Transcript MedPhys963_RadiationPet_06

Medical Physics
Option 9.6.3
2006
Syllabus - Contextual Outline
Contextual Outline
The use of other advances in technology, developed from our understanding of the
electromagnetic spectrum, and based on sound physical principles, has allowed medical
technologists more sophisticated tools to analyse and interpret bodily process for diagnostic
purposes. Diagnostic imaging expands the knowledge of practitioners and the practice of
medicine. It usually uses non-invasive methods for identifying and monitoring diseases or
injuries via the generation of images representing internal anatomical structures and organs of
the body.
Technologies, such as ultrasound, compute axial tomography, positron emission tomography
and magnetic resonance imaging, can often provide clear diagnostic pictures without surgery. A
magnetic resonance image (MRI) scan of the spine, for example, provides a view of the discs
in the back, as well as the nerves and other soft tissues. The practitioner can look at the MRI
films and determine whether there is a pinched nerve, a degenerative disc or a tumour. The
greatest advantage of these techniques are their ability to allow the practitioner to see inside
the body without the need for surgery.
This module increases students’ understanding of the history of physics and the implications of
physics for society and the environment.
Syllabus 9.6.1
The properties of
ultrasound waves can
be used as diagnostic
tools
Syllabus 9.6.2
The physical
properties of
electromagnetic
radiation can be
used as diagnostic
tools
Syllabus 9.6.3
Radioactivity can be used as a diagnostic tool
Syllabus 9.6.4
The magnetic field
produced by
nuclear particles
can be used as a
diagnostic tool
Ultrasound
X-rays
Medical
Physics
MRI
Endoscopy
Nuclear - PET
Individual
1 minute
Group
2 minutes
Syllabus 9.6.3
Radioactivity can be used as a diagnostic tool
Radioactive Isotopes and Half-lives
The atom
• Electrons
• Nucleus
• Protons
• neutrons
• Quarks
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Radioactive Isotopes
• Atomic nuclei contain protons
and neutrons
• The number of protons in the
nucleus is called the atomic
number
• The atomic number determines
which element an atom is e.g. all
fluorine atoms have 9 protons in
the nucleus
• The number of neutrons in the
nucleus of a particular element
can vary
Only hydrogen has
isotopes with
special names
Atoms of the same element but with
different numbers of neutrons are
called isotopes
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
The atomic nucleus and isotopes
• Collectively, protons and
neutrons are called nucleons
• The total number of nucleons in
a nucleus is called the mass
number
• Nuclei are represented by the
symbol for the element, and the
mass and atomic numbers
• Atoms of the same element but
having different numbers of
neutrons are called isotopes
mass number
18F
atomic number 9
fluorine 18
99Tc
43
technetium 99
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Stable and unstable isotopes
• Isotopes having too
many or too few
neutrons, relative to
the number of protons
are unstable
14C
6
unstable nucleus
19F
9
stable nucleus
18F
9
unstable nucleus
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Radioactive Decay
• Unstable isotopes may become
more stable in several ways
• Alpha decay (decay)
• Beta decay (b decay)
• Gamma () emission
• Positron emission (b+ decay)
• Each unstable isotope
undergoes a particular type of
change, which is on average
constant and unique to that
isotope
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Radioactive Decay
• Unstable isotopes may become
more stable in several ways
• Alpha decay (decay)
• Beta decay (b decay)
• Gamma () emission
• Positron emission (b+ decay)
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Radioisotopes
• Radioisotopes may be natural
or they can be artificial
• Artificial radioisotopes are
produced in nuclear reactors
or using particle accelerators
called cyclotrons
• Sydney has one medical
cyclotron at Prince Alfred
Hospital near Sydney University
• Lucas Heights produces a
range of artificial isotopes for
medical use
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Radioisotopes
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Radioactive Decay
• The time taken for 50% of a
sample of the radioactive
material to decay is called
the half-life
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Radioactive Decay
Half-life
Tim e (s)
0
10
20
30
40
50
60
Ma ss (g)
128
73
41.9
23.9
13.7
7.8
4.4 8
P-210 has a half-life
of 12 seconds
12 s
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Review Question - Half-life
What is the half-life of strontium-90?
28 years
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Radioactive Decay
When beta decay occurs, a neutron in the
nucleus changes into a proton (which
remains in the nucleus) and an electron
(which is ejected from the nucleus at high
velocity).
In all nuclear reactions, mass and charge
are conserved.
Propose what may happen in the nucleus
to produce positron emission.
Answer
A proton changes into a positron and a
neutron, which remains in the nucleus and
the positron is ejected from the nucleus.
