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Nuclear Med Flashcards

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1
Q

SPECT - Compared to Gamma Camera

A

Better contrast

Worse spatial resolution than gamma

Higher noise than gamma

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2
Q

PET - Compared to SPECT

A

Better spatial resolution

Less noise

SPECT has better contrast

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3
Q

Decay Patterns

A

M -4 ALPHA DECAY (too many protons or neutrons)

A -2

M 0 BETA -VE DECAY (too many neutrons). Neutron converted to proton in nucleus and an electron and antineutrino released from the nucleus)

A +1

M 0 BETA +VE DECAY (too few neutrons) (positron produced)

A -1

M 0 ELECTRON CAPTURE (too few neutrons). A proton captures an electron from one of the shells

A -1 (starts with K) and converts to neutron. Iodine 123 decays this way producting a gamma wray and x-ray (given off as a result of hole in electron shell)

M 0 ISOMERIC TRANSITION (decay of radionuclide to ground state giving off gamma radiation as in Tn-99m)

A 0

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4
Q

Positrons

A

Same mass as electrons (1/1840)

Only exist while they have energy

Positive charge

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5
Q

PET

A

30,000 bismuth detectors

Gamma photons 511keV produced by annihilation between electron and positron

Fluorine decays by beta positive decay

The activity from each point source can be measured as a sine curve or sonogram

Spatial resolution 4-5mm

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6
Q

PET Detectors

A

Good energy resolution

High detection efficiency

Short scintillation decay time

High atomic number and physical density

High light output

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7
Q

SPECT

A

Camera moves in 3 or 6 degree increments

  • can be continuous or step and shoot

Produces 3D images

Spatial resolution 15-20mm

Contrast good in SPECT

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8
Q

Cyclotron

A

Cyclotron

67Gallium

111Indium

123Iodine

201Tl

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9
Q

Gamma Camera

A

Gamma Camera

  • 100 photomultiplier tubes that determine position and intensity of photons
  • Technetium is a pure GAMMA EMITTER
  • Lead collimator thickness varies:
  • 0.3mm
  • 1mm
  • 2mm

Typical sensitivity is up to 2%

(Around 75-90% of gamma photons are detected)

Scattered photons can still be accepted if energy is within the photopeak

Photomultiplier tube contains a series of dynodes

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10
Q

Gamma Camera

A

Gamma Camera

-spatial resolution is made better by either

—making collimator holes smaller

—making lead septa thicker

—making collimator thicker

This however will reduce the number of gamma photons reaching the scintillation crystal (will hit collimator instead) and therefore sensitivity will decrease

Photomultipliers and count rate do NOT affect spatial resolution

Metal objects can cause artefact in Gamma cameras bu absorbing some of the gamma rays which result in appearance of a possible ‘cold lesion’

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11
Q

Photopeak

A
  • Photopeak is produced by pulses that are a result of photoelectric interaction
  • ‘Pulse pile up’ is caused by two or more photons being detected at same time. This peak occurs to the right of the main photopeak
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12
Q

Photomultiplier

A

Needs a photocathode for operation (converts light into electrons)

  • Photocathode releases 1 electron for every 5-10 photons
  • Photomultipler contains a series of dynodes than amplify electrons
  • 10-12 dynodes, each more positive in potential than the last
  • each dynode has potential difference of 1.2kV (total potential 1-1.2kV
  • each dynode releases 4 electrons for every electron that hits it
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13
Q

Pulse Height Analyser

A

Reduces noise!!

Located between detector and counting system

Can detect more than one photo-peak

Wide window produces images faster but they are of lower quality due to scatter and lower energy photons

Narrow window produces better quality image

Collimator allows location of the source of gamma photons within patient (primary purpose is not to filter out scattered radiation. Pulse height analyser does this)

The use of collimators improves spatial resolution

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14
Q

Quality Control in Gamma

A

Field Uniformity (Test of detector)

-Field uniformity refers to the ability to produce a uniform distribution of activity

Can be tested using a flat, sealed plate of cobalt 57 called a flood field or sheet phantom

  • Intrinsic floods performed WITHOUT collimator (addition of a collimator reduces intrinsic resolution)
  • Extrinsic floods performed WITH collimator

Modern cameras have uniformity of at least 2%

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15
Q

Quality Control in Gamma

A

Spatial Resolution

Tested using a Line Source which results in creation of a graph called the Line Spread Function

  • From this we can measure FWHM which is used to compare resolutions of different systems
  • Gamma camera resolution generally 5-10mm
  • Intrinsic resolution 3-4mm (intrinsic resolution is always the best)
  • FWHM is a measure of spatial resolution of a system
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16
Q

Collimators in Gamma Camera

A

Collimator allows location of the source of gamma photons within patient (primary purpose is not to filter out scattered radiation. Pulse height analyser does this)

The use of collimators improves spatial resolution

There is a trade off between collimator sensitivity and spatial resolution

Increasing number of holes increases sensitivity!!

