EN 61828:2001

SLOVENSKI STANDARD
01-september-2002
Ultrasonics - Focusing transducers - Definitions and measurement methods for
the transmitted fields (IEC 61828:2001)
Ultrasonics - Focusing transducers - Definitions and measurement methods for the
transmitted fields
Ultraschall - Fokusierende Wandler - Definitionen und Messverfahren für die erzeugten
Felder
Ultrasons - Transducteurs focaliseurs - Définitions et méthodes de mesure des champs
transmis
Ta slovenski standard je istoveten z: EN 61828:2001
ICS:
11.040.55 'LDJQRVWLþQDRSUHPD Diagnostic equipment
17.140.50 Elektroakustika Electroacoustics
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN 61828
NORME EUROPÉENNE
EUROPÄISCHE NORM October 2001
ICS 17.140.50
English version
Ultrasonics -
Focusing transducers -
Definitions and measurement methods
for the transmitted fields
(IEC 61828:2001)
Ultrasons - Ultraschall -
Transducteurs focaliseurs - Fokusierende Wandler -
Définitions et méthodes de mesure Definitionen und Messverfahren
des champs transmis für die erzeugten Felder
(CEI 61828:2001) (IEC 61828:2001)
This European Standard was approved by CENELEC on 2001-09-01. 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 Central Secretariat 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 Central Secretariat has the same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Czech Republic,
Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands,
Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom.
CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
Central Secretariat: rue de Stassart 35, B - 1050 Brussels
© 2001 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61828:2001 E
Foreword
The text of document 87/196/FDIS, future edition 1 of IEC 61828, prepared by IEC TC 87, Ultrasonics,
was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as EN 61828 on
2001-09-01.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2002-06-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2004-09-01
Annexes designated "normative" are part of the body of the standard.
Annexes designated "informative" are given for information only.
In this standard, annex ZA is normative and annexes A, B and C are informative.
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 61828:2001 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 61161:1992 NOTE: Harmonized as EN 61161:1994 (not modified).
IEC 62092:2001 NOTE: Harmonized as EN 62092:2001 (not modified).
__________
- 3 - EN 61828:2001
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
This European Standard incorporates, by dated or undated reference, provisions from other
publications. These normative references are cited at the appropriate places in the text and the
publications are listed hereafter. For dated references, subsequent amendments to or revisions of any
of these publications apply to this European Standard only when incorporated in it by amendment or
revision. For undated references, the latest edition of the publication referred to applies (including
amendments).
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 60050-801 1994 International Electrotechnical --
Vocabulary (IEV) -
Chapter 801: Acoustics and
electroacoustics
IEC 61102 1991 Measurement and characterisation of EN 61102 1993
ultrasonic fields using hydrophones in
the frequency range 0,5 MHz to 15 MHz
IEC 61157 1992 Requirements for the declaration of the EN 61157 1994
acoustic output of medical diagnostic
ultrasonic equipment
IEC 61689 1996 Ultrasonics - Physiotherapy systems - EN 61689 1996
Performance requirements and methods
of measurement in the frequency range
0,5 MHz to 5 MHz
NORME CEI
INTERNATIONALE
IEC
INTERNATIONAL
Première édition
STANDARD
First edition
2001-05
Ultrasons – Transducteurs focalisants –
Définitions et méthodes de mesurage
pour les champs transmis
Ultrasonics – Focusing transducers –
Definitions and measurement methods
for the transmitted fields
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61828  IEC:2006 – 3 –
CONTENTS
FOREWORD.7
INTRODUCTION.11
1 Scope.13
2 Normative references .13
3 General .15
3.1 Focusing transducers .15
3.2 System and measurement requirements.19
3.3 General focused field descriptions.21
4 Focusing definitions.25
4.1 Background information.25
4.2 Definitions .25
5 List of symbols .51
6 Measurement procedures .53
6.1 General .53
6.2 Finding the beam axis .53
6.3 Determining if transducer is focusing.57
6.4 Measuring other focal parameters of a focusing transducer .59

