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Medical Imaging > PET/CT basic principles > Flashcards

PET/CT basic principles Flashcards

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USMLE® Step 1 flashcards
1
Q

What does a PET scanner consist of?

A

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

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2
Q

What are the 4 steps of the scintillation process?

A

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

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3
Q

what converts the incident light into a large electrical signal?

A

Photomultiplier tube

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4
Q

What technology converts light directly into a digital signal?

A

Digital photon counting technology

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5
Q

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

A

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

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6
Q

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

A

sensitivity gain
volumetric resolution
quantitative accuracy

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7
Q

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

A

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

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8
Q

What is the basis of coincidence detection & coincidence imaging?

A

Simultaneous emission of two photons in opposite directions

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9
Q

What do simultaneous pulses from detectors indicate?

A

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

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10
Q

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

A

How much activity there was in the LOR

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11
Q

When are radial artifacts visible (star effect)?

A

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

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12
Q

How are artifacts caused by low count data avoided?

A

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

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13
Q

What is the size of the detector related directly to?

A

Spatial resolution (smaller detector = better spatial resolution)

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14
Q

What causes blurring in the image?

A

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

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15
Q

What is regarded as a true coincidence event

A

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

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16
Q

What are 2 artificial coincidences?

A

random
scatter

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17
Q

What is the recorded count rate a combination of?

A

Trues
randoms
scatter

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18
Q

What do random and scatters degrade?

A

Image both qualitatively and quantitatively

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19
Q

What is attenuation?

A

Loss of true events due to scatter and absorption

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20
Q

What are the characteristics of resulting images when attenuation occurs?

A

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

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21
Q

what is the CT scan used for in PET scans?

A

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

22
Q

What do the attenuation values depend on?

A

Energy

23
Q

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

A

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

24
Q

What are truncation artifacts?

A

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

25
Q

What are respiration artifacts?

A

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

26
Q

What are 3 other potential artifacts?

A

CT contrast agents
metal implants
beam hardening

27
Q

What does time of flight make use of?

A

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

28
Q

What is the time difference due to?

A

Different distances travelled

29
Q

Is the location of annihilation site known?

A

No it must be recovered by image reconstruction

30
Q

How do you calculate SUV?

A

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

31
Q

What areas will have SUV’s > 1?

A

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

32
Q

What 5 things can influence SUVs?

A

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

33
Q

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

A

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

34
Q

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

A

Point spread functions (PSFs) in the field of view

35
Q

What does PSF-based reconstruction reconstruction cause?

A

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

36
Q

When is PET in radiotherapy most widely used?

A

For NSCL - identifies collapse region/consolidation

37
Q

What happens to GTV in many cases?

A

It is reduced which opens up the potential to dose escalate

38
Q

How is PET/CT used in dementia?

A

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

39
Q

How is PET/CT used in epilepsy?

A

Localization of epileptopgenic foci

40
Q

How is AI used in PET?

A

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

41
Q

How is PET/CT used in cardiology?

A

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

42
Q

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

A

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

43
Q

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

A

To suppress myocardial glucose utilization

44
Q

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

A

Patients with psoriasis
Patients with HIV

45
Q

What is the purpose of the iMAR algorithm?

A

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

46
Q

What is the purpose of respiratory motion correction?

A

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

47
Q

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

A

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

48
Q

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

A

reduces overall radiation dose
reduce positron range effect

49
Q

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

A

MR data

50
Q

What are the 5 different image classes of MR/

A

air
lungs
fat
fat-nonfat tissue mixture
non fat

51
Q

What are 4 advantages of PET?

A

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

52
Q

What are 4 limitations of PET?

A

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

Medical Imaging (10 decks)

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