Medical Ultrasound Exposure Measurements: Update on Devices, Methods, and
Problems
Gerald R. Harris
FDA, Center for Devices and Radiological Health, Rockville, MD
20850
Absfrac!- Medical ultrasound fields span a wide range of
exposure conditions, from the continuous, low-frequency
vibrations of surgical fragmentation needles, to the intense,
broadbandpressurepulsesproducedbyextracorporealshock
wave lithotripters. Each of these diverse conditionsof use
presents
its
own
exposure
measurement
challenges.
Furthermore, the sensors and techniques used to evaluate
ultrasound exposures have had
to evolve as new
or expanded
clinical applications have emerged.
In this paper some
ofthe
morenotableofthesedevelopmentsarepresentedand
discussed. Topics covered include recent work on devices
and
techniques,
methods
of calibration,
progress
in
standardization, and current problem areas, including the
effects of nonlinearpropagation.Emphasisisgivento
miniature hydrophones because of the prominent role they
continue to play in ultrasound exposure measurements.
I. INTRODUCTION
11. DIAGNOSTIC
IMAGING AND DOPPLERTO 15 MHz
Diagnosticultrasoundexposimetryhasreceivedmore
attention thatany other medical use category, paralleling the
success of diagnosticultrasound as amedicalimaging
modality. New and refined measurement approaches have
appeared with some regularity, as have problems associated
withcharacterizingtheultrasoundfields.
To emphasize
where these developments seem to be most applicable, the
discussion of diagnostic ultrasound has been separated into
two frequency ranges, the higher range being covered in the
next section. The choice of15 MHz is somewhat arbitrary,
although this frequency has been used to define the of
range
applicability
for
several
International
Electrotechnical
Commission (IEC) standards discussed below.
A . Nonlinear propagation effectson p , measuremen!
Forhydrophone-basedmeasurements, the presence
ofnonlinear propagation effects poses a serious impediment
to the accurate assessment of ultrasonic exposure [a]. An
As the medical uses of ultrasound have grown, so has the
need for quantifying the acoustic field variables thatdefine
obvious first consequence is that the harmonics generated
anincreasedmeasurementbandwidth.Aless
the extent of exposure. Added demands have been placednecessitate
on
apparent result is that for a distorted pulsed waveform, the
the measurement devices as well. For example, increased
bandwidth below the center frequencyof the pulse mustbe
bandwidths are necessary to faithfully reproduce the acoustic
extended as well, particularly for accurate measurement of
pressures associatedwithnonlinear wave propagation
orhigh
the peak negative pressure, p,[7], [g]. One reason is that the
frequency applications; small detector size must be achieved
shorter the pulse, the broader the
specmm below as well as
to measure highly focused or high frequency fields, or to
make measurementsin vivo; robust devicesor techniques are abovethecenterfrequency.Additionallyforp,,the
waveform asymmetry resulting from the combined effects of
in
required to withstandtheintensefieldsencountered
sourcediffractionandnonlinearpropagationcausethe
lithotripsyandfocusedsurgery;andefticientmeansare
portion of thepulsedwaveformwherep,occurs
to be
needed to characterize the acoustic output in the complex
dominated by low-frequency components. The accuracy of
scanning modes found in modem diagnostic imagers.
the Mechanical Index (MI) decreases as well, because it is
In this paper some of the more recent developments
defined as the derated peak rarefactional pressure, measured
inmedicalultrasoundexposimeCry
arepresented.
For
under specified conditions, divided by the square root
the of
background
on
fundamental
measurement
techniques,
centerfrequency (fJ [9], [IO]. From [7] and [g], which
instrumentation,andstandards, as well as definitions of
contain both simulations and measurements
ofthe effects of
[l], [2], [3], [4],
terms and concepts, the reader may consult
[5]. The paperis organized by medical application category. bandlimited response onp, and MI, it is seen that to reduce
errors
to
less
than
510%. the
hydrophone-amplifier
Within each section, problems associated with the clinical
bandwidth
should
extend
at
least
an order of magnitude
device or withmeasurement of therelevantexposure
belowthepulse center frequency.Currentmeasurement
quantities are identified, and the instruments and methods
standards require that the hydrophone-amplifiercombination
thathavebeendeveloped
to addresstheseproblems, or
have a bandwidth with lower comer frequency of fJ2[l l],
simply to improvecurrentmeasurementpractice,are
[12], although in [l I ] it is recommended that this value he
described.Also,progressinmeasurementandlabeling
decreased to fJ20.
standardizationisbrieflydiscussedwhereapplicable.A
Because of this result,at a minimum the combined
short description of in vivomeasurementsconcludesthe
frequency
response of all components used to condition,
presentation.
U S . Government work not protected by U.S. copyright
1999 IEEE ULTRASONICS SYMPOSIUM - 1341
the SI increases. TheSI is a field quantity rather than a beam
amplify, or recorddiagnosticpulsedwaveformsshould
descriptor likea,, and as such may be calculated
at any field
extenddown to approximatelyfJ20.Inaddition,needle
location from a single hydrophone measurement. In practice
hydrophones should be used with caution, because of the
roll-off in low-frequency response that has been observe
in d the upper integration limit is chosen to reduce errors from
high-frequency spectral noise (25MHz was usedin [25]). In
thesedevices[13],[14], [IS], 1161, [17]. Tbisroll-offseems
to he relatedto diffraction at the needle tip, and at least one [2S] it was found that the SI tracked fairly linearly withom,
modifieddesignhasbeendeveloped
to amelioratethis
beingnumericallyabout
a factorof IO lower. Thus,a
problem [IS]. A related issue is the need for low-frequency thresholdvalue of 0.0.5-0.1 forthe SI seemed to be
calibration standards, and in this regard several methods areappropriate for demarcating the quasi-linear region, but this
concept needs further study.
heingconsidered, includingthree-transducerreciprocity[13],
interferometry [13], time delay spectrometry (TDS) [16],
m
[ 171 and broadband pulse[ l S].
SI = ! P ( f ) d f f P ( S ) d f
1 6
(1)
B. Nonlinearpropagation efects on derated quanlities
Medical ultrasound fields usually are measured in
Acousticattenuators: It hasbeenproposedthat
water, and one international standard specifies that outputs
low-densitypolyethyleneattenuators
be placedinthe
must be declared to the user in that form [19]. In the US,
ultrasoundbeambetweenthesource
and hydrophone to
standards for diagnostic ultrasound equipment have been
simulatean in situ exposure[26],[27].Theattenuation
published that define acoustic output quantities in which a
coefficient of thismaterialhasafrequencydependence
derating factorof 0.3 dB cm-lMHz.’is applied to estimatein
similar to that of soft tissue, and the acoustic impedance is
situexposurevalues[g],
[ll], [20].Forcomputational
approximately
25%
greater
than
that
of
water.
