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 Collimator Lightguide PMTs Electronics Crystal

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  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 Patient Detector

Types of Collimators  Several types of collimator:  Parallel-Hole  Converging  Diverging  Pin-Hole

Energy Ranges of Collimators Type of Collimator Energy Range Typical Nuclide Low Energy (LE) keV Tc-99mTl-201 Medium Energy (ME) keV In-111 High Energy (HE) 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: 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 ~ 10 6  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-(Bones, Lungs)  Dynamic-(Renography)  Gated-(Cardiac)  Tomography  SPECT  PET  List Mode-(Cardiac)

Static Imaging  Camera FOV divided into regular matrix of pixels  Each pixel stores number of gamma rays detected at corresponding location on detector  Typical Matrix Sizes: 256 2, 128 2, 64 2 CameraComputer MemoryImage Display 1

Static Imaging  Camera FOV divided into regular matrix of pixels  Each pixel stores number of gamma rays detected at corresponding location on detector  Typical Matrix Sizes: 256 2, 128 2, 64 2 CameraComputer MemoryImage Display 1 1

Static Imaging  Camera FOV divided into regular matrix of pixels  Each pixel stores number of gamma rays detected at corresponding location on detector  Typical Matrix Sizes: 256 2, 128 2, 64 2 CameraComputer MemoryImage Display 1 1 1

Static Imaging  Camera FOV divided into regular matrix of pixels  Each pixel stores number of gamma rays detected at corresponding location on detector  Typical Matrix Sizes: 256 2, 128 2, 64 2 CameraComputer MemoryImage Display

Static Imaging  Camera FOV divided into regular matrix of pixels  Each pixel stores number of gamma rays detected at corresponding location on detector  Typical Matrix Sizes: 256 2, 128 2, 64 2 CameraComputer MemoryImage Display

Static Imaging  Camera FOV divided into regular matrix of pixels  Each pixel stores number of gamma rays detected at corresponding location on detector  Typical Matrix Sizes: 256 2, 128 2, 64 2 CameraComputer MemoryImage Display

Static Imaging  Camera FOV divided into regular matrix of pixels  Each pixel stores number of gamma rays detected at corresponding location on detector  Typical Matrix Sizes: 256 2, 128 2, 64 2 CameraComputer MemoryImage Display

Static Imaging  Camera FOV divided into regular matrix of pixels  Each pixel stores number of gamma rays detected at corresponding location on detector  Typical Matrix Sizes: 256 2, 128 2, 64 2 CameraComputer MemoryImage Display

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 ROIs Curves showing changing renal activity over time Split Renal Function

Gated Imaging  Several frames acquired covering the cardiac cycle  Acquired over many cycles