<s>
Microwave	B-Algorithm
imaging	I-Algorithm
is	O
a	O
science	O
which	O
has	O
been	O
evolved	O
from	O
older	O
detecting/locating	O
techniques	O
(	O
e.g.	O
,	O
radar	B-Application
)	O
in	O
order	O
to	O
evaluate	O
hidden	O
or	O
embedded	O
objects	O
in	O
a	O
structure	O
(	O
or	O
media	O
)	O
using	O
electromagnetic	O
(	O
EM	O
)	O
waves	O
in	O
microwave	O
regime	O
(	O
i.e.	O
,	O
~	O
300MHz-300GHz	O
)	O
.	O
</s>
<s>
Engineering	O
and	O
application	O
oriented	O
microwave	B-Algorithm
imaging	I-Algorithm
for	O
non-destructive	O
testing	O
is	O
called	O
microwave	B-Algorithm
testing	I-Algorithm
,	O
see	O
below	O
.	O
</s>
<s>
Microwave	B-Algorithm
imaging	I-Algorithm
techniques	O
can	O
be	O
classified	O
as	O
either	O
quantitative	O
or	O
qualitative	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
qualitative	O
microwave	B-Algorithm
imaging	I-Algorithm
methods	O
calculate	O
a	O
qualitative	O
profile	O
(	O
which	O
is	O
called	O
as	O
reflectivity	O
function	O
or	O
qualitative	O
image	O
)	O
to	O
represent	O
the	O
hidden	O
object	O
.	O
</s>
<s>
Synthetic	B-Application
aperture	I-Application
radar	I-Application
(	O
SAR	O
)	O
,	O
ground-penetrating	B-Algorithm
radar	I-Algorithm
(	O
GPR	O
)	O
,	O
and	O
frequency-wave	O
number	O
migration	O
algorithm	O
are	O
some	O
of	O
the	O
most	O
popular	O
qualitative	O
microwave	B-Algorithm
imaging	I-Algorithm
methods[1]	O
.	O
</s>
<s>
In	O
general	O
,	O
a	O
microwave	B-Algorithm
imaging	I-Algorithm
system	O
is	O
made	O
up	O
of	O
hardware	O
and	O
software	O
components	O
.	O
</s>
<s>
In	O
fact	O
,	O
a	O
synthetic	O
(	O
virtual	O
)	O
aperture	O
is	O
produced	O
by	O
moving	O
the	O
antenna	O
(	O
similar	O
to	O
the	O
synthetic	B-Application
aperture	I-Application
radar	I-Application
principle	O
)	O
.	O
</s>
<s>
Depending	O
on	O
the	O
applied	O
processing	O
algorithm	O
,	O
microwave	B-Algorithm
imaging	I-Algorithm
techniques	O
can	O
be	O
categorized	O
as	O
quantitative	O
and	O
qualitative	O
.	O
</s>
<s>
Microwave	B-Algorithm
imaging	I-Algorithm
has	O
been	O
used	O
in	O
a	O
variety	O
of	O
applications	O
such	O
as	O
:	O
nondestructive	O
testing	O
and	O
evaluation	O
(	O
NDT&E	O
,	O
see	O
below	O
)	O
,	O
medical	O
imaging	O
,	O
concealed	O
weapon	O
detection	O
at	O
security	O
check	O
points	O
,	O
structural	O
health	O
monitoring	O
,	O
and	O
through-the-wall	O
imaging	O
.	O
</s>
<s>
Microwave	B-Algorithm
imaging	I-Algorithm
for	O
medical	O
applications	O
is	O
also	O
becoming	O
of	O
more	O
interest	O
.	O
</s>
<s>
This	O
difference	O
translates	O
into	O
a	O
contrast	O
which	O
can	O
be	O
detected	O
by	O
microwave	B-Algorithm
imaging	I-Algorithm
methods	O
.	O
</s>
<s>
As	O
one	O
example	O
,	O
