(1) Concerns with bioeffects
Diagnostic ultrasound is recognized as being safe. In fact, there have been no reports of injuries to patients caused by diagnostic ultrasound.
It cannot be stated categorically that ultrasound is 100% safe. Studies have revealed that ultrasound with extremely high intensity is harmful to body tissues. This rapid advance has generated concerns about the potential risk of bioeffects when new applications or diagnostic technologies become available.
(2) Mechanical effects and thermal effects
Studies have revealed that two different kinds of ultrasound characteristics affect human bodies: one is the mechanical effects due to cavitation generated when the negative pressure of ultrasound exceeds a certain limit, and the other is the effects due to thermal energy generated when tissues absorb ultrasound. The levels of these two kinds of effects are represented by indices: the MI (Mechanical Index) and the TI (Thermal Index), respectively.
(3) MI/TI values
(4) Controlling the ultrasound output
In 1976, the FDA in the U.S.A. established limits to the acoustic output level for diagnostic ultrasound equipment (TRACK1), along with the rapid advance in diagnostic ultrasound technology, and the versatile proliferation of its applications, in order to ensure the safety of patients. Thereinafter, manufacturers were required to control the acoustic output level below the limits, when releasing new diagnostic ultrasound systems on the market.
Maximum acoustic power specified by FDA (TRACK1) Application Ispta.3
(mW/cm2) (W/cmIsppa.3 2)
MI
Peripheral blood vessels 720 190 1.9
Cardiovascular systems 430 190 1.9
Fetal and other 94 190 1.9
Eyes 17 28 0.23
The idea of MI/TI has been introduced recently to increase the diagnostic capability, promoting relaxation of these acoustic power limits (TRACK3).
Maximum limit for MI/TI display (TRACK3) Application Ispta.3 (mW/cm2) (W/cmIsppa.3 2) MI All regions (except eyes) 720 <190 or <1.9
With this trend, control of the level of acoustic power has been transferred from manufacturers to users. This means that users have a higher degree of
responsibility for safety than manufacturers. Against this background, users are required to understand the ultrasound bioeffects and their causes.
(5) ALARA (As Low As Reasonably Achievable)
It is required to practice ALARA when using ultrasound energy. Practicing ALARA ensures that the total energy level is controlled below a low enough level at which bioeffects are not generated while diagnostic information is being accumulated. The total energy is controlled by output intensity and total radiation time. The output intensity necessary for examinations differs depending on the patient and the clinical case.
Not all examinations can be performed with an extremely low level of acoustic energy. Controlling the acoustic level at an extremely low level leads to low-quality images or insufficient Doppler signals, adversely affecting the reliability of the diagnosis. However, increasing the acoustic power more than necessary does not always contribute to an increase in quality of information required for diagnosis, rather increasing the risk of generating bioeffects.
Users 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.
20.2 MI/TI Display Description
Mechanical Index (MI) formula is the spatial peak value of the peak rarefactional
pressure Pr.3 (Zsp), derated by 0.3 dB/cm-MHz at each point along the beam axis, divided by the square root of the center frequency fc (in MHz). That is:
( )
c sp 3 . r f Z P MI=Thermal Index (TI) is a quantity related to calculated or estimated temperature rise
under certain defined assumptions. TI is the ratio of total acoustic power to the acoustic power required to raise tissue temperature by 1 degree centigrade under defined assumptions.
TIS (Thermal Index Soft Tissue) is related to heating in abdominal, cardiac and
peripheral vascular applications.
TIB (Thermal Index Bone) is related to bone heating, which may be of particular
importance in fetal applications.
TIC (Thermal Index Cranial) is related to heating in adult cephalic applications. Display Accuracy is a manufacturer's specification as to how closely the displayed
values correspond to actual measured values. The display accuracy for acoustic output indices is +/-0.1.
Display Precision is rounded in units of 0.1.
20.3 Parameters Affecting the MI/TI Values
The MI/TI values are affected by transmission conditions (focus, drive frequency, voltage applied to piezoelectric elements, etc.), scan conditions, and settings of the main panel, and preset menu.
20.4 Operating Procedures for MI/TI
The control of ultrasound output and the most useful switches for this purpose are described below.
(1) ACOUSTIC POWER knob
The output intensity is one of the available ultrasound output control parameters. It can be adjusted with the ACOUSTIC POWER knob at the right end of the main panel. To decrease ultrasound output, turn the knob counterclockwise and to increase ultrasound output, turn the knob clockwise.
Low High
ACOUSTIC POWER
(2) Setting the defaults for ACOUSTIC POWER
Selection of diagnostic applications is the most important factor for controlling ultrasound output.
The permissible level of intensity of ultrasound differs depending on the region of interest. For fetal examinations, in particular, much care must be exercised. For this reason, the switch must be used to reset the parameters for each new patient, and the Preset menu must be used to select the settings appropriate for each new region of interest.
This system allows defaults to be set for MI, TIS, TIB, and TIC using the preset menu. The defaults automatically adjust the ACOUSTIC POWER level so that
MI, TIS, TIB, and TIC do not exceed their own settings at the time of power ON, , switching of the transducer, and other initializations.
Default choices Initial power 20% to 100%* * Definition of 100%:
The maximum acoustic power of a transducer determined by the
increase in transducer surface temperature in the selected mode and the acoustic power restrictions specified by the FDA.
When 100% is set as the default setting value, the default function does not operate.
20.5 References for MI/TI
For further information on measurement methods and MI/TI, refer to the followings: (1) "510 (K) Guide For Measuring and Reporting Acoustic Output of Diagnostic
Ultrasound Medical Devices" issued by FDA in 1993
(2) "Acoustic Output Measurement Standard for Diagnostic Ultrasound Equipment, Revision 2" issued by AIUM/NEMA in 1998
(3) "Standard for real-time display of thermal and mechanical acoustic output indices on diagnostic ultrasound equipment" issued by AIUM/NEMA in 1998
(4) "Bioeffects and Safety of Diagnostic Ultrasound" issued by AIUM in 1993 (5) "Medical Ultrasound Safety" issued by AIUM in 1994
(6) "Information for Manufacturers Seeking Marketing Clearance of Diagnostic Ultrasound Systems and Transducers" issued by FDA in 1997