<s>
Multi-spectral	O
optoacoustic	B-Algorithm
tomography	B-Algorithm
(	O
MSOT	B-Algorithm
)	O
,	O
also	O
known	O
as	O
functional	O
photoacoustic	B-Algorithm
tomography	I-Algorithm
(	O
fPAT	O
)	O
,	O
is	O
an	O
imaging	O
technology	O
that	O
generates	O
high-resolution	O
optical	O
images	O
in	O
scattering	O
media	O
,	O
including	O
biological	O
tissues	O
.	O
</s>
<s>
MSOT	B-Algorithm
illuminates	O
tissue	O
with	O
light	O
of	O
transient	O
energy	O
,	O
typically	O
light	O
pulses	O
lasting	O
1-100	O
nanoseconds	O
.	O
</s>
<s>
The	O
tissue	O
absorbs	O
the	O
light	O
pulses	O
,	O
and	O
as	O
a	O
result	O
undergoes	O
thermo-elastic	O
expansion	O
,	O
a	O
phenomenon	O
known	O
as	O
the	O
optoacoustic	B-Algorithm
or	O
photoacoustic	O
effect	O
.	O
</s>
<s>
This	O
expansion	O
gives	O
rise	O
to	O
ultrasound	B-Application
waves	O
(	O
photoechoes	O
)	O
that	O
are	O
detected	O
and	O
formed	O
into	O
an	O
image	O
.	O
</s>
<s>
acoustic	O
focusing	O
or	O
optical	O
focusing	O
)	O
or	O
computed	O
tomography	B-Algorithm
(	O
mathematical	O
image	O
formation	O
)	O
.	O
</s>
<s>
Unlike	O
other	O
types	O
of	O
optoacoustic	B-Algorithm
imaging	I-Algorithm
,	O
MSOT	B-Algorithm
involves	O
illuminating	O
the	O
sample	O
with	O
multiple	O
wavelengths	O
,	O
allowing	O
it	O
to	O
detect	O
ultrasound	B-Application
waves	O
emitted	O
by	O
different	O
photoabsorbing	O
molecules	O
in	O
the	O
tissue	O
,	O
whether	O
endogenous	O
(	O
oxygenated	O
and	O
deoxygenated	O
hemoglobin	O
,	O
melanin	O
)	O
or	O
exogenous	O
(	O
imaging	O
probes	O
,	O
nanoparticles	O
)	O
.	O
</s>
<s>
Computational	O
techniques	O
such	O
as	O
spectral	O
unmixing	O
deconvolute	O
the	O
ultrasound	B-Application
waves	O
emitted	O
by	O
these	O
different	O
absorbers	O
,	O
allowing	O
each	O
emitter	O
to	O
be	O
visualized	O
separately	O
in	O
the	O
target	O
tissue	O
.	O
</s>
<s>
In	O
this	O
way	O
,	O
MSOT	B-Algorithm
can	O
allow	O
visualization	O
of	O
hemoglobin	O
concentration	O
and	O
tissue	O
oxygenation	O
or	O
hypoxia	O
.	O
</s>
<s>
Unlike	O
other	O
optical	B-Algorithm
imaging	I-Algorithm
methods	O
,	O
MSOT	B-Algorithm
is	O
unaffected	O
by	O
photon	O
scattering	O
and	O
thus	O
can	O
provide	O
high-resolution	O
optical	O
images	O
deep	O
inside	O
biological	O
tissues	O
.	O
</s>
<s>
MSOT	B-Algorithm
has	O
been	O
described	O
as	O
a	O
6-dimensional	O
(	O
6-parametric	O
)	O
method	O
,	O
in	O
which	O
the	O
three	O
geometrical	O
dimensions	O
(	O
x	O
,	O
y	O
,	O
z	O
)	O
are	O
complemented	O
by	O
time	O
,	O
illumination	O
wavelengths	O
and	O
band	O
of	O
ultrasound	B-Application
frequencies	O
detected	O
.	O
</s>
<s>
MSOT	B-Algorithm
can	O
measure	O
over	O
time	O
,	O
allowing	O
longitudinal	O
studies	O
of	O
dynamic	O
processes	O
.	O
</s>
<s>
Illumination	O
wavelengths	O
in	O
MSOT	B-Algorithm
can	O
cover	O
the	O
entire	O
spectrum	O
from	O
ultraviolet	O
(	O
UV	O
)	O
to	O
infrared	O
(	O
IR	O
)	O
.	O
</s>
<s>
The	O
choice	O
of	O
ultrasound	B-Application
frequency	O
band	O
defines	O
resolution	O
and	O
overall	O
size	O
range	O
of	O
the	O
objects	O
that	O
can	O
be	O
resolved	O
.	O
</s>
<s>
 [ 6 ] 	O
Microscopic	O
resolution	O
is	O
also	O
possible	O
using	O
multi-spectral	O
optoacoustics	B-Algorithm
.	O
</s>
<s>
MSOT	B-Algorithm
has	O
now	O
been	O
used	O
in	O
a	O
broad	O
range	O
of	O
biological	O
applications	O
,	O
including	O
cardiovascular	O
disease	O
research	O
,	O
neuroimaging	B-Algorithm
and	O
cancer	O
research	O
.	O
</s>
<s>
The	O
development	O
of	O
real-time	O
hand-held	O
imaging	O
systems	O
has	O
enabled	O
clinical	O
use	O
of	O
MSOT	B-Algorithm
for	O
imaging	O
the	O
breast	O
,	O
vasculature	O
,	O
lymph	O
nodes	O
and	O
skin	O
.	O
</s>
<s>
MSOT	B-Algorithm
collects	O
images	O
at	O
multiple	O
wavelengths	O
