PET Project - Revision history

Dfe002 at 09:12, 12 June 2009

2009-06-12T09:12:53Z

← Older revision		Revision as of 09:12, 12 June 2009
Line 183:		Line 183:
	* '''Crosstalk''': in an avalanche multiplication process, photons might be generated which are different from photons initially incident on an APD pixel. If those generated photons are detected by other APD pixels, then the MAPD output shows a value higher than the number of photons that were actually input and detected by the MAPD. This phenomenon is thought to be one of the causes of crosstalk in the MAPD.		* '''Crosstalk''': in an avalanche multiplication process, photons might be generated which are different from photons initially incident on an APD pixel. If those generated photons are detected by other APD pixels, then the MAPD output shows a value higher than the number of photons that were actually input and detected by the MAPD. This phenomenon is thought to be one of the causes of crosstalk in the MAPD.

	=== Documentation ===		== Documentation ==


	[[User:Dfe002\|Dominik]] 09:06, 12 June 2009 (UTC)		[[User:Dfe002\|Dominik]] 09:06, 12 June 2009 (UTC)

	[[Category:Detector lab]]		[[Category:Detector lab]]

Dfe002 at 09:08, 12 June 2009

2009-06-12T09:08:28Z

← Older revision		Revision as of 09:08, 12 June 2009
Line 183:		Line 183:
	* '''Crosstalk''': in an avalanche multiplication process, photons might be generated which are different from photons initially incident on an APD pixel. If those generated photons are detected by other APD pixels, then the MAPD output shows a value higher than the number of photons that were actually input and detected by the MAPD. This phenomenon is thought to be one of the causes of crosstalk in the MAPD.		* '''Crosstalk''': in an avalanche multiplication process, photons might be generated which are different from photons initially incident on an APD pixel. If those generated photons are detected by other APD pixels, then the MAPD output shows a value higher than the number of photons that were actually input and detected by the MAPD. This phenomenon is thought to be one of the causes of crosstalk in the MAPD.

			=== Documentation ===

	[[User:Dfe002\|Dominik]] 09:06, 12 June 2009 (UTC)		[[User:Dfe002\|Dominik]] 09:06, 12 June 2009 (UTC)

	[[Category:Detector lab]]		[[Category:Detector lab]]

Dfe002 at 09:06, 12 June 2009

2009-06-12T09:06:50Z

← Older revision		Revision as of 09:06, 12 June 2009
Line 184:		Line 184:


	~~== Characterisation Cell ==~~		[[User:Dfe002\|Dominik]] 09:06, 12 June 2009 (UTC)
	~~== Characterisation setup and results ==~~

	[[User:Dfe002\|Dominik]] 09:04, 12 ~~March~~ 2009 (~~CET~~)

	[[Category:Detector lab]]		[[Category:Detector lab]]

St12361 at 07:51, 14 April 2009

2009-04-14T07:51:39Z

← Older revision		Revision as of 07:51, 14 April 2009
Line 186:		Line 186:
	== Characterisation Cell ==		== Characterisation Cell ==
	== Characterisation setup and results ==		== Characterisation setup and results ==

	~~== [[Photomultipliers]] ==~~

	[[User:Dfe002\|Dominik]] 09:04, 12 March 2009 (CET)		[[User:Dfe002\|Dominik]] 09:04, 12 March 2009 (CET)

	[[Category:Detector lab]]		[[Category:Detector lab]]

Dfe002 at 08:04, 12 March 2009

2009-03-12T08:04:26Z

← Older revision		Revision as of 08:04, 12 March 2009
Line 188:		Line 188:

	== [[Photomultipliers]] ==		== [[Photomultipliers]] ==

			[[User:Dfe002\|Dominik]] 09:04, 12 March 2009 (CET)

	[[Category:Detector lab]]		[[Category:Detector lab]]

