Imagine sampling tissues and flow behavior in real time at a rate of 5 thousand a second

This is the power of ultrasound for stroke diagnosis and management. It’s also non-invasive, portable, and versatile. Yet it is a woefully underused modality in stroke medicine.

The second edition of Cerebrovascular Ultrasound in Stroke Prevention and Treatment 2nd Edition allows all involved in treating stroke patients to learn how ultrasound can enrich their ability to diagnose, evaluate, and treat stroke. It helps sonographers and physicians understand what information can be gained from ultrasound imaging generally, as well as what is needed for a specific case. The authors present a full overview of the "how"s and "why"s, including:

• The principles of intra- and extra-cranial ultrasound
• Hemodynamic principles
• The criteria of interpretation
• Ultrasound in neurocritical care
• Ultrasound in acute stroke

A final chapter on Neurosonology Pearls provides diagnostic waveforms, images, and practical wisdom on best-practice use of ultrasound for monitoring cerebrovascular disease.

Cerebrovascular Ultrasound in Stroke Prevention and Treatment 2nd Edition provides neurologists, radiologists, vascular medicine specialists, and sonographers with the basics for developing the skills needed for truly modern stroke diagnosis and care.

Titles of Related Interest
Intracranial Atherosclerosis
Edited by: Kim, Caplan and Wong; ISBN 978-1-4051-7822-8

Stroke: Practical Management, 3e
Warlow et al.; ISBN 978-1-4051-2766-0

A Primer on Stroke Prevention and Treatment: An overview based on AHA/ASA Guidelines
Edited by: Goldstein; ISBN 978-1-4051-8651-3

Edited by Andrei V Alexandrov, MD, University of Alabama Hospital, Birmingham, AL, USA.

Cerebrovascular Ultrasound in Stroke Prevention and Treatment 2nd Edition

John Wiley & Sons

Copyright © 2011 Blackwell Publishing Ltd.
All right reserved.
ISBN: 978-1-4051-9576-8

Chapter One

Principles of Extracranial Ultrasound Examination

Andrei V. Alexandrov, Alice Robinson-Vaughn, Clotilde Balucani & Marsha M. Neumyer

Introduction

A simple observation gives origins to clinical examinations, analysis and scientific exploration. Being able to observe Nature at work or in distress often offers clues that clinicians and scientists need to get an idea as to what could be going on and come up with a hypothesis to explain it. Ultrasound, with its unprecedented temporal resolution, is an elegant tool to probe living tissues. This first chapter describes how we evaluate the extracranial vasculature, then subsequent chapters continue the journey into cerebral vessels, hemodynamics and specific disease states.

Anatomy of the cerebrovascular arterial system

The choice of transducer placement and subsequent repositioning determines the success of visualizing the target structures and staying with the spatial course of the pre-cerebral vessels. Therefore, sonographers performing vascular ultrasound examinations must think "in 3-D," or three dimensions, about the vessel being investigated and put together transducer positioning with vessel intercept and further "go with the structure or flow" to complete scanning.

A sonographer should further imagine how this arterial segment would look on an angiogram. We strongly encourage those learning and interpreting ultrasound to be familiar with cerebral angiograms since angiography is the gold standard for the assessment of the accuracy of ultrasound testing, and ultrasound performance is often judged by vessel appearance on invasive or non-invasive angiograms. The following section deals with normal vascular anatomy and describes a standard protocol for carotid and vertebral duplex testing on the neck. More details of anatomy and angiographic images are provided in Chapter 3.

The common carotid artery

Scanning starts with a quick brightness-modulated (B-mode) surveillance in a transverse transducer position of the common carotid artery (CCA) up to its bifurcation. Color flow or power mode can be added to visualize flow in the vessel lumen (Figure 1.1).

On the right, the brachiocephalic trunk or innominate artery, arises from the aortic arch and then bifurcates into the subclavian artery and the CCA. On the left side, both the common carotid artery and the left subclavian artery usually originate directly from the aortic arch. The CCA is easily assessable on the neck where it runs in parallel with the jugular vein and the initial assessment starts right above the clavicle (Figure 1.2). It is further assessed in its mid-portion as the transducer slides cephalad over the sternocleido-mastoid muscle (Figure 1.3).

