3. PET
imaging:
Ideal
case
= Annihilation event
Positron
emitting
nucleus
ϒ1
= event of detection
of photon in the
detector ring
e+
Detector
Ring
Line of
Response
(LOR)
ϒ2
Two simultaneous (within
6-12 ns time difference)
events in the detector make
a line of response.
4. Data
arrangement
ϒ1
θ
Angle (θ)
Sinogram
ϒ2
t
(t, θ)
Position (t)
Count on a Line of Response gets mapped to the corresponding position in a
sinogram.
5. DeviaEons
from
Ideal
case
–
scaGer
events
True counts
ϒ1
e+
Detector
Ring
True Line of
Response
(LOR)
ϒ2
ϒ2`
= Annihilation event
Detected scatter
event’s misplaced
LOR
= event of detection
of photon in the
detector ring
6. DeviaEons
from
Ideal
case-‐
Random
counts
True counts
ϒ1
+
Detector
Ring
False (LOR),
contributed from two
separate annihilation
events
ϒ2
= Annihilation event
= event of detection
of photon in the
detector ring
7. PET:
Noise
• Background
noise:
ScaGer
events
map
misplaced
counts
to
the
sinogram
and
Random
events
map
false
or
spurious
counts
• ScaGer
-‐
Supress
with
collimaEon
and
detectors
with
beGer
energy
resoluEon
• Randoms
-‐
Supress
by
smaller
sampling
window
of
coincidence,
i.e.,
faster
scinEllator
detectors;
and
CollimaEon
8. Factors
affecEng
counts
• Trues
↑
as
radio-‐nuclide
concentraEon
↑
(the
good
stuff)
↓
as
paEent
size
↑
(absorpEon
and
scaGer
effects)
• Randoms
↑↑
as
count
rate
↑
(varies
as
square
of
count
rate)
Effects
dominate
image
noise
at
high
injected
acEviEes
Reduce
by
faster
electronics,
faster
crystals
• Sca/ers
↓
with
collimaEon
(about
15%
for
2D
PET
and
50%
for
3D
PET)
Reduce
with
collimaEon
(energy
selecEon
not
efficient
in
PET)
9. PET
improvements
leading
to
improved
SNR
• Faster
electronics
• Faster
scinEllators:
shorter
coincidence
Eming
window,
reduced
dead
Eme,
improved
energy
resoluEon
window.
• PET/CT
technology,
ACFs
from
CT
scan
(reduced
paEent
movement
related
noise
factors
from
long
transmission
scans),
total
scanning
Eme
reduced
to
10-‐15
minutes.
• Fourier
re-‐binning
and
staEsEcally-‐based
algorithms
10. Resources
• DW
Townsend,
‘Physical
principles
and
technology
of
clinical
PET
imaging’,
BJR,
March
2004,
Vol
33
(2)
• Bushberg
book
• J
A
Anderson,
‘IntroducEon
to
PET/CT’,
CRCPD
winter
2004
meeEng
presentaEon
