proposed standard thermal test object for medical ultrasound

PII S0301-5629(98)00136-7

● Original Contribution

PROPOSED STANDARD THERMAL TEST OBJECT FOR MEDICALULTRASOUND

ADAM SHAW, NADINE M. PAY, ROY C. PRESTON, and ANTONY D. BOND

Centre for Mechanical and Acoustical Metrology, National Physical Laboratory, Teddington, UK

(Received20 April 1998; in final form 14 August1998)

Abstract—A general design for a thermal test object (TTO) is proposed. A number of novel features make thedesign particularly suitable for use as a standardised device for assessing the heating capability of diagnosticultrasound beams. To assess performance, soft-tissue TTOs have been made containing thin-film thermocouplessandwiched between discs of tissue-mimicking gel. Installed in an appropriate measurement system, these TTOsexhibit excellent thermal and spatial resolution, allowing the ultrasound beam to be located rapidly andreproducibly. The measured temperature rise after 3 minutes of heating has been compared with theoreticalpredictions based on measured pressure distributions, and agreement is within 10%. Other studies have shownthat soft-tissue– and bone-mimicking TTOs can be used to evaluate a wide range of ultrasound fields and thatdifferent physical tissue models can be simulated. It is concluded that this design would be suitable for providingreference assessments of the thermal hazard posed by diagnostic ultrasound under standardised conditions.Crown copyright. Reproduced by permission of the Controller of HMSO.

Key Words:Ultrasound, Ultrasonic heating, Thermal phantom, Thermal test object, Thermal hazard, Tissue-mimicking material, Thin-film thermocouple, Thermal index.

INTRODUCTION

In recent years, with the increasing acoustic power levelsavailable from certain types of medical equipment, therehas been growing interest in estimating the degree ofheating caused by the use of diagnostic ultrasound inclinical practice. Future international standards beingprepared by the International Electrotechnical Commis-sion (IEC) are expected to include classification of ul-trasonic equipment based, in part, on the temperatureincrease generated by absorption of ultrasound. Boththeoretical and experimental methods have been sug-gested for estimating the heating potential, or thermalhazard, of diagnostic ultrasound. The approach adoptedby the National Council on Radiation Protection andMeasurements (NCRP 1992) and the American Instituteof Ultrasound in Medicine/National Electrical Manufac-turers Association (AIUM/NEMA 1992) has been toestablish theoretical prediction methods using simplifiedclinical models and acoustic output data, with the mostwidely quoted method being the AIUM/NEMA thermal

indices TIS, TIB, and TIC. This has been taken a stepfurther by Shaw (1994), who used measured intensity dis-tributions to predict the temperature rise in a number ofidealised propagation models. The main advantages of theNCRP and AIUM/NEMA approaches are that they arebased on readily available acoustic data, that the effects ofblood perfusion are included through the use of the linearbioheat transfer equation proposed by Pennes (1948), andthat the tissue models are simple. The major limitations areassociated with beam shape (Bacon and Shaw 1993), non-linear enhancement of heating (Bacon and Carstensen1990), uncertainties in the values for the acoustic parame-ters required for the model, and the form of the bioheatequation itself (Weinbaum and Jiji 1985; Wulff 1980).These limitations also apply to the work of Shaw (1994),except that the beam shape is not simplified.

Due to the weaknesses of the theoretical methodsdescribed, there has also been great interest in establish-ing experimental methods for predicting temperaturerises. Measurement of temperature rise due to ultrasonicheating has been carried out by, among others, Fry andFry (1954a, 1954b), Dunn et al. (1969), Parker (1983),Drewniak et al. (1989), Carstensen et al. (1990), Wu etal. (1992), Bosward et al. (1993), Bacon and Shaw(1993), and O’Neill et al. (1994). In some cases, tem-

Address correspondence to: Adam Shaw, Centre for Mechanicaland Acoustical Metrology, National Physical Laboratory, QueensRoad, Teddington TW11 0LW, United Kingdom. E-mail:[email protected]

Ultrasound in Med. & Biol., Vol. 25, No. 1, pp. 121–132, 1999Copyright © 1998 World Federation for Ultrasound in Medicine & Biology

Printed in the USA. All rights reserved0301-5629/99/$–see front matter

121

perature rise has been measuredin vivo; in others, theexperimental measurement has involved the use of athermal test object (TTO) incorporating a thermal sensorembedded in an absorbing medium, the precise config-uration depending on the clinical situation being simu-lated. In vivo measurement clearly would not be appro-priate as a standard means for assessing equipment;however, TTOs would be. With the correct configura-tion, TTOs automatically could account for complica-tions such as beam shape, nonlinear propagation, andcomplex scanner pulsing sequences. Consequently, awell-designed TTO would have significant advantagesover theoretical methods as a reference means for eval-uating the heating potential of an ultrasound field.

The TTOs described in the literature have beendesigned for particular investigations and generally arenot appropriate for use as standard devices. A standarddevice must be reproducible and stable; its behaviourmust be predictable; it must not introduce significantmeasurement artefacts; and it must be applicable to thewide range of equipment and fields encountered in med-ical ultrasound. Importantly, it also must be recognisedas a standard by the user community, and it must bereadily available to that community. This article de-scribes a design for a standard TTO that meets thesetechnical requirements. TTOs have been built to thisdesign at the National Physical Laboratory (NPL) andtheir performance evaluated. The results of these evalu-ations are described, including tests for stability, repro-ducibility of manufacture, and validation against detailedtheoretical predictions. An outline of how these devicescan be used is given. It is hoped to make these TTOsavailable to other users in the near future.

