NBSIR 75 937

NIST

PUBLICATIONS

THE NATIONAL MEASUREMENT SYSTEM

FOR MEDICAL ULTRASONICS

Paul A. Hudson

Electromagnetics Division

Institute for Basic Standards

Boulder, Colorado 80302

February 1976

100

'976

NBSIR 75-937

THE NATIONAL MEASUREMENT SYSTEM

FOR MEDICAL ULTRASONICS

Paul A. Hudson

Electromagnetics Division

Institute for Basic Standards

Boulder, Colorado 80302

February 1976

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker Johnson , Assistant Secretary tor Science and Technology

NAIIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director
FOREWORD

This is one of the reports from the 1972-75 NBS Study of the National Measurement

System. It deals specifically with the current structure, status and trends of the

system used in this country for the measurement of physical quantities related to medical

ultrasonics

.

The "Concept of a National Measurement System" has been discussed by NBS authors

and spokesmen for a number of years, since the publication by Dr. R. D. Huntoon of

an article by that title in the October 6, 1967 issue of Science (Vol. 158, No. 3797,

pages 67-71). Dr. Huntoon began his article with the follov;ing paragraph:

"Concurrently with the grov;th and industrialization of this nation, there has

developed within it a vast, complex system of measurement which has made

possible the very growth that brought the system into being. This National

Measurement System (NMS) stands today as one of the key elements in a worldwide measurement system that links all major nations together in a consistent,

compatible network for communication and trade."

The U.S. National Measurement System is defined as comprising all of the activities

and mechanisms -- intellectual and operational, technical and institutional -- used

by this country to produce the physical measurement data needed to create the objective, quantifiable knowledge required by our society. This knowledge is used to describe, predict, communicate, control, and react, in many aspects of our personal

and social lives, science, and technology.

Dr. Huntoon continued his discussion in 1957:

"Our National Measurement System is one of a number of mutually interacting systems

within our technologically based society that form the environment in which the

individual citizen must live and function. Familiar examples are the communication, transportation, educational, medical and legal systems, all of which may

be included under the general heading of social systems."

"In view of the demonstrated value of the systems approach for the understanding and

improvement of hardware such as computers and weapons, some of these social systems

are being subjected to the same type of analysis. The National Measurement System,

which evolved in this country with little formal recognition as a system, is now

being examined in this way at the National Bureau of Standards (NBS) which undertook the study of the National Measurement System partly because of the all -pervasive nature and great economic importance of the nation's measurement activities,

and partly because of the challenge to NBS in putting its splended new facilities

to optimum use for the benefit of the nation. Such optimum use can be approached

only when the National Measurement System, of which NBS is a central element, and

the services it requires for effective operation are sufficiently well understood."

Over the years both before and since 1957, NBS has taken a number of approaches to the study of our national system for physical measurements. The current series of

studies was initiated in 1972 by Dr. Ernest Ambler, then Dr. Huntoon's successor as Director of the NBS Institute for Basic Standards, now Acting Director of the Bureau.

The reports of these studies are being presented to the public at large for the purpose of increasing national understanding, and the operational effectiveness, of our

National Measurement System. They are presented as an initial or interim, not final,

word on the subject. NBS welcomes contributions from all sectors of the System

which can aid it in increasing its understanding of and effectiveness in supporting

that System.

ii
• CONTENTS

Page

EXECUTIVE SUMMARY 1

1. INTRODUCTION 3

2. STRUCTURE OF THE MEASUREMENT SYSTEM 3

2.1 Conceptual System 3

2.2 Basic Technical Infrastructure 5

2.2.1 Documentary Specification System .5

2.2.1.1 Standardization Institutions 5 2.2.1.2 Survey of Voluntary (Documentary)

Standards and Recommendations 6 2.2.2.1 Measurement Tools and Techniques 7 2.2.2.2 The Instrumentation Industry 8

2.2.5 Science and People 9

2.3 Realized Measurement Capabilities 9

2.4 Dissemination and Enforcement Network 9

2.5 Organization Input-Output Transactions Matrix 10

3. IMPACT, STATUS AND TRENDS OF MEASUREMENT SYSTEM 10

3.1 Impact of Measurement 10

3.2 Status and Trends of the System 10

4. SURVEY OF NBS SERVICES 10

4.1 The Past 10

4.2 The Present -- Scope of NBS Services 10

4.2.1 Description of NBS Services 10

4.3 Impact of NBS Services 12

4.4 Evaluation of the NBS Measurement Program in

Medical Ultrasouncs 12

4.5 The Future -- Recommended NBS Actions 12

5. SUMMARY AND CONCLUSIONS 13

APPENDIX A. METHODOLOGY OF THE STUDY 13

ACKNOWLEDGMENT 13

REFERENCES 14

iii
THE NATIONAL MEASUREMENT SYSTEM FOR MEDICAL ULTRASONICS

Paul A. Hudson

Electromagnetics Division

Institute for Basic Standards

February 1976

ABSTRACT

The rapid increase in the use of ultrasonics in medical diagnostics has

been brought about by recent advances in imaging techniques employing

sophisticated electronic systems. Along with the proliferation of this

equipment in the medical community has come concern about possible tissue

damage due to this type of radiation. Research on damage thresholds is being

carried out in laboratories across this country and overseas. Agreement

between various researchers is hampered by lack of accurate standards for

measurement of transducer beam power and intensity.

The National Measurement System for medical ultrasonics is, at this

time, inadequate, fragmented and largely uncoordinated. The Electromagnetics

Division is in process of developing standards for measurement of transducer

beam power, intensity and beam pattern. These standards will be disseminated

widely throughout the country to provide the needed accuracy and measurement

agreement among biomedical researchers and ultimate users (hospitals,

physicians, clinics, etc.). Interim standards for beam power were developed

1 1/2 years ago and are already in use by standards laboratories of

manufacturers and universities. It is estimated that another 5 years will

be required to bring the measurement system under some form of control.

Key Words: Electroacoustics ; medical diagnostics; radiation safety;

radiation therapy (ultrasonic); ultrasonics.

i V
THE NATIONAL MEASUREMENT SYSTEM FOR MEDICAL ULTRASONICS

Paul A. Hudson

Electromagnetics Division

Institute for Basic Standards

February 1976

EXECUTIVE SUMMAR'.

