<s>
Thermoacoustic	B-Algorithm
imaging	I-Algorithm
was	O
originally	O
proposed	O
by	O
Theodore	O
Bowen	O
in	O
1981	O
as	O
a	O
strategy	O
for	O
studying	O
the	O
absorption	O
properties	O
of	O
human	O
tissue	O
using	O
virtually	O
any	O
kind	O
of	O
electromagnetic	O
radiation	O
.	O
</s>
<s>
But	O
Alexander	O
Graham	O
Bell	O
first	O
reported	O
the	O
physical	O
principle	O
upon	O
which	O
thermoacoustic	B-Algorithm
imaging	I-Algorithm
is	O
based	O
a	O
century	O
earlier	O
.	O
</s>
<s>
Shortly	O
after	O
Bowen	O
's	O
work	O
was	O
published	O
,	O
other	O
researchers	O
proposed	O
methodology	O
for	O
thermoacoustic	B-Algorithm
imaging	I-Algorithm
using	O
microwaves	O
.	O
</s>
<s>
Since	O
that	O
time	O
,	O
thermoacoustic	B-Algorithm
imaging	I-Algorithm
has	O
gained	O
widespread	O
popularity	O
in	O
research	O
institutions	O
worldwide	O
.	O
</s>
<s>
As	O
of	O
2008	O
,	O
three	O
companies	O
were	O
developing	O
commercial	O
thermoacoustic	B-Algorithm
imaging	I-Algorithm
systems	O
–	O
Seno	O
Medical	O
,	O
Endra	O
,	O
Inc	O
.	O
and	O
OptoSonics	O
,	O
Inc	O
.	O
</s>
<s>
The	O
term	O
"	O
photoacoustic	O
"	O
(	O
see	O
photoacoustic	B-Algorithm
imaging	I-Algorithm
in	I-Algorithm
biomedicine	I-Algorithm
)	O
applies	O
to	O
this	O
phenomenon	O
when	O
the	O
stimulating	O
radiation	O
is	O
optical	O
,	O
while	O
"	O
thermoacoustic	O
"	O
is	O
the	O
more	O
general	O
term	O
and	O
refers	O
to	O
all	O
radiating	O
sources	O
,	O
including	O
optical	O
.	O
</s>
<s>
Frequencies	O
greater	O
than	O
1MHz	O
are	O
referred	O
to	O
as	O
ultrasonic	O
,	O
and	O
are	O
also	O
associated	O
with	O
medical	B-Application
ultrasound	I-Application
applications	O
.	O
</s>
<s>
Any	O
thermoacoustic	B-Algorithm
imaging	I-Algorithm
device	O
requires	O
a	O
source	O
of	O
electromagnetic	O
radiation	O
,	O
be	O
it	O
a	O
laser	O
or	O
a	O
microwave	O
antenna	O
,	O
to	O
deliver	O
energy	O
to	O
the	O
anatomy	O
being	O
studied	O
,	O
and	O
one	O
or	O
more	O
acoustic	O
detectors	O
coupled	O
acoustically	O
to	O
the	O
outside	O
surface	O
of	O
the	O
anatomy	O
,	O
as	O
is	O
illustrated	O
in	O
Fig	O
.	O
</s>
<s>
The	O
typical	O
acoustic	O
detector	O
is	O
an	O
ultrasound	B-Algorithm
transducer	I-Algorithm
,	O
which	O
is	O
commonly	O
made	O
of	O
a	O
piezo-electric	O
material	O
that	O
converts	O
detected	O
pressure	O
to	O
an	O
electrical	O
signal	O
.	O
</s>
<s>
Linear	O
transducer	O
arrays	O
(	O
both	O
curved	O
and	O
straight	O
)	O
are	O
commonly	O
used	O
in	O
conventional	O
medical	B-Application
ultrasound	I-Application
.	O
</s>
<s>
They	O
are	O
easily	O
adapted	O
for	O
use	O
in	O
thermoacoustic	B-Algorithm
imaging	I-Algorithm
.	O
</s>
<s>
Figure	O
7	O
illustrates	O
how	O
a	O
linear	O
array	O
is	O
used	O
for	O
2D	O
thermoacoustic	B-Algorithm
imaging	I-Algorithm
.	O
</s>
