basics about pet machine and its basic working principle ,components of pet machine and its safety measurement

1. Bio Medical Radiation
PET

2. PET Machine

3. Outline
 Introduction
 Components of PET
 Basic working Principle of PET
 Medical application of PET
 Safety measurement of PET
 Dosimetry

4. Introduction
What Is a PET Scan ?
 PET is stand for positron emission tomography.
 PET used a principle of physics, chemistry, and medicine united in an effort
to measure physiologic parameters in invasively.
 History The first experiments to exploit positron-electron annihilation goes
back to 1953 when Gordon Brownell at MIT constructs the first detector
device. The first PET experiments were performed in 1974 by Michael E.
Phelps, Edward Hoffman, and Michel M. Ter-Pogossian.
 A positron emission tomography (PET) scan is an imaging test that allows your
doctor to check for diseases in your body.
Why is a PET scan performed ?
 Your doctor may order a PET scan to inspect your blood flow, your oxygen
intake, or the metabolism of your organs and tissues. PET scans show
problems at the cellular level, giving your doctor the best view of complex
systemic diseases.

5. Basic components of the machine
1.The Gantry:- is the ring-shaped part of the CT or PET, housing many of the
components necessary to produce and detect the radiation. The components are
mounted on the rotating scan frame.
2.Detector:- a PET scanner uses a ring of radiation detectors to produce images
of the distribution of radio-isotopes in the brain, and can measure cerebral
blood flow (CBF), oxygen extraction fraction (OEF), blood volume (CBV) and
glucose metabolic rate. Ex:-Gamma camera
3.Coinsindence circuit:- prepare electrical quantity which is suitable for the
conversion to display or imaging purpose.
4.Table:- the place where the patients lie down.
: 5.Computer- is the device which make basic calibration process and display the
result.
6.Cyclotron:- is a machine used to make short-lived radioactive isotopes that
can be used for medical imaging or research.

7. Physics of PET scanner
Basic working principle
1.Radio nucleotides 18F-FDG injected into the bloodstream, swallowed or
inhaled as a gas.
2. The nucleotides accumulates in the area of your body under examination,
where it gives off a small amount of energy in the form of gamma rays. Special
cameras detect this energy and, with the help of a computer, create pictures
that offer details on the structure and function of organs and tissues.

8. 3. PET exams focus on processes within the body, such as rates of metabolism or
levels of various other chemical activity. Areas of greater intensity, called "hot
spots," indicate where large amounts of the radiotracer have accumulated and
where there is a high level of chemical or metabolic activity. Less intense areas, or
"cold spots," indicate a smaller concentration of radiotracer and less activity.
4. Due to radionucleotide that patient annihilate the detector of the PET scanner
identify the material and try to send measurand to coincidence circuit to the further
processing of the signal which detect at the hot spot.
5. The coincidence circuit is try to simultaneously detect 2 photons. Which is the
major factor for basic image display. In this portion the major work of the scanner
done. Ex-projection, attenuation, signal amplification, resolution.
6. The coincidence circuit prepare electrical quantity which is suitable for the
conversion to display or imaging purpose.
7. Then the computer display the image either in 2D or 3D way.

9. Designing content
1.Positron Decay
When a nucleus undergoes positron decay, the result is a new nuclide with 1
fewer proton and 1 more neutron, as well as the emission of a positron and a
neutrino.
2.Positron Annihilation
As positrons pass through matter, they experience the same interactions as
electrons, including loss of energy through ionization and excitation of nearby
atoms and molecules. After losing enough energy, and having traveled a distance
in the neighborhood of 1 mm (depending on the initial positron energy), the
positron will annihilate with a nearby electron.
 The energy of a particle has 2 components:- its energy of motion and its
mass. In the annihilation process described above, the initial energy is from
the electron and positron masses, since they are moving relatively slowly at
the time of the interaction, and the final energy is the combined energies of
the photons, which have no mass.
.

