Medical ultrasonic standards at NPLR.C. Preston, Ph.D., B.Sc, and A.J. Livett, B.Sc.

Indexing terms: Ultrasonics, Biomedical engineering, Standardisation and standards

Abstract: Absolute measurement of the acoustic output of medical ultrasonic equipment in the megahertzfrequency range is discussed, together with the methods developed at NPL for the determination of the totalultrasonic power using a radiation pressure balance and the measurement of spatial and temporal field quan-tities with miniature piezoelectric hydrophones. The principles of the radiation pressure balance and the absol-ute calibration of hydrophones, which form the basis of the NPL ultrasonic standards programme, aredescribed. Finally, beam calibration systems, which have been specially developed for the rapid and quantitativeevaluation of ultrasonic fields, are briefly discussed.

1 Introduction

The use of ultrasound in medicine has increased dramat-ically in recent years. It is, however, generally restricted tothe frequency range 0.5-15 MHz. The lower limit is dic-tated by the velocity of ultrasound in tissue (approximately1500 m/s), which makes the wavelength and hence theresolution, or localisation, at lower frequencies unaccept-able. The upper limit is determined by the attenuation ofthe ultrasound which increases with frequency.

There are, essentially, three categories of application ofultrasound in medicine: imaging, monitoring and therapy.Imaging is based on the pulse-echo technique, where ashort pulse is sent into the body and the reflected acousticsignals are detected by the same transducer. Imaging tech-niques are now routinely applied to all soft tissues,although obstetric examination tends to be the majorapplication. The second category, which utilises theDoppler effect, is the measurement and monitoring of bothblood flow and foetal heart rate. Finally, ultrasound isfinding increased use in the third category, physiotherapy,and, in addition, the new technique of hyperthermia isbeing investigated; this involves the local heating of tissueby ultrasound in combination with drugs or X-ray irradia-tion for the treatment of certain types of tumour.

In virtually all medical applications the ultrasound isgenerated by the electrical excitation of piezoelectric trans-ducers usually made from ceramics. However, the physicalcharacteristics of the ultrasonic fields generated by theequipment in these various applications differ consider-ably. For instance, pulse-echo imaging systems use shortpulses of one or two cycles duration and high peakacoustic pressures, typically between 5 x 105 Pa and50 x 105 Pa, at repetition rates of, say, 1-3 kHz with afundamental frequency in the range 2 MHz to 5 MHz. Forophthalmic work, frequencies up to 15 MHz are oftenused. Doppler systems, however, use lower peak acousticpressures and the ultrasound is generally either continuousor in a gated sinusoidal (tone burst) mode at frequenciesbetween 2-8 MHz. Therapeutic equipment uses eithercontinuous-wave ultrasound or relatively long tone bursts(well over 100 cycles), at acoustic pressure levels somewhathigher than those used for Doppler systems and at fre-quencies typically between 0.5 MHz and 3 MHz.

Whereas a single-element transducer is employed inmuch medical ultrasonic equipment, in pulse-echo scan-ners there is a wide range of complex multielement trans-

Paper 3162A, first received 8th November 1983 and in revised form 2nd February1984

The authors are with the Division of Radiation Science & Acoustics, NationalPhysical Laboratory, Teddington, Middlesex TW11 0LW, England

ducers such as those used in linear and phased-arraysystems, and in various types of sector scanners with elec-tronic or mechanical beam steerage. These complexsystems generate acoustic fields which exhibit a wide rangeof temporal and spatial characteristics, as well as widelydiffering peak acoustic pressures.

With the rapid growth in the use of ultrasound in medi-cine, it has been realised that there are a number ofrequirements for the measurement and specification of theacoustic output of medical ultrasonic equipment. First, fordosimetric purposes, exposure levels can only be moni-tored, or, in the case of applications in therapy and hyper-thermia, controlled, if the absolute output is known. It isonly by careful beam characterisation combined with fullycontrolled clinical studies that the mode of interaction ofultrasound with tissue can be expected to become fullyunderstood. Secondly, the reliable assessment of acousticoutput, beam shape and pulse waveform can provide valu-able information on certain aspects of ultrasonicequipment performance. In the design and optimisation ofnew ultrasonic equipment, it is often essential to determinereliably these acoustic parameters as they play importantroles in the ultimate performance level of any particularsystem. Absolute measurement techniques provide accu-rate reference information and the results of measurementsare capable of being monitored over both short and longperiods, forming the basis for objective national and inter-national comparison and assessment.

The benefit derived from the use of a well studied mea-surement technique can be seen, for example, when mea-suring the acoustic pulse waveform from focused medicalultrasonic transducers. The high acoustic pressures and thenonlinear propagation of the ultrasound in water lead to asawtoothed distortion of the acoustic pulse waveform. Thiseffect can be observed if a broadband hydrophone is usedfor the measurement, but if a narrowband hydrophone isused the distortion may not be noticed. Furthermore, as aresult of diffraction effects in the field, the positive acousticpressure can be significantly larger than the negative(perhaps by a factor of 3). This phenomenon is most pro-nounced at the point of highest acoustic pressure, namelythe centre of the beam at the focus. These effects couldhave significant consequences for water-bath scanners andin other clinical applications where fluid constitutes amajor portion of the acoustic path length, and can only beobserved using broadband hydrophones such as the mem-brane hydrophone described later.