and	O
resolves	O
the	O
spectral	O
signatures	O
in	O
each	O
voxel	B-Algorithm
imaged	O
,	O
making	O
it	O
a	O
multi-spectral	O
method	O
.	O
</s>
<s>
Typically	O
,	O
MSOT	B-Algorithm
is	O
used	O
to	O
generate	O
three	O
images	O
:	O
one	O
anatomical	O
image	O
at	O
a	O
single	O
wavelength	O
,	O
one	O
functional	O
image	O
resolving	O
oxy	O
-	O
and	O
deoxy-hemoglobin	O
concentrations	O
,	O
and	O
a	O
third	O
image	O
resolving	O
additional	O
target	O
photoabsorber(s )	O
.	O
</s>
<s>
Optoacoustic	B-Algorithm
.	O
</s>
<s>
This	O
term	O
denotes	O
the	O
combination	O
of	O
optical	O
(	O
Greek	O
,	O
oπτικός	O
)	O
and	O
acoustic	O
(	O
Greek	O
,	O
ακουστικός	O
)	O
energy	O
(	O
or	O
components	O
)	O
in	O
a	O
single	O
modality	O
,	O
which	O
distinguishes	O
optoacoustic	B-Algorithm
imaging	I-Algorithm
from	O
optical	B-Algorithm
imaging	I-Algorithm
.	O
</s>
<s>
Photoecho	O
and	O
photoacoustic	O
are	O
to	O
optoacoustics	B-Algorithm
what	O
photon	O
is	O
to	O
optics	O
:	O
optical	O
methods	O
rely	O
on	O
photons	O
,	O
whereas	O
optoacoustic	B-Algorithm
methods	O
rely	O
on	O
photoechoes	O
or	O
photoacoustic	O
responses	O
.	O
</s>
<s>
Tomography	B-Algorithm
.	O
</s>
<s>
This	O
process	O
is	O
analogous	O
to	O
x-ray	O
computed	O
tomography	B-Algorithm
,	O
except	O
that	O
tomographic	B-Algorithm
mathematical	O
models	O
describe	O
light	O
and	O
sound	O
propagation	O
in	O
tissues	O
.	O
</s>
<s>
1	O
:	O
Operational	O
capabilities	O
of	O
MSOT	B-Algorithm
.	O
</s>
<s>
Hybrid	O
image	O
showing	O
an	O
optical	O
micrograph	O
of	O
part	O
of	O
a	O
mouse	O
kidney	O
cross-section	O
(	O
gray	O
)	O
,	O
overlaid	O
with	O
the	O
distribution	O
of	O
an	O
exogenous	O
fluorescent	O
agent	O
imaged	O
using	O
MSOT	B-Algorithm
(	O
right	O
)	O
.	O
</s>
<s>
Distribution	O
of	O
oxy-hemoglobin	O
(	O
red	O
)	O
and	O
deoxy-hemoglobin	O
(	O
blue	O
)	O
in	O
the	O
tumor	O
,	O
imaged	O
using	O
MSOT	B-Algorithm
(	O
left	O
)	O
.	O
</s>
<s>
MSOT	B-Algorithm
detects	O
photoechoes	O
,	O
i.e.	O
</s>
<s>
ultrasound	B-Application
waves	O
generated	O
by	O
thermo-elastic	O
expansion	O
of	O
a	O
sample	O
(	O
e.g.	O
</s>
<s>
This	O
creates	O
a	O
pressure	O
wave	O
in	O
the	O
form	O
of	O
a	O
broadband	O
ultrasound	B-Application
wave	O
.	O
</s>
<s>
The	O
ultrasound	B-Application
waves	O
emitted	O
by	O
the	O
sample	O
are	O
detected	O
by	O
transducers	O
positioned	O
near	O
the	O
sample	O
,	O
usually	O
at	O
multiple	O
positions	O
around	O
it	O
.	O
</s>
<s>
The	O
amplitude	O
of	O
the	O
pressure	O
wave	O
provides	O
information	O
about	O
the	O
local	O
absorption	O
and	O
propagation	O
of	O
energy	O
in	O
the	O
sample	O
,	O
while	O
the	O
time	O
interval	O
between	O
the	O
illumination	O
pulse	O
and	O
arrival	O
of	O
the	O
ultrasound	B-Application
wave	O
at	O
the	O
detector	O
provides	O
information	O
about	O
the	O
distance	O
between	O
the	O
detector	O
and	O
photoecho	O
source	O
.	O
</s>
<s>
Optoacoustic	B-Algorithm
data	O
collected	O
over	O
time	O
and	O
at	O
multiple	O
positions	O
around	O
the	O
sample	O
are	O
processed	O
using	O
tomographic	B-Algorithm
reconstruction	I-Algorithm
to	O
produce	O
images	O
of	O
the	O
distribution	O
of	O
photoabsorbers	O
in	O
the	O
sample	O
.	O
</s>
<s>
The	O
wavelengths	O
of	O
light	O
used	O
to	O
illuminate	O
samples	O
in	O
MSOT	B-Algorithm
are	O
selected	O
based	O
on	O
the	O
absorption	O
characteristics	O
of	O
the	O
target	O
photoabsorbers	O
.	O
</s>
<s>
MSOT	B-Algorithm
has	O
the	O
potential	O
to	O
provide	O
multi-parametric	O
information	O
involving	O
the	O
three	O
spatial	O
dimensions	O
(	O
x	O
,	O
y	O
,	O
z	O
)	O
,	O
time	O
,	O
optical	O
wavelength	O
spectrum	O
and	O
ultrasound	B-Application
frequency	O
range	O
.	O
</s>
<s>
This	O
dimensionality	O
has	O
been	O
