SIST EN 62555:2014

SLOVENSKI STANDARD
01-junij-2014
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Ultrasonics - Power measurement - Output power measurement for high intensity
therapeutic ultrasound (HITU) transducers and systems
/
Ultrasons - Mesurage de puissance - Transducteurs et systèmes ultrasonores
thérapeutiques de haute intensité (HITU)
Ta slovenski standard je istoveten z: EN 62555:2014
ICS:
17.140.50 Elektroakustika Electroacoustics
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 62555
NORME EUROPÉENNE
April 2014
EUROPÄISCHE NORM
ICS 17.140.50
English version
Ultrasonics -
Power measurement -
High intensity therapeutic ultrasound (HITU) transducers and systems
(IEC 62555:2013)
Ultrasons -  Ultraschall -
Mesurage de puissance - Leistungsmessung -
Transducteurs et systèmes ultrasonores Messung der Ausgangsleistung für
thérapeutiques de haute intensité (HITU) hochintensive, therapeutische
(CEI 62555:2013) Ultraschallwandler und -systeme
(IEC 62555:2013)
This European Standard was approved by CENELEC on 2013-12-24. CENELEC members are bound to comply
with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard
the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the CEN-CENELEC Management Centre or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and notified
to the CEN-CENELEC Management Centre has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus,
the Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany,
Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland,
Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

CEN-CENELEC Management Centre: Avenue Marnix 17, B - 1000 Brussels

© 2014 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 62555:2014 E
Foreword
The text of document 87/538/FDIS, future edition 1 of IEC 62555, prepared by IEC TC 87 "Ultrasonics"
was submitted to the IEC-CENELEC parallel vote and approved by CENELEC as EN 62555:2014.
The following dates are fixed:
• latest date by which the document has (dop) 2014-10-25
to be implemented at national level by
publication of an identical national
standard or by endorsement
• latest date by which the national (dow) 2016-12-24
standards conflicting with the
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC [and/or CEN] shall not be held responsible for identifying any or all such patent
rights.
Endorsement notice
The text of the International Standard IEC 62555:2013 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
IEC 62127-2 NOTE  Harmonized as EN 62127-2
IEC 60601-2-62 NOTE  Harmonized as EN 60601-2-62

- 3 - EN 62555:2014
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

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.

NOTE  When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD
applies.
Publication Year Title EN/HD Year

IEC 61161 2013 Ultrasonics - Power measurement - Radiation EN 61161 2013
force balances and performance requirements

IEC/TR 62781 Ultrasonics - Conditioning of water for - -
ultrasonic measurements
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

– 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

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

– 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

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
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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
– 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.
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.
– 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.

62555 © IEC:2013 – 9 –
3.1
acoustical efficiency
η
a
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
S
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,
-1
such as mg J .
-1
Note 4 to entry: Buoyancy sensitivity is expressed in Newton per Joule, N J .
3.4
expansion ratio
R
V
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
temperature dependent.
3 -1
Note 2 to entry: Expansion ratio is expressed in cubic metre per Joule, m J .
3.5
expansion target
a liquid-filled device specially designed to intercept and absorb substantially all of the
ultrasonic field and to undergo thermal expansion
3.6
free field
sound field in a homogeneous isotropic medium whose boundaries exert a negligible effect on
the sound waves
[SOURCE: IEC 60050-801:1994, 801-23-28, modified – the term no longer contains “sound”]
3.7
high intensity therapeutic ultrasound (HITU) equipment
equipment for the generation and application of ultrasound to a patient for therapeutic
purposes with the intention to destroy, disrupt or denature living tissues or non-tissue
elements (for example, liquids, bubbles or micro-capsules) and which aims notably at making

