the transducer is first placed on the common carotid artery longitudinally, and the image is shifted laterally until the shadows representing the transverse processes of the cervical vertebrae are identified. The vertebral artery is identified between the transverse processes. A vertebral vein may occasionally accompany the artery. Cephalad flow within the vertebral is confirmed by comparing the vertebral and carotid flow directions. The entire length of the vessel is surveyed as far as possible. Origin is studied carefully. The diameter of the normal appearing segment is measured and waveforms and peak systolic velocities are documented. Finally, the artery is scrutinized for any abnormal colour flow or high velocity jets.[20],[32]

Vertebral Arteries : Duplex Features

Colour Doppler demonstrates cephalad flow in the vertebrals throughout the cardiac cycle and a uniform flow pattern. The mean vertebral artery diameter is 4 mm, however, the arteries are assymmetric in size in 73% of normal individuals. In case of asymmetric sizes, the left is larger in 80% of cases.

Vertebral veins noted adjacent to the arteries are usually components of the vertebral venous plexus and not directly related to cerebral circulation.

Duplex doppler signals in the vertebral arteries exhibit low resistance characteristics identical to the internal carotid artery. Normal peak systolic velocities range from 20-40 cm/sec, and peak systolic velocities below 10 cm/sec are considered abnormal.[20],[31]

Abnormalities of vertebral arteries

1. Absence of flow : In a successfully imaged vertebral artery, absence of flow indicates occlusion of the vessel.

2. Nonvisualisation : Failure to identify the vertebral artery may indicate an occlusion. However, this finding should be very cautiously reported if at all.

3. Increased velocity : Peak systolic velocities 40 cm/sec with disturbance of flow in the adjascent downstream portion indicates a stenosis of atleast 50%.

4. Reduced velocity : Dampened waveforms within the vertebrals suggest stenosis at the arterial origin.

5. Vertebral steal phenomenon : Stenosis or occlusion of the subclavian artery proximal to the vertebral artery origin results in the vertebral to subclavian steal phenomenon. There is reversal of flow in the vertebral artery throughout the cardiac cycle. Sometimes the flow may be normal at rest or bi-directional i.e. forward in systole and reversed in diastole.[20],[31],[32]

TRANSCRANIAL DOPPLER

Introduction

Transcranial doppler ultrasound is an excellent way of studying the haemodynamics of intracranial vessels.

Over a period of time it has evolved rapidly and has become a complimentary tool to the study of extracranial carotid system. Transcranial Carotid Doppler was earlier done with the blind technique, assessing known windows and identifying the main vessels blindly. This was however time consuming and depended a lot on knowing direction of flow and depth of vessels. Now with better resolution machines and the advent of colour doppler it has become easier to identify the main vessels, sample them for the velocities; accuracy and reproducibility has increased.

Further, Power doppler is a useful adjuvant in identifying the main vessels.

Contrast studies will be useful in identifying transcranial vessels. As with any other doppler technique there is a definite learning curve, but once one gets used to the technique it is an excellent modality.[21]

Indications

1. To detect intracranial stenosis and occlusions.

2. To facilitate the detection and evaluation of collaterals capacity in patients with carotid occlusion.

3. It is also useful in studying gross abnormalities close to the circle of Willis like vascular malformation.

Technique and Anatomy

It is worth revising the basic anatomy before discussing the technique. The Internal Carotid Artery bifurcates into anterior cerebral artery (ACA) and middle cerebral artery (MCA). The Anterior communicating artery joins the two Anterior Cerebral arteries as they converge medially towards the interhemispheric fissure. The anterior communicating artery constitutes demarkation between the A1 and A2 segments of ACA. Only the A1 and at times the A2 segments of ACA can be identified by transcranial doppler.