Traditionally, measurements of the acoustic outputfrom medical ultrasonic equipment are made with theultrasound propagating through water. At first sight thisseems to be an inappropriate choice, because the attenu-ation of ultrasound in tissue is significantly larger than for

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water. However, the velocity of sound is similar in the twomedia and, hence, if scattering is neglected, any focusingand beam direction properties should not differ signifi-cantly. In addition, the media have similar densities and,hence, their acoustic impedances are similar; and theamount of acoustic energy transmitted from a transducerinto the media will, therefore, be approximately the same.Owing to its low attenuation, measurements made in waterwill yield maximum values for the acoustic field par-ameters such as could occur in low attenuating regions ofthe body like amniotic fluid and urine. Of course, this pre-supposes that there are no highly focusing reflections at,say, bone surfaces, or highly focusing refractive effects,both of which could cause additional beam convergenceand even higher acoustic pressures. Finally, water is plenti-ful and cheap, and can be easily specified.

There is, as has been shown, a need for absolute tech-niques suitable for the determination of the acousticoutput parameters for the wide range of acoustic fieldsgenerated in water by medical ultrasonic equipment. Tomeet this requirement, it is necessary to develop and tomaintain a common basis for the measurement and specifi-cation of such fields. It is for this reason that the NationalPhysical Laboratory (NPL) has established an ultrasonicssection to develop the necessary national standards and todisseminate them through the provision of a measurementservice. To ensure consistency with other national labor-atories, NPL also collaborates in international inter-comparisons. In addition, to meet the requirements of acommon basis for the specification of fields, NPL partici-pates in the discussion and preparation of written stan-dards for international organisations such as theInternational Electrotechnical Commission (IEC).

2 Measurement methods for ultrasonic fields

There are a number of widely differing techniques whichmay be used for. ultrasonic field measurements, and thereader is referred to the literature [1-7] for a detailed dis-cussion of the relative merits of each. For the purpose ofthis paper, it should be noted that, while each techniqueoffers certain advantages over others, most have funda-mental limitations or restrictions. For instance, Schlierenvisualisation is a powerful and versatile technique, but itintegrates the effect of the refracted light passing throughthe ultrasonic field along a line of sight, and hence quanti-tative assessment is very difficult. Thermocouples andthermistors offer convenient and potentially small-sizedsensing elements, but they only determine temporally aver-aged quantities.

Of all the techniques, two have found most commonusage in recent years, mainly because of their potentialaccuracy and reproducibility, and these two techniquesform the basis of the standards work at NPL. The first isthe radiation pressure balance which measures directly theforce exerted by the ultrasound on a suspended target inwater. The change in the apparent weight of the target canbe related to the ultrasonic power incident on the target.The second technique is the use of a calibratedpiezoelectric hydrophone. A hydrophone is a devicedesigned for the reception of ultrasound propagating inwater and permits the determination of the spatial andtemporal distribution of acoustic pressure in the field. Thisis because the electrical signal generated by the hydro-phone, at any instant, is proportional to the instantaneousacoustic pressure at the active element.

234

3 Establishment of primary standards

Both a primary standard radiation pressure balance andspecial high-quality hydrophones, which have performanceproperties appropriate to the characterisation of acousticfields generated by medical ultrasonic equipment, havebeen developed at NPL. As hydrophones are not absolutedevices, it is also necessary to develop absolute hydro-phone calibration techniques. To some extent, the type ofhydrophone calibration technique is dictated by the typeof hydrophone; and, therefore, the requirements for hydro-phones and the different types which have been developedwill be briefly described in Section 3.2.

3.1 Total power measurement based on radiationpressure balances

The time-averaged force per unit area on the surface of anobstacle placed in an ultrasonic field is commonly giventhe name radiation pressure. This radiation pressure isexactly related to the time-averaged momentum flux of theultrasonic wave [8]. Therefore, any object which absorbsor reflects an ultrasonic wave, and hence changes the wavemomentum, will experience a force equal to the rate ofchange of that momentum. For a perfectly absorbingtarget, this force can be related to the time-averaged totalpower in the ultrasonic wave using the relation

F= W/c (1)

where F is the force on the target, W is the total powerand c is the velocity of the ultrasonic wave. This equationis, in general, only valid for infinitely small particle dis-placement amplitudes and plane travelling waves, butappears to be applicable to many of the fields frommedical ultrasonic transducers, although the exact relationbetween the force on a target and the total power is stillsubject to some controversy.

