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*FRCR1 Physics > Mersey Nuc Med > Flashcards

Mersey Nuc Med Flashcards

1
Q

Radioactive Decay

  • Radioactive atoms have unstable nuclei
  • Radioactive decay is a spontaneous attempt by a nucleus
    to become more stable
  • The arrangement of protons & neutrons in the nucleus is restructured to achieve a more stable configuration and ionising radiation (energetic particles and/or high energy EM radiation) is emitted
  • The emitted radiation is easily detected and can be used
    to form an image.
A
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2
Q

Radioactive Decay modes

Beta minus decay

MASS number does not change just atomic number increases by one

A
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3
Q

Isomeric transition

Tc99m decays via isomeric transition into Tc99 with the emission of a 140keV gamma ray

A
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4
Q

Internal Conversion (IC)

Excess energy in nucleus is TRANSFERRED TO K SHELL electron - then emitted

Tc99m

A
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5
Q

Positron Decay

MASS number does not change just atomic number DECREASES by one

A
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6
Q

Electron Capture (EC)

➢ a proton-rich nucleus captures a K-shell
electron to neutralise its excess positive
charge

A
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7
Q

Activity

  • A radioactive sample contains millions of unstable atoms.
  • Each second some of these atoms will undergo a radioactive disintegration into a “daughter” atom and give off radiation.
  • The activity of a sample is how many disintegrations occur each second.
A
  • Activity is measured in units of “becquerel” (Bq)
  • 1 Bq = 1 disintegration/sec
  • 1 MBq (mega-becquerel) = 1 million dis/sec
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8
Q

Radioactive Decay Constant
* Radioactive atoms don’t all disintegrate at once.
* In any sample, only a fraction of the atoms disintegrate each second.
* This fraction is called the decay constant, λ
* λ changes from one substance to another.

A

Half-Life

  • This is a measure of how quickly the activity of a
    source diminishes with time.
  • It is the time needed for half the activity to disappear.
  • After 1 half-life the activity drops to 50%
  • After 2 half-lives the activity drops to 25%
  • After 3 half-lives the activity drops to 12.5%
  • The most common radionuclide in nuclear medicine
    (Tc99m) has a half-life of 6 hours.
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9
Q

How the activity of a source changes with time

A
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10
Q

Radiopharmaceuticals (RPs)

  • RPs are radioactive drugs (“tracers”) given to patients
  • Radiopharmaceutical = drug + radionuclide
A
  • Drug – determines where the RP will localise in the body
  • Radionuclide – produces a detectable signal (diagnostic study) or delivers a radiation dose (therapy)
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11
Q

Technetium-99m (Tc99m)

A
  • Short half-life (6 hours)
  • Only emits gamma radiation
  • Decays into a “mildly” radioactive daughter
  • Easily obtainable
  • Can be attached to a wide range of drugs to make radiopharmaceuticals

GAMMA RAYS with a photon energy of 140 keV

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12
Q

Mo99/Tc99m Generator

  • A fresh supply of Tc99m is needed each morning.
  • Molybdenum99 (Mo99) is radioactive and decays into Tc99m with a half-life of 66 hours.
  • A Mo99/Tc99m generator contains a glass tube filled with tiny spheres coated
    with alumina & Mo99.
A
  • As Mo99 decays, Tc99m builds up in the tube
  • The 2 radionuclides are in
  • Passing saline through the glass tube extracts the Tc99m but not the Mo99
  • Optimum delay between successive elutions of the generator is 23hr
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13
Q

Mo99/Tc99m Generator

A
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14
Q

Radiopharmaceuticals

  • Tc99m can be added to commercial pharmaceutical kits to make
    specific radiopharmaceuticals.
  • Different radiopharmaceuticals are needed to study different
    organ systems.
A
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15
Q

Nuclear Medicine Imaging

A
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16
Q

The Gamma Camera

  • Generates an image of the pattern of gamma radiation emitted from a patient injected with a radiopharmaceutical
A
  • It consists of:
    ➢ A collimator – forms an image of the activity distribution in the patient
    ➢ A scintillation crystal – absorbs g-rays & emits light
    ➢ An array of photomultiplier tubes – detects light emitted by the crystal
    ➢ Pulse processing electronics – produces spatial X,Y coordinates and an energy signal (Z)
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17
Q

