<s>
Photon-counting	B-Algorithm
computed	I-Algorithm
tomography	I-Algorithm
(	O
PCCT	O
)	O
is	O
a	O
form	O
of	O
X-ray	B-Library
computed	O
tomography	O
(	O
CT	O
)	O
in	O
which	O
X-rays	B-Library
are	O
detected	O
using	O
a	O
photon-counting	O
detector	O
(	O
PCD	O
)	O
which	O
registers	O
the	O
interactions	O
of	O
individual	O
photons	B-Application
.	O
</s>
<s>
By	O
keeping	O
track	O
of	O
the	O
deposited	O
energy	O
in	O
each	O
interaction	O
,	O
the	O
detector	O
pixels	B-Algorithm
of	O
a	O
PCD	O
each	O
record	O
an	O
approximate	O
energy	O
spectrum	O
,	O
making	O
it	O
a	O
spectral	O
or	O
energy-resolved	O
CT	O
technique	O
.	O
</s>
<s>
In	O
contrast	O
,	O
more	O
conventional	O
CT	O
scanners	O
use	O
energy-integrating	O
detectors	O
(	O
EIDs	O
)	O
,	O
where	O
the	O
total	O
energy	O
(	O
generally	O
from	O
a	O
large	O
number	O
of	O
photons	B-Application
as	O
well	O
as	O
electronic	O
noise	O
)	O
deposited	O
in	O
a	O
pixel	B-Algorithm
during	O
a	O
fixed	O
period	O
of	O
time	O
is	O
registered	O
.	O
</s>
<s>
These	O
EIDs	O
thus	O
register	O
only	O
photon	B-Application
intensity	O
,	O
comparable	O
to	O
black-and-white	O
photography	O
,	O
whereas	O
PCDs	O
register	O
also	O
spectral	O
information	O
,	O
similar	O
to	O
color	O
photography	O
.	O
</s>
<s>
These	O
include	O
improved	O
signal	O
(	O
and	O
contrast	O
)	O
to	O
noise	O
ratio	O
,	O
reduced	O
X-ray	B-Library
dose	O
to	O
the	O
patient	O
,	O
improved	O
spatial	O
resolution	O
and	O
,	O
through	O
use	O
of	O
several	O
energy	O
bins	O
,	O
the	O
ability	O
to	O
distinguish	O
multiple	O
contrast	B-Algorithm
agents	I-Algorithm
.	O
</s>
<s>
Due	O
to	O
the	O
large	O
volumes	O
and	O
rates	O
of	O
data	O
required	O
(	O
up	O
to	O
several	O
hundred	O
million	O
photon	B-Application
interactions	O
per	O
mm2	O
and	O
second	O
)	O
the	O
use	O
of	O
PCDs	O
in	O
CT	O
scanners	O
has	O
become	O
feasible	O
only	O
with	O
recent	O
improvements	O
in	O
detector	O
technology	O
.	O
</s>
<s>
As	O
of	O
January	O
2021	O
photon-counting	O
CT	O
is	O
in	O
use	O
at	O
five	O
clinical	O
sites	O
.	O
</s>
<s>
Some	O
early	O
research	O
has	O
found	O
the	O
dose	O
reduction	O
potential	O
of	O
photon-counting	O
CT	O
for	O
breast	O
imaging	O
to	O
be	O
very	O
promising	O
.	O
</s>
<s>
On	O
September	O
29	O
,	O
2021	O
the	O
FDA	O
cleared	O
the	O
first	O
photon-counting	O
CT	O
(	O
developed	O
by	O
Siemens	O
Healthineers	O
)	O
for	O
clinical	O
use	O
.	O
</s>
<s>
When	O
a	O
photon	B-Application
interacts	O
in	O
a	O
PCD	O
,	O
the	O
amplitude	O
of	O
the	O
resulting	O
electrical	O
pulse	O
is	O
roughly	O
proportional	O
to	O
the	O
photon	B-Application
energy	O
.	O
</s>
<s>
By	O
comparing	O
each	O
pulse	O