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 Conservation of energy and momentum dictate, therefore, that the 2
photons are emitted each with an energy of 511 keV (the electron mass times
the speed of light squared) and in opposite directions.
3.Coincidental Detection:-The simultaneous emission of the 2 photons in
opposite directions is the basis of coincidence detection and coincidence
imaging. By using Gamma camera the realized radiation was detect.
i.coincidental Event:- (true event) :-Imagine a ring of radiation detectors
Within the ring is a patient in whom a positron emission has occurred.

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 Although most of the annihilation photons will not be detected, some will
remain in the plane of the detector ring, and 2 of the detectors will be hit,
yielding electronic signals.
 The simultaneous pulses from the detectors indicate that an annihilation
occurred somewhere along the path between the detectors. This is because
the photons leave the annihilation point in opposite directions. The path
between 2 detectors is referred to as a line of response (LOR). The
simultaneous detection of 2 photons is referred to as a “coincidence” .The
number of coincidence. events occurring between detectors indicates how
much radioactivity there was on the LOR between the detectors.

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ii. Scattered considence
One photon from an annihilation leaves the body un scattered , and the other
scatters once before leaving the body. In this prosses the antiparallel photon
travel to ward opposite direction to and are absorbed by detector.
iii.Random Event
Is phenomena in which different photon from different nuclear decay are
detected simultaneously.

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4.Projections
Each pair of detectors in the ring defines a possible emission path. Over the
course of a PET scan, the system is counting how many times each pair of
detectors is hit in coincidence. For a ring with n detectors, there are n2/2 ways
to pair up the detectors, so a great deal of information is recorded.
 The composite grouping of all angles is called a sinogram. In the sinogram,
which is a matrix that can be displayed as an image, the first row of pixels
represents the number of counts at a single angle. The first row typically
represents the angle made from vertical LORs. The next row represents the
next angle, which is only slightly different.

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5.Image reconstruction
The raw PET data can be reconstructed into cross-sectional images with the
same algorithms as SPECT and x-ray CT. Although it is beyond the scope of this
article to discuss reconstruction algorithms, it is important to note the recent
addition of iterative algorithms to the capabilities of most commercial systems..
6.Degrading Factor
The quality of images produced by a PET system is degraded by several physical
factors. Some can be corrected.
7.Attenuation
Attenuation is the loss of true events due to scatter and absorption. Fig shows
an event in which photons were directed toward detectors, but one detector is
not hit because the photon is somehow stopped or deflected. This scattered
photon may or may not be detected in another detector.

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8.Attenuation correction
Two general approaches are used to correct attenuation: calculated correction
and measured correction.
 A calculated attenuation correction assumes that the outer body contour can
be known and that, within this contour, the attenuation properties are
constant (e.g., no lungs, no gas, no substantial bone). The outer contour can
be determined automatically from the data, or defined by an operator by
using an image without attenuation correction.
 A measured attenuation correction is done by performing an additional scan.
This transmission scan typically uses a radioactive source and the same
detectors used for emission scanning to measure the attenuation of the body
along all the LORs. Reference scan (called the “blank”) is performed before
any patient transmission scans, and the ratio of the blank counts to the
transmission counts during a patient scan yields a correction factor for each
emission LOR. The blank scan also serves as a quality assurance measure for
the scanner on a daily basis.
 9.Normaliaztion:- the prosses of making purifying the signal to best image
 Display.

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17. Designing content
3D image Display

18. Medical application
Pet scan commonly used to investigate :-
 Epilepsy: It can reveal which part of the brain the epilepsy is affecting. This
can help doctors decide on the most suitable treatment, and it can be useful
if surgery is necessary.
 Alzheimer’s disease: PET scans can help diagnose Trusted Source Alzheimer’s
disease by measuring the uptake of sugar in specific parts of the brain.
 Brain cells that are affected by Alzheimer’s tend to use glucose more
slowly than normal cells.
 Cancer: PET scans can reveal the presence and stage of a cancer, show
whether and where it has spread, and help doctors decide on treatment.
A PET scan can give an idea of how well chemotherapy is working, and it can
detect a recurring tumor sooner than other techniques.
 Heart disease: A PET scan can help detect which parts of the heart have
been damaged or scarred, and it can help identify circulation problems in the
working of the heart.
This information can help plan treatment options for heart disease.
 Medical research: Researchers can learn vital information by using PET
scans, especially .