The most proximal segment of MCA, M1 divides into two or three branches, M2 which courses into the convexity of the cerebral hemisphere. Often both, M1 and M2 can be visualized by ultrasound.[23,24] The vertebral artery in its last segment (v4) pierces the dura and inclines medially into the medulla oblongata where at the lower body of pons it unites with the contra lateral vertebral artery to form the basilar artery. The posterior inferior cerebellar artery is the largest branch of the vertebral artery. Before bifurcating into posterior communicating artery, the basilar artery (BA) has several branches like the anterior inferior and superior cerebellar artery. The basilar artery divides into two PCAs each of which is linked to ipsilateral ICA by the posterior communicating artery. The flow in the posterior communicating artery can be in any direction depending upon whether it is supplied by the ICA or the vertebrobasilar system, usually blue. The proximal segment of the PCA is the P1 segment and is well visualized on transcranial Doppler. However, the P2 segment courses posteriorly and superiorly and is not well visualised. The circle of Willis is an anastomosis between the vertebrobasilar system and the two ICAs is situated in the interpedicular and supracellar cisterns at the base of the skull. The collateral circulation of the brain is established primarily through the Circle of Willis. The terminal branches of the basilar artery the posterior carotid artery (PCA) constitute the circle of Willis together with the posterior communicating arteries, the anterior communicating artery and the A1 segment of the ACA. In the majority of patients the anatomic arrangement is not present and anomalies such as hypoplasia, aplasia, or atresia commonly affect anterior communicating and posterior communicating arteries. These variations may be of critical importance.[21]

Technique

It is essential to have a special probe (2 to 2.5 MHz) as well as necessary software in the machine.

The various parameters which allow us to identify vessels during transcranial doppler are, the depth at which the vessel is seen, direction of blood flow at that depth, the velocity, noting the probe position and direction of ultrasound beam, traceability of the vessel and response to carotid compression. Each vessel is individualized by colour doppler or by power angio. Once a vessel is identified, peak systolic velocities and end diastolic velocities are usually noted; also RI and PI can be calculated. The patient is placed in supine position; usually the examination begins at the temporal window. The first artery examined is the MCA, its M1, M2 positions identified. It is then traced in steps from shallow to deeper depths.

Transtemporal window

The tranducer is placed on the temporal bone just superior to the zygomatic arch and anterior to the targus of the ear. The probe can be directed at the anterior, middle and posterior landmarks. Usually there is an indicator, which comes on the image, which gives us an idea about the orientation of the probe. Before switching colour, it is worth noting some more landmarks; the commonest landmark in the identification is axial plane of the cerebral peduncles. Anterior approach identifies M1,M2, segments of middle cerebral artery. C1 segment of ICA and A1 segment of anterior cerebral artery. The anterior communicating artery is the most common collateral connection between the hemispheres and is important in CCA occlusion. The flow in the ipsilateral artery is inverted and can be demonstrated in colour coded transcranial doppler or pulsed Doppler. The patency of anterior communicating artery can be assessed by compression on the cervical carotid artery (compression test). Flow in the ipsilateral PCA, P1 can be seen. when the probe is directed posteriorly the P1 and P2 segment of PCA, the tip of basilar artery and posterior communicating arteries are perceived.[21]

Orbital window

Lot of jelly is put on transducer and kept gently on the closed eye. This allows us to visualise the ophthalmic artery and various segments of carotid siphon. Knowing the depth of the segment and the direction of flow, help in identifying the vessels. Ophthalmic artery is the most significant collateral between the external and internal carotid arteries in case of internal carotid artery occlusion, it can allow collateral flow to brain through facial and angular anastomosis with ECA. On transcranial colour doppler, flow direction appears reverse.[21]

Sub-occipital approach

This is usually done with the patient sitting. Basilar artery is imaged with transducer placed between the posterior margins of foramen magnum and the palpable spinal processes of first cervical vertebra and oriented towards the bridge of the nose.

