RTB simulation on a linear array with the USTB built-in Fresnel simulator

In this example we show how to use the built-in fresnel simulator in USTB to generate a Retrospective Transmit focus Beamforming (RTB) dataset for a linear array and a linear scan and show how it can be beamformed with USTB.

This tutorial assumes familiarity with the contents of the 'CPWC simulation with the USTB built-in Fresnel simulator' tutorial. Please feel free to refer back to that for more details.

% by Alfonso Rodriguez-Molares alfonso.r.molares@ntnu.no and Arun Asokan Nair anair8@jhu.edu 11.03.2017

Contents

Phantom

We start off defining an appropriate phantom structure to image. Our phantom here is simply a single point scatterer. USTB's implementation of phantom comes with a plot method to visualize the phantom for free!

pha=uff.phantom();
pha.sound_speed=1540;            % speed of sound [m/s]
pha.points=[0,  0, 20e-3, 1];    % point scatterer position [m]
fig_handle=pha.plot();

Probe

The next UFF structure we look at is probe. It contains information about the probe's geometry. USTB's implementation of probe comes with a plot method too. When combined with the previous figure we can see the position of the probe respect to the phantom.

prb=uff.linear_array();
prb.N=128;                  % number of elements
prb.pitch=300e-6;           % probe pitch in azimuth [m]
prb.element_width=270e-6;   % element width [m]
prb.element_height=5000e-6; % element height [m]
prb.plot(fig_handle);

Pulse

We then define the pulse-echo signal which is done here using the fresnel simulator's pulse structure. We could also use 'Field II' for a more accurate model.

pul=uff.pulse();
pul.center_frequency=5.2e6;       % transducer frequency [MHz]
pul.fractional_bandwidth=0.6;     % fractional bandwidth [unitless]
pul.plot([],'2-way pulse');

Sequence generation

Now, we shall generate our sequence! Keep in mind that the fresnel simulator takes the same sequence definition as the USTB beamformer. In UFF and USTB a sequence is defined as a collection of wave structures.

For our example here, we define a sequence of 31 diverging waves. The wave structure too has a useful plot method.

N=31;                               % number of waves
x0=linspace(-19.2e-3,19.2e-3,N);
z0=20e-3;
seq=uff.wave();
for n=1:N
    seq(n)=uff.wave();
    seq(n).probe=prb;
    seq(n).source.xyz=[x0(n) 0 z0];
    seq(n).sound_speed=pha.sound_speed;

    % show source
    fig_handle=seq(n).source.plot(fig_handle);
end

The Fresnel simulator

Finally, we launch the built-in simulator. The simulator takes in a phantom, pulse, probe and a sequence of wave structures along with the desired sampling frequency, and returns a channel_data UFF structure.

sim=fresnel();

% setting input data
sim.phantom=pha;                % phantom
sim.pulse=pul;                  % transmitted pulse
sim.probe=prb;                  % probe
sim.sequence=seq;               % beam sequence
sim.sampling_frequency=41.6e6;  % sampling frequency [Hz]

% we launch the simulation
channel_data=sim.go();

USTB's Fresnel impulse response simulator (v1.0.7)
---------------------------------------------------------------

Scan

The scan area is defines as a collection of pixels spanning our region of interest. For our example here, we use the linear_scan structure, which is defined with two components: the lateral range and the depth range. scan too has a useful plot method it can call.

scan=uff.linear_scan('x_axis', linspace(-2e-3,2e-3,200).', 'z_axis', linspace(18e-3,22e-3,200).');
scan.plot(fig_handle,'Scenario');    % show mesh

Midprocessor

With channel_data and a scan we have all we need to produce an ultrasound image. We now use a USTB structure midprocess, that takes an apodization structure in addition to the channel_data and scan, and returns a beamformed_data strcuture.

mid=midprocess.das();
mid.dimension = dimension.both;
mid.channel_data=channel_data;
mid.scan=scan;

F_number=1.7;
mid.receive_apodization.window=uff.window.hanning;
mid.receive_apodization.f_number=F_number;
mid.receive_apodization.minimum_aperture = 3e-3;

mid.transmit_apodization.window=uff.window.hanning;
mid.transmit_apodization.f_number=F_number;
mid.transmit_apodization.minimum_aperture = 3e-3;

b_data=mid.go();
b_data.plot();

USTB General beamformer MEX v1.1.2 .............done!

Analyse the transmit apodization used

We can plot the apodization and get a GUI to choose which pixel in the scan we want to plot the apodization across the aperture by using the transmit_apodization.plot() function. To exit press "enter".

%mid.transmit_apodization.plot()

Published with MATLAB® R2020b