Unit 3 - Radiation Electronics and Detection Flashcards
What are the units for Rontgens?
C/kg
What are rontagens measuring?
Exposure
Define exposure?
The intensity of radiation upon the surface of an object (i.e. the number of ionized electrons)
What is the units for Gry and what is it measuring?
J/Kg-measuring the absorbed dose
How do you count the x ray photons before they are absorbed by an attenuator?
Find their number of photons by recording the ionizations that take place in air near the material that is being attenuated
What is exposure (i.e. Rontgens) useful for measuring?
The intestisity of the “raw” x ray beam emmited from the tube
Is the measurement of rontgens useful for measuring the biological imact of radiation exposure? Why or why not?
No
1. Air and soft tissue have diff. densities therefore the dose absorbed by the patient is different with varying tissue types
2. P.E. effect contributes most to dose, and we cannot tell the difference in a measurement between P.E. and scatter.
True or false?
All radiation detection instruments have a transducer? What is a transducers purpose?
True. Transducer measures the # of ionizations that are occuring.
What are the 2 different ways that radiation detectors operate based off of?
- # Ionizations of atoms
- The excitation of electrons
How do detectors that operate on the basis of excitation of electrons count the number of photons?
As the e- returns to the pre-excitation stage, they release energy which is captured and transferred into an electrical current which is then measured.
How do detectors that operate on the basis of the ionizations of atoms count the number of photons?
Free e- that have been knocked out of orbit travel to a positive anode plate that creates a charge that can be measured.
What factors determine whether ionization or excitation is favored?
- The physical state of the transducer
- The denisty of the transducer
The transducer is coupled to an ___________ where the radiation is converted into a useful electronic signal
Electronic component
What are the 3 functions that radiation detectors can preform?
- Can detect radiation
- Can measure radiation
- Can do both
What is the function of pulse or rate mode detectors?
Detect the presence of radiation (and general intensity)
Can pulse mode detectors measure radioactivity without a meter or a counter?
No
When using a pulse mode detector, how do we know that radiation is present?
When radiation is present, a clicking/beeping/ticking will occur and the meter will move to indicate the general amount of radiation present
What type of mode is a geiger counter?
It is a pulse mode detector
What is rate mode measuring?
The rontgens present per hour
In order for a rate mode detector to be useful, what enviroment must it be in?
-Useful when there is a steady flow of radiation
True or false?
Rate mode detectors are useful for detecting natural sources of radioactivity. Why or why not?
False, natural sources of radioactivity have varying rates which does not allow for an accurate reading
What is them most useful mode of detectors?
Integrate mode
What does integrate mode measure?
The number of ionizations per set unit of time
True or false?
Integrate mode is accurate.
True, they can count events at an extremely high speed and with great accuracy
What mode of radiation detection can be derived from the integrate mode?
Rates can be accurately derived
What is the most accuate way to obtain exposure rates?
Use integrated measurements taken over long periods of time and then divide by the seconds or minutes expired
What are the characteristics of radiation detection devices that makes a good device?
- Sensitivety
- Accuracy
- Resolving (interogation) time
- Range
What is sensitivity?
The ability to dectect small amounts of radiation OR the amount of electrical current/charge produced by radiation
What are the 2 main modicfications that would make a dtection instrument more sensitive?
- Larger detection chamber
- Increasing the amplification of the electronic signal (proportionally)
What is the only downside of having a larger detector?
We cannot make it portable
What is accuracy in realtion to radiation detection devices?
The precision with which measurements are obtained
True or false?
Decreasing sensitivity increases accuracy.
False, increasing sensitivity increases accuracy
What are the factors that could affect accuracy negatively?
- Printed scale being missaligned
- Electronic noise contributing to the signal causing the read out to be higher
- Fluctuations in the amount of power being supplied such as the batteries running down
What is the resolving (interrogration) time?
The minimum time that must elpase before another ionization can be detectted by the detector
True or false?
Resolving (intograttion time) affects the accuracy of the instrument
True
What mode of detector is the least susceptible to dead times during the elapsed/resolving time
Integrate mode
What is a range detector?
A range mode detector takes into consideration the expected range of intensity and only displays a certain amount of radiation that it is programmed to expect
What are geiger counter detectors generally used for?
Low amounts of radiation that are typical of naturally occurring radioactivity
Cannot use these for measuring x ray intensity
What does higher accuracy detectors result from?
- Increased sensitivity
- Increased range (not just low range)
- Faster resoving time
- Higher DQE
What makes a detector have a high DQE?
- Intrinic factor: Transducer should have sufficient stopping power (ability to absorb) and detect the radiation
- Geometric factor: Transducer should be optimally positioned relative to the radiation source (SID)
What is absoulte efficency?
