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Radiation Physics > Week 8 - Radiation Detection and Image Formation > Flashcards

Week 8 - Radiation Detection and Image Formation Flashcards

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1
Q

Principles of Radiation Detection and Image Formation

A
  • 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
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2
Q

Absorption Efficiency

A

 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

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3
Q

Conversion Efficiency

A

 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)

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4
Q

Capture Efficiency

A

 Percentage of the area of the detection that is ‘active’ detector
 Greater Area Active = Greater amount of x-ray detection

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5
Q

Dose Efficiency

A

 Dependent on conversion and capture efficiency, how much of the dose incident on the detector contributes to the image

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6
Q

Temporal Response

A

 Fast Response = Low dead-time (period where ionisation is not detected)

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7
Q

Timing of Phosphorescence or Afterglow

A

 Length of burst of light signal after x-ray absorption (quanta of energy)
 Short pulses preferable – smaller dead time, clearer image

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8
Q

Wide Dynamic Range

A

 Range of exposures the detector is sensitive to

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9
Q

High Reproducibility and Stability

A

 Consistency of measured signal and images

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10
Q

Detective Quantum Efficiency

A

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)
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11
Q

Noise

A
  • 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
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12
Q

Spatial Frequency

A
  • 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

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13
Q

Gas-Filled Detectors

A
  • 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

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14
Q

Ionisation Chambers

A
  • 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
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15
Q

Ionisation Chamber: Uses

A

o Radiation Dosimetry

o Automatic Exposure Control Circuits

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16
Q

Ion Chambers and Automatic Exposure Control

A
  • 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
17
Q

Xenon Gas Detectors

A
  • 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
18
Q

Scintillators and Photomultiplier Tube

A

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

19
Q

Photomultiplier Tube

A

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

20
Q

Scintillator Crystals and Photo Diodes

A
  • 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
21
Q

Scintillation Crystal/Photocathode Image Intensifier

A
  • 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
22
Q

Indirect Detectors

A

o X-rays are first converted to light
o Light is then converted to an electrical signal
o Multiple stage detection process

23
Q

Direct Detectors

A

o X-rays are converted directly into electrical signal in the detector
o Single stage detection process

24
Q

Electronic Band Theory of Solids

A

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

25
Q

Computed Radiography (CR)

A
  • 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
26
Q

Indirect Digital Radiography

A
  • 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)

27
Q

Phosphors and Scintillators

A

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)

28
Q

Scintillators with Thin Film Transistor Technology

A
  • 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
29
Q

Thin Film Transistor Arrays

A
  • 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
30
Q

Scintillators and Charged Couple Device

A
  • 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
31
Q

CCD Problems

A

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

32
Q

Direct Digital Radiography

A
  • 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
33
Q

Digital Fluoroscopic Systems

A
  • 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
34
Q

Fluoroscopic Flat Panel Detectors

A
  • 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)

Radiation Physics (9 decks)

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