produced	O
in	O
a	O
pixel	B-Algorithm
with	O
a	O
suitable	O
low-energy	O
threshold	O
,	O
contributions	O
from	O
low-energy	O
events	O
(	O
resulting	O
from	O
both	O
photon	B-Application
interactions	O
and	O
electronic	O
noise	O
)	O
can	O
be	O
filtered	O
out	O
.	O
</s>
<s>
This	O
effectively	O
eliminates	O
contributions	O
from	O
electronic	O
noise	O
at	O
the	O
expense	O
of	O
discarding	O
photons	B-Application
with	O
energy	O
comparable	O
to	O
the	O
noise	O
level	O
(	O
which	O
are	O
of	O
little	O
use	O
since	O
they	O
are	O
indistinguishable	O
from	O
noise	O
counts	O
)	O
.	O
</s>
<s>
In	O
an	O
EID	O
,	O
on	O
the	O
other	O
hand	O
,	O
the	O
contributions	O
from	O
individual	O
photons	B-Application
are	O
not	O
known	O
.	O
</s>
<s>
Therefore	O
,	O
an	O
energy	O
threshold	O
cannot	O
be	O
applied	O
,	O
making	O
this	O
technique	O
susceptible	O
to	O
noise	O
and	O
other	O
factors	O
which	O
can	O
affect	O
the	O
linearity	O
of	O
the	O
voltage	O
to	O
X-ray	B-Library
intensity	O
relationship	O
.	O
</s>
<s>
This	O
can	O
either	O
be	O
used	O
to	O
increase	O
the	O
image	O
quality	O
at	O
the	O
same	O
X-ray	B-Library
exposure	O
level	O
,	O
or	O
to	O
lower	O
the	O
patient	O
X-ray	B-Library
dose	O
whilst	O
maintaining	O
the	O
same	O
image	O
quality	O
.	O
</s>
<s>
Second	O
,	O
it	O
is	O
difficult	O
to	O
manufacture	O
energy-integrating	O
detectors	O
with	O
smaller	O
pixel	B-Algorithm
size	O
than	O
approximately	O
1×1mm2	O
without	O
compromising	O
dose	O
efficiency	O
.	O
</s>
<s>
The	O
reason	O
for	O
this	O
is	O
that	O
reflective	O
layers	O
must	O
be	O
placed	O
in	O
the	O
scintillator	O
between	O
the	O
pixels	B-Algorithm
to	O
prevent	O
cross-talk	O
between	O
pixels	B-Algorithm
,	O
and	O
these	O
cannot	O
be	O
made	O
too	O
thin	O
.	O
</s>
<s>
In	O
addition	O
,	O
the	O
measured	O
signal	O
is	O
proportional	O
to	O
the	O
pixel	B-Algorithm
area	O
whereas	O
the	O
electronic	O
noise	O
is	O
fairly	O
independent	O
of	O
pixel	B-Algorithm
size	O
,	O
so	O
that	O
noise	O
will	O
dominate	O
the	O
measured	O
signal	O
if	O
the	O
pixels	B-Algorithm
are	O
made	O
too	O
small	O
.	O
</s>
<s>
These	O
problems	O
do	O
not	O
occur	O
in	O
a	O
photon-counting	O
detector	O
with	O
a	O
low-energy	O
threshold	O
,	O
which	O
can	O
therefore	O
achieve	O
higher	O
detector	O
resolution	O
.	O
</s>
<s>
Each	O
registered	O
photon	B-Application
is	O
thus	O
assigned	O
to	O
a	O
specific	O
bin	O
depending	O
on	O
its	O
energy	O
,	O
such	O
that	O
each	O
pixel	B-Algorithm
measures	O
a	O
histogram	B-Algorithm
of	O
the	O
incident	O
X-ray	B-Library
spectrum	O
.	O
</s>
<s>
First	O
,	O
it	O
makes	O
it	O
possible	O
to	O
quantitatively	O
determine	O
the	O
material	O
