Positron Emission Tomography
Positron Emission Tomography (PET) scans are medical imaging techniques that show how tissues and organs are functioning by detecting radioactive tracers inside the body. They are commonly used to identify abnormal metabolic activity, such as cancer, neurological disorders, and heat disease
How do PET Scans work?
PET Scan Applications
Preparing for a PET Scan
Limitations & Risks
Deeper Physics
How do PET Scans Work?
Patient
Ring of Detectors
Computer Imaging
Patient Bed
What Are PET Scans Used For?
Heart Disease
Cancer Detection
Medical Research
Brain Disorder
Preparing for a PET Scan
Before getting a PET scan you will be asked to stop eating 6 hours before the scan and drink only water. You should avoid doing any strenuous exercise for 24 hours before the scan as this may decrease the accuracy of the scan. It is also important to inform the hospital if you are pregnant, breastfeeding, or take any medication.More than anything, you should not worry! PET scans are perfectly safe and highly tested.
At the start of the appointment, you will be asked to remove all jewellery and other metal objects and may be asked to change into a gown. Your blood sugar levels will be tested and the radioactive tracer will be given to you via an intravenous drip. You will then rest for a short while to allow for the tracer to travel around your body. During the scan itself, you will lie flat on a bed for a painless, roughly 30 minute scan while the detectors collect the data needed.
Once the scan is complete, you will usually be able to go home immediately and be able to eat and drink straight away. The radiation from the tracer will be gone from your body within a few hours but be sure to drink plenty of fluids to aid this. To be safe, avoid close contact with young children or anyone who is pregnant. After 1 to 2 weeks, you will receive your scan results and a radiologist will analyse the images and explain any necessary steps.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Next
Title
Before the Scan
Title
During the Scan
Title
After the Scan
Write a brief description here
What you should and should not do before the scan
Write a brief description here
What will happen during the scan
Write a brief description here
What to expect after the appointment
Limitations and Risks of PET Scans
As with all medical procedures, altough subtle, PET Scans do come with some risks...
Radiation Exposure
Injection-Related Risks
Before the scan, the tracer is given through a standard injection, which may cause minor discomfort. Serious side effects or allergic reactions from this are very rare.
PET Scans involve injecting a small amount of radiation into your body. This is very safe as the amount is very low, however any radiation can slightly increase your risk of getting cancer in the future.
Limited Structure Detail
Pregnancy and Breastfeeding
PET Scans show how organs and tissues are functioning, rather than providing detailed images of their structure. For this reason, PET scans are often complementary to CT or MRI scans to give a more complete picture.
If you are breastfeeding at the time of your scan, the hospital will tell you what to do beforehand to prevent the scan affecting your child, such as expressing milk before the scan. It is not recommended that you have a PET Scan if you are pregnant to avoid the risk of the radiation affecting the baby.
Accuracy Limitations
Areas of high tracer uptake do not always indicate disease and some conditions may not be clearly visible. PET scans must therefore be interpreted carefully by a radiologist and clarified with other tests for accurate diagnosis
Physics Behind PET Scans
The Physics behind PET Scans begins at the synthesis of the radioactive tracer. Using a cyclotron, radioactive isotopes certain elements are produced and then chemically bonded to a glucose molecule or an amino acid. The most common isotope used is Fluorine-18 which is attached to a glucose molecule to get a Fluorine-18 fluorodeoxyglucose (18F-FDG) molecule.
The emitted positron travels a short distance (roughly 1-2mm of tissue) before encountering an electron. When they meet, they annihilate each other and two gamma photons are produced, with energy 511keV. This is due to the difference in mass-energy before and after the decay, following Einstein’s equation: E=mc2.
To conserve momentum, these gamma ray photons are emitted in almost directly opposite directions. The ring of scintillation detectors surrounding the patient detect these two photons. A computer system can then deduce where the annihilation occurred by analysing the time between detection and the line connecting the two photons. Thousands of photons are detected during the imaging process, and a 3D image of the activity in the body can be produced and analysed by a radiologist.
