Ultrasound is a specialized diagnostic procedure which allows clinicians to visualize internal body parts or structures. What is the science behind this fascinating tool? Just as a violin string vibrates to produce sound, a solid ceramic “crystal” inside a protected casing (the probe) is set to vibrate by an electrical signal. The frequency, or pitch, is higher than humans can recognize. These sound waves travel outward and are reflected back as echoes, then being “heard” by the same probe which released them. Computer-based processing allows the sending and receiving of signals from multiple sites on the crystal to occur so rapidly that use of the entire array to form a picture makes it almost instantaneous. Thus, “real-time” movement is observed as the image is updated at a rate of many times per second. The two-dimensional image is created because, just like in fish-finding sonar, the length of time it takes for the sound to travel to and from the surface reflecting the echo indicates the depth. The returned echo is translated electronically into digital information, which can be processed further. What one sees is that a very dense structure will create a strong echo which appears white. Full transmission of the sound waves (no echo) will leave the image black. A spectrum of gray indicates the range of densities and the whole result is like an artist’s canvas and shows a “slice” of whatever the target tissue is.
The quality of ultrasound has improved dramatically since the first two-dimensional images in the mid-1970s. The electronics, computerized processing, speed, fine detail, and the display of motion using graphs and color have made it not only easier to perform but also widened the applications. Not content to settle for only one two-dimensional picture at a time, the acquisition of the same type of sound echo information from multiple adjacent “slices” forms a volume of digital data. These data can be selectively processed to reveal any chosen slice or the surface of a solid structure inside water, such as the fetal face inside amniotic fluid.
Contrast between water (fluid), which transmits sound waves, and dense tissue, which reflects sound waves, provides the clearest images. Ultrasound is thus most useful in examining structures where that difference is present, such as unborn babies in amniotic fluid, heart and vessels containing blood, joints with fluid, gallbladders with bile, kidneys and the collecting system containing urine, and the like. Where such contrast does not naturally exist, it has been specially introduced, such as the infusion of saline into the nonpregnant uterine cavity to perform sonohysterography.
A variety of probe sizes, shapes, and sound wave frequencies are available to “see” at shallow and deep levels or inside body cavities. One example is the intravaginal probe, which provides good views of early pregnancy and the nonpregnant uterus. Due to probe technology, a full bladder is rarely needed anymore for obstetrical or gynecologic exams and in fact may hinder or mislead the examiner.
In obstetrics, the number of babies, their sizes, their organ structures including brain, the shape of their limbs and bones, the appearance of the face and other features are eagerly sought by prospective parents. From such things, we postulate due date, adequacy of growth, and probability of being “normal.” Also important to the medical caregivers is information about the baby’s environment: placenta, amniotic fluid, uterus, and cervix.
Such things may imply an increased risk later on in the pregnancy. Sometimes we can be virtually certain about what these observations mean; at other times we must consider a range of possibilities with no answer expected during the pregnancy. To accomplish further testing or even treat a few conditions, ultrasound can be used to guide placement into the uterus of a needle, catheter, or fiber-optic scope. As pregnancy advances, observations of fetal movement, blood flowing to the placenta through the umbilical cord, and blood distribution to fetal brain or other organs may reassure or conversely reveal a problem indicating whether delivery or some other action on behalf of the baby is needed. One big caution bears repeating: Fetal weight estimated from ultrasound measurements in late pregnancy should be taken with a big grain of salt.
Although obstetrics was the first use to rapidly grow, ultrasound has become essential to gynecology as well as cardiology and almost all areas of medicine. Abnormal vaginal bleeding, pelvic pain, a mass or large lump felt on physical examination, possible cancer, fibroids, and many other gynecologic concerns can be clarified by looking with ultrasound.
Ultrasound examinations are rarely uncomfortable. Patients are often examined while on their backs, and a change of position is used either to see better in a specific way or to assist in patient comfort. No harmful effects have been observed in the many years of ultrasound use, even heavy use during a complicated pregnancy. Based on research, for example, effects on tissue culture, the possibility of biological effects has not been completely ruled out, however, particularly with extensive use in early pregnancy. Thus, casually looking at an unborn baby just for fun is not recommended.