Calculation of B-mode image of synthetic fetus
The anatomic phantoms are attempts to generate images as they will
be seen from real human subjects. This is done by drawing a bitmap
image of scattering strength of the region of interest. This map
then determines the factor multiplied onto the scattering amplitude
generated from the Gaussian distribution, and models the difference
in the density and speed of sound perturbations in the tissue.
Simulated boundaries were introduced by making lines in the scatterer map
along which the strong scatterers were placed. This is marked by completely
white lines shown in the scatterer maps. The model is currently two-dimensional,
but can readily be expanded to three dimensions. Currently,
the elevation direction is merely made by making
a 15 mm thickness for the scatter positions, which are
randomly distributed in the interval.
A phantom for a 3 month old fetus has been made.
200,000 scatterers were randomly distributed within
the phantom, and with a Gaussian distributed scatter amplitude
with a standard deviation determined by the scatter map. The
phantom was scanned with a 5 MHz 64 element phased array transducer
with lambda/2 spacing and Hanning apodization. A single
transmit focus 70 mm from the transducer was used, and focusing
during reception is at 40 to 140 mm in 10 mm increments. The
images consists of 128 lines with 0.7 degrees between lines.
The fetus image is shown below. Note how the anatomy can be clearly seen
at the level of detail of the scatterer map. The same boundary
features as for the kidney image is also seen.
The images have many of the features from real scan images, but still
lack details. This can be ascribed to the low level of details
in the bitmap images, and that only a 2D model is used. But the
images do show great potential for making powerful fully synthetic
phantoms, that can be used for image quality evaluation.
Artificial fetus in 12th week.
The m-files can be found at:
The routine field.m initializes the field system, and should be modified
to point to the directory holding the Field II code and m-files. The routine
mk_pht.m is then called to make the file for the scatterers in the
phantom. The file kidney6.bmp holds the description of the kidney.
The script sim_phas.m is then called. Here the field simulation
is performed and the data is stored in RF-files; one for each RF-line done.
The data for the scatteres are read from the file pht_data.mat,
so that the procedure can be started again or run for a number of workstations.
The data must then subsequently be processed to do the polar to rectangular
mapping to yield the image.