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Decay producing a gamma ray
Technetium-99m nucleus
• Gamma decay does not change
either the atomic number or the mass
number
• The nucleus is left in a lower energy
state as a result of losing energy in
the form of gamma radiation
• Gamma rays are very important in
nuclear medicine
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Radioactive Isotopes and Half-lives
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Beta+ Decay of Artificial Isotopes
Positron (b+) decay occurs only in certain man-made isotopes
• b+ decay is medically very important (used in the process of PET scanning)
• Positrons are anti-electrons
• Positrons have the same mass as an electron, but the opposite charge (+1)
•
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Penetration by Radiation
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Review
1.
2.
3.
4.
Define the term “isotope”.
Outline the characteristic of an
isotope that causes it to be unstable
and give an example of an element
having stable and unstable isotopes.
Technetium-99 is an important
medical isotope. Identify the type of
radiation produced when Tc-99
decays.
Fluorine-18 decays to produce a
positron. Describe the main
characteristics of positrons and
identify the other main decay product
from F-18.
1.
Isotopes are atoms of the same
element, having different numbers of
neutrons
e.g. carbon-12 and carbon-14
2.
3.
4.
Unstable isotopes are characterised
by having too many or too few
neutrons, relative to the number of
protons. e.g. F-19 is stable however
F-18 is unstable.
Tc-99 produces gamma radiation
when it decays.
Positrons have the same mass as an
electron but the same positive charge
as a proton. When F-18 decays to
produce a positron, an oxygen-18
nucleus is produced.
Properties of Radioactive Isotopes
Alpha: highly ionising (high mass
and charge), low penetration
Not used medically
Beta: less ionising (low mass and
single charge), low penetration
Not used medically
Gamma: least ionising (photon, no
charge), high penetration
Used for imaging (gamma scan)
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Properties of Radioactive Isotopes
Source: ANSTO brochure
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Properties of Radioactive Isotopes
Source: ANSTO brochure
• outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
Metabolism of Isotopes - Organ Accumulation
Source: ANSTO brochure
• describe how radioactive isotopes may be metabolised by the body to bind or accumulate in the target organ
Metabolism of Isotopes - Organ Accumulation
•
Medical radioisotopes are metabolised to
bind to specific organs either as elements
or as a part of a molecule used by the
body
e.g.
•
iodine-131 is used to monitor thyroid
gland function because iodine is
metabolised by this gland
•
Iodine-131 decays with a half-life of
8.0197 days with beta and gamma
emissions
•
Source: ANSTO brochure
fluorine-18 is incorporated into a modified
glucose (18 FDG) molecule to investigate
a wide range of organ function, especially
the brain
• describe how radioactive isotopes may be metabolised by the body to bind or accumulate in the target organ
Metabolism of Isotopes - Organ Accumulation
Factors influencing the choice of isotopes for medical use
• Half-life should be less than a few hours
• Should be easily attached to a pharmaceutical agent
• Can be delivered to hospital in a form to last several days
• describe how radioactive isotopes may be metabolised by the body to bind or accumulate in the target organ
Metabolism of Isotopes - Organ Accumulation
• Technetium-99m is a widely-used isotope in nuclear medicine
• It decays from excited to the ground state by emitting a ray
• The parent isotope is molybdenum 99, having a half-life of 2.7 days
• Half-life of technetium-99m is 6.01 hours
Tc+m
-
99
42
Mo

99
43
Tc  Tc +  (140 keV)
m
99
43
99
43
+b
Half life 2.7 days
Half life 6.01 hours
• describe how radioactive isotopes may be metabolised by the body to bind or accumulate in the target organ
Milking the Cow
• Molybdenum-99 is delivered to hospital inside the generator (cow)
Radionuclide
generator
• Decays to the excited state of technetium 99
• NaCl solution passed through cow
removed in solution
• Mixed with pharmaceutical for labelling
• Injected into patient
•
99Tcm
• describe how radioactive isotopes may be metabolised by the body to bind or accumulate in the target organ
Comparison of Bone Scan and X-ray Images
•
•
•
Inject patient with radiopharmaceutical
labelled with radioactive isotope
Detect where gamma rays are coming
from within the body
• Use a gamma camera
Local concentration in patient permits
diagnosis
• ‘Hotspot’ may indicate tumour*
* Tumours contain rapidly dividing cells…
therefore metabolism occurs at a higher rate
in the tumour… producing a higher than
normal concentration of metabolites, in the
cells. If tagged with radioisotopes this can
be detected.