DESIGN CONCEPT

In general, a TTO may be used for a range ofdifferent purposes. The NPL standard TTO is intended toidentify the maximum temperature rise that can be pro-duced by a given ultrasound field in a specified timeunder clinically relevant conditions. For the conditions tobe clinically relevant, first it is necessary to select theclinical site that is to be modelled. In general, this sitewill consist of many types of tissue in a complex geom-etry. In order to construct a practicable TTO, the keyproperties of the site must be established so that a rela-tively simple physical tissue model can be described,which will form the basis of the TTO. Typically, thephysical tissue model will comprise one or two types ofmaterial in a planar or concentric arrangement.

Consider the simplest physical tissue model: this isa homogeneous material with properties similar to typi-cal soft tissue. A TTO could be constructed by embed-ding a thermal sensor at a fixed depth in a block of

suitable tissue-mimicking material (TMM), and the tem-perature rise at a single distance from the surface of theTTO could be measured. Identifying the distance atwhich the maximum temperature rise occurs would re-quire a large number of TTOs, each with the sensorembedded at a different depth (Fig. 1a). Alternatively, anarray of thermal sensors could be embedded, each atdifferent depth. This has advantages, but it would requirethat all the sensors were aligned, not just with each other,but also with the axis of the ultrasound beam (Fig. 1b). Italso is possible that reflection or scattering of the ultra-sound by sensors closer to the transducer would affectthe temperature rise at more distant sensors.

A TTO based on a more complex physical tissuemodel would consist of two types of TMM, soft tissueoverlying bone, with a thermal sensor near the bonesurface (Fig. 2a). We would want to know the distancebetween the transducer and the bone mimic that producesthe maximum temperature rise. In this case, an array ofsensors would be less useful, because the distance fromthe transducer to the soft tissue–bone interface remainsfixed (Fig. 2b), and the array gives no information onwhat would be the effect of changing this distance. Thephysical tissue model for a first-trimester obstetric ex-amination would be still more complicated, with a soft-

Fig. 1. Homogeneous soft-tissue model with (a) Single ther-mocouple and (b) Thermocouple array. TMM5 tissue-mim-

icking material.

Fig. 2. Soft tissue–bone model with (a) Single thermocoupleand (b) Thermocouple array.

122 Ultrasound in Medicine and Biology Volume 25, Number 1, 1999

tissue layer followed by a region of low attenuation andthen more soft tissue.

To allow a single TTO to be used to locate themaximum temperature rise in a range of physical tissuemodels, it is useful to consider the tissue model asconsisting of three parts: these are the pre-target region,the target region, and the post-target region (Fig. 3).

The requirements of each region are different. TheTTO is intended to act as the target and post-targetregions, which means that the acoustic and thermal prop-erties of the material around the sensor must be appro-priate to the tissue that is being mimicked, and that thebacking should prevent acoustic reflections reaching thesensor. The acoustic properties of the pre-target regionare important, in that they determine the fraction of thefree-field acoustic intensity that reaches the target region.The thickness of the pre-target region will determine thedistance of the target region and, hence, the sensor, fromthe transducer. This concept of three distinct regions isused in the design of the NPL TTO.

GENERAL CONSTRUCTION

The NPL TTO (Fig. 4) has been designed as aself-contained, sealed device, which can be connecteddirectly to a suitable DC voltmeter or preamplifier. Thebasic format of the TTO allows for a variety of materials

and sensors to be used; the particular components used inthe NPL standard soft-tissue TTO are described later.TTOs can be built to any size, but the dimensionsadopted were chosen to be suitable for diagnostic ultra-sound fields and for heating times of up to 3 minutes.

The outer case of the TTO is a plastic cylinder,approximately 150 mm in diameter and 55 mm in height.The top surface of the case, on which the ultrasound isincident, is sealed with a thin membrane. Inside the case,there is a central thermal sensor sandwiched betweenlayers of tissue-mimicking material and a peripheral sen-sor that compensates for variation in the ambient tem-perature. Different materials, or combinations of tissue-mimicking material, can be used to match the targetregion of any particular tissue model. The cable from thesensor exits the case through a hole that is sealed with apotting compound. Spacing rings locate the sensor andTMM centrally in the TTO; a sprung backing platemaintains contact between the TMM and the sensor, andbetween the TMM and the top membrane. The inside ofthe TTO is fluid filled to provide acoustic couplingbetween the components of the TTO and to preventdrying out or degradation of the TMM. An absorberprevents acoustic reflections from the rear of the TTO.

The novel features that are improvements over pre-vious designs are as follows:● The thin entry membrane to eliminate refraction and

absorption of the ultrasound beam.● The use of a sprung backing plate to maintain constant

acoustic and thermal coupling throughout the TTO.● The use of a pair of thermal sensors to provide automatic

compensation for changes in ambient temperature.

