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

What is lower limb?
Humans are bipeds, animals with 2 legs. We also have 2 arms. In medical terminology each of the legs and arms is called a limb or an extremity, the arms being upper limbs (or upper extremities) and the legs, the lower limbs (or lower extremities).

What are arteries of the lower limbs?
An artery is a vessel that carries fresh blood from the heart to other parts of the body, while a vein carries oxygenated blood to the heart to be pumped to the lungs for oxygenation. The aorta is the large artery that carries blood from the heart and distributes it to the rest of the body. The portion of the aorta in the chest is called the thoracic aorta and the portion in the abdomen or belly, the abdominal aorta. In the chest and the abdomen the aorta lies in the back, next to the spinal column. It is the largest artery in the body, measuring about 3 cm in the chest and 2.5 cm in the abdomen. These measurements vary for men and women.   Somewhere about the level of the belly button the abdominal aorta splits into two to become the common iliac arteries. These arteries carry on past the waist line and enter into the lower extremities, becoming known by different names as they pass through and branch in the thighs, the knees, the legs proper, until they arrive feet, much like some long major roads are known by different names as they pass through different cities and towns. (Now you see why city roads are sometimes called arterials.) The arteries that pass through the lower extremities are called lower limb (extremity) arteries.

What is Doppler ultrasound examination?
If you yell loud enough in an empty hall way or warehouse you will notice that the echo of your voice comes back to you. This is because the walls around 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 some loose 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 US Navy used knowledge of these principles of sound behavior to detect objects in the bowels of the sea in the Second World War. 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 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 represents complete absence of any returned waves as happens when all the dispatched waves pass through a simple cyst; white represents 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.

As you know, the body is made of tissues of varying compositions that lie at varying depths from the skin. 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 draws nearer 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 cells and their directions of movement. A computer uses these differences in frequencies to describe the speed and direction of the cells traveling through the arteries and veins. Narrowing, occlusion, and abnormal dilation of arteries and veins affect the speed and direction of the red blood cells.

Thus, arterial Doppler ultrasound of the lower extremities is a special ultrasound examination of the structure of the arteries of the lower limbs and the way the red blood cells flow through them.

Why do I need arterial Doppler ultrasound examination of my lower limbs?
Normally, the pressure of blood flow through the mid arm of an upper extremity equals the amount flowing across the ankle into the foot, all things being equal.

Diseases like diabetes mellitus, hypertension, and high cholesterol, especially when poorly controlled, and cigarette smoking damage arterial walls. In time, the damage translates from bulges in the walls of the arteries to complete occlusion, causing diminished or complete lack of flow, respectively. Normally, you don’t notice these events until the disease obstructs more than 50% of the diameter of the artery. This is when you may begin to feel cramps in your buttocks, thighs, or legs, initially when you are walking and, later, at rest. Some men may develop erectile dysfunction. As time goes on, a certain percentage of people with this problem will begin to notice changes in the color of the skin of their legs and feet and some may experience odd sensations too. Ultimately, if nothing is done, the skin may break down, wounds may not heal or delay doing so, and in extreme cases, gangrene sets in and the patient looses a limb. This spectrum of diseases is known as peripheral arterial disease to distinguish it from similar conditions that occur in arteries that feed the brain (carotid artery stenosis) and the heart (coronary artery disease). It includes obstructive disease of the arteries to the kidneys and intestines.

Arterial Doppler ultrasound allows us to screen for peripheral arterial disease without invading the body with needles, wires, and catheters and provides us with template information on which to build treatment strategies when the disease is present.

What should I expect at arterial Doppler ultrasound of my lower limbs?
Your only preparation is to show up for the study. It is simple, painless, quick, and useful. A 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 and the transducer. Most patients go home soon after the examination unless its findings suggest otherwise, in which case some one will notify you immediately.