MOD 4 - Digital Imaging Process

Computed Radiography

= exposure of an imaging receptor in a cassette and a computer system for image processing

• both are digital (not film)

• uses IP

CR Imaging Receptor / Cassette

= lightweight tight plastic container that protects the digital imaging plate from damage from handling

• backed by a thin sheet of Al that absorbs x-rays

• front or tube side of the cassette is constructed of a radiolucent carbon fibre

• also contains an antistatic material that protects against static electricity build up, dust collection and mechanical damage to the IP

Cassette

• the backside contains brackets and hinges

• only use front side when imaging

• main function is to protect the IP from damage

  • however, it can’t protect the IP from light damage bc its not sensitive to light

CR Imaging Plate

• capable of storing an image formed by the incident x-ray photon (a single x-ray in the primary x-ray beam that is traveling towards the interested part) that excites the photostimulable phosphors (PSP: materials that store energy from X-rays and gamma rays and release it as light when stimulated)

Composition of IP

• PSP are deposited in layers on the imaging plate

  • common PSP materials:

    • europium activated fluorohalide crystals

    • cesium bromide crystals

• Europium: element that allows for energy storage

  *ability of the PSP to retain energy is time limited, THEREFORE IPs should be processed soon after exposure *

CR IP Layers

1. Protective Layer

• thin, tough, clear plastic top layer

• protects the underlying phosphor layer and rest of IP

• improves the signal to noise ratio of an image

2. Phosphor layer / F-Center / storage phosphor / colour layer

• active layer of the IP as it traps/stores the -e when struck by x-rays

• may contain a dye to absorb the stimulating light to prevent as much spread

• phosphor gradually releases stored energy naturally (phosphorescence) but this is acc during processing by the exposure to intense infrared laser light (photostimulated luminescence - PSL)

• made up of barium fluorohalide or cesium bromide crystals, europium azelcleaer (?)

3. Light reflective layer

• below phosphor layer and reflects light

• reflects and focuses the light heading to the back of the IP to the crystal structure in the phosphor layer

• energy is bounced back to the crystal structure (phosphor layer)

4. Conductive Layer

• absorbs and reduces static electricity that can cause light emission (light emission can produce artifacts or a false image)

5. Supportive Layer

• semi-rigid material that gives the imaging plate some strength

• constructed of a polymethylene terephthalate (PET) base- a strong, pliable plastic

6. Colour or Anti-halation Layer

• consists of a blue tinted dye that absorbs unwanted stimulating laser light while reflecting emitted light

7. Backing Layer

• soft, plastic layer constructed of a soft polymer that also serves to protect the base of IP from damage during processing (as the IP is removed from the cassette to be processed)

8. Barcode

• allows the user to match the image information with the RIS patient demographics

CR Image Formation Steps

1. Expose

   • phosphors of the IP retains energy based on the frequency and number of photon interactions

2. Stimulate

   • cassette is inserted into the CR reader

   • the IP is removed from the IR

   • a laser scans the IP to release the energy stores as bursts of light (photostimulable luminescence- PSL)

   • light emitted is converted into an electrical signal

3. Read

   • electrical signal is detected by PMT and measured, then converted to a digital signal by ADC (analog-to-digital converter) and digitized to be displayed on the workstation monitor

4. Erase

   • then the IP is erased by exposure to intense light that releases any retained energy (causes it to return to ground-state energy level)

   • not all areas can be erased, some portions will be missed

   • CR receptor is ready for reuse

CR Processor / Reader

= designed to process a single cassette perhaps within a radiography room or multiple cassettes in a hub of multiple x-ray rooms

CR Processing Steps

1. exposure of CR plate

2. scan barcode of CR plate at workstation to associate to patient exam

3. insert plate into CR processor

4. imaging plate processed

   1. IP removed, scanned by laser, 

   2. light emitted detected by photomultiplier tube (PMT)

   3. PMT analog signal converted to digital signal (analog-to digital-conversion)

   4. digital signal transmitted to monitor viewing station

5. IP exposed to intense light to release any residual energy stored in phosphors

6. IP inserted into cassette and ejected from CR processor, ready to reuse

• dark features = energy absorbed by IP

• white features = energy blocked, absorbed by bone/parts

Direct Radiography

= more streamline, does not require a separate processing step

• both digital

• flat-panel detectors

  • uses a large area active-matrix array of electronic components

two image production methods for DR

• direct conversion

  • uses photoconductor

• indirect conversion

  • scintillator material

  • high sensitivity of CsI → significant exposure reductions → substantially reduce patient dose

Three elements to all digital radiography

• Capture element (crystal structures that absorb energy, creates light)

• Coupling element (layer that binds the light produced to a collection system)

• Collection element (reads the light produced which is read by chips or in an amorphous silicon)

Indirect DR Detectors 

= system absorbs x-rays and first converts it to light before creating a digital signal

1. Scintillating (light producing) materials (cesium iodide) are used to convert photon energy into visible light

2. The light is detected and converted to an electrical signal that is sent to the computer for processing

Two methods for this light conversion

• charge-coupled device (CCD)

• thin-film transistor (TFT)

Direct DR Detectors

• no scintillation layer (no CsI) involved, therefore no light-emitting process that is detected and amplified

• x-ray photon energy/exit radiation is directly (by a-Se) converted to electrical energy/electrical charges

• this system relies on a layer of amorphous selenium (a-Se) for both the capture element and the coupling element

• Electrical signals are sent to the ADC for digitization

Post Processing Functions

image control quality

• includes a thorough critique of each image, as well as post-processing if necessary

Examples of Post Processing

• annotation (markers, position)

• zoom (zoom data is not saved, just for viewer instance)

• image flip

• image rotation (to correct orientation)

• window level/width (adjusting the grey scales and contrast to improve visualization, this adjusted image must not be sent to PACS for diagnosis)

• measurement (for length or volume of a structure)