Nuclear Medicine: Nuclear Medicine: Planar Imaging and the Planar Imaging and the

Gamma CameraGamma CameraKatrina CockburnKatrina Cockburn

Nuclear Medicine PhysicistNuclear Medicine Physicist

Methods of AnalysisMethods of Analysis

Once tracer has traced – need some Once tracer has traced – need some method of analysing distributionmethod of analysing distribution

Imaging Imaging Gamma Camera, Gamma Camera,

PET CameraPET Camera

Compartmental Compartmental AnalysisAnalysis

Sample CounterSample Counter

Radiation DetectorsRadiation Detectors

Converts incident photon into electronic Converts incident photon into electronic signalsignal

Most commonly used detectors are Most commonly used detectors are scintillationscintillation Photon interacts with crystal to convert Photon interacts with crystal to convert

incident photon into light photonsincident photon into light photons PMT changes light into electrical signalPMT changes light into electrical signal Electrical signal recorded and analysedElectrical signal recorded and analysed

Imaging Equipment Imaging Equipment

The Gamma CameraThe Gamma Camera Basic principle hasn’t Basic principle hasn’t

changed since 1956!changed since 1956!

Scintillation ImagingScintillation Imaging

Administration of IsotopeAdministration of Isotope

Scintillation ImagingScintillation Imaging

Localisation and UptakeLocalisation and Uptake

Scintillation ImagingScintillation Imaging

Localisation and UptakeLocalisation and Uptake

Scintillation ImagingScintillation Imaging

Localisation and UptakeLocalisation and Uptake

Scintillation ImagingScintillation Imaging

Localisation and UptakeLocalisation and Uptake

Scintillation ImagingScintillation Imaging

Localisation and UptakeLocalisation and Uptake

Scintillation ImagingScintillation Imaging

Localisation and UptakeLocalisation and Uptake

Scintillation ImagingScintillation Imaging

Localisation and UptakeLocalisation and Uptake

Enhanced contrast Enhanced contrast between Organ of between Organ of Interest and rest of Interest and rest of bodybody

Scintillation ImagingScintillation Imaging

Imaging distributionImaging distributionGamma-rays emitted by Gamma-rays emitted by radiopharmaceuticalradiopharmaceutical

Collimator ‘selects’ only Collimator ‘selects’ only those rays travelling at those rays travelling at right angles to face of right angles to face of cameracamera

Scintillation events in Scintillation events in crystal recorded crystal recorded

Early Scintillation StudyEarly Scintillation Study

Components of a Modern Gamma Components of a Modern Gamma CameraCamera

The components of a modern gamma The components of a modern gamma cameracamera Lead ShieldLead Shield

CollimatorCollimator

LightguideLightguide

PMTsPMTs

ElectronicsElectronics

CrystalCrystal

The CollimatorThe Collimator The collimator consists of:The collimator consists of:

a lead platea lead plate array of holesarray of holes

It selects the direction of It selects the direction of the photons incident on the photons incident on the crystalthe crystal

It defines the geometrical field of view of the It defines the geometrical field of view of the cameracamera

The CollimatorThe Collimator

In the In the absenceabsence of collimation: of collimation: no positional relationship between source – destination no positional relationship between source – destination

In the In the presencepresence of collimation: of collimation: all all γ-γ-rays are excluded except for those travelling rays are excluded except for those travelling

parallel to the holes axis – true image formationparallel to the holes axis – true image formation

Patient Patient

Detector Detector

Types of CollimatorsTypes of Collimators

Several types Several types of collimator:of collimator: Parallel-Hole Parallel-Hole Converging Converging DivergingDiverging Pin-HolePin-Hole

Energy Ranges of CollimatorsEnergy Ranges of Collimators

Type of Type of CollimatorCollimator

Energy Energy RangeRange

Typical Typical NuclideNuclide

Low Energy Low Energy (LE)(LE)

0 - 200 keV0 - 200 keV Tc-99mTc-99mTl-201Tl-201

Medium Medium Energy (ME)Energy (ME)

200 - 300 keV200 - 300 keV In-111In-111

High Energy High Energy (HE)(HE)