The
simplicitythisderatingprocessassumesthatultrasound
procedure recommended involves first measuring a, and fc
propagationislinear,butmostdiagnosticscannerscan
inwateratthelocationwherethe
in situ value is to be
produce output levelsin which nonlinear propagation effects determined. Then the attenuator thickness is chosen so that
are significant, not only in water but
in tissue as well [6]. A
its attenuation matches that of the desired
tissue type. If a,
problem arises because nonlinear propagation can lead
to
< 1, the attenuator is placed in the beam next to the source
acoustic saturation, in which
an increase in source voltage is transducer and the hydrophone measurement is made.a,If
not accompanied byan equivalent increase in field pressure. 2 I, an additional measurement is made with the attenuator
Attenuation in the propagation medium tends to counteract
movednext to thehydrophone,andthe
two resultsare
this effect, so saturation is relatively more pronounced in
averaged. While this technique has advantages, it has not
water than in tissue. Thus, a derated quantity calculated from
been widely evaluated.
a hydrophone-measured pressure at full output in water can
Tissue mimicking materials: An obvious but to date
lead to a significant underestimateof that quantity in tissue difficult approach for obtaining accurate estimatesinofsitu
1211. Severalapproachesarebeingstudied
to dealwith
exposure values when nonlinear conditions existtoismake
this phenomenon.
measurements in a material that simulates of
allthe relevant
Linear extrapolation of low-outputlevels:One
properties oftissue, including attenuation. Recent work with
proposal for making estimates of in situ exposures from
atissue mimickingliquid hasbeen reported inwhicha
water-based hydrophone measurements in the presence of
60140 mixture of deionized water and evaporated
roughly
nonlinear propagation isto perform a linear extrapolation of milk was used, along with a small amount of preservative
derated “small-signal” measurements; i.e., measurements
at
[28]. At room temperature (22°C) the material had a speed
source output levels in the linear regime [22].
To define this of sound of 1526 d s , a density of 1.03 &c, an attenuation
small-signal, quasi-linear region, it has been suggested that
coefficient of 0.27 dB cm-’MHz-’,andanonlinearity
a value of shock parameter a < 0.5 be used [D]. One such parameter (B/A) of 5.7. Theattenuationcoefflcientwas
parameter used for focused medical ultrasound beams and designed to match the commonly used derating factor
of 0.3
adopted in [1 l], [ 121is the nonlinear propagation parameter, [9], [ 101, [l l]. The other properties area reasonable match
am. Calculation of omrequires measurement of the focal
to soft tissue. While this and other similar materialsdo not
distanceandpressure,centerfrequency,andfocalgain.
havetheconvenience,universalavailability,andwellOther,possiblymoreconvenientnonlinearityparameters
characterizedproperties of water,theyarevaluablefor
have been introduced [24], [25]. In[25] afrequency domain
evaluating errors that may be introduced in making in situ
estimator of nonlinearity, the spectral index (SI),is defined
exposureestimatesusingderatedwater-based measurements.
(seeEq. (I)). Here P(f) isthepowerspectralvalueat
Also, they provide amedium for experimental verification of
frequency f, and f6 is the frequency above f, at which the
various theoretical models that have been developed that
spectrumfirstfalls 6 dB belowitsvalueat
F,. Asthe
include the effectsof diffraction, attenuation, and nonlinear
measured hydrophone waveform becomes more nonlinear
propagation
on the radiated field.
and generates more high-frequency harmonics, the value of
1:
1342 - 1999 IEEE ULTRASONICS SYMPOSIUM
change in n, which is integrated over the path
of interaction
The abilityofmodern diagnostic scanners to operate of the light and sound. For small pressure variations, the
local instantaneouschange in refractiveindex is proportional
not only with multiple focal zones, but in several modesat
to thepressure,theproportionalityconstantbeingthe
once, has madedeterminingthepeakvalues
of acoustic
adiabaticpiezo-opticcoefficient, K. Thus,withpasthe
quantities a challenging task, especially I,
[29]. Many
acoustic
pressure,
operator control settings alter the location
of the maximum
values,andgenerallynotinaneasilyidentifiableway.
(2)
v=kKJpdl.
Several approach have been takento alleviate this problem.
Establishing measurementprotocols: As a start to
Thelightintensityinthescatteredbeamis
a
determining the control settings that lead
to a maximum
function of v, and this relationship has beenforused
rapid 2state, a systematic approach for stepping through the process
ofmeasuring I,
has been proposed for B-mode [30], [31].D mapping and visualization of the ultrasonic field pattern
fieldscanbe
The protocol involves, among other things, looping throughusingaCCDcamera[35],[36].Pulsed
measured
by
proper
synchronizing
of
the
light
and
changes in focal position, number
of foci, and depth
of field.
ultrasoundbeams.Thefielddistributionintheresultant
The flow chart format is designed to reduce measurement
optical imagerepresents integratedpressure,
so tomographic
that the maximum IsPTA has
time while increasing confidence
reconstruction
has
been
used
to
obtain
the
pressure
been located. Others have used this procedure with some
dismbution by making multiple optical measurements at a
success,butinitsimplementationtheimportanceofan
series of angles through the ultrasoundbeam.Theoptical
inductive coil pick-up for reliable triggering
of the acoustic
system in [35]uses schlierenimaging(withzerothdiffracti0n
pulse measurement system was noted, not only for providing
the3-Dpressure
a trigger, but also
for providing information about pulse and ordersuppressed), andreconshuction yields
amplitudedistribution,fromwhichIsPTAandultrasonic
frame timing [32]. It also was mentioned that a way to avoid
triggering difficultiesis to measure untriggered hydrophone power canbe derived. In [36] the instantaneous perturbation
ofn (and pressure per Eq.
(2)) is found via optical processing
output with an RF power meter to obtain the I,,,, as has
in whichan iterative algorithm recoversthe integrated phase
been described previously (pp. 140-145 in [l]).
change at instant t, from measurements of optical intensity
in
Measurement
via
hydrophone
arrays:
As
an
two planesbeyondthesoundfieldandnormaltothe
additionalaidtofacilitatingmeasurementsinscanning
modes, both l-D and 2-D hydrophone arrays are being used direction of optical wave propagation. After reconstruction
the result is p(x,y,z)at ti, and the complete field p(x,y.z,t)
is
[33],
[34].
The
l-D array systemcontains a PVDF
obtained by varying t,. (This result may be compared to a
2 I elementsof0.4 mmdiameter
membrane hydrophone with
which p(t) is measuredatpoints
and 0.6 mm spacing. The
-3 dB bandwidth is approximately hydrophonescan,in
23 MHz. Scanning mechanics and processing and control
(X,,YpZk).)
In addition to its ability
to provide rapid 2-D beam
electronicsandsottware are included.Upgradesfroma
previous design include the ability
to transfer acoustic output plots (albeit of an integrated field quantity), advantages of
this technique are its inherently non-perturbing nature and
datainASCIIformat,areductioninnoiseequivalent
goodspatialresolution
(< 0.1 mm).Asthepressure
100
MHz
A-D
converter.
A
and
a
new
to
1
kPa,
pressure
amplitude
increases,
the
validity
of Eq. (2) diminishes, a
RF
power
meter
as
the
of
an
addition
is
interfacing
planned
mentioned above to make easier the identification of systemconsiderationthatapplies to some of thehigheroutput
settings that result in maximumIsWC The more recent 2-D therapy and diagnostic applications. In these large amplitude
ofthe field have been made using
array uses a vinylidene fluoride copolymer membrane withcases useful optical images
the
higher
orders
of
diffracted
light [37], but the relationship
64 (8x8) elements having0.2 mm-diameter elements and a
between
light
intensity
and
pressure
needs fiuther study.
