<s>
X-ray	B-Algorithm
detectors	I-Algorithm
are	O
devices	O
used	O
to	O
measure	O
the	O
flux	B-General_Concept
,	O
spatial	O
distribution	O
,	O
spectrum	O
,	O
and/or	O
other	O
properties	O
of	O
X-rays	B-Library
.	O
</s>
<s>
Detectors	O
can	O
be	O
divided	O
into	O
two	O
major	O
categories	O
:	O
imaging	O
detectors	O
(	O
such	O
as	O
photographic	B-Algorithm
plates	I-Algorithm
and	O
X-ray	B-Library
film	O
(	O
photographic	O
film	O
)	O
,	O
now	O
mostly	O
replaced	O
by	O
various	O
digitizing	O
devices	O
like	O
image	O
plates	O
or	O
flat	O
panel	O
detectors	O
)	O
and	O
dose	O
measurement	O
devices	O
(	O
such	O
as	O
ionization	O
chambers	O
,	O
Geiger	O
counters	O
,	O
and	O
dosimeters	O
used	O
to	O
measure	O
the	O
local	O
radiation	O
exposure	O
,	O
dose	O
,	O
and/or	O
dose	O
rate	O
,	O
for	O
example	O
,	O
for	O
verifying	O
that	O
radiation	O
protection	O
equipment	O
and	O
procedures	O
are	O
effective	O
on	O
an	O
ongoing	O
basis	O
)	O
.	O
</s>
<s>
To	O
obtain	O
an	O
image	O
with	O
any	O
type	O
of	O
image	O
detector	O
the	O
part	O
of	O
the	O
patient	O
to	O
be	O
X-rayed	O
is	O
placed	O
between	O
the	O
X-ray	B-Library
source	O
and	O
the	O
image	O
receptor	O
to	O
produce	O
a	O
shadow	O
of	O
the	O
internal	O
structure	O
of	O
that	O
particular	O
part	O
of	O
the	O
body	O
.	O
</s>
<s>
X-rays	B-Library
are	O
partially	O
blocked	O
(	O
"	O
attenuated	O
"	O
)	O
by	O
dense	O
tissues	O
such	O
as	O
bone	O
,	O
and	O
pass	O
more	O
easily	O
through	O
soft	O
tissues	O
.	O
</s>
<s>
Areas	O
where	O
the	O
X-rays	B-Library
strike	O
darken	O
when	O
developed	O
,	O
causing	O
bones	O
to	O
appear	O
lighter	O
than	O
the	O
surrounding	O
soft	O
tissue	O
.	O
</s>
<s>
The	O
contrast	O
compounds	O
have	O
high	O
atomic	O
numbered	O
elements	O
in	O
them	O
that	O
(	O
like	O
bone	O
)	O
essentially	O
block	O
the	O
X-rays	B-Library
and	O
hence	O
the	O
once	O
hollow	O
organ	O
or	O
vessel	O
can	O
be	O
more	O
readily	O
seen	O
.	O
</s>
<s>
Typical	O
x-ray	B-Library
film	O
contains	O
silver	O
halide	O
crystal	O
"	O
grains	O
"	O
,	O
typically	O
primarily	O
silver	O
bromide	O
.	O
</s>
<s>
Grain	O
size	O
and	O
composition	O
can	O
be	O
adjusted	O
to	O
affect	O
the	O
film	O
properties	O
,	O
for	O
example	O
to	O
improve	O
resolution	B-Algorithm
in	O
the	O
developed	O
image	O
.	O
</s>
<s>
Silver	O
ions	O
are	O
attracted	O
to	O
these	O
defects	O
and	O
reduced	B-Operating_System
,	O
creating	O
clusters	O
of	O
transparent	O
silver	O
atoms	O
.	O
</s>
<s>
The	O
first	O
radiographs	O
(	O
X-ray	B-Library
images	O
)	O
were	O
made	O
by	O
the	O
action	O
of	O
X-rays	B-Library
on	O
sensitized	O
glass	O
photographic	B-Algorithm
plates	I-Algorithm
.	O
</s>
<s>
X-ray	B-Library
film	O
(	O
photographic	O
film	O
)	O
soon	O
replaced	O
the	O
glass	B-Algorithm
plates	I-Algorithm
,	O
and	O
film	O
has	O
been	O
used	O
for	O
decades	O
to	O
acquire	O
(	O
and	O
display	O
)	O
medical	O
and	O
industrial	O
images	O
.	O
</s>
<s>
The	O
metal	O
silver	O
(	O
formerly	O
