Transcript FRCR 2011 - 12 Lecture 2-

Nuclear Medicine:
Planar Imaging and the
Gamma Camera
Katrina Cockburn
Nuclear Medicine Physicist
Methods of Analysis
 Once tracer has traced – need some
method of analysing distribution
Imaging
Gamma Camera,
PET Camera
Compartmental
Analysis
Sample Counter
Radiation Detectors
 Converts incident photon into electronic
signal
 Most commonly used detectors are
scintillation
 Photon interacts with crystal to convert
incident photon into light photons
 PMT changes light into electrical signal
 Electrical signal recorded and analysed
Imaging Equipment
 The Gamma Camera
 Basic principle hasn’t
changed since 1956!
Scintillation Imaging
 Administration of Isotope
Scintillation Imaging
 Localisation and Uptake
Scintillation Imaging
 Localisation and Uptake
Scintillation Imaging
 Localisation and Uptake
Scintillation Imaging
 Localisation and Uptake
Scintillation Imaging
 Localisation and Uptake
Scintillation Imaging
 Localisation and Uptake
Scintillation Imaging
 Localisation and Uptake
Enhanced contrast
between Organ of
Interest and rest of
body
Scintillation Imaging
 Imaging distribution
Gamma-rays emitted by
radiopharmaceutical
Collimator ‘selects’ only
those rays travelling at
right angles to face of
camera
Scintillation events in
crystal recorded
Early Scintillation Study
Components of a Modern Gamma
Camera
 The components of a modern gamma
camera
Lead Shield
Electronics
PMTs
Lightguide
Crystal
Collimator
The Collimator
 The collimator consists
of:
 a lead plate
 array of holes
 It selects the direction of
the photons incident on
the crystal
 It defines the geometrical field of view of the
camera
The Collimator
Detector
Detector
Patient
Patient
 In the absence of collimation:
 no positional relationship between source –
destination
 In the presence of collimation:
 all γ-rays are excluded except for those travelling
parallel to the holes axis – true image formation
Types of Collimators
 Several types
of collimator:




Parallel-Hole
Converging
Diverging
Pin-Hole
Energy Ranges of Collimators
Type of
Collimator
Energy
Range
Low Energy
(LE)
0 - 200 keV
Medium
Energy (ME)
High Energy
(HE)
Typical
Nuclide
Tc-99m
Tl-201
200 - 300 keV In-111
300 – 400 keV I-131
The Scintillation Crystal
 First step of image
formation
 Photon detected by its
interaction in the
crystal
 γ-rays converted into
scintillations
Scintillation
 Can be thought of as
“partial ionisation”
 Electrons excited and
gain energy
 As electrons fall
back to ground
state, photons
emitted
 Use of doping (eg
NaI:Tl) creates
smaller gaps
Scintillation Crystal Properties
 High stopping efficiency
 Stopping should be without scatter
 High conversion of γ-ray energy into visible light
 Wavelength of light should match response of
PMTs
 Crystal should be transparent to emitted light
 Crystal should be mechanically robust
 Thickness of scintillator should be short
Properties of NaI(Tl) Scintillator
 The crystal – NaI(Tl)
 emits light at 415 nm
 high attenuation
coefficient
 intrinsic efficiency:
90% at 140 keV
 conversion efficiency:
10-15%
 energy resolution:
15-20 keV at 150 keV
Disadvantages of NaI(Tl) crystal
 NaI(Tl) crystal suffers from the
following drawbacks:
 Expensive (~£50,000 +)
 Fragile
 sensitive against mechanical stresses
 sensitive against temperature changes
 Hygroscopic
 encapsulated in aluminium case
Lightguide and Optical Coupling
 Lightguide acts as optical coupler
 Quartz doped plexiglass (transparent
plastic)
 The lightguide should:
 be as thin as possible
 match the refractive index of the scintillation crystal
 Silicone grease to couple lightguide, crystal
