Does the ultrasound therapy work

From inaudible sound to visible image - how does ultrasound work?

The first image that is taken of a person today is usually an ultrasound image in the womb. These images are important because doctors can use them to identify problems in the unborn child. Ultrasound also plays an important role in tumor prevention and when examining the thyroid, liver or heart. Even in emergency medicine, lightweight, portable ultrasound devices are increasingly being used.

Propagation of a sound wave

Noises and tones in ultrasound come about when the air pressure in a place fluctuates very quickly when a sound wave passes through it. With their vibrations, the air molecules move away from each other sometimes more, sometimes they are pressed closer together. The pressure changes between two values ​​several hundred or thousand times per second. This frequency is called frequency. The dial tone in the phone, for example, oscillates at a frequency of 425 times per second. Physicists specify the oscillations per second in Hertz. People hear sound between the bass tones from around 20 hertz and the highs at around 10,000 to 20,000 hertz, depending on age. Ultrasound is sound with a significantly higher frequency above the audible range.

In most other substances, too, molecules and atoms can vibrate like in air, so there are also sound waves in solids and liquids. Sound usually travels in all directions. In liquids and in air, sound is reflected when it hits something solid or viscous. This also applies to ultrasound: Medical ultrasound images are possible because sound signals in the human body are reflected and scattered by the organs.

Natural sonar

Animals such as dolphins and bats can emit and perceive ultrasound. Just as humans can estimate the distance of the rock from an echo in the mountains, the animals are able to "hear" the direction and distance of a school of fish or a rock slit. The distance is the result of a simple formula: You multiply half the travel time of the sound by the speed of sound, similar to how you derive the distance of a thunderstorm. Of course, mammals don't count off seconds and neither know the formula nor the speed of sound. As they grow up they develop something like an "ear measure" for their natural sonar. In contrast, in order to determine distances with artificial sonar - for example in the case of ships - seafarers rely on electronic support.

Reflection and scattering of ultrasound

Medical ultrasound signals also pass through electronic assemblies before they are displayed as an image. The actual ultrasound transmitter is usually located in a handpiece, the so-called transducer head, which the doctor holds up to the part of the body to be examined. Ultrasound impulses are generated in the transducer from electrical impulses, which are conducted into the body via a contact gel on the skin and spread there. Different structures in the body reflect the original ultrasonic pulse to different degrees and in different ways, depending on their size.

Where the impulses meet interfaces, they are reflected. The closer an organ is to the transmitter, the earlier the pulses reflected by it reach the transducer again. This also serves as a receiver (detector) for the echoes, i.e. the reflected and scattered ultrasonic signals. The transmitter and receiver mostly work according to the same principle: They are based on the piezo effect, in which small movements of crystals are converted into electrical voltages or vice versa (see box “Piezo effect”).

Reflection of ultrasound on organs

Picture gallery: From the ultrasonic measurement to the finished picture

The detector records when reflected signals arrive and how strong they are. After the transmitter has emitted a uniform impulse, the detector receives changed shapes again. These signals each come from a strip-shaped section of the body directly below the transmitter. Since these signals are converted into electrical voltages in the detector, they can be recorded in a voltage-time diagram. In order to make the signal visible, it is changed: First it is rectified, i.e. all negative voltages are converted into positive ones. Then the signal is smoothed, voltage peaks lying close together are connected with one another. The signal converted in this way can finally be displayed as a strip with light and dark sections.

For the complete picture, a hundred or more such strips are usually displayed next to one another. In the transmitting and receiving part there is not only one transmitter and detector, but several next to each other. They are activated one after the other at short intervals of fractions of a second. The multitude of strips thus obtained ultimately results in the ultrasound image on the monitor.