X-ray Detectors

what is an analogue detector and give examples

produces a continuous electrical signal

examples:

• screen film

• image intensifier

what is a digital detector and give examples

converts incoming x-ray energy directly into numerical data, allowing instant image processing and storage

Examples:

• computed radiography , use a CR plate

• integrated digital systems : direct conversion using a-se Amorphous Selenium , indirect conversion , x-rays converted to light then to electrical signal , uses charge coupled device or a-si amorphous silicon

what are phosphors

substances that emit light when exposed to radiant energy

what are xray phosphors

covert x-ray photons into visible light

what are scintillators

solids that emit light when exposed to ionizing radiation

what 2 step system do x-ray detectors usually have

• phosphor layer

• light detection device

what are fluorescent phosphors

emit light very quickly after simulation by x-rays

what are phosphorescent phosphors

emit light after a delay

which materials are ideal as phosphors

materials with a higher Z , photoelectric effect dominates, best conversion of x-rays to light photons

the x-eays must first interact with phosphors atoms mainly through which 2 effects?

• Photoelectric effect

• Compton scattering

what is a valence band

where electrons normally are, bound to the atom

what id s conduction band

where electrons can move freely, conduct electricity or generate light when dropping back

explain a fluorescene energy level diagram

• high energy electron generated by x0ray interaction gives up energy through atomic interaction

• electrons promoted from valence band to conduction band

• electrons return to valence band and energy is emitted as visible light

explain phosphorescence energy level diagram

• high energy electron generated by x-ray interaction give up energy through atomic interaction

• electrons excited from valence to conduction band

• some electrons return immedistly to the valence band and fluorescnece takes place

• some electrons fill energy traps

• after a delay some electrons in the energy traps return to valence band and light is produced

what is photo-simulated luminescence

when irridation with IR laser light causes electrons in energy traps to return to valence band and light is emitted

what is bright white light used for

to remove any residual charge and the phosphor is returned to its ground state

what are the two types of common phosphors and give what they are used for

• turbid phosphor goddlinium oxysulphide, used for screen/film

• structured phosphor caesium iodide, used for integrated digital systems

what is a screen/film

a film coated in silver halide crystals suspended in gelation , suspended in an emulsion. This is being phased out

how does a film work

• light from the phosphor screen hits the silver halide crystals

• some silver ions are reduced to metallic silver→ image invisible

• chemical treatment reduces exposed silver halide ions to metallic silver → forms visible black image

• unexposed silver halide ions are removed

what is an image intensifier

a device used in fluoroscopy to convert x-rays into bright visible images in real time , also being phased out

how does an image intensifier work

• x-rays exit a patient and hit a phoshor layer

• emitted light hits a photocathode, coverts photons to electrons

• electrons are accelerated and focused by an e.f which increases brightness

• electrons hit output phosphor+window which convert back into light

what is computed radiography

a digital x-ray imaging system that uses phosphor plates instead of traditional film

how does computed radiography work, what is it only for and what are some properties

• image is stored in the phosphor plate as a latent image and then read by a laser scanner to produce a digital image.

• only for radiography

• reusable and digital

what is an integrated digital system CCD

indirect digital x-ray detectors that use phosphor to convert x-rays to light which is then captured by a CCD camera, converts light to a digital signal and then its digitised

what happens in CCD, what is it used for

• CSI scintillator converts x-ray to light

• lens or fibre optics transmit light to ccd panel

• ccd commonly used in cameras turns light into a electrical signal that is then digitised

• used for radiography and fluoroscopy

• replaced by flat panels

how does an indirect flat panel detector work

• Csi scintillator converts x-rays to light , this makes it indirect

• a-si (Amorphous silicon) detects light and produces electric charge proportional to light intensity and stores it. (acts as a photodiode)

• each photodiode is paired with an TFT switch (thin film transistors)

• TFT activated row by row , transferring stored charge to electronics which is then digitised

how does an indirect flat panel detector work and whats it used for

• no Csi scintillator

• a-se used instead

• added capacitors

• mainly for low energy applications i.e mamography

• x-ray generates charge in a-se photoconductor

• charge detected by pixel electrode

• charge stored by capacitor

• TFT switch reads charge via data line and its digitised

what is FPD array operation

a large matrix of tiny detector elements, converts x-ray to electrical signals which are then digitised to form a digital x-ray image

how does FPD array operation work

• a row of TFT’s is turned on electronically

• stored charge in those pixels flows out into electronics and digitised

• the row is turned off, next row is activated

• entire matrix is read out row by row

what are some FPD properties

• used for radio and fluoroscopy

• high spatial resolution

• high temporal resolution

• excellent image quality

what is spatial resolution

How much detail you can see.

what is temporal resolution

How well the system freezes motion / how fast it can image.

ability of an imaging system to accurately capture motion over time.

what is FPD pixel binning , give advantages and disadvantages

combining signals from neighboring pixels to form a superpixel

• allows faster frame rate (fewer superpixels need to be read)

• increases signal to noise ratio ( summing charges reduces effect of electronic noise)

• lower spatial resolution (fine details blurred)

what is meant by signal to noise ratio

How clear the image is compared to the amount of graininess or random noise.

Higher SNR → clearer, smoother, more diagnostic image

Lower SNR → grainy, noisy, poor-quality image