Dr. Mohammed AlnafeaDr. Mohammed Alnafeaalnafea@ksu.edu.saalnafea@ksu.edu.sa

Nuclear MedicinePrinciples & Technology_I

Nuclear medicine imagesNuclear medicine images

1. Single photon imaginga) Planar 2D imageb) Single Photon Emission Computed

Tomography (SPECT) 3D image2. Positrons Emission Tomography

(PET).

All reveal the spatial and temporal distribution of target-specific pharmaceuticals in the human body.

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Nuclear MedicinePrinciples & Technology

Non-invasive determination of physiologic processes

Tracer principle:

Radiopharmaceuticals are distributed, metabolized, and excreted according to their chemical structure

Display of biological functions as:

-Images -Numerical data -Time-activity curves

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Nuclear medicine imagesNuclear medicine images

Depending on the application, the nuclear medicine data can be interpreted to yield information about physiological processes such as :

1.glucose metabolism.2.blood volume, flow and perfusion.3.tissue and organ uptake.4.receptor binding, and oxygen utilization.

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Tracer Principle

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Common Radio-nuclides

Nuclides Half lives TypeEnergy (keV)

TC-99m 6 h

140

Tl-201 73 h

70

I-123 13 h

159

I-131 8 d

364

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RadiopharmaceuticalRadiopharmaceutical

Selection of pharmaceutical based on organ-specific question.

Labeling of pharmaceutical with radioactive isotopes.

Radiopharmaceuticals should not disturb the process under investigation

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Ideal Ideal RadiopharmaceuticalsRadiopharmaceuticals

Low radiation doseLow radiation doseHigh target/non-target activityHigh target/non-target activitySafetySafetyConvenienceConvenienceCost-effectivenessCost-effectivenessOnly emit gammaOnly emit gammaProduced by generatorProduced by generator

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Mechanisms of Localization

Compartmental localization Compartmental localization and leakageand leakage

Cell sequestrationCell sequestrationPhagocytosisPhagocytosisPassive diffusionPassive diffusionMetabolismMetabolismActive transportActive transport

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Localization (cont.)Localization (cont.)

Capillary blockadeCapillary blockadePerfusionPerfusionChemotaxisChemotaxisAntibody-antigen complexationAntibody-antigen complexationReceptor bindingReceptor bindingPhysiochemical adsorptionPhysiochemical adsorption

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Half-Life (HL)Half-Life (HL)• Physical Half-Life

Time (in minutes, hours, days or years) required for the activity of a radioactive material to decrease by one half due to radioactive decay

• Biological Half-LifeTime required for the body to eliminate half of the radioactive material (depends on the chemical form)

• Effective Half-LifeThe net effect of the combination of the physical & biological half-lives in removing the radioactive material from the body

• Half-lives range from fractions of seconds to millions of years

• 1 HL = 50% 2 HL = 25% 3 HL = 12.5%

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NuclearNuclear ImagingImaging

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Interaction of Photons with Interaction of Photons with MatterMatter

Pass through unaffected (i.e. penetrate)

Absorbed (and transfer energy to the absorbing medium)

Scattered (i.e. change direction and possibly lose energy)

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Main Interactions of gamma-Main Interactions of gamma-rays with matter when used rays with matter when used

for imagingfor imaging

Photo-electric absorption

Compton Scattering

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Photoelectric absorptionPhotoelectric absorption

An incident photon is completely absorbed by an atom in the absorber material, and one of the atomic electrons is ejected. This ejected electron is known as a photoelectron.

The electron must be bound to the atom, to conserve energy and momentum.

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The Photoelectric EffectThe Photoelectric Effect

In the photoelectric effect the photon interacts with an orbital electron and disappears, while the electron is ejected from the atom thus ionising it. The energy of the photoelectron is given by

EEkk = h = hνν – E – EBB

Where Ek is the kinetic energy of the ejected electron, hν the energy of the photon and EB the binding energy of the electron.

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Compton ScatteringCompton Scattering

In this case, an incident In this case, an incident gamma ray scatters from gamma ray scatters from an outer shell electron in an outer shell electron in the absorber material at the absorber material at an angle , and some of an angle , and some of the gamma ray energy is the gamma ray energy is imparted to the electron. imparted to the electron.