Annex A (informative) Background for the transmission/ Characteristics of focusing
transducers .81
Annex B (informative) Methods for determining the beam axis for well-behaved
beams .91
Annex C (informative) Methods for determining the beam axis for beams that are not
well-behaved .99

Bibliography.103

Figure 1 – Transducer options – Top: Transducer with a radius of curvature R and a focal
length equal to R – Middle: Transducer with a plano-concave lens – Bottom: Transducer
with a plano-convex lens.63
Figure 2 – Definitions for focusing measurements when the transducer geometry is
unknown .65
Figure 3 – Field parameters for non-focusing and focusing transducers .67
Figure 4 – Beam contour plot – Contours at –6, –12, and –20 dB for a 5 MHz transducer
with a diameter of 25 mm and a radius of curvature of 50 mm centred at location 0,0
(bottom centre of graph) .69
Figure 5 – Parameters for describing a focusing transducer of a known geometry.71
Figure 6 – Path difference parameters for describing a focusing transducer of a known
geometry .73
Figure 7 – Beamwidth focus in a principal longitudinal plane.75
Figure 8 –Types of geometric focusing.77
Figure 9 – Pressure focus in a principal longitudinal plane.79

61828  IEC:2006 – 5 –
Figure B.1 – X-axis scan at 9 cm depth for the first focal zone with beam centre .93
Figure B.2 – X-axis scan at 4,4 cm depth for the second focal zone.95
Figure C.1 – Asymmetric beam showing beamwidth midpoint method .101

Table B.1 – Standard deviations for x and y scans using three methods of determining
the centre of the beam .93
Table B.2 – –dB beamwidth levels for determining midpoints .97

61828  IEC:2006 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ULTRASONICS – FOCUSING TRANSDUCERS –
DEFINITIONS AND MEASUREMENT METHODS
FOR THE TRANSMITTED FIELDS
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
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
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 61828 has been prepared by IEC technical committee 87:
Ultrasonics.
This bilingual version (2006-07) replaces the English version.
The text of this standard is based on the following documents:
FDIS Report on voting
87/196/FDIS 87/204/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.
The French version of this standard has not been voted upon.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

61828  IEC:2006 – 9 –
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
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
61828  IEC:2006 – 11 –
INTRODUCTION
Focusing transducers are essential in medical applications for obtaining high-resolution
images, Doppler and flow data and for concentrating ultrasonic energy at desired sites for
therapy. Present terminology for focusing transducers is inadequate for communicating
precisely the characteristics of the focused fields of the wide variety of transducers and
transducer array types and focusing means in common usage.
This International Standard provides specific definitions appropriate for describing the
focused field from a theoretical viewpoint for transducers with known characteristics intended
by design. Other specific definitions included in this standard, based on measurement
methods, provide a means of determining focusing properties, if any, of a transducer of
unknown field characteristics. The measurement method and definitions provide criteria for
determining if the transducer is focusing, as well as a means of describing the focusing
properties of the field. Beam axis alignment methods are given for focusing transducers.

61828  IEC:2006 – 13 –
ULTRASONICS – FOCUSING TRANSDUCERS –
DEFINITIONS AND MEASUREMENT METHODS
FOR THE TRANSMITTED FIELDS
1 Scope
This International Standard
− provides definitions for the transmitted field characteristics of focusing transducers for
applications in medical ultrasound;
− relates these definitions to theoretical descriptions, design, and measurement of the
transmitted fields of focusing transducers;
− gives measurement methods for obtaining defined characteristics of focusing transducers;
− specifies beam axis alignment methods appropriate for focusing transducers.
This International Standard relates to focusing ultrasonic transducers operating in the
frequency range appropriate to medical ultrasound (0,5 MHz to 40 MHz) for both therapeutic
and diagnostic applications. It shows how the characteristics of the transmitted field of
transducers may be described from the point of view of design, as well as measured by
someone with no prior knowledge of the construction details of a particular device. The
radiated ultrasound field for a specified excitation is measured by a hydrophone in either a
standard test medium (for example, water) or in a given medium. The standard applies only to
media where the field behaviour is essentially like that in a fluid (i.e. where the influence of
shear waves and elastic anisotropy is small), including soft tissues and tissue-mimicking gels.
Any aspects of the field that affect their theoretical description or are important in design are
also included. These definitions would have use in scientific communications, system design
and description of the performance and safety of systems using these devices.
This standard incorporates definitions from other related standards where possible, and
supplies new, more specific terminology, both for defining focusing characteristics and for
providing a basis for measurement of these characteristics.
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 60050(801):1994, International Electrotechnical Vocabulary (IEV) – Chapter 801:
Acoustics and electroacoustics
IEC 61102:1991, Measurement and characterization of ultrasonic fields using hydrophones in
the frequency range 0,5 MHz to 15 MHz
___________
Specifically, IEC 61102 and IEC 61157 (see clause 2).