COMPONENTS

Although the general design of the TTO offers anumber of advantages, it is also essential that the partic-ular components used in a standard device are appropri-ate. Details of the thermal sensor, TMM, coupling fluid,and entry membrane that are used in the NPL standardsoft-tissue TTO are given here.

Thermal sensorThe phenomenon of viscous heating, which can

occur when focused ultrasound is incident on wire ther-mocouples in fluid or semifluid media, is well known(Fry and Fry 1954a, 1954b) and can produce errors of upto 300% in the measured temperature rise (Bacon andShaw 1993). To eliminate this major problem, Bacon andShaw (1993) used a thin-film thermocouple (TFT) fortheir measurements of temperature rise.

As shown by Fay (1992) and Esward and Bacon(1998), TFTs offer substantial advantages. Figure 5shows an example comparison of the measured temper-

Fig. 3. Schematic diagram of a general physical tissue modelillustrating the three main regions and their key properties.

Fig. 4. Schematic diagram of the construction of a NationalPhysical Laboratory standard thermal test object. TMM5

tissue-mimicking material.

Thermal test object● A. SHAW et al. 123

ature rise and viscous heating artefact for a 50-mm wirethermocouple and a TFT. The TFT used by Bacon andShaw (1993) was slightly different from the design usedin this study (Fig. 6). It consisted of gold and nickelelectrodes evaporated onto a thin (0.004 mm) plasticsubstrate. The electrodes overlapped over a region ofapproximately 0.2 mm3 0.2 mm at the centre of thefilm, which was supported by a 3-mm thick, plastic ring.The outer diameter of the ring was 85 mm and the innerdiameter was 50 mm. A copper lead was connected to thegold electrode and a nickel lead to the nickel electrode;

these were connected to a thermocouple preamplifier.This TFT worked well, and measurement showed goodagreement with theory. However, in order to use them ina standard soft-tissue TTO, two important modificationswere made.

The first modification required was to protect theTFT from chemical attack. Many soft-tissue mimics con-tain small amounts of corrosive material, which can reactwith the thermocouple. In this case, formaldehyde isused as a fixative in the TMM, and the formic acidproduced by catalytic breakdown of the formaldehyde onthe nickel electrode dissolved the nickel after severalweeks of continuous contact. The solution adopted wasto laminate the electrode side of the TFT with a secondlayer of Mylar and to coat both sides of the device witha submicrometre layer of gold. Lamination with Mylaralone protects the device from physical abrasion but doesnot prevent loss of the nickel electrode. Gold coating ofthe device prevents the acid from reaching the electrodesand has the added advantage of providing electricalshielding and, therefore, improving thermal resolution.

The second modification was to introduce a com-plementary thermocouple junction at the periphery of theTFT by replacing the nickel lead with copper (Fig. 6).This has two advantages: it allows a simpler measure-ment system to be used to monitor the TFT voltage and,by providing automatic compensation for changes in theambient temperature of the TTO, it ensures that themeasured voltage is governed by the temperature in-crease that occurs when the TFT is insonated. The elec-trical measurement system is simplified because a stan-dard twisted copper–copper cable (shielded, to take ad-vantage of the gold coating on the TFT) can beconnected directly from the TTO to the measurementsystem, without the need for an external reference ther-mocouple junction. Additionally, because the voltagegenerated by the TFT depends on the temperature dif-ference between the centre and the edge of the device,DVTFT } [Tcentre 2 Tedge], the voltage prior to in-sonation of the TTO is close to zero (typically within 1mV), irrespective of the ambient temperature within theTTO. Consequently, high-gain amplification can be usedwithout the need to introduce a variable DC offset volt-age, which further improves the signal-to-noise ratio.

The sensor is calibrated by immersing the centraljunction in a temperature-controlled water bath while theperipheral junction remains outside the bath. The tem-peratures of the bath and the peripheral junction aremonitored, and the output voltage is measured as afunction of temperature difference between the two junc-tions for bath temperatures typically between 16° and35°C. The sensitivity, which is the rate of change ofoutput voltage with temperature difference, generally isbetween 10 and 16mV/K.

Fig. 5. Comparison of temperature rise measured with (a)Fine-wire and (b) Thin-film thermocouples. The very rapid

initial temperature rise in (a) is caused by viscous heating.

Fig. 6. Schematic diagram showing positions of the central andperipheral junctions on thin-film thermocouple.


Tissue-mimicking materialThe material used as a soft-tissue mimic has been

described previously (Bacon and Shaw 1993; Madsen etal 1982; Ruf 1988). It is a gel made from animal hidegelatine dissolved in water. The important physical prop-erties of the gel are given in Table 1 and compared withtypical values for soft tissue. The attenuation coefficientcan be controlled by adding varying quantities of castorand olive oil to the solution to form an emulsion. Form-aldehyde and alcohol are added to fix the gel after settingand to inhibit bacteriological degradation. For use in thestandard TTO, the gel is made in discs of approximately50 mm in diameter and between 4 mm and 8 mm thick.In the past, there have been three major difficulties withusing this type of gel as a standard material: first, it isdifficult to manufacture reproducibly; second, it is diffi-cult to keep it for extended periods without it eitherdrying out (which starts to occur within minutes if the gelis left in air) or decaying; and third, it is sufficientlycorrosive that small thermocouples have a limited usefullife when in contact with it.