there	O
are	O
several	O
research	O
groups	O
all	O
around	O
the	O
world	O
working	O
on	O
developing	O
efficient	O
microwave	B-Algorithm
imaging	I-Algorithm
techniques	O
for	O
early	O
detection	O
of	O
breast	O
cancer	O
.	O
</s>
<s>
Recently	O
,	O
microwave	B-Algorithm
imaging	I-Algorithm
has	O
shown	O
great	O
potential	O
to	O
be	O
used	O
for	O
structural	O
health	O
monitoring	O
.	O
</s>
<s>
If	O
there	O
is	O
any	O
rust	O
on	O
the	O
rebar	O
,	O
since	O
rust	O
reflects	O
less	O
EM	O
waves	O
in	O
comparison	O
with	O
sound	O
metal	O
,	O
the	O
microwave	B-Algorithm
imaging	I-Algorithm
method	O
can	O
distinguish	O
between	O
rebars	O
with	O
and	O
without	O
rust	O
(	O
or	O
corrosion	O
)	O
.	O
</s>
<s>
Microwave	B-Algorithm
imaging	I-Algorithm
also	O
can	O
be	O
used	O
to	O
detect	O
any	O
embedded	O
anomaly	O
inside	O
concrete	O
(	O
e.g.	O
,	O
crack	O
or	O
air	O
void	O
)	O
.	O
</s>
<s>
These	O
applications	O
of	O
microwave	B-Algorithm
imaging	I-Algorithm
are	O
part	O
of	O
non-destructive	O
(	O
NDT	O
)	O
testing	O
in	O
civil	O
engineering	O
.	O
</s>
<s>
More	O
on	O
microwave	B-Algorithm
imaging	I-Algorithm
in	O
NDT	O
is	O
described	O
in	O
the	O
following	O
.	O
</s>
<s>
Microwave	B-Algorithm
testing	I-Algorithm
uses	O
the	O
scientific	O
basics	O
of	O
microwave	B-Algorithm
imaging	I-Algorithm
for	O
the	O
inspection	O
of	O
technical	O
parts	O
with	O
harmless	O
microwaves	O
.	O
</s>
<s>
Microwave	B-Algorithm
testing	I-Algorithm
is	O
one	O
of	O
the	O
methods	O
of	O
non-destructive	O
testing	O
(	O
NDT	O
)	O
.	O
</s>
<s>
Microwave	B-Algorithm
testing	I-Algorithm
can	O
be	O
used	O
to	O
inspect	O
components	O
also	O
in	O
a	O
built-in	O
state	O
,	O
e	O
.	O
g	O
.	O
built-in	O
non-visible	O
gaskets	O
in	O
plastic	O
valves	O
.	O
</s>
<s>
Microwave	O
tests	O
are	O
possible	O
with	O
constant	O
frequency	O
(	O
CW	B-Algorithm
)	O
or	O
with	O
continuously	O
tuned	O
frequency	O
(	O
FMCW	B-Algorithm
)	O
.	O
</s>
<s>
FMCW	B-Algorithm
is	O
advantageous	O
to	O
determine	O
the	O
depth	O
of	O
defects	O
within	O
the	O
DUT	O
.	O
</s>
<s>
Microwave	B-Algorithm
testing	I-Algorithm
is	O
a	O
useful	O
NDT	O
method	O
for	O
dielectric	O
materials	O
.	O
</s>
<s>
Microwave	B-Algorithm
testing	I-Algorithm
is	O
used	O
in	O
many	O
industrial	O
sectors	O
:	O
</s>
<s>
In	O
recognizing	O
this	O
growing	O
importance	O
,	O
in	O
2011	O
the	O
Expert	O
committee	O
for	O
microwave	O
and	O
THz	O
procedures	O
of	O
the	O
German	O
Society	O
of	O
Non-Destructive	O
Testing	O
(	O
DGZfP	O
)	O
was	O
founded	O
as	O
in	O
2014	O
the	O
Microwave	B-Algorithm
Testing	I-Algorithm
Committee	O
of	O
the	O
American	O
Society	O
for	O
Non-Destructive	O
Testing	O
(	O
ASNT	O
)	O
.	O
</s>
