PET/CT basic principles

Question 1

What does a PET scanner consist of?

Answer 1

Ring of scintillation detectors arranged so pairs of detectors on opposite sides of the ring operate in coincidence with one another

Question 2

What are the 4 steps of the scintillation process?

Answer 2

Electrons in valence band can absorb energy by interaction of photoelectron or Compton scatter electron with an atom & get excited into conduction band
electron de-excites by releasing scintillation photons & returns to ground state
Impurities or activators are added to modify the band structure
scintilliation is in visible range

Question 3

what converts the incident light into a large electrical signal?

Answer 3

Photomultiplier tube

Question 4

What technology converts light directly into a digital signal?

Answer 4

Digital photon counting technology

Question 5

Why is the digital photon counting technology better than traditional analog photomultipliers?

Answer 5

System improves signal-to-noise ratio which enables improved sensitivity & image clarity

Question 6

What does 1:1 coupling of crystals to digital detectors double?

Answer 6

sensitivity gain
volumetric resolution
quantitative accuracy

Question 7

What are the 4 main properties of scintillator which are crucial for PET application?

Answer 7

High stopping power to stop 511 keV gamma rays
decay time of excited states must be short
high light output
good intrinsic energy resolution

Question 8

What is the basis of coincidence detection & coincidence imaging?

Answer 8

Simultaneous emission of two photons in opposite directions

Question 9

What do simultaneous pulses from detectors indicate?

Answer 9

Annihilation occurred somewhere along the path between 2 detectors, referred to as line of response

Question 10

What does the no. of coincidence events occurring between detectors indicate?

Answer 10

How much activity there was in the LOR

Question 11

When are radial artifacts visible (star effect)?

Answer 11

When count rates are low or a region of interest with low activity is near an organ with high activity

Question 12

How are artifacts caused by low count data avoided?

Answer 12

by using alternative reconstruction method that calculates the activity image iteratively in an algebraic fashion

Question 13

What is the size of the detector related directly to?

Answer 13

Spatial resolution (smaller detector = better spatial resolution)

Question 14

What causes blurring in the image?

Answer 14

because the annihilation site and not the site of radioactive atoms are imaged by PET

Question 15

What is regarded as a true coincidence event

Answer 15

if 2 gamma rays from a single annihilation event enter the PET detector without undergoing any significant interactions within the imaging field of view

Question 16

What are 2 artificial coincidences?

Answer 16

random
scatter

Question 17

What is the recorded count rate a combination of?

Answer 17

Trues
randoms
scatter

Question 18

What do random and scatters degrade?

Answer 18

Image both qualitatively and quantitatively

Question 19

What is attenuation?

Answer 19

Loss of true events due to scatter and absorption

Question 20

What are the characteristics of resulting images when attenuation occurs?

Answer 20

artificially-depleted radioactivity deep in the body while outer contour of the body shows an artificially high amount of radioactivity

Question 21

what is the CT scan used for in PET scans?

Answer 21

used to correct PET data for loss of counts due to attenuation

Question 22

What do the attenuation values depend on?

Answer 22

Energy

Question 23

What is a major advantage of the PET/CT scanner?

Answer 23

Provides an anatomical image which can be superimposed on the functional PET image

Question 24

What are truncation artifacts?

Answer 24

If a part of the patient’s anatomy extends beyond the boundaries of transverse CT FOV the part of the anatomy is not reconstructed

Question 25

What are respiration artifacts?

Answer 25

Mismatches of breathing patterns are particularly severe with breath-hold techniques

Question 26

What are 3 other potential artifacts?

Answer 26

CT contrast agents metal implants beam hardening

Question 27

What does time of flight make use of?

Answer 27

The small difference in time between the detection of the 2 photons emitted from the same positron annihilation event

Question 28

What is the time difference due to?

Answer 28

Different distances travelled

Question 29

Is the location of annihilation site known?

Answer 29

No it must be recovered by image reconstruction

Question 30

How do you calculate SUV?

Answer 30

(activity in ROI (MBq)/vol(ml)) / injected activity (MBq)/patient weight (g)

Question 31

What areas will have SUV's > 1?

Answer 31

areas with higher average uptake higher the SUV greater the risk of disease

Question 32

What 5 things can influence SUVs?

Answer 32

Reconstruction algorithm Blood glucose level ROI used Uptake period Patient motion/breathing

Question 33

What is the partial effect due to and what does it affect?

Answer 33

Due to low spatial resolution affects quantitative accuracy of PET/CT images

Question 34

What is incorporated into the reconstruction to address partial volume effect?

Answer 34

Point spread functions (PSFs) in the field of view

Question 35

What does PSF-based reconstruction reconstruction cause?

Answer 35

edge artifacts & a high degree of overestimation of radioactivity in small regions

Question 36

When is PET in radiotherapy most widely used?

Answer 36

For NSCL - identifies collapse region/consolidation

Question 37

What happens to GTV in many cases?

Answer 37

It is reduced which opens up the potential to dose escalate

Question 38

How is PET/CT used in dementia?

Answer 38

As an indicator to differentiate Alzheimer's disease from other types of dementia in its early stages

Question 39

How is PET/CT used in epilepsy?

Answer 39

Localization of epileptopgenic foci

Question 40

How is AI used in PET?

Answer 40

Assist in classifying tumours/areas in brain with less/more uptake than expected

Question 41

How is PET/CT used in cardiology?

Answer 41

For coronary artery assessment - evaluates significance of individual stenoses and provide valuable prognostic information

Question 42

What does the combined PET multi-detector CT system allow for in cardiology?

Answer 42

Direct comparison of a PET study with CT coronary angiography performed in the same sitting

Question 43

In cardiac sarcoid imaging why is it crucial for patients to follow an appropriate diet?

Answer 43

To suppress myocardial glucose utilization

Question 44

What are 2 new applications of PET/CT for inflammation?

Answer 44

Patients with psoriasis Patients with HIV

Question 45

What is the purpose of the iMAR algorithm?

Answer 45

Increases the confidence of the interpretation of the PET/CT scan & influences SUV

Question 46

What is the purpose of respiratory motion correction?

Answer 46

AI does automatic amplitude-based gating which virtually freezes respiratory motion

Question 47

What are 2 challenges of dual modality PET/MRI scans?

Answer 47

Radiofrequency interference between 2 systems PET detectors working in high magnetic fields

Question 48

What are 2 advantages for dual modality PET/MRI scanners?

Answer 48

reduces overall radiation dose reduce positron range effect

Question 49

What data is used to perform AC of PET/MRI?

Answer 49

MR data

Question 50

What are the 5 different image classes of MR/

Answer 50

air lungs fat fat-nonfat tissue mixture non fat

Question 51

What are 4 advantages of PET?

Answer 51

Differentiate malignant from benign determine extent and stage of cancer detect unsuspected metastatic disease differentiate between residual scar tissue, radiation necrosis and tumour recurrence

Question 52

What are 4 limitations of PET?

Answer 52

Range of cancers do not accumulate FDG also accumulates in inflammatory lesions lesion < 4mm not seen Scan takes 10-15 mins not a screening test