Positron Emission Tomography

Positron Emission Tomography


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P E T


Introduction
What is PET? Revision of beta decay and isotopes. How a positron annihilates with an electron. How PET works. Medical uses for PET scanning.


Introduction
What organs appear dark in this scan?


What Is PET?


Having a PET Scan
Minute amount of radioactive isotope injected into patient.Patient lies still 30 to 60 minutes in a chamber like the one illustrated.Scan is looked at by an imaging specialistResults passed to patients doctor.


Revision: Beta Decay
When a nucleus is unstable, there are three ways it can decay. It can emit:

An alpha particleA beta particleA gamma ray

or


Revision: Beta Decay
Beta-minus decay ( -)
Beta-plus decay ( +)


Revision: Isotopes
Atoms of the same atomic number but with different mass numbers (i.e. with different numbers of neutrons) are called isotopes.

Example: carbon atoms always have 6 protons in the nucleus, but there are different isotopes, each of which has a different mass number.
These are called carbon-11, carbon-12 etc.


Revision: Isotopes
Isotopes can be plotted on a graph like this.Isotopes that lie on this line are stable. Isotopes that are above or below the line are unstable or radioactive.


How PET works: Positron-electron Annihilation
When a positron and electron collide, they annihilate each other and emit gamma rays at 180 degrees to each other.


How PET Works: Cyclotron


How PET Works: Labelling and Tracers
Positron-emitting isotopes label molecules.An image can be acquired showing the location of these molecules.The most common molecule labelled is Fluoro-deoxyglucose (FDG), which behaves almost exactly like glucose in the body.


Applications of PET:Cancer Diagnosis
PET scanning can be used to find out where cancerous tumours are, and how far they have spread in the body.

This is very important in choosing the best treatment for the patient.

tumour


Applications of PET:Cancer Diagnosis
18-FDG behaves almost exactly like glucose.

Cancer cells use up a lot of glucose as they rapidly divide.

Can be used to stage the disease, and decide the best treatment.


Applications of PET:Alzheimers Disease
The most common type of dementiaCan gradually destroy people's memory. Patients with Alzheimers have protein deposits in the brain called Amyloid plaques.PET can be used to detect the presence of Amyloid plaques.


PET Scanning with 18-FDG(made with Fluorine-18)
Normal Alzheimers


PET Scanning with 11C-PIB(made with Carbon-11)
Normal Alzheimers


PET Scanning with 11C-PIB(made with Carbon-11)
Normal Alzheimers


Applications of PET:Heart Disease
PET can be used to work out whether or not it is worth performing an operation on heart muscle.


Applications of PET:Heart Disease
After a heart attack, heart muscle can be:StunnedHibernatingDeadHeart muscle that is alive is often called viable.


Summary
PET scanning is one of the most important medical applications of radioactivityIt involves a type of beta decay called + or positronA positron emitting chemical is injected into the patient, who is then imagedPositrons dont travel far inside a patient, but annihilate when they meet an electronThe resulting gamma rays are detected by a PET scanner.


Positron Emission Tomography is a medical imaging technique which produces a three-dimensional image of the body functioning. It is commonly referred to by its three initials PET.
This Powerpoint presentation covers the following information about PET scanning.

What is PET? Revision of beta decay and isotopes. How a positron annihilates with an electron. How PET works. Medical uses for PET scanning.

More advanced material is included on the In Depth slides.
[Note: The labels will appear upon clicking]
Beta-plus decay causes positrons to be emitted.

These positrons collide with electrons close by to produce gamma rays that are emitted at 180 degrees to each other.

Since the gamma rays are emitted simultaneously and in opposite directions, you can work out where they came from.