The medical applications of ultrasonics

have increased at a rapid rate during the

past five years. Ultrasonics is used for

diagnostics, therapy (diathermy) and surgery. The most important of these is the

diagnostic applications. Recent advances

in signal processing techniques and hardware

have made possible the conversion of ultrasonic images to real-time optical images of

good quality. Systems employing complex

phased arrays of transducers are nov/ in the

laboratory stage and will find their way to the practitioner within the next two years.

A large amount of activity is going on in these develop-ents in university laboratories,

manufacturers, and non-profit organizations.

The size of the field of medical ultrasonics

is estimated to be between $50M and SIOOM.

These figures include doctor's fees, research

and equipment production. Projections by

knowledgable people indicate that by 1985

sales of ultrasonic diagnostic instruments

will equal or exceed sales of X-ray

equi pment.

Reviews of the literature have uncovered

a considerable concern with regard to the

toxicity of ultrasonic waves on certain

parts of the body. Since ultrasonics is widely used in fetal studies, a large

amount of the concern v/as in this area. Certain consumer groups (e.g., Ralph Nader)

have testified before congressional com- mittees detailing their concern over this new medical tool

.

Research into the toxic effects and

damage thresholds of ultrasonic radiation

are in progress. However, it is expected

to require several years before definitive,

quantitative results are obtained. In the

meantime, measurements are har.oered by a lack of standards ana reasure~ent con- sistency between various researchers.

The National Measurement System as it

applies to medical ultrasonics is fragmented, uncoordinated and inadequate.

Measurement capability for the more important parameters, power, intensity, and

beam pattern, is mostly centered in development laboratories, certain manufacturers, and government laboratories. The

Bureau of Radiological Health (DHEW/FDA)

maintains measurement capability designed'

to aid in their role as the regulatory

agency under provisions of the Radiation

Health and Safety Act of 1368. The Electromagnetics Division of NBS began standards development three years ago and now offers a limited calibration service at 1,

2, 3 and 5 MHz for total beam power. Also

offered is a measurement service for

efficiency of high-0 quartz transducers in the band 0.5 to 10 MHz. Accuracies at NBS are b\ or better as compared to to

3Grt in the field. A serious hindrance to more widespread standards dissemination is the lack of a suitable transfer standard

device. It is believed that a low-cost,

rugged and reliable device can be developed

for use by both technical and non-technical

(medical ) personnel

.

National laboratories in foreign countries, notably West Germany and Australia,

have developed standards for important

ultrasonic quantities and offer a more com- plete measurement service than does NBS at this time. At the PTE in West Germany,

for example, the ultrasonics group consists

of 15 people and they have been working in

the field for more tnan 12 years.

As a result of announcement of NBS

services (August, 1974), numerous inquires

have been received, calibrations have been

performed, and negotiations are in progress

for i ntercompari sons with two leading university laboratories. Intercomparisons

with BRH have been carried out. This was important to that agency in that they were

able to proceed with the preparation of

regulatory documents. Information received

at NBS indicates that these documents will

prescribe that all ultrasonic medical equipment be metered for power output and that

the accuracy of the metering be within

certain limits. This is expected to create

a demand for NBS measurement services from

manufacturers who wish to insure that their

equipment meets BRH requirements.

Close cooperation and exchange of information is maintained between NBS and BRH,

For example, information gained by BRH in

their field surveys and contacts with manufacturers is passed on to NBS. The BRH
program in ultrasonics standards is aimed

at development of working standards for use

in their regulatory role. They look to NBS

for development and maintenance of basic

standards

.

For the future, the Electromagnetics

Division has under development a precision

scanning system (computer controlled) to yield accurate measurements of field and

beam patterns of ultrasonic transducers

and arrays. This system is expected to become a major factor in solving measurement problems which now exist. A new calorimeter which is to serve as a basic

standard for power measurement, is nearing

completion. Thus, the potential impact of

NBS on the important field of medical

ultrasonics appears to be substantial.

2
1. INTRODUCTION

Ultrasonic techniques are becoming

increasingly important in medical applications including diagnosis, therapy and

surgery. In diagnosis, for example,

ultrasonic techniques are gaining ever greater acceptance by radiologists as a unique diagnostic modality that complements and is often superior to medical

procedures involving the use of ionizing

radiation (1). Each day ultrasound is in

routine clinical use in hospitals and

medical centers througnout the world for

the diagnosis and treatment of such

diverse soft-tissue structures as the

brain, heart, liver, kidneys, breast, the

reproductive systems, the vascular system,

the fetus and the eye (2).

Because ultrasound may be used non- invasively, it is a nearly ideal tool for

the study of blood flow and valve function

in the larger vessels (e.g., by means of

doppler and pulsed doppler techniques)

and for observing the motion and character of organs (e.g., by pulse-echo mapping with mechanically scanned systems

or phased arrays, by the use of doppler

techniques and by systems based upon

holographic principles). As one doctor put

it, ...Once these machines come out of the

laboratory and are put to use in screening

for circulatory disease, there is a good

chance that we can decrease the death rate

from this type of disease by 30%.

The hazards associated with ultrasonic

radiation are the subject of continuing re- search, (although it is generally conceded

that ultrasonic diagnosis at conservative

power levels is safer) than diagnosis by means of ionizing radiation, where the re- sults of genetic damage can often be severe. A recent survey oy the Bureau of Radiological

Health indicates that at present, fetal scans

are performed on approximately 35% of pregnant women in the United States. This percentage is expected to increase in the future

as improved imaging techniques come into use. Much of the present impetus for characterization of the acoustic fields of

transducers and for precise and convenient

measurements of acoustic field quantities

comes from researchers engaged in the

assessment of the effects of ultrasound

upon tissue. However, the present proliferation, in number and kind, of commercial

ultrasonic equipment suggests the necessity

and inevitability of controls by government regulator agencies. This, in turn,

implies an even greater future need for

precise and convenient methods for measuring the fields of medical transducers.

Those dealing with ultrasound hazards

are faced with a difficult problem. It is not generally possible for measurement to

be made in vivo and therefore measurements

of ultrasonic quantities are typically

made in water tanks. The problem thus

remains of relating these measurements

to the complex propagation phenomena that occur in tissue. Little work has been

done in defining the relative roles of

phenomena such as scattering and relaxation

effects upon sound propagation (e.g., upon

sound speed and attenuation) in tissue.

