Ultrasound Potential PDF Ppt

RSNA 2008

Ultrasound Bioeffects

Dolores H. Pretorius, MDThomas R. Nelson, Ph.D.

Professorsof Radiology

University of California. Son Diego

Ang ESBC. Gluncic V. Duque A. Schafer ME. Rakic P,Prenatal exposure to ultrasound waves impactsneuronal migration in mice, Proceedings of the NationalAcad of Sci, August 2006; 103: 12903-12910. Links

335 mice

Ultrasound for 30 - 420 min

A small number of neurons failed to acquire theirproper position

They remain scattered within inappropriate corticallayers

Consequences such as epi lepsy, schizophrenia andautism

~gcts in Ani as

Lung hemorrhage in young mice andneonatal/adult pigsIntestinal hemorrhage in adult miceBleeding near developing bone in youngmice

AlUMConsensus Report on Potential Bioeffects of DiagnosticUltrasound. JUM 2008: 27:503-515

Ultrasound Bioeffects

_~~W.arnina

Concerns have been raised in the past relatedto. Autism.Abnormal hearing, vision or language

development. Intrauterine growth retardation. Childhood cancer. Increase in non-right handedness

Abramowicz J, UOG 29:363, 2007

Approximately 250 million fetal ultrasoundexaminations performed per year in US.To date no adverse effects on fetusreportedFetal ultrasound considered to be safeUltrasound exposure can producebioeffects at diagnostic levels inlaboratory studiesValid experimental studies difficult toperformCaution advises following ALARA principle

Ultrasound Bioeffects: Reasons for Concern

Ultrasound is mechanical energy:. thermal effects. mechanical effects

Energy level depends on machinesetting

. MI - mechanical index

. TI - thermal indexBioeffects studies based on oldlimits - 94 mW/cm2New equipment limits much higher -720 mWlcm2

Use As Low As ReasonablyAchievable technique

Ultrasound Bioeffects: Background1 ,

TI is an on-screen measure of the potential fortissue heatingMI is an on-screen measure of the potential toinduce cavitation in tissues

3D ultrasound does not introduce additionalsafety considerations4D ultrasound with continuous exposure offerspotential to prolong examination times

ClinICal S<>f~fy Slote_ntfO<' Di<Ig"".tk

Ulfr<tlound, ECMUS (Europull Co",,,,ittu 01 M.d;cool Ull'NsoYnd$ofety) (2OQt.)

Ultrasound Bioeffects: Some Ph sics

Medical ultrasound basic properties:. frequency 1 - 15 MHz. pulse length 3-5 cycles. speed of sound in tissue ~ 1540 mls. attenuation ~ 1dBI (em-MHz). wavelength & resolution ~ 0.5 mm. image rates up to 150 fps (30 typical)

2.5 MHz 3.5 MHz 4.0 MHz 5.0 MHz 6.0 MHz

Ultrasound Bioeffects: Background

Ultrasound has diagnostic value

Wider range of US studies and new technologies havinghigher acoustic output levels in more patients

Current acoustic output much greater than earlierequipment

No evidence that diagnostic ultrasound produces harm

Subtle or transient effects not well understood

Diagnostic ultrasound should be used prudently

Ultrasound examinations should only be performed bytrained. competent personnel

Essential to maintain vigilance to ensure continued safety

Ultrasound Bioeffects: Some Physics

Characteristics of SoundSound is mechanical energyPropagates longitudinally through elastic media. Alternating zones of compression and

rarefactionUltrasound imaging typically uses short pulsesEnergy is reflected at interfaces


Doppler.Pulses reflecting off moving interfaces, (e.g. blood cells, heart valves

or contrast) exhibit a phase shift that can be used to measure thevelocity of motion along the path of the sound beam.