1 mm spacing. A -3 dB bandwidth of
> 30MHzanda
combinedacoustic-electroniccrosstalk
of -30 dB are
D. Temperature rise phantoms
reported.
Thethermalindices(TI)oftheOutputDisplay
Measurement
acousto-optic
via
dt@raction:
Another approach that has been used
as an efficient means to Standard [g], [IO] have been in use since 1992. While they
obtain ultrasound field distribution measurements employs have proved useful as a predictor of thermal risk during
ultrasound
procedures,
they
are
based
on
the phenomenon ofacousto-optic diffraction (pp. 143-162 indiagnostic
theoreticalmodelsratherthanactualmeasurements
of
[3]).
When
ultrasonic
pressure
waves
propagate
in
temperature
rise
in
tissue.
Furthermore,
to
facilitate
transparent
media,
the
density
perturbations
produce
implementation of these
indices
on
clinical
systems,
fluctuations in the optical refractive index, n. A beam of
simplifyingassumptionsregardingtissuepropertiesand
lightpassingthroughtheselocalinhomogeneitieswill
as ameans to
experience velocity (and phase) deviations. The total optical beamgeometriesweremade.Therefore,
phase shift, v, for a monochromatic light beam is given by evaluate the theoretical models,as well as to estimate tissue
from
diagnostic
ultrasound
exposures
v=kJ 6n dl, where k is the optical wave number
6nand
is the heating
C. Measurements in complex scanning modes
1999 IEEE ULTRASONICS SYMPOSIUM - 1343
experimentally,athermaltestobject(TTO)hasbeen
conductivity, and the acoustic impedance of the plexiglass
proposed 1381. (Note that, strictly speaking, temperature rise absorber. The sensor has been tested for CW beams from 3is notan exposure quantity.)This TT0 comprises a thin-film 17 MHz [44]. The sensitivity over this range varied 10from
thermocouple (TFT) positioned between
two layers of tissue
30 W,and the timeto reach thermal equilibrium was-13
mimickingmaterial,each4-8mmthickand
50 mm in
minutes. Thistime could be reduced by decreasing the
diameter. The top surface of the TT0 is protected by a 6
absorber thickness, but sensitivity would be decreased
as
pm-thickmetallizedmylarsheet.Thebottomsurfaceis
well. Also, for
measurement
of pulsed
waveforms,
of 2% ethanol
coupled to an acoustic absorber by a mixture
knowledge of the frequency spectrum is necessary.
in water.
The
TFT
was
chosen
over
bulkier
thermal
F. Intercomparison measurements
configurationstominimizeviscousheating due to interaction
In Europe two measurement studies on diagnostic
between the thermocouple body and the ultrasound beam, fields, each coordinated by a national measurement institute,
andthermalconductionalongtheconnectingwires,
two
have been performed in accordance withlEC61157 [ 191. In
artifacts that become significant as the beam dimensions
both, a diagnostic ultrasound device was circulated among
decrease. The TT0 is scanned in the ultrasound field much palticipating laboratories and the resultscompared, In 1451
as a hydrophone would he. The sensitivity was foundto he
10 groupsmademeasurementsona
5 MHzlineararray
10-16 p V K , with a temperature resolutionof 2 mK.
scanner and a check source.
It was concluded that the single
The advantage of a TT0 approachisthatno
mostimportantfactorcontributing
to thedifferences in
assumptions about the radiated field are made, only
the about
results was the variety
of hydrophone and amplifier systems
tissue model. Also, transducer self-heatingis accounted for, used. In 1461 a commercial diagnostic instrument was sent
a factor ignored in the TI models. The
TT0 in [38] contains to 9 participants. Measurements were restricted to M-mode
no perfusion, although it has been shown that perfusion mon
aythe3.5MHztransducerused.Whilethemajority
of
beaccountedforcomputationally
1391. Futureworkis
results were in good agreement with one another, significant
proposed for comparing temperature risein the TMM with
deviations were found, indicating the importance ofperiodic
that in tissue both in vitro and in situ,
checking of the measurement devices and procedures.
E. Measurement ofultrasonicpower
111. DIAGNOSTICIMAGING AND DOPPLER
ABOVE15 MHZ
Theradiationforcebalance
(FWB) andplanar
scanning techniques remain the most common methods for High frequency
diagnostic
applications
include
measuring
the
temporal-average
power
in medical
a
ophthalmology, dermatology, and intravascular ultrasound.
ultrasound beam [l l], [40], [41]. In the RFB measurement, The challenges here have been to improve the spatial and
errorsourcesincludeforcesduetotargetbuoyancy,
temporal
measurement
resolution,
perform
and
to
streaming, surface tension of target support structures, and hydrophone calibrationsat higher frequencies.
external vibration. One RFB approachtominimizethese
confounding forces involves modulating the ultrasound and A . New hydrophone designs
measuringtheresultantradiationforce
at afrequency
Both needleand membrane hydrophones have been
removed from their influence (20 Hz) (421. Errors of less
redesigned for measurements in higher-frequency focused
than a few percent are reported for powers from O.lmW-30fields. Thinner films to increase measurement bandwidth,
andsmalleractiveelements
to reducespatialaveraging
W, and frequencies from0.5-30 MHz.
Both the RFB and planar scanning methods requireeffects, have been used. To date, needle hydrophones with
9 pm PVDF film and diameters
of 0.04,0.75,0.15, and 0.2
a somewhat complex set-up. A simpler technique that has
beenproposed,especiallyforroutinecalibrationchecks
mm are obtainable commercially. Inthemodifiedneedle
mentioned in Sec.1I.A [IS], 4 pm copolymer film was used
wherethehighestaccuracyisnotrequired,isbasedon
in probe an effective
measuring the temperature rise inan absorber placed in the in an ellipsoidal configuration, and one
ultrasoundbeam.Thisthermoacousticsensingtechnique
diameter of 0.13 mm was measured, along withe a0 . 3 dB
response
from
0.2-40
MHz.
Bilaminar
membrane
[43] comprisesan ultrasound-absorbing piexiglass disk 1 cm
thick x3 cm in diameter. The water-home ultrasound waveshydrophones made from9 pm PVDF sheets are available
as
impingeuponthefrontfaceofthedisk.Theotherdisk
well, one basedon [47] witha 0.2 mm electrode diameter.
In
surfaces must be thermally insulated, and an air layer has
[48] a membrane hydrophone is described in which
37 pma
been used in the reported device.