composition	O
of	O
each	O
pixel	B-Algorithm
in	O
the	O
reconstructed	O
CT	O
image	O
,	O
as	O
opposed	O
to	O
the	O
estimated	O
average	O
linear	O
attenuation	O
coefficient	O
obtained	O
in	O
a	O
conventional	O
CT	O
scan	O
.	O
</s>
<s>
These	O
arise	O
because	O
of	O
the	O
higher	O
linear	O
attenuation	O
of	O
most	O
materials	O
at	O
lower	O
energy	O
which	O
shifts	O
the	O
mean	O
energy	O
of	O
the	O
X-ray	B-Library
spectrum	O
towards	O
higher	O
energies	O
as	O
the	O
beam	O
passes	O
through	O
the	O
object	O
.	O
</s>
<s>
Finally	O
,	O
using	O
more	O
than	O
two	O
energy	O
bins	O
allows	O
to	O
discriminate	O
between	O
on	O
the	O
one	O
hand	O
dense	O
bone	O
and	O
calcifications	O
and	O
on	O
the	O
other	O
hand	O
heavier	O
elements	O
(	O
commonly	O
iodine	O
or	O
gadolinium	O
)	O
used	O
as	O
contrast	B-Algorithm
agents	I-Algorithm
.	O
</s>
<s>
This	O
has	O
the	O
potential	O
reduce	O
the	O
amount	O
of	O
X-ray	B-Library
dose	O
from	O
a	O
contrast	O
scan	O
by	O
removing	O
the	O
need	O
for	O
a	O
reference	O
scan	O
before	O
contrast	O
injection	O
.	O
</s>
<s>
Although	O
spectral	O
CT	O
is	O
already	O
clinically	O
available	O
in	O
the	O
form	O
of	O
dual-energy	O
scanners	O
,	O
photon-counting	O
CT	O
offers	O
a	O
number	O
of	O
advantages	O
.	O
</s>
<s>
Also	O
,	O
it	O
can	O
be	O
shown	O
that	O
at	O
least	O
three	O
energies	O
are	O
necessary	O
to	O
simultaneously	O
decompose	O
both	O
tissue	O
and	O
contrast	B-Algorithm
medium	I-Algorithm
.	O
</s>
<s>
More	O
energy	O
bins	O
also	O
allow	O
for	O
simultaneously	O
differentiating	O
between	O
different	O
contrast	B-Algorithm
agents	I-Algorithm
.	O
</s>
<s>
As	O
an	O
example	O
,	O
each	O
mm2	O
of	O
a	O
CT	O
detector	O
may	O
receive	O
several	O
hundred	O
million	O
photon	B-Application
interactions	O
per	O
second	O
during	O
a	O
scan	O
.	O
</s>
<s>
To	O
avoid	O
saturation	O
in	O
areas	O
where	O
little	O
material	O
is	O
present	O
between	O
the	O
X-ray	B-Library
source	O
and	O
the	O
detector	O
,	O
the	O
pulse	O
resolving	O
time	O
must	O
be	O
small	O
compared	O
to	O
the	O
average	O
time	O
between	O
photon	B-Application
interactions	O
in	O
a	O
pixel	B-Algorithm
.	O
</s>
<s>
Even	O
before	O
saturation	O
,	O
the	O
detector	O
functionality	O
starts	O
to	O
deteriorate	O
because	O
of	O
pulse	O
pileup	O
(	O
see	O
figure	O
to	O
the	O
left	O
)	O
,	O
where	O
two	O
(	O
or	O
more	O
)	O
photon	B-Application
interactions	O
take	O
place	O
in	O
the	O
same	O
pixel	B-Algorithm
too	O
close	O
in	O
time	O
to	O
be	O
resolved	O
as	O
discrete	O
events	O
.	O
</s>
<s>
Such	O
quasi-coincident	O
interactions	O
lead	O
to	O
a	O
loss	O
of	O
photon	B-Application
counts	O
and	O
distorts	O
the	O
