Radiation Electronics and Detection Flashcards
What are radiation units?
Roentgens (R) is the conventional unit of measuring exposure to X-rays
- Roentgens are not the best indicator for the biological impact of the exposure
Exposure definition Roentgens
- Intensity of radiation incident upon the surface of an object
Measuring X-ray photons
extrapolate the number of photons hitting something from the ionizations caused in air molecules near the surface
- free electrons attracted to anode of detection device - current is measured
Radiation detection instruments
In all radiation detectors, radiation is incident on a transducer (Radiation-sensitive region) of the device
Radiation detection instruments operate based on?
- the ionization of atoms, which frees up electrons that can be measured as a charge or as a current in a circuit
- the basis of excitation of electrons. As the electrons return to the pre-excitation stage, they release energy which is captured and transferred into an electrical current
Properties of radiation detectors
the physical state and density of the transducer determines whether ionization or excitation is favoured
Stages of radaition detection
the transducer is coupled to an electric component, where the radiation effect in the transducer is converted into a useful electronic signal, which is then processed, amplified, analyzed and counted
Radiation detectors: modes of operation
instruments are designed to: detect radiation, measure radiation or do both
Those designed for detection usually operate in the pulse or rate mode and are used to indicate the presence of radiation
Pulse mode (Detection only)
clicks or beeps indicate the presence of radiation
clicks can overlap or occur in extremely rapid sequence, extremely difficult to count/measure
without a meter or a counter such devices cannot be used to meaure radioactivity
PRESENCE AND GENERAL INTENSITY
Rate mode
fairly accurate measurement if the rate is reasonably constant
- x-ray machines generate a steady flow of radiation so can be used for this
natural sources of radioactivity have varying rates; this does not allow for accurate reading
integrate mode
measure the intensity of radiation
devices accumulate the count of radiation events over a set period
electronic devices can count these events at extremely high speed and with great accuracy
read outs give a total exposure, divide by the seconds or minutes that transpired during measurement
characteristics of radiation detection devices
- sensitivity - able to detect small amounts of radiation
- accuracy - precision with which measurements are obtained; increasing sensitivity is one way to increase accuracy
- resolving (interrogation) time - requires a fractional amount of time to reset between ionizations - integrate mode are less susceptible to dead times
- Range - sensitivity of detection instrument must be matched to the expected intensity levels and type or radiation
sensitivity for radiation detection
larger detection chamber makes the instrument more sensitive
increase electronic amplification of incoming signal makes the instrument more sensitive
- any electronic detection instrument requires a certain threshold current to generate a read out
factors that affect accuracy of radiation detection devices
alignment of printed scale behind needle of a meter
electronic noise - can increase reading
supply power fluctuations - i.e. batteries running low
Range
the range of a detection instrument may be set too high for the intensities of radiation expected
- low intensities will not be picked up and adequately displayed on the read out
properties of ideal detectors
high absolute detection efficiency (quantum DE) = absorbs radaition
what 2 things does quantum DE require?
- transducer should have sufficient stopping power to absorb (and therefore detect) the radiation - intrinsic
- transducer should be optimally positioned relative to the radiation source - geometric
absolute efficiency
ratio of the number of radiation events detected in each interval t the number of photons emitted by the radiation source in the same interval
only a small fraction of events reach the detector (isotropic emission) - of these events, a smaller fraction interact with the transducer
absolute efficiency can be defined as the product of intrinsic and geometric efficiency
Gas filled detectors
radiation passes through gas, ionizes atoms, create ion pairs
free electrons are then attracted to and strike a positively charged anode within the chamber
the electrons released in ionization are detected as an electrical signal that is proportional to the radiation
types of gass-filled detectors
ionization chambers
proportional counters
Geiger-muller detectors
Ionization Chamber
Cylinder filled with air or pressurized gas (Xenon/argon)
larger chamber = more gas molecules available for ionization = more sensitive instrument
- typically, one electron is released from the gas in the chamber for each x-ray that interacts within it
High accuracy
Proportional counters
proportional counters take advantage of the “cascade” effect
- An electron ejected from the event has enough energy to ionize another atom
- secondary electrons produced
the result is a cascade in which several electrons eventually reach the anode
extremely high sensitivity
- little application for clinical imaging
Geiger-Muller Tube
operates based on the saturation of the detction chamber
like the cascade effect, except that so much energy is imparted that all the original gas molecules within the chamber are ionized from a single radiaition exposure event
- Argon
longer resolving time
high sensitivty - low accuracy
not able to do integrates dose
Scintillation Detectors
converts x-rays to light
only materials with a particular crystalline structure scintillate
some materials emit a flash of visible or UV light in response to radiation absorption
Scintillation involves the rearrangement of valence electrons into traps
- if valance electrons return to normal position immediately: scintillate
- if delayed: phosphorescence
What are Scintillation detectors used in?
- digital image receptor in radiography and fluoroscopy
- detector array in CT
- Gamma camera in nuclear medicine
Scintillation detector compounds
Thallium activated sodium iodide ad thallium activated cesium iodide crystals are the most widel used scintillation phosphors
What compound is commonly used in radiography imaging detectors?