.Typical Doppler shift range is 10 to 1000 Hz


Velocity Doppler. computes the velocity in each pixel and displays acolor whose hue depends on the direction and whosesaturation depends on the velocity component measured


Ultrasound Contrast Materials. Improved visualization of vessels and tissues is possible

using gas-filled micro-bubbles to produce a largeimpedance discontinuity. Contrast agents offer

promise in enhancingmasses, visualizing bloodflow, measuring perfusion,and delivering drugs andgenetic agents to specificsites

Ultr~sound Bioeffects: Some Ph sics

Focused Excitation

..1I3"~! Weak Focus.---.-.-.-.-.-.-.-.-.-.-.-.-.---.-.-.-.-.-


Power Doppler. computes the integral of the entire velocity distribution

and displays the magnitude as a color and brightness value. has less angular dependence and better sensitivity toslow flow


Beam PropertiesUltrasound propagates as longitudinal wave

Two primary zones

Near field (Fresnel) - Complex field pattern

Far field (Fraunhofer) - More coherent field pattern

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Diar.wI"",d ':;'.. d'/4;.

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Image formation. Scan lines sweeping through different directions are used to producea two-dimensionalimage


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ISAPA = IsATA/duty cycle. IpA = ITA/duty cycle

ISPTA = ISATA (Isp/IsA). Indicator of thermal effectsISPPA=ISATA(Isp/IsA)/duty cycle

Indicator of potential mechanical bioeffects andcavitation

ISPPA > ISPTA > ISAPA > ISATA

SA = Spatial AverageSP = Spatial Peak

PA = Pulse AverageTA = Temporal Average

TP = Temporal Peak


Attenuation

1.0

.~

~ 0.5.o"'"

The lowerthe frequency,

the morethe penetration

4.5

Penelr'otion depth, em

(Dist:~nce traveled =2 pellstlation depth)



Acoustic Power

Rate of energy production, absorption or flow. SI unit - Watt (1 J oule/sec)

Acoustic Intensity.Rate of energy flow through a unit area

(Watts/cm2)

Power OutputOutput power

PRF = pulsed repetition frequencyFrequencyOperating mode


100

10

.5

~~.~

E 01

001

Attenuation



SPTA =Spacial peak temporal averageSPPA =Spacial peak pulse average


10 100 1000Expo$ure time, minutes

10000


FDA Output Limits. ISPTA< 100 mW/cm2. ISPTAcan exceed 1000 mW/cm2 for Doppler

IndicesThermal Index (TI). Ratio of acoustic power produced to power required

to raise tissue by 1° C. TIs - soft tissue, TIb - bone, TIc - cranium

Mechanical Index (MI). Estimate of likelihood of cavitation. Related to intensity

Ultrasound Bioeffects: Recommendations=p~

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Nelson TR, Fowlkes B, Abramowicz. Ultrasound Biosafety Considerations forthe Practici Son ra her/Sonol ist. JUM, 2008.

Ultrasound Bioeffects: Thermal Hazard

Thermal hazard exists with some equipment if usedimprudently

Temperature elevation < 1.5° C does not present harm toembryo

Temperature elevation> 1.5° C may cause harm

Temperature elevation> 4° C for 5 minutes potentiallyhazardous to embryo

Spectral pulsed Doppler modes can reach these levels

Color Doppler modes may reach these levels

TI provides rough guide to sonographer/sonologistregarding thermal hazard

Ultrasound Bioeffects: Non-thermal Hazard

Non-thermal hazard exists with some equipment if usedimprudently

Damage demonstrated in tissues with gas pockets at MI >0.3

Avoid unnecessary exposure to neonatal lung

Keep MI as low as possible

MI provides rough guide to sonographer/sonologistregarding non-thermal hazard

Contrast agents increase probability of cavitation

Single beam modes (A-mode, M-mode and spectral pulsedDoppler) more likely than scanned modes (B-mode, ColorDopp ler)

BMU5$<1fctyStGtc...n'(2000)

Ultrasound Bioeffects: Ultrasound Contrast Aqents

Ultrasound contrast agents typically are stable gas-filledmicrobubblesPotentially produce cavitation and micro-streamingMicrovascular damage or rupture is possible (animalstudies)Premature ventricular contractions induced in contrastenhanced echocardiography with high MI and triggeredexcitationRisk increases with increasing MI

Ultrasound Bioeffects: Literature Studies

pulsed wave Doppler TI was 1.5 ! 0.5: range 0.9-2.8

color flow Doppler TI was 0.8 ! OJ: range 0.6-1.2Significantly greater than

B-mode sonography TI was 0.3 ! OJ: range 0.1-0.7 (p<.01)

190 B-mode MI variations 1.1 ! 0.1

31 color flow Doppler variation I.O! 0.1 (p=.09)

190 pulsed wave Doppler MI variations 0.9!0.2 (p<.OOI).