61828  IEC:2006 – 15 –
IEC 61157:1992, Requirements for the declaration of the acoustic output of medical
diagnostic ultrasonic equipment
IEC 61689:1996, Ultrasonics – Physiotherapy systems – Performance requirements and
methods of measurement in the frequency range 0,5 MHz to 5 MHz
3 General
The information contained in this clause is an introduction to the definitions given in clause 4
and the measurement methods given in clause 6.
3.1 Focusing transducers
The term "focusing transducer" is commonly used for a device which has a smaller
beamwidth in some regions of the field than a device which is "non-focusing". A "non-
focusing transducer" can still have a natural focus, so it is necessary to distinguish a
focusing transducer as having a greater concentration of pressure amplitude (for a given
power output) than a non-focusing transducer at its natural focus. For example, a non-
focusing transducer made of a simple disc of uniformly poled piezoelectric material has a
beam whose intensity at its natural focus can be as much as four times the average intensity
at the source, and whose –6 dB beamwidth can be approximately half of that at the source. A
definition of a focusing transducer is given in 4.2.33 to make a quantitative distinction
between focusing and non-focusing transducers.
3.1.1 Focusing methods
The simplest means of intentionally focusing an ultrasonic transducer, borrowed from
analogous optical principles, is that of shaping the ultrasonic transducer into a concave form
or adding to it a physical lens as illustrated in figure 1. In the top part of this figure, a
transducer curved with a radius R is shown focusing to the centre of curvature, where R is
positive by convention. By the geometrical-optics approximation, the focal length F is equal to
R and hence is also positive. In the middle of figure 1 is shown a transducer with a plano-
concave lens made of a material with longitudinal velocity, c , which is curved on one side
L
with a radius, R , and radiates into a medium in which the velocity is c . In acoustics, c
LENS W W
is typically less than c , i.e., the index of refraction n (equal to c /c ) is less than 1. When
L W L
this is true, the radius is considered to be negative and the focal length, given by the
geometric-acoustics approximation as R divided by (n – 1), is positive. At the bottom of
LENS
the figure, for comparison, the typical situation for a convex lens in optics is shown: n is
greater than 1 and the radius is considered to be positive, so the focal length is positive.
3.1.2 Known and unknown focusing transducers
For ultrasonic transducers currently used in medical ultrasound applications, it is difficult to
determine from physical observation if an ultrasonic transducer is focusing, because
additionally many other focusing methods such as geometric shaping and arrangement,
reflectors, arrays with electronic phasing and delay, Fresnel lenses, shading, etc. may be
used singly or in combination. Because of inherent natural focusing and the potential
complexity of additional focusing means used, any generally useful definition of a focusing
transducer must be in terms of its field rather than its construction. If a focusing source were
to be defined in terms of its pressure field, then this would be relatively easy to apply in
practice, since the pressure can be measured directly with a hydrophone.
___________
Terms in bold print are defined in clause 4.

