EDAN F6 User manual - A4.1 Ultrasound In Medicine

F Series Fetal & Maternal Monitor User Manual                   Ultrasound Intensity and Safety 

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Appendix 4 Ultrasound Intensity and Safety 

A4.1 Ultrasound in Medicine 

The use of diagnostic ultrasound has proved to be a valuable tool in medical practice. Given its 

known benefits for non-invasive investigations and medical diagnosis, including investigation of 

the human fetus, the question of clinical safety with regards to ultrasound intensity arises. 

There is no easy answer to the question of safety surrounding the use of diagnostic ultrasound 

equipment. Application of the ALARA (As Low As Reasonably Achievable) principle serves as a 

rule-of-thumb that will help you to get reasonable results with the lowest possible ultrasonic 

output. 

The American Institute of Ultrasound in Medicine (AIUM) states that given its track record of 

over 25 years of use and no confirmed biological effects on patients or instrument operators, the 

benefits of the prudent use of diagnostic ultrasound clearly outweigh any risks. 

A4.2 Ultrasound Safety and the ALARA Principle 

Ultrasound waves dissipate energy in the form of heat and can therefore cause tissue warming. 

Although this effect is extremely low with Doppler, it is important to know how to control and 

limit patient exposure. Major governing bodies in ultrasound have issued statements to the effect 

that there are no known adverse effects from the use of diagnostic ultrasound, however, exposure 

levels should always be limited to As Low As Reasonably Achievable (the ALARA principle).   

A4.3 Explanation of MI/TI 

A4.3.1 MI (Mechanical Index) 

Cavitations will be generated when ultrasound wave passes through and contacts tissues, 

resulting in instantaneous local overheating. This phenomenon is determined by acoustic pressure, 

spectrum, focus, transmission mode, and factors such as states and properties of the tissue and 

boundary. This mechanical bioeffect is a threshold phenomenon that occurs when a certain level 

of ultrasound output is exceeded. The threshold is related to the type of tissue. Although no 

confirmed adverse mechanical effects on patients or mammals caused by exposure at intensities 

typical of present diagnostic ultrasound instruments have ever been reported, the threshold for 

cavitation is still undetermined. Generally speaking, the higher the acoustic pressure, the greater 

the potential for mechanical bioeffects; the lower the acoustic frequency, the greater the potential 

for mechanical bioeffects. 

The AIUM and NEMA formulate mechanical index (MI) in order to indicate the potential for 

mechanical effects. The MI is defined as the ratio of the peak-rarefactional acoustic pressure 

(should be calculated by tissue acoustic attenuation coefficient 0.3dB/cm/MHz) to the acoustic 

frequency.