MI = 1.0 and Tis = 0.2

Ultrasound Bioeffects: Preanancy Exposure

Embryo/fetus is particularly sensitive

Little information available regarding possible subtlebiological effects at diagnostic levelsTemperature rise greatest at bone surfaces andadjacent soft tissuesIncreasing mineralization raises possibility of heatingsensitive tissues such as brain and spinal cordCare should be taken to minimize eye exposureImportant to limit exposure time and the MI and TIcommensurate with acceptable diagnostic evaluationNo reason to withhold diagnostic scanning duringpregnancy provided it is medically indicated and is usedprudently by fully trained operators


Sheiner et.al. Increased TI can be achieved during Doppler OBSonography. J Ultrasound Med 2007: 26:71-76

Patients with suspected fetal growth problems undergoing Dopplerstudies of the fetal circulation in addition to B-mode sonography

Examinations took place between 21 and 40 weeks' gestation

HDI 5000 scanners (Philips Medical Systems, Bothell, WA)

transvaginal (4-8 MHz) probe. transabdominal (2-5 MHz) probe

63 examinations were evaluatedgestational age was 31.6 ! 5.1 weeks

total examination time was 17.6 ! 8.6 minutesDoppler examination time was O.9:tO.8 minutes


Increased acoustic output levels, as expressedby TI levels, are reached during obstetricDoppler studiesTI levels may reach 1.5 and higher

Doppler procedures should be performed withcautionDoppler procedures should be as brief aspossible during obstetric sonography


Color and pulsed Doppler should be avoidedin the first trimester of pregnancy due topotential for tissue heating

AILIM Cons sus Report on Potential Bioef; ts of DiagnosticLJltrasound. JLlM 2008: 27;503- 15

Ultrasound Bioeffects: Literature Studies-~

11first-trimester14 second-trimester12 third-trimester examinations were evaluatedFirst-trimester examination was 8.9 minutes. MI was 0.73; range 0.3-1.3. TI was 0.34; range 0.1-1.7

Second-trimester examination was 31.8 minutes. MI was 1.04; 0.5-1.5. TI was 0.28; 0.1-2.4

Third-trimester examination was 16.3 minutes. MI was 1.06; 0.2-1.5. TI was 0.32; 0.1-2.4


Output levels during routine obstetric ultrasoundexaminations, as expressed by the MI and TI,are generally lowHowever, higher output levels, particularly TIlevels of greater than 1.5, can be achievedTI greater than 1.5 typically account for only asmall proportion of the examination timePrudent use requires ALARA principles. Minimize study time consistent with diagnostic

goals


Sheiner et. al. Acoustic output as measured by MI and TIduring routine OB exams, J Ultrasound Med 2005;24:1665-1670

Quantify acoustic output of clinical ultrasoundinstruments during routine obstetric examinations. thermal index (TI). mechanical index (MI)

Prospective, observational studySonographers were unaware of the data being sought

HDI 5000 scanners (Philips Medical Systems, Bothell,WA). transvaginal (4-8 MHz) probe. transabdominal (2-5 MHz) probe


Statistical significance existed acrosstrimesters with regard to examination durationsand MI (P < .001)No statistical significance existed in the TIacross trimesters3.5~o of third trimester examinations had a TIof greater than 1.0. 2.4 ~owere between 1.0 and 1.49. 1.1~owere greater than 1.5

TI ~ 1 changes were brief (0.17 :t 0.08 minutes)TI changes were observed during color Dopplerimaging

er

Tricuspid valve interrogation for aneuploidy risk

Requires certification

Should be done rapidly, by experienced sonographers


Sheiner et.al., What Da Clinical Users KnowRegarding Safetyof Ultrasound During Pregnancy? J Ultrasound Med 2007;26:319-325

determine end users' knowledge regarding safety aspects ofdiagnostic ultrasound during pregnancy