**A product of intrinic efficiency and geometric effficency **
(The number of photons emmitted vs the events detected)
How do gas filled detectors work?
- As x rays pass through the gas, it ionizes atoms of the gas, creting ion pairs (free e-)
- e- then strike a positively charged anode within the chamber
- The e- released in ionization cause an electrical signal proportional to the raidation intensity
What are the types of gas filled detectors?
- Ionization chambers
- Proportional counters
- Geiger Muller Detectors
What type of gas is located within the ionization chambers?
-Filled with air
-Filled with pressurized gas (xenon or argon)
True or false?
The larger the chamber, the more sensitive the instrument.
True
In ionization chambers, how many e- are produced for every photon interacting with it?
1 e- for every 1 photon
What are the benefits of ionization chambers?
- Good for portable work for radiation surveys around floro, nuclear medicine generators and syringes
- Good for checking integrity of protective barriers
- Good for checking the output and callibration of the x ray machines
What are proportional counters?
Take advantage of the cascade effect by using high E e- that has other interactions with e- in the ionization chambers and create secodnary e-.
(in a way amplifies e-)
What are proportional counters useful for? Why?
Useful for lower range amounts of raidaiton (in geiger counters) since they have high sensitivity and are able to detect small amounts of radiation
True or false?
Proportional counters are useful in clinical imaging due to thier high sensitivity.
False-Although they do have high sensitivity
What type of gas filled detector is used in the geiger mueller tube?
Proportional counters
What happens in a geiger-muller tube when an e- becomes ionized from a single radiation exposure event?
- The energy imparted from the e- being ionized causes all of the e- in the gas chamber to become ionized
- Entire chamber is discharged
- The entire chamber must now be regarched before it can detect another radiation event
What is one way that we can slightly resolve the resting/dead time in geiger-muller tubes?
By using quenching agents (Butane/ethanol vaopor, bromine/chlorine)
How long is the dead time in geiger muller tubes with quenching agents?
50-100 microseconds
True or false?
Geiger muller tubes have low sensitivity, but high accuracy.
Geiger muller tubes have high sensitivity, but low accuracy. They are not capable of an integrated/accumulated dose calcualtion.
What is the rate of the range for the intenisty of gieger muller tubes?
100mr/hr
What are geiger muller tubes primarily used for?
- Portable survey instruments for enviormental raidaiton
- Detect raidaiton contamination on work surfaces in nuc.med. and laboratories
What is recombination?
When the voltage is too low to cause an ionized e- in the chamber to reach the anode, the e- travels back to its original atom and reattaches
What is the ionization region?
When photons are at a good voltage such that for every 1 ionization, 1 e- will be released and go to the anode
What is the proportional region?
Photons have a much higher voltage such that for every ionization, 10 electrons will be released
What is the geiger muller region?
When the photon energy is so high such that 1 ionization discharges the whole tube
What is the CD region?
When the Voltage energy is so high such that it causes contrast discharge without ionization
Where are scintillation detectors primarily used in?
- Digital image receptor in radiography and floro
- Detector array in CT
- Gamma camera in nuc. med.
What materials scintillate?
Particular crystalline structures scintillate
What is scintillation?
The conversion of x ray energy into light photons
What is it called when the ground state return is delayed for trapped e- in a detector?
Phosphorescence (CR)
What is the most widely used scintillation phosphors?
Thallium activated cesium iodide crystals
What are the 2 types of scintiallation phosphors?
- Csl: TI (Thallium activated cesium iodide)
- Na:TI (Thallium activated sodium ionide)
What is Na:TI (Thallium activated sodium ionide) scintillation detectors generally used for?
Nuclear medicine
What are . Csl: TI (Thallium activated cesium iodide) scintillation detectors generally used for?
Radiography
What is the atomic number for iodine?
z#=53
What does it mean that crystals are hygroscopic?
They absorb moisture which can cause damage (crack and break)
What prevents crystals from cracking and breaking?
The hermetic seal that prevents the crystal from coming into contact with air or moisture
In radiography, when scinitillation crystals are used as detectors, how to we prevent isotropic emmission of light?
The scinitillation crystals are enclosed in an aluminum seal with a polished inner surface in contact with the crystal . This allows the isotropic light to be refflected internally only one way.
Where is the PMT located in relation to the CR? Why?
Coupled to the window of the crystal scintillator (optical coupling). This allows for maximum light transmission.
What does the PMT do?
Converts the light produced from the scitillator into electrical signals (is an e- vaccum)
How do we decide what type of PMT to use?
We choose the PMT based off of its wavelegth so that it matches the wavelegth (spectral sensitivity) of the scintillator
What is the PMT coupled to? What is the function of the thing its coupled to?
A photocathode that converts the light into e-
True or false?