The gamma rays can be detected inside a PET scanner, and an image can be constructed like this.
The procedure The patient may need to go without food for 4 to 6 hours prior to their appointment, and only drink water. For some scans there may not be any preparation needed. The patient must travel to their nearest hospital that is equipped with a PET scanner. However there isn't one at every hospital. There is a shortage of PET scanners in UK hospitals for three main reasons - PET scanners are expensive, PET is a new technique that is constantly changing, and there is a lack of staff who are suitably trained to operate the equipment. The patient is injected with a minute amount radioactive tracer. After the injection the patient rests for a few minutes to allow the tracer to take effect. It is important to lie as still as possible during the scan, which typically lasts half an hour. It should not be painful at all. The staff conducting the scan watch the patient at all times.

The results It could take a few weeks for the results to come back to the patient. The scan is examined by a specialist in radiology or nuclear medicine, and a report is typed up. The specialist will then pass these results to the patient.

Are there any side effects? There are no side effects. However, lying still for the scan can be difficult. It could also be boring. To combat boredom, patients are encouraged to bring music to listen to while they are in the scanner.

Can a PET scan be dangerous? Though PET involves a radioactive injection - which sounds dangerous - it is actually very unlikely that a PET scan will be harmful. The amount of radioactive isotope injected is minute, and it decays quickly. However, some doctors do advise that, after a scan, the patient avoids close contact with pregnant women and children for the rest of that day or overnight.
When a nucleus is unstable, there are three ways it can decay. It can emit an alpha particle, a beta particle or a gamma ray.
Positron Emission Tomography (PET) involves beta decay. There are two kinds of beta decay - "beta-minus" and "beta-plus".

Beta-minus (-) decay happens when an unstable nucleus emits an electron as a neutron becomes a proton. The atomic mass is unchanged and the atomic number increases by 1.

Beta-plus decay (+) happens when an unstable nucleus emits a positively-charged electron (a positron). This is because a proton has become a neutron. The atomic mass is unchanged and the atomic number decreases by 1.
Atoms of the same element all have the same number of protons in their nuclei. This number is called the atomic number. Almost all atoms also contain neutrons. Atoms of the same element may have different numbers of neutrons. The total number of protons and neutrons in the atom is called the mass number. Atoms of the same element with different mass numbers (i.e. with different numbers of neutrons) are called isotopes.

Isotopes can be plotted on a graph like this.Isotopes that lie on this line are stable. Isotopes that are above or below the line are unstable or radioactive.
The mass of the electron and positron is converted into the energy of the gamma rays The conversion between mass and energy is governed by the most famous equation there is: E = mc
[Note: The opening question asks how many protons and how many neutrons a Fluorine-18 nucleus has. These can be entered with a keyboard. Correctly answering this question activates the rest of the animation.]

In Positron Emission Tomography (PET), a patient is injected with a radioactive isotope which undergoes beta-plus (positron) decay. As this isotope has to have a short half-life, it must be made inside (or close by) the hospital. Positron emitting isotopes are made in a machine called a cyclotron by bombarding stable nuclei with protons.

The animation illustrates a proton being accelerated around the cyclotron until it emerges. The proton collides with a (stable) Oxygen-18 nucleus. Resultantly, a nucleus is knocked off and a Fluorine-18 nucleus is made.
Once positron emitting isotopes have been created, then can be used to label molecules, so that an image can be acquired showing the position of those molecules in the body. These are radioactive tracers. These molecules can be as simple as water (labelled with a positron emitting isotope of oxygen) or carbon dioxide (labelled with a positron emitting isotope of carbon).

Alternatively much larger molecules such as neurotransmitters or amino acids can be labelled. The most common molecule labelled is Fluoro-deoxyglucose (FDG), which behaves almost exactly like glucose in the body. goes to the places in the body that are using a lot of energy. can be labelled with the positron emitting isotope of Fluorine, Fluorine-18, to form 18-FDG.
[Note: The label and arrow appear upon clicking.]

This example shows a scan of a patient with lung cancer.

Knowing how far cancerous tumours have spread in the body is vitally important, so an appropriate level of treatment can be chosen.
Fluorine-18 labelled Fluoro-deoxyglucose (18-FDG) is widely used to work out how extensively cancer has spread in patients. 18-FDG behaves in almost exactly the same way as glucose in the body.