A questionnaire was distributed to ultrasound end usersattending review courses and hospital grand rounds betweenApril and June 2006

130 end users completed the questionnaires (63~o responserate).63% were physicians81.7~o were obstetricians-18~o of participants routinely performed Doppler ultrasoundexaminations during the first trimester


-70/0 of users disapproved of"keepsake/entertainment" ultrasoundKnowledgeable users support limitations onthe number of ultrasound examinations inlow-risk pregnancies. Concern regarding possible adverse

effects during pregnancy

Sheiner et.ol., J Ultrasound Med 2007__26:319-325

Ultrasound Bioeffects: Guidelines for Safe Use

Doppler for fetal heart monitoring. The power levelsused for fetal heart monitoring are sufficiently low thatuse of this modality is not contra-indicated, on safetygrounds, even when it is to be used for extended periods.

Peripheral pulse monitoring. The output from CW Dopplerdevices intended for monitoring peripheral pulses issufficiently low that their use is not contra-indicated, onsafety grounds.

Transcranial ultrasound investigations Transcranialultrasound investigations may require higher acousticoutput than other applications. Safety Guidelines shouldbe applied for both imaging and stand-alone Dopplerequipment. TIC should be monitored.


32.2/0 of the participants were familiar with theterm TI. only 17.7/0 correctly described the nature of

TI~22/0 were familiar with the term MI. only 3.810 correctly described the nature of MI

8010 of end users did not know where to find theacoustic indices. Only 20.8/0 knew they are displayed on the

sonographic display

Sheiner et.ol., J Ultrasound Med 2007,' 26:319-325


Ultrasound end users are poorly informedregarding safety issues during pregnancy

Further efforts in the realm of education andtraining are needed to improve end userknowledge about the acoustic output of themachines and safety issues

That's why we are here today!

Thank 'Ou

Ultrasound Bioeffects: AIUM Guidelines

AIUM Practice Guideline for the Performance of an AntepartumObstetric Ultrasound Examination. (2003)http://www .aium.org/publicatio 051clinicall obstetri cal.pdf

Diagnostic ultrasound studies of the fetus are generally consideredto be safe during pregnancy.This diagnostic procedure should be performed only when:

there is a valid medical indicationthe lowest possible ultrasonic exposure setting should be used togain the necessary diagnostic information under the as low asreasonably achievable (ALARA) principle.

The promotion, selling, or leasing of ultrasound equipment for making'keepsake fetal videos' is considered by the US Food and DrugAdministration to be an unapproved use of a medicol device.Use of a diagnostic ultrasound system for these purposes, without aphysician's order, may be in violation of state laws or regulations.

Ultr(ls0':l.nd B.i.<>effects: Background

This Output Display Standard (ODS) consists of the thermalindex (TI) and the mechanical index (MI)

The TI expresses the potential for a rise in temperature atthe ultrasound's focal point.The MI indicates the potential for the ultrasound to induceinertial cavitation in tissues.There are 3 TIs:

the TI for soft tissues (TIS), when the ultrasound beamdoes not impinge on bone, appropriate mostly for the firsttrimesterthe TI for bones (TIB), when the beam impinges on bone ator near its focus, which should be displayed in the secondand third trimestersTI for cranial bone (TIC), when the transducer is veryclose to the bone, such as when scanning in the adult.


GUIDELINESFOR THE SAFE USE OF DIAGNOSTIC ULTRASOUNDEQUIPMENT,Safety Group of the British MedicalUltrasoundSociety, http://www .bmus.orglultras-safe ty Ius -safety04 .asp

Medical endorsement. Ultrasound should only be used for medicaldiagnosis if endorsed by a medical practitioner

Operator training. Diagnostic ultrasound procedures should becarried out only by persons who are fully trai ned in the use of theequipment, the interpretation of its results and images and in thesafe use of ultrasound, including an appreciation of its potentialhazards.

Awareness of machine factors influencing hazard. Operators shouldunderstand the likely influence of the machine controls, theoperating mode (e.g. B-mode, color Doppler imaging or spectralDoppler) and probe frequency on the thermal and cavitation hazards.

