1. PET Positron Emission Tomography
Most sensitive method for quantitative measurement of
Physiologic processes in vivo
Lecture (4)
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2. Lecture out lines What is Positron emission tomography (PET)
Positron emission tomography principle
PET camera main components
Solid scintillation detectors in PET
Photomultiplier tube
Pulse Height Analyzer
Arrangement of detectors
Tracers or radionuclides for PET
PET camera design
PET Camera main specification
PET images quality (resolution, signal to noise ratio)
Image corrections
What Can PET Detect?

3. History 1975 the first commercial PET scanner was introduced
70s and 80s PET was mainly used for research
1990s being used in clinics regularly
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4. PET IMAGING
Positron Emission Tomography: is a tomographic nuclear imaging procedure, which uses positrons as radiolabels and positron - electron annihilation reaction-induced gamma rays to locate the radiolabels
PET imaging, an imaging method introduced earlier into clinical practice, is also based on ring detector systems, but recently, manufacturers also have started to fit dual-head camera systems with the coincident detection circuitry necessary for PET imaging.

5. PET Principle 1. the positron is emitted by a beta decay,
2. it is slowed down to small speeds which are necessary for the annihilation reaction between the positron and a shell electron of a neighbouring atoms to occur. The distance the positron travels (mean free path) depends on the energetics of the beta decay but is typically one or a few millimeters.
4. The annihilation reaction produces two 511 keV gamma rays which travel in almost exactly opposite directions (this is due to the conservation of energy and momentum laws).
5. The two gamma rays are detected by a coincidence counting detection system.
6. After proper filtering the collected raw data sinograms are reconstructed into a cross-sectional image.

6. PET Principle: Positrons
When a positron meets an electron the collision creates two gamma rays that have the same energy but go in opposite directions. PET detects the gamma rays as they leave the patients body. The information is fed to a computer and makes a picture.
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7. More of PET PRICIPLES
The PET principle is as follows. A low dose of a radiopharmaceutical labelled with a positron emitter such as C-11, N-13, O-15 or F-18 is injected into the patient, who is scanned by the tomographic system.
Scanning consists of either a dynamic series or a static image obtained after an interval during which the radiopharmaceutical enters the biochemical process of interest.
The scanner detects the spatial and temporal distribution of the radiolabel by detecting gamma rays during the so-called emission scan.

8. PET Components
A positron tomography is an imaging device with which to recover the spatial distribution of a radioisotope decaying by positron emission.
The positron tomography comprises:
Solid scintillation detectors in PET
Photomultiplier tube
Pulse Height Analyzer
see the text) ) Arrangement of detectors
an array of radiation detectors
timing circuitry
position circuitry
energy discrimination circuitry.
For annihilation photons detection, the detectors are usually made of bismuth germanate (BGO), lutetium oxyorthosilicate (LSO), or lutetium – yttrium oxyorthosilicate (LYSO). The detectors are from 20 to 30 mm thick to stop efficiently the 511 keV annihilation photons .
Circa 1977
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9. More on PET Principle
PET principle showing annihilation reaction between positron and electron, production of two gamma rays and detection in coincidence detection system.

10. Annihilation reaction
Electron and a positron meet, annihilate and form two gamma rays

11. ANNIHILATION
511 keV
positron + - +
511 keV
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12. More on PET Principle
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13. Coincidence counting
Method of counting employing a coincidence circuit so that an event (the 2 of 511 kev of γ rays) is recorded only if events are detected in two sensing devices simultaneously.
Such counting methods may be used to reduce background noise if a radioisotope emits more than one detectable radiation event in coincidence.
The requirement for a coincidence between two detectors eliminates background counts that occur in only one detector at a time.

