Ultrasonics - Power measurement - High intensity therapeutic ultrasound (HITU) transducers and systems

IEC 62555:2013 establishes general principles relevant to HITU fields for the use of radiation force balances in which an obstacle (target) intercepts the sound field to be measured; specifies a calorimetric method of determining the total emitted acoustic power of ultrasonic transducers based on the measurement of thermal expansion of a fluid-filled target; specifies requirements related to the statement of electrical power characteristics of ultrasonic transducers; provides guidance related to the avoidance of acoustic cavitation during measurement; provides guidance related to the measurement of HITU transducers of different construction and geometry, including collimated, diverging and convergent transducers, and multi-element transducers; provides guidance on the choice of the most appropriate measurement method; and provides information on assessment of overall measurement uncertainties. This International Standard is applicable to the measurement of ultrasonic power generated by HITU equipment up to 500 W in the frequency range from 0,5 MHz to 5 MHz. HITU equipment may generate convergent, collimated or divergent fields. For frequencies less than 500 kHz, no validations exist and the user should assess the uncertainties of the power measurement and measurement system at the frequencies of operation. This International Standard does not apply to ultrasound equipment used for physiotherapy, for lithotripsy for general pain relief.

Ultrasons - Mesurage de puissance - Transducteurs et systèmes ultrasonores thérapeutiques de haute intensité (HITU)

La IEC 62555:2013 établit les principes généraux, applicables aux champs HITU, permettant d'utiliser les balances de forces de rayonnement dans lesquelles un obstacle (cible) intercepte le champ acoustique à mesurer; spécifie une méthode calorimétrique de détermination de la puissance acoustique totale émise par des transducteurs ultrasonores, en se fondant sur le mesurage de la dilatation thermique d'un fluide cible; spécifie les exigences relatives à la déclaration des caractéristiques de la puissance électrique des transducteurs ultrasonores; fournit des recommandations permettant d'éviter la cavitation acoustique au cours du mesurage; donne des lignes directrices pour le mesurage des transducteurs HITU de différentes constructions et géométries, y compris les transducteurs non focalisants, divergents et focalisants ainsi que les transducteurs à éléments multiples; fournit des recommandations quant au choix de la méthode de mesurage la plus appropriée; et fournit des informations concernant l'évaluation des incertitudes de mesure globales. La présente Norme Internationale s'applique au mesurage de la puissance ultrasonore générée par des appareils HITU, atteignant jusqu'à 500 W, dans la gamme des fréquences de 0,5 MHz à 5 MHz. Les appareils HITU peuvent générer des champs focalisés, non focalisés ou divergents. Il n'existe aucune validation pour les fréquences inférieures à 500 kHz. Il convient que l'utilisateur évalue les incertitudes de mesure de la puissance et du système de mesure aux fréquences de fonctionnement utilisées. La présente Norme internationale ne s'applique pas aux appareils à ultrasons utilisés pour la physiothérapie, pour la lithotritie ou pour soulager la douleur de manière générale.

General Information

Status
Published
Publication Date
18-Nov-2013
Technical Committee
Current Stage
PPUB - Publication issued
Start Date
19-Nov-2013
Completion Date
19-Nov-2013
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IEC 62555
Edition 1.0 2013-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Ultrasonics – Power measurement – High intensity therapeutic ultrasound (HITU)
transducers and systems
Ultrasons – Mesurage de puissance – Transducteurs et systèmes ultrasonores
thérapeutiques de haute intensité (HITU)
IEC 62555:2013
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC 62555
Edition 1.0 2013-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Ultrasonics – Power measurement – High intensity therapeutic ultrasound (HITU)
transducers and systems
Ultrasons – Mesurage de puissance – Transducteurs et systèmes ultrasonores
thérapeutiques de haute intensité (HITU)
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XA
ICS 17.140.50 ISBN 978-2-8322-1163-2

Warning! Make sure that you obtained this publication from an authorized distributor.

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé.

® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
---------------------- Page: 3 ----------------------
– 2 – 62555 © IEC:2013
CONTENTS

FOREWORD ........................................................................................................................... 5

INTRODUCTION ..................................................................................................................... 7

1 Scope .............................................................................................................................. 8

2 Normative references ...................................................................................................... 8

3 Terms and definitions ...................................................................................................... 8

4 List of symbols .............................................................................................................. 11

5 Power measurement for HITU equipment....................................................................... 12

6 Radiation force on a target ............................................................................................ 13

6.1 General ............................................................................................................ 13

6.2 Requirements for equipment ............................................................................ 14

6.2.1 Target type ..................................................................................... 14

6.2.2 Target diameter .............................................................................. 15

6.2.3 Balance / force measuring system .................................................. 15

6.2.4 System tank .................................................................................... 15

6.2.5 Target support structures ................................................................ 15

6.2.6 Transducer positioning ................................................................... 15

6.2.7 Anti-streaming foils ......................................................................... 15

6.2.8 Transducer coupling ....................................................................... 16

6.2.9 Calibration and stability .................................................................. 16

6.3 Requirements for measuring conditions ........................................................... 16

6.3.1 Lateral target position ..................................................................... 16

6.3.2 Transducer/target separation .......................................................... 16

6.3.3 Water ............................................................................................. 16

6.3.4 Water contact ................................................................................. 17

6.3.5 Environmental conditions ................................................................ 17

6.3.6 Thermal drifts ................................................................................. 17

6.4 Measurement uncertainty ................................................................................. 17

6.4.1 General .......................................................................................... 17

6.4.2 Non-planar ultrasound field ............................................................. 17

6.4.3 Balance system with target suspension........................................... 17

6.4.4 Linearity and resolution of the balance system ................................ 17

6.4.5 Extrapolation to the moment of switching the ultrasonic

transducer ...................................................................................... 18

6.4.6 Target imperfections ....................................................................... 18

6.4.7 Reflecting target geometry .............................................................. 18

6.4.8 Lateral absorbers in the case of reflecting target

measurements ................................................................................ 18

6.4.9 Target misalignment ....................................................................... 18

6.4.10 Ultrasonic transducer misalignment ................................................ 18

6.4.11 Water temperature .......................................................................... 18

6.4.12 Ultrasonic attenuation and acoustic streaming ................................ 19

6.4.13 Foil properties ................................................................................ 19

6.4.14 Finite target size ............................................................................. 19

6.4.15 Environmental influences ................................................................ 19

6.4.16 Excitation voltage measurement ..................................................... 19

6.4.17 Ultrasonic transducer temperature .................................................. 19

6.4.18 Nonlinearity .................................................................................... 19

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62555 © IEC:2013 – 3 –

6.4.19 Other sources ................................................................................. 19

6.5 Calculation of output power .............................................................................. 20

7 Buoyancy change of a target ......................................................................................... 20

7.1 General ............................................................................................................ 20

7.2 Requirements for equipment ............................................................................ 21

7.2.1 Target type ..................................................................................... 21

7.2.2 Entry window diameter .................................................................... 22

7.2.3 Balance / force measuring system .................................................. 22

7.2.4 System tank .................................................................................... 22

7.2.5 Target support structures ................................................................ 22

7.2.6 Transducer positioning ................................................................... 22

7.2.7 Anti-streaming foils ......................................................................... 22

7.2.8 Transducer coupling ....................................................................... 23

7.2.9 Calibration ...................................................................................... 23

7.3 Requirements for measuring conditions ........................................................... 23

7.3.1 Lateral target position ..................................................................... 23

7.3.2 Transducer/Target separation ......................................................... 23

7.3.3 Water ............................................................................................. 23

7.3.4 Water contact ................................................................................. 24

7.3.5 Environmental conditions ................................................................ 24

7.3.6 Thermal drifts ................................................................................. 24

7.4 Measurement uncertainty ................................................................................. 24

7.4.1 General .......................................................................................... 24

7.4.2 Buoyancy sensitivity ....................................................................... 24

7.4.3 Non-planar ultrasound field ............................................................. 24

7.4.4 Balance system including target suspension ................................... 24

7.4.5 Linearity and resolution of the balance system ................................ 24

7.4.6 Curve-fitting and extrapolation ........................................................ 25

7.4.7 Water temperature .......................................................................... 25

7.4.8 Ultrasonic attenuation and acoustic streaming ................................ 25

7.4.9 Foil properties ................................................................................ 25

7.4.10 Finite target size ............................................................................. 25

7.4.11 Environmental influences ................................................................ 25

7.4.12 Excitation voltage measurement ..................................................... 25

7.4.13 Ultrasonic transducer temperature .................................................. 26

7.4.14 Nonlinearity .................................................................................... 26

7.4.15 Other sources ................................................................................. 26

7.5 Calculation of output power .............................................................................. 26

8 Electrical characteristics ................................................................................................ 26

