Many of you will be familiar with ultrasound if you have given birth or caused the birth of a child in the past 30 years. Ultrasound is a ubiquitous and crucial imaging modality in the modern radiology department. I have always thought “Ultrasound” could be a formidable new superhero, with the abilities to speak beyond the audible range of humans and gently heat soft tissues. Intrigued? Confused? Either way, I encourage you to keep reading, and all will be explained.
Ultrasound creates two-dimensional images, in real time, using high frequency sound waves far above the audible range of human hearing. As I write this, I wonder if dogs would be able to hear the frequencies emitted by an ultrasound probe (I later checked: they can hear above the human range, but nowhere near the diagnostic ultrasound range). The waves travel at high speed—the speed of sound—penetrating through and bouncing off the different types of tissue in your body directly below the ultrasound probe. Think of it as sonar inside your body.
A treatise on ultrasound physics is beyond the scope of what I would like to present here. Most radiologists understand the basics—enough to fully interpret ultrasound studies, and troubleshoot if necessary—but frankly, it is a rare and admirable radiologist who fully comprehends all the subtleties of this challenging topic. One might call what follows “The Cliffs Notes of Ultrasound Physics,” and I will leave a more detailed and informed discussion to those better versed in the topic.
In space, no one can hear you scream (also, ultrasound would not work)
You might recall from high school physics that acoustic waves—including sound waves and ultrasound—need to travel through matter, i.e., they cannot travel through a vacuum such as outer space (in contrast to X-rays). Thankfully, ultrasound departments are located on the Earth’s surface. But in order to understand how ultrasound waves can be used to create images, we do need a basic grasp of what is happening when they travel through matter, such as our bodies. Before we address this issue, let us define what exactly we mean by the word “ultrasound.”
Ultrasound refers to acoustic waves with a frequency above the range of human hearing, frequency being the number of waves that pass by a given point in a second. The upper range of human hearing is 20 kilohertz, which means 20,000 waves per second. If you think that is fast, modern diagnostic ultrasound ultrasound machines use waves of 2-15 megahertz (MHz), or 2,000,000-15,000,000 waves per second!
What? How? Why?!
First, why does the frequency of ultrasound need to be so high? In short: image resolution. For reasons beyond the scope of our discussion, smaller waves equal better image resolution. Frequency is inversely related to wavelength (the distance between waves) such that the higher the frequency, the shorter the wavelength; thus, ultrasound waves are very tiny.
How about the “how?” The magic of ultrasound occurs primarily within the ultrasound transducer, or probe, which is placed on a patient’s skin surface during the scan. Near the surface of the transducer is a crystal with a unique property: Pizoelectricity! While this may sound like something shouted by a snake oil salesman of old, rest assured it is real and important. A pizoelectric crystal is able to both create the ultrasound waves, and receive and interpret the echoes of the ultrasound waves after they are reflected off tissue. Thus, transducers containing this crystal can both “shout” ultrasound waves and then “listen for” the reflected waves, which ultimately form the ultrasound image.
About the size of a computer mouse, transducers come in several different shapes, lengths, and frequencies which relate to the optimal use of each type. Transducers are named by frequency (5Mhz, 12Mhz, etc.), and we need these different frequency probes because of a property of the waves called attenuation. Although the tiny size of ultrasound waves is beneficial in regard to image resolution, the tradeoff is that they cannot travel far though tissue—they are attenuated—compared to larger waves like audible sound. Even among ultrasound waves, there are significant differences in how far a given transducer can send waves into the body (and create the associated image). Lower frequency transducers (2-5MHz) are able to image more deeply than higher frequency transducers (12-15 MHz). Although higher frequency transducers cannot image as deeply, they have better resolution than lower frequency transducers. So, it is always a tradeoff between depth of imaging and resolution when choosing an appropriate ultrasound transducer for a given study.
During an ultrasound study, waves are sent out by the transducer—held against the skin surface—and travel through and reflect off different types of tissue in slightly different ways. That same transducer can listen for the ultrasound echoes coming back from the tissue, and this information is processed into an image. Organs and tissues are displayed in shades of gray based on internal structure and composition; the brightest tissues correspond to areas reflecting the most ultrasound waves, referred to as “echogenic” or “hyperechoic.” Darker tissues (reflecting the least waves) are “hypoechoic.”