• perform an investigation to compare an image of bone scan with an X-ray image
Comparison of Bone Scan and X-ray Images
• Collimator restricts direction
from which photons reach
the camera
• Made of lead with parallel
holes running through it
• Bigger, shorter holes
means more photons, but
more blurring and visa
versa
Details need not be memorised
Gamma camera
Details need not be memorised
Comparison of Bone Scan and X-ray Images
• Detector made from single crystal of
sodium iodide
• 250 mm to 250 mm diameter
• 6 mm thick
• Enveloped in thin aluminium can
(light tight)
• Light guide takes all light directly to
photomultipliers
• Glass plate with non-reflective
coating
• Improves efficiency of light
coupling
Details need not be memorised
Delivery of Radioisotopes into the Body
• Radioisotopes are usually injected* into the body
• Short half-life isotopes are used to reduce risk
• Isotopes that are excreted rapidly are used
• Specific isotopes are chosen specific organs
* sometimes they are inhaled or swallowed
Delivery of Radioisotopes into the Body
Whole body bone scan made using a
gamma ray camera. The image was
produced using gamma emissions
from phosphate tagged with
technetium–99m.
The tracer was injected intravenously
and the phosphate is metabolised
mainly in the bones.
The test was done to determine
whether cancer had spread to the
patient’s bones.
The image is normal, showing that
metastasis has not occurred.
Bone Scan vs X-Ray
• perform an investigation to compare an image of bone scan with an X-ray image
Comparison of Diseased and Healthy Organs
Question
Compare a bone scan with an X-ray.
Similarities
• Both x-ray and bone scans (gamma or PET) use high energy electromagnetic waves to
produce the image
• Both types of image show 3-dimensions projected onto a 2-dimensional image
Differences
• A radiograph uses x-ray radiation whereas a bone scan uses gamma radiation
• The source of x-rays for a radiograph is outside the body whereas the source of gamma
rays for a bone scan is inside the body
• A radiograph has a higher resolution than a bone scan image [due to the need to use a
collimator for the bone scan]
• A radiograph is a produces an image that shows structure whereas a bone scan produces
an image resulting from functional differences (difference in metabolism - which may reflect
structural differences)
• perform an investigation to compare an image of bone scan with an X-ray image
Comparison of Diseased and Healthy Organs
What caused this hot spot?
Answer
The radioactive tracer was
injected into the vein in the arm.
Traces of the radioactive
material remain at the injection
site.
• gather and process secondary information to compare a scanned image of at least one healthy body part or organ with a
image
of its diseased
counterpart
• scanned
perform an
investigation
to compare
an image of bone scan with an X-ray image
Comparison of Diseased and Healthy Organs
Bone scan
Recount how a bone scan is produced.
Answer
1.
A radiopharmaceutical is put in the patient by
injection, inhalation or ingestion. The patient waits
while the radiopharmaceutical is metabolised. The
radioisotope accumulates in the target organ.
2.
The patient lies down and remains stationary on or
under a gamma camera.
3.
Gamma rays are emitted in all directions from the
body, however the collimator allows only gamma
rays following parallel paths to reach the gamma
detector in the camera.
4.
Signals from the detector are processed by a
computer to produce an image showing functional
information.
• gather and process secondary information to compare a scanned image of at least one healthy body part or organ with a
scanned image of its diseased counterpart
Positron Emission Tomography (PET)
PET is used for
• bone imaging
• monitoring tumours
• monitoring the function of the heart
• monitoring blood-flow in the heart
• studying brain activity
Overview
A radiopharmaceutical that produces positrons is placed in the body, targeting
a particular organ. The radioisotope accumulates in the desired organ.
Positrons are emitted into the body as the isotope decays. Positrons annihilate
electrons, producing a pair of gamma rays that travel in opposite directions.
The pairs of gamma rays are detected by a ring of sensors. Signals from the
sensors are processed by a computer to produce an image showing the
location and concentration of the radioisotope in the body. The radioactive
material is excreted or decays to form harmless products.
Radioactive Decay - Positron Production
• Phosphorus-30 and fluorine-18 are
two artificial radioisotopes that
undergo decay that produces
positrons
• As a result of the radioactive decay of
18
9
F O +b
30
15
fluorine-18, a positron is emitted from
the nucleus
Explanation - because there are too
many protons in the nucleus for it to be
stable, one of the protons decays,
producing a positron and a neutron
which remains in the nucleus
• identify that during decay of specific radioactive nuclei positrons are given off
18
8
30
14
+
P Si + b
+
Positron Annihilation - Gamma Ray Production
•
A positron interacts with an electron,
annihilating both and producing two -photons
[energy = 0.511 MeV]
positron + electron => gamma rays
The -photons travel in opposite directions
Gamma photons produced use in PET
• Gives high resolution compared with other
gamma imaging technologies [but not as good
as X-ray, CT, ultrasound, MRI]
• Typical isotopes used: 18F, 68Ga, 15O
• Isotopes produced using cyclotron
• Main advantage - the image produced is a
functional image i.e. it shows HOW the body
is working
•
The most commonly used tracer is
18FDG
(18F fluorodeoxyglucose)
• discuss the interaction of electrons and positrons resulting in the production of gamma rays
PET Scanner
This imaging technology is called tomography because the image is obtained
and usually viewed as slices perpendicular to the long axis of the body.