and PMT
 No air bubbles trapped in the grease
The Photomultiplier Tube
 A PMT is an evacuated
glass envelope
 It consists of:
 a photocathode
 an anode
 ~ 10 dynodes
The Photomultiplier Tube
 Photocathode of PMT emits 1
photoelectron per ~ 5 – 10 photons
 Photoelectron accelerated towards first
dynode
 Dynode emits 3 – 4 secondary e- per
photoelectron
 Secondary e- accelerated towards next
dynode
 Multiplication factor ~ 106
 Output of each PMT proportional to the
number of light photons
PMT Properties
 The photocathode
should
 be matched to blue light
 have high quantum
efficiency
 High stability voltage
supply: ~1kV
Positional and Energy Co-ordinates
 PMT signals processed
 spatial information – X and Y signals
 energy information – Z signal
 Z signal – the sum of the outputs of all PMTs
 proportional to the total light output of the crystal
 Light output proportional to the energy of incident
gamma
 Pulse height analyser accepts or rejects the
pulse
Pulse Height Analysis
 Z-signal goes to PHA
 PHA checks the energy of the γ-ray
 If Z-signal acceptable
 γ-ray is detected
 position
determined by
X and Y signals
 20% window still
includes 30% of
scattered photons
Determining the Position of Events
Image Acquisition Techniques
 Static
 Dynamic
 Gated
 Tomography
-
(Bones, Lungs)
(Renography)
(Cardiac)
-
(Cardiac)
 SPECT
 PET
 List Mode
Static Imaging
1
Camera
Computer Memory
Image Display
 Camera FOV divided into regular matrix of pixels
 Each pixel stores number of gamma rays
detected at corresponding location on detector
 Typical Matrix Sizes: 2562, 1282, 642
Static Imaging
1
1
Camera
Computer Memory
Image Display
 Camera FOV divided into regular matrix of pixels
 Each pixel stores number of gamma rays
detected at corresponding location on detector
 Typical Matrix Sizes: 2562, 1282, 642
Static Imaging
1
1
1
Camera
Computer Memory
Image Display
 Camera FOV divided into regular matrix of pixels
 Each pixel stores number of gamma rays
detected at corresponding location on detector
 Typical Matrix Sizes: 2562, 1282, 642
Static Imaging
1
1
1
1
Camera
Computer Memory
Image Display
 Camera FOV divided into regular matrix of pixels
 Each pixel stores number of gamma rays
detected at corresponding location on detector
 Typical Matrix Sizes: 2562, 1282, 642
Static Imaging
2
1
1
1
Camera
Computer Memory
Image Display
 Camera FOV divided into regular matrix of pixels
 Each pixel stores number of gamma rays
detected at corresponding location on detector
 Typical Matrix Sizes: 2562, 1282, 642
Static Imaging
2
1
1
1
Camera
Computer Memory
1
Image Display
 Camera FOV divided into regular matrix of pixels
 Each pixel stores number of gamma rays
detected at corresponding location on detector
 Typical Matrix Sizes: 2562, 1282, 642
Static Imaging
3
1
1
1
Camera
Computer Memory
1
Image Display
 Camera FOV divided into regular matrix of pixels
 Each pixel stores number of gamma rays
detected at corresponding location on detector
 Typical Matrix Sizes: 2562, 1282, 642
Static Imaging
3
1
1
1
Camera
Computer Memory
1
Image Display
 Camera FOV divided into regular matrix of pixels
 Each pixel stores number of gamma rays
detected at corresponding location on detector
 Typical Matrix Sizes: 2562, 1282, 642
Dynamic Imaging
 Series of sequential
static frames
 E.g. 90 frames each
of 20s duration
 Image rapidly
changing distribution
of activity within the
patient
 Used in Renography
Dynamic Imaging Analysis
Split Renal
Function
ROIs
Curves
showing
changing
renal activity
over time
Gated Imaging
 Several frames
acquired covering the
cardiac cycle
 Acquired over many
cycles