Two parts of vertebral artery are identified with probe displaced laterally. Flow in all arteries is directed away from probe.[21]

Sub-mandibular approach

The head is slightly extended. Tranducer is placed below the angle of mandible directed medially and posteriorly. This allows assessment of the distal segments of the extra cranial ICA artery just before it enters the skull base.[21]

Clinical Applications

1. Vasospasm : Transcranial doppler is very useful in detecting and following up intracranial vasospasm in patients with subarachnoid haemorrhage. MCA velocities are typically accelerated and are more than 120-140 cm/sec. Velocities above 140 cm/sec are accompanied by delayed ischaemic deficits. This being a non-invasive and an easy technique, follow up is easy.[25]

2. Intracranial occlusion and stenosis : Transcranial doppler can reliably diagnose occlusions of main trunks of ACA and PCA. Cerebral vascular stenosis can be detected by demonstrating localised high velocities on colour and spectral wave form.[26,27]

3. Collateral : It is an excellent technique to study collateral circulation in patients with internal carotid occlusion.

4. Carotid cavernous fistulas are diagnosed and followed up. Prominent venous channels are seen with pulsatile (arterial like) flow.

5. Cerebral micro emboli : They can be detected by transcranial doppler as high intensity transient signals (HITS) in the MCA. Patients with high grade stenosis and dissection with micro emboli have more risk of cerebral infarction. Transcranial doppler can help identify subgroup of patients at highest risk of ischaemia during cerebral revascularisation or during carotid angioplasty with or without stenting.[28]

6. AV malformation and aneurysms close to circle of Willis can be diagnosed. Contrast enhancement is promising in picturing small aneurysms and AV malformation.

7. Cerebral tumours : There have been attempts to study tumour circulation and haemodynamics especially with contrast.[29]

Future

Transcranial doppler is one field where contrast media are going to play a significant role, this with harmonics and newer techniques like 3 dimensional reconstruction promise to give MRI angiography like pictures at patient’s bedside.