Dfe002 at 08:08, 25 February 2009

2009-02-25T08:08:19Z

@@ Line 30: / Line 30: @@
 [[Image:MAPD_crystaltransition.JPG‎ |frame|left|200px|Electronic transition in a crystal]]
 A scintillator is a substance that absorbs high energy and then, in response, emits photons. Scintillators are defined by their light output (number of emitted photons per unit absorbed energy), short fluorescence decay times, and optical transparency at wavelengths of their own specific emission energy. The high Z-value of the constituents and high density of inorganic crystals favour their choice for gamma-rays spectroscopy (rather than organic crystal) because heavy nucleuses accept better gammas than light nucleus. The scintillation mechanism in inorganic materials depends on energy states determined by the crystal lattice of the material. Absorption of energy can result in the elevation of an electron from its normal position in the valence band across the gap in the conduction ban, leaving a hole in the valence band. A charged particle passing through the detection medium will form a large number of electron-hole pairs, created by the elevation of electrons from the valence to the conduction band. The positive hole will quickly drift to the location of an impurity and ionize it, because the ionization energy of this impurity will be less than that of a typical lattice  site. Meanwhile, the electron is free to migrate through the crystal and will do so until it encounters an ionized activator. At this point,  the electron can drop into the impurity site, creating a neutral impurity configuration which can have its own set of excited energy states. If the activator state that is formed is an excited configuration with an allowed transition to the ground state, its deexcitation will occur very quickly and with high probability for the emission of the corresponding photon. The migration time for the electronics is shorter than the drop-out time: therefore, the decay time of these states determines the time characteristics of the emitted scintillation light. In order to fully utilize the  scintillation light, the spectrum should fall near the wavelength region of maximum sensitivity for the device used to detect the light.
 {| border="1" cellpadding="2" cellspacing="0"
@@ Line 66: / Line 67: @@
 === Coincidence detection ===
 In a PET camera, each detector generates a time pulse when it registers an incident photon. These pulses are then combined in coincidence circuitry, and if the pulses fall within a short time-window, they are deemed to be coincident. A diagram of this process is shown below:
 A coincidence event is assigned to a line of response (LOR) joining the two relevant detectors. Coincidence events in PET fall into 3 categories: true, scattered and random.
 * '''True coincidences''' occur when both photons from an annihilation event are detected by detectors in coincidence, neither photon undergoes any form of interaction prior to detection, and no other event is detected within the coincidence time-window.
@@ Line 77: / Line 82: @@
 === Time of Flight ===
 Time-of-flight PET takes advantage of the difference in arrival times of two photons from the same annihilation event to infer spatial information of this event. A detected coincidence between two crystals will have a time difference for any annihilation event that does not occur at the midpoint between the detectors, this time difference is used to place the position of the event.
 == Technology ==
 === Avalanche Photodiodes ===
 An APD is basically a p-n junction diode operated at large reverse bias voltage. The physical mechanism which avalanche gain depends, is the impact ionization. It occurs when the electric field in the depletion region is strong enough: an electron colliding with a bound valence electron transfers enough energy to this electron to ionize it. The additional carriers can gain sufficient energy from the electric field to cause further impact ionization, creating an avalanche of carriers.
 * '''proportional mode'''
 In a proportional counter, each original electron leads to an avalanche which is basically independent of all other avalanches formed from the other electrons associated with the original ionizing event. The collected charge remains proportional to the number of original electrons.
 * '''Geiger mode'''
 With a higher electric field, a situation is created, in which one avalanche trigger itself a second avalanche at a different position.
 The difference between both modes relies on the holes: in Geiger mode, they trigger avalanches, whereas in proportional mode they have not enough energy to do so. From the critical value of the electric field (corresponding to the breakdown voltage), a self propagating chain reaction occurs. In principle, an exponentially growing number of avalanches could be created.
 * '''b- Passive quenching'''
 The avalanche photodiode (i. e. pixel for the silicon photomultiplier) is connected to the power supply through a large series resistor Rs. If the current through the diode tends to zero, the voltage across the diode equals Vbias, which will be larger than the breakdown voltage. So, when the diode breaks down, the series resistor reduces the voltage across the APD, what quenches the avalanche. After the breakdown is quenched, the diode is recharged through the resistor. The APD is now ready to receive another photon.
 === different MAPDs ===
@@ Line 119: / Line 187: @@
 == Characterisation setup and results ==
-[[Photomultipliers]]
+== [[Photomultipliers]] ==
 [[Category:Detector lab]]

Dfe002 at 07:54, 25 February 2009

2009-02-25T07:54:24Z

← Older revision		Revision as of 07:54, 25 February 2009
Line 15:		Line 15:

	=== Radiotracers ===		=== Radiotracers ===
			[[Image:MAPD_radiotracer.JPG‎\|frame\|right\|200px\|Fluorodeoxyglucose-FDG]]
			[[Image:MAPD_positronelectronannihilation.JPG‎‎\|frame\|left\|200px\|Positron - Electron Annihilation]]
	The PET technology is based on radioactive emission. Radioactive substances are combined to molecules that the studied cells use particularly in their metabolism. These tracers are radioactive substances. The first step in PET imaging is the production of radionuclides by a cyclotron. These		The PET technology is based on radioactive emission. Radioactive substances are combined to molecules that the studied cells use particularly in their metabolism. These tracers are radioactive substances. The first step in PET imaging is the production of radionuclides by a cyclotron. These
	radionuclides will be attached to molecules used by the body before being injected to the patient by intravenous way. The molecule and the adionuclide form the radiotracer. The tracer is injected to the patient and, following the half life of the radionuclide, it will become stable by		radionuclides will be attached to molecules used by the body before being injected to the patient by intravenous way. The molecule and the adionuclide form the radiotracer. The tracer is injected to the patient and, following the half life of the radionuclide, it will become stable by
	emitting a positron and a neutrino (the proton which stays in excess will become a neutron). Then, the emitted positron travels a short distance before		emitting a positron and a neutrino (the proton which stays in excess will become a neutron). Then, the emitted positron travels a short distance before
	encountering an electron. When they meet each other, the two particles combine and annihilate each other resulting in the emission of two 511 keV gamma rays in opposite directions.		encountering an electron. When they meet each other, the two particles combine and annihilate each other resulting in the emission of two 511 keV gamma rays in opposite directions.






	=== Scintillators ===		=== Scintillators ===
			[[Image:MAPD_crystaltransition.JPG‎ \|frame\|left\|200px\|Electronic transition in a crystal]]
	A scintillator is a substance that absorbs high energy and then, in response, emits photons. Scintillators are defined by their light output (number of emitted photons per unit absorbed energy), short fluorescence decay times, and optical transparency at wavelengths of their own specific emission energy. The high Z-value of the constituents and high density of inorganic crystals favour their choice for gamma-rays spectroscopy (rather than organic crystal) because heavy nucleuses accept better gammas than light nucleus. The scintillation mechanism in inorganic materials depends on energy states determined by the crystal lattice of the material. Absorption of energy can result in the elevation of an electron from its normal position in the valence band across the gap in the conduction ban, leaving a hole in the valence band. A charged particle passing through the detection medium will form a large number of electron-hole pairs, created by the elevation of electrons from the valence to the conduction band. The positive hole will quickly drift to the location of an impurity and ionize it, because the ionization energy of this impurity will be less than that of a typical lattice site. Meanwhile, the electron is free to migrate through the crystal and will do so until it encounters an ionized activator. At this point, the electron can drop into the impurity site, creating a neutral impurity configuration which can have its own set of excited energy states. If the activator state that is formed is an excited configuration with an allowed transition to the ground state, its deexcitation will occur very quickly and with high probability for the emission of the corresponding photon. The migration time for the electronics is shorter than the drop-out time: therefore, the decay time of these states determines the time characteristics of the emitted scintillation light. In order to fully utilize the scintillation light, the spectrum should fall near the wavelength region of maximum sensitivity for the device used to detect the light.		A scintillator is a substance that absorbs high energy and then, in response, emits photons. Scintillators are defined by their light output (number of emitted photons per unit absorbed energy), short fluorescence decay times, and optical transparency at wavelengths of their own specific emission energy. The high Z-value of the constituents and high density of inorganic crystals favour their choice for gamma-rays spectroscopy (rather than organic crystal) because heavy nucleuses accept better gammas than light nucleus. The scintillation mechanism in inorganic materials depends on energy states determined by the crystal lattice of the material. Absorption of energy can result in the elevation of an electron from its normal position in the valence band across the gap in the conduction ban, leaving a hole in the valence band. A charged particle passing through the detection medium will form a large number of electron-hole pairs, created by the elevation of electrons from the valence to the conduction band. The positive hole will quickly drift to the location of an impurity and ionize it, because the ionization energy of this impurity will be less than that of a typical lattice site. Meanwhile, the electron is free to migrate through the crystal and will do so until it encounters an ionized activator. At this point, the electron can drop into the impurity site, creating a neutral impurity configuration which can have its own set of excited energy states. If the activator state that is formed is an excited configuration with an allowed transition to the ground state, its deexcitation will occur very quickly and with high probability for the emission of the corresponding photon. The migration time for the electronics is shorter than the drop-out time: therefore, the decay time of these states determines the time characteristics of the emitted scintillation light. In order to fully utilize the scintillation light, the spectrum should fall near the wavelength region of maximum sensitivity for the device used to detect the light.

Dfe002 at 07:41, 25 February 2009

2009-02-25T07:41:37Z

← Older revision		Revision as of 07:41, 25 February 2009
Line 80:		Line 80:
	With a higher electric field, a situation is created, in which one avalanche trigger itself a second avalanche at a different position.		With a higher electric field, a situation is created, in which one avalanche trigger itself a second avalanche at a different position.