One final physics concept which bears mentioning is doppler ultrasound. Have you ever noticed how a police siren or train whistle seems to change pitch as the vehicle moves toward you, increasing as it approaches and decreasing as it moves away? This phenomenon is a result of the doppler effect. Sound waves get more scrunched together as the object moves toward you, and more spaced apart as the object moves away. The same principle applies to ultrasound waves, only the moving objects are inside the human body, most commonly the red blood cells in flowing blood. Doppler ultrasound can be turned on with the push of a button, allowing measurement of the speed at which blood is moving inside a vessel, or the movement of structures in the heart—information that can help detect disease and other pathologic conditions.
Doppler ultrasound generates a graphic representation of blood flow (or other moving object), and also creates an audible whooshing sound approximating the character of the flow. You will, in a sense, “hear” the flow of your blood when the technologist turns on doppler. This effect is also what allows you to hear the beating of an unborn baby’s heart.
Why are you slopping gel all over me?
During your ultrasound examination, the flat or slightly curved transducer is placed on the skin, and a pleasantly warm gel is spread over the area of interest. Of course, there is a reason for this slimy bath. Ultrasound waves travel poorly through air; in fact, air effectively blocks our view of what lies beneath. Gel fills in the air gaps between the transducer and your skin, and allows ultrasound waves to reach the area of interest.
Gel dispensing duties—not to mention the critical task of obtaining relevant images—fall to the ultrasound technologist. This specially-trained, usually nice person will greet you pleasantly and lead you into the ultrasound suite, after which you will get your clothing out of the way of the ultrasound transducer and begin the scan. During the exam, the technologist will take pictures of the area of interest. If you look at the monitor, you will see real-time images of your underlying tissues. Occasionally, the technologist will freeze the picture and save images to show to the radiologist. Most ultrasound images are two-dimensional, with more recent technology allowing 3-D images (4-D is just a fancy term for a 3-D video clip). The ultrasound technologist may show some of your images to the radiologist before you leave the department. The radiologist then examines the images, and dictates a report for your doctor.
Babies and hearts and bellies, oh my
Ultrasound has several unique advantages which contribute to its popularity as an imaging modality. It is particularly suited to imaging certain parts of the body, and is an attractive choice in children and pregnant women due to its lack of ionizing radiation (ionizing radiation, produced by X-rays and CT scans, can increase the risk of cancer in higher doses, particularly in young children and fetuses). Because the ultrasound transducer is small and handheld, it can be variably angled and positioned on the skin to more fully assess an abnormality. Basic ultrasound machines can be very small and portable, and ultrasound is frequently used in remote or underserved parts of the world that otherwise have little access to advanced medical imaging.
Probably the most renowned type of ultrasound study is screening of the developing fetus in pregnant women. In the United States, this is routinely done in the second trimester to evaluate fetal size and anatomy, and identify abnormalities that may need further attention. Not infrequently, fetal ultrasound is also performed in the first trimester to ensure the fetus is alive and well, and in the third trimester to further assess fetal size and growth.
Other types of ultrasound studies include:
- Echocardiogram: ultrasound evaluation of the heart
- Arterial and Venous Doppler: evaluation of blood vessels for narrowing or the presence of blood clots
- Abdomen: evaluation of abdominal organs including the pancreas, liver, gallbladder, kidneys, and spleen
- Pelvis: evaluation of the uterus, ovaries, and urinary bladder
- Thyroid: evaluation of the thyroid and adjacent lymph nodes
- Breast: often used in conjunction with mammograms to evaluate breast abnormalities
There are quite a few more indications and body parts for which ultrasound works well, but the types of studies listed above are certainly the most common.
Although very safe overall, ultrasound can have demonstrable effects on the tissues being imaged. The main potential effect is the heating of tissue due to absorption of energy from the ultrasound beam. More rarely, ultrasound can create mechanical irritation, particularly in tissues containing a large amount of gas such as the lungs or intestines. Both of these potential side effects are significantly decreased by relatively short scan times. Ultrasound, when used following the appropriate regulations, appears to pose minimal risk of meaningful tissue injury to the patient.
A round of applause for ultrasound
Ultrasound is a fast, versatile method to evaluate many different conditions, and often gives the radiologist a different kind of look at the body part in question when compared to its main “competitors,” CT and MRI.
Do you think “Ultrasound” should be the next Marvel superhero? What other questions do you have about this article or ultrasound in general? Let me know in the comments below.