made	O
possible	O
by	O
key	O
advances	O
in	O
laser	O
source	O
and	O
detector	O
technology	O
,	O
computed	O
tomography	B-Algorithm
and	O
unmixing	O
techniques	O
.	O
</s>
<s>
The	O
capabilities	O
and	O
challenges	O
of	O
each	O
MSOT	B-Algorithm
dimension	O
are	O
described	O
below	O
.	O
</s>
<s>
Optoacoustic	B-Algorithm
(	O
photoacoustic	O
)	O
imaging	O
is	O
intrinsically	O
a	O
three-dimensional	O
imaging	O
method	O
,	O
since	O
photoechoes	O
(	O
optoacoustic	B-Algorithm
waves	O
)	O
propagate	O
in	O
all	O
three	O
spatial	O
dimensions	O
.	O
</s>
<s>
Optimal	O
tomographic	B-Algorithm
imaging	I-Algorithm
is	O
therefore	O
achieved	O
by	O
recording	O
time-resolved	O
pressure	O
waves	O
along	O
a	O
closed	O
surface	O
volumetrically	O
surrounding	O
the	O
target	O
tissue	O
.	O
</s>
<s>
Typically	O
,	O
three-dimensional	O
imaging	O
systems	O
achieve	O
this	O
by	O
scanning	O
a	O
single	O
ultrasound	B-Application
sensor	O
around	O
the	O
sample	O
,	O
or	O
by	O
using	O
one-dimensional	O
or	O
two-dimensional	O
ultrasound	B-Application
sensor	O
arrays	O
to	O
parallelize	O
detection	O
.	O
</s>
<s>
To	O
reduce	O
these	O
requirements	O
,	O
the	O
three-dimensional	O
problem	O
is	O
often	O
simplified	O
to	O
a	O
quasi-two-dimensional	O
problem	O
by	O
using	O
focused	O
ultrasound	B-Application
detectors	O
to	O
limit	O
ultrasound	B-Application
detection	O
to	O
a	O
two-dimensional	O
plane	O
in	O
the	O
illuminated	O
volume	O
.	O
</s>
<s>
2	O
:	O
Volumetric	O
optoacoustic	B-Algorithm
imaging	I-Algorithm
and	O
comparison	O
with	O
reflection-mode	O
ultrasound	B-Application
computed	O
tomography	B-Algorithm
.	O
</s>
<s>
Cross-sectional	O
tomographic	B-Algorithm
ultrasound	B-Application
(	O
right	O
)	O
and	O
optoacoustic	B-Algorithm
(	O
middle	O
)	O
whole-body	O
image	O
stacks	O
of	O
a	O
living	O
mouse	O
.	O
</s>
<s>
Mercep	O
,	O
N.C.	O
Burton	O
,	O
J	O
.	O
Claussen	O
,	O
D	O
.	O
Razansky	O
,	O
«	O
Whole-body	O
live	O
mouse	O
imaging	O
by	O
hybrid	O
reflection-mode	O
ultrasound	B-Application
and	O
optoacoustic	B-Algorithm
tomography»	O
,	O
Opt	O
Lett	O
.	O
</s>
<s>
Early	O
optoacoustic	B-Algorithm
imaging	I-Algorithm
involved	O
scanning	O
a	O
single	O
ultrasound	B-Application
detector	O
along	O
one	O
or	O
two	O
dimensions	O
,	O
resulting	O
in	O
acquisition	O
times	O
of	O
several	O
seconds	O
,	O
minutes	O
or	O
longer	O
.	O
</s>
<s>
Technological	O
advances	O
in	O
detector	O
arrays	O
and	O
analog-to-digital	O
converters	O
allow	O
simultaneous	O
data	O
collection	O
over	O
512	O
parallel	O
elements	O
,	O
substantially	O
shortening	O
the	O
amount	O
of	O
time	O
needed	O
to	O
acquire	O
a	O
tomographic	B-Algorithm
dataset	O
,	O
even	O
to	O
the	O
point	O
of	O
allowing	O
video-rate	O
imaging	O
.	O
</s>
<s>
In	O
addition	O
,	O
lasers	O
have	O
been	O
developed	O
that	O
allow	O
switching	O
between	O
wavelengths	O
within	O
20	O
ms	O
,	O
enabling	O
video-rate	O
MSOT	B-Algorithm
.	O
</s>
<s>
(	O
b	O
)	O
Maximal-intensity	O
projections	O
along	O
the	O
axial	O
direction	O
following	O
single-wavelength	O
illumination	O
before	O
(	O
upper	O
)	O
and	O
after	O
injection	O
of	O
two	O
concentrations	O
of	O
contrast	B-Algorithm
agent	I-Algorithm
(	O
10nmol	O
in	O
the	O
middle	O
and	O
50	O
nmol	O
lower	O
)	O
,	O
indocyanine	O
green	O
.	O
</s>
<s>
Inflow	O
of	O
the	O
contrast	B-Algorithm
agent	I-Algorithm
can	O
be	O
followed	O
in	O
real	O
time	O
.	O
</s>
<s>
A	O
key	O
strength	O
of	O
MSOT	B-Algorithm
is	O
its	O
ability	O
to	O
resolve	O
the	O
photoechoes	O
obtained	O
in	O
response	O
to	O
excitation	O
with	O
different	O
wavelengths	O
of	O
illuminating	O
light	O
.	O
</s>
<s>
Since	O
the	O
photoechoes	O
depend	O
on	O
the	O
optical	O
absorption	O
characteristics	O
of	O
