M y L a b
–
S A F E T Y A N D S T A N D A R D
2-5
On-screen Real-Time Acoustic Output Display
Until recently, application-specific output limits
2
established by the USA Food and
Drug Administration (FDA) and the user's knowledge of equipment controls and
patient body characteristics have been the means of minimizing exposure. Now,
more information is available through a new feature, named the Acoustic Output
Display. The output display provides users with information that can be specifically
applied to ALARA. It eliminates some of the guesswork and provides both an
indication of what may actually be happening within the patient (i.e. the potential
for bioeffects), and what occurs when system control settings are changed. This
makes it possible for the user to get the best image possible while following the
ALARA principle and thus to maximize the benefits/risks ratio.
MyLab
incorporates a real-time acoustic output display according to the
AIUM
3
/NEMA
4
"Standard for Real-Time Display of Thermal and Mechanical
Acoustic Output Indices on Diagnostic Ultrasound Equipment" publication,
adopted in 1992 by both institutions. This
output display standard
is intended to
provide on-screen display of these two indices, which are related to ultrasound
thermal and cavitation mechanisms, to assist the user in making informed risk (i.e.
patient exposure)/benefit (diagnostically useful information) decisions.
Considering the type of exam, patient conditions and the case study level of
difficulty, the system operator decides how much acoustic output to apply for
obtaining diagnostically useful information for the patient; the thermal and
mechanical indices real-time display is intended to provide information to the
system operator throughout the examination so that exposure of the patient to
ultrasound can be reasonably minimized while maximizing diagnostic information.
For systems with an output display, the FDA currently regulates only the
maximum output.
MyLab
system has been designed to automatically default the
proper range of intensity levels for a particular application. However, within the
limits, the user may override the application specific limits, if clinically required.
The user is responsible for being aware of the output level that is being used. The
MyLab
real-time output display provides the user with relative information about
the intensity level.
The Mechanical Index
The Mechanical Index (
MI
) is defined as the peak rarefactional pressure in MPa
(derated by a tissue attenuation coefficient of 0.3 dB/cm/MHz) divided by the
square root of the probe central frequency in MHz.
With the MI, the user can keep the potential for mechanical bioeffects as low as
reasonably achievable while obtaining diagnostically adequate images. The higher
the index, the larger the potential. However, there is not a level to indicate that
2
Also known as the preamendments limits, those values were established on the basis of acoustic output
of equipment on the market before 1976.
3
American Institute for Ultrasound in Medicine.
4
National Electric Manufacturers Association.
O D S
Thermal and Mechanical
Indices display to assist in
making informed
risk/benefit decisions
M I
Estimates mechanical
bioeffects
