Sending sound waves into the body and recording what was reflected was an idea that arose from wartime. Just as we had used sonar to detect submarines and other submerged objects, scientists wondered if sonar could be used to detect objects in the human body, which is an ocean of its own. There was another advantage: unlike x-rays, which are a form of radiation that we know is not completely harmless, sound waves have never been shown to cause significant damage, even to tender, growing fetuses.

What happens when a sound pulse is sent into the body? If you send a sound wave into the side of the head, the reflected beam is a line with three distinct peaks that form where the sound encounters something hard: the skull adjacent to where the beam starts; the other side of the skull at the far end; and in the middle is a prominent blip that turns out to be a midline structure in the brain, the falx. The falx is a fibrous septum that separates the right and left sides of the brain. The falx is not as hard as bone, but it is hard enough to deflect sound, so the falx can be seen between the two sides of the skull. That is, unless, of course, something in the brain is causing the midline septum to shift, like blood or a tumor. Such echoencephalograms were the mainstays for figuring what was going on in the skull as late as the 1960s. Ultrasound sampling of a single line across the brain is known as A-mode testing.

Suppose a device allowed you to sample multiple lines across the body. Doing an ultrasound of multiple lines is almost like putting a piece of paper over the face of a coin and drawing lines over it with a soft pencil. The picture develops line by line. If it's done right, you end up with the outline of some organs that aren't visible with conventional x-ray (without dye), such as the gallbladder (in an ultrasound of the upper abdomen). You might even get lucky and see a gallstone. The stone, which is solid, completely reflects the sound coming and has a characteristic appearance. Sampling multiple lines is known as B-mode testing, today's ultrasound technology. The first B-mode images were simple black or white pictures, with no shades of gray. There was either a line or no line, making them very hard to interpret. Gray-scale images were a huge step forward in the quality of ultrasound pictures. Instead of setting a threshold that determines where a dot or line is stuck onto a blank screen, each intensity of the reflected sound is assigned a gray-scale value. A very strong reflector looks white, while a very poor reflector looks almost black, and all the others are various shades of gray.

Another very important improvement was the development of real time ultrasound. Originally, the technologist had to move the B-mode transducer all over the body to get images of internal structures. Moving a transducer with the hand often produced a jerky, unreliable scan. Why not put a bunch of transducers together that fire at different times to get different pieces of information? This is ultrasound in motion, in real time. This is the technology that enables us to watch the truly astounding image of a fetus sucking his or her thumb.

How Ultrasound Works

It works with sound. The ultrasound technologist places a handheld transducer up against the part of the body to be examined. Mineral oil or acoustic jell, both of which should be heated, are spread on the body surface to provide a good seal between the transducer and the skin surface. The transducer sends a sound signal into the body that is transformed by whatever it comes into contact with. The reflected signal is processed and made into something that looks like a picture. If you have ever seen a real-time ultrasound of a fetus, you have probably been amazed at just how good a picture it is. But some things wreak havoc on a signal of sound, such as gas in the bowel.

Among countless other tasks, the image processor must assign shades of gray to the degree of reflected sound. Most ultrasound departments assign white to bright reflectors such as bone, gas, and stones, and black to the low reflectors that allow sound to pass fairly freely, like fluid in the gallbladder and urinary bladder. The result is that a gallbladder will look black and stones will look white, and a fetus in utero will look white floating in black amniotic fluid.

It is surprising when you think that there is so little color used in radiology. It has been said that color is distracting, and instead of improving information, it actually degrades it. There is one place that color has become invaluable, ultrasound. We can not only locate a structure in the body with sound waves, but we can also determine how fast and in which direction that structure is moving. When the sound hits a moving object, it is deflected differently, and by analyzing that difference we can get the computer to calculate all sorts of things about the motion of that structure. In ultrasound, movement is colorcoded. Flow toward the transducer is red and flow away, blue.

For most organs in the body that sit relatively motionless, flow information is not such a big deal. But for other structures, say the red cells in the blood flowing through arteries and veins, color-coded flow information is a big plus. In the neck, for example, we can see the blood flowing through the carotid arteries and can get all sorts of information about that flow, such as how fast the blood is flowing and how turbulent it is, all of which give us a remarkably accurate view of what is happening. Such flow information can be important in many places, including the head, neck, heart, liver, and just about everywhere in the abdomen and pelvis, the legs, and the arms.

When is ultrasound needed?

Ultrasound is valuable in many circumstances. There are situations where physicians can't get information in any other way, such as with pregnant women. The safety of the sound, coupled with the incredible images of the fetus it produces, makes ultrasound the test of choice.

For gallbladder disease, ultrasound is standard. CT and US work together in other parts of the abdomen. Ultrasound is still standard in the female pelvis. From the patient's perspective, if you have ever had a contrast venogram where the dye was injected into a vein in your foot, you will appreciate the value of the noninvasive ultrasound study that can make the same reliable diagnosis in a fraction of the time with no contrast and no needles.

Prostate biopsies are most often performed under ultrasound guidance.

Ultrasound is used to examine the blood flowing through the carotid artery in the neck, which nourishes the brain.

Ultrasound if often used as the first line of tests for appendicitis.

Risks and Potential Complications

There are no side effects to ultrasound, serious or otherwise.

What actually happens?

What actually happens during an ultrasound depends on the type of test you are going to have. One of the most common studies is the abdominal, or so-called gallbladder, ultrasound. To prepare for this test, the ultrasound technologist will explain the test and ask you a number of questions. You will be asked to climb up on an examining table after you have removed whatever clothing is necessary. Next will come the jelly; though sticky and slimy, it is a water-soluble substance. If it hasn't been warmed up, it will feel cold. Try to keep your clothing away from it; it will not stain, but it will feel cold and funny when you're back at work. The technologist will then scan by sliding the transducer over the areas of interest. Because there is so much gas on the left side of the abdomen, the technologist will usually focus on the right side where the liver offers a "window" into the abdomen. You will be asked to take deep breaths so that certain structures like the gallbladder come into view. This may go on for some time. If the technologist forgets to tell you to breathe, by all means take a breath.

When the test ends, can you ask the technologist about the result? Technically and probably legally… no. Remember, telling you the results is the responsibility of the radiologist. But if the test is absolutely negative and you are leaving right away for a trip to Timbuktu, why shouldn't the technologist tell? If the technologist says no, it's no. If you have a hard time contacting your radiologist, you should go to your primary care physician to find out the results.

There are a few extra wrinkles to the pelvic ultrasound. Whereas in the upper abdomen, the liver is the acoustic window; in the pelvis, it is the bladder. To see the pelvic structures, most departments first do a study over the lower abdomen with a dilated urinary bladder, meaning you have to have a full bladder. You also may have a transvaginal exam, which is done by inserting a probe into the vagina and scanning the pelvic structures. These transvaginal studies are very well tolerated, except in the young and very old, and they provide much more detailed information about the uterus and ovaries.

If you are going to have a Doppler study (an ultrasound that studies movement) for your carotid arteries or a Doppler study has been added to your abdomen and pelvis ultrasound, you will also probably hear the sound as well. The pitch of the sound tells the technologist a lot about the flow of blood in whatever structure is being imaged.

What is it like afterwards?

You should experience no significant side effects to ultrasonography.