Download 8th_lecture_-_311_rad_28_4_2010

Nuclear Medicine
Principles & Technology_I
Dr. Mohammed Alnafea
[email protected]
Nuclear medicine images
1. Single photon imaging
a) Planar 2D image
b) Single Photon Emission Computed Tomography
(SPECT) 3D image
2. Positrons Emission Tomography (PET).
All reveal the spatial and temporal distribution of
target-specific pharmaceuticals in the human
body.
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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
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Nuclear 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
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Nuclides
Half lives
TC-99m
6h
Tl-201
73 h
I-123
13 h
I-131
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Type
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
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
Energy (keV)
140
70
159
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Radiopharmaceutical
 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 Radiopharmaceuticals
 Low radiation dose
 High target/non-target activity
 Safety
 Convenience
 Cost-effectiveness
 Only emit gamma
 Produced by generator
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Mechanisms of Localization
 Compartmental localization and leakage
 Cell sequestration
 Phagocytosis
 Passive diffusion
 Metabolism
 Active transport
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Localization (cont.)
 Capillary blockade
 Perfusion
 Chemotaxis
 Antibody-antigen complexation
 Receptor binding
 Physiochemical adsorption
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Half-Life (HL)
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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
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Biological Half-Life
Time required for the body to eliminate half of the radioactive
material (depends on the chemical form)
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Effective Half-Life
•
•
The net effect of the combination of the physical & biological halflives 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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Nuclear Imaging
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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)
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Main Interactions of gamma-rays with
matter when used for imaging
 Photo-electric absorption
 Compton Scattering
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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.
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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.
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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.
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All interaction
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General-Purpose Circular
Detector
High-Performance
Circular Detector
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The gamma camera
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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.
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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
- lead shield to minimize
background radiation
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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
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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
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Camera component
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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
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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.
Detail of the pin-hole collimator
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Collimator
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Collimator
 Defines the spatial resolution of the system
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Collimator
Septa designed for specific gamma ray energy:
e.g. length 35 mm
distance 1.5 mm
thickness 0.2 mm
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Again Camera components
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Gamma Camera Components
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Principle of Scintillation detector
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Scintillator
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Anger (gamma) camera
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Energy Signal
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Ideal Energy Spectrum
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Projections
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Real Energy Spectrum
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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
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Examples
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Nuclear medicine image
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Renal Scan
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My time is up!
Any questions ??
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