Thus, good basic experimental and theoretical work is most urgently needed in this area

.

Historically, the clinical applications

of ultrasound date back to the 1930' s. For a time, improvement of the early sys- tems was slow and their acceptance was

not widespread. This was because valid

results required highly trained technicians.

Therefore, results often appeared to have

a subjective quality. In more recent years,

the advent of solid-state technology, microcircuits and information processing techniques has resulted in improvements which

permit relatively unskilled personnel to obtain good results.

Many of the more advanced systems (e.g.,

the more sophisticated phased array instrumentation) are currently undergoing development or preliminary clinical evaluation.

Hov/ever, the Federal Government has provided $3 million to the National Science

Foundation to accelerate refinements and

provide more rapid availability of equipment to hospitals and other users. Thus,

there appears to be little doubt that the

medical applications of ultrasound will in- crease rather rapidly. Indeed, some have

predicted a boom to the extent that uses

of ultrasound will equal or exceed X-rays

within ten years (3)(4).

2. STRUCTURE OF THE MEASUREMENT SYSTEM

2.1 Conceptual System

In medical applications of ultrasonics,

mechanical waves at ultrasonic frequencies

are transmitted as a beam through body

tissue for therapeutic purposes, or to obtain information on soft tissue structures,

or to destroy a region of tissue. To obtain the desired effect without toxicity,

users of ultrasonic equipment generally

are most interested in the power, the intensity (power per unit area), and the

field pattern (intensity as a function of

location in the field) of the ultrasonic

3
radiation. For specific needs, there is often an interest in acoustic pressure, . particle velocity, sound speed, absorption

(i.e., conversion to heat), attenuation,

and transducer efficiency. Another quantity of interest is the specific acoustic

impedance to plane waves in the medium.

Table 1 shows the most important quantities used in medical ultrasonics, the

quantity symbol, and the appropriate SI units.

Physical laws provide relationships

among these measurement quantities. For example, for plane v;aves the acoustic intensity (power per unit area) is related

to acoustic pressure, particle velocity,

and specific acoustic impedance by the

simple algebraic formula I = P/S = pu = u^p^c = p2/p^c. However, in the near field of an ultrasonic transducer, wherein

most medical applications lie, such plane- wave relations are not appropriate. The

field quantities are most accurately defined and measured using methods that

properly account for the complex structure

in the near field of ultrasonic beams.

Power

Intensity

Amplitude attenuation

coefficient

Efficiency (transducer)

Note: In the units column of the table above:

s = second, N = newton, Pa - pascal, W =

Field quantities most often of interest

are intensity at a specified point in the

field, total beam power, acoustic pressure,

and field pattern. Methods exist for

determining these quantities in terms of

other measurable quantities.

Intensity and acoustic pressure are usually measured with a small piezoelectric,

capacitive or thermal probe previously

calibrated in a field of known intensity

of pressure, or by using a very small tar- get as a sensor of radiation pressure.

Beam power is often measured with radiation force devices, or with calorimetric

devices, or with acousto-optic equipment.

The field pattern of a transducer is usually measured with a probe mounted on

a precision scanning mechanism. A laseracoustic scanning system now under development (by RCA) (5) will measure displacement

amplitude. A precision scanning system

under construction (by NBS-Boulder) will

be used to provide probe-corrected information on nearly all field quantities,

based on a plane wave scattering matrix

(PWSM) formulation developed at NBS (6).

-1

P W

I W-m"^

a Np/m

n dimensionless

kg = kilogram, m = metre,

watt, Np = neper.

Table 1. Symbols and SI Units for Ultrasonic Quantities.

Quanti ty

Quantity

Symbol

SI Unit

(Symbol

)

Particle displacement y m

Particle velocity (vector) u

-1 m*s

Speed of sound c

Density p

t -3

kg-m

Specific acoustic impedance

for progressive plane waves.

pQ is the equilibrium density

-3

N-m •s=kg'm

Force F N

Pressure P Pa=N-m"^

4
The velocity of sound in, and density

of, water are near that of soft tissue in

the body, and water is commonly used as the convenient standard medium for evaluation of medical ultrasonic equipment.

Sound speed and density information for

water are available as reference data.

Information on ultrasonic absorption and

other forms of attenuation is obtainable

for some, but not all, liquids and solids.

Specific acoustic impedances for plane wave propagation in some materials are available as reference data, and the quantity can be calculated as the product

of the density times the speed of sound

in the material

.

Several types of standard test objects

have been described for use in checking

resolution of ultrasonic diagnostic equipment. Most laboratories have constructed

their own test objects, and at least one

of these is now commercially available.

The only commercially-available measuring

equipment for medical ultrasonic field

quantities (e.g., power, intensity, pressure) are piezoelectric probes and

radiation-force power meters. The latter

are useful for power levels from about 5 to

25 watts. A number of laboratories in universities, hospitals, industry, and in

government have built their own equipment

to perform required measurements, often at great expense, and sometimes with uncertain results. A few interlaboratory intercomparisons have been carried out (7).

Careful workers have attained uncertainties

believed to be in the r5": to ±15" range

for the field quantities of power, intensity,

and pressure. Uncertainties of ±15% are

not adequate for laboratories concerned

with meeting and enforcing performance

standards for ultrasonic equipment, yet,

±5% uncertainties are difficult to achieve.

2.2 Basic Technical Infrastructure

2.2.1 Documentary Specification System

2.2.1.1 Standardization Institutions

Attempts by standardization organizations to establish uniform methods of mea- surement for power and intensity of

therapeutic ultrasonic transducers date

back 20 years or more. For example, the

current standard was promulgated by the

American Standards Association in 1956.

This standard follows very closely that

recommended by the International Electrotechnical Commission which is based upon

techniques available in the 1950's.

As shown by the list below, a surprisingly large number of organizations are active in the standards area. Communication and coordination between these groups

is needed and NBS is represented in at least three of the more important ones.