8.1 Electrical impedance ........................................................................................ 26

8.2 Radiation conductance ..................................................................................... 26

8.3 Efficiency ......................................................................................................... 27

Annex A (informative) Other measurement methods ............................................................ 28

Annex B (informative) Target size ........................................................................................ 29

Annex C (informative) Formulae for radiation force .............................................................. 31

Annex D (informative) Expansion method ............................................................................ 36

Annex E (informative) Influence of attenuation and acoustic streaming on determining

incident and output powers ................................................................................................... 42

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– 4 – 62555 © IEC:2013

Annex F (informative) Avoidance of cavitation ...................................................................... 45

Annex G (informative) Transducer efficiency ........................................................................ 46

Bibliography .......................................................................................................................... 54

Figure 1 – Linearity check: balance readout as a function of the input quantity ..................... 20

Figure C.1 – Correction factor of plane wave for the acoustic field of a circular plane

piston ultrasonic transducer as a function of the product of the circular wavenumber

and transducer radius ........................................................................................................... 33

Figure D.1 – Schematic diagram of an expansion target. ...................................................... 36

Figure D.2 – Example of weight vs time sequence ................................................................ 37

Figure D.3 – Time history of the apparent mass of the castor oil target at different

frequencies following an insonation of approximately 1 W acoustic power for a period

of 10 s .................................................................................................................................. 40

Figure G.1 – Electrical voltage source under different loading conditions .............................. 52

Figure G.2 – Electrical voltage source and electrical matching network and transducer

equivalent circuit ................................................................................................................... 52

Figure G.3 – Diagram illustrating electrical loss. ................................................................... 53

Table D.1 – Selected properties of Acros® Organics castor oil in the range 10 °C to

60 °C .................................................................................................................................... 39

Table D.2 – Absorption coefficient of castor oil as a function of temperature ......................... 41

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62555 © IEC:2013 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ULTRASONICS – POWER MEASUREMENT –
HIGH INTENSITY THERAPEUTIC ULTRASOUND (HITU)
TRANSDUCERS AND SYSTEMS
FOREWORD

1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

all national electrotechnical committees (IEC National Committees). The object of IEC is to promote

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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is

indispensable for the correct application of this publication.

9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of

patent rights. IEC shall not be held responsible for identifying any or all such patent rights.

International Standard IEC 62555 has been prepared by IEC technical committee
87: Ultrasonics
The text of this standard is based on the following documents:
FDIS Report on voting
87/538/FDIS 87/543/RVD

Full information on the voting for the approval of this standard can be found in the report on

voting indicated in the above table.

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

NOTE The following print types are used:
• Requirements: roman type
• Notes: in small roman type
---------------------- Page: 7 ----------------------
– 6 – 62555 © IEC:2013
• Words in bold in the text are defined in Clause 3.

• The numbers in square brackets refer to references given in the Bibliography which follows Annex G.

The committee has decided that the contents of this publication will remain unchanged until

the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data

related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
---------------------- Page: 8 ----------------------
62555 © IEC:2013 – 7 –
INTRODUCTION

In ultrasound fields at megahertz frequencies, output power is typically determined by

measuring the force on a target using a radiation force balance [1],[2],[3]. However, the

relationship between the radiation force and the output power is affected by the focusing or

other geometrical aspects of the field, by the type and shape of the target, by the distance of

the target from the transducer, by absorption (including ‘shock-loss’) in the water path, and by

acoustic streaming currents. Whilst many of these effects are small for typical diagnostic or

physiotherapy ultrasound fields, they cannot generally be ignored for HITU fields (particularly

for those often referred to as high intensity focused ultrasound HIFU) [4]. Furthermore, in

HITU, the quantity of interest is the power incident on the patient rather than the output power

at the transducer face. Since it is common to have a water stand-off between the transducer

and the patient, attenuation and shock-loss in the water path may be significant and will vary

depending upon the chosen distance.

The purpose of this International Standard is to establish standard methods of measurement

of ultrasonic power of HITU devices in liquids in the lower megahertz frequency range based

on the measurement of the radiation force using a gravimetric balance, and calorimetry

(based on the measurement of thermal expansion). This standard identifies the sources of

errors and describes a systematic step-by-step procedure to assess overall measurement

uncertainty as well as the precautions that should be undertaken and uncertainties that should

be taken into account while performing power measurements. Practical guidance is given for

the determination of acoustic power from the very wide range of transducer geometries used

for HITU. Unlike radiation force approaches in IEC 61161 that deal with “time average power,”

other power measurement methods are described in this document.