pulse	O
shape	O
,	O
skewing	O
the	O
recorded	O
energy	O
spectrum	O
.	O
</s>
<s>
Due	O
to	O
these	O
effects	O
,	O
the	O
demands	O
on	O
the	O
physical	O
response	O
time	O
of	O
the	O
detector	O
material	O
as	O
well	O
as	O
on	O
the	O
electronics	O
responsible	O
for	O
pulse-shaping	O
,	O
binning	O
and	O
recording	O
pixel	B-Algorithm
data	O
become	O
very	O
high	O
.	O
</s>
<s>
Using	O
smaller	O
image	O
pixels	B-Algorithm
decreases	O
the	O
per-pixel	O
count	O
rate	O
and	O
thus	O
alleviates	O
the	O
demands	O
on	O
pulse	O
resolving	O
time	O
at	O
the	O
expense	O
of	O
requiring	O
more	O
electronics	O
.	O
</s>
<s>
Partial	O
energy	O
deposition	O
and	O
single	O
photons	B-Application
causing	O
signals	O
in	O
multiple	O
pixels	B-Algorithm
poses	O
another	O
challenge	O
in	O
photon-counting	O
CT	O
.	O
</s>
<s>
Charge	O
sharing	O
,	O
where	O
an	O
interaction	O
takes	O
place	O
close	O
to	O
a	O
pixel	B-Algorithm
boundary	O
,	O
causing	O
the	O
released	O
energy	O
to	O
be	O
shared	O
between	O
neighboring	O
pixels	B-Algorithm
and	O
thus	O
be	O
interpreted	O
as	O
several	O
lower-energy	O
photons	B-Application
,	O
is	O
one	O
cause	O
of	O
such	O
events	O
(	O
see	O
figure	O
to	O
the	O
right	O
)	O
.	O
</s>
<s>
Others	O
include	O
the	O
emission	O
of	O
K-escape	O
X-rays	B-Library
and	O
Compton	O
scattering	O
,	O
where	O
the	O
escaping	O
or	O
scattered	O
photon	B-Application
results	O
in	O
a	O
partial	O
energy	O
deposition	O
in	O
the	O
primary	O
pixel	B-Algorithm
and	O
may	O
go	O
on	O
to	O
cause	O
further	O
interactions	O
in	O
different	O
pixels	B-Algorithm
.	O
</s>
<s>
In	O
contrast	O
to	O
saturation	O
and	O
pileup	O
effects	O
,	O
problems	O
caused	O
by	O
partial	O
energy	O
deposition	O
and	O
multiply	O
interacting	O
photons	B-Application
is	O
aggravated	O
by	O
smaller	O
pixel	B-Algorithm
size	O
.	O
</s>
<s>
Anti-coincidence	O
logic	O
,	O
where	O
simultaneous	O
events	O
in	O
nearby	O
pixels	B-Algorithm
are	O
added	O
,	O
can	O
be	O
used	O
to	O
somewhat	O
counteract	O
counting	O
the	O
same	O
photon	B-Application
in	O
different	O
pixels	B-Algorithm
.	O
</s>
<s>
The	O
fundamental	O
challenge	O
of	O
tomographic	B-Algorithm
reconstruction	I-Algorithm
is	O
the	O
inverse	O
problem	O
of	O
reconstructing	O
the	O
3D	O
,	O
tomographic	O
information	O
of	O
a	O
volume	O
using	O
2D	O
projections	O
from	O
different	O
angular	O
positions	O
.	O
</s>
<s>
The	O
same	O
fundamental	O
methods	O
conventionally	O
used	O
for	O
tomographic	B-Algorithm
reconstruction	I-Algorithm
can	O
be	O
used	O
without	O
alteration	O
on	O
data	O
acquired	O
from	O
PCDs	O
.	O
</s>
<s>
This	O
is	O
typically	O
performed	O
by	O
writing	O
each	O
pixel	B-Algorithm
