Week 8 - Radiation Detection and Image Formation

Question 1

Principles of Radiation Detection and Image Formation

Answer 1

• 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

Question 2

Absorption Efficiency

Answer 2

 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

Question 3

Conversion Efficiency

Answer 3

 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)

Question 4

Capture Efficiency

Answer 4

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

Question 5

Dose Efficiency

Answer 5

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

Question 6

Temporal Response

Answer 6

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

Question 7

Timing of Phosphorescence or Afterglow

Answer 7

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

Question 8

Wide Dynamic Range

Answer 8

 Range of exposures the detector is sensitive to

Question 9

High Reproducibility and Stability

Answer 9

 Consistency of measured signal and images

Question 10

Detective Quantum Efficiency

Answer 10

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)

Question 11

Noise

Answer 11

• 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

Question 12

Spatial Frequency

Answer 12

• 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

Question 13

Gas-Filled Detectors

Answer 13

• 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

Question 14

Ionisation Chambers

Answer 14

• 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

Question 15

Ionisation Chamber: Uses

Answer 15

o Radiation Dosimetry

o Automatic Exposure Control Circuits

Question 16

Ion Chambers and Automatic Exposure Control

Answer 16

• 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

Question 17

Xenon Gas Detectors

Answer 17

• 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

Question 18

Scintillators and Photomultiplier Tube

Answer 18

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

Question 19

Photomultiplier Tube

Answer 19

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

Question 20

Scintillator Crystals and Photo Diodes

Answer 20

• 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

Question 21

Scintillation Crystal/Photocathode Image Intensifier

Answer 21

• 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

Question 22

Indirect Detectors

Answer 22

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

Question 23

Direct Detectors

Answer 23

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

Question 24

Electronic Band Theory of Solids

Answer 24

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

Question 25

Computed Radiography (CR)

Answer 25

- 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

Question 26

Indirect Digital Radiography

Answer 26

- 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)

Question 27

Phosphors and Scintillators

Answer 27

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)

Question 28

Scintillators with Thin Film Transistor Technology

Answer 28

- 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

Question 29

Thin Film Transistor Arrays

Answer 29

- 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

Question 30

Scintillators and Charged Couple Device

Answer 30

- 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

Question 31

CCD Problems

Answer 31

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

Question 32

Direct Digital Radiography

Answer 32

- 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

Question 33

Digital Fluoroscopic Systems

Answer 33

- 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

Question 34

Fluoroscopic Flat Panel Detectors

Answer 34

- 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)