Sonography of the carotid arteries. Ken U. Ekechukwu, MD, MPH, FACP.

What is Carotid artery?
An artery is a vessel that carries fresh blood from the heart to another part of the body, while a vein returns used blood to the heart to be pumped to the lungs for oxygenation. (The exceptions to this rule are the pulmonary artery, which carries impure blood from the heart to the lungs and the pulmonary veins, which return oxygenated blood from the lungs to the heart.)

Two arteries, one on each side of the neck, carry blood from the thoracic aorta to the head and neck region of the body. They are called the carotid arteries (from the Greek word, karotis, which means deep sleep). Somewhere around the angle of the jaw bone these arteries split into two: one branch, the smaller, feeds all the tissues of the face and most of the tissues of the neck, while the second and larger branch, entering through the base of the skull, participates in supplying blood to the brain. This portion of the brain’s blood supply is called the anterior circulation. Another pair of arteries, much smaller than the carotid arteries, arises from the arteries that carry blood into the upper extremities and supply the posterior circulation of the brain. These are called the vertebral arteries because they travel through holes in the wings of the vertebral bodies that make the spinal column of the neck.

What is Doppler ultrasound examination?
If you yell loud enough in an empty hall way or warehouse you will notice that after a pause the echo of your voice comes back to you. This is because the walls surrounding you reflect the sound waves back to you.  Did you know that when sound waves travel through media of different compositions like water, air, and solid they change their directions and are reflected, scattered or absorbed, while loose some energy?

Did you also know that there are sound waves called ultrasound (or ultrasonic) waves, whose frequencies of oscillation (number of back-and-forth movements in a given second) put them beyond perception by the human ears, though they behave in much the same way as the waves we hear? The Navy used knowledge of these principles of sound behavior in the Second World War to detect objects in the bowels of the sea. Today, we use them in examining practically all parts of the body in what is called ultrasonography (ultra- is Latin prefix that denotes “beyond”, sonus is Latin for sound, -graphia is Greek suffix derived from graphein, which means to write). In a sense we write or express on paper for the eyes to see the way these ultrasonic waves are transmitted through and reflected from the body tissues.

A small hand-held instrument, generally rectangular in configuration and called an ultrasound transducer, contains a small strip of crystal in one of its shorter ends which converts electrical energy into short bursts of ultrasonic waves of predetermined frequency. It then “listens” briefly for reflected echoes of these waves coming from tissues of the body and repeats the cycle. The transducer each time reconverts the reflected waves into electrical energy that a computer in the ultrasound machine then expresses on a screen as pixels of black, white, and varying degrees of gray: black representing complete absence of any returned waves as happens when all the dispatched waves pass through a simple cyst; white representing intense or complete reflection of the waves back to the transducer as happens when the waves hit an impenetrable solid structure like bone or calcium; and the varying levels of gray express the different intensities of the reflected waves as happens when the waves strike soft parts of the body.

Now, as you can know, the body is made of tissues of varying compositions which lie at varying depths from the transducer. Using mathematical equations which incorporate the speed of the waves through body tissues, the time lapse between the sending and receiving of the waves, and the difference in intensity between the sent and received waves, the computer graphically expresses on a screen an “image” of the examined part of the body for the eyes to see. This is gray-scale sonography.

Have you noticed that when an ambulance approaches you blaring its siren that the pitch of the siren rises in intensity as the ambulance approaches and gradually fades as it recedes from you? Well, Christian Johann Doppler, an Austrian physicist and mathematician, constructed a mathematical equation that describes this experience and we use this equation in building ultrasound concepts specifically designed to examine arteries and veins. This method of ultrasound examination of vessels is called Doppler ultrasonography in honor of the Austrian. Qualifications like color Doppler ultrasonography and pulsed Doppler ultrasonography are color-coded and graphic expressions of this mathematical equation, respectively. 

What happens is that we send ultrasonic waves of predetermined frequency from a transducer to examine moving red blood cells in the arteries and veins. A change occurs in the frequencies of the reflected waves governed, in part, by the angle at which the sound hits the blood cells and their directions of movement. A computer uses these differences in frequencies to describe the speed and direction of the blood cells through the vessels. Narrowing, occlusion, and abnormal dilation of arteries and veins affect the speed and direction of the red blood cells.

Why do I need carotid ultrasound examination?
Stroke is the third leading cause of death and a principal maimer in the western hemisphere as well as the rest of the world. One of its causes is disease of the carotid and vertebral arteries (see above). So we use this examination to screen or search for causes (s) of a stroke. Besides neurological deficits that constitute a stroke, other warning symptoms and signs of diseased carotid and vertebral arteries are dizziness, syncope, and near-syncope. By performing ultrasound examination of the carotid and vertebral arteries one can diagnose or suggest these problems. There is a good chance that one of such considerations must have prompted your doctor to recommend this examination for you.

What should I expect during carotid ultrasound examination?
Your only preparation for the examination is to show up for it. It is simple, painless, and quick. Gel will be placed on the skin over the artery in question to “bond” the transducer (the little instrument the technologist will place on your body) to your skin so that the sound waves can pass well across the skin into and from the transducer.  Most patients go home soon after the examination unless its findings rule otherwise, in which case some one will notify you immediately.