These difficulties now, in most part, have beenovercome. Preventing corrosion of the thermocouple al-ready has been discussed, and keeping the TMM in goodcondition is dealt with in the next subsection on thecoupling medium. To ensure reproducibility, the majorcomponents are carefully degassed and covered duringmixing to limit the amount of air in the final material. Inaddition, it has been observed at NPL that the tempera-ture at which the gel sets greatly affects its attenuationcoefficient. To minimise this variation, the liquid mixtureis poured into moulds and then placed in a refrigerator toset. The refrigerator temperature is adjusted so that it isbetween 3° and 5°C. By taking these steps and carefullyfollowing a well-defined procedure during the rest of themanufacturing process, the attenuation coefficient can becontrolled to within 65% of the desired value. Gelshaving different attenuation coefficients can be producedby varying the amount of oil added to the mixture: moreoil leads to higher attenuation. This is useful because it

allows the same basic recipe to be used to mimic differ-ent types of soft tissue.

Coupling fluidA number of fluids were investigated as to their

suitability as both a coupling and a storage medium forthe TMM. Olive oil and castor oil were used initially,and the gel can be stored successfully in castor oil forseveral years without suffering bacterial assault. Thereare problems: first, there is a gradual reduction in thevolume of the TMM sample as water is absorbed fromthe sample by the oil; and second, castor oil is not aconvenient material to use inside a TTO. As well asbeing messy, it is very difficult to avoid layers of highlyattenuating oil between the TTO components; oil alsotends to make the TMM stick to the TFT and the entrymembrane, and it stiffens the insulation of the TFT cable.Distilled water on its own can be used for no more than2 or 3 days before biological degradation occurs. Variousdisinfectants and water bath treatments were tried unsuc-cessfully; one of these had the interesting effect of al-lowing the TMM to grow to approximately twice itsoriginal diameter by absorbing water! Sodium azide, anantibacterial agent routinely used for biological work,was not considered suitable because it reacts with metals.The medium finally settled on was a mixture of 2%ethanol in water, which is easy to handle, safe, and doesnot react with the components of the TTO. Over a periodof 9 months kept in the alcohol–water mixture, the TMMsamples displayed no visible evidence of bacterial attack,such as changes in surface texture or cloudiness in themedium. The diameter of each sample was reducedby ,8%, and the attenuation coefficient at 5 MHz wasconstant to within67%. It should be noted that, duringthe 9-month period, the samples were handled occasion-ally, were characterised in water that was not disinfected,and necessarily were exposed to air for short periods. Itseems likely, therefore, that the TMM properties wouldbe even more stable once permanently installed in aTTO.

MembraneThe top surface of the TTO is very important,

because it is through this that the ultrasound interactswith the TMM and the TMM interacts with the outsideworld. Although it is possible to imagine an open TTO,the standard design proposed here, with the soft-tissue–mimicking gel, needs to be sealed both to keep the insidefree from bacterial and algal growth and to prevent waterloss from the TMM. The thermal phantom made byO’Neill et al. (1994) was sealed with low-density poly-ethylene 3.2 mm thick. This minimises water loss butwas observed to cause distortion of the transmitted beamdue to refraction. There also is substantial absorption

Table 1. Properties of tissue-mimicking gel compared withthose of soft tissue.

Property Gel Soft tissue

Volume heat capacity (MJ K21 m23) 3.9 3.8–3.9Thermal diffusivity (1027 m2 s21) 1.33 1.2–1.4Thermal conductivity (W m21 K21) 0.52 0.47–0.56Speed of sound (m s21) 1540 1560–1590Typical attenuation coefficient

at 5 MHz (dB cm21) 2.45 2.2

The tissue attenuation is taken from NCRP (1992); the other tissuedata are from Duck (1990).


and, consequently, heat generation in the window mate-rial. Diagnostic imaging phantoms generally have afairly thick polyethylene (or similar polymer) window ofapproximately 0.25 mm thick. Although this reflects andattenuates the ultrasound beam, it is thin enough thatrefraction effects are minimal; therefore, the material issuitable for imaging purposes. The phantoms do, how-ever, lose water and need frequent topping up and “re-generating.” The ideal material would be thin, strong,waterproof, nonattenuating, nonreflecting, and nonre-fracting. This is a demanding list of requirements, butalmost any material, if it is thin enough, becomes non-attenuating, nonreflecting, and non-refracting, at leastover the frequency range of interest. Very thin materials,however, generally are permeable to water.

The material used for the membrane on the NPLTTOs is Mylar (a brand name of polyethylene terephtha-late), which is readily obtainable in thin films. On itsown, it is permeable to water, but it can be made water-proof by coating one surface with a submicrometer metallayer. Even at a thickness of 6mm, Mylar is relativelystrong under tension, although it can be punctured bysharp or abrasive objects, so some care is needed duringhandling and storage of the TTOs. The amplitude reflec-tion coefficient is calculated to be,5% for frequenciesup to 10 MHz, so a thicker and stronger film could beused with a relatively insignificant increase in the reflec-tion (Fig. 7). There is still some small water loss from thetest objects after assembly, but it is much reduced com-pared to uncoated Mylar (approximately 2 g/month forcoated Mylar; 9 g/month for uncoated mylar) and can bereduced further by covering the TTO when it is not inuse.

