DW simulation with a curvilinear array using the USTB built-in Fresnel simulator

In this example, we show how to use the built-in Fresnel simulator in USTB to generate a Diverging Wave (DW) dataset on a curvilinear array, and then beamform it 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 23.02.2017

Contents

close all;

Phantom

First step - define our phantom. Here, our phantom a collection of point scatterers in the shape of a cross. 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=[zeros(11,1),  zeros(11,1), linspace(10e-3,160e-3,11).', ones(11,1);...
            linspace(-70e-3,70e-3,11).',  zeros(11,1), 70e-3*ones(11,1), ones(11,1)];    % point scatterer position [m]
fig_handle=pha.plot();

Probe

The next step is to define the probe structure which contains information about the probe's geometry. This too comes with a plot method that enables visualization of the probe with respect to the phantom. The probe we will use in our example is a curvilinear array transducer with 128 elements.

prb=uff.curvilinear_array();
prb.N=128;                  % number of elements
prb.pitch=508e-6;
prb.element_width=408e-6;
prb.radius=60e-3;
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=3.2e6;       % 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 15 diverging waves. The wave structure has a plot method which plots the direction of the transmitted waves.

N=15;                             % number of diverging waves
x0=linspace(-30e-3,30e-3,N);
z0=-prb.radius;
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 sector_scan structure, which is defined with two axes - the azimuth axis and the depth axis, along with the position of the apex. scan too has a useful plot method it can call.

scan=uff.sector_scan();
scan.azimuth_axis=linspace(-prb.maximum_angle,prb.maximum_angle,256).';
scan.depth_axis=linspace(prb.radius,prb.radius+180e-3,256).';
scan.apex=uff.point('xyz',[0 0 -prb.radius]);
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.

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

mid.receive_apodization.window=uff.window.tukey50;
mid.receive_apodization.f_number=1.7;

% beamforming
b_data=mid.go();
h_fig=b_data.plot();hold on;

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

Published with MATLAB® R2020b