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All interactionAll interaction

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General-Purpose Circular Detector

High-Performance Circular Detector

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The gamma cameraThe gamma camera

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Gamma Camera Components Gamma Camera Components

PhotomultipliersPhotomultipliers

CollimatorCollimator

ScintillatorScintillator

Organ to be imagedOrgan to be imaged

Typically: 40cm × 55cm NaI(Tl) scintillatorTypically: 40cm × 55cm NaI(Tl) scintillator Spatial resolution ~ a few mmSpatial resolution ~ a few mm Use of large collimator not efficient Use of large collimator not efficient relatively large relatively large

radiation dose needed to be given to patient.radiation dose needed to be given to patient.

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The modern gamma camera consists of:

- multihole collimator

- large area NaI(Tl) (Sodium Iodide - Thallium activated) scintillation crystal

- light guide for optical coupling array (commonly hexagonal) of photo-multiplier tubes

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Features and parameters of the Features and parameters of the scintillation crystal scintillation crystal

The following are the typical features of the scintillation crystal The following are the typical features of the scintillation crystal used in modern gamma cameras used in modern gamma cameras

most gamma cameras use thallium-activated (NaI (Tl)) most gamma cameras use thallium-activated (NaI (Tl)) NaI(Tl) emits blue-green light at about 415 nm NaI(Tl) emits blue-green light at about 415 nm the spectral output of such a scintillation crystal matches the spectral output of such a scintillation crystal matches

well the response of standard bialkali photomultipliers .well the response of standard bialkali photomultipliers .

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the linear attenuation the linear attenuation coefficient of NaI(Tl) at 150 coefficient of NaI(Tl) at 150 KeV is about 2.2 1/cm . KeV is about 2.2 1/cm . Therefore about 90% of all Therefore about 90% of all photons are absorbed within photons are absorbed within about 10 mmabout 10 mm

NaI(Tl) is hyrdoscopic and NaI(Tl) is hyrdoscopic and therefore requires hermetic therefore requires hermetic encapsulation encapsulation

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• NaI(Tl) has a high refractive index ( ~ 1.85 ) and thus a light guide is used to couple the scintillation crystal to the photomultiplier tube

• the scintillation crystal and associated electronics are surrounded by a lead shield to minimize the detection of unwanted radiation

• digital and/or analog methods are used for image capture

Camera componentCamera component

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A crucial component of the modern gamma camera is the collimator.

The collimator selects the direction of incident photons.

For instance a parallel hole collimator selects photons incident the normal.

The action of a parallel hole collimator

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collimator

Detail of the pin-hole collimator Detail of the pin-hole collimator

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Pinhole Collimator

Other types of collimators include pinhole collimator often used in the imaging of small superficial organs and structures (e.g thyroid,skeletal joints) as it provides image magnification.

CollimatorCollimator

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CollimatorCollimator

Defines the spatial resolution of the Defines the spatial resolution of the systemsystem

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CollimatorCollimator

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Septa designed for specific gamma ray energy: e.g. length 35 mm distance 1.5 mm thickness 0.2 mm

Again Camera Again Camera componentscomponents

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Gamma Camera ComponentsGamma Camera Components

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Principle of Scintillation Principle of Scintillation detectordetector

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ScintillatorScintillator

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Anger (gamma) cameraAnger (gamma) camera

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Energy Signal

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Ideal Energy SpectrumIdeal Energy Spectrum

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ProjectionsProjections

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Real Energy SpectrumReal Energy Spectrum

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Camera specificationCamera specification

Detector size ca. 50 cm x 60 cmDetector size ca. 50 cm x 60 cmca. 60 photomultiplier tubes per ca. 60 photomultiplier tubes per

detectordetectorEnergy resolution @140 keV< 10%Energy resolution @140 keV< 10%Intrinsic spatial resolution: 3,5 -4 Intrinsic spatial resolution: 3,5 -4

mmmmExtrinsic spatial resolution Extrinsic spatial resolution

(Collimator): 8 -20 mm(Collimator): 8 -20 mm

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ExamplesExamples

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Nuclear medicine imageNuclear medicine image

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Renal ScanRenal Scan

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My time is up!

Any questions ??

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