PERFORMANCE

Measurement systemA photograph of the NPL thermal test rig is shown

in Fig. 8. The TTO is mounted in anx,y,z-coordinatepositioning system driven by stepper motors under com-puter control. The stepper motors and controllers arestandard commercial components; the rest of the hard-ware was designed and built at NPL. The software waswritten in Visual Basic and runs on a 486 personalcomputer. This system allows the TTO to be scannedthrough the ultrasound field to locate the beam axis. Thesensor output from the TTO is connected via a Keithley1801 preamplifier to a Keithley 2001 digital voltmeter.

Examination of Fig. 9 shows that, with the com-puter and the stepper motors turned on, the temperatureresolution of the TTO is approximately 2 mK, with thepreamplifier configured for a bandwidth of DC to 30 Hz.Additional improvement can be gained by de-energisingthe stepper motors, but a resolution of 2 mK is consid-ered more than adequate. The Keithley 2001 DVM has asufficiently low noise floor that it is possible to connectthe sensor output directly to it, without the need for apreamplifier; however, this decreases the temperatureresolution to about 20 mK.

To locate the center of the beam at a given distance,the TTO initially is scanned over a horizontal grid oftypically 2 mm spacing. The transducer is energizedbriefly at each point and the temperature rise measured.Because the initial rate of temperature increase is pro-portional to the local intensity, the location of the max-imum temperature rise is generally the centre of thebeam. The position identified in the planar scan is then

Fig. 7. Calculated reflection and transmission coefficients for 0.2mm gold on (a) 6mm and (b) 12mm Mylar.


used as the central point of a linear scan across the beam.Typically, the scan would have a step size of 0.5 mm andextend over 4 mm, although the details depend upon thesize of the ultrasound field. The resulting temperatureprofile is examined to identify the centre, and this pointis used as the central point of a perpendicular scan.Additional sets of orthogonal scans are carried out, ifnecessary, to give confidence in the location of the centreof the beam. (Note: sometimes the centre of the beammay not be an intensity maximum. When this is the case,e.g., in the near field, the centre can be located byexamining the symmetry of the temperature profiles.)

Axial scans can be carried out “left to right” or“right to left” to check for systematic errors in locatingthe centre, although, as Fig. 10 shows, the position of the

peak is independent of the direction of the scan. Locatingthe centre is repeatable to within 0.1 mm for beams ofaround 2 mm in diameter. For narrower beams, locationcan be more accurate, because the discrimination be-tween adjacent sampling points is better; however, it maybe necessary to use smaller step sizes.

Having found the centre of the beam, the tempera-ture within the TTO is allowed to equilibrate for 1 or 2min, so that the TFT voltage is close to zero. Thetransducer is then turned on, and the temperature rise ismeasured as a function of heating time (Fig. 11). Due tothe restricted size of the TMM in the TTO, a heating time

Fig. 8. Photograph of the National Physical Laboratory thermal test rig.

Fig. 9. Temperature difference between central and peripheraljunctions as a function of time without insonation.

Fig. 10. “Left-to-right” and “right-to-left” scans across theultrasound beam demonstrating the reproducibility of locating

the beam axis.


of 3 min is generally used, as recommended by Shaw etal. (1996).

To assess repeatability, a number of investigationswere carried out at the focus of a circular, 5-MHz,focused Panametrics transducer driven by an H.P. 8116Asignal generator as a reference source. The estimatedvariation in the output power of the transducer was65%as a semirange. Figure 12 shows the temperature risemeasured at the focus of the transducer using a numberof soft-tissue TTOs over a period of 17 months. For theTTO labelled 96/19, all the measurements fell within therange 0.35–0.51 K. The mean was 0.43 K, and thepopulation standard deviation was 0.032 K, which isequivalent to a 95% confidence random variation of615%. There is an indication of a decrease in the mea-sured temperature rise with time, at a rate of approxi-mately 1.2% per month. This is believed to be caused bya gradual change in the absorption coefficient of theTMM and is in agreement with the figures given later inthe section on uncertainty assessment.

Comparison with theoryThe temperature rise on the beam axis at the posi-

tion of maximum spatial-peak, temporal-average inten-

sity (Ispta) was measured for four transducers and com-pared with the predicted values. Acoustic parametersdescribing the transducers are given in Table 2. Trans-ducers A and B were circular transducers operating incontinuous or long tone-burst mode, C was an 21211aphased array operating in pulsed Doppler mode on anHewlett Packard 77020 scanner, and D was an 21200bphased array on the same scanner. For transducers C andD, parameters D and d are the geometric mean of thein-plane and out-of-plane diameters (also called azi-muthal and elevational diameters).

