Ultrasonics - Field characterization - In situ exposure estimation in finite-amplitude ultrasonic beams

This Technical Specification describes means to allow "attenuated" acoustic quantities to be calculated under conditions where the associated acoustic measurements, made in water using standard procedures, may be accompanied by significant finite-amplitude effects. This Technical Specification establishes: - the general concept of the limits of applicability of acoustic measurements in water resulting from finite-amplitude acoustic effects; - a method to ensure that measurements are made under quasi-linear conditions in order to minimise finite-amplitude effects; - the definition of an acoustic quantity appropriate for establishing quasi-linear conditions; - a threshold value for the acoustic quantity as an upper limit for quasi-linear conditions; - a method for the estimation of attenuated acoustic quantities under conditions of nonlinear propagation in water.

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Status
Published
Publication Date
26-Nov-2007
Technical Committee
Current Stage
PPUB - Publication issued
Start Date
27-Nov-2007
Completion Date
27-Nov-2007
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IEC/TS 61949
Edition 1.0 2007-11
TECHNICAL
SPECIFICATION
Ultrasonics – Field characterization – In situ exposure estimation
in finite-amplitude ultrasonic beams
IEC/TS 61949:2007(E)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC/TS 61949
Edition 1.0 2007-11
TECHNICAL
SPECIFICATION
Ultrasonics – Field characterization – In situ exposure estimation
in finite-amplitude ultrasonic beams
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
ICS 17.140.50 ISBN 2-8318-9463-8
---------------------- Page: 3 ----------------------
– 2 – TS 61949 © IEC:2007(E)
CONTENTS

FOREWORD...........................................................................................................................3

INTRODUCTION.....................................................................................................................5

1 Scope...............................................................................................................................6

2 Normative references .......................................................................................................6

3 Terms and definitions .......................................................................................................6

4 List of symbols ...............................................................................................................10

5 Equipment required ........................................................................................................11

6 Test methods .................................................................................................................11

6.1 Establishing quasi-linear conditions.......................................................................11

6.1.1 The local distortion parameter ...................................................................11

6.1.2 Upper limit for quasi-linear conditions for σ ..............................................12

6.1.3 Range of applicability for quasi-linear conditions .......................................12

6.2 Measurement procedure for estimated in situ exposure .........................................13

6.2.1 Identification of quasi-linear conditions ......................................................13

6.2.2 Tables of limiting mean peak acoustic pressure.........................................14

6.2.3 Measurement of acoustic quantities under quasi-linear conditions .............14

6.2.4 Measurement of the scaling factor .............................................................14

6.2.5 Calculation of attenuated acoustic quantities .............................................15

6.3 Uncertainties .........................................................................................................16

Annex A (informative) Review of evidence ...........................................................................18

Annex B (informative) Review of alternative methods for managing finite-amplitude

effects during field measurement ..........................................................................................21

Annex C (informative) Parameters to quantify nonlinearity ...................................................23

Annex D (informative) Tables of upper limits for mean peak acoustic pressure for

quasi-linear conditions ..........................................................................................................26

Bibliography..........................................................................................................................30

Figure 1 – Flow diagram for obtaining values of attenuated acoustic quantities.....................13

Table A.1 – Experimental evidence of nonlinear loss associated with the propagation

of ultrasound pulses under diagnostic conditions in water .....................................................19

Table A.2 – Theoretical evidence of nonlinear loss associated with the propagation of

ultrasound pulses under diagnostic conditions in water .........................................................19

Table B.1 – Methods for estimation of in-situ exposure in nonlinear beams...........................22

Table C.1 – Parameters for quantification of nonlinearity in an ultrasonic field ......................23

Table D.1 – The upper limit for mean peak acoustic pressure (MPa) associated with

quasi-linear conditions. σ ≤ 0,5. Acoustic working frequency, f = 2,0 MHz .......................26

q awf

Table D.2 – The upper limit for mean peak acoustic pressure (MPa) associated with

quasi-linear conditions. σ ≤ 0,5. Acoustic working frequency, f = 3,5 MHz .......................27

q awf

Table D.3 – The upper limit for mean peak acoustic pressure (MPa) associated with

quasi-linear conditions. σ ≤ 0,5. Acoustic working frequency, f = 5,0 MHz .......................28

q awf

Table D.4 – The upper limit for mean peak acoustic pressure (MPa) associated with

quasi-linear conditions. σ ≤ 0,5. Acoustic working frequency, f = 7,0 MHz........................29

awf
---------------------- Page: 4 ----------------------
TS 61949 © IEC:2007(E) – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ULTRASONICS –
FIELD CHARACTERIZATION –
IN SITU EXPOSURE ESTIMATION
IN FINITE-AMPLITUDE ULTRASONIC BEAMS
FOREWORD

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

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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.