Increasing collimator sensitivity reduces patient dose (more gama rays passing through collimator measn less radionuclide needed hence lower patient dose. This results in lower spatial resolution however

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17
Q

Internal conversion

A

Gamma ray interacts with inner shell electrons via photoelectric effect releasing

Photoelectrons

Characteristic x rays

Examples: Iodine 125 and 123

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18
Q

Beta Particles

A

Beta Particles

  • Are made up of electrons
  • Negative electrons are negative beta particles
  • Positive electrons are positrons

Beta particles follow a random path in matter like secondary electrons

Release as a continuous spectrum of energies (like bremmst)

NOT specific energies

Can only travel a few mm in tissue

Result in a trail of ionized atoms

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19
Q

Ideal Radiopharmaceutical Properties

A

Pure gamma emitter between 50 - 300keV

Half life suitable for examination

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20
Q

Half Lives

A

Krypton - 13 seconds

Fluorine - 110mins

Technetium 99 - 6 hours

Molybdenum 99 - 67 hours

Biological half life is dependent on the rate if ELIMINATION of the radionuclide from the body (NOT rate of radioactive decay)

The dose received by the organ is PROPORTIONAL to the effective dose

The pharmaceutical used does NOT affect the half life

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21
Q

Gamma Camera Colliamtors

A

Collimators locate radioactive source within patient along line of sight (primary function is NOT to reduce scatter)

In parallel hole - the field of view and in air sensitivity are the same at all distances

Divergent and Convergent collimators suffer from geometric distortion at edges and reduced spatial resolution

Fish tail collimators cause distortion

22
Q

Gamma Camera Sodium Iodide Crysal

A

Sodium iodide crystal

  • 500mm wide
  • Approx 10mm thick
  • emits 5000 light photons when struck by gamma photon

Absorbs gamma rays by photoelectric absorption

Fragile, can be damaged by damp and is susceptible to changes in temp

23
Q

Dose in Radionuclide Imaging

A

Dependent on emitted gamma energy

Proportiontal to administered activity

Dependent on all half lives

Effective dose is measured in sieverts

24
Q

Dose for different examinations

A

Renal studies - 2mSv

Lung scans - 2mSv

Bones Scan - 5mSv

Brain scan - 8mSv

Thalium heart scan - 18mSv

25
Q

Creating Radioactivity

A

Nucleur Reactor - neutron added to a stable nucleus

Cyclotron - addition of a proton into a nucleus causing a neutron to be ejected out

26
Q

Separating Radionuclides

A

Can be separated from carrier in Cyclotron as have different atomic numbers and therefore different properties

Cannot be separated in Nuclear Reactor as both have same atomic numbers and therefore same properties

27
Q

Radioactive Decay

A
  • Not an energy dependent process, occurs independently
  • After decay can result in metastable, unstable or stable daughter nucleus

Radioactive decay of a radionuclide decreases by equal fractions in equal intervals of time

Number of radioactive atoms in a sample is proportional to rate of decay

The higher the rate of decay, the higher number of beta and gamma particles produced

28
Q

Isomer

A

Isomer

Following beta decay in some radionuclides, gamma ray is not released immediately. Before and after the emission of the gamma ray, these are called isomers

  • Different energy states and half lives
  • Same atomic number
  • Same mass number
29
Q

Electron Capture

A

Where there is a a neutron deficit

Electron taken from inner shell and combined with proton to form a neutron

No net charge. Negative charge or electron with positive proton charge cancel eachother out

Characteristic radiation released

30
Q

Ideal properties of a pharmaceutical agent (that attaches to radionuclide)

A
  • Should accumulate in target tissue which is its function
  • Should remain stable after labelling with radionuclide
31
Q