A temperature sensor at the diameter spot-poled electrode was formedon a single sheet
back face (i.e., the planar
Plexiglas-air interface) measures of4pmP(VDF-TrFE)film.Themeasuredeffective
diameter was <l00 pm, and the -3 dB bandwidth was 150
the temperature rise, whose steady-state value is proportional
MHz. In all of these needle and membrane designs reduced
to the powerof the incident ultrasonic wave. Calibration of
this thermoacoustic sensor requires knowledge of the sound sensitivity is an important factor, and submersible integral
150 MHz
absorptioncoefficient asafunction offrequency, the thermal amplifiers or short cable lengths are required. The
1344 - 1999 IEEE ULTRASONICS SYMPOSIUM
bandwidth in this latter device was achievedat the expense
down to 25 pm have been reported. An added advantage
of
of increased noise susceptibility, because only a single layer
thismethod is thatthemeasurementscharacterizethe
of polymer film was used. The noise canceling feature of a transmit-receive response of the system. However, relating
to offset this themeasuredwaveformsandprofiles
differential amplifier design has been proposed
to transmit-only
effect [49], and one such membrane device having0.2amm
quantities is difficult, so direct measurements of exposure
spot size and 14 dB-gain differential amplifier is available, fields are preferred[l l], [12].
although no comparative test results have been reported.
Whenhydrophonesizeguidelinescannotbemet,an
alternativeapproach is to performaspatialaveraging
B. Hydrophone calibration at high frequencies
correction.Thegeneralprocedureis
first tochoosea
No standardsexistforhydrophonecalibrations
theoretical model to describe the acoustic pressure field at
above IS MHz, although the IEChasinprocessadraft
z from the source
radialdistance r andaxialdistance
document that addresses some aspects of this topic [50].
transducer (assumed circular). Next, taking the hydrophone
Calibration in this range is an active of
area
study, and some output as proportionalto the average pressure overits active
current work is described below.
surface, the spatial averageofacoustic quantity Q(r,z) (e.g.,
The use of laserinterferometry for calibrating
pressure) over that surface is calculated (denoted <Q(r,z)>).
hydrophones has become standard practiceat some national Then correction factor Cis computed as C= Q(r,z)/<Q(r,z)>
measurementinstitutes,andrecentlyextensionofthe
= l+&,, 6, being the spatial-average deviation.
calibration range to
60-70 MHz has been reported
[5 I], [52],
A simple approach recommend in an IEC Technical
[53]. Both interferometer systems are
ofthe Michelsontype,
Report [59] assumes a jinc radial beam shape. The most
but inone the optically reflective pellicle that is displaced by
comprehensive
method
proposed
date,
tohowever,
the ultrasonic wave is located at the water surface instea
of d incorporates the effects of nonlinear propagation into the
analysis of the pressure distribution and 6, [60]. In this
beingtotallysubmerged.Thestatedadvantage
of this
to whichthe
approach :S thattheopticalphaseshift
procedure 6, is expressedas a functionof om and new factor
interferometer responds is not affected by changes in the
a, = b.&ddd,, where b.wis
themeasured(i.e.,spatial
refractive indexofwater caused by the propagating acoustic averaged) beamwidth for quantity Q, andde is the effective
n
wave (see Eq.(2) and discussion). Thus, no correction for
hydrophonediameter. In [60] 6, was computedforp,as well
as the peak compressional pressure (p,) and pulse pressureis necessary.
Inbothsystems,focusedtransducersdrivento
squared integral.As an example result,forp, with om = I, 6,
produce
harmonically
rich
waveforms
via
nonlinear
increased from~ 5 at
% a, =5 to >40% for a, = 1. In general,
propagation are used to obtain a sufticient pressure SNRat
6, (or C) is a complex functionof the hydrophone size and
work is needed for
the higher frequencies, the main problem being the inverse degree of distortion, and more simulation
both circular and rectangular fields, as well as experimental
dependenccofparticledisplacementon frequencyforagiven
verification of results.
pressure. For one system the calibration uncertainty (at
95%
C.L.) increased from*4.6% at 20 MHz to*25% at 60 MHz.
N.EXTRACORPOREAL
PRESSUREPULSELITHOTRIPSY
Thisprimarycalibrationmethodcanprovidereference
hydrophones
for
use secondary
in
calibrations
via
substitution.
A. Hydrophone developments
The highly focused, large amplitude pressure fields
Two other less vetted but still useful methods for
high frequency calibration have been reported of late. In
produced by extracorporeally induced lithotripsy devices
providean efficientmeansforfragmentingurinarytract
to that
one,thehydrophoneresponsewascompared
calculi. To address the characterization and safe use
ofthese
predicted from theoretical nonlinear field calculations[54],
devices, two IEC standards are now available, one covering
[S]. Inamoredirectapproach,
TDS hasbeenused to
calibrate a membrane hydrophone from 1-40 MHz, and the field measurements [61] and theother dealing with labeling
maximum difference from an interferometric calibration of and other safety aspects[62]. The field quantities calledfor
the same hydrophone was 1.3 dB [56].
include p, and p,. the energyperpulse,andthefocal
dimensions. As withdiagnosticpulsemeasurements,the
hydrophonesusedfordeterminingthesequantitiesmusthave
C. Sparial averagingby hydrophone
awide,uniformfrequencyresponseandasmallactive
At high frequencies the measurement of “point”
field quantities is hampered by spatial averaging effects dueelement, although the latter is not
as critical, 0.5-1 mm being
to the finite sizeof the sensing element. Ideally,
the receiver sufficient in most cases. Because
ofthe potential for damage
dimensions should be less than one wavelength, hut at 15
tothehydrophone,additionalfeaturesalsoareimportant
MHzinwaterthisvalueis
0.1 mm. One approachfor
[63], [64]: robustness, to withstand the intense fields and
overcoming the size limitations of availzble hydrophones has
possible cavitation effects; electrical shielding, to suppress
been to performpulse-echobeamprofilemeasurements
RF interference and provide insulation from the conductive
using fine wire or spherical targets [57], [S]. Wire sizes
propagation media sometimes used; and a large dynamic
1999 IEEE ULTRASONICS SYMPOSIUM - 1345
pulse fidelity, especially the trailing edge, is degraded by
range. In [61] a high-performance hydrophone, termed a
attenuation in the front plate as well as its two-way travel
“focus”hydrophone,isdefinedformeasurements
at the
time (about 3.3
focuswheretherequirementsaremostdemanding.Itis
ps for l 0 mm-thick steel), plus the generation
stated that the focus hydrophone shall be equivalent to a
of slower velocity shear waves in this plate. However, the
to
be
relatively
unaffected
by
single-filmpiezoelectricpolymerspot-poledmemhranetype measured p, seemed
with
film
thickness
not
greater
than
25 pm.
While
thicknesses of from6-18mm,andthedurabilityofthe
membrane hydrophones appearto provide generally faithful hydrophone was good, with small pinholes being observed
after 1000 pulses having peak amplitudes of 20-45 MPa.
reproductionsoflithotripsypressurepulses,theyarefarfrom
The effective diameterof the probe was calculated
to be 2.1
ideal. In addition to the damage issue, an artifact has been
seen in the hydrophone waveform due to reflections at the
mm duetothefringeelectricalfield
at thecapacitor
boundary.
hydrophonehousing[65].Thepresenceofthisartifact
Electromagnetic hydrophone: Magnetic induction
determineshowfarawayfromthelithotripsybeamaxis
measurements can he made, especially if that structure is
as a basisforconstructing a robust
alsohasbeenused
hydrophone. In [75]a probe is described in which a 5 mm
highly
reflective
[66].