The main task of IEC technical committees is to prepare International Standards. In

exceptional circumstances, a technical committee may propose the publication of a technical

specification when

• the required support cannot be obtained for the publication of an International Standard,

despite repeated efforts, or

• The subject is still under technical development or where, for any other reason, there is

the future but no immediate possibility of an agreement on an International Standard.

Technical specifications are subject to review within three years of publication to decide

whether they can be transformed into International Standards.

IEC 61949, which is a technical specification, has been prepared by IEC technical committee

87: Ultrasonics.
---------------------- Page: 5 ----------------------
– 4 – TS 61949 © IEC:2007(E)
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
87/349/DTS 87/364A/RVC

Full information on the voting for the approval of this technical specification 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.

This publication is being issued as a technical specification (according to 3.1.1.1 of the

IEC/ISO directives, Part 1) as a “prospective standard for provisional application” in the field

of finite-amplitude ultrasonic beams, because there is an urgent need for guidance on how

standards in this field should be used to meet an identified need.

This document is not to be regarded as an “International Standard”. It is proposed for

provisional application so that information and experience of its use in practice may be

gathered. Comments on the content of this document should be sent to the IEC Central

Office.

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

the maintenance result 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

• transformed into an International standard,
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
---------------------- Page: 6 ----------------------
TS 61949 © IEC:2007(E) – 5 –
INTRODUCTION

Acoustic waves of finite amplitude generate acoustic components at higher frequencies than

the fundamental frequency. This provides a mechanism for acoustic attenuation which is not

significant at lower acoustic pressure, and for which there is substantial experimental and

theoretical evidence (Tables A.1 and A.2). The generation of harmonic frequency

components, and their associated higher attenuation coefficient, can occur very strongly when

high amplitude pulses, associated with the use of ultrasound in medical diagnostic

applications, propagate through water. This fact is of importance when measurements of

acoustic pressure, made in water, are used to estimate acoustic pressure in another

medium, or when intensity derived from hydrophone measurements in water is used to

estimate intensity within another medium. In particular, errors occur in the estimation of the

acoustic pressure and intensity in situ, if it is assumed that the propagation of ultrasound

through water, and through tissue, is linear.

Standards for measurement of frequency-rich pulse waveforms in water are well established

(IEC 62127-1). Whilst means to quantify nonlinear behaviour of medical ultrasonic beams are

specified, no procedures are given for their use. Since that time IEC 60601-2-37 and

IEC 62359 have introduced “attenuated” acoustic quantities, which are derived from

measurements in water and intended to enable the estimation of in situ exposure for safety

purposes.

This Technical Specification describes means to allow “attenuated” acoustic quantities to be

calculated under conditions where the associated acoustic measurements, made in water

using standard procedures, may be accompanied by significant finite-amplitude effects. A

number of alternative methods have been proposed (Table B.1).The approach used in this

Technical Specification is aligned with the proposal of the World Federation for Ultrasound in

Medicine and Biology [1] , that “Estimates of tissue field parameters at the point of interest

should be based on derated values calculated according to an appropriate specified model

and be extrapolated linearly from small signal characterization of source-field relationships.”

___________
Figures in square brackets refer to the Bibliography.
---------------------- Page: 7 ----------------------
– 6 – TS 61949 © IEC:2007(E)
ULTRASONICS –
FIELD CHARACTERIZATION –
IN SITU EXPOSURE ESTIMATION
IN FINITE-AMPLITUDE ULTRASONIC BEAMS
1 Scope
This Technical Specification establishes:

• the general concept of the limits of applicability of acoustic measurements in water

resulting from finite-amplitude acoustic effects;

• a method to ensure that measurements are made under quasi-linear conditions in order to

minimise finite-amplitude effects, which may be applied under the following conditions:

− to acoustic fields in the frequency range 0,5 MHz to 15 MHz;

− to acoustic fields generated by plane sources and focusing sources of amplitude gain

up to 12;

− at all depths for which the maximum acoustic pressure in the plane perpendicular to

the acoustic axis lies on the axis;
− to both circular and rectangular source geometries;
− to both continuous-wave and pulsed fields;

• the definition of an acoustic quantity appropriate for establishing quasi-linear conditions;

• a threshold value for the acoustic quantity as an upper limit for quasi-linear conditions;

• a method for the estimation of attenuated acoustic quantities under conditions of nonlinear

propagation in water.
2 Normative references

The following referenced documents are indispensable for the application of this document.

