B-K Medical Pro Focus 2202, User Manual

Page: 28 / 356

28 Chapter 1
Monitor Display
The Mechanical Index (MI) and Thermal Index (TI) can be viewed in all scanning
modes.
Thermal and Mechanical Indices
The MI and TI indices are intended to allow users to implement the ALARA [10, 11]
(As Low As Reasonably Achievable) principle using an indicator related to a
potential bioeffect.
The full details of the indices are given in references [3] and [9], but the formulas are
given below.
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.
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)
M
I
P
r0.3
z
sp
( )
f
c
------------------------ =
TI
W
0
W
deg
------------- =