International

1. International Electrotechnical

Commission

2. International Federation of

Medical Ultrasonics

-- U.S. Government Legal and Regulatory

1. U.S. Dept. of Health, Education

& Welfare (HEW)

(a) Health Services and Mental

Health Administration

National Institute for

Occupational Safety & Health

(NIOSH)

(b) Food and Drug Administration

(FDA)

Bureau of Radiological

Health (BRH)

2. U.S. Dept. of Labor

(a) Occupational Safety and

Health Administration (OSHA) — U.S. Government - Non Regulatory - Dept. of Comm. NBS/Dept. of Defense

Veterans Administration

-- Voluntary Standards Institutions

1. American National Standards

Institute (ANSI)

2. Institute of Electrical and Electronic Engineers, Inc. (IEEE)

3. Instrument Society of American (ISA)

4. Acoustical Society of America (ASA)

5. Association for the Advancement

of Medical Instrumentation (AAMI)

6. American Society for Testing and

Materials (ASTM)

7. American Institute for Ultrasound

in Medicine (AIUM)

8. Alliance for Engineering in

Medicine and Biology (AEMB)

5
— Other Standardizing and Measurement

Insti tutions

1. Foreign

(a) Standards Association of

Austral ia (b) British Standards Institution

(c) Canadian

(d) Physikal isch-Technische

Bundesanstal

t

2.2.1.2 Survey of Vol unt ary (Documentary)

Standards and Recommendations C

I. American National Standards Institute

A. Sponsored by the Acoustical Society

of America (ASA)

1 . Procedures f or Calibration of

Underwater El ectroacousti

c

Transducers . ANSI SI. 20-1972

2. Acoustical Terminology (Including Mechanical Shock and Vibration) USAS SI .1-1960 (Revision

of Z24. 1-1951 and Includes

Z24.1a)

3. Specifications for Laboratory

Standard Microphones USAS

SI .12-1967 (Revision of

Z24. 8-1949)

4 . Method for Speci fying the

Characteristics of Analyzers

Used for the Analysis of Sounds

and Vibrations . USAS Z24. 15-1955

5. Calibration of Microphones . USAS SI. 10- 1966 (Revision of

Z24. 4-1949 and Z24. 11-1954)

6. Calibration of El ectroacoustic

Transducers . USAS Z24.24-1957.

Sponsored by the Acoustical

Society of America

7. Ultrasonic Therapeutic Equipment. ASA Z24. 18-1955

B. Sponsored by the Institute for Electrical and Electronic Engineers (IEEE)

1 . Standard Definitions and Methods

of Measurement for Piezoelectric Vibrators . ANSI C83.17-1970

(IEEE Na 177-1966)

2. IRE Standards on Piezoelectric

Crystals: Determination of the

Elastic, Piezoelectric and Dielectric Constants - The Electromechanical Coupling Factor, 1958

ANSI C83. 23-1960 (IEEE Std.

178-1958, Reaffirmed 1971)

3. Standards on Piezoelectric

Crystals, 1949 ANSI C83. 3-1951

(Reaffirmed 1972) IEEE Std.

176-1949 (Reaffirmed 1971)

SH-0106-5

4. IRE Standards on Piezoelectric

Crystals: Measurements of

Piezoelectric Ceramics, 1961

ANSI C83. 24-1962 (Reaffirmed

1972) IEEE Std. 179-1961 (Reaffirmed 1971) SH-0109-9

Other Voluntary Standards and their

Sponsor

1 . IEEE Standard on Magnetostricti ve Material : Piezomagnetic Nomenclature IEEE Std. 319-1971

SH-0236-0 Institute for Electrical & Electronic Engineers

2. IRE Standards on Piezoelectric

and Ferroelectric Crystals:

Definitions of Ferroelectric

Crystal Terms 1962 62 IRE 14. SI SH-02527 Institute of Electrical

& Electronic Engineers

3. Specifications and Tests for

Piezoelectric Pressure and Sound

Pressure Transducers ISA-S37.10

1969 Instrument Society of

America

4 . Specification and Tests for

Piezoelectric Pressure and Sound

Pressure Transducers ISA-S37.10

1969 Instrument Society of

America

5. Testing and Calibration of Ultrasonic Therapeutic Equipment

Publication ISO 1963 International El ectrotechnical

Commi ssion

6. Ultrasonic Therapeutic Equipment

AST 40-1969 Standards

Association of Australia

7. Standard Specifications of Echoscope Sensitivity and No ise Level

Including Recommended Practice

for Such Measurements Adopted

October, 1973 American Institute

of Ultrasound in Medicine

6
2.2.2.1 Measurement Tools and Techniques

Within the past year at least three

techniques have been reported in the

literature fc measurement of the near and

far field characteristics of ultrasonic

transducers in a water medium. These include two o;:tical rr.ethods (8.9) and a steel

ball reflector scheme (10) which employs

conventional i'^strumentation. The data

provided by these iKethods is r.ostly in terms of relative field intensity rather

than absolute values. Nevertheless, the

data does provide useful information

concerning transducer quality.

Powerful techniques have been developed

for the evaluation of sonar arrays and

electromagnetic antennas. Although, these

array and antenna problems are somewnat

different than those in !T-:edical ultrasonics,

they have served to suggest analogous methods

of equal (and greater) utility for the evaluation of transducer-fluid systems employing

ultrasound.

Also, several methods for measuring

acoustic field quantities have evolved to meet the needs of individual laboratories.

Intercompariscns of techniques used by

various laboratories have taken place.

The present state of knowledge in this

field is such that no single measurement

system has em.erged as a primary standard.

It is possible that more flexible and perhaps less convenient methods of field

specification 'e.g., a determination re- quiring detailed scanning of the radiation

field) may emerge as such a primary standard. It will then be possible to evaluate

the performance of less flexible measure- ment systems and more precisely calibrate

convenient secondary standards for dissemination to laboratories, hosoitals and

medical centers.

The following paragraphs will outline

existing tools and techniques in medical

ultrasonics. These will include existing

techniques •^or determining field quantities,

field scanning and mappinn technology, and

instrumentation performance stanaaras.

Field Scanning '•'.ethodol ogy and Equipment

The necessary equipment for the mapoing

of acoustic fields must include a positioning mechanism with an inherent uncertainty

in position that is a small fraction of a wavelength, and an "ideal" field probe.

An ideal probe should have a critical

dimension that is a small fraction of a wavelength to prevent spatial averaging

effects. It m'jst also be nearly omnidirectional and nave a flat frequency response.

