Ultrasound
Sound waves with a higher frequency than can be heard by the human ear. Ultrasound is used to examine many parts of the body as these waves can penetrate through our tissues (exceptions bone and air), and will be reflected from tissue interfaces (=where one tissue borders to another).
Ultrasound utilises high-frequency sound waves, which are reflected in specific ways by different tissues, normal or pathological, in the body. Ultrasound is mechanical high frequency longitudinal vibration of molecules, and differs from usual sound only by its frequency. It is not ionising and not harmful at the energy levels used for diagnostic purposes. The reflected sound (echo) is processed by a computer to produce a real-time image which is displayed on a screen instantly.
The principle for ultrasound, or ultrasonography, is the same as for underwater sonar or echo sounding. An apparatus sends an ultrasonic wave through the body at a speed of about 1,500 meters per second. At the interface between two types of tissue, the wave will be refracted or 'broken up', and part of the wave will be reflected back and detected by the apparatus. The rest of the ultrasonic wave continues deeper into the body, and is reflected as an echo from the surface of tissues lying further inside the body. How much is reflected depends on the densities of the respective tissues, and thus the speed of the sound wave as it passes through them. The time taken for the reflected wave to return indicates how deep the tissue lies within the body. In this way, one obtains a picture of the relative locations of the tissues in the body, in the same way that one may visualise the contours of a school of fish with a sonar.
Not all of the sound waves will be reflected by the first interface they encounter; quite a few of them will continue until they strike a new surface, etc. Using the same analogy, this explains why we can see stones lying at the bottom of a stream, because the water-stone interface reflects some of the light that has penetrated through the water-air interface. Some structures will also absorb the sound waves, so that in the end there is little or nothing left to reflect. In this way, a kind of "sound-shadow" is formed. Based on these properties, we can create a picture of what the structures inside the body look like.
The location of the various structures is calculated based on the speed of sound as it travels through tissue, which in almost all tissues (and water) is about 1500 m/s (300 m/s in air). The distance between the skin and the interfase can be calculated from the time it takes for a reflected sound wave (echo) to return to the ultrasound probe.This again constitutes the basis for building up an image.
The time a sound wave uses from when it is transmitted, encounters an interface, is reflected and returns to the acoustic source which both transmits and receives the signals, is divided by 2 (because the sound has travelled the distance twice, back and forth) and multiplied by the velocity. If something produces an echo after 0.0002 seconds has elapsed, then it is located 15 cm inside the body.
Ultrasound examinations are best suited for investigations of soft tissues. Ultrasonic waves penetrate poorly through air and bone, and ultrasonography is therefore not suitable for examining organs behind bony structures, such as the brain within the skull. Clear advantages of the technique, though, are that the equipment is small and easily portable, and that ultrasound produces 'living pictures', i.e. 'real-time' images.
Blood and other fluids reflect sound waves rather poorly, allowing them to pass through more or less unweakened (unattenuated) until they encounter the surface of something denser. For this reason, ultrasound has traditionally been used in examinations of the liver, kidneys, various abdominal organs and, last but not least, the heart.
An ultrasound examination is often done of the fetus in the eighteenth week of pregnancy.
GE Healthcare Glossary