To overcome
these
problems,
particularlywithregard
to hydrophonedamage,several
length (I) of 30 pm-thick copper wire was fused into the front
face ofa 5 mm-diameterplastic rod.The front ofthe rod then
alternatives
the
classical
to spot-poled
membrane
hydrophone design have been explored, as discussed below. was placed in the field of a permanent magnet suchthat the
While no single hydrophone design has yet emergedas the
fused section of wire was oriented normally to the acoustic
optimal choice, each of these variations has found use in
and static magnetic(B=200 mT) fields. In this configuration
the vibration of the wire due to the ultrasonic wave will
lithotripsy field measurements.
induceatimevaryingvoltage
(e) at the wireterminals.
Modijed membrane hydrophone: In one modified
Assumingthatthewirethicknessisnegligibleandthe
membraneapproach,unmetallizedPVDF
film wasspotpoled and mounted inan enclosed chamber with an acoustic acousticandmagneticfieldsareuniformoverits5-mm
window. The chamber was filled with either dielectric fluid length, p(t) is proportional to eB1. This electrical signal was
[67] or low-resistivity electrolyte [68], [69]. Electrodes were amplified by a preamp located close to the magnet. Input
located in the chamber away from the film, close enough to limiting ofthe preamp was necessary to clip the large signals
collect the pressure-induced charge(via capacitive coupling arising from the lithotripter excitation pulse.
in the former case), hut far enough away to minimize damage The high frequency response was limited by the
-6 dBcorner
from the shock waves. In a different approach, the goal has wirethickness,30pmgivingatheoretical
been to produce an inexpensive, disposable membrane [70], frequency of - 5 MHz. Also, errors will be caused by the
of uniform
[71], [72]. In [72] a commercial shock gauge element was transverse resolutionof 5 mm and the assumption
no
fields over this length. However, a prototype showed
mounted on a housing containing degassed petroleum as a
signs of failure after exposureto 4000 pulses havinga p, of
backing material for electrical isolation. Inanotherdesign the
resistance of themetal electrodes and leads is monitored to 30 MPa.
Another electromagnetic probe designed primarily
indicate when the film should he replaced
[71].
for quality control measurements employed
a 12.7 mm steel
Spot-poledceramic hydrophone:Inthisvariationon
[76]. The ball was attached to a
ballplacedatthefocus
a spot-poled membrane, a lead titanate disk(15 or 30 MHz
stainless steel rod containing the electromagnetic transducer
resonance frequency) having a diameter of less than - I cm
located between the polesofa permanent magnet. Although
was spot-poled and brass-backed, and this assembly was
connected to a coaxial cable [73]. Lead titanate was chosen theoutput of theprobedoesnotfollowthelithotripter
pressurepulse, it has beenrelated to both p, andstone
for its low radial mode coupling. Rise times
<50 ns and
effective diameters<0.5 mm have been achieved. A typical disintegration, and it was used to monitor the output of a
lithotripter system over a 12-month period.
lifetime of between 200 and 1000 shots at the focusofa 100
Fiber-opric hydrophone: The various deficiencies
MPa
(p,)
lithotripter
is
reported,
the
major
damage
mechanismbeingpiningoftbeceramicduetocavitationjets. with hydrophones mentioned above (e.g., fragility in intense
Capacitance hydrophone:To increase resistance to fields, susceptibility toRF interference, and, particularly for
diagnostic fields, inadequate spatial resolution) have inspired
hydrophone damage, a hydrophone bas been designed in
which the incoming pressure wave modulates the air gap of the development of acoustic sensor designs basedon fibera parallel plate capacitor formed by a steel front plate and aoptic (FO) detection techniques. In the most successfulF 0
to form a rightcircular
2 mm diameter brass cylinder
as the rear platelelectrode [74]. approaches,thefiber,cleaved
cylinder,
is
aligned
so
that
the
endface
isnormaltothe
Assuming that the acoustic particle displacement is small
compared to the air gap thickness
d,, the measured pressure, direction of acoustic propagation, just as a piezoelectric
needle hydrophone would be.In this configuration the fiber
p(t), is proportional to d,e’iEoC,, where C, is the electrode
behaves an extrinsic rather than intrinsic sensor, in that its
is the
capacitance, E, is the capacitor DC bias voltage, e’and
basic function is simply to carry coherent light to the tip
timederivativeofthemeasuredcapacitorvoltage. Measured
1346 - 1999 IEEE ULTRASONICS SYMPOSIUM
where the transduction processis initiated. The tip has been acoustic displacement or, when a heterodyne technique is
of which lead toa calculation of
treated in several ways to create a reflected light signal that used, particle velocity, both
the pressure. To correct for the tip diffraction noted above,
can be related to the acoustic field.
as well as other acoustic effects associated with the fiber, the
1) Bare fiber tip - In the simplest approach, light
magnitudeofthedisplacementtransferfunctionwas
from a laser diode is coupled to a multimode fiber with a
measured experimentally using TDS
both and interferometry
bare endface [77]. To a good approximation, the light is
[82]. Then, assuming minimum-phase behavior for the
F0
reflectedaccordingtotheFresnelintensityreflection
system so thatthephaseresponsecouldbeuniquely
coefficient, R = [(nc-nw)/nc+nJ]*, wheren, and n, are the
indices ofreflection ofthe fiber core and water, respectively.determined,themeasuredlithotripsypulsedatawere
corrected via deconvolution.
As discussed above in Sec. KC, pressure variations in the
3) Fabry-Perot interferometer- As an alternative to
a changein density, which inturn alters
acoustic wave cause
the refractive indices, and thus the reflected light intensity. the optical complexity of two-beam interferometry, but still
Over most of the pressure range associated with lithotripsy with improved sensitivity compared to the bare fiber design,
aFabry-Perot(FP) interferometercan formed
by attachingan
pulses,thechangeinreflectedlightintensityhasan
FP cavity to the end of the fiber[83], [S4],[ 8 5 ] , [86], [87].
approximatelylineardependenceonacousticpressure,
When light is sent down the fiber, optical reflections occur at
neglecting the acoustic scattering effects described below
the interfaces between the fiber and cavity, and the cavity
[77]. It should be noted that a change in n, will occur as
andload(water).Theopticalreflectioncoefficientsare
well, hut due to the much lower compressibility of the fiber
determined either by the Fresnel formula for uncoated cavity
material, this effect is negligible[77].
surfaces [83], [84], or by the type and thickness of metal
The device in [77] had a bandwidth of 20 MHz,
limited by the photodetector amplifier, an effective aperture coatings on these faces[ES], [87]. In general an FP cavity is
a multiple-beam interference device, but its operation can be
of 0.1 mm, and good immunity to
RF interference. Due to a
relatively low sensitivity, the minimum detectable pressure explainedbyconsideringonlythetwoinitialreflectionsfrom
thecavitysurfaces.Theintensityofthereflectedlight
was 0.5 MPa,
value
a acceptable
for
lithotripsy
of these reflected beams,
which
measurements, but one that could be reduced with a higher depends on the interference
in tum depends on the optical phase shift between them.
light source power more
or sensitive photodetector. Also,
in
Acoustically-induced changes in cavity thickness will vary
comparison
a
with
a PVDF
membrane
hydrophone
measurement, it was noted that the negative pressure tail or this phase shift, and the acoustic pressure can be deduced
from the optical, acoustic, and elastic properties of the cavity
trough was shorter in the membrane-measured waveform.