molecules	O
within	O
the	O
target	O
tissue	O
(	O
or	O
added	O
to	O
the	O
tissue	O
)	O
,	O
MSOT	B-Algorithm
can	O
image	O
the	O
distributions	O
of	O
specific	O
photoabsorbing	O
molecules	O
.	O
</s>
<s>
Several	O
exogenous	O
contrast	B-Algorithm
agents	I-Algorithm
have	O
also	O
been	O
used	O
in	O
MSOT	B-Algorithm
,	O
including	O
some	O
common	O
histology	O
dyes	O
,	O
fluorescent	O
dyes	O
,	O
novel	O
metal-based	O
agents	O
and	O
non-metallic	O
nanoparticles	O
.	O
</s>
<s>
Transfecting	O
target	O
tissue	O
with	O
reporter	O
genes	O
to	O
express	O
contrast	B-Algorithm
agents	I-Algorithm
in	O
situ	O
has	O
also	O
been	O
reported	O
,	O
such	O
as	O
transfection	O
with	O
the	O
tyrosinase	O
gene	O
to	O
produce	O
melanin	O
.	O
</s>
<s>
Through	O
spectral	O
unmixing	O
and	O
other	O
techniques	O
,	O
MSOT	B-Algorithm
data	O
can	O
be	O
used	O
to	O
generate	O
separate	O
images	O
based	O
on	O
the	O
contrast	O
provided	O
by	O
different	O
photoabsorbers	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
a	O
single	O
MSOT	B-Algorithm
data	O
collection	O
run	O
provides	O
separate	O
images	O
showing	O
the	O
distribution	O
of	O
oxy	O
-	O
or	O
deoxy-hemoglobin	O
.	O
</s>
<s>
By	O
using	O
hemoglobin	O
as	O
an	O
intrinsic	O
oxygen	O
sensor	O
,	O
MSOT	B-Algorithm
is	O
the	O
only	O
method	O
available	O
that	O
can	O
provide	O
high-resolution	O
images	O
of	O
tissue	O
oxygenation	O
without	O
the	O
need	O
for	O
exogenous	O
labels	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
MSOT	B-Algorithm
can	O
image	O
additional	O
endogenous	O
photoabsorbers	O
such	O
as	O
lipids	O
and	O
water	O
,	O
as	O
well	O
as	O
exogenous	O
contrast	B-Algorithm
agents	I-Algorithm
.	O
</s>
<s>
Ultimately	O
,	O
though	O
,	O
the	O
frequencies	O
that	O
can	O
be	O
collected	O
and	O
processed	O
for	O
image	O
reconstruction	O
are	O
determined	O
by	O
the	O
ultrasound	B-Application
detector	O
.	O
</s>
<s>
Macroscopic	O
MSOT	B-Algorithm
typically	O
uses	O
detectors	O
operating	O
in	O
the	O
frequency	O
range	O
from	O
0.1	O
to	O
10MHz	O
,	O
allowing	O
imaging	O
depths	O
of	O
approximately	O
1	O
–	O
5cm	O
and	O
resolution	O
of	O
0.1	O
–	O
1mm	O
.	O
</s>
<s>
Images	O
are	O
then	O
generated	O
using	O
computed	O
tomography	B-Algorithm
.	O
</s>
<s>
Ultrasound	B-Application
detectors	O
have	O
been	O
developed	O
that	O
collect	O
bandwidths	O
of	O
10-200MHz	O
or	O
wider	O
,	O
which	O
allows	O
unprecedented	O
mesoscopy	O
at	O
tissue	O
depths	O
of	O
0.1	O
–	O
1cm	O
with	O
resolution	O
that	O
can	O
exceed	O
10	O
microns	O
even	O
at	O
depths	O
of	O
several	O
millimeters	O
.	O
</s>
<s>
Images	O
are	O
generated	O
using	O
computed	O
tomography	B-Algorithm
.	O
</s>
<s>
The	O
possibility	O
of	O
applying	O
optoacoustics	B-Algorithm
to	O
the	O
microscopic	O
regime	O
has	O
been	O
suggested	O
.	O
</s>
<s>
The	O
imaging	O
depth	O
(	O
typically	O
<	O
1mm	O
)	O
and	O
quality	O
of	O
the	O
resulting	O
image	O
are	O
limited	O
by	O
optical	O
diffraction	O
and	O
scattering	O
,	O
not	O
by	O
ultrasound	B-Application
diffraction	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
optoacoustic	B-Algorithm
microscopy	O
has	O
the	O
same	O
limitations	O
as	O
conventional	O
optical	O
microscopy	O
.	O
</s>
<s>
MSOT	B-Algorithm
can	O
operate	O
in	O
three	O
imaging	O
modes	O
:	O
</s>
<s>
functional/dynamic	B-Algorithm
imaging	I-Algorithm
,	O
which	O
can	O
be	O
performed	O
at	O
video	O
rates	O
to	O
reveal	O
changes	O
in	O
tissue	O
contrast	O
caused	O
by	O
hemodynamics	O
,	O
motion	O
such	O
as	O
vessel	O
pulsation	O
,	O
and	O
uptake	O
of	O
drugs	O
(	O
pharmacodynamics	O
)	O
.	O
</s>
<s>
biological	O
/	O
molecular	B-Algorithm
imaging	I-Algorithm
,	O
which	O
exploits	O
the	O
multi-spectral	O
capability	O
of	O