The number of electrons ionized in the detector is inversely proportional to the power of 3 to the intensity of light produced from the scinitillator.
False. The # of e- is directly proportional to the intenisty of light.
Where are dynodes located?
Within the PMT after the photocathode
What is a dynode?
A series of plate like elements with increasing potential thats function is to amplify the accelerated e- so that several secondary e- are emmitted to the collective electrode.
What is dynode gain?
The ratio of secondary electrons to the incident electrons (i.e. the number of electrons that have been created after going through the dynode)
What is the function of the collecting electrode?
It is at the anode end that absorbs the electron pulse from the last dynode and conducts it to a preamplifier
What is the function of a preAmp?
Provides an initial pulse amplification after the collecting electrode
True or false?
Portable scinitillation detectors have a low DQE.
False. Portable scinitillation devices have high detector efficiency.
Other than radiography, what else can scintillation dectors be used for?
- Presence of contamination
- Low level radiation
What is thermoluminescence dosimetry?
The delayed emission of light (phosphoresencce) by a thermally stimulated crystal following irradiation.
What is the process of heating a crystalline substance to induce it to glow called?
Annealing
What allows thermoluminesence to occur? How does this happen?
Heat provides e- with the E to escpae the trap and emmit a burst of light in the process
What machine induces thermoluminesence?
An annealing oven
How and where is the light emmitted from thermoluminesence measured?
In the annealing oven by a PMT after it goes through the heating process by measuring the intensity of light emmitted
What is a glow curve graph?
Light intensity plotted as the temperature of the oven is increased
What occurs with the TLD’s as the temperature of the planchet is increased?
The amount of light emitted by the TLD increases in an irregular manner
What is the planchet?
The tray on which the crystaline dosimiters are read
What happens after the light emmitted from the dosimeter has been measured by the PMT?
The output signal from the PMT is amplified and displayed.
What is this graph demonstrating?
As the temperature of the planchet is increased, the amount of light emitted by the TLD increases
True or false?
The height of the highest temperature peak and the total area under the curve are directly proportional to the energy deposited in the TLD by ionizing radiation.
True
What happens after the PMT measures the amount of light emmitted in TLDs?
Recieved dose is found through a conversion factor.
What is the most widely used thermoluminescent phosphor used in TLDs?
Lithium Fluoride
What is the relative density and atomic number of Lithium Fluoride?
-Similar to soft tissue density
-Z number of 8.1 (simular to soft tissue 7.4)
What device isused to measure absorbed dose?
Lithium Floride dosimeters
What are the 2 advantages to thermoluminesence dosimetery with lithium floride?
- LiF suffers negligible fading of stored dose information.
- Can detect a wide range of doses.
True or false?
TLDs can only be annealed and used 5 times before they need to be replaced.
False. TLDs can be used up to 300 times.
Before TLDs are used again, what process do they need to go through?
Before they are used again, LiF TLDs are subjected to a simple annealing process to release residual energy stored from previous exposures.
What is the PSP made of in the phosphor plate in CR?
Fluorohalides
What is the phosphor plate doped with in CR cassetes? Why?
A small amount of europium
-It causes the crystal to develop a series of tiny defects called metastable sites or F centers
What is the purpose of F centers?
To act like small electronic holes in the crystal that capture the e- that are realeased from incident radiation
What is the stored energy of the remnant beam in CR cassetes known as?
The latent image
What are the phosphor screens that store the the latent image known as?
Storage phosphor screen
What is the term that describes the process of a latnent image being exposed to a laser, causing the electrons to return to thier ground state?
Phosphorescence
In CR cassetes, if more light is released from a pixel, what shade would be shown in that area primarily?
The area would be more black
During exposure with a CR cassete, how many electrons immediately return to thier ground state instead of being stored?
50%
How much of the latent image remains after 8 hours?
75% of the image remains
What is PSP stimulation?
The process of the phosphor plate being exposed to a laser light
What determines the spatial resolution of in CR imaging? Why?
The diameter of the laser beam; the smaller the laser beam diameter the higher the spatial resolution of the system
What is a photocathode?
The first area that the light hits in a detector
How are electrons emitted from the photocathode in CR cassetes?
By photoemission; light triggers the release of electrons
True or false?
The number of electrons emitted is directly proportional to the intensity of the light.
True
Label the image
What type of light is emitted from the PSP as it is being read by the laser beam?
Blue light is emitted
During the read process in CR systems, are all light photons captured? Why or why not?
No, not all lgiht is captured due to the scattering of emitted light and the lack of collection efficiency of the photodetector
What is the light detector of choice for CR?
Photodiodes as opposed to photocathodes
What is the term that describes the residual latent image remaining on the IP due to the subsequent use of the detector?
Ghosting
How is ghosting prevented in CR radiography?
