Ultrasound examination can do more than visualize sucking babies in the womb. It allows the assessment of organs, tissues, joints, soft tissues and blood vessels, is inexpensive, painless and, according to current knowledge, does not stress the human body.
The development of ultrasound
Ultrasound exists in nature – animals such as bats generate it themselves and use it to orient themselves in space. Humans began using it in the early 20th century, first to detect icebergs and submarines underwater, and later to test materials for integrity.
Attempts to use ultrasound for therapeutic purposes followed in the 1930s and 1940s. In 1938, the physician Dussik came up with the idea of using ultrasound for diagnostic purposes, but he tried it on the brain, of all things. This was not a good idea, since the brain – except in infants – is completely surrounded by bones through which sound cannot penetrate.
In 1950, it was possible to image organs: the patient to be examined was placed in a vat of water, and the transducer was mounted on a motorized wooden rail – a method that proved only partially suitable for use on patients.
In 1958, the gynecologist Donald succeeded for the first time in obtaining images with an ultrasound device in which the transducer was placed directly on the patient’s skin and moved by hand. A principle that has been continuously developed since then, and since the 1980s (and the availability of powerful computers) has allowed the wide diagnostic application of sonography.
How does sonography work?
Ultrasound has a frequency of 20 kHz-1GHz, which humans cannot hear. With a sonography device, such sound waves are generated in a probe (transducer) and emitted in a directed manner. When they hit structures, they are reflected and scattered.
This so-called echogenicity varies depending on the type of tissue – it is low for liquids such as blood and urine, and high for bones and air, e.g. intestinal gases. The extent of the reflection is measured by the probe, converted into electrical pulses and displayed on a screen as gray values: Liquids appear black, bones very bright, organ tissues are in between.
To prevent the first sound waves from being deflected by the air between the skin and the transducer before they even reach the structures to be imaged, a gel containing water is applied to the skin. In the meantime, very fine imaging of the tissues has become possible with high resolution and, since recently, even as a 3-D image.
In addition, the Doppler effect is used: The frequency of the echo depends on the distance of the structure from the transducer, which makes it possible, for example, to visualize the flow velocity of the blood (whose solid components move either toward or away from the transducer).