Wavelengths on the order of one millimeter are CGr.monly used in medical ultrasound. For imaging eye structures the

wavelengths may be less by a factor of ten or more. Thus, the requirement that the

probe size be a small fraction of a v/avelength imposes a serious fabrication

DroDlem. In addition prooes having sucn

a small size are generally very insensitive.

Recent work at NBS - Boulder has provided the theoretical basis and the means

for correcting a known receiving probe

for non-ideal characteristics. This solves

the sensitivity and fabrication problems,

because a larger probe may be used. The

resulting data may then be corrected to that of an ideal, omnidirectional, point

probe having a flat frequency response.

The basis of this work is a plane-wave

scattering matrix (PWSM) formulation involving the relatively mild assumptions

of linearity, a homogeneous medium, and

negligible secondary reflections. This

v/ork also allows more efficient scanning

since a scan over a single plane (e.g., in

the near field of a transducer) is shown

to be sufficient for complete specification

of the sound field.

Since errors due to the directional

characteristics, spatial averaging effects,

and freauency response of a orobe may be

removed, the remaining errors for in-vitro

measurements at diagnostic power levels

should reside in the sampling process and

the precision of the traversing mechanism.

These errors may be estimated and hence

there is no conceptual difficulty in making

them arbitrarily small. Therefore, efficient scanning employing a PVJSM formulation

has the potential of being a more precise,

Dossibly primary, measurement system for

medical diagnostic transducers.

Existi ng '-'ethods 0"^ Ultrasonic Field

Veasurer'e nt

At this time the following techniques

are being examined and used for measuring

quantities in the acoustic fields of

medical transducers. There is no current

agreement concerning the relative suoeriority of any one of these techniques. In fact, there has been no definitive work

outlining the sensitivity of these methods

over ranges of e.g., acoustic power,

acoustic intensity, or frequency. In at least some cases, this information is not yet known.

7
However, it seems certain that some combination of these methods will provide

the basis for a convenient system of primary or secondary standards for the specification of the important field quantities

associated with the diverse and expanding

field of medical ultrasonic equipment.

These methods are:

1. Radiation Force Measurements -- field

quantities are determined by observing

the deflection of a reflecting or absorbing target in the acoustic field.

2. Calorimetry -- field quantities are determined from observation of the

temperature rise in the fluid due to sound absorption.

3. Methods Involving Capacitive Transducers -- field quantities are determined from the modulation of the spacing

of a charged parallel plate capacitor

by and incident sound wave.

4. Methods Involving Reciprocity Calibration Techniques -- field quantities are determined using the field-theoretic

principle of reciprocity. For example,

the measurement could involve two identical transducers, or three transducers, one of which is known to be

reciprocal. This technique has involved far-field relationships.

5. Optical Diffraction Techniques -- field

quantities are determined from the

diffration of light by the acoustic

field.

6. The Electrical or Equivalent Circuit

Method -- evaluation of the parameters

in an equivalent circuit model of

resonant transducers under different

loacing conditions permits determination of the radiated power directly

from electrical measurements.

Instrumentation Performance Standards

Recently, the American Institute of

Ultrasound in Medicine adopted a standard

test object (i.e., a wire gauge for use in

tanks of fluid) for the clinical evaluation of azimuthal and range resolution

provided by diagnostic pulse-echo equipment.

Consideration has been given by the AIUM

standards committee to tissue phantoms,

which would help in the evaluation of an ultrasonic system's ability to discriminate

between soft tissue structures. However,

this idea has been set aside for the

present because not enough is known about

both sound propagation in tissue and the

stability of the acoustical properties of

polymers over extended time periods.

The method is more qualitative than

quantitative. With the advent of, for

example, phased array instrumentation involving signal processing, compression of

the dynamic range of echo-information, the

use of switched delay lines, and gray-scale

CRT displays, evaluation of resolution

characteristics alone is overly simplistic

and inadequate.

Other groups, such as the Acoustical

Society of America and the International

Electrotechnical Commission, are working on the development of performance standards for

ultrasound diagnostic equipment.

2,2.2.2 The Instrumentation Industry

One of the clearest pictures to emerge

as a result of this study is the non- availability from commercial sources of

accurate, reliable ultrasonic measurement

equipment. At present there are less than

a dozen companies which make measuring

instruments and such equipment is usually

a sideline. The performance characteristics of available instruments have not been evaluated at NBS so that we are un- able at this time to assess their adequacy.

However, as a result of conversations with

colleagues and perusal of the literature,

it appears that present instruments leave

something to be desired in the way of

accuracy, reliability, and user convenience.

At least three manufacturers of ultrasonic equipment are now attempting to manu- facture ultrasonic power measurement

equipment. As of this writing their instruments have not appeared on the market.

Doubtless, more manufacturers will enter

the field as measurement needs become more acute with the growth in numbers of ultrasonic equipment users.

At present, manufacturers do not maintain traceability for ultrasonic power and

intensity to national reference standards.

Within the next year, NBS will attempt to establish a Measurement Assurance Program

(MAP) with manufacturers of both measuring

instruments and medical ultrasonic equipment. The new BRH regulation on therapeutic

equipment is expected to be issued very

soon. This regulation is expected to be a strong incentive for manufacturers to seek

traceability to national standards. We

believe that they will welcome any help

that they can get to insure that equipment

meets BRH requirements.

8
2.2.5 Science and People 2.3 Realized Measurement Capabilities

The professional societies relevant

to this field of measurement activity and

their interrelationship are as follows:

International Electrotechnical Commission

Technical Committee on Electroacoustics

Sub-Committee 29D: Ultrasonics

Working Group 4: Medical Applications

Alliance for Engineering in Medicine

and Biology (AEMB)

AEMB's Professional Society Membership

includes the following organizations:

4.

5.6.

7.8.

9.

*10.

n.

12.

13.

*14.

15.

16.

17.

*18.

*19.

*20.

21.

22.

23.

24.

Aerospace Medical Association

American Academy of Orthopedic

Surgeons

American Association of Physicists

in Medicine

American College of Chest Physicians

American College of Physicians

American College of Radiology

American College of Surgeons

American Institute of Aeronautics

and Astronautics

American Institute of Biological

Sci ences American Institute of Ultrasonics

in Medicine

American Society for Engineering

Education

American Society for Heating,

Refrig. & Air-Cond. Engineers

American Society for Hospital Engineers of Am. Hosp. Assoc.