-Any residual latent image is removed by the flooding of the phosphor with very intense white light from fluorecent lamps
What is this image demonstrating?
Ghosting-pelvis overlaying chest on a CR cassette
How much background radiation is normally produced in general, and how much background radiation can cause “fogging” of the phosphor plate in CR cassettes?
-80uR generally per day
-100uR to cause fogging in a CR cassette
What improves the effiency of CR cassettes?
- Absorption efficiency
- Conversion efficiency
- Emission efficiency
What improves absorption efficency in CR plates?
-Increased atomic number of the plate
-Increased thickness of the plate
What is the downside of having a thicker phosphor layer?
-Light is released isotropically, so thicker phosphors causes more time for the light to diverge
What is conversion efficiency?
The percentage of energy form the absorbed x ray photons that is converteed to light rather than into infared/heat (wasted energy)
How can conversion efficiency be improved in CR cassettes?
By changing the chemcial compund used in the PSP
What is emission efficiency?
The ability of the light produced by the phosphor to especape the phosphor layer adn reach the photocathode/photodiode in the CR reader
How is emission efficiency improved in CR cassettes?
- Reflective layer reflects light that was backwards, forwards
- Needle shaped crystals act as tubular guides directing the light upward
Where are active matrix arrays (AMA) located?
In both indirect and direct aquesition
What is an active matrix array?
It is a layer of microscopic pixel elements each containing its own TFT located in flat pannel detectors
What makes up an active matrix array?
Dexels
What is the function of dexels?
To collect inofmation from the image
What makes up a dexel?
- A storage capacitor
- A TFT
How much of a dexel is a semiconductor that allows for the signal of x rays to pass through?
80%
True or false?
The larger the dexel, the less efficiency
False; the larger the dexel, the more efficient (DQE)
What is the fill factor?
The percentage of the square of the dexel that is devoted to the semiconductor dectector layer
True or false?
The higher the DQE, the higher contrast resoution.
True
What is the function of the microscopic storage capacitor in the dexels?
Determines the dexels ability to store electric sharges
What is the function of the TFT in dexels?
Acts as a switching gate to release the electrical charge when the dexel is read
True or false?
Direct DR has a scinitalltion phosphor.
False
What element converts the x ray signal into an electrical signal in direct DR?
Amorphus selenium
What type of element is amorphus selenium in direct DR?
The capture and coupling element
What occurs in the semiconductor layer after ionization of e- in direct DR?
Each ionzing event that occurs with the amorphus selenium creates an electron hold pair consisting of the freed e- and the +atom. The + atom is the collected by the storage capacitor
What is the function of the gate lines in an AMA? How does it work?
-Gate lines control the order in which the dexels are read out
1. When V is changed across gate lines, the TFT gates open up and sequentially dump the stored charge from each dexel
What happens after the charge is let out throught the TFT gates?
- The charge flows along the data line to an amplifier
- The signal is boosted and is then sent to an ADC in the computer
What is the array in indirect conversion systems made of?
Amporphus silicon
What is the phosphor screen made of in indirect conversion systems? What is its purpose?
-Made of a phosphor screen of Csl or gadolinium oxysulfide
-The phosphor screen scintillates when exposed to x rays, emitting light
What confines the isotropic dispersion of light in indirect conversion systems?
Vertical crystaliline channels in the Csl layer that form light channels
True or false?
SR is better in indirect conversion systems?.
False; SR is worse in indirect conversion systems
What is the capture element in CR systems?
PSP
What is the coupling element in CR systems?
Fiber optics (light guide)
What is the collection element in CR systems
Photodiode
What is the capture element in indirect DR systems?
Scintillation phosphors
What is the coupling element in indirect DR system
Contact layer with a-Si
What is the collection element in indirect DR systems?
TFT via a-Si
What is the capture element for direct DR systems?
A-Se
What is the coupling element direct DR systems?
The charge is transferred through the a-se, so not contact layer is required
What is the collection element in direct DR systems?
TFT
What converts light to an electrical current in indirect DR?
The photo diode (A-Si)
What is absorption efficiency in the case of AMA?
The ratio of the photons absorbed by the selenium or silicon detector surfaces to the photons incident upon the layres of the P.E. material
True or false?
Only a small % of the incident x ray beam is absorbed by the ultra thin detector elements.
True
True or false?
The AMA layers must be thin
True
What are direct DR systems dependant on to do a good job of absorbing x rays ?
They are dependent on high atomic number and the k-edge effect
True or false?
Indirect systems can save patient dose. Why or why not?
True;
A much higher % of light photons from the phosphor layer above can be absorbed by the AMA, therefore saving patient dose.
True or false?
Indirect systems have an overall higher absorption efficiency becuse the phosphor layer can be thicker than the direct conversion detectors.
True
