PET Physics

What Radioisotopes are used for PET Radiopharmaceuticals

• F-18

• Ga-68

• C-11

Positron emitters that are typically produced through a cyclotron

PET Corrections

• normalization

• scatter correction

• random correction

• attenuation correction

• dead time correction

• decay correction

Types of Coincidences

• True Coincidences

• Scatter Coincidences

• Random Coincidences

True Coincidence

Both 511keV photons originate from the same annihilation event and reach the detectors at the same time without interactions

Scatter Coincidence

One of the 511keV photons from the annihilation event undergoes Compton scattering, leading to an incorrect position for the line of response

Random Coincidence

Two separate annihilation events are detected within the same coincidence timing window creating a line of response between them

PET Imaging Process

1. patient is administered the radiopharmaceutical and waits for uptake

2. patient is positioned on the PET scanner

3. tracer decays via positron emission and annihilation event occurs sending two 511keV photons in opposite directions hitting the detector rings

4. photons arrive within the coincidence timing window (5-12 ns) and creates a line of response

5. PET scintillation detectors turn the photons into visible light

6. light photons are then converted into a electrical signal via PMT and SiPM

7. the electrical signal is analyzed by the pulse heigh analyzer to see whether it fits within the energy windows (around 350-650keV)

8. accepted signals is passed through a coincidence processor

9. data is recorded in views and each pixel in the sinogram corresponds to a LOR

10. image is reconstructed using algorithms

Annihilation Event

• proton suddenly becomes a neutron and emits a positive electron (positron)

• positron travels and annihilates with an electron

• this reaction causes two photons of 511keV to travel in opposite directions

Noise Equivalent Count Rate (NECR)

gives the equivalent count rate of the observed count rate after scatter and random coincidences are corrected.

where is true counts, is scatter, is randoms, and is the fraction of the sinogram used for scatter estimation.

What does NECR do?

• a higher NECR will mean better image quality

• NECR helps gives the optimal dose to give

• increasing the patient dose will increase the amount counts however this will also increase the amount of noise in the image from randoms and scatter. the NECR peak will show where increasing activity until it reaches a point where there’s an increase in noise causing a decrease in image quality

Cross Calibration

• calibrates PET counts/ml into units of MBq/ml and against the dose calibrator that is used to measure the activity

• a uniform phantom is filled with activity measured by the dose calibrator and the phantom is scanned to estimate the cross calibration factor

PET Scintillation Crystal Characteristics

• high density and effective atomic number - high stopping power for interactions

• high light output

• fast light decay - reduces dead time and increases randoms rejection

• visibile light photon matches response of PMT

• low self-absorption

• non-hydroscopic

examples include: BGO or LSO

Dead Time

is the time required to process individual detected events

if a second pulse occurs when the first one has not finished it will be disregarded

Non-Paralyzable System

when an event occurs during dead time, the second event is ignored and there is no effect on subsequent events

• digital counters

• pulse-heigh analyzers

• computer interfaces

Paralyzable System

when an event occurs during another events dead time it is not counted however it still introduces its own dead time

• “extendable“ dead time in most radiation detectors

Standard Uptake Value (SUV)

normalizes the activity concentration measured by the PET scanner with the injected dose and patients weight

• the acitviity concentration (MBq/ml) and density of tissue (1g/ml) and estimates the activity in the tissue per mass (MBq/kg)

Limitations of SUV

• physiological factors

• time post injection the SUV is measured

• partial volume effects for smaller tumors may result in underestimation

• assumes all activity administered goes into circulation (extravasated injections)

• FDG uptake in fat is usually lower than muscle or other tissue meaning there is possible overestimation in obese patients

Time of Flight

measures which of the two detectors received the annihilation photons first, and uses that tiny time difference to better localize where along the line of response the event happened

• this helps improve signal-to-noise ratio

PET Sensitivity

is the ability to detect true annihilation events per unit of activity of radioactivity present in the FOV (cps/MBq)

depends on

• intrinsic detector efficiency - stopping power of crystal and light yield

• geometric efficiency - proportion of emitted photons that reach the detectors (influenced by detector rings diameter and axial coverage)

high sensitivity allows for shorter scan time and lower injected activity

PET/CT Daily QC

PET QC

• normalization

• singles, coincidence, timing, energy

• sinograms

CT QC

• air calibrations

• uniformity with water at HU of 0

• noise (standard deviation of HU)

• slice thickness

Partial Volume Effect

is the loss of accuracy in small structures due to a decreased spatial resolution. it occurs when an objects size is similar to systems spatial resolution leading to underestimation of activity concentration and spill over into adjacent regions

• spill-out effect - activity in hot lesions spills out into surrounding tissue

• spill-in effect - activity from high uptake regions spill into nearby low uptake regions

Energy Resolution

is the ability to distinguish photons of different energies

• having a tighter energy window can reduce scatter nut increases scan time

• defined by full-width at half-maximum (FWHM)

• you want a lower energy resolution and low energy threshold

Scatter Fraction

is the fraction of how much scatter is present in the final image

Spatial Resolution

is the ability to see small objects and measured in full-width at half-maximum (FWHM)

Temporal Resolution

is the time between each successful electrical pulse

• it shows how quickly a crystal can decay light

• coincidence timing window is 6–10 ns

• TOF timing resolution is 300–600 ps

better timing → better localization → improved SNR