Ultrasound is very useful for imaging structures in the body. Ultrasound is a nonionizing form of energy that propagates through a medium as pressure waves. If you could measure the very small pressure disturbance when an ultrasound wave travels by, you would find it fluctuates rapidly about the ambient, normal background pressure until the wave is past. One way we characterize a sound wave is by its frequency, that is, the number of oscillations or fluctuations per second in the medium. Audible sounds have frequencies between about 15 cycles per second (15 hertz, Hz) and 20,000 cycles per second (20 kHz). The upper limit of human hearing is usually taken to be 20 kHz, and ultrasound refers to sound waves whose frequency is above this level. Other mammals are not nearly as limited as man in terms of the useful sonic frequency range. For examples, bats and dolphins utilize ultrasound waves that have frequencies as high as 125 kHz for navigating and sonar visualization. Medical ultrasound applications have been described that use frequencies from as low as 500 kHz to as high as 30 MHz. For most imagiing applications, ultrasound devices operate between 3.5 MHz and about 10 MHz. The exception is for intravascular imagers, tiny catheter tipped probes operating at frequencies as high as 30 MHz. The optimal ultrasound frequency for any application represesents a tradeoff between a) the need to acquire ultrasound images with a high degree of spatial resolution, dictating use of higher frequencies, and b) the need to obtain adequate "penetration" in the tissue. Imaging depth into tissue is limited by attenuation of the ultrasound waves, and this becomes more severe as the ultrasound frequency is increased. The most commonly used modality in medical ultrasound is called B-mode imaging. An ultrasound transducer is placed against the patients skin surface, directly over the region to be imaged. The transducer sends a very brief pulse of ultrasound into the tissue. The pulse travels along a beam, very much like the beam of a search light at an airport. Interfaces along the way reflect some of the ultrasound energy back to the transducer. The transducer, in turn, converts the reflected energy into echo signals, which are sent into amplifiers and signal processing circuits inside the imaging machine's hardware. The exact, microsecond delay between when the transducer first launched the ultrasound pulse and when it picked up an echo tells the machine how far the reflecting interface is from the transducer. After all the echoes are picked up from along the first beam, the transducer sends a second pulse along a slightly different beam direction into the tissue. Echoes are picked up the same way and sent into the machine hardware. Then another pulse is launched in still a different direction, and so forth. Very much like the beam of a search light is swept across the night sky, the pulsed beam from an ultrasound transducer is swept throughout the body, mapping out reflectors and other interfaces and forming 2-dimensional images. Beams are swept and ultrasound images are formed very rapidly, essentially in "real-time. As the operator holds the transducer in contact with the skin, the image appears live on a video monitor. By moving and manipulating the transducer different internal views are provided. The images represent cross-sectional, or tomographic views of the plane that the beam was swept across in. Ultrasound provides images of any region in the body where there is a soft tissue path between the probe - or ultrasound transducer - and the region of interest. In abdominal imaging, for example, with the transducer placed on the skin surface beneath the rib cage the sonographer can view the liver, blood vessels inside the liver, aorta, kidneys, pancreas and spleen. With special oral preps, the stomach and even the intestines can be viewed. Diffuse disease conditions, such as cirrhosis and fatty infiltrated liver, focal disease, such as cancerous tumors and vessel abnormalities are detected. Other common ultrasound exampination areas include the heart, the pelvis, the neck and the arms and legs. In some applications it is advantageous to utilize intra cavitary transducers for close up views of the uterus, the ovaries, the prostate and the colon. More to come
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