Ultrasound Safety and Biological Effects

ULTRASOUND SAFETY

ULTRASOUND SAFETY

BASIC RULES AND PRINCIPLES

IS ULTRASOUND SAFE?

So far ultrasound has not been proven to be unsafe in humans. However, ultrasound is now being very widely used and advances in technology are leading to increasing power levels being used to obtain diagnostic information. This can lead to measurable biological effects The interaction of ultrasound with biological tissue can result in effects which may cause heating, interfere with normal functioning of cells, and cause structural damage.

It is important for people using ultrasound to do all they can to minimize the exposure of the patient to ultrasound to reduce these risks.

OPERATING MODES AND THEIR RISKS

B Mode ultrasound imaging records returning echoes strength as a brightness display on the screen. This is the normal grey scale imaging used for real time ultrasound.

This uses the lowest output power and intensities and is generally considered safe in all applications,

PULSED DOPPLER AND POWER DOPPLER

Pulsed Doppler uses higher intensity of ultrasound than B Mode, and is therefore more likely to cause biological effects, particularly from heating.

Pulsed Doppler ultrasound focuses the beam in a small area for longer periods increasing the dose to those tissues.

Power Doppler also uses intensities higher than B Mode, but less than pulsed Doppler

BIOLOGICAL EFFECTS

• There are two important effects which can be caused by ultrasound:
• 1/ Thermal (heating)
• 2/ Non thermal effects (cavitation)

1/ As the ultrasound beam travels through the tissue some of its energy is lost through absorption. This absorbed energy is converted into heat which causes a temperature rise of the tissues.

• The heat rise will depend upon:
• * the attenuation coefficient of the tissue
• * the operating frequency of the transducer
• * power/ intensity of the ultrasound beam

* the length of operating exposure time

BIOLOGICAL EFFECTS

2/ Non thermal effects ( cavitation ) The oscillating pressure wave caused by an ultrasound beam creates areas of compression and rarefaction. This can cause small bubbles to form in tissues. These can grow, oscillate in size, an at high intensity and pressure these can collapse.

This causes high pressures and temperatures in the localized area with the potential to cause biological damage. While this risk is very small in general ultrasound, it is wise to be careful in early pregnancy where the tissues are more vulnerable.

CAVITATION

SAFE USE

Changes in power output and increased use of Doppler ultrasound have led to awareness of the need to take every measure to ensure safe practice.

The accepted guideline for safe use is known as the ALARA principle ( “As Low As Reasonably Achievable” ) This is a guide to minimizing exposure times and exposure intensity.

ALARA

SOME GUIDELINES

1/ Examinations should use B mode real-time in the first instance and employ color and spectral Doppler only when and if required.

2/ The overall examination should be kept as short as possible to minimize exposure times.

• 3/ The transducer should not be held in a fixed position any longer than necessary.

4/ The transducer should never be left resting on the patients body when not in use.

5/ The output power setting should be as low as possible consistent with obtaining a satisfactory image.

6/ Ultrasound examinations should only be performed for a specific medical indication to obtain useful diagnostic information.

7/ Only suitably trained people should be allowed to operate ultrasound machines.

CONCLUSION

Although harmful effects have not been shown to result from the use of ultrasound at normal diagnostic levels, the potential for harm does exist.

More research still needs to be done. For example, we do not know if the effects from ultrasound exposure are cumulative.

As in the use of x-rays, a very small risk is justified when the risk is minimal and the benefit high.