<s>
Ultrasound	B-Algorithm
computer	I-Algorithm
tomography	I-Algorithm
(	O
USCT	O
)	O
,	O
sometimes	O
also	O
Ultrasound	B-Application
computed	O
tomography	B-Algorithm
,	O
Ultrasound	B-Application
computerized	O
tomography	B-Algorithm
or	O
just	O
Ultrasound	B-Application
tomography	B-Algorithm
,	O
is	O
a	O
form	O
of	O
medical	B-Application
ultrasound	I-Application
tomography	B-Algorithm
utilizing	O
ultrasound	B-Application
waves	O
as	O
physical	O
phenomenon	O
for	O
imaging	B-Application
.	O
</s>
<s>
It	O
is	O
mostly	O
in	O
use	O
for	O
soft	O
tissue	O
medical	B-Application
imaging	I-Application
,	O
especially	O
breast	B-Algorithm
imaging	I-Algorithm
.	O
</s>
<s>
Ultrasound	B-Application
computer	O
tomographs	B-Algorithm
use	O
ultrasound	B-Application
waves	O
for	O
creating	O
images	O
.	O
</s>
<s>
In	O
the	O
first	O
measurement	O
step	O
a	O
defined	O
ultrasound	B-Application
wave	O
is	O
generated	O
with	O
typically	O
Piezoelectric	O
ultrasound	B-Algorithm
transducers	I-Algorithm
,	O
transmitted	O
in	O
direction	O
of	O
the	O
measurement	O
object	O
and	O
received	O
with	O
other	O
or	O
the	O
same	O
ultrasound	B-Algorithm
transducers	I-Algorithm
.	O
</s>
<s>
While	O
traversing	O
and	O
interacting	O
with	O
the	O
object	O
the	O
ultrasound	B-Application
wave	O
is	O
changed	O
by	O
the	O
object	O
and	O
carries	O
now	O
information	O
about	O
the	O
object	O
.	O
</s>
<s>
Unlike	O
X-ray	B-Library
or	O
other	O
physical	O
properties	O
which	O
provide	O
typically	O
only	O
one	O
information	O
,	O
ultrasound	B-Application
provides	O
multiple	O
information	O
of	O
the	O
object	O
for	O
imaging	B-Application
:	O
the	O
attenuation	O
the	O
wave	O
's	O
sound	O
pressure	O
experiences	O
indicate	O
on	O
the	O
object	O
's	O
attenuation	O
coefficient	O
,	O
the	O
time-of-flight	O
of	O
the	O
wave	O
gives	O
speed	O
of	O
sound	O
information	O
,	O
and	O
the	O
scattered	O
wave	O
indicates	O
on	O
the	O
echogenicity	O
of	O
the	O
object	O
(	O
e.g.	O
</s>
<s>
Unlike	O
conventional	O
ultrasound	B-Application
sonography	B-Application
,	O
which	O
uses	O
phased	O
array	O
technology	O
for	O
beamforming	B-Algorithm
,	O
most	O
USCT	O
systems	O
utilize	O
unfocused	O
spherical	O
waves	O
for	O
imaging	B-Application
.	O
</s>
<s>
Most	O
USCT	O
systems	O
aiming	O
for	O
3D-imaging	O
,	O
either	O
by	O
synthesizing	O
(	O
"	O
stacking	O
"	O
)	O
2D	O
images	O
or	O
by	O
full	O
3D	O
aperture	B-General_Concept
setups	O
.	O
</s>
<s>
Another	O
aim	O
is	O
quantitative	O
imaging	B-Application
instead	O
of	O
only	O
qualitative	O
imaging	B-Application
.	O
</s>
<s>
The	O
idea	O
of	O
Ultrasound	B-Algorithm
computer	I-Algorithm
tomography	I-Algorithm
goes	O
back	O
to	O
the	O
1950s	O
with	O
analogue	O
compounding	O
setups	O
,	O
in	O
the	O
mid	O
1970s	O
the	O
first	O
"	O
computed	O
"	O
USCT	O
systems	O
were	O
built	O
up	O
,	O
utilizing	O
digital	O
technology	O
.	O
</s>
<s>
The	O
"	O
computer	O
"	O
in	O
the	O
USCT	O
concept	O
indicates	O
the	O
heavy	O
reliance	O
on	O
computational	O
intensive	O
advanced	O
digital	O
signal	O
processing	O
,	O
image	B-Algorithm
reconstruction	I-Algorithm
and	O
image	B-Algorithm
processing	I-Algorithm
algorithms	I-Algorithm
for	O
imaging	B-Application
.	O
</s>
<s>
USCT	O
systems	O
designed	O
for	O
medical	B-Application
imaging	I-Application
of	O
soft	O
tissue	O
typically	O
aim	O
for	O
resolution	B-Algorithm
in	O
order	O
of	O
centimeters	O
to	O
millimeters	O
and	O
require	O
therefore	O
ultrasound	B-Application
waves	O
in	O
the	O
order	O
of	O
mega-hertz	O
frequency	O
.	O
</s>
<s>
This	O
requires	O
typically	O
water	O
as	O
low-attenuating	O
transmission	O