necessary	O
to	O
the	O
radiographic	O
&	O
photographic	O
industries	O
)	O
is	O
a	O
non-renewable	O
resource	O
although	O
silver	O
can	O
easily	O
be	O
reclaimed	O
from	O
spent	O
X-ray	B-Library
film	O
.	O
</s>
<s>
Where	O
X-ray	B-Library
films	O
required	O
wet	O
processing	O
facilities	O
,	O
newer	O
digital	O
technologies	O
do	O
not	O
.	O
</s>
<s>
Phosphor	O
plate	O
radiography	O
is	O
a	O
method	O
of	O
recording	O
X-rays	B-Library
using	O
photostimulated	O
luminescence	O
(	O
PSL	O
)	O
,	O
pioneered	O
by	O
Fuji	O
in	O
the	O
1980s	O
.	O
</s>
<s>
A	O
photostimulable	O
phosphor	O
plate	O
(	O
PSP	O
)	O
is	O
used	O
in	O
place	O
of	O
the	O
photographic	B-Algorithm
plate	I-Algorithm
.	O
</s>
<s>
The	O
PSP	O
plate	O
can	O
be	O
reused	O
,	O
and	O
existing	O
X-ray	B-Library
equipment	O
requires	O
no	O
modification	O
to	O
use	O
them	O
.	O
</s>
<s>
X-rays	B-Library
are	O
also	O
used	O
in	O
"	O
real-time	O
"	O
procedures	O
such	O
as	O
angiography	O
or	O
contrast	O
studies	O
of	O
the	O
hollow	O
organs	O
(	O
e.g.	O
</s>
<s>
Angioplasty	O
,	O
medical	O
interventions	O
of	O
the	O
arterial	O
system	O
,	O
rely	O
heavily	O
on	O
X-ray-sensitive	O
contrast	O
to	O
identify	O
potentially	O
treatable	O
lesions	O
.	O
</s>
<s>
Solid	O
state	O
detectors	O
use	O
semiconductors	O
to	O
detect	O
x-rays	B-Library
.	O
</s>
<s>
Direct	O
digital	O
detectors	O
are	O
so-called	O
because	O
they	O
directly	O
convert	O
x-ray	B-Library
photons	B-Application
to	O
electrical	O
charge	O
and	O
thus	O
a	O
digital	O
image	O
.	O
</s>
<s>
Indirect	O
systems	O
may	O
have	O
intervening	O
steps	O
for	O
example	O
first	O
converting	O
x-ray	B-Library
photons	B-Application
to	O
visible	O
light	O
,	O
and	O
then	O
an	O
electronic	O
signal	O
.	O
</s>
<s>
Since	O
the	O
1970s	O
,	O
silicon	O
or	O
germanium	O
doped	O
with	O
lithium	B-Language
(Si(Li )	O
or	O
Ge(Li )	O
)	O
semiconductor	B-Algorithm
detectors	I-Algorithm
have	O
been	O
developed	O
.	O
</s>
<s>
X-ray	B-Library
photons	B-Application
are	O
converted	O
to	O
electron-hole	O
pairs	O
in	O
the	O
semiconductor	O
and	O
are	O
collected	O
to	O
detect	O
the	O
X-rays	B-Library
.	O
</s>
<s>
When	O
the	O
temperature	O
is	O
low	O
enough	O
(	O
the	O
detector	O
is	O
cooled	O
by	O
Peltier	O
effect	O
or	O
even	O
cooler	O
liquid	O
nitrogen	O
)	O
,	O
it	O
is	O
possible	O
to	O
directly	O
determine	O
the	O
X-ray	B-Library
energy	O
spectrum	O
;	O
this	O
method	O
is	O
called	O
energy-dispersive	O
X-ray	B-Library
spectroscopy	O
(	O
EDX	O
or	O
EDS	O
)	O
;	O
it	O
is	O
often	O
used	O
in	O
small	O
X-ray	B-Library
fluorescence	O
spectrometers	O
.	O
</s>
<s>
Silicon	O
drift	O
detectors	O
(	O
SDDs	O
)	O
,	O
produced	O
by	O
conventional	O
semiconductor	B-Architecture
fabrication	I-Architecture
,	O
provide	O
a	O
cost-effective	O
and	O
high	O
resolving	O
power	O
radiation	O
measurement	O
.	O
</s>
<s>
Unlike	O
conventional	O
X-ray	B-Algorithm
detectors	I-Algorithm
,	O
such	O
as	O
Si(Li )	O
,	O
they	O
do	O
