in the risk of potential thermal bioeffects and it provides a relative magnitude that can be
used to implement ALARA. In addition to tissue heating due to the generated ultrasound
field, the temperature of the probe head itself can also increase during the examination.
The operator shall be aware, that in the tissue region near the ultrasonic transducer,
there will be a superposition with the heating due to the ultrasound field, which is not
considered by the TI values.
•
Nonthermal effects refer to mechanical phenomena such as cavitation
Nonthermal bioeffects are caused by the interaction of ultrasound fields with very small
pockets of gas (stabilized gas bodies), i.e. the generation, growth, vibration and possible
collapse of microbubbles within the tissue. This behavior is referred to as cavitation
(Medical Ultrasound Safety, 2nd Edition, AIUM 2009/American Institute of Ultrasound in
Medicine Consensus Report on Potential Bioeffects of Diagnostic Ultrasound, AIUM
2008/Guidelines for the safe use of diagnostic ultrasound equipment, Safety Group of
the British Medical Ultrasound Society 2010). The potential of cavitation increases with
the rarefactional peak pressure but decreases with the pulse frequency. Therefore the
Mechanical Index MI was introduced to take account of both the pressure and the
frequency. The higher the MI the greater is the risk of nonthermal bioeffects.
2.10.3 Regulated Parameters
Relevant parameters having physiological effects (
For more information see
'Bioeffects'
on
page 2-22.
) are regulated according to FDA and IEC guidelines and standards. These
parameters are
Parameter Meaning Limit Displayed
MI Mechanical Index 1.9 Yes
TIs, TIb, TIc Thermal Indices TI – one of the
following values can be displayed:
6 Yes
TIs: soft tissues
TIb: bone in focal region
TIc: bone at surface (e.g. cranial)
Ispta.3 Averaged intensity at spatial peak
with a derating of 0.3dB/(cm MHz)
720 mW/cm2 No
T Temperature at the patient’s side of
the probe – lower limit during patient
contact, higher limit for rest position
43˚C/50˚C
(109.4˚F/122˚F)
No
2.10.4 Interpretation of displayed parameters MI and TI
During obstetric examinations these displayed values shall be observed very critically,
because there may be conditions that are potentially hazardous even below the regulatory
limits.
Some guidelines recommend that embryonic and fetal in situ temperatures of 41˚C (4˚C above
normal temperature) should be limited in time by 5 min or less. Thus, for a reasonable safety
margin, TI values above 1 should be avoided. Additional factors, like fever of the mother, are
again reasons to keep the TI values as low as possible on the one hand, and go only as high
as necessary to achieve the desired clinical results (
'Prudent Use – ALARA Principle'
on page
2-22
).
The mechanical index, which indicates the risk of cavitation, becomes important at the
interface between gas and soft tissue (nonfetal lung and bowel), but also with the use of gas
Safety
Voluson® S6/S8 Basic User Manual
5433669-100 Revision 4 2-23
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