medium	O
between	O
ultrasound	B-Algorithm
transducers	I-Algorithm
and	O
object	O
to	O
retain	O
suitable	O
sound	O
pressures	O
.	O
</s>
<s>
USCT	O
systems	O
share	O
with	O
the	O
common	O
tomography	B-Algorithm
the	O
fundamental	O
architectural	O
similarity	O
that	O
the	O
aperture	B-General_Concept
,	O
the	O
active	O
imaging	B-Application
elements	O
,	O
surround	O
the	O
object	O
.	O
</s>
<s>
For	O
the	O
distribution	O
of	O
ultrasound	B-Algorithm
transducers	I-Algorithm
around	O
the	O
measurement	O
object	O
,	O
forming	O
the	O
aperture	B-General_Concept
,	O
multiple	O
design	O
approaches	O
exist	O
.	O
</s>
<s>
There	O
exist	O
mono-	O
,	O
bi	B-Algorithm
-	I-Algorithm
and	O
multistatic	O
setups	O
of	O
transducer	O
configurations	O
.	O
</s>
<s>
Common	O
are	O
1D	O
-	O
or	O
2D	O
-	O
linear	O
arrays	O
of	O
ultrasound	B-Algorithm
transducers	I-Algorithm
acting	O
as	O
emitters	O
on	O
one	O
side	O
of	O
the	O
object	O
,	O
on	O
the	O
opposing	O
side	O
of	O
the	O
object	O
a	O
similar	O
array	O
acting	O
as	O
receiver	O
is	O
placed	O
,	O
forming	O
a	O
parallel	O
setup	O
.	O
</s>
<s>
While	O
cost	O
efficient	O
to	O
build	O
the	O
main	O
disadvantage	O
of	O
such	O
a	O
setup	O
is	O
the	O
limited	O
ability	O
(	O
or	O
inability	O
)	O
of	O
gathering	O
reflectivity	O
information	O
,	O
as	O
such	O
an	O
aperture	B-General_Concept
is	O
limited	O
to	O
only	O
transmission	O
information	O
.	O
</s>
<s>
Another	O
aperture	B-General_Concept
approach	O
is	O
a	O
ring	O
of	O
transducers	O
,	O
sometimes	O
with	O
the	O
degree	O
of	O
freedom	O
of	O
motorized	O
lifting	O
for	O
gathering	O
additional	O
information	O
over	O
the	O
height	O
for	O
3D	O
imaging	B-Application
(	O
"	O
stacking	O
"	O
)	O
.	O
</s>
<s>
Full	O
3D	O
setups	O
,	O
with	O
no	O
inherent	O
need	O
for	O
aperture	B-General_Concept
movements	O
,	O
exist	O
in	O
form	O
of	O
apertures	B-General_Concept
formed	O
by	O
semi-spherical	O
distributed	O
transducers	O
.	O
</s>
<s>
Tomographic	B-Algorithm
reconstruction	I-Algorithm
methods	O
used	O
in	O
USCT	O
systems	O
for	O
transmission	O
information	O
based	O
imaging	B-Application
are	O
classical	O
inverse	B-Algorithm
radon	I-Algorithm
transform	I-Algorithm
and	O
fourier	B-Algorithm
slice	I-Algorithm
theorem	I-Algorithm
and	O
derived	O
algorithms	O
(	O
cone	O
beam	O
etc	O
.	O
)	O
.	O
</s>
<s>
As	O
advanced	O
alternative	O
also	O
ART-based	O
approaches	O
are	O
utilized	O
.	O
</s>
<s>
For	O
high	B-Algorithm
resolution	I-Algorithm
and	O
speckle	B-Algorithm
noise	I-Algorithm
reduced	O
reflectivity	O
imaging	B-Application
Synthetic	O
Aperture	B-General_Concept
Focusing	O
Techniques	O
(	O
SAFT	O
)	O
,	O
similar	O
to	O
radar	B-Application
's	O
SAR	B-Application
and	O
sonar	B-Application
's	O
SAS	O
,	O
are	O
widely	O
used	O
.	O
</s>
<s>
Iterative	O
wave	O
equation	O
inversion	O
approaches	O
as	O
imaging	B-Application
method	O
coming	O
from	O
the	O
seismology	O
are	O
under	O
academic	O
research	O
,	O
but	O
usage	O
for	O
real	O
world	O
applications	O
is	O
due	O
to	O
the	O
enormous	O
computational	O
and	O
memory	O
burden	O
still	O
a	O
challenge	O
.	O
</s>
<s>
Many	O
USCT	O
systems	O
are	O
designed	O
for	O
soft	O
tissue	O
imaging	B-Application
and	O
for	O
breast	O
cancer	O
diagnosis	O
specifically	O
.	O
</s>
<s>
As	O
ultrasound	B-Application
based	O
method	O
with	O
low	O
sound	O
pressures	O
,	O
USCT	O
is	O
a	O
harmless	O
and	O
risk-free	O
imaging	B-Application
method	O
,	O
suitable	O
for	O
periodical	O
screening	O
.	O
</s>
<s>
Breast	B-Algorithm
ultrasound	I-Algorithm
)	O
which	O
rely	O
on	O
the	O
individual	O
examiners	O
performance	O
and	O
experience	O
.	O
</s>