PET Scans rely on β⁺ (beta-plus) decay, where a proton in the nucleus of the F-18 atom transforms into a neutron and releases a positron in the process. p→ n + e+ + νe Where p is a proton, n is a neutron e+ is a positron, and ve is an electron neutrino.
Patient Bed
The patient bed begins out of the ring of detectors and then moves in slowly such that your whole body can be imaged. If the scan is only looking at your brain, for example, sometimes the scan will remain stationary once you are in the correct position.The patient must remain as still as possible during the scan for the imaging process to be accurate, as moving will change the location of where decay happens in accordance to the detectors, leading to an inaccurate image.
Medical Research
PET scans are often used to research metabolic and cognitive function. They allow for clear indications of bodily functions in different situations.Often combined with MRI and/or CT scans, cognitive function is investigated, particularly to improve knowledge of treatment of conditions such as Alzheimer's. Recovery from heart injuries (after surgery) is also monitored. More broadly, PET scans support the testing of new drugs and treatments by allowing researchers to monitor biological changes over time, helping translate research findings into improved clinical care.
Computer Imagine
How does the image get produced?
After the annihilation of the released positron from the decay of the tracer, two particles are produced in opposite directions. Once the ring of detectors receives the particles that are released from the body, a computer is able to figure out where the original decay occured by looking at the time difference between the detection of these two particles. As the particles are emitted in opposite directions, the position of decay must be along the line connecting the two positions of detection.
With millions of particles being detected, the computer is able to build a 3D map of what is going on inside the body, looking at areas of high activity and allowing for diagnosis of conditions such as cancer.
Brain Disorders
As with cancer diagnosis, brain activity can be studied in great depth. Some cognitive disorders, such as Alzheimer's and Parkinson's disease, result in abnormal brain activity. PET scans allow for the visualisation of brain activity and, consequently, the diagnosis of these cognitive disorders. For example, Figure (X) shows the results of a PET scan of somebody with Alzheimer's Disease against a brain scan of somebody with normal cognitive function.
Cancer Diagnosis
PET Scans measure areas of high activity in the body. Cancer cells use a lot more energy than normal cells, so PET Scans are able to highlight cancerous regions in the body. For example, Figure (X) is an image of a patient with a brain tumour. The image clearly shows a strong area of high activity, allowing for easy diagnosis and evaluation of the next steps for treatment.
The Patient in a PET Scan
Before the PET Scan:
In advance to the scan, the patient is injected with what is called a "radioactive tracer". This is essentially a glucose molecule with a radioactive atom attached. After 60-90 minutes, the patient is ready for the scan. They lay down on a table which then moves them into the ring of detectors, where they lay still for 30-60 minutes. During this time, the body treats the radioactive tracer as a normal glucose molecule, transporting it to areas of high activity. Once it reaches said areas, the radioactive atoms start decaying and, in the process, release a type of atom called a positron. When this positron collides with a nearby electron (typically 1-2mm away from the point of decay), they "annihilate", meaning they disappear and release 2 new atoms called photons. These photons travel out of the body and reach the ring of detectors surrounding the patient, where a computer is able to pinpoint where the photons came from and map out areas of activity in the body.
Detectors in a PET Scan
Detector Structure
Inside, each detector is built from several layers that work together to caputre the signals coming from the body. At the front is a type of crystal known as a "scintillation crystal", which produces a small flash of light when it is hit by the radiation emitted by the tracer. This flash is extremeley faint, so is passed to a "photocathode" within a "photomultipler tube", which are pieces of technology that convert light into an electrical signal and then greatly amplify it. By using many of these crystals and detectors arranged around the patient, the PET scanner can precisely detect where the signals are coming from and combine them to form an image of inside the body.
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Heart Disease
In research on heart diseases, PET scans are used to assess conditions such as coronary artery disease helping researchers understand how heart conditions develop and respond to treatment. They are widely used to study reduced blood flow to the heart muscle, to distinguish between permanently damaged tissue and tissue that may recover after a heart attack, and to evaluate the impact of interventions such as drugs, stents, or bypass surgery. PET scans are also important in research on heart failure and inflammation of the heart, allowing scientists to track disease progression and compare treatment effectiveness over time.