19. Who interpret the result
A radiologist or other doctor specially trained in nuclear medicine will interpret
the images and send a report to your referring physician.
What risks are involved with a PET scan?
 The PET scan involves radioactive tracers, but the exposure to harmful
radiation is minimal, but it Has some disadvantage.
 Injection of the radiotracer may cause slight pain and redness. This should
rapidly resolve.
 It’s possible to have an allergic reaction to the tracer. People who are allergic
to iodine, aspartame, or saccharin should alert their doctor.
 People who are pregnant
Radiation is not considered safe for developing fetuses. If you’re pregnant or
think you may be pregnant, you should not get a PET scan.
Safety Measurement
 PET scanning is non-invasive, but it does involve exposure to ionizing
radiation.
 18F-FDG, which is now the standard radiotracer used for PET neuroimaging
and cancer patient management, has an effective radiation dose of 14 mSv.

20. Safety Measurement
 You may be asked to change into a hospital gown. Because metal can
interfere with the testing equipment, you’ll also need to remove any jewelry
you’re wearing, including body-piercing jewelry.
 However, you cannot undergo a PET with nonapproved medical devices or
metal implant.
Preparation
 Before the scan, you’ll get tracers through a vein in your arm, through a
solution you drink, or in a gas you inhale. Your body needs time to absorb the
tracers, so you’ll wait about an hour before the scan begins.
 While you wait, you’ll want to limit any movement, relax, and try to stay
warm. If you’re undergoing a brain scan, you’ll want to avoid television,
music, and reading.
The day before:-Twenty-four hours before your appointment, you’ll be asked to
stick to a low-carbohydrate, no-sugar diet. Foods and beverages you should
avoid include: cereal, pasta, bread, rice, milk and yogurt, whether dairy or
nondairy, fruit and fruit juices, alcohol, caffeinated beverages, candy, including
chewing gum and mints. Foods you can eat include meat, tofu, nuts, and non
starchy vegetables.

21. Safety Measurement
Hours before
 If you’re receiving anesthesia for the procedure, don’t eat or drink anything the
entire morning of your PET scan. Drink only a few sips of water if you need to
take any medications.
 If you’re not receiving anesthesia, you’ll still want to refrain from eating
anything for six hours before your scan. Remember to avoid chewing gum or
sucking on hard candy, cough drops, or mints.
You’ll be able to drink water, however, and take any medications as recommended.
After
 You should not be bothered by any side effects after the exam. You will be
encouraged to drink plenty of fluids to help flush any remaining tracer from your
body. If you were sedated you will need to arrange for someone to drive you
home.
Radiation dosimetry
 Radiation dosimetry in the fields of health physics and radiation protection is the
measurement, calculation and assessment of the ionizing radiation dose absorbed
by an object, usually the human body.

22. Radiation dosimetry
Results:- The absorbed doses (mean +/- SD) ranged from 0.00057 +/- 0.000281
cGy/ MBq (to skin) to 0.00868 +/- 0.00481 cGy / MBq (to bladder wall) (voiding
intervals, 1-2 h), and the effective dose equivalent was 0.00177 +/- 0.000152
cSv MBq.
Conclusion:- The maximum absorbed dose among all tissues in all 10 studies,
0.0151 cGy/MBq, occurred for the urinary bladder wall (with hydration and 1- to
2-h voiding intervals). To ensure that the maximum normal-tissue absorbed dose
is kept below the recommended maximum permissible dose of 5 cGy per single
administration, a maximum administered activity of 331 MBq (5 cGy/[0.0151
cGy/MBq]) is recommended for (18)F-FDHT.

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