Ultrasound Bioeffects: Backqround

National Council on Radiation Protection and Measurements(NCRP). . NCRP report 140, Exposure Criteria for MedicalDiagnostic Ultrasound, II: Criteria Based on All KnownMechanisms. Bethesda, MD: NCRP; (2002)

Before 1976, there were no limits to the permissible acousticoutput from diagnostic ultrasound equipment.

In 1976, the US Food and Drug Administration beganregulating the output levels of machines to be no more than 94mW/cm2 spatial-peak temporal-average intensity for fetaluse.

In 1992, under pressure from manufacturers and end users,the Food and Drug Administration changed this limit to 720mW/cm2 but mandated that machines capable of producinghigher outputs display to the diagnostician some indication ofthe likelihood of ultrasound-induced bioeffects (OutputDisplay Standard)

Ult~(lsoun~.<>eff~cts: Background

The TI and MI are not perfect indicators of the actualthermal and non-thermal risks

At present, they should be accepted as the most sensiblemethods of risk estimation

The implementation of the ODS puts much greaterresponsibility for patient safety on the ultrasound enduser

The ALARA (as low as reasonably achievable) principlewas also recommended.

Nevertheless, for the acoustic indices to be meaningful,the diagnostician must be familiar with safety issues andimplications

In fact, one of the major recommendations of the ODSdocument was education of end users.


Initial power setting. Machines should be set up so that the default(switch-on) setting of the acoustic output power control is low. If alow default setting cannot be achieved, a low setting should beselected after switching on. A low setting should be selected foreach new patient. The output should only be increased during theinvestigation if this is necessary to produce Qsatisfactory result.

Exposure time. The overall examination times should be kept asshort as is necessary to produce a useful diagnostic result.

Stationary probe. The probe should not be held in a fixed positionfor any longer than is necessary, and should be removed from thepatient whenever there is no need for a real-time image or spectralDoppler acquisition.

For example, using the freeze frame or cine loop facilities allowsimages to be reviewed and discussed without continuing theexposure.


Probe self-heating. Endo-probes (e.g. vaginal, rectal or oesophagealprobe) should not be used if there is noticeable self heati ng of theprobe when operating in air.

applies to any probeparticular care should be taken if trans-vaginal probes are to beused to investigate a pregnancy during the first eight weekspost-conception.

Pre-existing temperature elevation. Particular care should be takento reduce output and minimise exposure time of an embryo or fetuswhen the temperature of the mother is already elevated.

Sensitive tissues. Particular care should be taken to reduce the riskof thermal hazard when exposing the following to diagnosticultrasound:

an embryo less than eight weeks after conception:the head. brain or spine of any fetus or neonate:an eye (in a subject of any age).


Thermal and Mechanical Indices.MI> 0.3 There is a possibility of minor damage to neonatal lung orintestine. If such exposure is necessary, try to reduce the exposuretime as much as passible.MI> 0.7 There is a risk of cavitation if an ultrasound contrast agentcontaining gas micro-spheres is being used.

There is a theoretical risk of cavitation without the presence ofultrasound contrast agents.The risk increases with MI values above this threshold.

TI> 0.7 The overall exposure time (including pauses) of an embryo orfetus should be restricted in accordance with Table 1.

TI> 1.0 Eye scanning is not recommended, other than as part of afetal scan.

TI 3.0 Scanning of an embryo or fetus is not recommended, howeverbriefly


Non-diagnostic uses of diagnostic ultrasound equipment. Examplesinclude repeated scans for training. equipment demonstration usingnormal subjects. and production of fetal souvenir pictures or videos

for equipment that display safety indicesthe TI should always be less than 0.5. the MI should always be less than 0.3

for equipment that do not display safety indicesTm~ should be less than I' CMIm~ should be less than 0.3.frequent exposure of the same subject is to be avoidedfollow safe scanning guidelines and ALARA

first trimester scans should not be carried out for:the sole purpose of producing souvenir videos or photographstheir prod.uction should not increase exposure levels or extendingthe scan times beyond those needed for clinical purposes.