– 10 – 62555 © IEC:2013
treatments through actions of ultrasound having mechanical, thermal or more generally
physical, chemical or biochemical effects
Note 1 to entry: Essentially HITU equipment comprises a generator of electric high-frequency power and a
transducer for converting this to ultrasound. In a lot of cases this equipment also includes a targeting and
monitoring device.
Note 2 to entry: HITU may as a side effect by its operation induce hyperthermia, however it should not be
confused with this technique, which heats much less rapidly and to much lower therapeutic temperatures (in
general 42 °C to 50 °C and thermal equivalent times of 0,2 min to 120 min). HITU equipment typically causes
temperature rises in excess of 55 °C and for much shorter times: alternatively, HITU may also induce bioeffects by
non-thermal mechanisms.
Note 3 to entry: This definition does not apply to: ultrasound equipment used for physiotherapy, ultrasound
equipment used for lithotripsy or ultrasound equipment used for general pain relief.
[SOURCE: IEC 60601-2-62:2013, 201.3.218, modified – the Note 3 to entry refers to "general
pain relief" instead of "dedicated hyperthermia".]
3.8
incident power
P
i
time-average ultrasonic power reaching a specified plane or surface after being emitted by an
ultrasonic transducer into an approximately free field, under specified conditions and in a
specified medium, preferably water
Note 1 to entry: Incident power is expressed in watt, W
3.9
multi-element transducer
a source of ultrasound comprising two or more spatially separated ultrasonic transducers
Note 1 to entry In this context, a single piezoelectric element in a phased array is considered to be an ultrasonic
transducer.
3.10
nonlinear loss
loss of energy from an ultrasound beam due to the absorption of harmonic components which
arise from nonlinear propagation effects
Note 1 to entry: In general, nonlinear loss does not occur uniformly throughout an ultrasound field but occurs
preferentially where the pressure amplitude is greatest, resulting in a change in the relative distribution of
ultrasound energy.
3.11
output power
P
time-average ultrasonic power emitted by an ultrasonic transducer into an approximately
free field under specified conditions in a specified medium, preferably water
Note 1 to entry: Output power is expressed in watt, W
[SOURCE: IEC 61161:2013, 3.3]
3.12
radiation conductance
G
ratio of the acoustic output power and the squared r.m.s. transducer input voltage.
Note 1 to entry:  It is used to characterize the electrical to acoustical transfer of ultrasonic transducers.
Note 2 to entry: The r.m.s. drive voltage is used (rather than, for instance, peak-to-peak drive voltage) because
its value is less affected by distortion of the applied electrical signal.
Note 3 to entry: This term is not the same as the real part of transducer admittance.

62555 © IEC:2013 – 11 –
Note 4 to entry: Radiation conductance is expressed in siemens, S
[SOURCE: IEC 61161:2013, 3.8, modified –two notes to entry relevant to HITU have been
added]
3.13
radiation force
acoustic radiation force
F
time-average force acting on a body in a sound field and caused by the sound field, excluding
the component due to acoustic streaming
Note 1 to entry: More generally: time-average force (excluding the component due to acoustic streaming) in a
sound field, appearing at the boundary surface between two media of different acoustic properties
Note 2 to entry: Radiation force is expressed in Newton, N
[SOURCE; IEC 61161:2013, 3.4 modified – the second part of the original definition is
presented as a note to entry, but without the phrase "or within a single attenuating medium"]
3.14
radiation force target
device specially designed to intercept substantially all of the ultrasonic field and to serve as
the object which is acted upon by the radiation force
3.15
target
device specially designed to intercept substantially all of the ultrasonic field
3.16
transducer electrical power
P
el
rate at which time-average electrical energy is converted by an ultrasonic transducer into
other forms of energy (typically into heat and the energy of the ultrasonic field)
Note 1 to entry: Electrical power which is reflected from the ultrasonic transducer is not part of the transducer
electrical power.
Note 2 to entry: Transducer electrical power is expressed in Watt, W
3.17
ultrasonic transducer
device capable of converting electrical energy to mechanical energy within the ultrasonic
frequency range and/or reciprocally of converting mechanical energy to electrical energy
Note 1 to entry: An ultrasonic transducer may include connected cables and components for electrical
matching.
4 List of symbols
a radius of a circular ultrasonic source transducer
b and b half-dimensions of a rectangular ultrasonic transducer in x and y direction,
x y
respectively (so that 2b and 2b are the transducer's side lengths)
x y
B change in the buoyancy force acting on an expansion target immersed in a
sound propagating medium (usually water)
c speed of sound (usually in water)
d and d geometrical focal lengths of a convergent ultrasonic transducer in the x-z and the
x y
y-z plane, respectively
– 12 – 62555 © IEC:2013
d geometrical focal length of a convergent ultrasonic transducer, in the case
that d = d = d
x y
C  the volumetric heat capacity
E  the volumetric expansion coefficient
F radiation force on a target in the direction of the incident ultrasonic wave
g acceleration due to gravity
G radiation conductance
2 2 1/2
h half the diagonal of a rectangular transducer, h = (b + b )
d d x y
h harmonic mean of b and b , h = 2 / (1/b + 1/b )
h x y h x y
k circular wavenumber (2π/λ)
L the fraction of acoustic streaming momentum recovered by a target
M the time-varying weight of a target or expansion target as it is displayed by the
supporting balance (often in mass-equivalent units)
P output power of an ultrasonic transducer
P the transducer electrical power
el
P incident power on a target or expansion target
i
R radius of curvature of a focused bowl transducer
c
R the expansion ratio of an expansion target
V
s normalized distance from an ultrasonic transducer (s = z λ / a )
S the buoyancy sensitivity of an expansion target
t the duration of insonation
z distance between a target and the radiating surface of an ultrasonic
transducer measured along the beam-axis
α amplitude attenuation coefficient of plane waves in a medium (usually water)
β and β focus (half-)angles of a convergent ultrasonic transducer in the x-z and the y-z
x y
plane, respectively; β = arctan(b /d ), β = arctan(b /d ) if the transducer is
x x x y y y
planar and the focal lengths are counted from the planar transducer surface
γ focus (half-)angle of a circular convergent ultrasonic transducer; γ = arcsin(a/d)
if the transducer is spherically curved and the focal length is counted from the
"bottom" of the "bowl"; γ = arctan(a/d) if the focal length is counted from the
plane defined by the rim of the active part of the "bowl" or if the transducer is
planar
η the acoustic efficiency of an ultrasonic transducer
a
θ angle between the direction of the incident ultrasonic wave and the normal to the
surface of a target
φ angle between the direction of the incident ultrasonic wave and the sensitive
axis (usually vertical) of a balance
λ ultrasonic wavelength in the sound-propagating medium (usually water)
ρ (mass) density of the sound-propagating medium (usually water).
NOTE The direction of the incident wave mentioned above under F and θ is understood to be the direction of the
field axis, i.e., it is understood in a global sense rather than in a local sense.
5 Power measurement for HITU equipment
Measurement of output power is well established for collimated (and weakly convergent or
weakly divergent) ultrasound fields at powers up to 20 W using the radiation force method
[IEC 61161]. Clause 6 of this International Standard is based on IEC 61161:2013 but some

