Week 8 - Radiation Detection and Image Formation Flashcards
Principles of Radiation Detection and Image Formation
- Desirable characteristics of radiation detectors
- Detective Quantum Efficiency (DQE)
- Gas Detectors
o Ionisation chambers
o Xenon gas detectors - Scintillator Detectors
- Large Field Detectors
- Indirect, Direct and Computed Digital Radiography
- Digital Fluoroscopic Systems
Absorption Efficiency
Percentage of x-rays incident on the detector that are absorbed
Depends on the physical density and thickness
Number of X-Rays stopped / Total Number of Incident X-Rays
Conversion Efficiency
How much of the absorbed x-ray energy is converted to a usable electronic signal?
Efficiency of the conversion into secondary particles/waves (charged particles or optical photons)
Capture Efficiency
Percentage of the area of the detection that is ‘active’ detector
Greater Area Active = Greater amount of x-ray detection
Dose Efficiency
Dependent on conversion and capture efficiency, how much of the dose incident on the detector contributes to the image
Temporal Response
Fast Response = Low dead-time (period where ionisation is not detected)
Timing of Phosphorescence or Afterglow
Length of burst of light signal after x-ray absorption (quanta of energy)
Short pulses preferable – smaller dead time, clearer image
Wide Dynamic Range
Range of exposures the detector is sensitive to
High Reproducibility and Stability
Consistency of measured signal and images
Detective Quantum Efficiency
A measure how well the available information (incident x-rays) are transferred into useful information (the image)
o Ideal detector: DQE = 1.0 or 100%
o In reality - Information is lost at different stages of the imaging system
o A DQE of 0.5 (50%) means only 50% of the available information in the form of x rays incident on the detector are used by the system in producing an output signal (image)
- DQE is affected by change in input signal (mAs, kV) and patient
- A system could have different DQE for different patient and anatomy (spatial frequency)
Noise
- No two images will ever be the same
- Noise in the image manifests as random variations in the recorded signal from pixel to pixel
- Is proportional to the number to the quanta (x-rays) involved in forming the recorded signal
- Ability to detect an object depends on the contrast of the object and the noise
Spatial Frequency
- The ability to see features in the images that are small or close together
- A line-pair phantom can be used to find the upper limit of spatial resolution in terms of the maximum spatial frequency that is resolvable by the imaging system
E.g., Students in a room
o Low Spatial Frequency = can identify that there are 20 students
o High Spatial Frequency = can identify details which are unique to each individual
Gas-Filled Detectors
- Enclosed volume of detection medium (gas)
Charged electrodes
o Potential difference (V) across electrodes
As the radiation passes through, ionisation results from interactions
o The electrons and positive ions (caused by the ionisation) are collected by the charged electrodes
Collected charge is measured by the external electronics
o Negative Electron –> positive terminal, Positive Ions –> negative terminal
o Resistor and Capacitor assist in the measuring process
The number of ion pairs produced depends on the LET of the radiation
o High LET = more densely ionising
Ionisation Chambers
- X-rays interact in the chamber wall surrounding the air cavity
o Generate electrons which transverse the air in the cavity causing ionisations - High voltage (electrical potential) applied across the air cavity
- Ionised atoms (+ve) move to the cathode (-ve terminal)
o Heavier and slower - Electrons (-ve) move to anode (+ve terminal)
o Lighter and Faster - Electron current forms the electrical signal and is proportional to the amount of ionisation in the air cavity of the ion chamber
Ionisation Chamber: Uses
o Radiation Dosimetry
o Automatic Exposure Control Circuits
Ion Chambers and Automatic Exposure Control
- Thin transmission ion chambers can be used to control the exposure of an image
- Measured signal is proportional to the number (fluence) and energy of x-rays passing through it
- Measured AEC signal is fed back to x-ray tube
- When signal reaches predetermined level (X-ray tube is switched off)
- Multiple ions chambers may be sued as the intensity of the x-rays may vary across the field of view
o E.g., three chambers shown here for this abdominal x-ray
Xenon Gas Detectors
- Used in older CT scanner
Xenon gas molecules widely spaced in cavity
o Low absorption efficiency
Need small detectors
o Physical space on the scanner and high spatial resolution
- Increase density of gas molecules by increasing pressure (10-20 atm)
- Fast response
Scintillators and Photomultiplier Tube
Used in nuclear medicine (gamma camera’s) and 1st and 2nd generation CT scanners
X-rays and gammas are detected in three stages
o A solid scintillation crystal absorbed the x-rays and converts their energy to light
o Light is then converted into a very small electrical signal by the photocathode
o The small electrical signal is amplified by a photo-multiplier tube
Photomultiplier Tube
Photocathode
Converts the pulse of light photons into a pulse electron
Electrode Multiplier
Amplifies the number of electrons into a measurable electronic signal
o Typical scintillation pulse will give rise to 10^7 – 10^10 electrons after amplification
Linear amplification
Amplification proportional to increase in voltage
Scintillator Crystals and Photo Diodes
- More recently photodiodes are used instead of photo-multiplier tubes to convert the scintillator light into electrical signal
- High stability
- Can be made small
- Uses
o CT scanners
o Large field of view radiography
Scintillation Crystal/Photocathode Image Intensifier
- Being phased out of use
- Solid scintillation crystal lines inside of the vacuum tube face plate: x-rays -> light
- Light -> small electric signal at the photocathode
- Voltage accelerates and focusses electrons onto the output phosphor screen
- Highly focussed energetic signal strikes the phosphor and converted to light
- Light is then detected using a video camera or CCD camera
Indirect Detectors
o X-rays are first converted to light
o Light is then converted to an electrical signal
o Multiple stage detection process
Direct Detectors
o X-rays are converted directly into electrical signal in the detector
o Single stage detection process