300 – 400 keV300 – 400 keV I-131I-131

The Scintillation CrystalThe Scintillation Crystal

First step of image First step of image formationformation

Photon Photon detected by its detected by its interaction in the interaction in the crystalcrystal

γ-γ-rays converted into rays converted into scintillationsscintillations

ScintillationScintillation Can be thought of as Can be thought of as

“partial ionisation”“partial ionisation” Electrons excited and Electrons excited and

gain energygain energy As electrons fall As electrons fall

back to ground back to ground state, photons state, photons emittedemitted

Use of doping (eg Use of doping (eg NaI:Tl) creates NaI:Tl) creates smaller gapssmaller gaps

Scintillation Crystal PropertiesScintillation 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) ScintillatorProperties 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) crystalDisadvantages of NaI(Tl) crystal

NaI(Tl) crystal suffers from the NaI(Tl) crystal suffers from the following drawbacksfollowing drawbacks:: Expensive (~£50,000 +)Expensive (~£50,000 +) FragileFragile

sensitive against mechanical stresses sensitive against mechanical stresses sensitive against temperature changessensitive against temperature changes

Hygroscopic Hygroscopic encapsulated in aluminium caseencapsulated in aluminium case

Lightguide and Optical CouplingLightguide and Optical Coupling Lightguide acts as optical coupler Lightguide acts as optical coupler Quartz doped plexiglass (transparent Quartz doped plexiglass (transparent

plastic)plastic) The lightguide should:The lightguide should:

be as thin as possiblebe as thin as possible match the refractive index of the scintillation crystalmatch the refractive index of the scintillation crystal

Silicone grease to couple lightguide, crystal Silicone grease to couple lightguide, crystal and PMTand PMT

No air bubbles trapped in the greaseNo air bubbles trapped in the grease

The Photomultiplier TubeThe Photomultiplier Tube

A PMT is an evacuated A PMT is an evacuated glass envelopeglass envelope

It consists of:It consists of: a photocathodea photocathode an anodean anode ~ 10 dynodes~ 10 dynodes

The Photomultiplier TubeThe Photomultiplier Tube Photocathode of PMT emits 1 Photocathode of PMT emits 1

photoelectron per ~ 5 – 10 photonsphotoelectron per ~ 5 – 10 photons PhotoelectronPhotoelectron accelerated towards first accelerated towards first

dynodedynode Dynode emits 3 – 4 secondary eDynode emits 3 – 4 secondary e-- per per

photoelectronphotoelectron Secondary eSecondary e-- accelerated towards next accelerated towards next

dynodedynode Multiplication factor ~ 10Multiplication factor ~ 1066

Output of each PMT proportional to the Output of each PMT proportional to the number of light photonsnumber of light photons

PMT PropertiesPMT Properties The photocathode The photocathode

shouldshould be matched to blue lightbe matched to blue light have high quantum have high quantum

efficiencyefficiency High stability voltage High stability voltage

supply: ~1kVsupply: ~1kV

Positional and Energy Co-ordinatesPositional and Energy Co-ordinates PMT signals processedPMT signals processed

spatial spatial informationinformation – – X and Y signalsX and Y signals energy information – Z signalenergy information – Z signal

Z signal – the sum of the outputs of all PMTsZ signal – the sum of the outputs of all PMTs proportional to the total light output of the crystalproportional to the total light output of the crystal Light output proportional to the energy of incident gamma Light output proportional to the energy of incident gamma

Pulse height analyser accepts or rejects the pulsePulse height analyser accepts or rejects the pulse

Pulse Height AnalysisPulse Height Analysis Z-signal goes to PHAZ-signal goes to PHA PHA checks the energy of the PHA checks the energy of the γγ-ray-ray If Z-signal acceptableIf Z-signal acceptable

γ-γ-ray is detectedray is detected position position

determined by determined by X and Y signalsX and Y signals

20% window still20% window stillincludes 30% ofincludes 30% ofscattered photonsscattered photons

Determining the Position of EventsDetermining the Position of Events

Image Acquisition TechniquesImage Acquisition Techniques

StaticStatic -- (Bones, Lungs)(Bones, Lungs) DynamicDynamic -- (Renography)(Renography) GatedGated -- (Cardiac)(Cardiac) TomographyTomography