Thallium activated Cesium iodide (CsI:Tl)
What compound is commonly used in nuclear medicine?
Thallium activated Sodium Iodide (NaI:Tl)
what is the proportionality between the amount of light emitted to the amount absorbed in scintillation detectors?
proportional
Scintillation crystals
crystals are hygroscopic (Absorb moisture), which can cause damage
the crystals are contained within a hermetic seal, one that prevents crystal from encountering air or moisture
how is light emitted during scintillation
isotropically
Photomultiplier tube (PMT)
vacuum tube
coupled to the window of the crystal
converts flashes of light into electrical signals
window of PMT lines up with window of scintillator crystal (optical coupling): allows for maximum light transmission
Light is incident on a thin metal coating called? (PMT)
photocathode
electrons are emitted from the photocathode by? (PMT)
photoemission
- like thermionic emission but from light
the number of electrons emitted is ____________ to the intensity of the light (PMT)
directly proportional
in the PMT what is the sensitivity?
closely matches that of the scintillation crystal
PMT (dynodes)
photoelectrons are accelerated by a series of plate like elements called dynodes
for each electron incident on the dynode, several secondary electrons are emitted and directed to the next stage
What is Dynode gain?
ratio of secondary electron to incident electrons
what is PMT gain?
PMT gain = g^n, g is dynode gain and n is equal to the number of dynodes
An 8 stage PMT (8 dynodes) has a dynode gain of 3 (3 electrons emitted for each incident electron). What is the PMT gain?
PMT gain = g^n
PMT gain = 3^8
PMT gain = 6561
what is the collecting electrode?
- anode
absorbs the electron pulse from the last dynode and conducts it to the preamplifier
What does the preamplifier provide (PMT)?
an initial pulse amplification
Scintillation detectors
scintillation detectors are sensitive devices capable of measuring radiation intensities as low as single photon interactions
portable scintillation devices have a high detection efficiency
can be used to monitor the presence of contamination and low-level radiation
Thermoluminescence Dosimetry (TLD)
- The emission of light by a thermally stimulated crystal following irradiation
- In some crystalline materials ionization from x-rays cause valance electrons to elevated into electron traps and remain there for an extended period
- this delayed emission of light is called thermoluminescence
- radiation induced thermoluminescence has been developed into a sensitive and accurate method of radiation dosimetry for personnel radiation monitoring
Thermoluminescence annealing
- heat can be used to provide these electrons with the energy to escape
- a burst of light is emitted in the process
- this process of heating a crystalline substance to induce it to glow is called annealing
how is the TLD read?
- After irradiation, the TLD
phosphor is placed on a
special planchet for
analysis in an instrument
called a TLD analyzer
1. Exposure to ionizing
radiation.
2. Subsequent heating.
3. Measurement of the
intensity of emitted
light.
glow curve
- In the annealing oven, light emitted from the heated
crystal is picked up and measured by a photomultiplier
(PM) tube - A “glow curve” is plotted for the light intensity as the
temperature of the oven is increased
what does a TLD graph look like? what does this signify?
Several prominent peaks
can be seen on the
graph; each occurs as
valence electrons are
released from traps
- 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
Lithium Fluoride
- LiF is the most widely used thermoluminescent phosphor used in TLDs
- It has a crystal density similar to soft tissue density, and Z number of 8.1 and exhibits x-ray absorption properties similar to those of soft tissue ( Z = 7.4)
- LiF is considered a tissue equivalent radiation dosimeter and the dose measurement can be used to calculate absorbed dose
- Before they are used again, LiF TLDs are subjected to a simple annealing process to release residual energy stored from previous exposures
Advantages of TLD dosimetry
- If TLDs are annealed before use, they can be used repeatedly, almost 300 times
- LiF is a tissue equivalent radiation dosimeter
- LiF suffers negligible fading of stored dose information
- Can detect a wide range of doses
Computed Radiography Phosphor Plate
- The CR plate consists of a
photostimulable phosphor
(PSP) with the ability to store
and release image data - PSPs are made up of a class
of compounds known as
barium fluorohalides
(BaFBr/BaFCl)
Barium Fluorohalides
- The pure crystal of barium fluorohalides is “doped” or activated with a small amount of Europium (BaFBr: Eu or BaFCl: Eu)
- The Eu causes the crystal to develop a series of tiny defects called metastable sites or F centres throughout its crystal lattice
- The F centres act like small “electronic holes” in the crystal that capture electrons that are released from the phosphor atoms when x-
rays strike the phosphor plate - Therefore, the imaging plate can store the energy of the remnant beam of x-rays in the form of a latent image composed of electric charges stored within the F centres of the crystal lattice
Computed Radiography Phosphor Layer
- Because the latent image is stored in the phosphor in the form of excited electrons, these screens are called storage phosphor screens
- Over time, these excited electrons return to the ground state on their own
- However, this return can be accelerated by exposing the phosphor to intense infrared light from a laser – stimulable phosphorescence
PSP Exposure
- PSP exposed to x-ray beam; energy transfer excites electrons
- 50% of electrons return to the ground state immediately, releasing light
- remaining trapped electrons contribute to the image
- these electrons will return to ground state over time, causing image to fade; 75% remains after 8 hours, processed immediately after exposure
PSP Stimulation
- Finely focused beam of infrared light is directed at the PSP
- the diameter of the laser determines spatial resolution; smaller laser beam results in higher spatial resolution
PSP Read
- Laser beam adds energy to the trapped electrons in the F centers
- results in the emission of blue light
- some signal is lost due to the scattering of emitted light and collection efficiency
What is the light detector choice for CR?
