Health effects of medical ultrasound

Ultrasound in Fled.& Biol. Vol. 11, No. 3, pp. 551-558. 0301-5629/85 $3.00 + .OO Printed in the U.S.A. Copyright (c) 1985 Pergamon Press Ltd. All rights reserved

REPORTS HEALTH EFFECTS OF MEDICAL ULTRASOUND 14th March 1985 London U.K. by the College of Radiographers and held

This meeting was organised

at the London Hospital, the intention being to reflect British views on the recent media publicity from

these

reports

the

given

the

the USA.

reports

Arising

College had received queries from members

regarding safety and what they prompted

to

should

say

to

patients.

This had

Chairman of the Professional and Technical Committee

to contacted similar

world wide to seek their views.

organisations

The replies received stressed the

need

for training and competence

of operators so that examinations carried out with positive benefits to patients.

were done effectively

There were no

reports

of

any

organisation advising the cessation of scanning.

The

morning

session

was

opened by a paper by Dr Todd, Nottingham

entitled "The effects and dosimetry observed

that

harmful,

its

it

was

heating

well and

of

Medical

Ultrasound".

He

known that ultrasound was potentially microstreaming

effects

were

used

in

physiotherapy for example, and workers had experimented with its use in tissue destruction.

He was quick

to point out though, that the

levels of power used and the pulsing regimes employed were different from those used diagnostically.

In 30 years of experimentation into

bio-effects

the

we

cannot

relate

severity of

effects

in

such

experimental situations to the level of

ultrasound

the

phenomenological. Dosimetry

reports

in

is complicated by

the many

literature

are

used.

Most

of

factors including scatter of the ultrasound

field, production of heat in

the

transducer which transfers to the

551

Reports

552

sample,

radiation pressure

due to

gas

entrapped

creating sheer stresses, and cavitation

in

the fluid medium.

alter the acoustic loading on a

transducer

Gas entrapment can

such that the delivered

power can be quite different to that measured In

addition,

bubbles

gas

beam width and acting as

using degassed fluid.

act as secondary sources increasing the

scattering centres.

Bubbles can lead

to

cavitation and whilst it was thought that cavitation could not occur below

a threshold of 1 watt

per

cm

squared

Cartensen and Flynn contradicts this. that are linearly related

not

threshold.

references from other.

He

to

presented

literature,

the

recent

ultrasound a

each

series

by

of

which

of the field parameters and he said there was

a

level and do of

bioeffects

contradicted the

In his opinion, pressure amplitude was the

ultrasound

paper

Streaming and heat production

are effects have a

a

most important

need to develop an

dose equivalent similar to that developed in X radiation

dosimetry.

Mr Blackwell, London, gave "The

the

next

paper

and

his

subject was:

recent ultrasound scare and interpretations: fact and fancy in

the study general

of

hazard".

public

was

He introduced his topic by saying that the

presented

television or radio producers this

with perception

had to be delivered within

minutes.

Of

course,

but the public is left unfavourable.

within with

what

a

was

essentially

the

of a complex subject, and

short

time

frame

of

say

fade

a week or so, detailed memories an

30

ambience about the subject that is

He hesitates to say that ultrasound is

safe because

the intensities used in some scanners, duplex ones for instance, are quite

high.

important

He

in

thought

that

the

potential

reducing

benefits

of ultrasound were

to

which the pregnant

hazards

patient would be exposed if the clinical information obtainable from ultrasound was not available. The best advice he could give was

to be on the lookout for

committees Questions

studies

who that

statements

from

to

USerS

the various watchdog

consider carefully the relevance of published work. the

include:

user is

should

bear

in mind when considering any

the experimental design adequate;

results been reproduced:

is

there

the results: is the result relevant.

an

have

the

alternative explanation for

553

Reports

design

He also said that an adequate trial

and

subsequent

report

requires:

Results that are scored blind. Numerical results are given in the paper. The trial is randomised. Adequate numbers in the trials with appropriate controls. A full description of the exposure conditions.

