英语翻译2.2 Simulation of current densityTo obtain an estimate o

英语翻译
2.2 Simulation of current density
To obtain an estimate of the current density present at the beam axis,a 2D QuasiStaticSmallCurrents simulation in
FemlabTM (Comsol Inc.) was done for the plane orthogonal to the beam axis.The current electrodes were modeled as
squares 3 mm wide (the short axis of the electrodes),injecting current only in the direction facing each other.Fringing of
the current field parallel to the beam axis was neglected.
2.3 Verification of linearity of AEE signal with respect to current density and ultrasound amplitude
To verify relationships (3) and (4),both current and ultrasonic amplitude were swept independently.The same basic
setup of Figure2 was used except that instead of a pulse/receiver,a HP33120A signal generator coupled to a ENI
AP400B RF power amplifier generated the ultrasound signal.The HP33120A output a 5 cycle burst when triggered.The
triggering scheme used is illustrated in Figure 5.The current generator output a 100 Hz sinusoidal waveform,issuing a
trigger at the start of each cycle.Waveforms were then collected at both the maxima and minima of the current
waveform.640 waveforms were averaged.The average waveform collected at the minima was subtracted from the
average waveform collected at the maxima.The subtracted waveforms were filtered and converted into analytical form
in MatlabTM (The MathWorks Inc.).
To verify relationship (3),the current amplitude was swept from 0 mA to 50 mA peak while the ultrasound was
fixed at 3 MPa peak positive pressure.Similarly,relationship (4) was verified by sweeping the pressure amplitude (0-3
MPa) while keeping the current at 50 mA peak.At each point in each sweep,the maximum value of the absolute
analytical signal was calculated.2.4 Measurement of mock cardiac current waveform
A timing diagram for the experiment is shown in Figure 6.The FPGA issues a series of delayed pulses which are
used to trigger the ultrasound pulse/receiver and the DAQ board.Delays are chosen such that the low frequency current
waveform is regularly and adequately sampled (at 2kHz).At each delay point,two voltage traces are recorded,one with
the current amplitude set at 50 mA peak and another with the current turned off as a control.The mock cardiac signal
used is an internal function of the HP33120A.The mock cardiac signal output has the same peak value as a sinusoid.
Each waveform is averaged 128 times using the oscilloscope’s internal averaging before it is sent to the computer.
The 0 mA signal is subtracted from the signal with current.The subtracted waveforms are ordered into a matrix such that
one axis is proportional to fast-time (i.e.the time frame of the ultrasound) and the other is proportional to slow-time (i.e.
the time frame of the current waveform).This matrix will be referred to as the wavefield in the following discussion.
The wavefield is filtered in both fast and slow time.