<s>
Ballistic	B-Algorithm
light	I-Algorithm
,	O
also	O
known	O
as	O
ballistic	B-Algorithm
photons	I-Algorithm
,	O
is	O
photons	B-Application
of	O
light	O
that	O
have	O
traveled	O
through	O
a	O
scattering	O
(	O
turbid	O
)	O
medium	O
in	O
a	O
straight	O
line	O
.	O
</s>
<s>
When	O
pulses	O
of	O
laser	O
light	O
pass	O
through	O
a	O
turbid	O
medium	O
such	O
as	O
fog	O
or	O
body	O
tissue	O
,	O
most	O
of	O
the	O
photons	B-Application
are	O
either	O
scattered	O
or	O
absorbed	O
.	O
</s>
<s>
However	O
,	O
across	O
short	O
distances	O
,	O
a	O
few	O
photons	B-Application
pass	O
through	O
the	O
scattering	O
medium	O
in	O
straight	O
lines	O
.	O
</s>
<s>
These	O
coherent	O
photons	B-Application
are	O
referred	O
to	O
as	O
ballistic	B-Algorithm
photons	I-Algorithm
.	O
</s>
<s>
Photons	B-Application
that	O
are	O
slightly	O
scattered	O
,	O
retaining	O
some	O
degree	O
of	O
coherence	O
,	O
are	O
referred	O
to	O
as	O
snake	O
photons	B-Application
.	O
</s>
<s>
The	O
aim	O
of	O
ballistic	B-Algorithm
imaging	I-Algorithm
modalities	O
is	O
to	O
efficiently	O
detect	O
ballistic	B-Algorithm
photons	I-Algorithm
that	O
carry	O
useful	O
information	O
,	O
while	O
rejecting	O
non-ballistic	O
photons	O
.	O
</s>
<s>
To	O
perform	O
this	O
task	O
,	O
specific	O
characteristics	O
of	O
ballistic	B-Algorithm
photons	I-Algorithm
vs.	O
non-ballistic	O
photons	O
are	O
used	O
,	O
such	O
as	O
time	O
of	O
flight	O
through	O
coherence-gated	O
imaging	O
,	O
collimation	O
,	O
wavefront	O
propagation	O
,	O
and	O
polarization	O
.	O
</s>
<s>
Slightly	O
scattered	O
"	O
quasi-ballistic	O
"	O
photons	B-Application
are	O
often	O
measured	O
as	O
well	O
,	O
to	O
increase	O
the	O
signal	O
'	O
strength	O
 '	O
(	O
i.e.	O
,	O
signal-to-noise	O
ratio	O
)	O
.	O
</s>
<s>
Ballistic	B-Algorithm
photons	I-Algorithm
have	O
many	O
applications	O
,	O
especially	O
in	O
high-resolution	B-Algorithm
medical	B-Application
imaging	I-Application
systems	O
.	O
</s>
<s>
Ballistic	O
scanners	O
(	O
using	O
ultrafast	O
time	O
gates	O
)	O
and	O
optical	B-Application
coherence	I-Application
tomography	I-Application
(	O
OCT	O
)	O
(	O
using	O
the	O
interferometry	O
principle	O
)	O
are	O
just	O
two	O
popular	O
imaging	O
systems	O
that	O
rely	O
on	O
ballistic	B-Algorithm
photon	I-Algorithm
detection	O
to	O
create	O
diffraction-limited	O
images	O
.	O
</s>
<s>
Advantages	O
over	O
other	O
existing	O
imaging	O
modalities	O
(	O
e.g.	O
,	O
ultrasound	B-Application
and	O
magnetic	O
resonance	O
imaging	O
)	O
is	O
that	O
ballistic	B-Algorithm
imaging	I-Algorithm
can	O
achieve	O
a	O
higher	O
resolution	B-Algorithm
in	O
the	O
order	O
of	O
1	O
to	O
10	O
micro-meters	O
,	O
however	O
it	O
suffers	O
from	O
limited	O
imaging	O
depth	O
.	O
</s>
<s>
Due	O
to	O
the	O
exponential	O
reduction	O
of	O
ballistic	B-Algorithm
photons	I-Algorithm
as	O
thickness	O
of	O
the	O
scattering	O
medium	O
increases	O
,	O
the	O
images	O
often	O
have	O
a	O
low	O
number	O
of	O
photons	B-Application
per	O
pixel	O
,	O
resulting	O
in	O
shot	O
noise	O
.	O
</s>
<s>
Digital	B-Algorithm
image	I-Algorithm
processing	I-Algorithm
and	O
noise	O
reduction	O
are	O
often	O
applied	O
to	O
reduce	O
that	O
noise	O
.	O
</s>
