Transcript 8533049_Nuclear Medicine

Nuclear Medicine
By Nazli Gharraee
8533049
April 2008
Outline
 History
 Introductions
 Imaging
 Treatment
 PET
 Image Reconstructions
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History
 1946
 1950s
 1960s
 1970s
 1980s
 1990s
first uses of nuclear medicine
Widespread clinical use of
nuclear medicine began
measuring blood flow to the
lungs and identifying cancer
most organs of the body
could be visualized with
nuclear medicine procedures
Radiopharmaceuticals,
monoclonal antibodies, FDG
PET
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Introduction
 Nuclear Medicine is a branch of
medicine that uses radiation to
provide information about a person’s
anatomy and the functioning of
specific organs. In most cases, the
information enables physicians to
provide a quick, accurate diagnosis of
conditions such as cancer, heart
disease, thyroid disorders and bone
fractures. In some cases, radiation is
used to treat the condition.
 More specifically, nuclear medicine is
a part of molecular imaging because it
produces images that reflect
biological processes that take place at
the cellular and subcellular level.
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Some Definitions
 Red spots (hot spots)
 Blue Spot (cold spots)
 Various other colors may be used for 'in between' levels of gamma rays
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emitted.
Photomultiplier tubes
Avalanche photodiodes (APDs)
Thermal neutron
Coincidence events
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What are radioisotopes?
 The isotopes of an element have the same number of protons in their atoms
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(atomic number) but different masses due to different numbers of neutrons.
In an atom in the neutral state, the number of external electrons also equals
the atomic number.
These electrons determine the chemistry of the atom.
The atomic mass is the sum of the protons and neutrons.
There are 82 stable elements and about 275 stable isotopes of these
elements.
When a combination of neutrons and protons, which does not already exist
in nature, is produced artificially, the atom will be unstable and is called a
radioactive isotope or radioisotope.
There are also a number of unstable natural isotopes arising from the decay
of primordial uranium and thorium.
Overall there are some 1800 radioisotopes.
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What are radioisotopes?
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At present there are up to 200 radioisotopes used on a regular basis, and
most must be produced artificially.
Radioisotopes can be manufactured in several ways.
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The most common is by neutron activation in a nuclear reactor. This
involves the capture of a neutron by the nucleus of an atom resulting in an
excess of neutrons (neutron rich).
Some radioisotopes are manufactured in a cyclotron in which protons are
introduced to the nucleus resulting in a deficiency of neutrons (proton rich).
The nucleus of a radioisotope usually becomes stable by emitting an
alpha and/or beta particle (or positron). These particles may be
accompanied by the emission of energy in the form of electromagnetic
radiation known as gamma rays. This process is known as radioactive
decay.
Radioactive products which are used in medicine are referred to as
radiopharmaceuticals.
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Radioisotopes
PET
 Radionuclides used in PET scanning are typically isotopes with
short half lives
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Oxygen-15 (~2 min)
Nitrogen-13 (~10 min)
Carbon-11 (~20 min)
Fluorine-18 (~110 min)
 These radionuclides are incorporated either into compounds normally used
by the body such as glucose (or glucose analogues), water or ammonia, or
into molecules that bind to receptors or other sites of drug action
(radiotracers).
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FDG
 Most clinical PET studies in oncology utilize [18F] 2-Fluoro-2-Deoxy-D-
Glucose - more commonly known as “FDG”.
 FDG, an analog of glucose, becomes trapped in the cells that try to
metabolize it. Its concentration is tissue builds up in proportion to the rate
of glucose metabolism. Because tumors have a high rate of glucose
metabolism, they concentrate FDG, and appear as “hot spots” in PET
images.
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Procedure
 Introducing small amount of radiopharmaceuticals into the body by
 Injection
 Swallowing
 Inhalation
 The amount of radiopharmaceuticals used is carefully selected to provide
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the least amount of radiation exposure to the patient but ensure an accurate
test.
Time taken for the radionuclide to travel to the target.
Taking pictures of the body by special cameras (PET,SPECT or gamma
camera) .
Detecting the emitted gamma rays from inside the body by the gamma
camera , converting it into an electrical signal and send to a computer.