Because the data are analysed by a computer, it is also possible to create a
computer-generated 3-D image from the scans.
• describe how the positron emission tomography (PET) technique is used for diagnosis
Diagnosis Using PET
• describe how the positron emission tomography (PET) technique is used for diagnosis
How PET Scanning is Performed
• PET produces an image of a 2-
• Contrast this with a gamma
D section through the body
• Tomography: “slice” or “section”
scan, which produces a 2-D
image of a 3-D volume
• Similar to CT - HOW?
• Many slices can be digitally
combined to produce a virtual 3D image
• describe how the positron emission tomography (PET) technique is used for diagnosis
How PET Scanning is Performed
• Positron + electron annihilate
•
each other
• Two photons are emitted
in opposite directions
•
PET uses annihilation coincidence
detection (ACD)!!
• Ring of gamma ray detectors around
body
• Time of arrival and intensity of gamma
rays is recorded
• 2 simultaneous events give line of
response (LOR)
Image is built up from LOR data
• describe how the positron emission tomography (PET) technique is used for diagnosis
How PET Scanning is Performed
•
Positrons emitted from the tracer
annihilate electrons producing
gamma ray pairs
1. The gamma ray pair strikes
detectors on opposite sides of the
patient
2. A flash of light is produced by the
detectors arranged in a circular
ring
3. A computer compares the
intensity of gamma ray pairs over
time, and calculates the location
of the positron-electron
interactions to produce an image
• describe how the positron emission tomography (PET) technique is used for diagnosis
How PET Scanning is Performed
The greater the depth
of tissue through which
the gamma rays travel,
the greater the
attenuation of the
gamma rays - enabling
the source point to be
located by comparing
intensities of gamma
rays on opposite sides
of the body.
• describe how the positron emission tomography (PET) technique is used for diagnosis
How PET Scanning is Performed
• describe how the positron emission tomography (PET) technique is used for diagnosis
How PET Scanning is Performed
• describe how the positron emission tomography (PET) technique is used for diagnosis
Diagnosis Using PET
• PET Scan
• gather and process secondary information to compare a scanned image of at least one healthy body part or organ with a
scanned image of its diseased counterpart
Diagnosis Using PET
• gather and process secondary information to compare a scanned image of at least one healthy body part or organ with a
scanned image of its diseased counterpart
Diagnosis Using PET
• gather and process secondary information to compare a scanned image of at least one healthy body part or organ with a
scanned image of its diseased counterpart
Diagnosis Using PET
• describe how the positron emission tomography (PET) technique is used for diagnosis
PET Images of the Brain
Heart with myocardial infarction.
Arrows indicate diseased tissue
Normal heart (left)
• gather and process secondary information to compare a scanned image of at least one healthy body part or organ with a
scanned image of its diseased counterpart
PET Images of the Brain
Brain of 9 year old girl suffering from
epilepsy.
Arrow indicates problem area, which
was removed.
Normal brain (left)
• gather and process secondary information to compare a scanned image of at least one healthy body part or organ with a
scanned image of its diseased counterpart
Questions
1.
2.
3.
4.
Identify the particle produced
by radioactive decay that is
used for to produce PET
images. (1M)
Identify the radiation detected
to produce a bone scan. (1M)
Compare the processes of
taking an X-ray image to
producing a PET image. (4M)
Compare the type of images
produced using ultrasound
and PET. (2M)
1.
2.
3.
4.
Positron
Gamma rays
The radiation source is outside the body
in the case of X-rays but it is inside the
body in the case of a PET scan. Both
processes use e/m radiation to produce
the image; X-rays and gamma rays,
however the gamma rays are produced
inside the body by positron-electron
annihilation whereas X-rays are
produced using an X-ray tube.
Images produced using ultrasound are
structural (showing anatomy) whereas
PET produces functional images
(showing physiology)
PET Images … or not?
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Answers to Questions
Question
Compare a bone scan with an X-ray.
Similarities
• Both x-ray and bone scans (gamma or PET) use high energy electromagnetic waves to
produce the image
• Both types of image show 3-dimensions projected onto a 2-dimensional image
Differences
• A radiograph uses x-ray radiation whereas a bone scan uses gamma radiation
• The source of x-rays for a radiograph is outside the body whereas the source of gamma
rays for a bone scan is inside the body
• A radiograph has a higher resolution than a bone scan image [due to the need to use a
collimator for the bone scan]
• A radiograph is a produces an image that shows structure whereas a bone scan produces
an image resulting from functional differences (difference in metabolism - which may reflect
structural differences)
Answers to Questions