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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