as	O
a	O
linear	O
combination	O
of	O
M	O
base	O
materials	O
of	O
known	O
properties	O
such	O
as	O
water	O
,	O
calcium	O
,	O
and	O
a	O
contrast	B-Algorithm
agent	I-Algorithm
such	O
as	O
iodine	O
.	O
</s>
<s>
Although	O
intuitive	O
,	O
this	O
pixel-wise	O
approach	O
relies	O
heavily	O
on	O
the	O
spectral	O
fidelity	O
(	O
or	O
spectral	O
calibration	O
)	O
of	O
the	O
individual	O
detector	O
pixels	B-Algorithm
across	O
thresholds	O
and	O
does	O
nothing	O
to	O
remove	O
common	O
image	O
artifacts	O
.	O
</s>
<s>
Using	O
projection-based	O
material	O
decomposition	O
,	O
the	O
material	O
composition	O
measured	O
by	O
a	O
detector	O
pixel	B-Algorithm
for	O
a	O
given	O
projection	O
is	O
expressed	O
as	O
a	O
linear	O
combination	O
of	O
M	O
basis	O
materials	O
(	O
e.g.	O
</s>
<s>
soft	O
tissue	O
,	O
bone	O
and	O
contrast	B-Algorithm
agent	I-Algorithm
)	O
.	O
</s>
<s>
This	O
is	O
determined	O
from	O
the	O
recorded	O
energy	O
histogram	B-Algorithm
,	O
for	O
example	O
through	O
maximum	O
likelihood	O
estimation	O
.	O
</s>
<s>
Instead	O
,	O
novel	O
iterative	O
algorithms	O
specific	O
to	O
photon-counting	O
CT	O
are	O
required	O
.	O
</s>
<s>
Research	O
in	O
the	O
field	O
of	O
deep	B-Algorithm
learning	I-Algorithm
has	O
also	O
introduced	O
possibilities	O
of	O
performing	O
material	O
decomposition	O
using	O
convolutional	B-Architecture
neural	I-Architecture
networks	I-Architecture
.	O
</s>
<s>
Experimental	O
PCDs	O
for	O
use	O
in	O
CT	O
systems	O
use	O
semiconductor	O
detectors	O
based	O
on	O
either	O
cadmium	O
(	O
zinc	B-Application
)	O
telluride	O
or	O
silicon	O
,	O
neither	O
of	O
which	O
need	O
cryogenic	O
cooling	O
to	O
operate	O
.	O
</s>
<s>
Cadmium	O
telluride	O
and	O
cadmium	O
zinc	B-Application
telluride	O
detectors	O
have	O
the	O
advantage	O
of	O
high	O
attenuation	O
and	O
relatively	O
high	O
photoelectric-to-Compton	O
ratio	O
for	O
X-ray	B-Library
energies	O
used	O
in	O
CT	O
imaging	O
.	O
</s>
<s>
However	O
,	O
detectors	O
made	O
of	O
Cadmium	O
telluride	O
(	O
zinc	B-Application
)	O
have	O
longer	O
collection	O
times	O
due	O
to	O
low	O
charge	O
carrier	O
mobility	O
,	O
and	O
thus	O
suffer	O
more	O
from	O
pileup	O
effects	O
.	O
</s>
<s>
They	O
do	O
not	O
suffer	O
from	O
K-escape	O
X-rays	B-Library
but	O
have	O
a	O
lower	O
photoelectric-to-Compton	O
ratio	O
at	O
X-ray	B-Library
energies	O
used	O
in	O
CT	O
imaging	O
,	O
which	O
degrades	O
the	O
collected	O
energy	O
spectrum	O
.	O
</s>
<s>
Further	O
,	O
silicon	O
attenuates	O
X-rays	B-Library
less	O
strongly	O
and	O
therefore	O
silicon	O
detectors	O
have	O
to	O
be	O
several	O
centimetres	O
thick	O
to	O
be	O
useful	O
in	O
a	O
CT	O
system	O
.	O
</s>