Some

work which has been extensively reported does not give

recent

It is important to realise that when looking for

numerical results. small changes in studies

order

in

a

in

follow up

population that

is necessary to carry out large

it

effects

any

Examples

significance. underway

a

are

detected

large

the

Canada where one study

have

statistical

epidemiological 10,000

embraces

studies

children

basis and 1 million on a questionaire basis.

on

He said

that a selected truth taken as the whole truth is an untruth! He also observed that

with some

1500

a statistical probability that some

papers now published there was 36

results purely by chance. We must ask

papers whether

could find positive the conditions of an

experiment are relevant to the conditions existing in vivo.

This is

important because in test tube experiments there are various factors that

can

alter

the

experimental situation, for example: focusing

effects due to the test tube

walls

producing

tube

walls;

heat:

mode

conversion at the test

higher than diagnostic levels giving

rise to non linear propogation

effects:

exposure times much longer

than those used diagnostically and additionally concentrated sample

is

being

continually sonicated;

occur

scanning patients.

could

not

say

that

It

ultrasound

was was

his

as the

situations that would not conclusion

safe, because

that while we this

was

an

impossible statement to prove, there was no justification for saying it was harmful on the basis of the work reported in the media.

Reports

554

The last paper London,

spoke

who

ultrasound.

of the morning session was given by Dr Hilton Meire, on

the

clinical

like

He said that

many

used

techniques

people use ultrasound because they believe it useful: assessed

its

objectively

value

and

for

evidence

against

clinically, they have not

against other techniques but have We can however ask

relied on value judgements as to its usefulness. certain questions about it such as:1.

Does it reduce the incidence of other tests.

2.

Does it enable a diagnosis to be made more quickly and does it influence patient management.

The literature suggests that the answers to both questions is yes. He described the influence of

ultrasound on obstetrical practice at

Northwick Park Hospital, where its introduction of

the pregnant patient from 26% to

routine obstetrical

6%.

reduced radiography

When,

in

addition,

a

scanning service was offered to the patients of

one of a group of three consultants, the rate of X-rays taken in the ultrasound group dropped

to

l-l%, whereas the rate

groups stayed at a S-6% level.

in the control

However he could not prove that the

group receiving ultrasound benefited over those who

did

not.

He

went on to discuss the role of ultrasound in the management of first trimester bleeding.

In his experience 50% of such cases were found

by ultrasound to have a viable pregnancy and would thus benefit from bed

rest

etc.

pregnancies

The

others

could be sent home.

patient care

in

such

a

with

non-viable

situation.

He contrasted the perinatal UK national statistics;

the last 10 years perinatal mortality generally

50%.

At

Northwick

Park,

the

the

has

fallen

by

rate before the ultrasound service

commenced was 22 per thousand patients, since

non-existent

Ultrasound was clearly influencing

records of Northwick Park Hospital with the in

or

it

is

now

6%.

However,

service has been provided by technical staff the results

have not been

so

favourable

and

he

wondered

if this was due to

different equipment being used and the level of operator skills.

555

Reports

At the end of the morning session

there was a general discussion on

all the papers and some of the points rely

greatly

on

operator

made

expertise

were:

in

reporting routine scans,

unlike other imaging modalities which lent from film or hard copy; is

impossible

to

in ultrsound we

themselves

to reporting

while in North America or Western Europe it

carry out a control trial

on

the

benefits

of

Such a study might

ultrasound scanning of the pregnant population.

still be possible in East Germany where ultrasound is only now being It

introduced into clinical practice.

was asked if there was any

evidence to support the contention that ultrasound slowed down fetal growth; the panel thought not because in the case of benign maternal hypertension bed rest induces catch up fetal growth.

opened

by

Dr

obstetrician who set out to explain,

in

his

The

session

afternoon

was

P

London,

Smith, paper

entitled

an "The

values of ultrasonic scanning in obstretrics", the positive benefits to patient management that ultrasound had brought. ultrasound

was

not

He said that if

availabe some of the problems the obstetrician

might encounter included placenta

praevia, which normally does not

cause bleeding until the onset

labour,

emergency

Caesarean

section

of for

fetal

the

possibility

of an

distress which involved a

gross fetal abnormality, undiagnosed twins in labour and respiratory distress syndrome due to pre-term Caesarean section.