Analysis with the computer.
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Radiotherapy
 Rapidly dividing cells are particularly sensitive to damage by radiation.
 For this reason, some cancerous growths can be controlled or eliminated by
irradiating the area containing the growth.
 Many therapeutic procedures are palliative, usually to relieve pain. For
instance, strontium-89 and (increasingly) samarium 153 are used for the
relief of cancer-induced bone pain. Rhenium-186 is a newer product for
this.
 Treating leukemia may involve a bone marrow transplant, in which case the
defective bone marrow will first be killed off with a massive (and otherwise
lethal) dose of radiation before being replaced with healthy bone marrow
from a donor.
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TAT
 A new field is Targeted Alpha Therapy (TAT), especially for the control of
dispersed cancers. The short range of very energetic alpha emissions in
tissue means that a large fraction of that irradiative energy goes into the
targeted cancer cells, once a carrier has taken the alpha-emitting
radionuclide to exactly the right place.
 An experimental development of this is Boron Neutron Capture Therapy
using boron-10 which concentrates in malignant brain tumors. The patient
is then irradiated with thermal neutrons which are strongly absorbed by the
boron, producing high-energy alpha particles which kill the cancer.
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Advantages
 It provides doctors with information about both structure and function.
 It’s a way to gather the medical information that would otherwise be
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unavailable, require surgery, or necessitate more expensive diagnostic tests.
Often identify abnormalities very early in the progress of a disease, long
before many medical problems are apparent with other diagnostic test.
It determines the presence of a disease based on biological changes rather
than changes in anatomy.
Combination with CT scan can give the image of both bone and soft tissue.
It’s extremely safe because
 The radioactive tracers, or radiopharmaceuticals, used are quickly eliminated
from the body through its natural functions.
 The tracers used rapidly lose their radioactivity.
 The dose of radiation necessary for a scan is very small.
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PET
 Positron emission tomography (PET) is a nuclear medicine imaging
technique which produces a three-dimensional image or map of functional
processes in the body.
 The system detects pairs of gamma rays emitted indirectly by a positronemitting radioisotope, which is introduced into the body on a metabolically
active molecule.
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As
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up
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These
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athe
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which
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(i.e.,
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creating
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bywithin a
counterpart
(gamma)
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of an electron.
moving in opposite directions.
few
nanoseconds)
areorignored.
photomultiplier
tubes
Silicon Avalanche Photodiodes (Si APD).
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Localization of the positron annihilation event
 Line Of Response (LOR)
 Width of LOR
 Time of flight
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Image reconstruction using coincidence statistics
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Coincidence events
LOR
Similar to CT scan reconstruction
A normal PET data set has millions
of counts for the whole acquisition,
while the CT can reach a few billion counts.
 PET data suffer from scatter and random
events much more dramatically than CT data does.
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Image Reconstruction
Attenuation correction
 As different LORs must traverse different thicknesses of tissue, the photons
are attenuated differentially.
 The result is that structures deep in the body are reconstructed as having
falsely low tracer uptake.
 While attenuation corrected images are generally more faithful
representations, the correction process is itself susceptible to significant
artifacts. As a result, both corrected and uncorrected images are always
reconstructed and read together.
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Image Reconstruction
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Early PET scanners had only a single ring of detectors, hence the
acquisition of data and subsequent reconstruction was restricted to a
single transverse plane. More modern scanners now include multiple
rings, essentially forming a cylinder of detectors.
There are two approaches to reconstructing data from such a scanner:
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2D/3D
Treat each ring as a separate entity, so that only coincidences within a ring
are detected, the image from each ring can then be reconstructed
individually (2D reconstruction)
Allow coincidences to be detected between rings as well as within rings,
then reconstruct the entire volume together (3D).
3D techniques have better sensitivity (because more coincidences are
detected and used) and therefore less noise, but are more sensitive to the
effects of scatter and random coincidences, as well as requiring
correspondingly greater computer resources.
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Source
 www.nuclearimaging.com.au
 www.snm.org
 www.biomedpet.org
 www.wikipedia.com
 www.nuclear.kth.se
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