	The difference between both modes relies on the holes: in Geiger mode, they trigger avalanches, whereas in proportional mode they have not enough energy to do so. From the critical value of the electric field (corresponding to the breakdown voltage), a self propagating chain reaction occurs. In principle, an exponentially growing number of avalanches could be created.		The difference between both modes relies on the holes: in Geiger mode, they trigger avalanches, whereas in proportional mode they have not enough energy to do so. From the critical value of the electric field (corresponding to the breakdown voltage), a self propagating chain reaction occurs. In principle, an exponentially growing number of avalanches could be created.

	* '''b- Passive quenching'''		* '''b- Passive quenching'''
	The avalanche photodiode (i. e. pixel for the silicon photomultiplier) is connected to the power supply through a large series resistor Rs. If the current through the diode tends to zero, the voltage across the diode equals Vbias, which will be larger than the breakdown voltage. So, when the diode breaks down, the series resistor reduces the voltage across the APD, what quenches the avalanche. After the breakdown is quenched, the diode is recharged through the resistor. The APD is now ready to receive another photon.		The avalanche photodiode (i. e. pixel for the silicon photomultiplier) is connected to the power supply through a large series resistor Rs. If the current through the diode tends to zero, the voltage across the diode equals Vbias, which will be larger than the breakdown voltage. So, when the diode breaks down, the series resistor reduces the voltage across the APD, what quenches the avalanche. After the breakdown is quenched, the diode is recharged through the resistor. The APD is now ready to receive another photon.

	=== different MAPDs ===		=== different MAPDs ===
Line 104:		Line 104:

	* '''Afterpulse''': afterpulses are spurious pulses following the true signal, which occur when the generated carriers are trapped by crystals defects and then release at a certain delay time. A fterpulses cause detection errors. The lower the temperature, the higher the probability that carriers may be trapped by crystal defects, so afterpulses will increase.		* '''Afterpulse''': afterpulses are spurious pulses following the true signal, which occur when the generated carriers are trapped by crystals defects and then release at a certain delay time. A fterpulses cause detection errors. The lower the temperature, the higher the probability that carriers may be trapped by crystal defects, so afterpulses will increase.

	* '''Crosstalk''': in an avalanche multiplication process, photons might be generated which are different from photons initially incident on an APD pixel. If those generated photons are detected by other APD pixels, then the MAPD output shows a value higher than the number of photons that were actually input and detected by the MAPD. This phenomenon is thought to be one of the causes of crosstalk in the MAPD.		* '''Crosstalk''': in an avalanche multiplication process, photons might be generated which are different from photons initially incident on an APD pixel. If those generated photons are detected by other APD pixels, then the MAPD output shows a value higher than the number of photons that were actually input and detected by the MAPD. This phenomenon is thought to be one of the causes of crosstalk in the MAPD.

Dfe002 at 07:32, 25 February 2009

2009-02-25T07:32:04Z

@@ Line 59: / Line 59: @@
 === Coincidence detection ===
 In a PET camera, each detector generates a time pulse when it registers an incident photon. These pulses are then combined in coincidence circuitry, and if the pulses fall within a short time-window, they are deemed to be coincident. A diagram of this process is shown below:
 A coincidence event is assigned to a line of response (LOR) joining the two relevant detectors. Coincidence events in PET fall into 3 categories: true, scattered and random.
 * '''True coincidences''' occur when both photons from an annihilation event are detected by detectors in coincidence, neither photon undergoes any form of interaction prior to detection, and no other event is detected within the coincidence time-window.
@@ Line 68: / Line 68: @@
 === Time of Flight ===
 Time-of-flight PET takes advantage of the difference in arrival times of two photons from the same annihilation event to infer spatial information of this event. A detected coincidence between two crystals will have a time difference for any annihilation event that does not occur at the midpoint between the detectors, this time difference is used to place the position of the event.

Dfe002 at 07:24, 25 February 2009

2009-02-25T07:24:19Z

@@ Line 1: / Line 1: @@
 == Goal ==
 The MEDUSA project focuses on R&D for high energy physics instrumentation with two important and dependant goals. One is to contribute to the research for future particle detectors and develop new improved detectors for the LHC upgrade as well as the planned international linear collider. The other is to provide new technologies for medical imaging devices such as PET. With this, we hope to contribute to bridging the gap between the particle physics research and the medical technology to fully take advantage of the latest development.
@@ Line 37: / Line 30: @@
 !LYSO
 |-
-|ZE
+|'''ZE'''
 |50
 |74,2
 |65
 |65
 |}
 == Characterisation Cell ==