Nuclear Medicine Principles & Technology_I Dr. Mohammed Alnafea alnafea@ksu.edu.sa

Nuclear medicine images Single photon imaging Planar 2D image Single Photon Emission Computed Tomography (SPECT) 3D image Positrons Emission Tomography (PET). All reveal the spatial and temporal distribution of target-specific pharmaceuticals in the human body. 9th lecture

Nuclear Medicine Principles & 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 9th lecture

Nuclear medicine images Depending on the application, the nuclear medicine data can be interpreted to yield information about physiological processes such as : glucose metabolism. blood volume, flow and perfusion. tissue and organ uptake. receptor binding, and oxygen utilization. 9th lecture

Tracer Principle 9th lecture

Common Radio-nuclides Energy (keV) Type Half lives Nuclides 140  6 h TC-99m 70 73 h Tl-201 159 13 h I-123 364 8 d I-131 9th lecture

Radiopharmaceutical Selection of pharmaceutical based on organ-specific question. Labeling of pharmaceutical with radioactive isotopes. Radiopharmaceuticals should not disturb the process under investigation 9th lecture

Ideal Radiopharmaceuticals Low radiation dose High target/non-target activity Safety Convenience Cost-effectiveness Only emit gamma Produced by generator 9th lecture

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Mechanisms of Localization Compartmental localization and leakage Cell sequestration Phagocytosis Passive diffusion Metabolism Active transport 9th lecture

Localization (cont.) Capillary blockade Perfusion Chemotaxis Antibody-antigen complexation Receptor binding Physiochemical adsorption 9th lecture

Half-Life (HL) Physical Half-Life Biological 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-Life Time required for the body to eliminate half of the radioactive material (depends on the chemical form) Effective Half-Life The 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% 9. Half-Life In any sample of radioactive material, the amount of radioactive material constantly decreases with time because of radioactive decay. The physical half-life is the amount of time required for a given amount of radioactive material to be reduced to half the initial amount by radioactive decay. The biological half-life is the time required for the human body to eliminate half of the radioactive material taken into it. For many radioactive materials, the elimination from the body occurs via urination. However, depending on the chemical composition of the radioactive material, other pathways can also help to eliminate the radioactive material from the body. The effective half-life is a measure of the time it takes for half the radioactive material taken into the body to disappear from the body. Both the physical half-life and the biological half-life contribute to the elimination of the radioactive material from the body. The combination of these two half-lives is called the effective half-life. After one half-life, half of the material remains. After a second half-life, a half of a half, i.e. 25% of the initial amount remains. After 10 half-lives, about 1/1000 remains. After 20 half-lives, only one millionth of the material remains. 9th lecture

Nuclear Imaging 9th lecture

Interaction of Photons with Matter Pass through unaffected (i.e. penetrate) Absorbed (and transfer energy to the absorbing medium) Scattered (i.e. change direction and possibly lose energy) 9th lecture

Main Interactions of gamma-rays with matter when used for imaging Photo-electric absorption Compton Scattering 9th lecture

Photoelectric 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. 9th lecture

The 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 Ek = hν – EB Where Ek is the kinetic energy of the ejected electron, hν the energy of the photon and EB the binding energy of the electron. 9th lecture

Compton Scattering In this case, an incident gamma ray scatters from an outer shell electron in the absorber material at an angle , and some of the gamma ray energy is imparted to the electron. 9th lecture

All interaction 9th lecture

9th lecture General-Purpose Circular Detector                                       High-Performance Circular Detector 9th lecture

The gamma camera 9th lecture

Gamma Camera Components Photomultipliers Scintillator Collimator Organ to be imaged Typically: 40cm × 55cm NaI(Tl) scintillator Spatial resolution ~ a few mm Use of large collimator not efficient  relatively large radiation dose needed to be given to patient. 9th lecture

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 - lead shield to minimize background radiation 9th lecture

Features and parameters of the scintillation crystal The following are the typical features of the scintillation crystal used in modern gamma cameras most gamma cameras use thallium-activated (NaI (Tl)) NaI(Tl) emits blue-green light at about 415 nm the spectral output of such a scintillation crystal matches well the response of standard bialkali photomultipliers . the linear attenuation coefficient of NaI(Tl) at 150 KeV is about 2.2 1/cm . Therefore about 90% of all photons are absorbed within about 10 mm NaI(Tl) is hyrdoscopic and therefore requires hermetic encapsulation 9th lecture

digital and/or analog methods are used for image capture 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 9th lecture

Camera component 9th lecture

The action of a parallel hole collimator 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 9th lecture

Detail of the pin-hole 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. Detail of the pin-hole collimator 9th lecture

Collimator 9th lecture

Collimator Defines the spatial resolution of the system 9th lecture

Collimator Septa designed for specific gamma ray energy: e.g. length 35 mm distance 1.5 mm thickness 0.2 mm 9th lecture

Again Camera components 9th lecture

Gamma Camera Components 9th lecture

Principle of Scintillation detector 9th lecture

Scintillator 9th lecture

Anger (gamma) camera 9th lecture

Energy Signal 9th lecture

Ideal Energy Spectrum 9th lecture

Projections 9th lecture

Real Energy Spectrum 9th lecture

Camera specification Detector size ca. 50 cm x 60 cm ca. 60 photomultiplier tubes per detector Energy resolution @140 keV< 10% Intrinsic spatial resolution: 3,5 -4 mm Extrinsic spatial resolution (Collimator): 8 -20 mm 9th lecture

Examples 9th lecture

Nuclear medicine image 9th lecture

Renal Scan 9th lecture

My time is up! Any questions ?? 9th lecture