Pulse Height Analysis

  • Scatter degrades contrast – so need to reject it
  • Scattered radiation has a lower energy than primary (unscattered) radiation
  • Analyse the size of the Z (energy) pulses detected
A
  • Only accept Z pulses that lie within a preset range
  • This is the pulse height analyser window
  • Window width is typically ±10% of the γ ray energy
  • For Tc99m window is from 126-154 keV
  • The collimator does not reject scatter
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18
Q

PHA Window Placement

Because of limited energy resolution there will always be some scattered
radiation accepted by the PHA window. Using a narrow window will reject
more scatter but will also reject some primary events too (again due to limited
energy resolution).

A
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19
Q

Nuclear Medicine Image Quality

  • Spatial resolution
    ➢ The ability to distinguish two adjacent objects in the patient as separate
    structures
    ➢ Nuclear Medicine images have low resolution due to the collimator properties
A
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20
Q
  • Contrast
    ➢ Difference in signal strength between target and background
    ➢ Contrast in nuclear medicine is due to the bio-distribution of the
    radiopharmaceutical
    ➢ Reflects patient physiology rather than anatomy
A
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21
Q
  • Noise - Main issue
    ➢ Any unwanted variations in signal strength
    ➢ Nuclear medicine images are noisy – relatively few γ-rays are detected due to
    the inefficiency of using a collimator
    ➢ Main source of noise is quantum mottle (random variations in signal strength)
    ➢ Images look “grainy”
A
22
Q

Spatial Resolution in NM Images

For optimum spatial resolution….
* Position the patient as close as
possible to the collimator face
(gamma cameras are short-sighted)
* Use a high-resolution collimator

A
  • Make the patient comfortable
    (acquisition times are long so need
    to minimise patient motion)
  • Spatial resolution is typically ~10mm
    FWHM (0.05lp/mm)
23
Q

Contrast in NM Images

  • Contrast is mainly controlled by the
    relative uptake of the tracer in
    different tissues
  • Relative uptake depends on the
    tracer & the condition of patient
  • Scatter reduces contrast
  • Patient size is important
A
  • A narrow PHA window rejects more
    scatter & improves contrast (some
    unscattered γ-rays are also rejected)
  • PHA window width is typically 15-
    20% of the photopeak as a
    compromise
24
Q

Noise in NM Images

  • “Poisson” or random noise
    (quantum mottle) is always
    apparent in Nuc Med images
  • To reduce random noise more -
    rays need to be detected:
    1) Use a lower resolution
    collimator (more efficient at
    detecting γ-rays, but poorer spatial
    resolution)
    2) Longer acquisition time (more
    chance of patient motion artifacts)
    3) Increase the injected activity
    (increases patient dose)
A
25
Q

Factors Affecting Patient Dose
in Nuclear Medicine

Each disintegration that occurs within the patient will result in a certain amount
of energy being deposited (ie a radiation dose). Factors that affect either the
number of disintegrations in the patient or the energy deposition per
disintegration will affect patient dose.

A
26
Q

Single Photon Emission Computed
Tomography

  • SPECT - Tomographic studies performed with a gamma camera & tracers labelled with Tc99m, I123 or In111
  • Multiple views (60-120) taken at different angles around
    the patient.
A
  • Use dual detector systems to speed up acquisition.
  • Tomographic images reconstructed using filtered
    backprojection (FBP) or iterative reconstruction (IR)
27
Q

Backprojection

A
28
Q

Filtered Backprojection

A
29
Q

FBP – Choice of Filter

A
30
Q

Iterative Reconstruction

A
31
Q

Iterative Reconstruction (OSEM)

A
  • Iterative reconstruction advantages
    – More accurate reconstructions than FBP
    – Can incorporate detector modelling during reconstruction to
    correct for poorer spatial resolution at depth; known as
    “resolution recovery”
    – Can correct for attenuation & scatter
  • Iterative reconstruction disadvantages
    – Can be very slow (lots of iterations to compute on lots of
    projections)
    – Additional cost (hardware & software)
  • OSEM is a faster version of iterative reconstruction
32
Q