The method of predicting the expected temperaturerise essentially follows that of Shaw (1994). The acousticoutput of each transducer was first measured using theNPL ultrasound beam calibrator (Preston 1988). Thissystem comprises a 21-element membrane hydrophonecoupled to a 60 MHz digitizer and controlled by apersonal computer. Each hydrophone element has a di-ameter of 0.4 mm, and the spacing between centers is 0.6mm. The computer calculates a range of acoustic param-eters from the digitized hydrophone signals. The tempo-ral-average intensity was recorded at each hydrophoneelement and saved in an ASCII file to allow temperaturecalculations to be carried out separately. Measurementswere performed at different distances along the beamaxis (z-axis), giving a grid of points spaced at 0.6 mmintervals in the radial direction and typically 5 mmintervals in thez-direction. A nearest neighbor interpo-lation was performed, followed by a modest degree ofsmoothing to give data on a new grid with intervals of0.12 mm in the radial direction and 0.5 mm in thez-direction. To allow for the asymmetry of transducers Cand D, this mapping process was repeated at 0°, 30°, 60°,and 90° to the scan plane. Figure 13 shows grey-scaleplots of the intensity distributions up to 70 mm from thetransducer and up to a radial distance of 6 mm from thebeam axis for transducer D; the distribution is shown inboth the in-plane and out-of-plane directions, where“plane” refers to the scan direction when the transduceris used in a scanning mode. Note that measurementswere not carried out closer than about 5 mm from the

Fig. 11. Typical variation of temperature rise with time insidethe thermal test object during insonation.

Fig. 12. Reproducibility of measured temperature rise in soft-tissue thermal test object (TTO) over a period of 17 months.

Table 2. Measured values of acoustic parameters fortransducers A, B, C, and D.

Symbol Parameter A B C D

D Transducer width (mm) 13 15 10 16W Acoustic power (mW) 37 24 22 57f Acoustic frequency (MHz) 3.6 4.9 5.0 2.4zfoc Focal distance (cm) 5.4 6.4 3.3 5.3d 26 dB beamwidth atzfoc (mm) 2.6 2.4 2.23 2.8

I spta

Spatial-peak temporal-averageintensity (mW/cm2) 770 350 660 960


transducer; instead, it was assumed that the distributionat small distances was the same as that at 5 mm.

Calculation programs were written using the soft-ware package Mathcad (Mathsoft Inc., MA, USA) topredict the temperature rise that would occur in the TTOas a result of exposure to each of the ultrasound fields.Locally, the heat generated per unit volumeqv(x,y,z)byabsorption of ultrasound within the TTO is given approx-imately by2a(f)I9(x,y,z), whereI9(x,y,z) is the intensitywithin the medium at a point(x,y,z) and a(f) is theabsorption coefficient expressed in nepers per unit lengthat the acoustic frequencyf. In the soft-tissue mimic usedhere,a(f) ' a0F

1; 0 f 1; this is also approximately trueof most biological soft tissues (Duck 1990). The temper-ature rise at a point in the medium can be calculated froma three-dimensional integral based on work by Nyborg(1988):

T~ x,y,z,t! 5 EEE 1

8pKs2a0 f I 9~ x9,y9,z9!

3 erfcS s

4ktD0.5

dx9dy9dz9 (1)

where s5 [(x 2 x9)2 1 (y 2 y9)2 1 (z 2 z9)2]1/2, andthe unperfused solution is used. If the point of interest ison the beam axis and the beam is assumed to be cylin-drically symmetric, eqn (1) can be written as:

T~ z,t! 5 E0

1`E-`

1` pr 9

8pKs2aof I 9~r 9,z9!

3 erfcS s

4ktD0.5

dr9dz9 (2)

where s5 [(r9)2 1 (z 2 z9)2]1/2.For each transducer, the TTO then was placed in a

thermal test tank with the TFT at the position of maxi-mum Ispta. The TMM used in the TTO was 6.5 mmthick and had an absorption coefficient of 0.057 Np cm21

MHz21 (equivalent to 0.48 dB cm21 MHz21). The in-tensity distributionI9(x,y,z)within the TMM was calcu-lated from the distribution measured in water by multi-plying the water values by the transmission loss of theTMM between the surface of the TTO and the position(x,y,z). No allowance was made for nonlinear propaga-tion effects or enhanced heating (Bacon 1984; Bacon andCarstensen 1990; Carstensen et al. 1980) within theTMM.

Calculation and measurement are compared in Ta-ble 3. For transducers C and D, three sets of calculationsare given: “\” assumes that the field is circularly sym-metric and that the in-plane intensity distribution applies;“z§” assumes that the out-of-plane intensity distributionapplies; and “mean” takes a weighted mean of the dis-tribution at 0°, 30°, 60°, and 90° to the scan plane (formore details, see Shaw 1994).

The results show good agreement between mea-sured and predicted values for transducers A and B andfor the “mean” values for C and D. The parallel andperpendicular values for C and D showed substantialdisagreement, which demonstrates the importance of al-lowing for the effects of beam asymmetry in any theo-retical predictions.

Uncertainty analysisThere are a number of sources of potential error

both in predicting and measuring the temperature in-crease; these are discussed and evaluated below as itemsa to n (Table 4). The figures given are assumed to besemirange values.

Prediction involves measuring the pressure distribu-tion (a) and acoustic frequency (b) at discrete points witha hydrophone and deriving the free-field intensity distri-bution. The free-field intensity is interpolated to generatea continuous spatial intensity function (c) and, if nonlin-ear enhancement (e) is ignored, a heat source functionwithin the TTO is calculated by multiplying the intensity

Fig. 13. Normalized temporal-average intensity as a function ofposition (in mm) in the field (a) In-plane and (b) Out-of-plane.

In each case, the transducer is on the left.

Table 3. Comparison of predicted and measured temperatureincreases (K) for the four transducers.