62555 © IEC:2013 – 13 –
changes are introduced to make it more appropriate for HITU equipment which in general is
not collimated and has higher output power. IEC 61161 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. 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 (guidance is given in
Annex E). Although the buoyancy change method determines the time-average power incident
on the target during the insonation time, the radiation force method actually determines the
turn-on and turn-off power. These two values may be different to each other, and the average
of the two is not necessarily equal to the time-average power. In general, insonation time is
adjusted as appropriate to the measuring device to account for device limitations.
6 Radiation force on a target
6.1 General
The radiation force balance shall consist of a target which is connected to a balance. The
ultrasonic beam shall be directed vertically upwards or downwards or horizontally on the
target and the radiation force exerted by the ultrasonic beam shall be measured by the
balance. The incident ultrasonic power shall be determined from the difference between the
force measured with and without ultrasonic radiation. Calibration of the balance can be
carried out by means of small precision weights of known mass.
The target shall be chosen so as to closely approach one of the two extreme cases, i.e.
perfect absorber or perfect reflector.
For a plane incident wave only, the acoustic incident power P from the ultrasonic
i
transducer shall be calculated from the radiation force component F on the target in the
propagation direction using Equation 1 or 2 as appropriate:
For a perfectly absorbing target:
P = cF (1)
i
For a perfectly reflecting target:
P = cF / (2 cos θ) (2)
i
where
c is the speed of sound in the sound-propagating fluid (water);
θ is the angle between the propagation direction of the incident wave and the normal to the
reflecting surface
NOTE 1 The direction of the incident wave mentioned above is understood to be the direction of the field axis,
i.e., it is understood in a global sense rather than in a local sense.
The relationship between radiation force and incident power depends in principle on
assumptions about the radiated field and its interaction with the target and the measurement
tank. For any non-plane wave (e.g. convergent, divergent or arising from multiple
simultaneous sources), the correct relationship between radiation force and incident power
shall be determined. The uncertainty in the incident power due to the non-plane nature of the
field shall be estimated.
– 14 – 62555 © IEC:2013
In some cases, forces on the target due to acoustic streaming may be significant compared
to the radiation force. In order to determine the magnitude of the radiation force in these
cases, corrective measures shall be taken which may include applying a theoretical correction
or the use of a streaming foil close to the target. Guidance is given in Annex E. The
uncertainty in the incident power due to streaming forces shall be estimated.
NOTE 2 The appropriate formulae for certain simple idealised transducer configurations are given in Annex C.
The incident power should be measured with the transducer driven in a way similar to its
intended clinical use (e.g. continuous wave or with the usual clinical pulsing sequence,
provided this is compatible with the time response of the balance).
If it is necessary to use a different pulsing sequence to avoid damage to the target or to the
transducer, the effect of different thermal loading on the power output of the transducer shall
be investigated
Further background information about the requirements in the remainder of Clause 6 can be
found in Annex A of IEC 61161:2013.
6.2 Requirements for equipment
6.2.1 Target type
6.2.1.1 General
The use of an absorbing target is recommended. The use of a conical reflecting target is not
recommended in general but may be necessary in some situations.
The target shall have known acoustic properties, these being relevant to the details of the
relation between ultrasonic power and radiation force. (See also A.5.2 of IEC 61161:2013)
6.2.1.2 Absorbing target
An absorbing target shall have:
• an amplitude reflection factor of less
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