Ultrasound involves the use of sound waves to capture images of the body beneath the skin. Most people associate the medical use of ultrasound with a way of visualizing an unborn fetus in pregnancy. While the skull makes it difficult for these sound waves to be used to evaluate the brain directly, there are still several uses for ultrasound in neurology.
How Ultrasound Works
A probe placed on the head emits a high-frequency sound wave. This bounces off materials in the body, and the echo is received by the probe. This is often used to take pictures of different tissue densities. Trained technicians can find blood vessels and bones, for example, that would otherwise be difficult to identify.
However, ultrasound has another use as well. Due to the Doppler effect, in which the frequency of sound changes depending on the velocity of the source, the echo of the sound may have a different frequency that is related to the speed of the blood flow. For this reason, ultrasound can be a useful way of ensuring that blood is flowing in an expected manner through the body.
Transcranial Doppler (TCD) is a technique that uses sound waves to measure the velocity at which blood flows through the arteries of the brain. There are several uses of transcranial Doppler in neurology, including screening for vasospasm after a subarachnoid hemorrhage, looking for lack of blood flow in brain death, and evaluating the risk of stroke in sickle cell disease. Compared to other imaging methods, transcranial Doppler is inexpensive and portable, making it easy to use in doctors’ offices and hospital wards.
Although the skull blocks the sound waves needed for TCD, there are regions where the bone is very thin, through which the sound waves can be directed. An experienced technician may locate the blood flow just based on velocity measurements, though many people use a different mode of imaging in order to locate the desired blood vessel first. Overall, the test is painless and noninvasive.
The brain receives its blood supply from four arteries in the neck. Two vertebral arteries fuse into the basilar artery that supplies blood to the brainstem and the back of the brain, and the larger front portion of the brain receives blood from the internal carotid arteries that branch from the carotid arteries in the neck. If any of these arteries are narrowed or otherwise damaged, it can lead to ischemic stroke.
There are many ways of looking at these blood vessels, including conventional cerebral angiography, MR angiogram (MRA) and computed tomographic angiography. Duplex ultrasound is another frequently used method to evaluate blood flow through these blood vessels.
Advantages of ultrasound include relatively low cost and the easy portability of the needed equipment. Furthermore, ultrasound does not require the use of any kind of contrast agent, whereas most forms of angiography require a contrast in order to acquire the best possible image.
On the other hand, while ultrasound can give good information about the carotid arteries in the front of the neck, it may offer more limited information about the vertebral arteries in the back of the neck. This is because the vertebral arteries run through loops of bone that can block the sound waves from the ultrasound probe.
Carotid ultrasound depends a lot on the skill of the technician, and interpretations of the results may vary depending on the expertise of those involved. If abnormal results are found on ultrasound, it is probably a good idea to confirm those results with other imaging modalities before proceeding to vascular surgery or other invasive interventions. This is particularly true since carotid ultrasound may systematically overestimate the degree of arterial narrowing.
An echocardiogram is an ultrasound of the heart. This can be done by placing a probe on the chest, or more invasively by having a probe slipped into the patient’s esophagus. While more invasive, this leads to a better picture of parts of the heart that lay further away from the chest wall, including the aorta and left atrium.
It may seem unusual to discuss an image of the heart in an article devoted to neurology, but ultimately the division of brain and heart is somewhat artificial. The brain depends on the heart to receive blood flow. After a stroke, protocol requires that the heart is imaged to look for potential sources of clots that could have traveled up into the brain to stick in an artery and stop the blood supply to part of the brain.
In conclusion, there are several ways that ultrasound technology is used to evaluate patients with neurological disease, even though only one of those methods (transcranial doppler) looks directly at blood flow in the brain itself. Together with the physical exam and other techniques, ultrasound can help doctors better understand what’s going on under your skin and behind your skull.