Cancer cells divide rapidly, so they use glucose at an unusually high rate. This means that these cells take up a lot of the 18-FDG tracer, and so they emit a lot of gamma rays.

Knowing how extensively the cancer has spread can be used to identify how advanced the disease is (sometimes called staging the disease). Correct staging is important to select the most appropriate treatment.

The PET scan can pinpoint disease that can be removed by surgery or treated with precision radiotherapy. It can also show whether the disease has spread widely in the body.

For very widespread disease, there is a very low chance of obtaining a cure, so there is little point in exposing a patient to painful, disfiguring and dangerous surgery, and it may be better just to treat the patient in order to make the remainder of their life as pleasant as possible. This is called palliative care.
Dementia is a disease of aging. It causes people's brains to stop working properly. The most common type of dementia, Alzheimer's disease, can gradually destroy people's memory.

Patients with early Alzheimer's disease seem a bit forgetful and might forget what they are doing in the middle of a task, of forget to do something important like collect their children from school. Patient's with advanced Alzheimer's disease might not even remember the names of their children, and are unable to look after themselves.

Around 20% of people will develop Alzheimer's disease over their lives, and as people live longer, Alzheimer's disease will become more common. There is currently no cure for Alzheimer's disease.

Patients with Alzheimer's disease have protein deposits called amyloid plaques building up in their brain. PET scanning can be used to image these protein deposits, and help scientists and doctors develop new treatments that could eventually provide a cure for Alzheimer's.

PET scanning can be used to image these protein deposits, and help scientists and doctors develop new treatments that could eventually provide a cure for Alzheimer's.
[Note: the ringed areas will appear upon clicking.]

This picture shows two PET scans. The first shows a normal brain and the second shows a brain affected by Alzheimers disease.

18-FDG shows subtle difference between the normal brain and the brain affected by Alzheimers. For example, look at the ringed regions of the brain.

Photo credit: The Cyclotron Unit, Insitute of Neurology, UCL
This picture shows how a different isotope can illustrate the difference better.

Another tracer, made with Carbon-11, and called Pittsburgh Imaging Compound B (PIB). This shows a dramatic difference between Alzheimers and normal.

Photo credit: The Cyclotron Unit, Institute of Neurology, UCL
Pittsburgh Imaging Compound B (PIB), made with Carbon-11, can be used to label the amyloid plaques, and provide a much more accurate diagnosis than 18-FDG.

New molecular tracers like PIB could improve our understanding of Alzheimer's disease, and help scientists invent new treatments for Alzheimer's disease by speeding up the process of testing new drugs on patients.

Photo credit: The Cyclotron Unit, Institute of Neurology, UCL
Heart disease is the greatest killer in the western world. The most common type of heart disease arises when a patient's coronary arteries (the blood vessels that supply blood to the heart muscle) get blocked. This cuts the oxygen supply to the heart muscle, which may then die.

Surgeons must then decide on a suitable treatment. They may widen the coronary arteries, or perform a heart bypass graft operation.

PET scanning can be used work out which procedure is most appropriate. There is no point in exposing a patient to a risky procedure to re-supply blood to their heart muscle, if that heart muscle is already dead.
After a heart attack, the muscle of the heart (myocardium) can be temporarily damaged (stunned) and then recover after a few days or weeks; it can be permanently inactive but still alive (hibernating); or it can be dead.

Imaging techniques like ultrasound and X-rays can be used to detect whether heart muscle is moving or not after a heart attack.

The challenge in planning the treatment of patients after a heart attack is to work out whether the myocardium that isn't moving is still alive, and if given a better blood supply, would start working again normally, or whether it is dead.

PET scanning with 18-FDG can be used to distinguish viable from non-viable myocardium.

The patient would arrive at the scanning unit, have some blood tests, be given an injection of 18-FDG, which behaves like glucose, and the be asked to rest for a half an hour while the 18-FDG spreads around their body.

They will then lie on their back on the scanner for about an hour for the scan.

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Positron Emission Tomography