bk ultrasound Flex Focus 1202, User Manual

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38 Chapter 2 December 2015 Flex Focus 1202 User Guide (BB1756-Q)
MI Formula
where the variables are defined in the table below.
TI Formula
where the variables are defined in the table below.
Blood
perfusion and
TI
As a rule of thumb, the Thermal Index (TI) indicates the highest expected
temperature increase in degrees Celsius. It is based on an average level of blood
perfusion. The displayed TI may underestimate the temperature rise in poorly
perfused tissues; you must take this into account when deciding on the maximum TI
you will allow. Conversely, in areas with a rich perfusion of blood the temperature
increase will be less than the displayed TI indicates.
Fever
A temperature increase of one degree Celsius increase in a patient with fever may
cause complications in certain circumstances; it may be safer to delay the
investigation.
Acoustic Output Measurement
All values are measured in water according to the EN 60601-2-37 [3] and
AIUM/NEMA display standards [9, 10]. For some of the acoustic parameters, an
estimated in situ derated value is given. This is derived assuming a tissue attenuation
of 0.3 dB/(cm·MHz) when the estimated in situ derated value (I) is described by the
following equation:
I formula
where the variables are defined in the table below.
Variable Definition
P
r0.3
(z
sp
) Peak Rarefactional Pressure (MPa), derated by 0.3dB/cm·MHz, measured at z
sp
,
the point on the beam axis where pulse intensity integral (PII
0.3
) is maximum
f
c
measured center frequency (in MHz)
Variable Definition
W
0
time-averaged acoustic power of the source or other power parameter (W)
W
deg
estimated power necessary to raise the temperature of the target tissue one
degree Celsius (W/°C)
MI
P
r0,3
z
sp
 
f
c
------------------------
=
TI
W
0
W
deg
------------- =
Variable Definition
I
W
Intensity in water at the position where I is maximum
f transducer frequency (in MHz)
z distance (in cm) from the transducer face to the position where I is maximum
I I
w
exp 0,069fz –
 
=