Transducer A B

C D

\ z§ Mean \ z§ Mean

Predicted 0.69 0.42 0.67 0.50 0.56 0.58 1.03 0.71Measured 0.67 0.46 NA NA 0.60 NA NA 0.68Ratio 1.03 0.92 0.93 1.04


by the absorption coefficient (d). The spatial integralgiven in eqn (1) requires, in addition, knowledge of thethermal conductivity and diffusivity of the TMM (f andg); the uncertainty ind, f, andg come primarily from thepossibility that these properties vary with temperatureand may change slowly with time. Combining these

contributions in quadrature and multiplying by1.96

Î3to

convert from a semirange to a 95% confidence levelresults in an uncertainty of626%, even with perfectspatial sampling of the beam. In a complex, rapidlyvarying, or nonlinear field, when an imperfect map of theintensity distribution is formed, contributionsc and ewould be nonzero, and the overall uncertainty could besignificantly larger.

Determining the maximum temperature rise at theTFT junction from the measured voltage requires that theTFT be calibrated (h) and that the junction be very smallcompared to the ultrasound beam (i) to avoid spatial-averaging effects. The junction must also be well alignedwith the centre of the beam to register the maximumtemperature rise (j). The importance of spatial averagingand misalignment will depend on the beam geometry, sothe figures given are only to give an estimate for a typicalbeam with a26 dB diameter of around 2 mm. Thepresence of the thermal sensor may produce heatingartefacts, due to either reflection/absorption (k) or vis-cous heating (l), although the latter is insignificant forthin-film sensors. Combining these contributions givesan overall uncertainty of65% at the 95% confidencelevel, if it can be assumed that the temperature within theTTO has equilibrated fully and there is no residual tem-perature change (m) following previous scans or heatingmeasurements.

When comparing prediction with measurement, theuncertainty in each is added in quadrature to give anestimate of the uncertainty in the ratio of one to the other.In this case, the combined uncertainty is627% at the95% confidence level, which means that theory andexperiment agree to well within the uncertainties.

When comparing nominally identical measurementsover an extended period, contributionsd, f, g, j, andmmust be considered; in addition, the transducer poweroutput (n) may vary. This implies that measurementsshould be repeatable to within616% at the 95% confi-dence level, as long as care is taken to allow the TTO toequilibrate before making measurements. This is inagreement with the results shown in Fig. 12.

DISCUSSION

Although it is possible to make predictions of thetemperature rise under defined conditions from hydro-phone measurements of the ultrasonic field, a standardTTO offers a number of advantages. First, the uncertain-ties associated with making the prediction are at least626%, which is much greater than the uncertainty andreproducibility of making a measurement of temperature.In very complex scanned or distorted fields, the uncer-tainty in the prediction would be even greater. Second,the measured temperature rise includes the effects oftransducer self-heating, whereas prediction cannot in-clude this without knowing a great deal of informationabout the transducer construction, material properties,and electrical efficiency. Third, TTOs would be cheaperand easier to use than hydrophone scanning systems andcould be used by medical physics departments as aroutine quality assurance tool.

The previous sections have shown that the soft-tissue TTO is a suitable basis for a standard device forevaluating the thermal hazard posed by diagnostic ultra-sound. This design of TTO has, in fact, already been usedand evaluated on a range of pulsed Doppler fields fromcurrent ultrasound scanners (Shaw et al. 1998). Themajor outstanding problem relates to the lifetime of thethin-film sensor. Although the sensors can be made withreproducible performance, once immersed in the cou-pling fluid, their lifetime is very variable: some last onlya few weeks, while others have lasted more than a year.The major cause of failure appears to be due to wateringress around the periphery of the device, damagingeither the connections to the copper leads or the elec-trodes on the unlaminated part of the device close to theconnection points. Work is continuing to find a morereliable method for sealing this part of the sensor.

The possibility of constructing a bone-mimickingTTO of this general design has been mentioned. Such adevice has been constructed and was used in the study by

Table 4. Estimated systematic uncertainty contributions tomeasurement and prediction of temperature rise.

Source Uncertainty Contribution toDT

a. Measurement of pressure 9% 18%b. Measurement of frequency 2% 2%c. Sampling of field Variable Variabled. Absorption coefficient 10% 10%e. Nonlinear enhancement Variable Variablef. Thermal conductivity 5% 5%g. Thermal diffusivity 15% 7%h. Calibration of TFT 3% 3%i . Spatial averaging by TFT 0.2 mmB 1%j . Misalignment with beam axis 0.25 mm 1%k. Intensity reflection

coefficient of TFT(at 5 MHz) 5% 2%

l . Viscous heating Negligible Negligiblem. Residual cooling Variable Variablen. Transducer variation 5% 5%

TFT 5 thin-film thermocouple.A 5 diameter.


Shaw et al. 1998. The bone-mimicking material used wasglass-filled polytetrafluoroethylene (PTFE). A report out-lining the properties of this material and possible alter-native bone-mimicking materials is in preparation (Payet al. 1998). A major point that remains to be resolved isthe establishment of agreed-upon properties for typicalbone. Different types of bone appear to exhibit signifi-cantly different properties, and the storage and fixingmethods used prior to measurement of the properties alsomay influence the final values. For a particular formula-tion of a bone TTO to become standardised, there is aclear need to agree upon the desired properties for abone-mimicking material. An ICRU report (ICRU 1997)makes this same point and includes some values thatseem to be representative of the limited informationcurrently available.