Most single-element medical ultrasonic transducershave element diameters of between 5 mm and 25 mm,which is large compared with the acoustic wavelength, andhence, unlike the situation that exists for acoustical sourcesin the audible range, a target that intercepts the entireultrasonic beam need not be of an unreasonable size. Thetotal power from an ultrasonic transducer in water may,therefore, be determined by suspending a target in a watertank and measuring the force on the target due to theultrasound, usually in a vertical direction. The total powerfrom medical diagnostic ultrasonic equipment may be aslow as 1 mW, and an ultrasonic beam of this power inci-dent normally on a totally absorbing target produces achange in the apparent weight of the target of approx-imately 69 fig. Hence, a microbalance for the accuratedetermination of total ultrasonic power must have a sensi-tivity of 1 fig or better.

The first instrument used to determine total outputpower by the measurement of radiation pressure wasdescribed by Altberg in 1903 [9]. Since then, many differ-ent measurement devices have been mentioned in the liter-ature and these can be divided into two classes: the targetis allowed to move and the amount of movement is noted,or the force required to keep the target stationary is mea-sured. Systems with moving targets, in general, tend to beinsensitive and can only be used at therapeutic powerlevels. Such a system called a tethered-float radiometer hasbeen designed and built at NPL [10] for determining theoutput power from therapeutic equipment. This radi-ometer consists of a target, which just floats in water, heldin place within a water bath by three light silver chainslinking the body of the target to the walls of the bath.

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Each of the chains hangs in a loop, and the fraction of theweight of the chains supported by the target increases asthe target rises, with the result that the target assumes anequilibrium position. The beam from the ultrasonic trans-ducer is directed downwards towards the target, and theadditional force exerted by the ultrasound is compensatedby a downward movement of the target to a new equi-librium position. This displacement can be measured andrelated to the total time-averaged power from the trans-ducer.

One problem with many moving target devices is themovement of the target suspension wire through the air/water interface, where surface tension effects will be aproblem. As an example, the surface tension force at anair/water interface for a 0.1 mm diameter wire is approx-imately equal to the force exerted on a totally absorbingtarget by an ultrasonic beam of 35 mW total power. Anyproblem of variable surface-tension forces is largely over-come in the second class of systems, where the forcerequired to keep the target stationary is measured. A ser-vocontrolled microbalance is sometimes used, and waschosen as the basis of the radiation pressure balance atNPL, as this type of balance not only keeps the targetfixed but also has adequate sensitivity.

As a microbalance measures only vertical forces and asit must be used above the water bath, it is necessary for theultrasonic beam to be vertical. It would be most conve-nient to position the transducer at the top of the tank,where it could easily be exchanged and a special watertank would not be required. In this position, however, thetransducer would interfere with the suspension of thetarget from the balance, necessitating a complex suspen-sion arrangement, and inevitably increasing any residualsurface-tension forces. To overcome this problem, thetransducer in the NPL balance is placed at the bottom of aspecial tank on a removable mounting plate. The alterna-tive of placing the transducer against a thin membrane inthe bottom of the tank was rejected because of the possi-bility of reflections from the membrane, and also becausesuch a membrane would not provide a reproducible loca-tion for the transducer.

A crucial part of any radiation pressure balance is thetarget. As any partially reflecting target will have a fre-quency dependent reflection coefficient, it is desirable toadopt a design in which the reflection is either almost totalor negligible. An air/water interface is a good approx-imation to a perfect reflector, the amplitude reflection coef-ficient from water to air being 0.9995. The lack of rigidityof a water surface if used as a target would, however, causeproblems and, therefore, the best approximation to a rigidperfect reflector is a water/rigid-material/air combination.The choice of rigid material is not critical, but it mustremain stable and rust-free, and therefore stainless steelwas chosen for the NPL target material. In addition, witha reflecting target, the reflected wave must not return andinterfere with either the target or the transducer. After con-sidering the various options, a right-angled cone with itstip pointing downwards was chosen as optimum for theNPL balance. The main disadvantage in adopting thisdesign is that the target will not be self-centering, therebysetting an upper limit to the measurable ultrasonic power.Figs. 1 and 2 show a schematic and photograph, respec-tively, of the NPL radiation pressure balance.

In addition, an absorbing target was constructed atNPL to investigate some of the sources of uncertainty inthe measurements with a reflecting target. A perfectlyabsorbing target must have both an exact impedancematch with water, eliminating reflections, and a high ultra-

sonic absorption coefficient. In practice, an absorbingmaterial from which there is minimal reflection is chosen,

Fig. 1 Schematic of NPL radiation pressure balance

although the amount of absorption will clearly depend onthe thickness of the material. Additional problems with anabsorbing target are due to the heat produced by theabsorbed ultrasound, as the consequent temperature risewill cause buoyancy changes and may also set up convec-tion currents. A rubbery substance called SOAB is claimed[11] to be a good approximation to a perfect absorber andthis material was used in the NPL absorbing target.

