Technical procedure of an Ultrasound measuring instrument for the power measurement at medical therapy devices

Introduction

A Introduction

Basics

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

Material and Methodology

E Investigation of Ultrasonic Therapy Devices
E.1. Results of the investigation

F Series of Experiments to Find Characteristics
F.1 “Voltage” test setup”
F.1.1 Series of voltage-measurements
F.1.2 Evaluation and discussion of the series of experiments
F.2 “Temperature” measurement setup
F.2.1 Series of temperature measurements
F.2.1.1 Results of the series of temperature measurements
F.2.2 Thermography of the ultrasonic transducer
F.2.2.1 Results of the thermography

Final Conclusions

G Final Conclusions
G.1 Future prospects of a handheld measuring instrument

List of Sources

H List of Abbreviations

I List of Equations and Formula Symbols

J List of Figures

K List of Tables and Graphs

L Bibliography

M Measuring Instruments and Test Equipment

Appendix
N.1 The Frequency-Wavelength Spectrum
N.2 Piezoceramic Oscillator Datasheet
N.3 Ultrasonic Therapy Device Datasheet
N.4 Extract from the Medical Devices Directive

A Introduction

This document is concerned with examining whether sound output levels from ultrasonic therapy devices, in particular their ultrasonic transducers, can be determined by means of piezoceramic sensors.

The objective of this feasibility study is to compile basic information in respect of a measuring instrument, envisaged for the future, which should take the form of a hand-held measuring unit and be capable of measuring the ultrasonic output from the transducers of various ultrasonic therapy devices.

The background to this examination 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.

These regular control measurements are taken on the respective medical devices at certain intervals by personnel trained in respect of medical equipment. The deadlines for the tests are prescribed by the device manufacturers and vary between intervals of 12 to 24 months. The aim of regular tests is to detect any faults on the therapy devices at an early stage and prevent any consequential injuries to patients and/or device operator.

Ultrasonic therapy devices are not only subjected to a function test and electrical safety checks but the output parameters and ultrasonic output at the transducers is measured and determined.

At present, the output level is completed using an extensive gravimetric procedure which, in practice, is relatively inconvenient to use. In addition, the purchase of a gravimetric measuring instrument involves high costs.

This study aims to examine whether piezoceramic sensors could be used to realise a measuring procedure which can be implemented to control ultrasonic output levels of ultrasonic therapy devices in everyday testing processes.

B Basics of Ultrasounf

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 propa­gation. 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 oscillat]ion 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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- Sound wavelength

The wavelength A 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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- Sound frequency

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

- 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

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 (also refer to Page 17 to 18). 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.

C.1 Effects on and in the human tissue

Ultrasonic therapy produces different effects on the human body and in human tissue. In general, a distinction is made between two effects:

- Mechanical effects

The ultrasound output or ultrasonic waves applied to human tissue produce contractions and expansions in the elastic human tissue using the same frequency (1 MHz or 3 MHz). This effect is also referred to as a micro­massage. The micromassage causes a volume change in cells, increases cell membrane penetrability and, thus, an increase in the exchange of metabolic products. Blood circulation is aided and accelerated by a micro­massage, thus achieving better “sustenance” of the tissue.

- Thermal effects

The thermal effects (warming of body tissue) are a result of the various characteristics (density, fat and water content) of the tissue types. The temperature increase is approximately proportional to the density and specific capacity of the tissue.

Together with the absorption coefficient ɑ², the sound energy radiated into the tissue is the decisive parameter with regard to the level of warming of the tissue [6]. The greatest thermal effect occurs at the boundary layer of various tissue structures due their differing impedance (resistance exerted on the sound wave). The higher the absorption coefficient, the greater the warming effect in the tissue.

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Table 02

Absorption coefficients depending on medium and frequency

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Table 03

Absorption coefficients depending on medium and frequency

The third effect of ultrasonic therapy should be mentioned at this point, the so called “physiological” effect.

This effect results from the two above effects and cannot be unequivocally defined or assigned. Even the two interdependent effects described above in conjunction with human tissue cannot always be observed and treated separately. Usually, the interaction of the two mechanisms plays a decisive role in the therapeutic effects of ultrasound treatment.

The application of ultrasound in treating the human body is mainly described by its frequency, modulation curve and output power.

Information on indications and treatment parameters or treatment period is generally provided in the respective device operating manuals enclosed with the various ultrasonic therapy devices. They also recommend device-specific settings for the therapeutic treatment indicated.