61828  IEC:2006 – 17 –
A distinction is also made between ultrasonic transducers whose construction is known and
transducers about which very little information is available. For the first category of ultrasonic
transducers, certain theoretical definitions, such as geometric focal length, are useful for
describing and modelling focusing characteristics. Ultrasonic transducers falling in the
second category function as an unknown "black box" and only the field may be accessible.
In the latter case, and in general, focusing parameters are determined from measurements,
and the measurement procedures of clause 6 are appropriate. In clause 6, measurement
methods are given for determining if a transducer system radiating into known propagation
media under specified excitation conditions is "focusing". Because of the lack of knowledge of
ultrasonic transducer construction and limited access to the ultrasonic transducer field, the
focusing definitions shown in figure 2 are required. These definitions are given in clause 4
and their use is explained in 3.1.5.
3.1.3 Focusing and beamwidth
Previously, hydrophone measurements of beam characteristics were based on regions of axial
peak pressure. For example, definitions for a depth-of-field were based on the fall-off in
intensity on either the near side or the far side of an axial peak on the beam axis. For axially
symmetric beams, this axial peak can be related to the geometric focal length. For typical
rectangular arrays, azimuthal plane electronic focusing and elevational plane mechanical lens
focusing can cause peaks of axial pressure at different locations along a beam axis. These
individual peaks can be dealt with separately by beamwidth measurements made in the
corresponding orthogonal planes: therefore, new definitions are based on beamwidths in a
specified longitudinal plane (refer to figure 7). Focusing definitions must also distinguish
between natural and intentional focusing.
3.1.4 New focusing parameter definitions
This document introduces new focusing parameters and provides more specific contexts for
existing terminology. For example, the terms "near field" and "far field" are often misapplied
to focusing transducers, though they have traditionally been defined for non-focusing
transducers only. The definitions near Fresnel zone, far Fresnel zone and focal
Fraunhofer zone, apply to focusing transducers. These definitions, explained in more detail
in 3.3 and derived from annex A, are illustrated in figure 3b and are applied to a strongly
focusing circular aperture in figure 4. Other concepts such as focusing in a particular plane
are also necessary to reduce ambiguity in usage.
For the purposes of this document, the following definitions for a focusing transducer will be
used.
For ultrasonic transducers of known construction (refer to figure 5 for transducer geometry
and terms), a focusing transducer is an electro-acoustic device that produces, at any
distance less than one-half of the transition distance from the transducer aperture, a –6 dB
beamwidth in a longitudinal plane that is less than half the transducer aperture width in
that plane. For measurement purposes and cases (see figure 2) where the geometry of the
ultrasonic transducer is not known or where there is no direct access to the ultrasonic
transducer (because of the device being used in some stand-off arrangement), a definition of
focusing based on data is more appropriate. For this second case, a focusing transducer is
an electro-acoustic device that produces, at any distance less than half of the transition

61828  IEC:2006 – 19 –
distance from the source aperture, a –6 dB beamwidth in a longitudinal plane that is less
than one-half the –20 dB source aperture width (measured in a plane as close as possible to
the ultrasonic transducer) in that plane. For arrays with a rectangular geometry, a specified
longitudinal plane is either an xz or yz plane with z along the beam axis. Non-focusing
transducers are those not meeting the conditions specified above.
3.1.5 Applications of focusing definitions
Two definitions of focusing are given in 3.1.4, which apply in two cases.
a) For transducers for which the construction is known, an ideal definition is given for
describing, modeling or design purposes.
b) A second definition applies to measurements of the focusing characteristics of real
transducers which either have an unknown construction or imperfect realization.
Use of the first definition is not a substitute for actual measurement. Whether or not a
transducer is focusing in practice must be determined by the second definition for transducers
of unknown construction and by the measurement procedures of clause 6. Knowledge about
the transducer (first definition) may be helpful in guiding measurements. If measurements
meet the criteria of the second definition, the transducer is focusing, irrespective of whether
focusing was intentional or accidental.
3.1.6 Relation of present definitions to physiotherapy transducers (treatment heads)
The definition of focusing in the present document is not related to the definitions of
“divergent, collimated and convergent” beams as described in IEC 61689. The definition of
beam type is based on energy and area considerations that are more important for
physiotherapy transducers. The definition of focusing in the present document is based on a
different parameter: –6 dB beamwidth. This definition is useful in identifying the existence and
location of the highest field concentrations. When the current document is applied to
physiotherapy transducers, focusing can be understood to correspond to high-beam non-
uniformity ratio “hot-spot” transducers.
3.2 System and measurement requirements
In 3.1 it was shown that the radiating device has to be considered as a whole, because it is
not possible to define a focusing transducer in terms of the properties of its component
elements. For clinical ultrasound systems, each of the measured focusing definitions only
applies for the field of a selected scan line generated by given electrical excitation conditions
and for a given medium.
3.2.1 Transmitted pressure waveforms
Because a wide variety of transmitted pressure waveforms are possible in an ultrasonic
transducer field, a measure of these waveforms must be robust enough to accommodate
broad-band or narrowband pulses, continuous-wave signals or even waveforms distorted by
non-linear propagation. For this reason, the pulse-pressure-squared-integral (see 3.33 of
IEC 61102) will be the field measurement used throughout this document. For certain types of
waveforms under the conditions of linear propagation, the pulse-pressure-squared-integral
can be related to more familiar pressure terminology. For example, for linear, continuous-
wave signals, the pulse-pressure-squared-integral divided by the period of one cycle is the
root-mean-square acoustic pressure squared. In other cases, when ratios of these integrals
are involved, these ratios can be thought of as ratios of equivalent squared pressures. In such
cases, ratios of the square roots of the pulse-pressure-squared-integral are analogous to
ratios of equivalent pressures.

