Sonoscape S30 - Mechanical and Thermal Indices

8 Service Manual
2 Safety
The operator must take responsibility for the safety of patients and utilize ultrasound deliberately.
Deliberate use of ultrasound means that output power of ultrasound must be selected based on
ALARA. Additional information regarding the concept of ALARA and the possible bioeffects of
Ultrasound is available in a document from the AIUM (American Institute of Ultrasound Medicine)
title “Medical Ultrasound Safety”.
2.3.3 Mechanical and Thermal Indices
The display of the ultrasound system consists of two parts: Thermal Index (TI) and Mechanical Index
(MI).

MI/TI Explanation
In October 1987, the American Institute of Ultrasound in Medicine (AIUM) ratified a report
prepared by its Bioeffects Committee (Bioeffects Considerations for the Safety of Diagnostic
Ultrasound, J Ultrasound Med.,Sept. 1988: Vol. 7, No. 9 Supplement), sometimes referred to
as the StoweReport, which reviewed available data on possible effects of ultrasound exposure.
Another report “Bioeffects and Safety of Diagnostic Ultrasound” dated January28, 1993, provides
more current information.
Mechanical Index (MI)
Mechanical bioeffects are threshold phenomena that occur when a certain level of output is
exceeded. The threshold level varies, however, with the type of tissue. The potential mechanical
bioeffects varies with peak pressure and ultrasound frequency. The MI accounts for these two
factors. The higher the MI value, the greater the likelihood of mechanical bioeffects occurring.
There is no specific MI value that means that a mechanical effect is actually occurring. The MI
should be used as a guide for implementing the ALARA principle.
Thermal Index (TI)
The TI value informs the operator about the conditions that might lead to an increase in
temperature at the surface of the body, within the body tissue, or at the point of focus of
the ultrasound beam on bone. That is, the TI value informs the operator about the potential
temperature rise in body tissue. It is an estimate of temperature increase in body tissue with
specific properties. The actual amount of any temperature rise is influenced by factors such as
tissue type, vascularity, mode of operation and others. The TI value should be used as a guide for
implementing the ALARA principle.
Depending on the examination and type of tissue involved, TI could be one of three types:
●
Soft Tissue Thermal Index (TIS) is used when imaging soft tissue only, it provides an estimate
of potential temperature rise in soft tissue.
●
Bone Thermal Index (TIB) is used when bone is near the focus of the image as in the third
cropester OB examination, it provides an estimate of potential temperature rise in the bone or
adjacent soft tissue.
●
Cranial Bone Thermal Index (TIC) is used when bone is near the skin surface as in
transcranial examination, it provides an estimate of potential temperature rise in the bone or
adjacent soft tissue.