Ultrasound Bioeffects: Guidelines for Safe Use1-------

Pulsed Doppler. The use of spectral pulsed Doppler, or color Dopplermode with a narrowwrite-zoom box selected, is not recommendedfor the investigation of any sensitive tissues, unless an e.stimate ofthe maximum likely temperature elevation has been obtained andconsidered in relation to the anticipated exposure time.

Thermal and Mechanical Indices. for machines which display on-screen thermal index (TI) and mechanical index (MI) values,operators should continually monitor their values and use controlsettings that keep them as small as is consistent with achievingdiagnostically useful results.In obstetric investigations, TIS (soft tissue thermal index) shouldbe monitored during scans carried out in the first eight weeks afterconception, and TIS (bone thermal index) thereafter.In applications where the probe is very close to bone (e.g. trans-cranial applications), TIC (cranial thermal index) should bemonitored.for eye scanning TIS should be monitored. In other applications,TIS should be monitored.


Thermal and Mechanical IndicesWhere an on-screen thermal index (TI) or mechanical index (MI) isnot displayed, try to obtain worst case estimates (considering allpossible combinations of control settings) of temperature elevation('Tm~) and mechanical index (MIm~) for the particular probe andmode in use.If these can be obtained, assume that the MI value is equal to MIm"and the TI value is equal to 0.5 "Tm~ and refer to Table 1.

Table 1Maximum recommended exposure times for an embryo or fetus

TI Maximum exposure time (minutes)0.7 601.0 301.5 152.0 42.5 1

Ultrasound Bioeffects: Mechanical Index---The mechanical index (MI) is intended to offer a roughguide to the likelihood of the occurrence of cavitation.

The MI is constantly updated by the machine, accordingto the control settings, using the formula

MI= maximum value of peak negative pressure / pulsecentre frequency,

NB: the maximum value of peak negative pressureanywhere in the ultrasound field measured in water isreduced by an attenuation factor equal to that whichwould be produced by a medium having an attenuationcoefficient of 0.3 dB cm-l MHz-!.

Gestation Gestatian fram Title af Majar relevant

Fram LMP Canceptlan / Canceptus eventsFertilisation

0.14 days NIL--

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multiplicatio.ns

29:70 days 15-56 days Embryo. ()rganagenesis

4.1-10 weeks 2.1-8 weeks Emb a

10-11 weeks 8-9 weeks Fetus Ossificatlan of

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13-14 weeks 11-12 weeks Fetus Ossification of

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bone starts

Ultrasound Bioeffects: Thermal Index

The thermal index (TI) is intended to give a rough guideto the likely maximum temperature rise that might beproduced after long exposure.

Three forms of TI may be displayed, according to theapplication.

TIS assumes that only soft tissue is insonated.TIB assumes bone is present at the depth wheretemporal intensity is greatest.TIC assumes bone is very close to the front face ofthe probe.

However, note that errors in calculating TI values, andthe limitations of the simple models on which they arebased, means that TI values can underestimate thetemperature elevation by a factor of up to two.

Ultrasound Bioeffects: Sensitive Tissues

Ultrasound Bioeffects: Conclusions

Study results at our perinatal, academic medical centersuggest that 3DUS targeted studies are shorter than general2DUS studies, especially far complex cases.

Extended scanning to watch fetal movement has the potentialto increase fetal US exposure without benefiting diagnosis ormanagement and thus caution is needed for these procedures.

-----

Ultrasound Bioeffects: Sensitive Tissues

Up to eight weeks after conception, organogenesis istaking place in the embryo.

This is a period when cell damage might lead to fetalanomalies or subtle developmental changes.

The brain and spinal chord continue to develop through tothe neonatal period.

The presence of bone within the beam greatly increasesthe likely temperature rise

due to direct absorption in the bone itself

due to conduction of heat from bone to adjacenttissues.

Ultrasound Bioeffects: Conclusions1.....---....

Technological advances enhance fetal scanningopportunities by providing improved visualization andimage quality that potentially improves fetal visualization,diagnosis and management

Notably, 3DUS equipment provides clear images of thedeveloping fetus that are recognizable to family andphysician.

4DUS equipment further facilitates observing fetalmovements similar to real-time 2DUS imaging -potentially inviting longer fetal viewing.

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