Positron Emission Tomography
Elliot Wright
Created on February 2, 2026
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Transcript
Positron Emission Tomography
Positron Emission Tomography (PET) scans are medical imaging techniques that show how tissues and organs are functioning by detecting radioactive tracers inside the body. They are commonly used to identify abnormal metabolic activity, such as cancer, neurological disorders, and heat disease
How do PET Scans work?
PET Scan Applications
Preparing for a PET Scan
Limitations & Risks
Deeper Physics
How do PET Scans Work?
Patient
Ring of Detectors
Computer Imaging
Patient Bed
What Are PET Scans Used For?
Heart Disease
Cancer Detection
Medical Research
Brain Disorder
Preparing for a PET Scan
Before getting a PET scan you will be asked to stop eating 6 hours before the scan and drink only water. You should avoid doing any strenuous exercise for 24 hours before the scan as this may decrease the accuracy of the scan. It is also important to inform the hospital if you are pregnant, breastfeeding, or take any medication.More than anything, you should not worry! PET scans are perfectly safe and highly tested.
At the start of the appointment, you will be asked to remove all jewellery and other metal objects and may be asked to change into a gown. Your blood sugar levels will be tested and the radioactive tracer will be given to you via an intravenous drip. You will then rest for a short while to allow for the tracer to travel around your body. During the scan itself, you will lie flat on a bed for a painless, roughly 30 minute scan while the detectors collect the data needed.
Once the scan is complete, you will usually be able to go home immediately and be able to eat and drink straight away. The radiation from the tracer will be gone from your body within a few hours but be sure to drink plenty of fluids to aid this. To be safe, avoid close contact with young children or anyone who is pregnant. After 1 to 2 weeks, you will receive your scan results and a radiologist will analyse the images and explain any necessary steps.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Next
Title
Before the Scan
Title
During the Scan
Title
After the Scan
Write a brief description here
What you should and should not do before the scan
Write a brief description here
What will happen during the scan
Write a brief description here
What to expect after the appointment
Limitations and Risks of PET Scans
As with all medical procedures, altough subtle, PET Scans do come with some risks...
Radiation Exposure
Injection-Related Risks
Before the scan, the tracer is given through a standard injection, which may cause minor discomfort. Serious side effects or allergic reactions from this are very rare.
PET Scans involve injecting a small amount of radiation into your body. This is very safe as the amount is very low, however any radiation can slightly increase your risk of getting cancer in the future.
Limited Structure Detail
Pregnancy and Breastfeeding
PET Scans show how organs and tissues are functioning, rather than providing detailed images of their structure. For this reason, PET scans are often complementary to CT or MRI scans to give a more complete picture.
If you are breastfeeding at the time of your scan, the hospital will tell you what to do beforehand to prevent the scan affecting your child, such as expressing milk before the scan. It is not recommended that you have a PET Scan if you are pregnant to avoid the risk of the radiation affecting the baby.
Accuracy Limitations
Areas of high tracer uptake do not always indicate disease and some conditions may not be clearly visible. PET scans must therefore be interpreted carefully by a radiologist and clarified with other tests for accurate diagnosis
Physics Behind PET Scans
The Physics behind PET Scans begins at the synthesis of the radioactive tracer. Using a cyclotron, radioactive isotopes certain elements are produced and then chemically bonded to a glucose molecule or an amino acid. The most common isotope used is Fluorine-18 which is attached to a glucose molecule to get a Fluorine-18 fluorodeoxyglucose (18F-FDG) molecule.
The emitted positron travels a short distance (roughly 1-2mm of tissue) before encountering an electron. When they meet, they annihilate each other and two gamma photons are produced, with energy 511keV. This is due to the difference in mass-energy before and after the decay, following Einstein’s equation: E=mc2.
To conserve momentum, these gamma ray photons are emitted in almost directly opposite directions. The ring of scintillation detectors surrounding the patient detect these two photons. A computer system can then deduce where the annihilation occurred by analysing the time between detection and the line connecting the two photons. Thousands of photons are detected during the imaging process, and a 3D image of the activity in the body can be produced and analysed by a radiologist.