At approximately the level of the fourth vertebra, which is at the level of the upper border of the thyroid cartilage, the CCA bifurcates into the internal and external carotid arteries (Figure 1.4). The carotid bulb represents dilatation at the distal CCA extending into the proximal internal carotid artery. The carotid bulb bears unique flow patterns yielding a boundary separation zone and its wall has numerous baro-and chemo-receptors. The size and location of the carotid bulb are variable.

Most atherosclerotic disease occurs at the level of the bifurcation due to marked changes in vessel geometry resulting in increased shear stress, fluid stagnation and increased particle residence time on the postero-lateral wall of the bulb.

The internal carotid artery

Beyond the carotid bulb, the internal carotid artery (ICA) returns to a normal caliber (Figure 1.5) and courses in a relatively straight line up the neck into the skull to supply blood flow to the eye and brain. As a rule, the ICA has no branches within the neck. After entering the skull, the ICA makes an S-shaped curve in the region of the carotid siphon. The ICA entrance to the skull marks a vulnerable area where arterial wall dissections may occur due to fixed ICA position. The first major branch of the ICA is the ophthalmic artery that supplies the eye. After giving off the ophthalmic artery, the ICA divides into the middle cerebral artery (MCA) and the anterior cerebral artery (ACA), a part of the circle of Willis. This is covered in Chapters 2 and 3.

The external carotid artery

The extracranial carotid artery (ECA) supplies the muscles of the face, forehead and scalp (Figure 1.6). To switch between the ICA and ECA, a sonographer often must reposition the transducer from lateral to more medial angulation since both vessels are seldom visualized together in one plane for sufficient interrogation. The ECA has branches that could be visible while scanning its proximal segment. The ICA divides into eight branches on the neck. Several of these branches, i.e. ascending pharyngeal, facial, internal maxillary and superficial temporal arteries, communicate via anastomoses with the ICA. The occipital artery is the only ECA branch that communicates with the vertebral artery circulation. It is important to recognize these branches because the ECA fairly often becomes a collateral source for blood flow to the brain when the ICA is critically stenosed or occluded.

The vertebral artery

The vertebral arteries (VA) arise from the subclavian arteries and its origin can be found with transducer sliding towards the clavicle and aiming deep and lateral to the CCA (Figure 1.7). The VA passes medially in the neck to enter the bony canal at the C6 vertebrae and it further courses cephalad through the transverse processes of the vertebrae (Figure 1.8). It enters the base of the skull by looping around the atlas and ascending through the foramen magnum. At this point, the right and left vertebral arteries join together to form the basilar artery. The basilar artery terminates in the posterior cerebral arteries, which make up the posterior portion of the circle of Willis.

Components of ultrasound examination

Continuous wave (CW) Doppler

Although this technology is now regarded more of historic interest due to technological advances in imaging, knowledge of CW Doppler is required for board examinations and occasionally the skill of using the so-called "pencil probe" may be useful.

Dr Eugene Strandness and co-workers first reported the use of a transcutaneous flowmeter to evaluate occlusive arterial disease in 1966 [3]. Extracranial carotid and vertebral examinations with CW Doppler were reported by Drs Merrill Spencer and Michael von Reutern and colleagues in the 1970s.

Insonation with CW probe

With this technology, one crystal continuously emits the signal and another crystal continuously receives returned echoes, and this "non-imaging" transducer looks like a pencil (Figure 1.9). CW Doppler displays Doppler frequency shifts including maximum frequency trace without an artifact due there being no limitations related to pulse repetition frequency. Current CW systems can differentiate between positive and negative Doppler shifts and create a bi-directional spectral signal that shows flow direction as towards or away from the transducer. However, CW Doppler shows no information regarding the structure, i.e. image, or the depth from which the signals originated.

The advantage of this ultrasound test is its ability to display Doppler frequency shifts from moving objects without an artifact called aliasing. With the recent development of direct imaging and pulsed...