The structure and content of parts of this International Standard are largely based on

IEC 61161:2013 but there are differences that are summarised below. In this standard the

prime measurand is considered to be the incident power, and not the output power. Output

power is always the quantity of interest in IEC 61161, which specifies that measurements are

made with the target placed close to the transducer. However, this may not always be

possible for strongly convergent transducers and there are cases where it is more relevant to

measure the incident power which reaches a specified surface at some substantial distance

from the transducer (this surface may represent the skin surface of the patient, for instance).

This extra distance may result in significant nonlinear loss in the water path even at low

megahertz frequencies. Consequently, in this International Standard the prime measurand is

considered to be the incident power, and not the output power. The incident power may of

course be the basis for determining the output power using an appropriate model with its own

uncertainties.
---------------------- Page: 9 ----------------------
– 8 – 62555 © IEC:2013
ULTRASONICS – POWER MEASUREMENT –
HIGH INTENSITY THERAPEUTIC ULTRASOUND (HITU)
TRANSDUCERS AND SYSTEMS
1 Scope
This International Standard

• establishes general principles relevant to HITU fields for the use of radiation force

balances in which an obstacle (target) intercepts the sound field to be measured;

• specifies a calorimetric method of determining the total emitted acoustic power of

ultrasonic transducers based on the measurement of thermal expansion of a fluid-filled

target;

• specifies requirements related to the statement of electrical power characteristics of

ultrasonic transducers;

• provides guidance related to the avoidance of acoustic cavitation during measurement;

• provides guidance related to the measurement of HITU transducers of different

construction and geometry, including collimated, diverging and convergent transducers,

and multi-element transducers;
• provides guidance on the choice of the most appropriate measurement method;
• provides information on assessment of overall measurement uncertainties.

This International Standard is applicable to the measurement of ultrasonic power generated

by HITU equipment up to 500 W in the frequency range from 0,5 MHz to 5 MHz. HITU
equipment may generate convergent, collimated or divergent fields.

For frequencies less than 500 kHz, no validations exist and the user should assess the

uncertainties of the power measurement and measurement system at the frequencies of

operation.
This International Standard does not apply to:

• ultrasound equipment used for physiotherapy, for lithotripsy for general pain relief.

2 Normative references

The following documents, in whole or in part, are normatively referenced in this document and

are indispensable for its application. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 61161:2013, Ultrasonics – Power measurement – Radiation force balances and
performance requirements
IEC/TR 62781, Ultrasonics – Conditioning of water for ultrasonic measurements
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
---------------------- Page: 10 ----------------------
62555 © IEC:2013 – 9 –
3.1
acoustical efficiency

ratio of the acoustic output power from an ultrasonic transducer to the transducer

electrical power
Note 1 to entry: Acoustical efficiency is unitless.
3.2
acoustic streaming
bulk fluid motion initiated by a sound field
[SOURCE: IEC 61161:2013, 3.1]
3.3
buoyancy sensitivity

ratio of the increase in the buoyancy force on an expansion target to the amount of absorbed

energy in the absence of thermal losses
Note 1 to entry: This ratio may be temperature dependent.

Note 2 to entry: The buoyancy sensitivity for a fluid filled expansion target immersed in water is most

conveniently and most accurately determined by calibration using electrical heating (see 7.2.9). It can also be

calculated from the product of the expansion ratio, the density of the water and the acceleration due to gravity

but, in practice, this leads to higher uncertainties.

Note 3 to entry: Since most sensitive balances display weight in grams or milligrams, the buoyancy sensitivity

is often more conveniently expressed as mass-equivalent buoyancy sensitivity in terms of a mass-equivalent unit,

such as mg J .
Note 4 to entry: Buoyancy sensitivity is expressed in Newton per Joule, N J .
3.4
expansion ratio

ratio of the increase in volume of the liquid inside an expansion target to the amount of

absorbed energy in the absence of thermal losses

Note 1 to entry: Subject to certain assumptions, the expansion sensitivity for a fluid-filled expansion target can

be calculated from the ratio of the volume expansivity of the fluid to its volumetric heat capacity. The ratio may be

tempera
...

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