American Society for Testing and

Material

s

American Society of Agricultural

Engineers

American Society of Internal

Medi ci ne American Society of Mechanical

Engineers

Association for the Advancement of

Medical Instrumentation

Biomedical Engineering Society

Institute of Electrical and Electronic Engineers

Instrumentation Society of America

National Associations of Bioengineers

Neuroel ectric Society

Society for Advanced Medical

Systems

Existing measurement capabilities for

ultrasonic power and intensity are located

mostly in university laboratories (many of

which are associated with medical schools),

certain manufacturers, research hospitals,

the Bureau of Radiological Health (BRH),

and at NBS. Much of this capability (outside BRH and NBS) is specialized for particular applications and little if any

capability exists in treatment hospitals

and clinics.

In a recent (1973) survey in Dada

County, Florida, and Montgomery County,

Maryland, the BRH found that the sonic

output of therapeutic transducers differed

by as much as 100% compared to the reading

of the meter contained in the equipment.

In one case, a therapeutic transducer used

almost daily by a certain hospital was

found to have no output even though the

internal meter indicated normal operation.

These examples serve to illustrate the

chaotic conditions which presently exist

in parts of the medical ultrasonics field.

Table 2 below shows the measurement

capability in various segments of the mea- surement system.

Table 2. Capability in Various Segments of

the Measurement System.

Segment

NBS

BRH

Research Hospitals

Manufacturers

Treatment Hospitals

CI inics

Capabil ity

(accuracy)

37o to 5%

5%

10% to 15%

20% to 30%

none

none

*Organizations which have standards committees

2.4 Dissemination and Enforcement Network

The Bureau of Radiological Health (BRH)

has responsibility under the Radiation Control for Health and Safety Act of 1968 for

enforcement of maximum radiated power and

intensity rules which have yet to be adopted.

For diagnostic equipment users have generally

agreed upon vpper limits for radiated intensity

of 0.5 mW/cm- time averaged or 0.5 W/cmpeak pulse.

9
The BRH maintains in-house standards

for accurate measurement of the output power

of medical ultrasonic equipment submitted to their laboratory for approval. They have no intention of disseminating standards or calibrating measuring equipment for other

organizations. Therefore, it will be the

responsibility of N8S to provide dissemination services as the first step in

realizing a coherent measurement system. A major shortcoming of the present system is

lack of a suitable transfer standard.

2.5 Organization Input-Output Transactions

Matrix

The organizations which perform measurements include the BRH, university laboratories, manufacturers and users such as researchers and hospitals. Results of

researchers investigating the biological

effects of ultrasound are not always in

agreement and one cause of this is the inaccuracies in measuring ultrasonic power

and intensity at various laboratories.

Users of ultrasonic equipment (i.e., hospitals

and clinics) also make measurements of power

especially for therapeutic applications. In addition to inadequate instrumentation, a large part of the problem also lies in the

lack of knowledge of proper measurement

techniques by medical personnel. Thus, a great amount of work must be done in the

development of both standard techniques

and their dissemination.

Measurement results are used by researchers in, for example, assessing the thresholds of tissue damage in animals and for

other uses. Manufacturers require meas- urement results to establish specifications

for their equipment and instruments sold in

the marketplace. Periodic measurements are required in hospitals, etc., to insure the

continued proper operation of ultrasonic

equipment. The results of the surveys in

Florida and Maryland, described above, are

an example of what can happen when frequent

monitoring is not carried out.

3. IMPACT, STATUS AND TRENDS

OF MEASUREMENT SYSTEM

3.1 Impact of Measurement

The maximum allowable error in the power

output metering for various types of medical ultrasonic equipment will be set by

the BRH. Thus, every manufacturer and user will need accurate measurements to insure that equipment metering indications

are operating within the established limits

of error. In research on tissue damage

by ultrasonics, it will be necessary that

all measurements of power and intensity

be referred to a common standard if observed damage levels are to correlate

between researchers.

In therapeutic applications of ultrasound, different power levels are required

to treat different parts of the body.

Therefore, accurate metering of the output

power is important if expected results' are

to be obtained.

3.2 Status and Trends of the System

As stated previously, the present status

of the system is fragmented, uncoordinated

and inadequate. A serious shortcoming

is the lack of a reliable, low-cost transfer standard which can be used by non- technical personnel.

The trend is toward increasing demand

for both quantity and quality of measurements from all segments of the industry,

researchers and medical users. Two brief

news releases from NBS during 1974 resulted

in a large number (60) of inquiries from

the field for more information, literature,

etc. This is indicative of the increased

interest in the measurement aspect of

medical ultrasound.

At least three manufacturers have, in

the past year, begun development of instruments to measure ultrasonic power. None of

these are in production as yet and when they

become available, they will be the first

U.S. made instruments to come on the market.

Scanning systems are available commer- cially for measurement of ultrasonic beam

profile and field pattern. The accuracy

of these systems is not known at this time.

4. SURVEY OF NBS SERVICES

4.1 The Past

The Electromagnetics Division began

development of standards for ultrasonic

power in 1972. First announced services

at 2 MHz came in January, 1974, with additional frequencies, 1, 3 and 5 MHz in

August, 1974. In addition, a 6 volume

set of bibliography on ultrasonics was published and widely distributed.

4.2 The Present -- Scope of NBS Services

4.2.1 Description of NBS Services

Present NBS services include the meas- urement of the response of ultrasonic total

beam power measuring instrument to inputs

of known level in the range 5 uW to 1 W,

10
continuous wave. The known power is emitted

from NBS-characterized quartz transducers

and the applied voltage is measured by NBS

calibrated voltmeters. An overall accuracy

of 5% or less is achieved at frequencies of

1, 2, 3 and 5 MHz. Maximum beam diameter

is 2.5 cm. The NBS standard is transportable and

may be taken to custoirer sites for calibrating of measuring equipment.

The resonant radiation conductance, Gr,

of quartz transducers can also be measured

at frequencies from 1 to 5 MHz. Such

characterized quartz transducers could be

used as a transfer standard by those laboratories which have the competence to make accurate high frequency voltage

measurements and have other measuring

equipment such as oscilloscopes.