not	O
need	O
to	O
be	O
cooled	O
with	O
liquid	O
nitrogen	O
.	O
</s>
<s>
Practical	O
application	O
in	O
medical	B-Application
imaging	I-Application
started	O
in	O
the	O
early	O
2000s	O
.	O
</s>
<s>
Amorphous	O
selenium	B-Library
is	O
used	O
in	O
commercial	O
large	O
area	O
flat	O
panel	O
X-ray	B-Algorithm
detectors	I-Algorithm
for	O
mammography	O
and	O
general	O
radiography	O
due	O
to	O
its	O
high	O
spatial	O
resolution	B-Algorithm
and	O
x-ray	B-Library
absorbing	O
properties	O
.	O
</s>
<s>
However	O
Selenium	B-Library
's	O
low	O
atomic	O
number	O
means	O
a	O
thick	O
layer	O
is	O
required	O
to	O
achieve	O
sufficient	O
sensitivity	O
.	O
</s>
<s>
Cadmium	O
telluride	O
(	O
CdTe	O
)	O
,	O
and	O
its	O
alloy	O
with	O
zinc	B-Application
,	O
cadmium	O
zinc	B-Application
telluride	O
,	O
is	O
considered	O
one	O
of	O
the	O
most	O
promising	O
semiconductor	O
materials	O
for	O
x-ray	B-Library
detection	O
due	O
to	O
its	O
wide	O
band-gap	O
and	O
high	O
quantum	O
number	O
resulting	O
in	O
room	O
temperature	O
operation	O
with	O
high	O
efficiency	O
.	O
</s>
<s>
Current	O
applications	O
include	O
bone	B-Algorithm
densitometry	I-Algorithm
and	O
SPECT	O
but	O
flat	O
panel	O
detectors	O
suitable	O
for	O
radiographic	O
imaging	O
are	O
not	O
yet	O
in	O
production	O
.	O
</s>
<s>
Current	O
research	O
and	O
development	O
is	O
focused	O
around	O
energy	O
resolving	O
pixel	B-Algorithm
detectors	O
,	O
such	O
as	O
CERN	O
's	O
Medipix	B-Algorithm
detector	O
and	O
Science	O
and	O
Technology	O
Facilities	O
Council	O
's	O
HEXITEC	O
detector	O
.	O
</s>
<s>
Common	O
semiconductor	O
diodes	O
,	O
such	O
as	O
PIN	O
photodiodes	O
or	O
a	O
1N4007	O
,	O
will	O
produce	O
a	O
small	O
amount	O
of	O
current	O
in	O
photovoltaic	O
mode	O
when	O
placed	O
in	O
an	O
X-ray	B-Library
beam	O
.	O
</s>
<s>
Indirect	O
detectors	O
are	O
made	O
up	O
of	O
a	O
scintillator	O
to	O
convert	O
x-rays	B-Library
to	O
visible	O
light	O
,	O
which	O
is	O
read	O
by	O
a	O
TFT	O
array	O
.	O
</s>
<s>
This	O
can	O
provide	O
sensitivity	O
advantages	O
over	O
current	O
(	O
amorphous	O
selenium	B-Library
)	O
direct	O
detectors	O
,	O
albeit	O
with	O
a	O
potential	O
trade-off	O
in	O
resolution	B-Algorithm
.	O
</s>
<s>
Each	O
of	O
these	O
thin-film	O
transistors	O
(	O
TFTs	O
)	O
is	O
attached	O
to	O
a	O
light-absorbing	O
photodiode	O
making	O
up	O
an	O
individual	O
pixel	B-Algorithm
(	O
picture	B-Algorithm
element	I-Algorithm
)	O
.	O
</s>
<s>
Photons	B-Application
striking	O
the	O
photodiode	O
are	O
converted	O
into	O
two	O
carriers	O
of	O
electrical	O
charge	O
,	O
called	O
electron-hole	O
pairs	O
.	O
</s>
<s>
Since	O
the	O
number	O
of	O
charge	O
carriers	O
produced	O
will	O
vary	O
with	O
the	O
intensity	O
of	O
incoming	O
light	O
photons	B-Application
,	O
an	O
electrical	O
pattern	O
is	O
created	O
that	O
can	O
be	O
swiftly	O
converted	O
to	O
a	O
voltage	O
and	O
then	O
a	O
digital	O
signal	O
,	O
which	O
is	O
interpreted	O
by	O
a	O
computer	O
to	O
produce	O
