Overview of Ultrasound
Ultrasound uses sound waves to produce images of internal structures. Unlike x-rays, which are a form of radiation and are not completely harmless, sound waves are not thought to cause any adverse effects.
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; and
- in the middle, a midline structure in the brain, called 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.
This test, called an echoencephalogram, was the mainstay 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). A gallstone may also be visible. The stone, which is solid, completely reflects the sound coming and has a characteristic appearance. Sampling multiple lines is known as B-mode testing. This is the ultrasound technology used today.
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.
Ultrasound works with sound waves. 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.
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 are able to not only locate a structure in the body with sound waves, but 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 are relatively motionless, flow information is not such a big deal. But for other structures (e.g., 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 information about that flow, such as how fast the blood is flowing and how turbulent it is, providing a remarkably accurate view of what is happening. Such flow information can be important in many places, including the head, neck, heart, liver, abdomen, pelvis, legs, and arms.