Fig. 2 NPL radiation pressure balanceNote: the absorber around the inside of the water tank has been removed so theconical reflecting target is visible

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Measurements were made at frequencies of 1 MHz and5 MHz using both the absorbing and reflecting targets,and, on average, values of total output power measuredwith the absorbing target were 2% higher than those for areflector. This difference is relatively small and could bedue either to reflections from the absorbing target or, asmentioned later, streaming in the water bath.

With a conical reflecting target, the ultrasound is reflec-ted to the sides of the water tank, where it must beabsorbed to prevent reflections from the tank wallsreturning the ultrasound towards the target and trans-ducer. A variety of absorbers have been used to line thewalls in radiation pressure balances, including rubber [12]and carpet [13], and, after some studies with a laser Sch-lieren system, long pile carpet was chosen for regular useat NPL.

Apart from the comparison of the reflecting and absorb-ing targets, several other tests for systematic errors wereperformed. Linearity of the radiation pressure balance withapplied ultrasonic power was confirmed using the square-law relationship between the output power from a trans-ducer and the applied electrical drive voltage. Possibleerrors caused by changes in surface-tension forces werealso investigated and found to be negligible.

Streaming in the water bath may be expected to occur ifthere is absorption of the ultrasonic wave; momentum isnecessarily conveyed to the medium as it is lost from thetravelling ultrasonic wave. Convection currents may alsooccur in the water above the ultrasonic transducer, bothfrom conversion of the electrical power applied to thetransducer into heat, and also from the transformation ofabsorbed ultrasonic energy into heat in the water. A mem-brane was placed at different distances between the trans-ducer and the target to investigate these sources ofuncertainty. At frequencies of 5 MHz and below, the sys-tematic uncertainty was found to be less than +2.5%.Above 5 MHz, the effects are more pronounced and workis still continuing at NPL to attempt to determine accu-rately the uncertainty.

From detailed investigations of the performance of theradiation pressure balance, the overall systematic uncer-tainty on the measurement of total ultrasonic power at fre-quencies of 5 MHz and below is estimated to be +3%.This uncertainty has been confirmed in an internationalintercomparison of techniques for the measurement ofultrasonic power organised by the National Bureau ofStandards in the United States [14]. This intercomparisonincluded other techniques such as calorimetry, and theNPL radiation pressure balance was found to give agree-ment to better than ±2% with the mean total outputpower obtained from all the different techniques.

3.2 Requirements for hydrophones and theirdevelopment

Ideally, for the accurate assessment of the output ofmedical ultrasonic equipment, a hydrophone should havestable performance properties, an active element of sizecomparable to or smaller than the acoustic wavelength,and an adequate and sufficiently broadband sensitivitywith little variation with frequency over the range 0.5 MHzto 15 MHz. In addition, for the very high acoustic press-ures generated by modern imaging systems, there is a needto extend this upper frequency to between 30 MHz and40 MHz, to be able to measure the severely distortedacoustic pressure waveform caused by nonlinear propaga-tion effects. Finally, the hydrophone should cause theminimum of disturbance to the acoustic field in which it isplaced.

Various developments of hydrophones have beendescribed in the literature and the reader is referred toPreston, Bacon, Livett and Rajendran [15] and Lewin[16] for detailed references. Essentially, three types ofhydrophone have been produced. First, there is theceramic hydrophone, which consists of an active elementmade from a piezoelectric ceramic mounted at the end of apencil-like or needle-like structure. The second type ofhydrophone is the PVDF needle hydrophone which con-sists of a device of a similar physical structure to theceramic hydrophone but uses a polyvinylidene fluoride(PVDF) active element. Finally, there is the PVDF mem-brane hydrophone consisting of a large sheet of PVDFstretched across an annular ring, with a small activeelement at the centre of the device defined by the overlapregion of electrodes evaporated onto the two surfaces ofthe film. While ceramic hydrophones have been producedwith active elements of diameter down to 0.2 mm, the twotypes of devices made from PVDF have not yet been pro-duced with elements less than 0.5 mm diameter.

In general, the three types of membrane hydrophonesknown as coplanar shielded, bilaminar and differentialoutput [15], and shown in Fig. 3, are used at NPL. These

Fig. 3 Various types of membrane hydrophoneTop left—coplanar shielded; top right—bilaminar; bottom—differential output

differ in the thickness of PVDF film and the degree of elec-trical shielding provided. Typically, a coplanar shieldedhydrophone made from 0.025 mm thick PVDF filmexhibits a flat frequency response to 40 MHz; and isalmost transparent to the ultrasound, thereby causing littleperturbation to the acoustic field. The bilaminar hydro-phone offers increased electrical shielding at the expense ofbeing thicker and, therefore, perturbing the ultrasonic fieldto a greater extent. The differential output device providesincreased electrical isolation and smaller perturbation tothe field, but requires a differential amplifier. All the mem-brane hydrophones have a predictable directionalresponse; an important property when characterisingcertain types of ultrasonic fields. Devices have been madewith active elements of diameter 4, 2, 1 and 0.5 mm. All

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these devices have been developed in a joint programmebetween NPL (UK Department of Trade and Industry)and GEC-Marconi Electronics Ltd, and are commerciallyavailable from the Marconi Research Centre, GreatBaddow, Chelmsford, Essex.