C.2 The effect spectrum and indications

Ultrasonic therapy is a technical, medical therapeutic procedure which is usually implemented for heat and stimulation applications locally or regionally. Due to the effects described above (Pages 7 to 8), a whole range of positive, healing effects can be provided to the human body.

Effects:

- Pain relief
- Stimulates blood circulation
- Releasing tissue adhesions
- Warming tissue areas
- Relaxing muscles
- Supporting healing of fractures. [9]

The above list only reflects a selection of effects on human tissue and does not claim to represent a complete list.

An ultrasonic therapy application can be considered appropriate for the following patterns and courses of illnesses.

Indications ³ :

- Muscle tension
- Arthrosis of the knee and hip
- Scar tissue adhesion
- Inflammation and strain of tendons
- Tennis and/or golfer’s elbow. [10]

The above list only reflects a selection of frequent indicators and does not claim to represent a complete list.

Pictures of example applications of ultrasonic therapy:

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Fig. 03

Application of ultrasound on human tissue with illustration of the layers of skin

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Fig. 04

The picture shows the application of ultrasonic therapy on a knee to treat gonarthrosis

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Fig. 05

The picture shows the application of ultrasonic therapy on an elbow to treat tennis or golfer’s elbow

D Basics of the Piezoceramic Sensor

D.1 Historical excursion

The piezoelectric effect was discovered by the French physicists Pierre and Jacques Curie in 1880. The two brothers experimented with natural crystals (salt, tourmaline, quartz) and discovered that when certain crystals are mechanically deformed, they are electrically charged or polarised.

They also found that by applying an electric current to a crystal, it deformed. This reciprocal effect was described as the inverse piezoelectric effect.

The discoveries of the French physicists formed the basis for the sonar, developed in around 1940 for navy vessels to locate submarines.

The breakthrough in piezotechnology was made by Russian and American scientists in the years leading up to the 1950s as they succeeded synthesising the first PZT (plumbum-zirconate-titanate) compounds.

These compounds remain the dominating materials for piezotechnology thanks to their excellent properties [11].

The discoverers of the piezoelectric effect

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D.2 Principles of functioning

Oscillating crystals (piezoceramic components) are used to produce ultrasound for medical applications. Oscillating crystals can be made of various materials, such as quartz, barium titanate and lead-zirconate-titanate. When piezoelectric materials are mechanically deformed by an external force, electric charges are produced on their surfaces due to the change of polarization. The greater the deformation or application of force, the greater the voltage which can be tapped. The generation of electrical charges related to deformation is referred to as the direct piezoelectric effect.

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Fig. 08

Illustration of an oscillating crystal (piezoelectric disc) on which a mechanical force is exerted. The mechanical deformation energy is converted to electrical energy (separation of charge) which can be measured This effect is also reversible. If, for example, an alternating voltage is applied to an oscillating crystal, the geometrical shape of the material changes as a result (stretching, compression). These changes of shape can cause pressure or sound waves in the ultrasonic range, whereby the alternating pressure is dependent on the alternating voltage applied, the frequency of the alternating voltage and the geometrical force of the oscillator. This process is referred to as the inverse piezoelectric effect.

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Fig. 09

Illustration of an oscillating crystal (piezoelectric disc) and its alternating alignment (pressure waves) when an alternating voltage is applied

D.3 Production and material

The production of piezoceramic sensors is heavily dependent on the use for which the sensors are intended. However, the typical sequence of processes for the production of piezoceramics is similar for all types. Production always begins with the selection and mixture of the basic materials (normally on a lead-zirconate- titanate basis). The mixture is then heated to approx. 800°C to 900 °C. This heating or firing process is also referred to as calcination. After calcination, the mixture, made up of various ingredients, is ground. This converts the mixture into fine, powdery particles, necessary for subsequent processing.

The powder is then pressed to produce the rough form required. Since the blanks cannot yet be exposed to any mechanical loads, they must be “baked” at approx. 1000°C to 1300°C. This process is also referred to as sintering. The polycrystalline ceramic structure is formed during the course of sintering. After sintering, the ceramics are set to their final form by means of a mechanical process. During this working process, the ceramics are sawn, ground and polished. When the ceramics have been brought to their final shape, the final working process is performed, whereby the crystals are polarized. During polarization, the dipole in the material is aligned by applying a direct current electrical field of approx. 2000 V to 3000 V.

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¹ Directive concerning the setting up, operation and use of medical devices (MPBetreibV); excerpts in Appendix N.4

² ɑ specifies the extent to which a physical magnitude is absorbed in a damping medium

³ Application of a specific medical measure for a specific cluster of symtoms