This has been attributed to the stronger water-fiber (silica) material, andthe undisturbed cavity thickness(L).
Both polymer films and harder dielectric materials
layer adhesion compared to that at the water-metal or waterhave
been
used for the cavity material, the former being
polymer boundaries. This effect has been noted elsewhere,
moresensitivedue to theirgreatercompressibility.The
where p,% of -25 MPa [78] and -59 MPa [79] have been
measured with an F 0 hydrophone, and it has served as the latter, however, canbe sputtered onto the endface[84], thus
avoiding the degradation in frequency response that can arise
[66]. The
basis for adjusting membrane hydrophone results
due to the adhesive layer that bondsthe polymer to the tip
integrity of thetipcanbecheckedbymonitoring
the
quiescent reflected light intensity. The fiber tip has a high
[85]. To avoid this adhesion problem, recently a polymer
damage threshold, but should it be damaged by cavitation, film has been vacuum-evaporated onto thefiber tip [E7].
of multiple
the fiher can
be recut without affecting sensitivity; however,Also, toincreasesensitivity,anFPstack
dielectric
layers
has
been
deposited
onto
the
fiher
[ES].
to maintain the minimum noise level, a clean, smooth fiber
However,
even
without
this
last
measure,
sensitivities
endface must he achieved.
comparable to piezoelectric polymer hydrophones have been
One problem reported in [77] is that diffraction at
realized, and with significantly smaller sensitive areas. As
the needle tip distorts the measured pulse, just as in the case
of piezoelectric needle hydrophones described in Sec. ILA. with piezoelectric hydrophones, atrade-off is involved in the
choice of L, in that sensitivity increases with increasingL,
This problem is common
to all F 0 hydrophones, and in[78]
a simple extrapolation is shown to for
correct
overshoot in p,. but for a broadband response,L should be smaller than the
A more rigorous deconvolution method is mentioned below.highest acoustic wavelength of interest. Also, both singleand multi-mode fibers have been used in these FP optical
Also, inthefollowing
F 0 hydrophonedescriptionsan
improvement in sensitivity is achieved by treating the fiber sensors. The former gives better spatial resolution (e.g., 6
in [87]), and is less susceptibleto changes in sensitivity
tip, but at the cost of design simplicity and replacement pm
ease.
due to fiber bending [ S ] , [87]. On the other hand, multi2) Two-beam
interferometer
- To
increase
mode F 0 hydrophones are easier to align with the laser
measurement sensitivity, the fiber tip has been mirrored and
before use [U]. Finally, as stated above, the tip diffraction
incorporatedintothemeasuring
arm of atwo-beam
interferometer [EO], [SI]. The optical phase change caused problem needs further investigation in order to make the use
of any type of
F 0 hydrophone more practical.
by the tip movement results in a signal proportional to the
1999 IEEE ULTRASONICS SYMPOSIUM - 1347
A . HIFU
B. Measurement ofpulse energy
There are no current standards for characterizing
ofthe
The energy impinging on the stone is the physical HIFU fields. Measurements typically have been made
power,
field quantity most closely associated with volume erosion ultrasonic
W,,
with
an
RFB, and
the
beam
distribution in water around the focus. The beam plots have
[a]This
. quantity usually is calculated from hydrophone
scans in the focal plane, and for a circularly symmetric beam
been obtained with both hydrophones (for derived I,A)
and
the suspended ball radiometer (for I, directly) (see [3], p.
withradialdimension
r, theenergyEperpulseis,
E=2nZ-'[jp2(r,t) dt rdr, where
2 is the acoustic impedance of207). The ball is more robust, but
the ball calibration factor
water. This quantity is not uniquely defined, however, until is a complicated functionof frequency, so its use is restricted
the temporal and spatial integration limits are specified
to narrow bandwidth fields. Therefore, errors can be large
[SS].
In [61] the temporal integration time may be either over the whenmeasuringhighintensityfieldsinwhichnonlinear
initial positive pressure spike (TJ, or over the entire pulse
propagation effects lead to significant harmonic generation.
(TT)(both definedby the first and last lO%-of-peak pressureAlso, acousticstreamingcanbeanimportantsourceof
points). Also, in [6 l] the radial scan limit may be defined
measurement error at high intensities.
To address these measurement problems, and
to
either by the-6 dB point (E=E,), or by a specifieddistance R
provide a single exposure quantity with relevance to thermal
that corresponds to a particular stone dimension @=E,). A
third scan limit has been described in which the integration lesion formation, a new intensity has been proposed [93].
Denoted I, it is the TA intensity spatially averaged over
is stopped at the point where p, = 5 MPa [64]. In one study
of these three spatially-defined pulse energies (using
TTand
the-6 dB beamarea,thelattermeasuredunderlinear
R=5 mm), it was found that
E a 5 MPa was highly correlated conditions. That is, ISAL= W.,/A,, where W., is the power
with fragmentation of model stonesin vitro, whereas E, was
within the contour defined by the half-maximum pressure,
and A, istheareaenclosedbythat
a poor indicator of stone destruction 1901. These and other
-6 dB contour.
results are summarized in [64], where it is emphasized that
Measuring A, at low output
as specified avoids the problems
if lithotripters are to he compared based on pulse energy,
associated with hydrophone instability and damage, and the
then it mustbe established that the integration limits are the hydrophoneneednotbecalibrated.Inpractice,
in situ
same.
intensities are calculated using linear derating[93], [94].
Using the theoretical jinc-squared radial intensity
distribution at the focus of a source with circular aperture, the
C. Optical techniques
Acousto-optic methods also have been explored forspatial-peak pressure is I,, = 1.8IsAL= I .56.WJ(d~,)', where
characterizing lithotripsy fields. These include a schlieren
d, = (4A.6/n)o.s.When these quantities wereused in models
of thermal lesioning threshold intensity and lesion size, the
system,inwhicharubylaserpulse
of 20 ns was
was
predictions agreed well with experimental results, itand
synchronized tothe acoustic pulse, giving a spatial resolution
of 30 p n in
water
[91].
Also, Michelson
a
type
concludedthat I,,, wasausefulexposurequantityfor
interferometer has been adapted
to measure the pellicle (i.e., focused surgery in terms of both measurement ease and
biophysical relevance.
particle)velocity,fromwhichtheacousticpressureis
Measurements of focal intensities at the highest
calculated [92]. Movement of the acousticallythinbut
levels
used
(several thousand W/cm2) have been performed
a
Doppler
shift
in
the
causes
opticallyreflectivepellicle
with
spot-poled
membrane
hydrophones,
either
the
frequency of thereflectedlight.