3.3 Hydrophone calibration methodsHydrophones are not absolute devices and the relationbetween the applied pressure in the ultrasonic field and thevoltage produced by the hydrophone must be determinedby calibration. Specifically, this requires a determination ofthe response of a hydrophone, as a function of frequency,to an applied acoustic pressure. Two techniques for hydro-phone calibration, the measurement of the total power of atransducer combined with beam plotting, and reciprocity,are commonly used and are being studied at NPL.

The first of these techniques, total power measurementcombined with beam plotting (commonly known as planarscanning) [17], provides a direct link between the hydro-phone calibration and the primary standard of totalpower. The sensitivity of the hydrophone is determined byscanning the hydrophone over the ultrasonic beam emittedby a transducer, and dividing the integral of the square ofthe hydrophone signal by the total power in the beam[18]. At NPL, the total power W from a transducer is firstdetermined using the radiation pressure balance. Thetransducer is then mounted horizontally in a beam-plotting tank and the hydrophone scanned over a planeperpendicular to the direction of the ultrasonic beam. Thisscanning plane is chosen to be, typically, between 1 and 5near-field distances. Here, the near-field distance is definedas the distance between the transducer and the lastmaximum in the axial pressure distribution. The receivedsignal at the hydrophone is measured as a function of theposition of the hydrophone in the ultrasonic beam, and thesensitivity S(f) of the hydrophone can be determined usingthe equation

= j V2dA exp (2<xf2d)/(WZ)

where { V2 dA is the square of the received hydrophonesignal V integrated over the area of the beam, exp (2<xf2d)is a correction for the attenuation of the water path oflength d between the transducer of frequency / and thehydrophone, Z is the acoustic impedance of water and a isits amplitude attenuation coefficient. This, therefore, givesa value for the sensitivity of the hydrophone at one fre-quency point. In most cases, a plane circular transducer isused, in which case, it may be possible to perform a dia-metrical scan and assume cylindrical symmetry. Scans maybe performed in different radial directions by rotating thetransducer in its mount to test the validity of the cylin-drical symmetry assumption. Alternatively, a raster scanover a plane in the ultrasonic field can be used and thisremoves any uncertainty due to lack of cylindrical sym-metry.

There are three major sources of uncertainty in theplanar scanning technique. The first of these occurs in thedetermination of the total output power and comprisesboth the overall systematic uncertainty in the determi-nation of total power using the radiation pressure balanceand variations in the total output power from the trans-ducer during the scanning procedure. Secondly, there is anuncertainty in the integration of the hydrophone signalover the ultrasonic-beam area, and, in addition, there is anuncertainty in the measurement of the hydrophone signalitself. In a set of measurements performed at NPL, all thesystematic uncertainties were combined linearly to give a

value for the total systematic uncertainty of approximately+ 9% at most frequencies. Random uncertainties werenegligible in comparison. For these measurements, trans-ducers were used with diameters, typically, 25 mm at1 MHz, 12.5 mm at 5 MHz and 6 mm at 10 MHz and 15MHz.

The second technique used at NPL is the two-transducer reciprocity method, which is a straightforwardtechnique requiring relatively little equipment, and is cur-rently recommended by the IEC [19]. This technique con-sists of two separate procedures. First, a plane circulartransducer is calibrated by self-reciprocity. To accomplishthis, the transducer is mounted in a water tank, and theultrasonic wave is reflected from a thick metal plate, posi-tioned, typically, between 0.5 and 3 near-field distances.The position and orientation of the transducer and thereflecting plate are adjusted for maximum received echosignal at the transducer, and then the drive current appliedto the transducer and the voltage of the received signal aredetermined. The time delay between the transmitted andreceived signals can be measured and used to calculate thetotal length d of the sound path. From the measurementsof current and voltage, a value for the acoustic pressure inthe field from the transducer may be determined. In thesecond procedure, the reflecting plate is removed, thehydrophone is placed in the ultrasonic field at a distance dfrom the transducer and the signal received by the hydro-phone is measured.

The sensitivity S(f) of the hydrophone can be deter-mined using the equation

S(f) = VJG2{2ArG1 exp {«f2d)/\;V0IZ]}1'2

where Vh and Vo are the open-circuit voltages at the hydro-phone and the transducer, respectively, / is the drivecurrent, Gl and G2 are corrections for diffraction effects, Ais the effective receiving area of the transducer, r is theamplitude reflection coefficient for the water/reflectorinterface, Z is the acoustic impedance of water andexp (ocf d) is a correction for the attenuation of the waterpath of length d.