One criticism of this design of the TTO is that it isunperfused and consequently will overestimate the tem-perature rise that would occurin vivo.Shaw et al. (1996)have shown that introducing a Pennes-type perfusionterm (Pennes 1948) with a perfusion time of 720 s, as isassumed in the output display standard (AIUM/NEMA1992), makes only a small reduction to the temperatureincrease after 180 s of insonation. For ultrasound beamsof radius,5 mm, the perfused temperature is expectedto be within 10% of the unperfused value in a soft-tissueTTO. In a bone TTO, the effects of perfusion would beeven smaller. Perfusion generally will become more im-portant for longer insonation times.

The idea of dividing the physical tissue model intopre-target, target, and post-target regions was introducedearlier (Fig. 3). The TTO is intended to mimic the targetand post-target regions, but little has been said about thepre-target region. The pre-target region fills the spacebetween the transducer and the target region and, conse-quently, its thickness and its transmission loss change asthe distance of the target region from the transducerchanges. The simplest pre-target material is water, whichhas minimal transmission loss, and the resulting temper-ature rises would therefore be considered “worst-case”values. The validation measurements reported in thisarticle are all “worst case.” For other pre-target regions,the variation of transmission loss with distance is themost important feature and can have a very strong influ-ence on the temperature increase at a sensor in the targetregion. The physical tissue models for each clinical sitecould show a different variation with distance. Preston etal. (1991a, 1991b) have shown that low-density polyeth-ylene attenuators can be used to provide the requiredtransmission loss at any distance for different physicaltissue models. This technique has been used to simulateexperimentally the attenuation coefficient of 0.3 dBcm21 MHz21 given in the output display standard (Shawet al. 1998) and could be used to simulate the NCRP

fixed attenuation obstetric models (NCRP 1992) or thephysical tissue models suggested by Ramnarine et al.(1993).

In most diagnostic ultrasound fields, the instanta-neous pressure can reach sufficiently high levels to pro-duce nonlinear propagation in the pre-target and targetregions. The importance of nonlinear propagation, andwhether it will tend to increase or decrease the temper-ature rise, depends upon the field and the distance fromthe transducer. There is currently no accepted way oftaking into account the effects of nonlinear propagationtheoretically. However, it is possible to take the effectsinto account experimentally, at least to a first approxi-mation, by choosing materials for the pre-target andtarget regions, which exhibit the same frequency depen-dence of absorption as the appropriate type of tissue. Formost biological materials, the absorption coefficient isapproximately proportional to frequency. This also istrue of the soft-tissue mimic used here and of the poly-ethylene attenuators used by Preston et al. (1991a,1991b).

As was discussed in the Introduction, the mostwidespread method for assessing thermal hazard cur-rently is by calculation of the thermal indices definedin the output display standard. By suitable choice ofpre-target and target materials, it is possible to simu-late experimentally the theoretical models underlyingthe output display standard while avoiding many ofthe simplifications relating to beam geometry, fre-quency content, nonlinear propagation, and transducerself-heating. Shaw et al. (1998) have compared ther-mal index and measured temperature rise values forthe output display standard soft tissue, bone, and cra-nial models. They have shown that, for any individualfield, the soft-tissue and bone thermal indices may besubstantially higher or lower than the measured value,and that the cranial thermal index is typically aroundhalf of the measured value. The difference observed inthe cranial model probably is due to transducer self-heating, which is not accounted for in the outputdisplay standard formula.

SUMMARY AND CONCLUSIONS

This article has proposed a general design for aTTO. A number of novel features make the design par-ticularly suitable for use as a standardised device forassessing the heating capability of diagnostic ultrasoundbeams. To assess performance, soft-tissue TTOs havebeen made containing TFTs sandwiched between discsof tissue-mimicking gel. Installed in an appropriate mea-surement system, these TTOs have been shown to exhibitexcellent thermal and spatial resolution, allowing theultrasound beam to be located rapidly and reproducibly.


The measured temperature rise after 3 min of heating hasbeen compared with theoretical predictions based onmeasured pressure distributions and agreement is within10%, which is well within the estimated uncertainty of627% for such a comparison. Studies elsewhere haveshown that soft-tissue– and bone-mimicking TTOs canbe used to evaluate a wide range of ultrasound fields andthat different physical tissue models can be simulated.

This article and other papers in preparation shouldprovide a stimulus for further research internationallyinto the assessment of thermal hazard from diagnosticultrasound. For the methods to gain wide acceptance,there is a need to gain consensus on the appropriateproperties for materials intended to mimic different typesof tissue and on establishing standard assessment meth-ods. On the basis of the findings in this report, it issuggested that the type of TTO described here could beused as a standard experimental method for providingreference temperature measurements. To establish this asa generally accepted technique first will require thatothers have the opportunity to use these test objects. Anyinterested researchers should contact the authors, whohope to be able to meet this demand in the near future.

Acknowledgements—The National Physical Laboratory gratefully ac-knowledges the financial support of the National Measurement SystemPolicy Unit of the UK Department of Trade and Industry.

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