SPECTSPECT PETPET

List ModeList Mode -- (Cardiac)(Cardiac)

Static ImagingStatic Imaging

Camera FOV divided into regular matrix of pixelsCamera FOV divided into regular matrix of pixels Each pixel stores number of gamma rays Each pixel stores number of gamma rays

detected at corresponding location on detectordetected at corresponding location on detector Typical Matrix Sizes: 256Typical Matrix Sizes: 25622, 128, 12822, 64, 6422

Camera Computer Memory

Image Display

1

Static ImagingStatic Imaging

Camera FOV divided into regular matrix of pixelsCamera FOV divided into regular matrix of pixels Each pixel stores number of gamma rays Each pixel stores number of gamma rays

detected at corresponding location on detectordetected at corresponding location on detector Typical Matrix Sizes: 256Typical Matrix Sizes: 25622, 128, 12822, 64, 6422

Camera Computer Memory

Image Display

1

1

Static ImagingStatic Imaging

Camera FOV divided into regular matrix of pixelsCamera FOV divided into regular matrix of pixels Each pixel stores number of gamma rays Each pixel stores number of gamma rays

detected at corresponding location on detectordetected at corresponding location on detector Typical Matrix Sizes: 256Typical Matrix Sizes: 25622, 128, 12822, 64, 6422

Camera Computer Memory

Image Display

1

1

1

Static ImagingStatic Imaging

Camera FOV divided into regular matrix of pixelsCamera FOV divided into regular matrix of pixels Each pixel stores number of gamma rays Each pixel stores number of gamma rays

detected at corresponding location on detectordetected at corresponding location on detector Typical Matrix Sizes: 256Typical Matrix Sizes: 25622, 128, 12822, 64, 6422

Camera Computer Memory

Image Display

1

1

1

1

Static ImagingStatic Imaging

Camera FOV divided into regular matrix of pixelsCamera FOV divided into regular matrix of pixels Each pixel stores number of gamma rays Each pixel stores number of gamma rays

detected at corresponding location on detectordetected at corresponding location on detector Typical Matrix Sizes: 256Typical Matrix Sizes: 25622, 128, 12822, 64, 6422

Camera Computer Memory

Image Display

2

1

1

1

Static ImagingStatic Imaging

Camera FOV divided into regular matrix of pixelsCamera FOV divided into regular matrix of pixels Each pixel stores number of gamma rays Each pixel stores number of gamma rays

detected at corresponding location on detectordetected at corresponding location on detector Typical Matrix Sizes: 256Typical Matrix Sizes: 25622, 128, 12822, 64, 6422

Camera Computer Memory

Image Display

2

1

1

11

Static ImagingStatic Imaging

Camera FOV divided into regular matrix of pixelsCamera FOV divided into regular matrix of pixels Each pixel stores number of gamma rays Each pixel stores number of gamma rays

detected at corresponding location on detectordetected at corresponding location on detector Typical Matrix Sizes: 256Typical Matrix Sizes: 25622, 128, 12822, 64, 6422

Camera Computer Memory

Image Display

3

1

1

11

Static ImagingStatic Imaging

Camera FOV divided into regular matrix of pixelsCamera FOV divided into regular matrix of pixels Each pixel stores number of gamma rays Each pixel stores number of gamma rays

detected at corresponding location on detectordetected at corresponding location on detector Typical Matrix Sizes: 256Typical Matrix Sizes: 25622, 128, 12822, 64, 6422

Camera Computer Memory

Image Display

3

1

1

11

Dynamic ImagingDynamic Imaging

Series of sequential Series of sequential static framesstatic frames E.g. 90 frames each E.g. 90 frames each

of 20s durationof 20s duration Image rapidly Image rapidly

changing distribution changing distribution of activity within the of activity within the patientpatient

Used in RenographyUsed in Renography

Dynamic Imaging AnalysisDynamic Imaging Analysis

ROIsCurves showing changing renal activity over time

Split Renal Function

Gated ImagingGated Imaging

Several frames Several frames acquired covering the acquired covering the cardiac cyclecardiac cycle

Acquired over many Acquired over many cyclescycles

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