The measured values of current and voltage need to becorrected for the electrical loading of the transmitting andreceiving circuitry and the corrections are calculated usingequivalent circuits. The values of the diffraction correc-tions Gi and G2 are equal to the ratio of the receivedacoustic pressure averaged over the surface of the trans-ducer (for GY) or the hydrophone (G2) to the pressure in aplane wave immediately in front of the transducer. A plotof this ratio was derived theoretically for an ideal piston-like source by Fay (see References 20 and 21). In measure-ments carried out at NPL, the various sources ofsystematic uncertainty, such as the electrical and the dif-fraction corrections, were combined linearly, and the totalsystematic uncertainty typically ranged from ±8% at1 MHz to ± 12% at 10 MHz and ±20% at 15 MHz. Thetransducers used for the planar-scanning calibration werealso used here.

The equivalence of the planar-scanning and reciprocitytechniques is demonstrated in Fig. 4, in which the sensi-tivity of a hydrophone determined using both techniques isplotted at various frequencies.

4 Specification of acoustic output of medicalultrasonic equipment

One of the main reasons for developing standards formedical ultrasound is the need to measure and therebyspecify the acoustic output of medical ultrasonic

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equipment. To do this, it is necessary to specify the precisequantity that is to be measured. For total ultrasonic power

0.11

0.10

2 0.09>

10.08

0.07

0.06-

4frequency, MHz

8 10

Fig. 4 Comparison of calibration of a hydrophone based on planar scan-ning and reciprocityError bars denote total systematic uncertaintyx reciprocity# total power and beam plotting

this is obvious and therefore straightforward, but for thespatial and temporal quantities, as measured using ahydrophone, the situation is somewhat more complicated.

Essentially, the hydrophone measures acoustic pressureand, therefore, it is logical to specify an ultrasonic field interms of parameters such as the peak acoustic pressure orthe RMS acoustic pressure measured over a specified timeinterval, such as the pulse duration, or over a finitenumber of pulse repetition periods. However, it has beenthe usual practice to convert the acoustic pressure to inten-sity using the relationship

2

pc

where / and p are the instantaneous values of intensity andacoustic pressure, respectively, p is the density of waterand c is the velocity of sound in water. However, thisrelationship is only valid for infinitely small particle dis-placement amplitudes and plane travelling waves. Forinstance, it can be used in the far field of a transducer, butnot in the near field [22]. In consequence, it is valid tospecify intensity as a means of characterising an acousticfield, only if it is legitimate to use the above relationship.Under these conditions, various intensity parameters maybe specified, such as spatial peak temporal peak, spatialpeak pulse average, spatial peak temporal average andspatial average temporal average. Detailed definitions ofthese parameters may be found in the AIUM/NEMA'Safety standard for diagnostic ultrasound equipment'[23]. Such a group of parameters, together with the beamwidth and acoustic frequency, permits a detailed descrip-tion of an acoustic field. However, because of therestrictions in the application of eqn. 2 mentioned in thepreceding text, a more general approach to the character-isation of fields when using hydrophones is to specify thephysical quantity, acoustic pressure. Furthermore, there isa need for an indication of the extent and importance ofany nonlinear propagation which may be present [24—26].Again, this is more readily achieved through the specifi-cation of acoustic pressure parameters.

5 General measurements of acoustic output ofultrasonic transducers

As already mentioned, the acoustic output of an ultrasonictransducer may be characterised using both the totaloutput power and also the spatial and temporal character-istics of the acoustic field.

For the total power measurement, the procedure is rela-tively straightforward, provided the target of the radiationpressure balance intercepts the whole of the ultrasonicbeam. For sector scanners, where the beam emerges in aradial fashion from the transducer, or in a linear array,where the beam pattern may extend over a width of over100 mm, problems of interpretation of measurements madewith a radiation pressure balance can occur. Character-isation of the system in a 'frozen' state, in which the beamis not in the scanning mode, is possible provided the elec-trical transducer excitations are identical in the scanningand 'frozen' mode. In addition, the scan repetition rateneeds to be known to convert the total power measure-ments made in the 'frozen' mode to values appropriate tothe scanning mode.

For field characterisation using a calibrated hydro-phone, it is necessary to be able to move the hydrophonethroughout the ultrasonic field so that its active elementcan be placed at any point at which measurements are tobe made. It is also necessary, because of the directionalresponse of the hydrophone [15], to be able to rotate thehydrophone for maximum received signal. Consequently, acomplex beam scanning tank and facilities are required.

(2) F'9- 5 Ultrasonic beam plotting tank at NPL

Fig. 5 shows such a system used at NPL which incorpo-rates translation and rotation control on both the trans-ducer and the hydrophone mounts. In this system, thevarious acoustic output quantities are determined from thehydrophone signal. At NPL, measurements are made witha Tektronix 7854 signal-processing oscilloscope with adelayed trigger time base. This system, combined withstepping motor controlled translation stages on the scan-ning tank and a controlling minicomputer, permits theautomatic acquisition of data.

6 Rapid evaluation systems—beam calibrators

Although the specialised equipment, such as the scanning-tank facility shown in Fig. 5, may be established at labor-atories such as NPL, and offers the most accurate meansof characterising an acoustic field, it is unlikely that suchfacilities can be established and used on a routine basis byeither manufacturers or ultrasonic equipment users,because of the complexity, cost and time necessary toundertake measurements.

