<s>
Medical	B-Algorithm
optical	I-Algorithm
imaging	I-Algorithm
is	O
the	O
use	O
of	O
light	O
as	O
an	O
investigational	O
imaging	O
technique	O
for	O
medical	O
applications	O
.	O
</s>
<s>
Examples	O
include	O
optical	O
microscopy	O
,	O
spectroscopy	O
,	O
endoscopy	B-Application
,	O
scanning	O
laser	O
ophthalmoscopy	O
,	O
laser	O
Doppler	O
imaging	O
,	O
and	O
optical	B-Application
coherence	I-Application
tomography	I-Application
.	O
</s>
<s>
Because	O
light	O
is	O
an	O
electromagnetic	O
wave	O
,	O
similar	O
phenomena	O
occur	O
in	O
X-rays	B-Library
,	O
microwaves	O
,	O
and	O
radio	O
waves	O
.	O
</s>
<s>
Optical	B-Algorithm
imaging	I-Algorithm
systems	O
may	O
be	O
divided	O
into	O
diffusive	O
and	O
ballistic	B-Algorithm
imaging	I-Algorithm
systems	O
.	O
</s>
<s>
A	O
model	O
for	O
photon	B-Application
migration	O
in	O
turbid	O
biological	O
media	O
has	O
been	O
developed	O
by	O
Bonner	O
et	O
al	O
.	O
</s>
<s>
Diffuse	B-Algorithm
optical	I-Algorithm
imaging	I-Algorithm
(	O
DOI	O
)	O
is	O
a	O
method	O
of	O
imaging	O
using	O
near-infrared	O
spectroscopy	O
(	O
NIRS	O
)	O
or	O
fluorescence-based	O
methods	O
.	O
</s>
<s>
When	O
used	O
to	O
create	O
3D	O
volumetric	O
models	O
of	O
the	O
imaged	O
material	O
DOI	O
is	O
referred	O
to	O
as	O
diffuse	B-Algorithm
optical	I-Algorithm
tomography	I-Algorithm
,	O
whereas	O
2D	O
imaging	O
methods	O
are	O
classified	O
as	O
diffuse	B-Algorithm
optical	I-Algorithm
topography	I-Algorithm
.	O
</s>
<s>
The	O
technique	O
may	O
also	O
be	O
referred	O
to	O
as	O
diffuse	B-Algorithm
optical	I-Algorithm
tomography	I-Algorithm
(	O
DOT	O
)	O
,	O
near	B-Algorithm
infrared	I-Algorithm
optical	I-Algorithm
tomography	I-Algorithm
(	O
NIROT	B-Algorithm
)	O
or	O
fluorescence	O
diffuse	B-Algorithm
optical	I-Algorithm
tomography	I-Algorithm
(	O
FDOT	O
)	O
,	O
depending	O
on	O
the	O
usage	O
.	O
</s>
<s>
In	O
neuroscience	O
,	O
functional	O
measurements	O
made	O
using	O
NIR	O
wavelengths	O
,	O
DOI	O
techniques	O
may	O
classify	O
as	O
functional	B-Algorithm
near	I-Algorithm
infrared	I-Algorithm
spectroscopy	I-Algorithm
(	O
fNIRS	B-Algorithm
)	O
.	O
</s>
<s>
Ballistic	B-Algorithm
photons	I-Algorithm
are	O
the	O
light	O
photons	B-Application
that	O
travel	O
through	O
a	O
scattering	O
(	O
turbid	O
)	O
medium	O
in	O
a	O
straight	O
line	O
.	O
</s>
<s>
Also	O
known	O
as	O
ballistic	B-Algorithm
light	I-Algorithm
.	O
</s>
<s>
If	O
laser	O
pulses	O
are	O
sent	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
randomly	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>
If	O
efficiently	O
detected	O
,	O
there	O
are	O
many	O
applications	O
for	O
ballistic	B-Algorithm
photons	I-Algorithm
especially	O
in	O
coherent	O
high	O
resolution	O
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
of	O
the	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	O
in	O
the	O
order	O
of	O
1	O
to	O
10	O
micro-meters	O
,	O
however	O
it	O
has	O
limited	O
imaging	O
depth	O
.	O
</s>
<s>
Furthermore	O
,	O
more	O
scattered	O
'	O
quasi-ballistic	O
'	O
photons	B-Application
are	O
often	O
measured	O
as	O
well	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>
Due	O
to	O
the	O
exponential	O
reduction	O
(	O
with	O
respect	O
to	O
distance	O
)	O
of	O
ballistic	B-Algorithm
photons	I-Algorithm
in	O
a	O
scattering	O
medium	O
,	O
often	O
image	B-Algorithm
processing	I-Algorithm
techniques	O
are	O
applied	O
to	O
the	O
raw	O
captured	O
ballistic	O
images	O
,	O
to	O
reconstruct	O
high	O
quality	O
ones	O
.	O
</s>
<s>
Ballistic	B-Algorithm
imaging	I-Algorithm
modalities	O
aim	O
to	O
reject	O
non-ballistic	O
photons	O
and	O
retain	O
ballistic	B-Algorithm
photons	I-Algorithm
that	O
carry	O
useful	O
information	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	O
gated	O
imaging	O
,	O
collimation	O
,	O
wavefront	O
propagation	O
,	O
and	O
polarization	O
.	O
</s>
