Electron – Positron Annihilation
e+ + e- = 2 * 511keV gammas at 180 degrees
Positioning the event
Ring of detectors
Coincidence detection – i.e. Electronic collimation
“Standard” tomographic imaging
Imaging
ring
Detector
gamma ray 1
Point of positron
emission
Event into
image
gamma ray 2
Detector
Coincidence
circuit
First
Second
2D
3D
Detector blocks
Consider for PET
Random events
Scatter
Attenuation
Resolution
Sensitivity
Detector properties
Random Events
Finite time window photons from two
separate decays may be
detected as coincident
events
Scattered events “blurring” of the image
Random Events
Co-incidence time window length selected to match
properties of the detector material
Fast detectors allows shorter windows – hence random
events are reduced
Short window time with a slow detector - reduce
sensitivity
Random Events
Random rate = 2 t R1R2
(where R1 and R2 are the count rates of the two
detectors individually)
As the real concidence rate increases the proportion of
randoms increases more quickly until it becomes
dominant
Dealing with Random Events
Adding a delay into one arm of the coincidence circuitry
can detect just randoms.
In theory these can be subtracted from the full data set to
leave the reals.
Poor count statistics means that data quality deteriorates
rapidly once randoms exceeds the number of real
events
Scatter
Scattering in the patient will
be result in events being
detected as real events but
in the wrong position
Scattered events will cause
image degredation
Scatter
In the gamma camera scattered events are rejected by
good pulse height analysis i.e. only accept gammas in
the photo peak
(e.g 140+-10% for Tc 99m on a GC)
Unfortunately -the detectors used in PET have poor
energy resolution (e.g BGO)
Energy windows are typically set at 350-650keV and
thus accept a lot of scatter
Scatter Correction
Often ignored for 2D imaging but some correction required
for 3D
Various approaches for approximate scatter correction images obtained from energy windows set to collect
scatter can be subtracted from the main image
Scatter Correction-iterative
More complex approaches can be used to give more
accurate correction
Model the physics of the whole system using an attenuation
map, the emission images and the characteristics of the
detector geometry.
As emission images contain scatter an iterative approach is
required
Attenuation is a major problem in a PET
2 photons need to be detected for a real event i.e if either is
attenuated the event is lost. Thus attenuation is
significantly greater than in single photon detection
Half value thickness in tissue is ~7cm at 511keV
Attenuation can be as high as 50% in large patients
Attenuation
Attenuation can be corrected for
Attenuation Correction
Attenuation is independent of the source distribution
Need to establish the distribution of the attenuating properties
of the tissue in the FOV to apply suitable correction factors
2 approaches:
Transmission source of isotope
CT Scan on PET/CT system
Resolution
Finite range of the positron in tissue
Annihilation photons are not exactly at 180
Resolution limit 1.5mm-2.5mm theoretically
In Practice technological limitations - image resolution is
~ 5-8mm
Sensitivity
As there is no in-plane collimation – sensitivity increased over
single photon imaging by ~50
With 3D imaging there is no physical collimation – further
increase in sensitivity by a factor of ~6
Thus image noise is greatly reduced as compared with standard
NM to th higher count densities
Detector materials
Initially NaI but inefficient at stopping 511keV photons
(Standard gamma cameras were used to perform PET imaging
but the poor count statistics & resolution at 511keV rapidly
lead to the procurement of PET imaging systems)
Bismuth germanate (BGO) – commonly used
Newer - Lutetium Oxyorthosilicate (LSO) - shorter decay time
– essential for 3D
Ideal Scintillator for PET
High light output in photons/event ie good scintillation
efficiency
Short decay time for fast light output (helps for short coincidence times)
High stopping power at 511keV
Refractive index close to glass to get good impedance
matching with PMTs
Robust, easy to manufacture and cheap
Typical Detector Characteristics at 511keV
light output
(%NaI)
decay constant attenution coefficient
(ns)
( per cm)
NaI
100
230
0.34
BGO
15
300
0.92
LSO
75
40
0.87
PET- advantages over standard NM
imaging
Higher sensitivity with resultant lower Poisson noise
Attenuation correction is easier
Can model PSF over the field of view for
Scatter correction methods readily available
Theortically accurate quantification of uptake is possible
Radionuclides used in PET
Cyclotron produced
e.g. 12C + 2H = 13N + n
Nuclide
18F
11C
13N
15O
1/2life
110m
20.4m
10m
122s
Common PET Radiopharmaceuticals and
applications
15 O2
oxygen extraction
13N-ammonia
blood flow measurement
11C-N-methylspiperone
receptor studies
18FDG (fluorodeoxyglucose) tissue metabolism
“Accuracy” of PET in Oncology
Investigation
Ring-PET
CT
Staging Lung
Cancer
92%
70%
Detecting Breast
Cancer
85%
67 %
(Mammography
Staging Ovarian
Cancer
87%
70%
Summary- Advantages of PET
Advantages
High Sensitivity – reduces Poisson noise
Potentially Absolute Quantification - modelling
Physiological Isotopes – radio chemistry
Disadvantages
Cost - both capital & running costs
Requires on site cyclotron (or limited to only FDG 18)
Currently 5 PET cameras in Scotland
Real Events; Random & Scattered events