61828  IEC:2006 – 21 –
3.2.2 Radiated fields
The radiated field of an ultrasonic transducer is dependent on the bandwidth of the
ultrasonic transducer as well as the type of excitation used. Frequently used models for
beam simulation such as that described in annex A are appropriate only for continuous-wave
excitation. For simulating the pulsed excitation of an ultrasonic transducer, the driving
waveform and the impulse response of the ultrasonic transducer element as well as the
boundary conditions need to be considered. As the bandwidth of a pressure acoustic pulse
waveform launched by an element increases, the resulting field becomes smoother compared
to a field from a continuous-wave signal.
In addition to the field depending on the waveform of the electrical driving function and the
propagation medium, it will also depend on the amplitude of the electrical input. This feature
is due to non-linear propagation, which is frequently present in the type of field being
considered. A parameter called the non-linear propagation parameter (3.25 of IEC 61102)
has been previously defined and, in general, the assumption of linearity can be made
provided that this parameter is less than approximately 0,2.
3.3 General focused field descriptions
New terms for describing the transmitted focused field of an ultrasonic transducer of known
construction are introduced in 4.2. Refer to figure 1, which shows the primary geometrical
relationships for the definitions. Background information for these definitions can be found
in annex A.
Measured focusing definitions, also in 4.2, can be used to characterize the focusing acoustic
field of an unknown acoustic source through measurements. In this case a measurement
plane, the source aperture plane, is chosen as close as possible to the source. An
equivalent acoustic aperture, the source aperture on this plane, is used for determining the
effective focusing characteristics of the field. As with the focusing definitions for an ultrasonic
transducer of known design, the ultrasonic transducer is considered as an ultrasonic
transducer system with a specified set of operating conditions and medium of propagation and
offset distance. Conditions of acoustic linearity are desirable but not necessary for these
measurements, and the non-linear propagation parameter must be specified. Figure 2
shows the relationship among several of these measurement definitions.
3.3.1 General field descriptions for transducers of known construction
From the fields of focusing transducers of known construction, it is possible to determine
general characteristics of focused fields. Ultrasound focusing is not well described by
geometric optics because of beam diffraction resulting from transducer sizes on the order of
wavelengths. Natural focusing of the beam is combined with the focusing of a lens or other
focusing device. The resulting combined effect is that the narrowest –6 dB beamwidth does
not in general occur at the geometric focal length of the focusing device, but approximately
at a distance
z F
T
z =  (1)
min
z + F
T
where
z is the transition distance, the natural focal length;
T
F is the geometric focal length (as explained in annex A, equation (A.11c)).

