Used in mri maging

1. POSITRON
EMISSION
TOMOGRAPHY

2. PET-keywords
Positron- is the antiparticle or the antimatter
counter part of the electron
Positron emission- is a particular type of radioactive decay
and a subtype of beta decay, in which a proton inside a
radionuclide nucleus is converted into a neutron while
releasing a positron and an electron neutrino.
Tomography- refers to imaging by sections or sectioning,
through the use of any kind of penetrating wave.

3. INTRODUCTION
• Positron-emission tomography (PET) is a nuclear
medicine functional imaging technique that is used to observe
metabolic processes in the body as an aid to the diagnosis of
disease.
• The system detects pairs of gamma rays emitted indirectly by
a positron-emitting radio tracer, most commonly fluorine-18,
which is introduced into the body on a biologically active
molecule called a radioactive tracer.
• Different tracer are used for different imaging purposes,
depending on what the radiologist/researcher wants to detect.

4. • The technique is based on the detection of radioactivity
emitted after a small amount of a radioactive tracer is injected
into a peripheral vein.
• The tracer is administered as an intravenous injection usually
labelled with oxygen-15, fluorine-18, carbon-11, or nitrogen-
13.
• The total radioactive dose is similar to the dose used in
computed tomography.
• It is the only imaging modality capable of providing
quantitative information about biochemical and physiologic
processes.
• Other techniques like magnetic resonance imaging (MRI) and
x-ray computerized tomography (CT) generally image the
anatomy or the structure of the body.

5. PRINCIPLE
• Positrons
• A positron is the anti-particle of an electron.
• It has the same mass as an electron except its electric charge is positive
instead of negative.
• Positrons (or positive beta particles) are naturally emitted by many
isotopes.
• According to the laws of modem physics, when a positron collides with an
electron, there is a chance that they both annihilate, and two gamma rays
are created simultaneously.
• The energy of each of these gamma rays is 511 keV.
• To conserve momentum, these two gamma rays travel in opposite
directions.
• The detectors in a PET scanner are tuned to accept coincident detection of
these two gamma rays.

6. • The positron is emitted from the nucleus of those isotopes which
have an excess number of protons compared to the number of
neutrons.
• What occurs inside the nucleus is that one of its protons decays to a
neutron along with ejection of a positron and a neutrino; therefore,
the nucleus decays to a more stable isotope.
• Eventually, it will collide with one of the many electrons in the
medium and spontaneous annihilation takes place:
• e+ + e- = Gamma + Gamma.

8. PRINCIPLE
• PET measure the two annihilation photons.
• The annihilation photons are produced back to back after positron
emission from a radionuclide tagged tracer molecule.
• This molecule is chosen to mark a specific function in the body on a
biochemistry level.
• Hence , unlike other techniques which provide anatomical detail
PET provide information regarding biological function.
• Most PET scans use FDG as a tracer.
• FDG is an unstable radioisotope with half life of approximately
110minutes.
• PET images are much blurrier or noisier, due to the relatively
limited number of photons that can be collected during an imaging
study as compared to CT & MRI.

11. HOW DOES PET WORKS
• PET works by using a scanning device (a machine with a large hole
at its center) to detect photons (subatomic particles) emitted by a
radionuclide in the organ or tissue being examined.
• The radionuclides used in PET scans are made by attaching a
radioactive atom to chemical substances that are used naturally by
the particular organ or tissue during its metabolic process.
• For example, in PET scans of the brain, a radioactive atom is
applied to glucose (blood sugar) to create a radionuclide called
fluorodeoxyglucose (FDG), because the brain uses glucose for its
metabolism. FDG is widely used in PET scanning.
• Other substances may be used for PET scanning, depending on the
purpose of the scan.
• You will need to wait nearby as the tracer is absorbed by your body.
This takes about 1 hour.

13. WHY PET IS
PERFORMED?
• In general, PET scans may be used to evaluate organs and/or tissues
for the presence of disease or other conditions.
• PET may also be used to evaluate the function of organs, such as the
heart or brain. The most common use of PET is in the detection of
cancer and the evaluation of cancer treatment.
• More specific reasons for PET scans include, but are not limited to,
the following:
• To diagnose dementias (conditions that involve deterioration of
mental function), such as Alzheimer's disease, as well as other
neurological conditions such as:
• Epilepsy. A brain disorder involving recurrent seizures.
• Cerebrovascular accident (stroke)
• To locate the specific surgical site prior to surgical procedures of the
brain

14. Detector Systems
• All existing PET scanners use scintillation crystals coupled to
photomultiplier tubes (PMTs) for detection of the gamma rays.
• Most companies building PET scanners use Bismuth
Germinate Oxide (BGO) crystals as the scintillator.
• BGO has higher density and average atomic number compared
to NaI (Tl), therefore it has higher stopping power for the 511
KeV gamma rays.
• Because of this fact, smaller individual detector elements can
be used; therefore, better image resolution can be achieved.

16. CLINICAL USES OF PET
• PET has been a powerful research tool in medicine, it has been shown to be a
unique and critical diagnostic modality for clinical use.
• This is because it provides unique physiologic and biochemical information about
the brain, heart, and the rest of the body.
• The origin of disease is fundamentally biochemical in nature. Therefore, to obtain
the most accurate diagnoses and most effective treatment, it is desirable to know the
biochemical status of the organ or organ system in question.
• The most common uses of PET in clinical studies are measurement of the metabolic
rate of glucose in different locations of the brain and the heart.
• These studies can:
 Help the clinician to pinpoint the location of epileptic foci in seizure disorder
patients.
 Identify different forms of dementia and degenerative diseases (i.e., Alzheimer's,
Parkinson's or Huntington's diseases).
 Identify the pathology of brain tumors and access the viability of the brain tissue
and evaluate the viability of the myocardium in patients with cardiac disease

17. • DATA ACQUISITION
• A filtered back projection is performed by the computer in
order to produce transvere images of
• the radio nuclide distribution in the patient.
• Modern scanners are capable of performing
simultaneous acquisition of greater than 45 slices over
16cm of axial distance.

18. THANK YOU