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Flashcards in PET & Spect Deck (57):
1

What does PET stand for?

Positron Emission Tomography

2

How are positron-emitting radionuclides produced? (3)

  • Produced in cyclotron
  • Tend to be useful nuclides such as OXygen, Nitrogen or Fluoride
    • O - 2mins
    • F - 110 mins
  • Radionuclides need to be delivered to hospital before half-life is too weak

3

Basis of PET scanning (5)

  • Positron is emitted within the body, where the electron & positron anihilate to produce two photons.
  • Two photons travel in opposite directions
  • Two photons detected within a few billionths of a second
  • A line is drawn where two photons are recorded
  • Gives rise to high-resolution image

 

4

What is Iterative Reconstruction?

Uses many different iterations to work out the image

5

What does FDG stand for?

FluoroDeoxyGlucose

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Benefits of FDG (2)

  • Provides a more accurate staging than other imaging procedures alone
  • FDG can be shipped from site with 2 hour half-life

 

7

What does SUV stand for?

Standardised Uptake Value

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What does SUV do? (2)

  • Useful semi-quantitive index of FDG uptake
  • Monitor progress of disease during therapy

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Calculation of SUV

SUV = counts / (injected dose x body weight)

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What value of SUV is good?

A low SUV value

11

How are radionuclides produced?

  • Accelerate charged particles (protons) to high energy
  • Make them collide with a target e.g. H2O with 18O
  • Nuclear rection sinside target
    • 18O plus a proton = 18F plus a neutron
    • Original 18O becomes 18F- / [18O]H2O
    • 18F has too much +'ve charge so emits e+
  • Extract radionuclides (18F) from target mixture

 

12

What equipment is used to accelerate a charged particle?

Cyclotron

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How much energy do charged particles need to have?

10MeV

14

Which two fields does a cyclotron use to accelerate a charged particle and their uses

  • Magnetic - causes particles to follow circular path
  • Electric - causes acceleration

 

15

Cyclotron Operation (4)

Q image thumb

  1. D's connected to AC
  2. Ions released into gap between D's
    1. Electric discharge in gas
    2. Accelerated right into D by E-field
    3. No E-field in D
  3. Steered back by M-field
  4. Emerge into Dee as e-field switches direction

 

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Two types of Cyclotron Developments (2)

  • Negative Ion technology
  • Targets

 

17

What is Negative Ion technology? (2)

  • H- ions accelerated to 10-50MeV
  • Pass through carbon foils that strips electrons to produce H+ (protons) beam to collide with target

 

18

How does the proton beam & H- ion beam diverge?

Through magnetic field depending on the sign of the ion

19

What is the target made out of?

A stable target nucleus

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Example of FDG production (3)

  • Irradiate [18O]H2O liquid --> 18F/[18O]H2O mixture washing down transfer line to hot cell
  • Can be up to 10-20 metres
  • Approx takes 20mins for new equipment

 

21

What does LOR stand for?

Line Of Response

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What does a PET scanner do? (3)

  • Patient surrounded by full/partial ring of detectors
  • Two photons detected in coincidence (LOR)
  • Want to maximise A, minimise B

A image thumb
23

PET Camera Design (2)

  • Ring of detectors around patient
    • BGO - good stopping power, long decay time
  • Each ring separated by septa 2D mode

A image thumb
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What is the resolution of a PET system determined by?

Detector size

25

Sensitivity for a PET camera @ low count rates is determined by (5)

  • Geometry
  • Total volume of detector material
  • Stopping power of detectors
  • Characteristics of septa
  • Low Compton Scatter

 

26

What do the Septa on a PET scanner do?

Minimise scatter

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What does the removal of Septa allow?

Take account of 3D imaging by allowing more scattering which allows more counts

28

Factors affecting PET camera performance (5)

  • 2D or 3D
  • Geometrical design
  • Choice of scintillator
  • Electronics
  • Attenuation Correction
    • Scatter Correction

29

What does IQ stand for?