For dated references, only the edition cited applies. For undated references, the latest edition

of the referenced document (including any amendments) applies.

IEC 61161, Ultrasonics – Power – Radiation force balances and performance requirements

IEC 62127-1:2007 Ultrasonics – Hydrophones – Part 1: Measurement and characterization of

medical ultrasonic fields up to 40 MHz
3 Terms and definitions
For the purposes of this document, the following definitions apply.
3.1
acoustic attenuation coefficient

coefficient intended to account for ultrasonic attenuation of tissue between the source and a

specified point
Symbol: α
–1 –1
Unit: decibels per centimetre per megahertz, dB cm MHz
[IEC 62359, definition 3.1]
---------------------- Page: 8 ----------------------
TS 61949 © IEC:2007(E) – 7 –
3.2
acoustic pressure

pressure minus the ambient pressure at a particular instant in time and at a particular point in

the acoustic field
Symbol: p
Unit: pascal, Pa
[IEC 62127-1, definition 3.33, modified]
3.3
acoustic working frequency

arithmetic mean of the frequencies, f and f , at which the acoustic pressure spectrum is 3 dB

1 2
below the peak value
Symbol: f
awf
Unit: Hertz, Hz
[IEC 62127-1, definition 3.3.2, modified]
3.4
attenuated acoustic pulse waveform

the temporal waveform of the instantaneous acoustic pressure calculated in a specified

attenuation model and at a specified point. See 3.1 of IEC 62127-1 for acoustic pulse

waveform
Symbol: p (t)
Unit: pascal, Pa
3.5
attenuated acoustic power

value of the acoustic output power calculated for a specified attenuation model and at a

specified point
Symbol: P
Unit: watt, W
[IEC 62359, definition 3.3]
3.6
attenuated peak-rarefactional acoustic pressure

the peak-rarefactional acoustic pressure calculated in a specified attenuation model and at

a specified point
Symbol: p
, α
Unit: pascal, Pa
[IEC 62359, definition 3.4, modified]
3.7
attenuated pulse-intensity integral

the pulse-intensity integral calculated for a specified attenuation model and at a specified

point
Symbol: I
, α
Unit: joule per metre squared, J m
[IEC 62359, definition 3.6, modified]
---------------------- Page: 9 ----------------------
– 8 – TS 61949 © IEC:2007(E)
3.8
attenuated spatial-peak temporal-average intensity

the spatial-peak temporal-average intensity calculated in a specified attenuation model

Symbol: I
, α
spta
Unit: watt per metre squared, W m
[IEC 62359, definition 3.7, modified]
3.9
attenuated temporal-average intensity

the temporal-average intensity calculated in a specified attenuation model and at a specified

point
Symbol: I
, α
Unit: watt per metre squared, W m
[IEC 62359, definition 3.8, modified]
3.10
beam area

area in a specified plane perpendicular to the beam axis consisting of all points at which the

pulse-pressure-squared integral is greater than a specified fraction of the maximum value of

the pulse-pressure-squared integral in that plane
Symbol: A
Unit: metre squared, m
[IEC 62127-1, definition 3.7, modified]
3.11
local area factor

the square root of the ratio of the source aperture to the beam area at the point of interest.