Electronic Band Theory of Solids
Valence band
o Outer shell electron bound to atom
Band Gap
o Can have differing magnitudes
Conductors – no band gap – many electrons in conduction band (free)
o E.g., copper
Semi-conductors – small band gap – approx. eV
o E.g., Silicon
Insulators – Large bad gap – (approx. 10 eV)
o Rubber, most plastics
To be a conductor, electrons need to be able to gain enough energy to move up into the conduction band
Computed Radiography (CR)
- X-rays interact in the phosphor screen
- Creation of election – hole pair in the crystal structure
o In valence band, electrons are ionised –> leaving behind a hole (vacancy)
o Hole is thought of as positive particle - Electrons have energy that allows them to be raised into the conduction band energy level
- They subsequently try to de-excite to valence energy level (band) but become trapped in defects in the forbidden band
o Atom is left in metastable state
o Results in stored energy proportional to the amount of ionisation - Red laser light gives them enough energy to escape the defect and subsequently de-excite back to valence band
o They then emit blue light - Emitted blue light intensity is proportional to x-ray intensity
- Red Laser scans across the imaging plate in x and y
- Light detector also scans and detects the blue light emitted at each (x, y) coordinate in the plate
- Typical CR resolutions 100 to 200 micro metres
- Readout times can be a few seconds
- Better than film
- Most photostimulable phosphors are in the barium fluorohalide family
Indirect Digital Radiography
- When a scintillator is exposed to x-rays it promptly produces fluorescent light in proportion to the quantity (number of x-ray) and quality (energy) of x-rays interacting in it
- X-ray interaction in the scintillator crystal create electron-hole pairs
o X-rays –> Photo- or Compton electrons cause ionisation (the electron-hole pairs) - Electrons gain enough energy to rise up to conduction band then promptly drop down to valence band emitting excess energy as light
o Each position on the generator will have measurable light
o Intensity is proportional to the exposure at that point
o No traps, defects or impurities as in CR and no need to read-out post-irradiation
Two methods/technologies used to detect the light
o Thin film transistor (TFT)
o Charged Couple Device (CCD)
Phosphors and Scintillators
Non-structured scintillator crystals (e.g., Gadolinium Oxysulfide)
o Crystals oriented randomly throughout the scintillator
o Individual crystals have relatively high light output or conversion efficiency
o Overall diffuse light output
Light goes in all directions
Small percentage of created light is collected by PMT or photodiode
Structured Scintillator Crystals
o Crystals in well defined aligned structures
o Individual crystals have low light output but overall, more focussed due to aligned structure
o More of the light created can be directed to the light collector (PMT or photodiode)
Scintillators with Thin Film Transistor Technology
- Scintillator Crystal convert x-ray –> light
- Photodiode converts light to electrical charge and stores it
- Charge generated is proportional to x-ray energy absorbed in scintillator
- Photodiode is connected to a thin film transistor (TFT) (a switch)
- Voltage can be applied to the TFT and charge can be read out
- Can construct large active pixel arrays
- Each pixel has a photodiode and TFT
Thin Film Transistor Arrays
- The TFT’s can be switched on very quickly
- Charge then flows – electronic signal
- Arrays read out pixel by pixel (scanning)
- Multiple pixels can be read out at once – multi-plexing
- The charge signal is each pixel is converted to greyscale and a digital image is generated
- An image would typically be made up of multiple frames that can be averaged
Scintillators and Charged Couple Device
- Produce a signal directly from the light
- Pixelated array – large number of individual CCD pixels
- Light strikes the surface of the CCD and knocks an electron out of an orbital
- A voltage (potential difference) is applied across the CCD pixel causing the electrons to travel to a region of the pixel where they are collected
- Each pixel then transfers the collected charge to its neighbouring pixel and so on down the line (charge coupling)
- Synchronisation against an internal clock allows the pixel to of origin to be determined
- No signal lines required (unlike TFT’s) more pixels/area = higher resolution
CCD Problems
o The CCD’s are sensitive to radiation
ScEven without a scintillator if you irradiate a CCD you get a signal
o Need to position the CCD out of the main x-ray beam
Two Techniques
Coupling of the scintillator – CCD using optical fibres
Mirror the lens system
Direct Digital Radiography
- X-rays interact with atoms in the amorphous selenium or silicon semiconductor layer creating electron-hole pairs
- Number of pairs is proportional to quantity and quality of x-rays interacting
- High voltage applied across the electrodes creates electric field lines that direct e-h pairs to collect at their respective electrodes
- Also prevent recombination of e-h pairs
- The charge is then stored in the TFT’s and can be read out
- Some area of the detector will be taken up with electronics
- Fill factor is the ratio of the sensitive area of detector (charge sensitive area) to the unused area
Digital Fluoroscopic Systems
- Fluoroscopy – sequence of images acquired creating a dynamic image or movie (30 or more frames per second
- 30 frames per second is require to create a smooth – real time image
- Image quality (and patient dose) has to be balanced with time resolution
- Early systems were a CCD camera connected to image intensifiers
- Pulsed x-ray tubed enabled significant dose reduction
- Slowly being replaced by flat panel technologies (scintillator and photodiodes)
- Early flat panel technology was subject to lag (delays in image generation) and slow refresh rates
o Not real time - More recent systems offer significant improvements
Fluoroscopic Flat Panel Detectors
- Need very fast read out to obtain 30 frames per second
- Requirement for low exposures per frame (minimise dose to patient)
- Flat panel detectors have poor signal-to-noise ratio compared to image intensifiers
- Pulsed sources can significantly reduce the dose
- Synchronise x-ray source with read-out of detector
- X-ray source only on when acquiring a frame – switched off during read out
- Large area arrays possible (up to 43 cm^2)
