CPWC (low-resolution) simulation 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 Coherent Plane-Wave Compounding (CPWC) dataset and how it can be beamformed with USTB.

Related materials:

Montaldo et al. 2009

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 , Ole Marius Hoel Rindal olemarius@olemarius.net and Arun Asokan Nair anair8@jhu.edu 16.05.2017

Phantom
Probe
Pulse
Sequence generation
The Fresnel simulator
Scan
Midprocess

Phantom

Our first step is to define an appropriate phantom structure as input. Our phantom here is simply a single point scatterer. USTB's implementation of phantom comes with a plot method for free!

pha=uff.phantom();
pha.sound_speed=1540;            % speed of sound [m/s]
pha.points=[0,  0, 40e-3, 1];    % point scatterer position [m]
fig_handle=pha.plot();

Probe

Another UFF structure is probe. You've guessed it, it contains information about the probe's geometry. USTB's implementation comes with a plot method. 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 3 plane-waves covering an angle span of $[-0.3, 0.3]$ radians. The wave structure has a plot method which plots the direction of the transmitted plane-wave.

N=3;                           % number of plane waves
angles=linspace(-0.3,0.3,N);    % angle vector [rad]
seq=uff.wave();
for n=1:N
    seq(n)=uff.wave();

    seq(n).source.azimuth=angles(n);
    seq(n).source.distance=Inf;

    seq(n).probe=prb;

    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. Go!
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 just two axes. scan too has a useful plot method it can call.

scan=uff.linear_scan();
scan.x_axis=linspace(-3e-3,3e-3,200).';
scan.z_axis=linspace(39e-3,43e-3,200).';
scan.plot(fig_handle,'Scenario');    % show mesh

Midprocess

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.

mid=midprocess.das();
mid.code = code.matlab;
mid.channel_data=channel_data;
mid.scan=scan;

mid.receive_apodization.window=uff.window.tukey50;
mid.receive_apodization.f_number=1.7;
mid.transmit_apodization.window=uff.window.none; % Setting transmit apodization to "none" since we want to look at the individual low quality PW's

The midprocess structure allows you to implement different beamformers. It returns yet another UFF structure: beamformed_data.

b_data=mid.go();

Finally, display each individual low quality plane wave image. Fin!

figure('Position',[100 100 1000 300])
h1 = subplot(1,3,1);
angle_1 = rad2deg(channel_data.sequence(1,1).source.azimuth);
b_data.plot(h1,sprintf('PW at angle = %0.1f',angle_1),[],[],[1 1]);

h2 = subplot(1,3,2);
angle_2 = rad2deg(channel_data.sequence(1,2).source.azimuth);
b_data.plot(h2,sprintf('PW at angle = %0.1f',angle_2),[],[],[1 2]);

h3 = subplot(1,3,3);
angle_3 = rad2deg(channel_data.sequence(1,3).source.azimuth);
b_data.plot(h3,sprintf('PW at angle = %0.1f',angle_3),[],[],[1 3]);

Index exceeds the number of array elements (2).

Error in uff/beamformed_data/plot (line 113)
                data=h.data(:,indeces(1),indeces(2),indeces(3));

Error in CPWC_linear_array_low_quality_PW_images (line 142)
b_data.plot(h1,sprintf('PW at angle = %0.1f',angle_1),[],[],[1 1]);