<s>
Terahertz	B-Algorithm
tomography	I-Algorithm
is	O
a	O
class	O
of	O
tomography	B-Algorithm
where	O
sectional	O
imaging	O
is	O
done	O
by	O
terahertz	B-Architecture
radiation	I-Architecture
.	O
</s>
<s>
Terahertz	B-Architecture
radiation	I-Architecture
is	O
electromagnetic	O
radiation	O
with	O
a	O
frequency	O
between	O
0.1	O
and	O
10	O
THz	O
;	O
it	O
falls	O
between	O
radio	O
waves	O
and	O
light	O
waves	O
on	O
the	O
spectrum	O
;	O
it	O
encompasses	O
portions	O
of	O
the	O
millimeter	O
waves	O
and	O
infrared	O
wavelengths	O
.	O
</s>
<s>
Because	O
of	O
its	O
high	O
frequency	O
and	O
short	O
wavelength	O
,	O
terahertz	B-Architecture
wave	I-Architecture
has	O
a	O
high	O
signal-to-noise	O
ratio	O
in	O
the	O
time	O
domain	O
spectrum	O
.	O
</s>
<s>
Tomography	B-Algorithm
using	O
terahertz	B-Architecture
radiation	I-Architecture
can	O
image	O
samples	O
that	O
are	O
opaque	O
in	O
the	O
visible	O
and	O
near-infrared	O
regions	O
of	O
the	O
spectrum	O
.	O
</s>
<s>
Terahertz	B-Architecture
wave	I-Architecture
three-dimensional	O
(	O
3D	O
)	O
imaging	O
technology	O
has	O
developed	O
rapidly	O
since	O
its	O
first	O
successful	O
application	O
in	O
1997	O
,	O
and	O
a	O
series	O
of	O
new	O
3D	O
imaging	O
technologies	O
have	O
been	O
proposed	O
successively	O
.	O
</s>
<s>
A	O
variety	O
of	O
materials	O
are	O
transparent	O
to	O
terahertz	B-Architecture
radiation	I-Architecture
,	O
which	O
allows	O
it	O
to	O
measure	O
the	O
thickness	O
,	O
density	O
,	O
and	O
structural	O
properties	O
of	O
materials	O
that	O
are	O
difficult	O
to	O
detect	O
.	O
</s>
<s>
Moreover	O
,	O
because	O
many	O
materials	O
have	O
a	O
unique	O
spectral	O
signature	O
in	O
the	O
terahertz	O
range	O
,	O
terahertz	B-Architecture
radiation	I-Architecture
can	O
be	O
used	O
to	O
identify	O
materials	O
.	O
</s>
<s>
Although	O
dangerous	O
metal	O
objects	O
,	O
such	O
as	O
knives	O
,	O
can	O
be	O
recognized	O
by	O
their	O
shapes	O
through	O
certain	O
pattern	O
recognition	O
algorithms	O
,	O
it	O
is	O
impossible	O
to	O
see	O
through	O
metallic	O
packages	O
with	O
terahertz	B-Architecture
waves	I-Architecture
.	O
</s>
<s>
For	O
example	O
,	O
terahertz	B-Architecture
waves	I-Architecture
made	O
it	O
possible	O
to	O
detect	O
metallic	O
and	O
nonmetallic	O
foreign	O
matter	O
in	O
chocolate	O
bars	O
,	O
since	O
food	O
with	O
low	O
water	O
contents	O
,	O
such	O
as	O
chocolates	O
,	O
are	O
almost	O
transparent	O
in	O
the	O
terahertz	B-Architecture
band	I-Architecture
.	O
</s>
<s>
Terahertz	B-Algorithm
tomography	I-Algorithm
is	O
also	O
useful	O
in	O
the	O
wine	O
and	O
spirits	O
industries	O
for	O
quantifying	O
moisture	O
and	O
analysing	O
cork	O
non-destructively	O
.	O
</s>
<s>
Terahertz	B-Algorithm
tomography	I-Algorithm
can	O
be	O
divided	O
into	O
transmission	O
and	O
reflection	O
mode	O
.	O
</s>
<s>
It	O
acts	O
as	O
an	O
extension	O
of	O
X-ray	O
computed	O
tomography	B-Algorithm
(	O
CT	O
)	O
to	O
a	O
different	O
waveband	O
.	O
</s>
<s>
It	O
mainly	O
studies	O
the	O
establishment	O
of	O
process	O
models	O
such	O
as	O
refraction	O
,	O
reflection	O
and	O
diffraction	O
