User's Manual

Acclarix LX9 Series Diagnostic Ultrasound System User Manual Ultrasound Intensity and Safety
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Line density
The acoustic output will be changed with the change of the number of the
scanning line (line density).
PRF
The acoustic power will be changed with the change of PRF.
Sample volume
The pulsed wave and the power will be changed with the change of the
sample volume, and acoustic output will be changed.
Presets
Presets contain all the parameters above, so any change of the presetting
will change acoustic output.
Restart, or power
on/off
System will return to the default setting when restarting, or powering on/off
the system, and acoustic output will be changed.
B.3 Explanation of MI/TI
B.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 square root of acoustic
frequency.
𝑀𝐼 =
𝑃
𝑟,𝑎
𝑓
𝑎𝑤𝑓
× 𝐶
𝑀𝐼
Where, 𝐶
𝑀𝐼
= 1 Mpa. MHz
−1/2
, 𝑃
𝑟,𝑎
is the Attenuated Peak-rare-factional Acoustic Pressure and 𝑓
𝑎𝑤𝑓
is Acoustic Working Frequency.
B.3.2 TI (Thermal Index)
Heating of tissues is caused by absorption of ultrasound when the ultrasound energy is applied. The
temperature rise is determined by the acoustic intensity, exposed area and thermophysical properties
of the tissue.
In order to indicate the potential for temperature rise caused by thermal effects, the AIUM and NEMA
formulate thermal index (TI). It is defined as the ratio of the total acoustic power to the acoustic power
required to raise the tissue temperature by 1ºC.
According to different thermophysical properties of the tissue, TI is divided into three kinds: TIS, TIB
and TIC.
TIS (Soft Tissue Thermal Index): It provides an estimate of potential temperature rise in soft or similar
tissues.
TIB (Bone Thermal Index): It provides an estimate of potential temperature rise when the ultrasound
beam passes through soft tissue and a focal region is in the immediate vicinity of bone.