To meet the need for rapid and quantitative evaluationsystems, multielement membrane hydrophones, such as thedevice shown in Fig. 6, have been developed, together withthe associated electronic circuitry. These are intended toprovide a simple-to-use system allowing the quantitative

238 IEE PROCEEDINGS, Vol. 131, Pt. A, No. 4, JUNE 1984

visualisation of beam profiles and the rapid determinationof absolute acoustic output parameters. Two such systems

Fig. 6 Multielement membrane hydrophone

have been developed at NPL and are referred to as beamcalibrators or BECAs.

Essentially, the BECA systems consist of a linear-arraymembrane hydrophone mounted horizontally in a versatilesmall test tank. A transducer is mounted at the top of thetank and the system is arranged so that the ultrasonic fieldgenerated by the transducer is sampled at each one of theactive elements of the hydrophone. The signal from the ele-ments is read sequentially using specially developed elec-tronic signal-processing systems and the distribution ofpressure amplitude is displayed. To date, linear-arrayhydrophones with 21 active elements of size 1 mm x 1 mmwith a pitch of 2 mm have been produced. Two electronicsystems BECA1 and BECA2 have been developed and aredescribed in the following. The detailed acoustic per-formance of these systems for the characterisation of bothcontinuous wave and pulsed ultrasonic fields is beyond thescope of this paper, and will be dealt with elsewhere.

BECA1: This is a small portable system designed tomeasure the temporal peak acoustic pressure distributionof a single-element transducer and is shown in Fig. 7. Theelectronic processing system detects the peak electrical

signal from each element of the hydrophone and presentsthe resulting beam profile information on an xy-display.Once this device has been calibrated, it is possible to deter-mine the spatial peak temporal peak pressure, and, if thepulse shape is measured using an oscilloscope and therepetition rate is known, the temporal average values canalso be determined. BECA1 has a wide range of potentialapplications. As the beam profile is presented instantane-ously, it has a unique capability for the optimisation of theoutput of a transducer as a function of frequency or electri-cal drive. For example, BECA1 can be used when it isnecessary to optimise the magnitude and phase of the elec-trical excitation of various elements of an array transducer,in order to achieve a particular beam shape.

BECA2: This system, shown in Fig. 8, utilises the samehydrophone, tank and first-stage electronic module as

Fig. 7 Ultrasonic beam calibration system BECA1

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Fig. 8 Prototype BECA2 system

BECA1, but is interfaced to a microcomputer, which storesthe waveform from each element of the hydrophone andallows the acoustic parameters to be calculated. All thepressure and intensity parameters can be determinedthrough postprocessing routines in seconds. In addition,frequency analysis of the data permits the beam profile tobe determined as a function of frequency. All data may berecorded on floppy discs for future reference.

Both BECA1 and BECA2 have an analogue frequencyresponse which is flat to 15 MHz, and the 3 dB roll-offoccurs at about 25 MHz. The linearity is better than±1 dB up to peak pressures of 30 x 105 Pa. As thesesystems are being developed as transfer standards, the sta-bility and the reproducibility of their performance proper-ties is of prime importance. To date, the stability of theanalogue systems is better than ±1.5 dB. Ultimately itshould be possible, in BECA2, to use interchangeablemultielement hydrophones with differing size and pitch ofactive elements; the different calibration factors beingincluded in the software. It should also be possible to copewith the characterisation of linear-array ultrasonic trans-ducers.

7 Conclusion

The two basic types of absolute measurement technique,which have been established at NPL as standards for thecharacterisation of the acoustic output of ultrasonic trans-ducers at megahertz frequencies, have been described. Bothtotal power determination, using a radiation pressurebalance, and the measurement of the temporal and spatialdistribution of acoustic pressure, using a hydrophone, have

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been considered, together with the basic principles of eachtechnique.

At frequencies of 5 MHz or below, the accuracy withwhich total power can be determined using the radiationpressure balance is, in most cases, adequate. The accuracywith which the sensitivity of a hydrophone can be deter-mined is, however, inadequate for most applications.Moreover, if the intensity is derived from the pressuremeasurements, the uncertainties are doubled. In additionto improvements in the two techniques for hydrophonecalibration described in this paper, new accurate tech-niques are being developed based on the interferometricmeasurement of the acoustic displacements generated byultrasonic fields.

The mapping of ultrasonic fields using single-elementhydrophones offers the most accurate field characterisationmethod. For lower accuracy, beam calibration systems(BECAl and BECA2) based on multielement hydrophoneshave been developed. These BECA systems provide rapidquantitative assessment of fields and are expected to bedeveloped further in the future.

The measurement techniques and standards discussedhere have been developed primarily with the needs ofmedical ultrasound in mind, although they could equallywell be used for the characterisation of certain types oftransducers used in nondestructive testing. For instance,the methods would be ideally suited to the detailed quanti-tative assessment of immersion transducers and those usedfor compressional wave testing.

