Medical Ultrasound Safety

The BMUS Physics and Safety Committee have put together the following top 5 tips on Medical Ultrasound Safety. 

1. Always scan for a justified reason.  Ask yourself….What is the clinical question? Could this scan change the clinical management for the patient?

2. Familiarise yourself with safety Indices and where on the ultrasound screen they are displayed.

Mechanical Index (MI) relates to the potential ability of the ultrasound beam to cause non thermal effects in the tissue, such as cavitation.It is not time linked, it can happen in any time frame but obviously the longer the scan the more chance of it occurring.

Thermal Index (TI) relates to the amount of heating generated by the ultrasound beam.This is directly time linked, the longer the insonation the increased heating effect.

There are 3 types of TI-

  1. TIS relates to scans where the ultrasound beam only passes through soft tissue.
  2. TIB relates to scans where the ultrasound beam focus lies near bone
  3. TIC relating to the cranium, where bone is near the surface of the skin, close to the transducer surface.

For a basic overview on Ultrasound safety go to the EFSUMB website and click on the safety tab and watch the Safety Powerpoint presentation.

3. When scanning observe the guidelines for ‘Safe use of Diagnostic Ultrasound Equipment’ prepared by BMUS.

?Table 1 (page 8) and table 2 (page 9) show advised safety limits.

One can alter settings to reduce the TI and MI if needed.   Reducing Output power will lower the TI and MI values, these along with reducing scan time reduces the risk of thermal and non thermal effects.  If you are routinely having to lower settings such as the Output Power from 100% before you start scanning, contact  your Applications Specialist or Medical Physicist so that they can alter the presets for you.  Be aware that some applications , such as Doppler involve higher TI and MI values and therefore more potential to cause Thermal and non Thermal effects. 

4. Consider the area being examined. Absorption of ultrasound by different tissues and therefore the degree of possible heating varies greatly, fully calcified bone absorbing the most.  The increase in temperature of the bone will also have an effect on adjacent tissues.  The sensitivity of tissue to thermal damage also varies depending on tissue type, particular care must be taken when insonating extra sensitive tissues which lie adjacent to bone such as the brain or growing epiphyses.  Some organs, such as the eyes and lungs, are always to be considered sensitive due to potential thermal and non-thermal hazards.

When scanning keep the probe moving to avoid overheating a specific area and do not hold an active transducer on the patient whilst not actually scanning.

5. Most importantly a diagnostic image must be obtained to enable you to answer the clinical question, therefore practice ALARA (as low as reasonably achievable).  If you cannot adhere to the recommended safety levels you should scan with the MI and TI as low as possible to achieve a diagnostic scan.