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REFERENCES

1. Clinical advisory : Carotid endarterectomy for patients with asymptomatic internal carotid stenosis. Stroke 1994; 25 : 2523-24.
2. North American Symptomatic Carotid Endarterectomy Trial Collaborators. Beneficial effect of carotid endarterectomy in symptomatic patients with high grade stenosis. N Eng Med 1991; 323 : 445-453.
3. European Carotid Surgery Trialists collaborative Group. MRC European carotid surgery trial- interim results for symptomatic patients with severe (70%-99%) and mild (0%—29%) carotid stenosis. Lancet 1991; 337 : 1235-1243
4. Carotid Doppler Sonography. E. Bluth. “Ultrasound in the new Millennium” AIUM Educational course.
5. Bluth E, Stavros A, Marik K, et al. Carotid Duplex Sonography. A multicentre recommendation for standardised imaging and Doppler. Criteria. Radiographics 1988; 487-506.
6. Fontanelle LJ, Simpes SC, Hanson TL. Carotid Duplex scan versus angiography in evaluation of carotid artery disease. American Journal of Surgery 1994; 60 : 864-868.
7. Freed, Brown, Carroll. The Intracranial Carotid vessels’. Diagnostic ultrasound. 2nd ed. Rumack, Wilson, Charboneau. Mosby.
8. Zwibel, William J. Cerebrovascular Doppler Applications. Clinical applications of doppler ultrasound. Editors : Taylor, Burns. Ravin Press.
9. Hennerici M, Rautenbery W, Trockel V, Kladezelky RG. Spontaneous progression and regression of small carotid atheroma. Lancet 1985; 2 : 1415-1419.
10. Polak JF, O’Leary DH, Kronmar RA, et al. Sonographic evaluation of carotid artery atherosclerosis in the elderly. Relationship of disease severity to stroke and transient ischemic attack. Radiology1993; 188 : 363-370
11. Veller MG, Fischer CM, Nicolaides AN, et al. Measurement of the ultrasonic Intima-media complex thickness in normal subjects. J Vasc Surg 1993; 17 : 719-725.
12. O’Leary DH, Polak JF. High resolution carotid Sonography-past, present and future. AJR 1989; 153 : 699-709.
13. Merritt CRB, Bluth EI. The future of carotid sonography. AJR 1992; 158 : 37-39.
14. Bluth E. Carotid Arteries - Preconvention Tutorial. Vascular Ultrasound. AIUM convention. 1998.
15. Hunink MGM, Polak KF, Barlan MM, et al. Detection and quantification of carotid artery stenosis : efficacy of various doppler velocity parameters. AJR 1993; 160 : 619-625.
16. Faught WE, Mattos MA, van Bemelen PS, et al. Color flow Duplex scanning of carotid Arteries : new velocity criteria based on receiver operator characteristic analysis for threshold stenoses used in the symptomatic and asymptomatic carotid trials. J Vasc Surg 1994;19:818-828
17. Impararates AM, Riles TS, Gorstein F. The carotid bifurcation plaque : Pathologic findings relating associated with cerebral ischemia. Stroke 1979; 3 : 238-245.
18. Knox RA, Philips DJ, Brestan PJ. Empirical findings relating volume size to diagnostic accuracy in pulsed Doppler cerebrovascular studies.
19. Middleton WD, Foley WD, Lawson TL. Flow reversal in the normal carotid bifurcation : Color Doppler flow imaging analysis. Radiology 167 : 207-210.
20. Tratting S, Schwaighoger B, Hiibsch P, et al. Color coded Doppler Sonography of vertebral arteries. J Ultrasound Med 1991, 10 : 221-226.
21. Khan Haleem G, Gailloud Phillip, Murphy Kieran. Anoverview of Transcranial Doppler : Ultrasound Quarterly. 14 (2) : 110-123.
22. Caplan CR, Brassl Dewitt, et al. Transcranial Doppler Ultrasound present status. Neurology 1990; 40 : 696-700.
23. Aslid R, Markwieder TM, Nosns H. Noninvasive Transcranial Doppler Ultrasound recording of flow velocity in basal cerebral arteries. 1982; 57 : 769-774.
24. Aaslid R. Transcranial Sonography. Vienna New York, Springer verlag. 1986.
25. Otis SM, Ringelstein EB. Transcranial Doppler Sonography. In : Zweibel WJ, et al. Introduction to vascular Ultrasonography. 3rd ed. Philadelphia : WB Saunders. 1992; 145-171.
26. Witerdink Feldmont Fusie KL, Bragoni M, Benavides JG. Transcranial Doppler Ultrasound Further reliably identifies severe internal Carotid artery stenosis. Stroke 1997; 28 : 133-136.
27. Kimura K, Hashimoto Hirano T. Uchinom. Random Diagnosis of MCA occlusion with TCD. Am J Neuroradiol 1996; 17 : 895-899.
28. Benichou H, Bergeron P. Carotid angioplasty and stenting will *prepro TCD monitoring be important? J Endovasc Surg 1996; 3 : 217-223.
29. Bogdah U, Frohlich T, Becker G, et al. Vascularisation of primary CNS tumors : detection with contrast enhanced TCD. Radiology 1994; 192 : 141-148.
30. Weinberger J, Marks SJJ, Gaul JJ, et al. Atherosclerotic plaque at the carotid artery bifurcation. Corelation of ultrasonographic imaging with morphology. J Ultrasound Med 1987; 6 : 363-366.
31. Zweibel WJ, Knighton R. Protocol for color-duplex examination of the carotid arteries. In : Zweibel WJ, ed. Introduction to vascular ultrasonography, 3rd ed. Philadelphia : WB Saunders. 1992; 95-104.
32. Zweibel WJ. Cerebrovascular doppler Applications. In : KJW Taylor, PN Burns, Wells, ed. Clinical applications of Doppler ultrasound; 2nd ed. Raven press. 1995; 109-131.
33. Steinke W, Kloetsch C, Hennerici M. Carotid artery disease assessed by color doppler flow imaging corelation with Standard Doppler Sonography and angiography. Am J Radiol 1990; 154 : 1061-1068.
    

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