14. Positron Tomography relies on the coincidence detection of the two 511 keV photons which are predominantly emitted as a result of the annihilation of a positron with an electron.
This measurement defines the line along which a nuclear disintegration must have occurred, and hence, along which the radionuclide of interest must have been located.
coincidence?
Yes
Register event
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15. Coincidence methods
In a PET camera , each detector generates a pulse when it registers an incident photon.
These pulses are then combined in coincidence circuitry.
If it fall within a short time-window, they are deemed to be coincident.
A coincidence event is assigned to a line of response (LOR) joining the two relevant detectors.
In this way, positional information is gained from the detected radiation without the need for a physical collimator, this is known as electronic collimation.
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16. Coincidence methods
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17. Scattered Coincidence
True Coincidence
True coincidences
contribute useful information.
Scattered Coincidence
Random Coincidence
Scattered and random coincidences degrade image quality.
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18. Coincidence Events
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19. Types of Coincidence Events
“True” events result from coincidence between 2 photons from the same annihilation. Such events provide valid data.
“Random” and “Scatter” events represent invalid
data. These events are recorded by the system as
misplaced “trues”, resulting in background noise
that reduces image contrast and resolution.
Line of
“scatter” response
Detected
Undetected
“random” response
Line of
“true”
response
Detected
Image Information
Background Noise
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20. Tracers and radionuclides for PET
Depending on the type of tracer, different gamma rays are given and detected. In this way multiple things can be evaluated.
Examples:
· Fluorodeoxyglucose [18F]-labeled 2-deoxyglucose (FDG) is used in neurology, cardiology and oncology to study glucose metabolism. FDG is potentially useful in differentiating benign from malignant forms of stimulated osteoblastic activity because of the high metabolic activity of many types of aggressive tumors.
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21. [15O]-labeled oxygen can also be used to measure tumor necrosis.
Oxygen [15O]-labeled water and oxygen are being evaluated for the quantification of myocardial oxygen consumption and oxygen extraction fraction.
[15O]-labeled oxygen can also be used to measure tumor necrosis.
· Ammonia [13N]-labeled ammonia can be used to measure blood flow.
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22. · Leucine [11C]-labeled methionine and leucine can be used to evaluate amino acid uptake and protein synthesis, providing an indicator of tumor viability.
· Fluorine Ion Radio-labeled fluorine ion [18F-] was once a standard agent for clinical bone scanning.
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23. Particle energy (mean)
RADIONUCLIDES
Radionuclide
Halflife
Particle energy (mean)
C-11
20.4 min
0.39 MeV
N-13
10 min
0.50 MeV
O-15
2.2 min
0.72 MeV
F-18
110 min
0.25 MeV
Cu-62
9.2 min
1.3 MeV
Ga-68
68.3 min
0.83 MeV
Rb-82
1.25 min
1.5 MeV
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24. Multi-crystals detector blocks
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25. PET Block Detector
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26. Scintillators for PET
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27. Camera Design
Many detectors are connected to a photomultiplier tube and arranged in circular, hexagonal, or orthogonal rings.
The field of view is defined by the width of the array of detectors.
The smaller the size of detectors the better the spatial resolution of the PET system.
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28. Coincidence Lines
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29. Image Resolution
The term resolution is often used as a pixel count in digital imaging
An image of N pixels high by M pixels wide can have resolution of N lines per picture height.
When the pixel counts are referred to as resolution, the convention is to describe the pixel resolution with the set of two positive integer numbers, where the first number is the number of pixel columns (width) and the second is the number of pixel rows (height), for example as 640 by 480.
Another popular convention is to cite resolution as the total number of pixels in the image, typically given as number of megapixels, which can be calculated by multiplying pixel columns by pixel rows and dividing by one million.

30. Image Quality: Resolution
Spatial resolution: is the ability to distinguish between two closely spaced objects on an image.
Spatial resolution depends on detector size, system geometry and detector material.
It is sum of intrinsic resolution and the collimator resolution.
Intrinsic resolution depends on the positioning of the scintillation events (detector thickness, number of PM-tubes, photon energy)
Collimator resolution depends on the collimator geometry (size, shape and length of the holes)
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31. Image resolution
An image that is 2048 pixels in width and 1536 pixels in height has a total of 2048×1536 = 3,145,728 pixels or 3.1 megapixels. One could refer to it as 2048 by 1536 or a 3.1-megapixel image.