Image Quality

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Benefits of 3D PET scanning (3)

  • Improved IQ
  • Shorter scan for same IQ
  • Reduce dose

 

31

Disadvantage of 3D over 2D scanning

More scatter but can be corrected

32

How does scatter correction work in 3D PET scanning? (3)

  • Energy window based
  • Convolution / deconvolution based
  • Direct estimate of scatter (through Monte-Carlo methods), however these are computationally intensive

 

33

What factor affects the geometry of PET scanner?

Whether or not the axis is aligned

34

What is a scintillator?

A material that fluoresces when struck by a charged particle or high-energy photon

35

What are required of Scintillators in the new generation of 3D scanners? (2)

  • Faster coincidence detection
  • Ideal Scintillators

 

36

What makes up an ideal scintillator? (6)

  • Short decay time
  • High light output
  • High atomic number 
  • Rugged
  • Low cost
  • High Availability

 

37

The reduction in detected photons is only dependent on...

... the total attenuation path, which is independent of the source.

38

What type of correction is used to find out the attenuation co-efficient along each LOR?

Transmission attenuation

39

What angles are the LORs for 2D & 3D?

  • 2D - LORs nearly all the same angle
  • 3D - LORs all different angles

 

40

What is determines the resolution of State of the Art PET systems?

The size of individual crystals

41

What do new LSO (Lutetium Oxyorthoscillate) crystals have over older formations?

higher light output

42

What sets the theoretical resolution limit?

Positron range in tissue ~ 1mm

43

Depth of interaction problem (2)

  • Causes parallax error or radial elongation
  • Effect worse for small-ring systems

 

44

What does TOF stand for?

Time-of-Flight

45

Why is TOF useful? (2)

  • Calculate the difference in time travelled by photos
  • Use timing information to restrict the LOR

 

46

What does TOF info improve? (2)

SNR

  • Reduces noise, therefore the 'noise equivalent count rate' (NEC) increases
    • Also known as sensitivity increase

Reduces overlapping of LORs

  • Overlapping causes blurring & noise
  • As patient size increases, # overlapping LORs within patient increases

47

What does SPECT stand for?

Single Photon Emission Computed Tomography

48

Advantages of PET over SPECT (2)

  • Sensitivity 
    • 2D PET 0.5% of all photon pairs
    • 3D PET 3 -5%
    • Gamma cameras 0.01 - 0.02% per detector head
  • Radio-pharmaceuticals
    • FDG is very flexible
    • Generally easier to label with positron emitters

 

49

Disadvantages of PET (4)

  • Logistically challenging due to short tracer half-life
    • 11C - 20.4mins & 18F - 109.5 mins
  • High capital cost due to large volume of detector material 
  • Extrememly dense detectors required due to high energy emissions
  • High radiation doses to both patients (particulate) & staff (high energy)

 

50

Advantages of SPECT (6)

  • Half-life better suited for logistics
  • Require less dense scintillators
  • Lower radiation dose to patients
  • Better spatial resolution than PET
  • Less signal loss due to attenuation & compton scattering
    • Single photon detected
  • Radiopharmaceutical production simpler

 

51

Gamma Camera: Design & Construction (5)

  1. Collimator
  2. Scintillation Detector Crystal
  3. Photomultiplier Tubes
  4. Positron/Eneergy Circuit
  5. Computer

 

52

What is a Collimator? (3)

  • A device that narrows a beam or particles or waves.
  • Cuts out any photons outside of a narrow window angle.
  • Resolution varies with distance

53

Gamma Camera Inner workings (6)

Q image thumb

  1. Ray parallel to hole, detected
  2. Penetrates collimator, detected
  3. Ray trapped by collimator
  4. Ray scatters once in patient, then detected
  5. Ray Scatters, not detected
  6. Multiple scatter, then detected

 

54

How does a Gamma Camera Work? (4)

  • Collimator lets some photons through
  • Crystal converts this to light, photomultiplier tubes to electrical pulses
  • Locations determined by electronics
  • Image is always square

 

55

What provides a better resolution for a collimator?

Smaller/longer holes

56

What is Voxel-based Morphometry (VBM) & method? 

A neuro-imaging technique that allows investigation of the brain

  • Take patient scan
  • Smooth images
  • Compare patient scan to database

 

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