The relevant beam area, A , is that for which the maximum pulse-pressure-squared integral is

greater than 0,135 (that is, 1/e ) times the maximum value in the cross-section. If the beam

area at the ־6 dB level, A , is known, the beam area can be calculated as
b,–6dB
A = A /0,69: (0,69 = 3ln(10)/10).
b b –6dB
0,69A
SAeff
F = .
b,−6dB
Symbol: F
3.12
local distortion parameter

an index which permits the prediction of nonlinear propagation effects along the axis of a

focused beam. The local distortion parameter is calculated according to 6.1.1
Symbol: σ
3.13
mean peak acoustic pressure
the arithmetic mean of the peak-rarefactional acoustic pressure and the peak-
compressional acoustic pressure
Symbol: p
Unit: pascal, Pa
---------------------- Page: 10 ----------------------
TS 61949 © IEC:2007(E) – 9 –
3.14
nonlinear threshold value

a value of any nonlinear propagation index Y, such that for Y≤τ the beam has quasi-linear

characteristics at the selected point and for Y>τ the beam has nonlinear characteristics at the

selected point
Symbol: τ
3.15
peak-rarefactional acoustic pressure

maximum of the modulus of the negative instantaneous acoustic pressure in an acoustic

field or in a specified plane during an acoustic repetition period. Peak-rarefactional acoustic

pressure is expressed as a positive number
Symbol: p
Unit: pascal, Pa.
[IEC 62127-1, definition 3.44, modified]
3.16
peak-compressional acoustic pressure

maximum positive instantaneous acoustic pressure in an acoustic field or in a specified

plane during an acoustic repetition period
Symbol: p
Unit: pascal, Pa.
[IEC 62127-1, definition 3.45, modified]
3.17
quasi-linear

a condition of the ultrasonic field between the source and a plane at a specified axial depth

for which, at every point, less than a specified, small proportion of the energy has transferred

from the fundamental frequency to other frequencies through nonlinear propagation effects.

3.18
scaling factor

the ratio between the mean peak acoustic pressure at a location close to the transducer to

the mean peak acoustic pressure at the same location under quasi-linear conditions, where

quasi-linearity is determined at the point of interest.
Symbol: S
3.19
source aperture

equivalent aperture for an ultrasonic transducer, measured within the –20 dB pulse-pressure-

squared-integral contour, in the source aperture plane
Symbol: A
SAeff
Unit: metre squared, m
[IEC 61828, definition 4.2.65, modified]
---------------------- Page: 11 ----------------------
– 10 – TS 61949 © IEC:2007(E)
3.20
source aperture plane

closest possible measurement plane to the external transducer aperture that is perpendicular

to the beam axis
[IEC 61828, definition 4.2.67]
3.21
transducer aperture width

full width of the transducer aperture along a specified axis orthogonal to the beam axis

Symbol: L
Unit: metre, m
[IEC 61828, definition 4.2.74, modified]
4 List of symbols
α acoustic attenuation coefficient
A beam area
A source aperture
SAeff
β nonlinearity parameter for water, ≅ 3,5
c speed of sound
f acoustic working frequency
awf
F local area factor
I attenuated pulse-intensity integral
, α
I reduced pulse-intensity integral
pi,q
I attenuated spatial-peak temporal-average intensity
spta
I reduced spatial-peak temporal-average intensity
spta,q
I attenuated temporal-average intensity
I reduced temporal-average intensity
ta,q
L discontinuity length
L transducer aperture width
P total acoustic output power
P attenuated acoustic power
p acoustic pressure
p (t) attenuated acoustic pulse waveform
p (t) acoustic pulse waveform under quasi-linear conditions
p peak-compressional acoustic pressure
p peak-compressional acoustic pressure close to the source for scaling
c,s
p pre-correction peak-compressional acoustic pressure close to the source for
c,s,m
scaling
p reduced peak-compressional acoustic pressure close to the source for scaling
c,s,q
p mean peak acoustic pressure
p peak-rarefactional acoustic pressure
p attenuated peak-rarefactional acoustic pressure
p reduced peak-rarefactional acoustic pressure
r,q
---------------------- Page: 12 ----------------------
TS 61949 © IEC:2007(E) – 11 –
p pre-correction peak-rarefactional acoustic pressure close to the source for
r,s,m
scaling
p reduced peak-rarefactional acoustic pressure close to the source for scaling
r,s,q
ρ density
S scaling factor
σ local distortion parameter
t pulse duration
nonlinear threshold value
τ nonlinear threshold for σ
q q
Y any nonlinear index
z axial distance of the point of interest from the transducer face
5 Equipment required

Measurements of the acoustic pulse waveform shall be carried out using hydrophones in

water, as specified in IEC 62127-1.