MSOT	B-Algorithm
.	O
</s>
<s>
MSOT	B-Algorithm
provides	O
anatomical	O
,	O
dynamic	O
and	O
molecular	O
information	O
,	O
but	O
quantifying	O
the	O
features	O
of	O
MSOT	B-Algorithm
images	O
is	O
not	O
straightforward	O
because	O
constituents	O
of	O
the	O
target	O
tissue	O
absorb	O
and	O
scatter	O
the	O
illuminating	O
light	O
.	O
</s>
<s>
Moreover	O
,	O
optoacoustic	B-Algorithm
imaging	I-Algorithm
,	O
with	O
a	O
resolution	O
in	O
the	O
range	O
of	O
1-100μm	O
,	O
cannot	O
resolve	O
individual	O
photoabsorbing	O
molecules	O
.	O
</s>
<s>
Recently	O
,	O
eigenspectra	O
MSOT	B-Algorithm
has	O
been	O
developed	O
to	O
model	O
more	O
accurately	O
the	O
spectral	O
responses	O
of	O
different	O
photoabsorbers	O
in	O
three-dimensional	O
tissue	O
.	O
</s>
<s>
MSOT	B-Algorithm
can	O
resolve	O
various	O
optoacoustic	B-Algorithm
moieties	O
based	O
on	O
their	O
absorption	O
spectrum	O
,	O
including	O
nanoparticles	O
,	O
dyes	O
and	O
fluorochromes	O
.	O
</s>
<s>
Most	O
fluorochromes	O
are	O
optimized	O
for	O
fluorescence	O
emission	O
and	O
are	O
sub-optimal	O
for	O
optoacoustic	B-Algorithm
detection	O
,	O
because	O
after	O
absorbing	O
the	O
illuminating	O
energy	O
,	O
they	O
tend	O
to	O
emit	O
fluorescence	O
rather	O
than	O
convert	O
it	O
to	O
heat	O
and	O
generate	O
a	O
photoecho	O
.	O
</s>
<s>
Dyes	O
with	O
higher	O
absorption	O
cross-sections	O
generate	O
stronger	O
optoacoustic	B-Algorithm
signals	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
sensitivity	O
of	O
MSOT	B-Algorithm
depends	O
on	O
the	O
contrast	B-Algorithm
agent	I-Algorithm
used	O
,	O
its	O
distribution	O
and	O
accumulation	O
in	O
the	O
target	O
tissue	O
,	O
and	O
its	O
resistance	O
to	O
photobleaching	O
by	O
the	O
illuminating	O
light	O
.	O
</s>
<s>
Sensitivity	O
also	O
depends	O
on	O
the	O
ultrasound	B-Application
detector	O
employed	O
,	O
the	O
amount	O
of	O
light	O
energy	O
applied	O
,	O
the	O
voxel	B-Algorithm
size	O
and	O
spectral	O
unmixing	O
method	O
.	O
</s>
<s>
As	O
imaging	O
depth	O
increases	O
,	O
light	O
and	O
ultrasound	B-Application
attenuation	O
together	O
reduce	O
the	O
optoacoustic	B-Algorithm
signal	O
and	O
therefore	O
the	O
overall	O
detection	O
sensitivity	O
.	O
</s>
<s>
Ultrasound	B-Application
attenuation	O
is	O
frequency-dependent	O
:	O
higher	O
frequencies	O
are	O
attenuated	O
faster	O
with	O
increasing	O
depth	O
.	O
</s>
<s>
Selecting	O
ultrasound	B-Application
detectors	O
that	O
are	O
most	O
sensitive	O
at	O
the	O
appropriate	O
frequency	O
can	O
improve	O
sensitivity	O
at	O
the	O
target	O
imaging	O
depth	O
,	O
but	O
at	O
the	O
cost	O
of	O
spatial	O
resolution	O
.	O
</s>
<s>
Earlier	O
calculations	O
predicted	O
that	O
MSOT	B-Algorithm
should	O
be	O
able	O
to	O
detect	O
concentrations	O
of	O
organic	O
fluorochromes	O
as	O
low	O
as	O
5	O
nM	O
.	O
</s>
<s>
These	O
calculations	O
did	O
not	O
properly	O
account	O
for	O
frequency-dependent	O
attenuation	O
of	O
ultrasound	B-Application
in	O
tissue	O
or	O
for	O
the	O
requirements	O
of	O
spectral	O
unmixing	O
.	O
</s>
<s>
Advanced	O
spectral	O
unmixing	O
methods	O
based	O
on	O
statistical	O
detection	O
schemes	O
can	O
improve	O
MSOT	B-Algorithm
sensitivity	O
.	O
</s>
<s>
Optoacoustic	B-Algorithm
imaging	I-Algorithm
in	O
general	O
,	O
and	O
MSOT	B-Algorithm
in	O
particular	O
,	O
have	O
been	O
applied	O
to	O
various	O
analyses	O
of	O
animal	O
models	O
,	O
including	O
imaging	O
of	O
organs	O
,	O
pathology	O
,	O
functional	O
processes	O
and	O
bio-distribution	O
.	O
</s>
<s>
This	O
range	O
of	O
applications	O
demonstrates	O
the	O
flexibility	O
of	O
MSOT	B-Algorithm
,	O
which	O
reflects	O
the	O
range	O
of	O
contrast	B-Algorithm
agents	I-Algorithm
available	O
.	O
</s>
<s>
Practically	O
every	O
