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1

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

2

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

3

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)

4

Capture Efficiency

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

5

Dose Efficiency

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

6

Temporal Response

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

7

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

8

Wide Dynamic Range

 Range of exposures the detector is sensitive to

9

High Reproducibility and Stability

 Consistency of measured signal and images

10

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)

11

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

12

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

13

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

14

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

15

Ionisation Chamber: Uses

o Radiation Dosimetry
o Automatic Exposure Control Circuits

16

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

17

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

18

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

19

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

20

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

21

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

22

Indirect Detectors

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

23

Direct Detectors

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

24

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

25

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

26

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)

27

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)

28

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

29

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

30

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