By Dr Lilius Chua, Radiologist, Rotheram GH, UK
 
All pregnant ladies should routinely have two ultrasound scans. One at the time of booking with the obstetrician usually in the 1st trimester (ie. up to 12weeks after the last menstrual period). This scan will confirm the dates and determine whether it is one baby or twins, whether the pregnancy is on-going and intrauterine.

The second routine scan is done at 18-20 weeks. Most babies are completely normal, but a scan is also used to look for any abnormalities. Although scans will not detect all abnormalities, many can be detected.

Sometimes an ultrasound scan is done earlier in pregnancy when there is bleeding. Scans are sometimes done later in pregnancy to check on the growth of the baby or as a follow up to a previous scan.

Is ultrasound dangerous? The strength of the ultrasound waves used are found not to be harmful to the baby. However, as with all modalities of imaging, use is only recommended when necessary and by trained personnel.
 

It was reported in the October 2002 issue of Seminars in Ultrasound, CT and MR that caution should be applied to routine ultrasound scanning in the first trimester in an uncomplicated pregnancy. This is because of the sensitivity of the embryo to physical damage together with uncertainties of both risk and benefit in ultrasound in healthy pregnancies.

Current facts show that it is unlikely that diagnostic ultrasound poses a significant thermal risk to the developing embryo when used according to published safety guidelines. Read more about safety guidelines in the section below.

However, the likelihood of producing some biological effects may be enhanced with the use of gas encapsulated echo-contrast agents.

Ref: Semin Ultrasound CT MR 2002 Oct; 23(5):387-91
 

Please go to Ultrasonography in the Radiology Inside the Human Body section of this website for a simple explanation of this imaging modality.

www.radiologymalaysia.org/Archive/RITHB/ultrasonography.html

Abridged from a lecture by Dr Ng Kwan Hoong, PhD, DABMP, Department of Radiology, UMMC, KL 

Ultrasound Biophysics and Bioeffects
When ultrasound propagates through human tissue, there are potential biological effects or bioeffects. There is very extensive research aimed at understanding basic mechanisms and evaluation of potential for tissue injury.  

Many studies are studies based on the effect in relation to the dose of ultrasound (intensity) and virtually all ultrasound-induced adverse bioeffects have occurred at higher intensities than diagnostic ultrasound. 

Diagnostic ultrasound refers to the ultrasound examinations that most people have in a radiology department for imaging their kidneys, liver, gallbladder, breasts and is also extensively used in infants and children. 

Setting up of Guidelines
As a result of potential “dangers” of ultrasound, International Guidelines and Regulations for the safe use of diagnostic ultrasound in Medicine have been drawn up. 

What are some of these effects of Ultrasound?

Thermal Effects
As a sound beam passes through tissue, some of the energy of this sound wave is absorbed by the tissue. However for low intensities of ultrasound, the heat deposited is quickly dissipated and does not build up.  

There is some concern with pulsed Doppler and color flow imaging equipment where high power levels and longer imaging times may increase the amount of heat deposited. 

The ultrasound therapy that is used in Physiotherapy is different from ultrasound used to image your organs (diagnostic US). The US therapy in physiotherapy for “tight” muscles, muscle ache, sprains and strains are of much higher intensity. If you have experienced this type of ultrasound, you would notice that the period of therapy is always short, about 10 -15 minutes and that the area which has been treated is “warm”. This does not happen in normal diagnostic US. 

Non-thermal (mechanical) Effects
Cavitation is due to the generation, growth, vibration and possible collapse of microbubbles in the tissue. These microbubbles are generated by the ultrasound waves. These bubbles may “move” with the sound beam or some may oscillate so strongly that the bubbles collapse suddenly producing local effects.

What is the bottomline? Is US safe for us and our unborn babies?

Evidence has been obtained by epidemiological studies, cell studies as well as animal studies. 

There have been no adverse effects, including no evidence of low birth weights from diagnostic ultrasound thus far in the history of ultrasound.  

The American Institute of Ultrasound in Medicine (AIUM) evaluated epidemiological studies and concluded that widespread clinical use over 25 years has not established any adverse effect arising from exposure to diagnostic ultrasound. “No confirmed biological effects on patients or instrument operators caused by exposure at intensities typical of present diagnostic ultrasound instruments have ever been reported. Although the possibility exists that such biological effects may be identified in the future, current data indicate that the benefits to patients of the prudent use of diagnostic ultrasound outweigh the risks, if any that may be present.” 

However, epidemiology data has its limitations. There is no data for modern powerful diagnostic equipment especially those with pulsed Doppler and harmonic imaging with use of contrast agents.

How do we balance Benefits and Risks?

Prudent use can be achieved by applying the simple concept of  ALARA, i.e. As Low As Reasonably Achievable 

Following ALARA principles, we try to keep total ultrasound exposure as low as reasonably achievable, while optimizing diagnostic information.  

Advice to operators (those performing the ultrasound examination)
As the threshold of bioeffect intensity is not known (i.e. we do not know exactly at what ultrasound intensity, dangerous effects start to occur), it is the responsibility of the operators to use his/her judgment and insight to adjust the intensity output of the equipment so as to get the most information at the lowest output power. 

AIUM 99 Statement on Non-medical use’
“The AIUM strongly discourages the non-medical use of ultrasound for psychosocial or entertainment purposes. The use of ultrasound (2D or 3D) to only view the fetus, obtain a picture of the fetus or determine the fetal gender without a medical indication is inappropriate and contrary to responsible medical practice….” 

Further Reading

For those interested in reading more into the safety of ultrasound, excellent review articles and books are available. References are given below: 

1.  AIUM,  Bioeffects Considerations for the Safety of Diagnostic Ultrasound, Journal of Ultrasound in Medicine 7/9 Supplement, 1988.

2. AIUM, Bioeffects and Safety of Diagnostic Ultrasound. 1992.

3. AIUM,  Bioeffects Committee: Bioeffects and Safety of Diagnostic Ultrasound, 1993.

4. AIUM, Mechanical Bioeffects from Diagnostic Ultrasound: AIUM Consensus Statements, Journal of Ultrasound in Medicine 19/2, 2000.

5.  BARNETT SB (ed), WFUMB Symposium on Safety of Ultrasound in Medicine. Conclusions and Recommendations on Thermal and Mechanical Mechanisms for Biological Effects of Ultrasound. Ultrasound Med Biol, 24, Supplement 1, 1998.

6. BARNETT SB. Biophysical Aspects of Diagnostic Ultrasound. Ultrasound Med Biol. 26, Supplement 1, S68-S70, 2000.

7. BARNETT SB AND KOSSOFF G (Eds), Safety of Diagnostic Ultrasound, Progress in Obstetric and Gynecological Sonography Series,  1998.

8. TER HAAR G AND DUCK FA, The Safe Use of Ultrasound in Medical Diagnosis, British Medical Ultrasound Society, British Institute of Radiology, 2000. 

Those interested in reviewing the whole lecture given by Dr Ng on the Guidelines for Safe Ultrasound Use.pdf (1.9Mb Adobe Acrobat PDF format) please click here!

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Last Updated:
Thursday, 21 August, 2003