molecule	O
that	O
absorbs	O
light	O
and	O
converts	O
it	O
to	O
a	O
pressure	O
wave	O
has	O
the	O
potential	O
to	O
be	O
detected	O
with	O
optoacoustics	B-Algorithm
.	O
</s>
<s>
Contrast	B-Algorithm
agents	I-Algorithm
absorbing	O
light	O
in	O
the	O
near-IR	O
are	O
particularly	O
attractive	O
,	O
because	O
they	O
enable	O
imaging	O
at	O
greater	O
depth	O
.	O
</s>
<s>
Hemoglobin	O
is	O
the	O
dominant	O
absorber	O
of	O
light	O
in	O
the	O
visible	O
and	O
near-IR	O
part	O
of	O
the	O
optical	O
spectrum	O
and	O
is	O
commonly	O
used	O
for	O
optoacoustic	B-Algorithm
imaging	I-Algorithm
.	O
</s>
<s>
Using	O
MSOT	B-Algorithm
further	O
allows	O
the	O
distinction	O
between	O
oxygenation	O
states	O
of	O
hemoglobin	O
,	O
enabling	O
label-free	O
assessment	O
of	O
tissue	O
oxygenation	O
and	O
hypoxia	O
,	O
both	O
of	O
which	O
are	O
useful	O
parameters	O
in	O
many	O
pathologies	O
and	O
functional	O
studies	O
.	O
</s>
<s>
Optoacoustic	B-Algorithm
imaging	I-Algorithm
of	O
melanin	O
has	O
been	O
used	O
to	O
assess	O
the	O
depth	O
of	O
melanoma	O
ingrowth	O
inside	O
epithelial	O
tissue	O
and	O
to	O
assess	O
the	O
metastatic	O
status	O
of	O
sentinel	O
lymph	O
nodes	O
in	O
melanoma	O
patients	O
.	O
</s>
<s>
MSOT	B-Algorithm
can	O
detect	O
several	O
other	O
endogenous	O
tissue	O
absorbers	O
,	O
as	O
long	O
as	O
the	O
correct	O
wavelength	O
range	O
is	O
used	O
to	O
illuminate	O
the	O
sample	O
.	O
</s>
<s>
A	O
multitude	O
of	O
exogenous	O
contrast	B-Algorithm
agents	I-Algorithm
have	O
been	O
developed	O
,	O
or	O
are	O
under	O
development	O
,	O
for	O
optoacoustics	B-Algorithm
.	O
</s>
<s>
These	O
contrast	B-Algorithm
agents	I-Algorithm
should	O
have	O
an	O
absorption	O
spectrum	O
different	O
from	O
that	O
of	O
endogenous	O
tissue	O
absorbers	O
,	O
so	O
that	O
they	O
can	O
be	O
separated	O
from	O
other	O
background	O
absorbers	O
using	O
spectral	O
unmixing	O
.	O
</s>
<s>
Different	O
classes	O
of	O
exogenous	O
contrast	B-Algorithm
agents	I-Algorithm
exist	O
.	O
</s>
<s>
Since	O
these	O
dyes	O
can	O
be	O
imaged	O
based	O
on	O
optoacoustics	B-Algorithm
and	O
fluorescence	O
,	O
the	O
two	O
types	O
of	O
microscopies	O
can	O
be	O
used	O
to	O
complement	O
and	O
verify	O
each	O
other	O
.	O
</s>
<s>
Photosensitizers	O
,	O
already	O
in	O
clinical	O
use	O
for	O
photodynamic	O
therapy	O
,	O
can	O
be	O
detected	O
using	O
MSOT	B-Algorithm
,	O
allowing	O
analysis	O
of	O
their	O
pharmacokinetics	O
and	O
bio-distribution	O
in	O
vivo	O
.	O
</s>
<s>
Gold	O
nanoparticles	O
,	O
silver	O
nanoparticles	O
,	O
carbon	O
nanotubes	O
,	O
and	O
iron-oxide	O
particles	O
have	O
been	O
used	O
for	O
optoacoustic	B-Algorithm
imaging	I-Algorithm
in	O
animals	O
.	O
</s>
<s>
Gold	O
nanoparticles	O
generate	O
strong	O
optoacoustic	B-Algorithm
signals	O
due	O
to	O
plasmon	O
resonance	O
,	O
and	O
their	O
absorption	O
spectrum	O
can	O
be	O
tuned	O
by	O
modifying	O
their	O
shape	O
.	O
</s>
<s>
Some	O
iron	O
oxide	O
nanoparticles	O
,	O
such	O
as	O
SPIO	O
,	O
have	O
already	O
been	O
approved	O
for	O
the	O
clinic	O
as	O
MRI	O
contrast	B-Algorithm
agents	I-Algorithm
.	O
</s>
<s>
Targeted	B-Algorithm
contrast	I-Algorithm
agents	I-Algorithm
combine	O
a	O
dye	O
or	O
nanoparticle	O
with	O
a	O
targeting	O
ligand	O
to	O
provide	O
MSOT	B-Algorithm
contrast	O
at	O
specific	O
tissues	O
or	O
in	O
the	O
presence	O
of	O
specific	O
cellular	O
or	O
molecular	O
processes	O
.	O
</s>
<s>
Such	O
agents	O
have	O
been	O
used	O
in	O
MSOT	B-Algorithm
imaging	O
of	O
integrins	O
within	O
tumors	O
in	O
animals	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
contrast	B-Algorithm
agent	I-Algorithm
activatable	O
by	O
matrix	O
metalloproteinase	O
(	O
MMP	O
)	O
cleavage	O
has	O
been	O
used	O
to	O
image	O