Photodiodes (PDs)
PSP Erasure
- the stimulation cycle does not fully release all excited electrons
- if residual latent image remained, ghosting could appear on subsequent use
- any residual latent image is removed by flooding the phosphor with very intense wight light from fluorescent lamps in the CR reader
Background and scatter radiation
- CR phosphors are highly sensitive
- can accumulate background radiation during periods of storage
- cassettes should be erased after periods of non-use or if they are present in an area of xray exposure
How much radiation can be gained as a background on a CR plate per day?
80 microR
how much radiation does it take on a CR plate can cause “fogging”?
100 microR
What are the 3 types of efficiency in CR plates?
- absorption efficiency
- conversion efficiency
- emission efficiency
Absorption efficiency
- ratio of x-ray photons absorbed by the phosphor to the x-ray photons incident upon the phosphor layer
- High Z and thicker phosphor layer = more x-rays absorbed
- more x-rays absorbed by the phosphor = more light is emitted when the plate is stimulated
Conversion efficiency
- % of energy from the absorbed x-ray photons that is converted to light instead or infrared or heat energy
- it is characteristic inherent to the chemical compound used in the PSP
Emission efficiency
- ability of the light produced by the phosphor to escape the phosphor layer and reach the light guides in the CR reader
- light emitted from specific crystals must penetrate out past other crystals and the chemical binder
- when light is emitted isotropically, any emitted laterally are lost
- light emitted backward is reflected forward by the reflective layer back to the light guides
- needle-shaped crystals improves emission efficiency buy guiding the light upwards
Active-matrix Array
direct conversion and indirect conversion DR systems are both based upon the active-matric array (AMA), a layer of microscopic pixel elements each containing its own thin film transistor (TFT)
What is Dexel?
- a DR image receptor made up of individual dexels
- information from within the patient is collected by the dexels which then become pixels in the final image
- 80% of the detector is a thing semiconductive layer that is sensitive to x-rays and light
-the larger the capture array the more efficient
What is fill factor?
the % of the square devoted to the semiconductor detector layer of the dexel
- higher the fill factor, better contrast resolution
- as dexel size is reduced, so does fil factor, requiring an increase in technique and therefore patient dose
what is thin film transistor (TFT)?
acts as a switching gate to release the electrical charge when the dexel is read
what is a microscopic storage capacitor?
determines the dexel’s ability to store electric charge
Direct DR
- No scintillation phosphor
- Amorphous Selenium sandwiched between charged electrodes
- X-rays interact directly with the a-Se producing a charged pair; the a-Se is both the capture and coupling element
- each ionizing event creates an electron hole pair, consisting of the freed electron and positively charges “hole” it leaves in the semi-conductor layer
- positive charge created is collected by the storage capacitor
How does Direct DR work?
- a network of gate lines and data lines criss-crosses between dexels in an AMA
- gate lines control the order in which the dexels are read out
- when the voltage is changed across gate lines, the TFT gates open sequentially and dump the stored charge from each dexel in succession
the charge flows along the data line to an amplifier, which boosts the signal before sending it through an analog to digital converter (ADC) into the computer
Indirect conversion systems
Same AMA layout, only this array uses amorphous silicon, and the entire array is overlaid with a phosphor screen of Csl
- this phosphor screen scintillates when exposed to x-rays, emitting light
Indirect DR
- Vertical crystalline channels in the CsI layer form light channels to confine the isotropic dispersion of light
- Spatial resolution less than direct system
- Light is directed towards the (a-Si) detector elements which acts as a photodiode (converts light to electrical current)
Efficiency
In the case of an AMA, absorption efficiency is the ratio of the photons absorbed by the detector surfaces to the photons incident upon the layers of photoelectric material
- layer must be kept very thin
- only a very small % of incident x-ray beam is absorbed by the ultra thin detector elements
For Direct DR what is efficiency dependant on due to thin layers of photoelectric material?
high Z and the k-edge effect to do a good job absorbing
efficiency of indirect
- higher % of light photons from the phosphor layer can be absorbed by the AMA - lower patient dose
- an overall higher absorption efficiency because the phosphor layer can be thicker than the direct surfaces
CR systems
- Capture element: PSP
- Coupling element: Fiber optics (light guide)
- Collection element: Photodiode a-Se
Indirect DR Systems
- Capture element: PSP
- Coupling element: Fiber optics (light guide)
- Collection element: Photodiode a-Se
Direct DR Systems