This frequencychange
results ina modulated light intensity, measurement of which disposable type 1951, [96], (as in [7l I), or with the pulse
duration reduced dramatically (e.g., tens of ps or less) to
allows the particle velocity can be calculated. The system
minimize the riskof hydrophone damage, [97], [98]. In the
bandwidth,limitedmainlybythephotodetectors,is
100
latter case, the intensity for pulse durations used clinically
is
MHz,andthevelocityrangeof1-100m/sconespondstothat
All
theshort-pulsemeasurements.
obtainedbyscaling
produced in lithotripsy.
measurements shouldbe madein clean, degassed water(1-2
ppm 0,) to reducethepotentialforcavitation-generated
V. THERAPEUTIC ULTRASOUND
damage or error (a statement applicableto all high-intensity
Clinical usesoftherapeutic ultrasound now encompass such measurement situations).
A more qualitative but very effective means that has
areas as hyperthermia,
enhanced
drug
delivery
for
been used to determine 2-D and 3-D beam distributions in
thrombolysis,
fracture
healing,
phacoemulsification,
waterforHIFUtransducersincorporatesacousto-optic
physiotherapy, and high intensity focused ultrasound (HIFU)
diffraction as in Sec. ILC[99]. Asstated there, this approach
for surgery and hemostasis. Lithotripsy, covered above in
Sec. IV, would be included as well,and the requirements for is inherently non-perturbing, has good spatial resolution, and
provide
rapid
a means
for observing
beam
hydrophones listed in Sec. 1V.A also apply when measuring can
high-intensity surgicalor therapeutic fields. Some important characteristics. Onceregionsofhigh intensity are identified,
they
can
be
probed
more
critically
using
calibrated
aspects of these measurements follow.
1348 - 1999 IEEE ULTRASONICS SYMPOSIUM
piezo-ceramic hydrophones are adequate for recording the
to the
transducer
being
pressure,
with
values
close
extrapolated from radial plots made normal to the cylinder
axis. The power can be derived from hydrophone scan data,
is
or via an RFB.In the latter case, a conical reflector has been
usedtodirectthecylindricallydivergingbeamontoan
absorbing RFB target.
The potentiationof chemotherapy by pulsed ultrasoundhasbeendemonstratedusingunfocused,plane
of a 2 MHz, 3.8 cm-diameter
transducers.Inonestudy
source [1051, a region of the field having a relatively smooth
distribution was identified, and the pressure waveform was
recordedoveracentral,transverseplaneusinga
B. Physiotherapy
2 mm
samplinginterval. Diagnosticpulsedefinitionsforintensities
Physicaltherapistsadjustthelevel
of exposure
and physiotherapy definitions for beam nonuniformity were
duringatreatmentusingaquantitydisplayedonthe
calculated. This hybrid group of exposure quantities was
equipment known as the effective intensity, defined as W,
divided by the effective radiating area,
or ERA. The ERA is then used to correlate exposure with biological response.
the areaof the surface close to the transducer through which
essentially all of the ultrasound power passes. The ERA is D. Low-frequency ultrasound surgery
determineddirectly fromfieldmeasurements,specificallyvia
In the 20-60 H z range, ultrasound finds use in
ophthalmic, neurological, dental, and plastic surgery, and
hydrophonescanning.ArecentIECstandardforthese
also
thrombolysis
and
angioplasty.
Device
operation
devices [l011 provides a more reproducible measurement
involves the vibration of a small wire
method for the ERA, in that it is not based on a perc
of entage
or hollow needle,
the maximum hydrophone output close to the transducer as resultingindestruction of tissueviamechanicalmeans,
in previous methods[ 1021, and thereforeit is notas sensitive
including cavitation. Measured acoustic quantities include
wire or needle tip displacement, frequency, pressure, and
to hydrophone sampling
to differences in hydrophone or
size
1998 IEC
power.Measurementmeansarespecifiedina
that might miss the point of highest pressure. In the new
standard [106],and are discussed in [l071 and [108]. The
approach,atseveralplanesa2-Darray
of hydrophonemeasured data with step size As is sorted from lowest to
power is derivedfromhydrophonemeasurements,using
either a monopoleor dipole source model depending on the
highest, and a running summation is computed. An area
proportional to the productWAS' is then found, whereN is
assumed depth of the vibrating tip in water or tissue. One
the number of data points associated with75% of the total problem identifiedwiththis
as tip
procedure isthat
displacement
increases,
cavitation
activity
causes
the
summation.TheERAisfoundfromanextrapolation
of
hue power.A
these areas back to the applicator surface. This method has calculatedpowertounderestimatethe
correction has been described in which the high frequency
been found to give reproducible results in a study among
components of the acoustic energy generated by cavitation
three measurement laboratories [102]. Also regarding the
implosionsaremeasuredwithahydrophonesensitiveat
ERA, a non-scanning method has been proposed recently
[103]. It involves RFB measurements of the power passing megahertz frequencies [107]. Future revisions of the IEC
standard may incorporate such a procedure.
throughasetofdifferent-sizedattenuatingapertures.
Acceptableagreement withthescanning-summation method
VI. IN VIVOMEASUREMENTS
was found, and this method is suggested
as a fast, relatively
simple procedure for measuring the ERA in a manufacturing
Piezoelectric needle hydrophones have been the traditional
or clinical setting.
means for in vivo pressure measurements; however, other
aproaches are being tried. For example, in [l091 a sevenC. Ultrasound-enhanced drug delivery
element linear array of PVDF elements was described that
Ultrasound has been found to increase the rate of
measures in vivoexposures intervaginally during an obstetric
clot
lysis
when
used
conjunction
in
with
various
ultrasound
examination.
The
OS-mm-diameter
PVDF
thrombolyticdrugs.Inonedesign,asmallcylindrical
elements were spaced 1.S mm apart along a stainless steel
piezoelectrictransducer is mounted in thedrug-delivery
tube.
For in vivo lithotripsy measurements, two
catheter and driven with both long-pulse and CW 1 MHz
modifications of thesingle-sheet,spot-poledmembrane
ultrasound[104].Themechanisms
of actionarenot
design have been used. In one, previously metallized and
completely understood, but both thermal and non-thermal
spot-poled PVDF film was formed into a hemispherical shell
contributions
are
likely,
including
stable
cavitation.
supportedbya soft, low-impedancebacking [IlO]. The
Accordingly, exposure measurements have included both
resultant bulb was mounted on the end of a stainless steel
power and pressure. Because the signals are narrowband,
hydrophones.
Lastly,anelectromagnetic pressuresensorhasbeen
developed for MRI-guided HIFU applications [loo]. The
transducercomprises6tumsofO.lmmdiametercopperwire
wound on a plastic core measuring 40x3.2 mm. A voltage
induced in the coil by ultrasonically-mediated movement of
the wire in theMM device's static magnetic field. For 750
kHz ultrasound the sensitivity was
1.g VMPa, and the noise
pressure level was 1 kPa for a 4.7 T magnet. The sensor is
saidtobeusefulformeasuringpressuretocalculate
temperature rise, or inQC testing of the HIFU transducer.