61828  IEC:2006 – 23 –
This equation shows that the distance to the location of the minimum beamwidth cannot
exceed the transition distance even when the geometric focal length is greater than the
transition distance.
An approximate relation exists between the characteristics of focused fields and non-focused
fields. In a longitudinal plane, beam profiles at an axial distance z in a focused field are
similar in shape to a beam profile in a non-focused field occurring at an equivalent depth, z
e
(see annex A for the approximations used in the derivation of equation (A.8)),
z
z =   (valid for z ≠ F, and F positive) (2)
e
z
1−
F
This equation indicates that to a good approximation, the field of a focusing transducer
(within a longitudinal plane) takes on all of the near-field and far-field beam profile shapes
of a non-focusing transducer of the same size in the distance between the ultrasonic
transducer and the geometric focus. At the geometric focus, the equivalent distance
becomes infinite and equation (2) no longer holds: the shape of the beam profile is the same
as that obtained in the far field of an identically sized non-focusing transducer.
The evolution of the field of a focusing transducer is accelerated by the scaling of equation
(2) compared to the field of a non-focusing transducer, and the transverse width of the
beam becomes narrower than that of a non-focusing transducer at distances close to the
geometric focus.
In a manner consistent with the determination of the transition distance, the distance
separating the near field and far field of a non-focusing transducer, transition distances
can be found for similar descriptions of a focused field. The focused field can be divided into
three regions, the near Fresnel zone, the focal Franhofer zone and the far Fresnel zone,
as shown in figure 3b. The corresponding distances separating these zones are the near
transition distance, z ,
NTD
1 1 1
= + (3)
z z F
NTD T
and the far transition distance, z ,
FTD
1 1 1
= − + (4)
z z F
FTD T
More information about these distances can be found in annex A (equation (A.11)).
3.3.2 The scan plane and the steering of beams
In addition to being focused, beams can also be made to change direction. This direction
corresponds to a scan line, the beam axis for a particular ultrasonic transducer element
group. The scan plane (or surface) is the plane or surface containing all the ultrasonic scan
lines. The scan plane is also known as the azimuth plane. For most cases, the elevation
plane is orthogonal to the azimuth plane and contains the central scan line – the beam
direction corresponding to an undeflected or unsteered beam.
The pattern of scan lines depends on the image format, the geometry of the ultrasonic
transducer and the method of transducer excitation. Several examples of scanning are
described below: sector (angular), linear (translation), and two-dimensional arrays.

61828  IEC:2006 – 25 –
Sector (angular) scanning is accomplished by either mechanically sweeping a single trans-
ducer in an arc or by changing the electronic excitation of active transducer elements, an
ultrasonic transducer element group, to produce angular deflections of the beam. The
resulting pattern of scan lines has a fan-like appearance and results in a sector image format.
An unsteered beam is one selected to be in the forward propagation direction without angular
deflection. The direction of this beam corresponds to the central scan line of a sector scan.
For the usual case in which the ultrasonic transducer is symmetric, the unsteered beam
may be chosen to be near the symmetry axis or a symmetry plane of the ultrasonic
transducer.
Linear scanning is the translation of active transducer elements, an ultrasonic transducer
element group, along the array surface (or by mechanically translating a single transducer).
When the array is flat and linear, a pattern of parallel scan lines forms a rectangular image
format. When a curved linear array geometry is used, the translation of the ultrasonic
transducer element groups results in scan lines which have an angular separation and
result in a sector type image format. In this case, angular deflection is caused by the
transducer geometry and not electronic steering.
In the most general case, a combination of methods for steering and focusing the beam
simultaneously may be used. In the situations described above, a mechanical lens with a
fixed focal length is applied to focus in the elevation plane. For a two-dimensional array, the
scan plane is not simply related to the shape or geometry of the array. Azimuth and elevation
focusing are coincident. Diagonal segments of the array or all elements of the array can be
employed to steer and focus the beam simultaneously at an arbitrary angle to the transducer
aperture. In the most common method used to form a three-dimensional image, the array
sweeps through a series of planes to fill a volume to be imaged. In this case, the position of
each scan plane is time-dependent, and by definition there is a corresponding orthogonal
elevation plane. Just as there is a central scan line in a scan plane, a central scan plane
can often be identified near the symmetry axis or a symmetry plane of the ultrasonic
transducer.
4 Focusing definitions
4.1 Background information
Definitions listed below fall into three general categories. The first are those definitions
directly applicable to describing and modelling focused fields. The second group of definitions
relates to measurements of focused fields. Some of the terms in the second group overlap
with those of the first. Third, additional commonly used terms about focusing have been
included. Previously used definitions have been modified or made more specific to remove
ambiguity in usage. Definitions related to focusing in 4.2 are not grouped but follow an
alphabetical order.
4.2 Definitions
For the purposes of this International Standard, the following definitions apply.