PET Scans rely on β⁺ (beta-plus) decay, where a proton in the nucleus of the F-18 atom transforms into a neutron and releases a positron in the process. p→ n + e+ + νe Where p is a proton, n is a neutron e+ is a positron, and ve is an electron neutrino.
Patient Bed
The patient bed begins out of the ring of detectors and then moves in slowly such that your whole body can be imaged. If the scan is only looking at your brain, for example, sometimes the scan will remain stationary once you are in the correct position.The patient must remain as still as possible during the scan for the imaging process to be accurate, as moving will change the location of where decay happens in accordance to the detectors, leading to an inaccurate image.
Medical Research
PET scans are often used to research metabolic and cognitive function. They allow for clear indications of bodily functions in different situations.Often combined with MRI and/or CT scans, cognitive function is investigated, particularly to improve knowledge of treatment of conditions such as Alzheimer's. Recovery from heart injuries (after surgery) is also monitored. More broadly, PET scans support the testing of new drugs and treatments by allowing researchers to monitor biological changes over time, helping translate research findings into improved clinical care.
Computer Imagine
How does the image get produced?
After the annihilation of the released positron from the decay of the tracer, two particles are produced in opposite directions. Once the ring of detectors receives the particles that are released from the body, a computer is able to figure out where the original decay occured by looking at the time difference between the detection of these two particles. As the particles are emitted in opposite directions, the position of decay must be along the line connecting the two positions of detection.
With millions of particles being detected, the computer is able to build a 3D map of what is going on inside the body, looking at areas of high activity and allowing for diagnosis of conditions such as cancer.
Brain Disorders
As with cancer diagnosis, brain activity can be studied in great depth. Some cognitive disorders, such as Alzheimer's and Parkinson's disease, result in abnormal brain activity. PET scans allow for the visualisation of brain activity and, consequently, the diagnosis of these cognitive disorders. For example, Figure (X) shows the results of a PET scan of somebody with Alzheimer's Disease against a brain scan of somebody with normal cognitive function.
Cancer Diagnosis
PET Scans measure areas of high activity in the body. Cancer cells use a lot more energy than normal cells, so PET Scans are able to highlight cancerous regions in the body. For example, Figure (X) is an image of a patient with a brain tumour. The image clearly shows a strong area of high activity, allowing for easy diagnosis and evaluation of the next steps for treatment.
The Patient in a PET Scan
Before the PET Scan:
In advance to the scan, the patient is injected with what is called a "radioactive tracer". This is essentially a glucose molecule with a radioactive atom attached. After 60-90 minutes, the patient is ready for the scan. They lay down on a table which then moves them into the ring of detectors, where they lay still for 30-60 minutes. During this time, the body treats the radioactive tracer as a normal glucose molecule, transporting it to areas of high activity. Once it reaches said areas, the radioactive atoms start decaying and, in the process, release a type of atom called a positron. When this positron collides with a nearby electron (typically 1-2mm away from the point of decay), they "annihilate", meaning they disappear and release 2 new atoms called photons. These photons travel out of the body and reach the ring of detectors surrounding the patient, where a computer is able to pinpoint where the photons came from and map out areas of activity in the body.
Detectors in a PET Scan
Detector Structure
Inside, each detector is built from several layers that work together to caputre the signals coming from the body. At the front is a type of crystal known as a "scintillation crystal", which produces a small flash of light when it is hit by the radiation emitted by the tracer. This flash is extremeley faint, so is passed to a "photocathode" within a "photomultipler tube", which are pieces of technology that convert light into an electrical signal and then greatly amplify it. By using many of these crystals and detectors arranged around the patient, the PET scanner can precisely detect where the signals are coming from and combine them to form an image of inside the body.
Heart Disease
In research on heart diseases, PET scans are used to assess conditions such as coronary artery disease helping researchers understand how heart conditions develop and respond to treatment. They are widely used to study reduced blood flow to the heart muscle, to distinguish between permanently damaged tissue and tissue that may recover after a heart attack, and to evaluate the impact of interventions such as drugs, stents, or bypass surgery. PET scans are also important in research on heart failure and inflammation of the heart, allowing scientists to track disease progression and compare treatment effectiveness over time.