Ultrasound for medical therapy devices

Technical Investigation

Scientific Essay 2010 24 Pages

Medicine - Biomedical Engineering




A Introduction

B Basics of Ultrasound
B.1 Selected definitions of terms

C Basics of the Ultrasonic Therapy
C.1 Effects at and in the human tissue
C.2 The effect spectrum and indications

D Basics of the Piezoceramic Sensor
D.1 Historical excursion
D.2 Principles of functioning
D.3 Production and material
D.4 Forms used and their basic oscillation

E Medical therapy transducer

List of Sources

F List of Abbreviations

G List of Equations and Formula Symbols

H List of Tables

I List of Figures

J Bibliography

A Introduction

The ultrasonic therapy appertains to the procedures of the Physical Therapy.

It is a part of mechanical therapy, because ultrasound consists of acoustic waves (longitudinal, wavelike spreading of smallest pressure variations of a medium as for example air or liquids) with a high frequency.

In addition the ultrasonic therapy can be used as thermotherapy. The ultrasonic therapy is also, in the widest sense, a form of the electrotherapy. This justifies itself, because the ultrasound is being obtained from electric energy. The therapy form is used particularly with chronically degenerative illnesses of the human movement apparatus.

The background to this technical summary is based on the legally prescribed tests on technical medical devices. According to the (MPBetreibV) Medical Devices Directive1, ultrasonic therapy devices must be subjected to regular safety inspections (STK). For these safety inspections, the basics of the technical ultrasonic therapy should be known.

This abstract shall give a basic overview of the ultrasound and of the transfer chain; from the ultrasound to the human skin.

B Basics of Ultrasound

Ultrasound relates to material oscillations (periodic, successive pressure fluctuations in conductive media) beyond the upper limit of human hearing. Sound waves above between 20 kHz to 1 GHz are referred to as ultrasonic waves. Distinction of the sound according to the frequency range[1]:

-Infrasound < 16 Hz (not audible for humans, too low-frequency).
-Audible sound from16 Hz to 20 kHz (audible for humans).
-Ultrasound from 20 kHz to 1 GHz (not audible for humans, too high-frequency).
-Hypersound > 1 GHz (sound waves with only a limited propagation capability).

B.1 Selected definitions of terms

Longitudinal wave

A longitudinal wave is a pressure wave in which the direction of oscillation of the moving particles (molecules) is identical to the direction of propa- gation. So called pressure fluctuations or density variations are produced within an elastic medium. Sound waves in gases and liquids are always longitudinal waves[2].

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Fig. 01 Illustration of a longitudinal wave. Direction of oscillation and direction of propagation are similar

Transverse wave

A transverse wave is a physical wave in which the direction of motion of the oscillating particles (molecules) is different from that of the direction of propagation. The molecules oscillate perpendicular to the direction of propagation.

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Fig. 02 Illustration of a transverse wave. Direction of oscillation and direction of propagation are dissimilar

Sound wavelength

The wavelength is the shortest distance between two points in the same phase (identical alignment and direction of movement) of a wave. In the example depicted in Fig. 02, the distance between two neighbouring peaks.

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The frequency of the wave corresponds to the number of waves which pass a certain point per unit of time. In the case of oscillation, they are measured per second. Within the scope of ultrasonic therapy, the frequency of the sound plays a major role because the lower the frequency, the deeper the penetration into human tissue (and vice versa)[3].

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Sound velocity

Sound velocity relates to the propagation rate of the sound wave in a specific medium. The sound velocity is determined by the density of the medium being penetrated and its bulk modulus. The bulk modulus, together with the rigidity modulus and torsion modulus, is responsible for the elastic behaviour of a medium. The bulk modulus is a temperature and pressure dependent material constant and can be obtained from mathematical tables. The sound velocity increases with the rigidity of the material or medium (Table 01).

In the case of gases and liquids (no rigidity), the rigidity and torsion modulus can be set equal to zero and, as a result, no transverse waves can propagate but only longitudinal waves.

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Acoustic impedance

Acoustic impedance or characteristic acoustic impedance is also referred to as the resistance of a specific material to a sound wave. The characteristic acoustic impedance is the product of the density of the medium to be penetrated and the specific sound velocity.

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Table 01

Summary of the density, sound velocity and acoustic impedance of media [4,14]

Sound pressure

Sound produces longitudinal pressure waves in liquids and gases. The pressure wave exerts a certain force on a defined surface. An ultrasonic therapy probe sends a sound wave on a defined area of tissue. In this case, the ultrasonic wave (pressure fluctuations) exerts a specific force per area of tissue. This quotient is referred to as sound pressure.

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Ultrasonic sensor

An ultrasonic sensor is a component capable of detecting changes in physical magnitudes (e.g. pressure fluctuations). The sensor converts these changes in physical magnitudes to electrical signals and sends them to the relevant evaluation unit. Refer to Fig. 08 on Page 12.

Sound actuator

A sound actuator is the counterpart of the sound sensor. A sound actuator is a component which converts electronic signals to a mechanical action. Refer to Fig .09 on Page 13.


Coupling relates to the adaptation of various impedances or characteristic acoustic impedances to one another. Ultrasonics, here, is analogous to electrotechnics (internal resistance = load resistance = optimum impedance adaptation). A coupling medium is used (water or ultrasound gel) in an attempt to eliminate, or reduce as far as possible, transition resistance from the ultrasonic transducer to the tissue. If the acoustic impedance of the ultrasonic transducer was practically the same as the acoustic impedance of the tissue, the sound could be transferred to 100% in an ideal case.

A reflection of the ultrasound against bordering surfaces could, thus, be prevented. However, a 100% adaptation is not possible in practical applications.

C Basics of Ultrasonic Therapy

In order to implement ultrasonics for medical treatment, an ultrasonic therapy device is required. A wide range of such therapy devices is currently available on the international market. However, the structure of the basic modules used in these devices is always the same. In order to produce an ultrasound, one needs a high-frequency generator and an ultrasonic transducer with an integrated oscillating crystal (piezoelectric actuator). The high-frequency generator produces an alternating voltage which is transferred to the oscillating crystal. This alternating voltage causes the oscillating crystal to change its geometrical shape (due to the piezoelectric effect). The oscillating crystal oscillates, and sends sound waves in the ultrasonic range. To achieve a broader range of application possibilities, the oscillating crystal can be excited either continuously or pulsed. The corresponding operating mode can then be selected on the ultrasonic therapy device according to the indicators. It is worth observing, however, that the human tissue warms up more when continuous ultrasound is transmitted than in the case of pulsed sound transmission.

The output frequency of most therapy devices is restricted to 1 MHz and 3 MHz. These two frequencies and their respective modulations are sufficient to treat numerous medical conditions. Normally, each device is equipped with a separate ultrasonic button for each frequency.

In order to be able to emit the sound waves on the human body, a coupling medium is required for the transfer (coupling the ultrasonic transducer to the human tissue). In practice, ultrasound gel or water is generally used as the ultrasound coupling medium. Since the various media (ultrasonic transducer, air, tissue) conducts and absorbs the ultrasound differently, selection of the right coupling medium is of great importance.


1 Directive concerning the setting up, operation and use of medical devices (MPBetreibV)


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Ultrasonic transducer medical therapy piezoeffect



Title: Ultrasound for medical therapy devices