a	O
digital	O
image	O
.	O
</s>
<s>
Although	O
silicon	O
has	O
outstanding	O
electronic	O
properties	O
,	O
it	O
is	O
not	O
a	O
particularly	O
good	O
absorber	O
of	O
X-ray	B-Library
photons	B-Application
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
X-rays	B-Library
first	O
impinge	O
upon	O
scintillators	O
made	O
from	O
such	O
materials	O
as	O
gadolinium	O
oxysulfide	O
or	O
caesium	O
iodide	O
.	O
</s>
<s>
The	O
scintillator	O
absorbs	O
the	O
X-rays	B-Library
and	O
converts	O
them	O
into	O
visible	O
light	O
photons	B-Application
that	O
then	O
pass	O
onto	O
the	O
photodiode	O
array	O
.	O
</s>
<s>
X-rays	B-Library
going	O
through	O
a	O
gas	O
will	O
ionize	B-Protocol
it	O
,	O
producing	O
positive	O
ions	O
and	O
free	O
electrons	O
.	O
</s>
<s>
An	O
incoming	O
photon	B-Application
will	O
create	O
a	O
number	O
of	O
such	O
ion	O
pairs	O
proportional	O
to	O
its	O
energy	O
.	O
</s>
<s>
Most	O
of	O
the	O
gas	O
volume	O
will	O
act	O
as	O
an	O
ionization	O
chamber	O
,	O
but	O
in	O
the	O
region	O
closest	O
to	O
the	O
wire	O
the	O
electric	O
field	O
is	O
high	O
enough	O
to	O
make	O
the	O
electrons	O
ionize	B-Protocol
gas	O
molecules	O
.	O
</s>
<s>
Since	O
every	O
electron	O
cause	O
an	O
avalanche	O
of	O
approximately	O
the	O
same	O
size	O
the	O
collected	O
charge	O
is	O
proportional	O
to	O
the	O
number	O
of	O
ion	O
pairs	O
created	O
by	O
the	O
absorbed	O
x-ray	B-Library
.	O
</s>
<s>
This	O
makes	O
it	O
possible	O
to	O
measure	O
the	O
energy	O
of	O
each	O
incoming	O
photon	B-Application
.	O
</s>
<s>
Geiger	O
–	O
Müller	O
counters	O
use	O
an	O
even	O
higher	O
electric	O
field	O
so	O
that	O
UV-photons	O
are	O
created	O
.	O
</s>
<s>
This	O
makes	O
the	O
signal	O
very	O
strong	O
,	O
but	O
causes	O
a	O
dead	O
time	O
after	O
each	O
event	O
and	O
makes	O
it	O
impossible	O
to	O
measure	O
the	O
X-ray	B-Library
energies	O
.	O
</s>
<s>
Gas	O
detectors	O
are	O
usually	O
single	O
pixel	B-Algorithm
detectors	O
measuring	O
only	O
the	O
average	O
dose	O
rate	O
over	O
the	O
gas	O
volume	O
or	O
the	O
number	O
of	O
interacting	O
photons	B-Application
as	O
explained	O
above	O
,	O
but	O
they	O
can	O
be	O
made	O
spatially	O
resolving	O
by	O
having	O
many	O
crossed	O
wires	O
in	O
a	O
wire	O
chamber	O
.	O
</s>
<s>
It	O
was	O
demonstrated	O
in	O
the	O
1960s	O
that	O
silicon	O
PN	O
solar	O
cells	O
are	O
suitable	O
for	O
detection	O
of	O
all	O
forms	O
of	O
ionizing	O
radiation	O
including	O
extreme	O
UV	O
,	O
soft	O
X-rays	B-Library
,	O
and	O
hard	O
X-rays	B-Library
.	O
</s>
<s>
This	O
form	O
of	O
detection	O
operates	O
via	O
photoionization	B-Algorithm
,	O
a	O
process	O
where	O
ionizing	O
radiation	O
strikes	O
an	O
atom	B-Language
and	O
releases	O
a	O
free	O
electron	O
.	O
</s>
<s>
Self-developing	O
radiochromic	O
film	O
can	O
provide	O
very	O
high	B-Algorithm
resolution	I-Algorithm
measurements	O
,	O
for	O
dosimetry	O
and	O
profiling	O
purposes	O
,	O
particularly	O
in	O
radiotherapy	O
physics	O
.	O
</s>