MMP	O
activity	O
within	O
thyroid	O
tumors	O
in	O
mice	O
.	O
</s>
<s>
Fluorescent	O
proteins	O
that	O
are	O
already	O
widespread	O
,	O
powerful	O
tools	O
for	O
biomedical	O
research	O
,	O
such	O
as	O
green	O
fluorescent	O
protein	O
,	O
can	O
also	O
be	O
visualized	O
using	O
MSOT	B-Algorithm
.	O
</s>
<s>
MSOT	B-Algorithm
based	O
on	O
in	O
situ	O
expression	O
of	O
fluorescent	O
proteins	O
can	O
take	O
advantage	O
of	O
tissue	O
-	O
and	O
development-specific	O
promoters	O
,	O
allowing	O
imaging	O
of	O
specific	O
parts	O
of	O
an	O
organism	O
at	O
specific	O
stages	O
of	O
development	O
.	O
</s>
<s>
This	O
transgenic	O
approach	O
is	O
not	O
limited	O
to	O
fluorescent	O
proteins	O
:	O
infecting	O
tissue	O
with	O
a	O
vaccinia	O
virus	O
carrying	O
the	O
tyrosinase	O
gene	O
allows	O
in	O
situ	O
production	O
of	O
melanin	O
,	O
which	O
generates	O
strong	O
optoacoustic	B-Algorithm
signal	O
for	O
MSOT	B-Algorithm
.	O
</s>
<s>
Because	O
of	O
its	O
ability	O
to	O
provide	O
spatial	O
and	O
spectral	O
resolution	O
in	O
real	O
time	O
on	O
multiple	O
scales	O
,	O
optoacoustic	B-Algorithm
imaging	I-Algorithm
in	O
general	O
and	O
MSOT	B-Algorithm
in	O
particular	O
are	O
likely	O
to	O
play	O
an	O
important	O
role	O
in	O
clinical	O
imaging	O
and	O
management	O
of	O
cancer	O
,	O
cardiovascular	O
disease	O
and	O
inflammation	O
.	O
</s>
<s>
MSOT	B-Algorithm
presents	O
numerous	O
advantages	O
over	O
other	O
radiology	O
modalities	O
because	O
of	O
its	O
ability	O
to	O
resolve	O
oxygenated	O
and	O
deoxygenated	O
hemoglobin	O
,	O
its	O
compatibility	O
with	O
a	O
broad	O
array	O
of	O
exogenous	O
contrast	B-Algorithm
agents	I-Algorithm
,	O
its	O
scalability	O
and	O
its	O
ability	O
to	O
image	O
rapidly	O
even	O
below	O
the	O
tissue	O
surface	O
.	O
</s>
<s>
MSOT	B-Algorithm
can	O
track	O
the	O
fate	O
of	O
administered	O
agents	O
in	O
blood	O
circulation	O
,	O
allowing	O
real-time	O
,	O
in	O
vivo	O
analysis	O
of	O
pharmacokinetics	O
.	O
</s>
<s>
Several	O
optoacoustic	B-Algorithm
studies	O
have	O
aimed	O
to	O
improve	O
on	O
the	O
poor	O
sensitivity	O
of	O
X-ray	O
mammography	O
in	O
dense	O
breast	O
tissue	O
and	O
the	O
low	O
specificity	O
of	O
ultrasound	B-Application
imaging	O
.	O
</s>
<s>
MSOT	B-Algorithm
may	O
miss	O
fewer	O
malignancies	O
in	O
dense	O
breast	O
tissue	O
than	O
these	O
conventional	O
modalities	O
because	O
optoacoustic	B-Algorithm
contrast	O
is	O
unaffected	O
by	O
breast	O
density	O
.	O
</s>
<s>
MSOT	B-Algorithm
studies	O
of	O
breast	O
cancer	O
typically	O
focus	O
on	O
detecting	O
the	O
increased	O
vascular	O
density	O
and	O
correspondingly	O
high	O
hemoglobin	O
concentration	O
thought	O
to	O
occur	O
in	O
and	O
around	O
tumors	O
.	O
</s>
<s>
The	O
flexibility	O
of	O
MSOT	B-Algorithm
may	O
also	O
allow	O
imaging	O
of	O
other	O
tissue	O
and	O
cancer	O
biomarkers	O
not	O
detectable	O
with	O
current	O
methods	O
.	O
</s>
<s>
The	O
hemoglobin	O
distribution	O
in	O
carotid	O
arteries	O
of	O
healthy	O
humans	O
has	O
recently	O
been	O
imaged	O
in	O
real	O
time	O
using	O
a	O
hand-held	O
device	O
similar	O
to	O
diagnostic	O
ultrasound	B-Application
systems	O
currently	O
in	O
the	O
clinic	O
.	O
</s>
<s>
4	O
:	O
MSOT	B-Algorithm
of	O
human	O
vasculature	O
.	O
</s>
<s>
The	O
handheld	O
MSOT	B-Algorithm
probe	O
shown	O
here	O
to	O
measure	O
photoechoes	O
from	O
hemoglobin	O
,	O
allows	O
more	O
sensitive	O
detection	O
of	O
small	O
blood	O
vessels	O
than	O
Doppler	O
ultrasound	B-Application
already	O
in	O
the	O
clinic	O
.	O
</s>
<s>
Optoacoustic	B-Algorithm
mesoscopy	O
is	O
suitable	O
for	O
imaging	O
skin	O
lesions	O
.	O
</s>
<s>
Combining	O
optoacoustic	B-Algorithm
mesoscopy	O
with	O
exogenous	O
agents	O
may	O
provide	O
further	O
useful	O