8 References

1 BRENDEL, K., and LUDWIG, G.: 'Diagnostic intensities and theirmeasurement'. Recent Advances in Ultrasonic Diagnosis 3, Pro-ceedings of the 4th European Congress on Ultrasound in Medicine,Dubrovnik, 1981, pp. 76-80 (Excerpta Medica, 1981)

2 'Interaction of ultrasound and biological tissues'. Workshop Pro-ceedings, US-DHEW Publication (FDA) 73-8008 BRH/DBE 73-1(US Government Printing Office, Washington DC, 1973)

3 SHOTTON, K.C., and LIVETT, A.J.: 'Measurement systems forultrasound', Acoust. Bull., 1979, pp. 18-21

4 STEWART, H.F., and STRATMEYER, M.E.: 'An overview of ultra-sound: theory, measurement, medical applications and biologicaleffects'. Bureau of Radiological Health, HHS Publication FDA 82-8190, 1982

5 WELLS, P.N.T.: 'Biomedical ultrasonics', (Academic Press, 1977)

6 WHO Environmental Health Criteria 22: Ultrasound (World HealthOrganisation, Geneva, 1982)

7 ZIENUIK, J., and CHIVERS, R.C.: 'Measurement of ultrasonicexposure with radiation force and thermal methods', Ultrasonics,1976, 14, pp. 161-172

8 LIVETT, A.J., EMERY, E.W., and LEEMAN, S.: 'Acoustic radiationpressure', J. Sound & Vib., 1981, 76, pp. 1-11

9 ALTBERG, D.: 'Uber die druckkrafte der schallwellen und die absol-ute messung der schallintensitat', Annalen der Physik, 1903, 11, pp.405-420

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11 ROONEY, J.A.: 'Determination of acoustic power outputs in themicrowatt-milliwatt range', Am. J. Phys., 1972, 40, pp. 1825-1830

12 KOSSOFF, G.: 'Balance techniques for the measurement of very lowultrasonic power outputs', J. Acoust. Soc. Am., 1965, 38, pp. 880-881

13 WEMLEN, A.: 'A milliwatt ultrasonic servocontrolled balance', Med.& Biol. Eng., 1968, 6, pp. 159-165

14 TSCHIEGG, C.E., GREENSPAN, M., and EITZEN, D.G.: 'Ultra-sonic continuous-wave beam-power measurements, internationalintercomparison', J. Res. Natl. Bur. Stand., 1983, 88, pp. 91-103

45 PRESTON, R.C., BACON, D.R., LIVETT, A.J., and RAJENDRAN,K.: 'Pvdf membrane hydrophone performance properties and theirrelevance to the measurement of the acoustic output of medical ultra-sonic equipment', J. Phys. £., 1983, 16, pp. 786-796

16 LEWIN, P.A.: 'Miniature piezoelectric polymer ultrasonic hydro-phone probes', Ultrasonics, 1981, 19, pp. 213-216

17 HERMAN, B.A., and HARRIS, G.R.: 'Calibration of miniature ultra-sonic receivers using a planar scanning technique', J. Acoust. Soc.Am., 1982, 72, pp. 1357-1363

18 JONES, S.M., CARSON, P.L., BANJAVIC, R.A, and MAYER,C.A.: 'Simplified technique for the calibration and use of a miniaturehydrophone in intensity measurements of pulsed ultrasonic fields', J.Acoust. Soc. Am., 1981, 70, pp. 1220-1228

19 'The characteristics and calibration of hydrophones for operation inthe frequency range 0.5 MHz to 15 MHz'. IEC draft standard pre-pared by Technical Committee 29D, (International ElectrotechnicalCommittee, Geneva)

20 BRENDEL, K., and LUDWIG, G.: 'Calibration of ultrasonic stan-dard probe transducers', Acustica, 1976, 36, pp. 203-208

21 BEISSNER, K.: 'Free-field reciprocity calibration in the transitionrange between near field and far field', ibid., 1980, 46, pp. 162-167

22 BEISSNER, K.: 'On the plane-wave approximation of acoustic inten-sity', J. Acoust. Soc. Am., 1982, 71, pp. 1406-1411

23 'Safety standard for diagnostic ultrasound equipment'. AIUM/NEMA, Publication UL 1-1981 (National Electrical ManufacturersAssociation, Washington, 1981)

24 BACON, D.R.: 'Finite amplitude distortion of the pulsed fields usedin diagnostic ultrasound', Ultrasound Med. & Biol. (to be published)

25 HARAN, M.E., and COOK, B.D.: 'Distortion of finite amplitudeultrasound in lossy media', J. Acoust. Soc. Am., 1983, 73, pp. 774-779

26 MUIR, T.G., and CARSTENSEN, E.L.: 'Prediction of nonlinearacoustic effects at biomedical frequencies and intensities', UltrasoundMed. & Biol., 1980, 6, pp. 345-357

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