61828  IEC:2006 – 27 –
4.2.1
acoustic pulse waveform
temporal waveform of the instantaneous acoustic pressure at a specified position in an
acoustic field and displayed over a period sufficiently long to include all significant acoustic
information in a single pulse, a single tone-burst, or one cycle of a continuous wave
[IEC 61102, definition 3.2]
NOTE In some cases such as an amplitude-modulated pulse, the overall pulse train may appear as a group of
nearly contiguous pulses with spacings much smaller than the overall pulse repetition time.
4.2.2
annular array
any ultrasonic transducer element group having radiating elements in the same plane or
curved surface and consisting of concentric elements which are electrically phased to control
the characteristics of an acoustic beam
4.2.3
aperture path difference
difference in path lengths from a specified geometric focus to the periphery of the
transducer aperture and to the intersection of the beam axis with the transducer aperture
plane for a specified longitudinal plane and for an unsteered beam
(See figure 6 and annex A for details.)
Symbol: Δ
Unit: metre, m
4.2.4
apodization
amplitude weighting or shading of the transducer aperture
4.2.5
arithmetic-mean working frequency
arithmetic mean of the frequencies f and f at which the amplitude of the acoustic pressure
1 2
spectrum is 3 dB below the peak amplitude,
[IEC 61102, definition 3.4.2, modified]
Symbol: f
awf
Unit: hertz, Hz
4.2.6
axial field-point path difference
difference in path lengths from a specified field point on the beam axis to the periphery of the
transducer aperture and to the intersection of the beam axis with the transducer aperture
plane. It is specified in the same longitudinal plane as the aperture path difference
(See figure 6.)
/
Symbol: Δ
Unit: metre, m
61828  IEC:2006 – 29 –
4.2.7
azimuth axis
axis formed by the junction of the azimuth plane and the source aperture plane
(measurement) or transducer aperture plane (design)
(Refer to figure 7.)
4.2.8
azimuth plane
for a scanning ultrasonic transducer: the scan plane, for a non-scanning ultrasonic
transducer: the principal longitudinal plane
NOTE Usually the principal longitudinal plane is chosen to coincide with the azimuth plane.
4.2.9
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. If the position of the plane is not
specified, it is the plane passing through the point corresponding to the spatial-peak temporal-
peak acoustic pressure in the whole acoustic field
[IEC 61102, definition 3.6]
NOTE The beam area may be composed of several sections.
Symbol: A
b
Unit: metre squared, m
4.2.10
beam area focal plane
plane perpendicular to the beam axis and containing the beam area focus
4.2.11
beam area focus
point on the beam axis at which the –6 dB beam area is a minimum
4.2.12
beam axis
straight line that passes through the pulse-pressure-squared-integral centroids (or beam
centrepoints) of two planes. The location of the first plane is the location of the pressure
(plane containing the maximum pulse-pressure-squared-integral) or, alterna-
focal plane
tively, is one containing a single main lobe which is in the focal Fraunhofer zone. The
location of the second plane is as far as is practicable from the first plane and parallel to the
first with the same two orthogonal scan lines (x and y axes) used for the first plane
NOTE This definition is appropriate for focusing transducers, whereas the beam-alignment axis in 3.5 of
IEC 61102 is more appropriate for non-focusing transducers. See 6.2 and figure 2.
4.2.13
beam centrepoint
position det
...