1999 IEEE ULTRASONICS SYMPOSIUM - 1349
[4] M.Hodnett and B. Zeqiri, “A strategy far lhedevelopmentand
tube, with electrical connection being made via a coaxial
standardization
of measurement methods for high poWCr/Cmitaling
cable as in the needle hydrophone. In another lithotripsy
ulWsonicfields:Reviewofhighpowerfieldmuvurementtechniques,”NPL
hydrophonedesignformeasurementsinpigs,apiece
of
ReporlCIRA-(EXT)O15. Teddington, UK: 1997, ISSN 1361-5053.
metallized, spot-poled film was stretched over a plastic ring [5]G.R.HarrisandP.A.Lewin,“Ultrasonicenposimehy,”Encvclooediaof
Electrical and Electronics Engineering.New York John Wiley, 1999, vol.
having an outsidediameter of 21.5 mm [ I l l ] . After
22, J.G. Webster,Ed.,pp. 634-646.
connecting
coaxial
a
cable,thisminiaturemembrane
[6] A.C. Balrer, “Nonlinear effects in ultrasound propagation.”
hydrophonewasthencoatedwithsiliconerubberbefore
inMedicine.Bfistol,UK: InstiruteofPhysics Publishing, 1998,F.A. Duck,
implantation to provide electrical insulation. More recently A.C. Baker, andH.C. Stanitt, Eds., pp. 23-38.
afiber-optichydrophonewasusedformeasurementsin
[7]G.R.Harris,”Pressurepulsedistonionbyhydrophonesduetodiminished
low frcqumcy response,” IEEE Trans. UFFC, vol. 42, pp. 989-992, 1995.
patientsundergoingclinicallithotripsytreatment
[U]. In
[SI G.R. Harris, “Arecurrent hydrophonelow frequency response standards
addition to small size, other advantages were: no electrical acceptableformeasuringmechanical/cavifationindices?,”Ult~osonics,vol.
contacts to patient; performance unaffected by conductive
34, pp. 649-654, 1996.
body fluids; easily replaceable so cross-infection hazards
[S] Slandord/oor Red-Time Display o/Thhermal and Meehonicd Acouslic
Output Indices on Diagnostic Ultraround Equipment, Rev. I,Am. Inst. of
minimized and alignment errors reduced by wide angular
in Med., Laurel, M D Nat. Electr. Manu. Assoc., Rosslyn. VA
response. Problems were experienced, as discussed in Sec. Ultrasound
(NEMA Publ. UD 31, 1998.
IV.A, but with further development these
F 0 devices seem [IO] 1. Abbott, “Rationale andderivation of MI and TI . A review;’
to hold promise for invasivein vivo exposimetly.
Oit~~~oundinMed&Biol.,vol.25,pp.431-441,
1999.
Regarding noninvasive measurements, quantitative [I11 Acoufic outpur meusurement slondard /or diagnostic dtrosound
equipment, Am. Inst. afUltrasound in Med., Laurel,MD; Nat. Electr. Manu.
mapping of ultrasound fields in tissue phantoms has been
Assoc., Rosslyn, VA (NEMA Publ. UD 2, Rev. Z), 1998.
demonstrated
via
MR
imaging
[112].
Absolute
[l21 Measurement
and
choroderimlion of ulrrasonlc fields using
measurements of acousticdisplacementamplitudeswere
hydropkonerinfhe/requencyrangeO.IMHrlo15MH~,IEC,Geneva,Publ.
achieved, and the noise-equivalentpressure, calculated via a IEC61102, 1991.
plane wave assumption at the
515 kHz frequency used, was [l31 S.P. Robinson, “A comparison ofthe frequency response ofmembrane
and needle probe PVDF hydrophones,”Physics in Medical Ultrasound 11.
19 kPa. Advantages include the abilityto make noninvasive
York, U K InStiNte ofPhysical Sciences in Medicine, 1988, D. Evans and
measurementsinopaquemedia,unliketheacousto-optic
K.Marlin, Eds.,pp.79-86.
methods in Sec. KC. However, the B-field gradient used
[l41 B. Fay, G. Ludwig, C. Lankjaer,and P.A. Lewin,”Frequencyresponse
ofPVDF needle-type hydrophones.”Ultrasound in Med. & B i d , YOI 20, pp.
wasabouttentimeshigherthanthatrecommendedfor
361-366, 1994.
human imaging.
[IS] G.R. Harris and P.M. Gammell, “Sensitivity meaSwements of
piewelectricpolymerhydrophonesfrom0.2-2MHrusingabraadbandpulse
VII. CONCLUSION
technique,”J. Acaust. Soc.Am., vol. 105,pp. 725-731, 1999.
[16]P.A.Lewin,R.Bautista,andV.Devaraju,“Voltagesensitivityrespanre
This paper contains a reviewof the progress and problems of ultrasonic hydrophones in the frequency range 0.25-2.5 MHz,”
U/t~~aundlnMed.&Bio/.,vol.25,pp.II3l-1137,1999.
associated with ultrasound exposure measurements. A wide [l71
P.M. Gammell and G.R. Harris, ”Time delayspectramctry for
array of topics was discussed, but coverage was by no means
hydrophone calibrations below I MHz,” J Aeousl. Soc. Am. (ARLO - in
press) 1999.
complete. For example, the measurement oftransient stress
waves induced purposefullyor not by medical laser radiation [l81 A.R.Selfridge andJ.P. Goetz, “Ellipsoidal hydrophonewith improved
characteristics.” Pmc. 1999 IEEE Ultroson. Symp.
was
omitted.
Also,
non-invasive
measurement
of
[l91 Reyuiremenlsfor the declorotion o/rhe acoustic oulput of medico1
temperature rise in tissue via
MFU or ultrasound was not
diagnostic ultromnic equipment, IEC, Geneva, Publ. IEC 61 157, 1992.
discussed; nor were means to monitor cavitation activity.
[201Aeousticoutputlobelingrtond~rd/~rdiagnosti~icltrosoundequipment:
These latter two are not exposure measurements per se, hut Aslnndard/~rhowmnnuf~cturersshouldspec~~ocowricoutpurdoia,Am.
Inst. ofUltmound in Med., Laurel, MD, 1998.
they shouldbe mentioned when discussing means toassess
[2I]T.ChristopherandE.L.Cantensen,“Fi~ileamplitudedistorlionandits
the effectiveness and risk potentialof medical procedures.
relationship to linear derating formulae for diagnostic ultrasound devices.”
Aproperunderstanding
of thecapabilities as well as
Ultrasound in Med. & Bioi., vol. 22, pp. 1103-1 116, 1996.
limitations of all these devices and methodsis essential for 1221 WorldFederationforUltrasoundinMedicineandBiology:Conclu~ionr
recommendations on lhermal and non-thermal mechanisms for
accurate ultrasound field characterization, and it is hoped and
that
biological eiTkcts of ulbasound. S.B. Bamett, Ed., Ullrasound in Ued. &
the material presented here will contributeto that goal.
B i d , vol. 24, suppl. I,p. S47, 1998.
[23] E.L. Cantensen, D. Dalccki, S.M. Gracewski,and T. Christopher,
“Nonlinearpropagationandtheoutputindicn,”J.UltrosoundMed.,vol.
18,
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