when	O
terahertz	B-Architecture
waves	I-Architecture
transmit	O
samples	O
,	O
which	O
has	O
certain	O
requirements	O
for	O
reconstruction	O
algorithms	O
.	O
</s>
<s>
According	O
to	O
the	O
different	O
transmission	O
delay	O
of	O
Terahertz	B-Architecture
wave	I-Architecture
reflected	O
signal	O
at	O
different	O
depths	O
inside	O
the	O
sample	O
,	O
the	O
depth	O
information	O
can	O
be	O
obtained	O
by	O
processing	O
the	O
reflected	O
signal	O
inside	O
the	O
sample	O
to	O
realize	O
the	O
tomography	B-Algorithm
.	O
</s>
<s>
Terahertz	O
time-of-flight	O
tomography	B-Algorithm
(	O
THz-TOF	O
)	O
and	O
THz	O
optical	O
coherence	O
tomography	B-Algorithm
(	O
Thz-OCT	O
)	O
are	O
mainly	O
used	O
in	O
implementation	O
.	O
</s>
<s>
In	O
diffraction	O
tomography	B-Algorithm
,	O
the	O
detection	O
beam	O
interacts	O
with	O
the	O
target	O
and	O
uses	O
the	O
resulting	O
scattered	O
waves	O
to	O
build	O
a	O
3D	O
image	O
of	O
the	O
sample	O
.	O
</s>
<s>
For	O
fine	O
samples	O
with	O
more	O
complex	O
surface	O
structure	O
,	O
diffraction	O
tomography	B-Algorithm
is	O
effective	O
because	O
it	O
can	O
provide	O
a	O
sample	O
refractive	O
index	O
distribution	O
.	O
</s>
<s>
However	O
,	O
there	O
are	O
also	O
drawbacks	O
:	O
although	O
the	O
imaging	O
speed	O
of	O
terahertz	O
diffraction	O
tomography	B-Algorithm
is	O
faster	O
,	O
its	O
imaging	O
quality	O
is	O
poor	O
due	O
to	O
the	O
lack	O
of	O
an	O
effective	O
reconstruction	O
algorithm	O
.	O
</s>
<s>
Tomosynthesis	O
is	O
a	O
technique	O
used	O
to	O
create	O
high-image	O
tomography	B-Algorithm
.	O
</s>
<s>
However	O
,	O
terahertz	O
fault	O
synthetic	O
tomography	B-Algorithm
is	O
not	O
affected	O
by	O
refraction	O
and	O
reflection	O
because	O
of	O
the	O
small	O
incidence	O
angle	O
during	O
projection	O
.	O
</s>
<s>
The	O
longitudinal	O
resolution	O
of	O
time-of-flight	O
tomography	B-Algorithm
depends	O
on	O
the	O
pulse	O
width	O
of	O
terahertz	B-Architecture
waves	I-Architecture
(	O
usually	O
in	O
the	O
tens	O
of	O
microns	O
)	O
;	O
therefore	O
,	O
the	O
vertical	O
resolution	O
of	O
flight	O
time	O
chromatography	O
is	O
very	O
high	O
.	O
</s>
<s>
designed	O
an	O
experimental	O
system	O
that	O
successfully	O
used	O
tomography	B-Algorithm
on	O
a	O
semiconductor	O
sample	O
consisting	O
of	O
three	O
sheets	O
of	O
superimposed	O
paper	O
and	O
a	O
thin	O
two-micron	O
thick	O
layer	O
of	O
GaAs	O
.	O
</s>
<s>
The	O
THz	O
beam	O
can	O
be	O
incorporated	O
into	O
3D	O
holography	O
if	O
the	O
differentiation	O
of	O
each	O
multiple	O
scattered	O
terahertz	B-Architecture
waves	I-Architecture
of	O
different	O
scattering	O
orders	O
is	O
enabled	O
.	O
</s>
<s>
As	O
their	O
focal	O
lengths	O
depend	O
on	O
frequencies	O
,	O
samples	O
can	O
be	O
imaged	O
at	O
various	O
locations	O
along	O
the	O
propagation	O
path	O
to	O
the	O
imaging	O
plane	O
,	O
which	O
can	O
be	O
applied	O
to	O
tomographic	B-Algorithm
imaging	I-Algorithm
.	O
</s>
<s>
Terahertz	O
computed	O
tomography	B-Algorithm
records	O
both	O
amplitude	O
and	O
spectral	O
phase	O
information	O
when	O
compared	O
to	O
X-ray	O
imaging	O
.	O
</s>