SPECT Image Quality

  • Spatial Resolution – ability to identify fine detail. Optimised by
    ➢ high resolution collimator
    ➢ non-circular orbits around the patient
    ➢ acquiring a large number of projections
    ➢ High cut-off filter during reconstruction
    ➢ Spatial resolution typically 10-15mm FWHM
A
  • Contrast – difference in image intensity between target & background
    ➢ Determined by the bio-distribution of the radiopharmaceutical
    ➢ Iterative reconstruction with attenuation correction & scatter correction will
    improve contrast
33
Q
  • Noise – SPECT images are noisy
    ➢ High resolution collimator & limited time per projection mean low number
    of detected gamma rays
    ➢ Can inject higher activities for SPECT (550MBq for WB bone vs 750MBq for
    SPECT bone) OR use resolution recovery reconstruction.
    ➢ Choice of filter applied during reconstruction can reduce noise
A
34
Q

SPECT Artifacts

Reduced signal due to poor camera performance - creates a ring artefact

Focal non-uniformities within gamma camera can lead to ring artefacts

A
35
Q

Attenuation Correction in SPECT

Low dose CT acquired before

Variable photon attenuation for different paths can cause artifacts. SPECT
data needs to undergo attenuation correction (AC). This is often provided
by a “low dose” CT scan (ie not diagnostic quality).

A

SPECT/CT Alignment

Poor aligment can lead to misregistrations

36
Q

SPECT Artifacts

A
37
Q

Positron Emission Tomography

  • PET - Tomographic studies performed with a dedicated
    scanner & tracers labelled with positron emitting radionuclides eg F18, C11, N13 etc
A
  • Scanner comprises multiple rings of detectors surrounding the patient – no rotation needed.
  • Coincidence detection - pairs of detectors used to record single events
  • Tomographic images reconstructed using iterative reconstruction (IR)
38
Q

Annihilation Radiation

A
39
Q

Coincidence Detection

A
39
Q

True Coincidences (“Trues”)

A GENUINE response

A
40
Q

Random Coincidences (“Randoms”)

Gamma from two different disintegration

A
41
Q

Scatter Coincidences (“Scatter”)

A
42
Q

PET Scanners use Block Detectors

Bismuth Germanate (BGO) or Lutetium Oxyorthosilicate (LSO)
crystals are better than NaI for detection of 511keV gamma rays.
NaI is not used in PET scanners

A
43
Q

PET Image Quality

  • High sensitivity for detecting radiation
    – ~100x better than SPECT
  • Spatial resolution
    – 4mm FWHM at centre
    – 5mm FWHM towards periphery
    – Better SNR with “Time of Flight”
A
44
Q

Spatial Resolution in PET

A

This is affected by three factors:

  • The distance travelled by the positron before it annihilates (~0.5-1.0mm).
  • Non-colinearity (the two annihilation -rays are not emitted at exactly 180o to one another).
  • The size of the individual detectors which detect the annihilation radiation.
45
Q

Positron Path Length

A
46
Q

Non Colinearity of Photon Emission

A
47
Q

Resolution loss due to detector size

The spatial resolution of a PET
scanner is best at the centre of the
image and degrades towards the
periphery

A
48
Q

Time-of-Flight PET

  • Using fast scintillators (LSO or
    LYSO) can measure the time
    difference between the arrival of
    the gammas.
  • This isn’t exact but can be used
    to improve the accuracy of the
    reconstruction
  • This leads to an improvement in
    the SNR

Compare the uniform probability
weighting of the LOR in standard
PET with the use of TOF
information to constrain the
location of annihilation site during
image reconstruction.

A
49
Q

18FDG Scans

  • The adult dose of 18FDG is based on body mass, & is typically 3MBq/kg
  • Administered activities are typically 200-350 MBq
  • Effective dose from 18FDG administration is 5-8mSv
A
50
Q

PET/CT Artifacts

A cold artifact (arrow) is commonly seen on the dome of the diaphragm/liver or at lung base. This only appears on PET images that have been corrected for attenuation. PET/CT Artifacts

This artifact is created due to different
patterns of breathing between the (much
shorter) CT scan and the longer PET scan.
Using shallow breathing during the CT
acquisition can help reduce the effect.

A
51
Q

Standard Uptake Value (SUV)

A

*FRCR1 Physics (19 decks)

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