Following are a series of exercises that will familiarize
you with the Field II GUI:
We can think of each element in an ultrasound transducer as a bandpass
filter. Thus, viewing its response to a unit impulse (delta function)
excitation provides useful information. Use the Plot pull-down
menu to plot the Element Impulse Response. Now experiment with changing
different transducer characteristics and observe what happens to the impulse
response. (NOTE: YOU MUST HIT ENTER AFTER YOU CHANGE A PARAMETER
IN THE GUI FOR IT TO TAKE EFFECT!)
What happens to the impulse response when you change the fractional bandwidth?
What happens to the impulse response when you change the center frequency?
View the electrical excitation pulse that is used to excite the elements
by selecting this plot from the plots pull-down menu. This is the
delta function that is being applied to generate the above impulse response.
You can use more complex excitation functions as well (more on that later).
We can use Field to calculate the transmitted (Tx) pressure field from
the defined transducer array. This is the pressure field that would
propagate through a series of hydrophones placed laterally (i.e. in a line
that is parallel to the transducer), placed at the focal depth (i.e. axial
focal position). You can modify the position of the line of hydrophones
by changing the calculation position options on the GUI. However,
for now, leave the calculation position at its default. Choose 'TX
Pressure Field' under the Calculate menu. This may take a few seconds
to compute, and once it is finished, there are more plots available under
the plots pull-down menu.
What does the TX Field image look like? This is a plot of the
pressure distribution. The x-axis represents the lateral dimension,
and is the spatial 'line' parallel with the transducer where the pressure
field is computed (there is one line for each hydrophone). The y-axis
is labeled in mm, corresponding to axial distance from the transducer.
However, we have used an assumption here. In fact, the y-axis is
computed to be the pressure through time at each lateral position.
A conversion to distance is made using the speed of sound (i.e. distance
= rate*time). This is the same conversion that an ultrasound machine
uses to determine axial depth when generating an image.
What does the TX Axial Waveform look like? This is a plot of the
pressure along the central axis of the transducer (the center column in
the TX Field image). Note the similarities and differences between
this waveform and the impulse response of the elements.
What does the TX Lateral Beamplot look like? This is a plot of the
maximum pressure that occurs at each hydrophone. These plots are typically
displayed in decibels (20*log10(beam/max(beam))).
Next we can look at the transmit-receive (TX/RX) field. This is also
referred to as the pulse-echo response since it incorporates the pulse
transmission and then the received echo. The defaults are set up
to position 1 point target at the focal point (using the Target Characteristics
menu). With one point target located at the focus, the resulting
TX./RX field is commonly referred to as a Point Spread Function (PSF).
This can be calculated using the TX/RX Pressure Field option under the
Calculate menu. Returning to the Plot menu will yield even more plotting
options.
What does the TX/RX Axial waveform look like? Note the similarities and
differences between this waveform and the impulse response of the elements.
This line represents the self-convolution of the element impulse response
- because the pressure field goes through the element on both transmit
and receive, and thus the excitation pulse is effectively going through
two identical bandpass filters.
What does the lateral acoustic beam plot look like for the TX/RX system?
Note the lateral width of the beam (at 6 dB down). This is indicative
of the lateral resolution of the imaging system.
What happens to the other plots for this system?
When computing the TX/RX Pressure Field, the GUI also computes the K-Space
representation of the TX/RX field. K-Space is the 2-D Fourier transform
of the TX/RX field image. It is often useful to view images in both
the spatial and frequency domains. Select and view the K-Space TX/RX
Field on the plot pull-down menu. You can also look at
a either an axial or lateral slice through K-Space. Some
K-Space exercises are also available.
Finally, try making images of more than one point target, by editing the
Target Characteristics menu. You can determine the resolution of
your transducer by evaluating how closely you can place two point targets
together, and still see 2 separate points in the TX/RX field.
NOTE: you can zoom in on the figures in the plot window using
the following commands in the matlab window: >>figure(2) >>zoom
