STA simulation on a phased 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 Synthetic Transmit Aperture (STA) dataset for a phased array and a sector scan and 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 14.03.2017
Contents
Close all previously opened plots.
close all;
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, 40e-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=64; % number of elements prb.pitch=300e-6; % probe pitch in azimuth [m] prb.element_width=270e-6; % element width [m] prb.element_height=7000e-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=3e6; % 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 64 (= number of elements) waves each emanating from a single element on the probe aperture. An appropriate apodization window is enforced through setting apodization.window = uff.window.sta. The wave structure too has a plot method.
seq=uff.wave(); for n=1:prb.N_elements seq(n)=uff.wave(); seq(n).probe=prb; seq(n).source.xyz=[prb.x(n) prb.y(n) prb.z(n)]; seq(n).apodization=uff.apodization(); seq(n).apodization.window=uff.window.sta; seq(n).apodization.origin=seq(n).source; 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=4*pul.center_frequency; % 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 components: the azimuthal(angle) range and the depth(radial) range. scan too has a useful plot method it can call.
sca=uff.sector_scan('azimuth_axis',linspace(-10*pi/180,10*pi/180,200).','depth_axis', linspace(35e-3,45e-3,100).'); sca.plot(fig_handle,'Scenario'); % show mesh
Beamformer
With channel_data and a scan we have all we need to produce an ultrasound image. We now use a USTB structure beamformer, that takes an apodization structure in addition to the channel_data and scan.
pipe=pipeline(); pipe.channel_data=channel_data; pipe.scan=sca; pipe.receive_apodization.window=uff.window.tukey50; pipe.receive_apodization.f_number=1.7; pipe.transmit_apodization.window=uff.window.tukey50; pipe.transmit_apodization.f_number=1.7;
The beamformer structure allows you to implement different beamformers by combination of multiple built-in processes. By changing the process chain other beamforming sequences can be implemented. It returns yet another UFF structure: beamformed_data.
To achieve the goal of this example, we use delay-and-sum (implemented in the das() process) as well as coherent compounding.
b_data=pipe.go({midprocess.das() postprocess.coherent_compounding()});
% show
b_data.plot();
USTB General beamformer MEX v1.1.2 .............done!