Nuclear Medicine - Imaging Systems

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IMAGING SYSTEMS


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Imaging in Nuclear Medicine

Radionuclide Imaging

Gamma Camera

PrinciplesSystem Components (Design)

Detector SystemCollimators

OperationHow Image is Formed

Event Detection


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Basic Performance Characteristics: Gamma Came

Intrinsic Spatial Resolution

Detection Efficiency

Energy Resolution

Limitations of DetectorNon-linearity

Non-uniformity


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SPECT

Principles

PET

Principles

PET ImagingHybrid Systems


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IMAGING IN NUCLEAR MED

Radionuclide imagingUse of radionuclides

Oral or IV

Recorded via external

radiation detectorsEnergy range ~ 80 to 500

keV

Photon annihilation ~ 511keV

Alpha particles and electrons Cannot penetrate tissues

Bremsstrahlung

More penetrating

Weak intensity


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IMAGING IN NUCLEAR MED

Imaging Detector

must have good detection efficiency for -rays

must be able to discriminate energy (for rays w/positional info)

NaI(Tl)Sodium Iodide doped with Thulium


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GAMMA CAMERA


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GAMMA CAM

Anger Scintillation Camera

Hal Ange


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GAMMA CAMERA: DETES System Components

Scintillation Crystal Light guide

Photomultiplier tubes

Collimator

Analog-to-Digital Converter (ADC)

Computer Monitor


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GAMMA CAMERA: SCINTILLC

Thallium dopedSodium IodideCrystal NaI(Tl)


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GAMMA CAMERA: SCINTILLC

Thallium doped Sodium Iodide Crystal NaI(Tl)Thickness: 6 to 12.5 mm

Tradeoff between: Detection efficiency and Intrinsic sparesolution

Size: 60 x 40 cm

Diameter (round crystals): 25 to 50 cm diameter

Highly reflective material (e.g. T maximize light output

Hermetically sealed in Al-casin protection from moisture


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GAMMA CAMERA: PM TUBELIGHT

Photomultiplier Tubes

Round PMT: arranged in hexagonal pattern tomaximize the area of NaI(Tl) crystal covered

Typical diameter: 5 cm

Most modern cameras: 30100 PMTs

Use of hexagonal or square cross-sections of PMTs eliminates the use of

light guides

Light guides Channels light away from the gaps

between PMTs

increases collection efficiency Improves uniformity of light

collection as function of position


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GAMMA CAMERA: COLLIM

CollimatorNarrows or aligns a beam or particle

Allows the focusing of gamma cameras

Pinhole Parallel Hole Diverging

Basic Collimator types


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GAMMA CAMERA: HOW IMFO


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GAMMA CAMERA: HOW IMAGE IS


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GAMMA CAMERA: EVENT DETE

(A) Valid Event

(B) Detector Scatter Event

(C) Object Scatter Event

(D) Septal Penetration


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GAMMA CAMERA:PERFORMANCECHARACTERISTICS


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PERFORMANCE CHARACTEINTRINSIC SPATIAL RESO

Spatial Resolution

Measure of sharpness and detail of image

Ability of the camera to separate two objects as tmoved closer to each other

Intrinsic Spatial Resolution

Limit of spatial resolution achievable by the detecelectronics


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Decreasing gamma-ray energydecrease in intrinsic resolution

Lower energy gamma raysproduce fewer light photons per

scintillation event


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Increasing crystal thicknessdecrease in intrinsic resolution

Thicker detectors result in greater spreadingof scintillation light before it reaches the PMtubes.

Greater likelihood of detecting multiple

Compton-scattered eventsparticularly inhigh energy radionuclides.


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Increasing efficiency of collection of scintillation pho

Improves intrinsic resolution


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PERFORMANCE CHARACTEDETECTION EFFIC


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PERFORMANCE CHARACTEENERGY RESO

Energy resolutionAbility of the detector to accurately determine the energy

incoming radiation

Ex: For a 10% Energy Resolution:


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DETECTOR LIMITA

Image NonlinearityResults when X- and Y- position signals do not change linearly with d

distant of radiation source across the face of the detector.

Pincushion Distortion Barrel Distortion


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DETECTOR LIMITA

Image Non-uniformity


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SPECT: SINGLE PHOTON

EMISSION COMPUTEDTOMOGRAPHY


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SPECT: PRINPrinciples

Similar to planar imagingusing gamma camera

Captures multiple 2Dimages (projection) frommultiple angles

Tomographicreconstruction is applied tothe projections

Yields a 3D dataset


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SPECT: PRIN


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SPECT: IM


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PET: POSITRON EMISSIOTOMOGRAPHY


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PET: PRIN

Functional imaging technique

Detects gamma rays indirectly

Emitted by a positron-emitting radionuclide


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PET: PRIN

Positron Emission and Annihilation

Positron annihilates with electron Two 511 keV photons

Photons are anti-parallel


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PET: PRIN

Annihilation CoincidenceDetectionEach detector generates

a timed pulse when itregisters an incidentphoton

When pulses are withinthe timed window, theyare consideredcoincident

Coincident events areassigned to a lineresponseprovidespositional informationwithout using a physicalcollimator


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PET: PRIN

Coincident Events


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PET IMA


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PET IMA


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HYBRID SYSTEMS:SPECT/CT AND PET/CT


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WHY USE HYBRID SYS

SPECT AND PET Provides functional information

using radiotracers Designed to measure

physiologic or metabolicparameters

Anatomical information isimportant to determine thelocation of activity

X-Ray CT Consists of x-ray tube with a

detector array mounted on arotating gantry

Can provide anatomicalinformation


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Nuclear Medicine - Imaging Systems

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