32. Image Resolution
Below is an illustration of how the same image might appear at different pixel resolutions, if the pixels were poorly rendered as sharp squares (normally, a smooth image reconstruction from pixels would be preferred, but for illustration of pixels, the sharp squares make the point better).

33. Spatial resolution
Image
Object
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34. Energy resolution
A measure of system ability to distinguish between interactions with different energies in its crystal.
if the camera has higher energy resolution it can reject more scattered photons.
Measured by full width half maximum (FWHM) of the peak.
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35. Window width
20%
40%
10%
Increased window width will result in an increased number of registered scattered photons and hence a decrease in resolution
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36. Resolution - distance
Sensitivity: is a measure of gamma-rays incident on the collimator which pass through to the detector.
The higher the sensitivity, the greater the count-rate recorded.
sensitivity
resolution
FWHM
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37. Image Quality: Signal to noise ratio
Information that is not useful is noise
signal-to-noise ratio, often written S/N or SNR.
Ideally, S must be greater than N, so S/N is positive.
Less noise More noise
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38. A high noise level will reduce contrast and image quality .
The noise level in nuclear medicine imaging is generally too high compared to other imaging modalities
The major source of noise is the random distribution of photons per picture element detected by the gamma camera.
A high noise level will reduce contrast and image quality .
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39. Noise Reduction To reduce noise:
increase either the examination time or the patient exposure.
increasing the pixel size (the pixel size should not be too large so as not to influence the spatial resolution of the image ).
image processing such as background subtraction and digital filtering should be applied.
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40. Clinical Positron Tomographs
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41. Image quality: Image correction
Necessary: Corrections for
detector differences
randoms
scatter
attenuation
and also for Calibration
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42. PET Scatter Correction
Methods for correcting scattered:
•Energy Windows
•Convolution/Filter and Subtraction
-measured scatter functions
-estimate from outside the object
•Calculation of Scatter Distribution
-model for single scatter
-Monte Carlo simulations
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43. What can PET Detect?
PET has several clinical indications, which include but are not limited to the following:
Cancer (eg, staging and evaluating specific types of cancer and evaluating response to treatment), which accounts for about 80% of PET usage
Cardiac function (eg, evaluating myocardial viability, detecting hibernating myocardium)
Neurologic function (eg, evaluation of dementia and seizures)
PET applications continue to be investigated.

44. What Can PET Detect?
Coronary Artery Disease PET imaging is unique in its ability to determine whether a patient's heart muscle will benefit from coronary artery bypass surgery.
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45. The second heart is normal
Example: Myocardial Viability
                                                                    
The first heart has a mycardial infarction. The arrows point to damaged areas (‘dead’ tissue).Therefore it is assumed that the patient will not benefit from heart surgery.
The second heart is normal
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46. Tumors
PET imaging is very accurate in differentiating malignant from benign growths, as well as showing the spread of malignant tumors.
PET imaging can help detect recurrent brain tumors and tumors of the lung, colon, breast, lymph nodes, skin, and other organs.
Information from PET imaging can be used to determine what combination of treatment is most likely to be successful in managing a patient's tumor.
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47. Example: Breast Cancer
                                                                            
The fist picture shows a malignant breast mass not shown in the conventional imaging techniques. (CT, MRI, mammogram)
Second picture is that of the same patient with enlarged left axillary lymph nodes. Through a biopsy they were found to be metastatic.
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48. FDG PET
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49. FDG PET: Metastases from Colon Carcinoma
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50. FDG IN CARDIOLOGY
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51. FDG IN NEUROLOGY
Alzheimers disease
Normal
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52. Metabolism in the Brain Surface Projections
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53. THE FUTURE Diagnostic methods
New radiopharmaceuticals based on positron
emitters.
Radiopharmaceuticals with high specificity.
More advanced application programs which
improve both sensitivity and specificity of the
examination.
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54. PET WITH GAMMA CAMERA