Measurement of acoustic output power shall be made using planar scanning by means of a

hydrophone. The methods described in this document do not apply to measurements of

acoustic output power using a radiation force balance as specified in IEC 61161.
6 Test methods
The following method shall be used for measurement of acoustic quantities, using

hydrophones in water, when these measurements are to be used for subsequent calculation

of attenuated peak-rarefactional acoustic pressure, attenuated pulse-intensity integral,

attenuated temporal-average intensity, and attenuated acoustic power.
6.1 Establishing quasi-linear conditions
6.1.1 The local distortion parameter

For the purpose of measurement at any axial point of interest, the local distortion

parameter, σ , is calculated from the measured pulse waveform in water from the following

expression
2πf β
awf
σ = zp (1)
q m
where
p is the mean peak acoustic pressure (p +p )/2
m r c
p is the peak-rarefactional acoustic pressure at the point of interest
p is the peak-compressional acoustic pressure at the point of interest
z is the axial distance of the point of interest from the transducer face
f is the acoustic working frequency
awf
β is the nonlinearity parameter for water, ≅ 3,5
F is the local area factor

NOTE 1 For 2< F <12, σ ≅σ , where σ is the nonlinear propagation parameter at the focus as defined in

a q m m
IEC 62127-1. Also see Annex C.
---------------------- Page: 13 ----------------------
– 12 – TS 61949 © IEC:2007(E)

NOTE 2 F = 2 may be associated with an unfocussed field. In an unfocussed field from a circular source, the

maximum axial amplitude is twice the acoustic pressure amplitude at the source.

NOTE 3 Alternative quantities to σ , that have been proposed elsewhere, are summarized in Table C.1

NOTE 4 Under some conditions, a value for the local area factor may not be available conveniently. Under these

conditions a conservative value F = 2 may be used.
6.1.2 Upper limit for quasi-linear conditions for σ

The field conditions shall be defined as quasi-linear if σ ≤ τ . τ is the nonlinear threshold

q q
for σ
For the purpose of this document, τ = 0,5 .

NOTE τ = 0,5 is the condition for which approximately 10 % of the energy (5 % of the amplitude at the acoustic

working frequency) has been transferred from the fundamental spectrum due to nonlinear propagation: see

Annex A.
6.1.3 Range of applicability for quasi-linear conditions

The procedures to establish quasi-linear conditions are applicable at all depths for which the

maximum mean peak acoustic pressure in the plane perpendicular to the acoustic axis lies

on the axis. Furthermore, having established quasi-linear conditions at any particular axial

point, together with an associated scaling factor, these conditions and scaling factor may

be used for measurements at all axial positions between the transducer and the selected

point. The procedures do not establish quasi-linear conditions for axial positions further from

the transducer than the selected point.

More generally, a procedure to establish quasi-linear conditions for all axial points of interest

in any particular field may be easily applied. This may be carried out by selecting a

measurement point at an axial distance greater than those of all axial points of interest in the

field under consideration. For example, for the purpose of establishing field maxima of any

acoustic quantity in a spherically-focused field, a single measurement point at the focus

should be sufficient. For fields with astigmatic focusing created, for example, by rectangular

ultrasound sources such as those commonly used for medical diagnostic purposes, for which

two focal depths exist, quasi-linear conditions shall be established at the focus of greater

axial distance from the transducer.
---------------------- Page: 14 ----------------------
TS 61949 © IEC:2007(E) – 13 –
6.2 Measurement procedure for estimated in situ exposure

Figure 1 shows the principle of the measurement procedure, which has four stages:

a) identification of quasi-linear conditions;
b) measurement under quasi-linear conditions;
c) measurement of the scaling factor;
d) calculation of attenuated acoustic quantities.
Start
Set non-linear Are measurement Attenuate
output
threshold value conditions quasi-linear?
YES
Measure and record the
acoustic pressure
waveform at the point of
interest
Measure pre-correction
and quasi-linear mean
peak acoustic
pressures at the source
Calculate and apply the
scaling factor
Select a tissue
exposure model
Calculate attenuated
acoustic quantities
IEC 2297/07
Figure 1 – Flow diagram for obtaining values of attenuated acoustic quantities
6.2.1 Identification of quasi-linear conditions

A calibrated hydrophone is positioned at the point of interest. The output from the transducer

is adjust
...

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