Funding for NBS services will be on a reimbursable basis. Services will be provided by measurement of customer transducers or power measuring instruments at NBS or measurement of the response at power measuring instruments at the cus- tomer site.

The level of reimbursable funds for mea- surement services is expected to increase

to $25 K/yr. by 1980. Other agency funding for specific development projects will

probably remain at the S20 K - $30 K level.

Certain foreign national laboratories,

notably West Germany and Australia, have

been working on ultrasonic standards for some 15 years and have developed a larger

capability in measurements than has NBS. Because of the boom in commercial diagnostic equipment, a corresponding need is developing for measurement techniques to evaluate and regulate this equipment.

This need is five-fold. It includes:

1. A need for more precise measurements of

pulsed acoustic fields, possibly

representing a primary measurement

system.

2. A need for more convenient secondary

standards for dissemination to medical

centers, hospitals and laboratories

(government, industrial and academic).

3. A need for better evaluation and intercomparison of existing methods for

measuring field quantities (especially more data on sensitivities as functions

of acoustic power or intensity, and as functions of frequency).

4. A need for more precise relationships

between ir. vivo acoustic propagation

phenomena and their relation to in vitro

(water tank) measurements.

5. A need for more and better test objects

to provide performance standards for

sophisticated medical ultrasonic

equipment.

Work is currently underway at NBS-Boulder

in areas 1 , 2, and 3.

Area 1

A scanning system and related theoretical work allowing probe correction and

characterization shows promise of providing

a precise (possibly a primary) measurement

system. The theoretical work is largely

complete for the acoustic case and the

analogous electromagnetic formulation has

been applied successfully in electromagnetic antenna calibration. The acoustical

scanning system is currently under con- struction and an interim system may be

complete in three months.

Area 2

This work is urgently needed and preliminary consideration is being given to the

design of such secondary standards for

field use.

Area 3

Efforts have been made to become aware

of the technology and methodology currently

in use in ultrasonics laboratories for

evaluation of ultrasonic transducer-field

systems

.

A preliminary survey has been made of

the ranges and types of balances applicable

to the measurement of radiation force.

Also, some understanding of the limitations

of optical diffraction techniques has been

gained. A theoretical generalization of

reciprocity calibration techniques has

been found which permits the determination

at a single frequency of the directional

character of each of three separate transducers from three transverse planar scans involving different pairs of the transducers .

Areas 4 & 5

No efforts, other than literature re- view, have taken place in these areas. However, expertise within the Electromagnetics Division (e.g., familiarity with

scattering, lossy media and relaxation

effects) could be brought to bear on problems of relating water-tank measurements

to tissue properties. The experimental

aspects of this work are probably beyond

the scope of NBS-Boulder' s activities.

n
However, there is a strong possibility

that a collaborative effort could be worked

out with a medical center or academic

laboratory.

Development and long-term evaluation of

tissue phantoms would correlate well with

the theoretical modeling of tissue

properties

.

4.3 Impact of NBS Services

There has not been adequate time to evaluate the impact of MBS services be- cause the announcement of these services

came only a few months ago. Nevertheless,

inquiries by letter, telephone and at con- ferences indicate that the NBS services

are most welcome and needed. Negotiations

are in progress with two university laboratories for i ntercompari sons and/or

calibrations of ultrasonic power measuring

systems developed at the universities.

The major user classes include medical

and biological researchers (e.g., university medical schools), manufacturers of

equipment and instruments, regulatory

agencies (i.e., BRH), and medical practitioners, hospitals, etc. The economic impact

at this early date after announcement of

NBS services is not known exactly but is bel ieved to be smal 1

.

The technological impact is somewhat

greater in that a new and independent

standard was developed at NBS for calibration of ultrasonic power measuring equipment. The new standard utilizes existing

electromagnetic standards to measure the

resonant radiation conductance, G^, of

special quartz transducers, by measuring

the input voltage to the transducer, the

radiated acoustic power, P, is calculated

from the equation P = Gp. Details of

this method have been disseminated via

presentation of a conference paper, and

subsequent publication in a widely circulated technical journal. Interest has

been expressed by those in the measurement

field to procure transducers similar to the

NBS type and use them as transfer standards

between NBS and their own laboratories.

4.4 Evaluation of the NBS Measurement

Program in Medical Ultrasonics

In the three years of its existence,

the NBS program in standards development for

medical ultrasonics has made significant

progress. The first step, a comprehensive

literature survey, was published and disseminated to approximately 50 organizations and individuals. Subsequently, mea- surement standards were developed in the

form of electrically characterized quartz

transducers which serve as sources of known

total beam power. These standards are especially useful for calibrating receivingtype transfer standards in a single measurement step. They were used in the 1973

i ntercompari sons with BRH. Unfortunately, there exist but few

receiving-type measuring instruments which

are suitable as transfer standards. As mentioned previously, this fact is a major

shortcoming in the effective operation of

the NMS in medical ultrasonics. Present

NBS capabilities are inadequate to meet

present needs.

Additional capability nearing completion is the calorimetric type basic power

standard developed with funding assistance

from BRH. This standard will allow calibration of transducers directly. Also,

the new scanning system for measurement

of beam profile, power, intensity, etc.,

is 60% complete and will be usable in

FY 76.With completion of these standards, NBS

will have a well-rounded basic capability

in ultrasonic power. The task of developing a low-cost, portable, reliable transfer standard will begin in late FY 76.

4.5 The Future -- Recommended NBS Actions

Since ultrasonics appears destined to become a major technology in medical diagnosis, it seems imperative that accurate

standards for measurement of the physical

quantities be provided by a competent

government laboratory. NBS should be that

laboratory.

There appears to be no authenticated

instances of damage to human tissue under

the conditions normally employed in diagnostic ultrasound. However, as usage ex- pands and users strive for ever better

sonic images, there will probably be a tendency to use higher intensities and

longer exposure times. Thus, the establishment of danger threshold is a high priority

necessity. A necessary tool in measuring

damage thresholds is accurate standards

for measurements of the levels of ultrasound which produce tissue damage. It is

clearly an NBS function to provide an accurate consistent measurement system for

ultrasound as has been done for other

physical quantities and phenomena.