information	O
.	O
</s>
<s>
Light	O
delivery	O
and	O
ultrasound	B-Application
detection	O
can	O
be	O
miniaturized	O
to	O
create	O
optoacoustic	B-Algorithm
endoscopy	O
systems	O
for	O
gastrointestinal	O
applications	O
.	O
</s>
<s>
A	O
system	O
combining	O
MSOT	B-Algorithm
and	O
ultrasound	B-Application
endoscopy	O
has	O
been	O
used	O
to	O
image	O
the	O
esophagus	O
and	O
colon	O
in	O
rats	O
and	O
rabbits	O
.	O
</s>
<s>
The	O
MSOT	B-Algorithm
images	O
revealed	O
vascular	O
features	O
and	O
hemoglobin	O
oxygenation	O
that	O
was	O
not	O
detectable	O
by	O
ultrasound	B-Application
.	O
</s>
<s>
Moreover	O
,	O
optoacoustic	B-Algorithm
endoscopy	O
can	O
detect	O
the	O
exogenous	O
dye	O
Evans	O
blue	O
after	O
injection	O
into	O
the	O
lymphatic	O
system	O
.	O
</s>
<s>
Ongoing	O
technological	O
progress	O
is	O
expected	O
to	O
allow	O
optoacoustic	B-Algorithm
imaging	I-Algorithm
of	O
the	O
gastrointestinal	O
tract	O
in	O
humans	O
in	O
the	O
near	O
future	O
,	O
which	O
may	O
allow	O
three-dimensional	O
analysis	O
of	O
suspicious	O
lesions	O
,	O
providing	O
more	O
complete	O
information	O
than	O
white	O
light	O
endoscopy	O
.	O
</s>
<s>
Miniaturized	O
optoacoustic	B-Algorithm
devices	O
are	O
also	O
expected	O
to	O
offer	O
interesting	O
possibilities	O
for	O
intravascular	O
imaging	O
[72-74],	O
improving	O
our	O
ability	O
to	O
detect	O
atherosclerosis	O
and	O
stent-related	O
biomarkers	O
.	O
</s>
<s>
Optoacoustic	B-Algorithm
imaging	I-Algorithm
is	O
likely	O
to	O
be	O
well	O
suited	O
to	O
this	O
application	O
,	O
since	O
it	O
can	O
detect	O
lipids	O
,	O
neovasculature	O
,	O
hemoglobin	O
oxygenation	O
and	O
contrast	B-Algorithm
agents	I-Algorithm
that	O
mark	O
inflammation	O
.	O
</s>
<s>
MSOT	B-Algorithm
may	O
provide	O
a	O
non-radioactive	O
,	O
non-invasive	O
alternative	O
for	O
examination	O
of	O
the	O
metastatic	O
status	O
of	O
the	O
sentinel	O
lymph	O
node	O
.	O
</s>
<s>
Initial	O
studies	O
have	O
shown	O
that	O
MSOT	B-Algorithm
can	O
detect	O
sentinel	O
lymph	O
nodes	O
based	O
on	O
indocyanine	O
green	O
(	O
ICG	O
)	O
accumulation	O
after	O
injection	O
in	O
the	O
tumor	O
,	O
as	O
well	O
as	O
melanoma	O
metastasis	O
inside	O
the	O
lymph	O
nodes	O
.	O
</s>
<s>
5	O
:	O
MSOT	B-Algorithm
for	O
determination	O
of	O
the	O
metastatic	O
status	O
of	O
sentinel	O
lymph	O
nodes	O
in	O
melanoma	O
patients	O
.	O
</s>
<s>
(	O
A	O
)	O
Indocyanine	O
green	O
(	O
ICG	O
)	O
is	O
injected	O
and	O
accumulates	O
inside	O
the	O
sentinel	O
lymph	O
node	O
,	O
which	O
is	O
detected	O
using	O
a	O
hand-held	O
two-dimensional	O
MSOT	B-Algorithm
device	O
.	O
</s>
<s>
(	O
b	O
)	O
MSOT	B-Algorithm
images	O
of	O
the	O
ICG	O
accumulating	O
in	O
the	O
sentinel	O
lymph	O
node	O
(	O
in	O
color	O
)	O
,	O
overlaid	O
on	O
a	O
background	O
image	O
of	O
tissue	O
illuminated	O
at	O
a	O
single	O
wavelength	O
.	O
</s>
<s>
(	O
c	O
)	O
MSOT	B-Algorithm
imaging	O
of	O
melanin	O
(	O
in	O
color	O
)	O
overlaid	O
on	O
a	O
background	O
image	O
of	O
tissue	O
.	O
</s>
<s>
Optoacoustic	B-Algorithm
imaging	I-Algorithm
in	O
general	O
and	O
MSOT	B-Algorithm
in	O
particular	O
may	O
address	O
a	O
number	O
of	O
challenges	O
for	O
surgical	O
procedures	O
by	O
providing	O
real-time	O
visualization	O
below	O
the	O
tissue	O
surface	O
.	O
</s>
<s>
In	O
particular	O
,	O
optoacoustic	B-Algorithm
imaging	I-Algorithm
can	O
provide	O
immediate	O
information	O
on	O
the	O
perfusion	O
status	O
of	O
tissues	O
based	O
on	O
analysis	O
of	O
hemoglobin	O
dynamics	O
and	O
oxygenation	O
.	O
</s>
<s>
MSOT	B-Algorithm
can	O
detect	O
exogenous	O
contrast	B-Algorithm
agents	I-Algorithm
up	O
to	O
a	O
depth	O
of	O
5cm	O
in	O
tissue	O
if	O
the	O
overlaying	O
tissue	O
is	O
sufficiently	O
compressed	O
.	O
</s>