Ultrasound is

most useful in establishing the correct time of delivery - menstrual dates are most unreliable in this judgement

is not good in detecting cases

retardation and

a

50%

Additionally, clinical

context.

success

rate

of

intra-uterine

growth

might be expected.

BY

comparison, ultrasound is 83% to 98% effective, though there is some indication

that measurement of the synthesis to

fundal

height

is

more accurate (63% to 70%) than fundal height alone.

He

discussed

patient

and

the

advantages

identified

as

of

an

benefits:

early scan of accurate

pregnancy which can be subsequently important in

the dating

evaluation

pregnant of

the

of the

556

Reports

maternal variety

alpha of

fetal

fetal

protein,

abnormalities

point, increased bonding is

more

accurate

retardation also

reduced

from

a

perinatal

between mother and fetus.

than

clinical

in

methods

The

view

late scan

detecting

growth

the risks associated with amniocentesis as it enables localise

perform the test at the

the placenta, isolate the liquor pool and correct

figures in support of this. complications which

as a case

and,

and placental localisation can be performed. Ultrasound

the operator to

that

multiple pregnancy, detection of a

gestational

age.

He quoted some

They have found at the London Hospital

associated with amniocentesis, which he defined

requires

two

or

more needle insertions, occured

18.7% when ultrasound was not used as opposed to 5.6% when done with ultrasound guidance.

The final

paper

of

the

afternoon was given by Dr Kinnear Wilson,

who spoke about obstetrical ultrasound and

childhood

commented that cancer is a rare disease in

children, therefore, the

incidence

is

not very high.

Three Region Study

She talked about two

carried

are assessed from when they

cancer.

studies,

She

the

out over three years in which children were

diagnosed

as

cancer

having

in

hospital and compared with a control group made up of other children in

hospital

population.

and

second control group obtained from the outside

a

The three regions participating

Birmingham, Leeds

and

Manchester.

embraces the whole country.

It

in

this

study

are

The second, the Oxford Survey

commenced in 1972 and is performed

on a questionnaire basis which includes a question about ultrasound. The

aim

children

of and

the

was

interview the parents of any dead

to

the parents of associated control children

been matched for regarding

study

age,

smoking,

sex,

drinking,

The

locality. etc

is

parental

obtained

and

who

background the mother's

history during pregnancy, exposure to X-rays, ultrasound etc. data

is

correlated with requests to hospitals for

X-rays and ultrasound. that

mother's

recall

hospital notes is not

It

is

data

This

regarding

retrospective and the draw back is

is often not always

have

good,

forthcoming.

and The

confirmation survey

is

from only

557

Reports

concerned

with

mortality and was finished in the year 1981.

matched pairs were

surveyed.

In the period 1970 to 1981 exposure

to ultrasound increased from 2.3%

to

died compared to an increase of .5% was

little

difference concluded

difference

between that

between

number

1605

of

25% in those patients who had to 75% in the controls.

cases scans

and

controls

between

each

and group.

There little She

there was no discernable difference between control

groups and childhood from the age zero

deaths, both having been exposed to ultrasound

to

six years.

doubt but the figures are very

After this period there could be

small:

from the mothers'

recall 20

children died of leukaemia and 15 of solid tumours against 16 in the control

group.

Enquiries to the patients' hospitals

could

only

confirm 9 of the leukaemia cases and 5 of the controls. Additionally the case mothers were older and there was complications of pregnancy and it seems the tip of the

probable that ultrasound is not to blame. iceberg

but

mortality in older children.

ANGUS J HALL

we

would

expect

to

see

It might be increased