Mo-99 Decay

A

Mo-99 decays to Tc-99m and releases a negative electron in the process

Technetium 99m then releases a gamma photon on its decay

32
Q

Gamma Camera Sensitivity

A

It is a function of both intrinsic and collimator efficiency

Below 100keV the system sensitivity is almost 100%

Sensitivity tested using a known amount of activity in a small source

Expressed as total counts per second per megabecquerel

33
Q

V/Q Imaging

A

Tc 99 MAA is used in CTPA

Microaggregated albumin particles are 20-40um

Ventilation and perfusion CANNOT be performed simultaneously

34
Q

Background Radiation

A
  • 85% are contributed from natural source
  • Account for 10% of all cancers
  • Air travel & watches contribute minimally
  • More than 50% of the natural background radiation comes from radon
  • Medical imaging contributes MORE than 10%
35
Q

Bone Scan

A

Uses Tc99m-MDP

  • Dose is approx 6mSv
  • Are non-specific for fractures
  • Dose is 555-750MBq dose
  • For evaluation of mets, the time from injection to scan is approx 3-4 hours
36
Q

Geiger Muller Counter

A

Requires a well stabilised voltage

Are used as part of quality control

NOT used for dose measurement

Usually coated in a metal such as lead (not glass)

37
Q

Collimator Energy Types

A

Low energy - photon energy 150keV (Technetium)

  • septal thickness of 0.3mm

Medium Energy - 300keV (Indium)

  • septal thickness of 1mm

High energy - 400keV (Iodine 131)

  • septal thickness of 2mm
38
Q

Dynamic Imaging examples

A

Renal and cardiac imaging mostly performed dynamically

  • MAG3 renal scan
    • MAG3 is a compound with high renal excretion. It is labelled to Technetium. Images taken every 2 mins fir 20 mins to demonstrate uptak followed by excretion.
  • A ROI over relavent organ needs to be obtained on first image and the number of counts present
  • Images can be displayed as the time it takes for x number of counts to take place

SHORTER TIME OF ACQUISITION THE LOWER THE SPATIAL RESOLUTION

39
Q

What is matrix size in SPECT?

A

Usually 64 x 64 or 128 x 128

  • This is to reduce Noise
  • Results in low resolution
40
Q

PET - Data Correction

A

Attenuation correction

  • A radioactive rod of gallium is rotated around inside the detector WITHOUT then WITH the patient
  • Then allows for attenuation correction

Normalisation

  • To ensure detectors are perfoming properly
  • A radioactive source is rotated around without the patient and correction factor applied

Correction Factor = measured counts for line of response / average counts for all lines

41
Q

What are the control rods in Nucleur reactors made of?

A

Cadium or Boron

42
Q

What type of collimator used in SPECT?

A

High resolution collimator

43
Q

Resolution in Gamma Cameras - what affects it?

A

Intrinsic made worse by:

  • Lower energy gamma photons
  • Thicker crystals
  • Smaller number of PMT

Collimator resolution made better by:

  • Longer thinner holes
  • Shorter distance from patient

What increases spatial resolution?

  • Smaller pixels
  • Imaging superficial rather than deep structures
44
Q

PET - inflammation imaging

A

Isotopes used are produced in Cyclotron

  • Indium and Gallium
  • Both produce gamma photons
  • Indium labelled to autologous white cells
  • Both decay by electron capture
45
Q

TOF PET

A

Emerging technique

  • Aimed to reduce the amount of random coincidences
  • Allow more accurate localisation of the line of response

Advantages

  • higher spatial resolution than PET
  • gives more accurate location of LOR

Disadvantages

  • Not all scintillation crystals can be used
  • FDG cannot be used for TOF PET
46
Q

Can isotopes only be used for ventilations scanning if they are gaseous?

A

Yes they have to be gaseous

Xenon must be used with rebreathe apparatus to avert bystander dose

  • Xenon has half life of 5 days
  • emits beta and gamma
  • is gas at room temp
  • produced in nucleur reactor
  • Krypton has half life of 13secs
  • gas at room temp
  • produced in generator
  • produces gamma rays
47
Q
A
48
Q

How does Technetium 99m decay?

A

Isomeric transition releasing gamma photons

49
Q

How does Mo 99 decay?

A

Beta decay

Releases beta particle

50
Q

What collimator used for technetium?

A

LEHR

Low energy high resolution

Physics (10 decks)

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