Within a year, NBS will have developed

a new calorimetric power standard to supplement our present technique and a scanning

system which will provide information on transducer field pattern, intensity distribution over the beam area and other quantiti

12
of interest. These systems are highly com- plex and require specialists for this proper

operation. There needs to be developed

simpler systems for use in the field. A very high priority item is development of

a reliable, rugged, low-cost transfer

standard for ultrasonic power so that NBS

accuracy can be disseminated to users on the broadest possible basis. This transfer

standard should be developed at NBS and

subsequently manufactured by private

industry.

Ultimately, NBS should provide a mea- surement assurance program in ultrasonics

with manufacturers, BRH, and other principal

laboratories such as universities and major

medical centers. These latter laboratories

could then provide measurement support to hospitals and other users.

To meet these objectives in a reasonable

time the NBS level of effort should be increased to $250K per year from the present

$100K per year for the next four to five

years

.

5. SUMMARY AND CONCLUSIONS

Diagnostic ultrasound is on the verge of

a technological boom. To obtain maximum

benefit from this technology and to protect

the public from possible hazardous radiation

levels, accurate standards must be developed

and disseminated widely. NBS has developed

standards for ultrasonic power at 1 , 2, 3,

and 5 MHz. Standards for a much broader

frequency range are in preparation and

will be available in 1976.

The projected size of the medical ultrasonics field requires that NBS provide

measurement services to include as a minimum,

total power, intensity, field pattern, and

transducer efficiency. Capability for

accurate measurement of these quantities

will form a basic national measurement system in ultrasonics and will be coordinated

by NBS both in the U.S. and with foreign

countries. Time-wise, the system is needed

as soon as possible. Funding should be

increased from the present level of SIOOK/

year to $250K/year for the next four or

' five years.

METHODOLOGY OF THE STUDY

APPENDIX A.

This study is an updated revision of the

Ultrasound Issue Study performed in 1970-71.

New information was gleaned from articles

in technical journals, popular press, and

conversations with other workers in the

field. Many of the conversations, interviews, etc., are documented in trip reports

from attendees at conferences, symposia,

and meetings.

ACKNOWLEDGMENT

The following people contributed to the

research and writing of this study:

T.L. Zapf 276.50

R.F. Metzker 276.50

Dr. Eric Miller 276 Postdoctoral

Research Associate

13
REFERENCES

[1] Erikson, K.R., Fry, F.J., and

Jones, J. P., Ultrasound in medicine --

a review, IEEE Trans, on Sonics and

Ultrasonics, Vol. SU-21 , No. 3,

July 1974, pp. 144-170.

[2] Jungen, Ronald K. , Ultrasonics in

medicine: strong; getting stronger,

IEEE Spectrum, Vol . 11 , No. 8,

August 1974, pp. 62-66.

[3] Marx, Jean L., Diagnostic medicine:

the coming ultrasonic boom. Science,

Vol. 186, 18 October 1974, pp. 247- ' 250.

[4] Prospectives for ultrasonic imaging

in medical diagnosis. National

Science Foundation Report, Washington,

D.C. , November 1973.

[5] Mezrich, R.S., Etzold, K.F., and

Vilkomerson, D.H.R., Ultrasonovision,

Paper C-2, IEEE Ultrasonics Symposium

1974, Milwaukee, Wise, November 1974.

[6] Kerns, D.M., Scattering-matrix

description and near-field measure- ments of electroacoustic transducers,

NBS Tech. Note 651 (1974). Also

J.A.S.A. (in press)

.

[7] Breazeale, M.A., and Dunn, F.,

Comparison of methods for absolute

calibration of ultrasonic fields,

J. Acoust. Soc. Am., 55, No. 3, pp. 671-672 (1974).

[8] Cook. B.D., and Berl inghieri , J.C.,

Experimental mapping of ultrasonic

fields by light diffraction,

Proceeding 1975 IEEE Ultrasonics

Symposium, pp. 133-135.

[9] Mezrich, R., Vilkomerson, D., and

Elgod, E., Ultrasonic imaging

exploits phase contrast, IEEE Spectrum, Vol. 12, No. 12, December

1975, pp. 35-39.

[10] Cousin, A.J., Smith, K.C., and

Rowe, I.H., A calibrated ultrasonic

field measuring system for general

use, IEEE Trans, on Instrumentation

and Measurement, Vol. IM24, No. 4,

December 1975, pp. 317-321.
NBS-lUA (REV. 7-73)

U.S. DEPT. OF COMM.

RIRI IflfiRAPHlC DATA

SHEET

1. m iM.u A ric)\ OK Ki:i'()K r .\o.

NBSIR 75-937

2. Ciov't Accession

No.

3. Recipient's Accession .\'o.

4. TITLL-; .\i\D SI H rn

The National Measurement System for Medical Ultrasonics

5. Publication Date February 1976

6. Performing Organization Code

276.02

7. AU THORiSi

Paul A. Hudson

8 • Pf* r f nr m int> (")ri7in R**r\r»rf N n

9. PHRFOR.MING ORGANIZATiO.X .NA.ME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Proiect/Task/Work Unit No. 2762111 1 11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same as Item 9

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14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or /ess [actual summary of most sigrtilicant information. If document includes a significant

bibliography or literature survey, mention it here.)

The rapid increase in the use of ultrasonics in medical diagnostics has been

brought about by recent advances in imaging techniques employing sophisticated

electronic systems. Along with the proliferation of this equipmeni in the medical

community has come concern about possible tissue damage due to this type of

radiation. Research on damage thresholds is being carried out in laboratories

across this country and overseas. Agreement betv/een various researchers is

hampered by lack of accurate standards for measurement of transducer beam power

and intensity.

The National Measurement System for medical ultrasonics is, at this time,

inadequate, fragmented and largely uncoordinated. The Electromagnetics Division

is in process of developing standards for measurement of transducer beam power,

intensity^ and beam pattern. These standards will be disseminated widely throughout

the country to provide the needed accuracy and measurement agreement among

biomedical researchers and ultimate users (hospitals, physicians, clinics, etc.).

Interim standards for beam power were developed 1 1/2 years ago and are already

in use by standards laboratories of manufacturers and universities. It is

estimated that another 5 years will be required to bring the measurement system

under some form of control

17. KEY VIORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Electroacoustics; medical diagnostics; radiation safety; radiation therapy

(ultrasonic); ultrasonics.

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