Computed Radiography & Digital Radiography

CR and DR

• Image
• Receptor:
• –Film
• screen
• –Computed
• Radiography
• –Direct
• Digital Radiography (DDR/DR)
• The
• computer processing means that systems have a far greater latitude compared to
• film/screen. But
• knowing what happens when you set/manipulate exposure factors still matters
• Increasing kVp also
• increases beam quantity to a
• certain extent: because the electrons with more energy are more efficient at
• removing K-shell electrons. Intensity
• ∝ kVp2
• CR has a tube potential (kVp) that determines the subject contrast in the resultant image - this is connected to K- edge of the receptors
• Anatomical tissues vary in absorption and
• transmission create a range of dark and light areas
• Remember it is the degree of
• photoelectric absorption that determines the range of greys visible on the
• radiographic image. This degree of absorption is strongly dependent on both the
• density and inversely proportional to beam energy
• This is what creates the radiographic
• image. Since it is the interaction of the outer electrons that usually
• influence the degree of absorption, transmission, or scatter, the radiographic
• can be thought of as a map of electron density
• And whilst CR and DR has a wide exposure latitude if there isn't enough information to make a diagnostic image then there isn't enough!

Exposure Latitude

• Formally
• it is the tonal range generated within an exposure.
• Sometimes
• called a characteristic curve, that has axes of transmitted light (ie
• range of greys very light to very dark), and exposure (logarithmic
• Sometimes
• called the dynamic range
• In
• digital imaging, changing kVp affects the amount of
• radiation reaching the image receptor and
• affects subject
• contrast (differential attenuation or inherent contrast)
• In
• digital imaging, brightness and image contrast are primary
• controlled during computer processing – but you still need to be aware of the
• effect of kVp on
• subject (inherent) contrast
• 
• Bildunterschrift: Speed of ‘film’ is related to the gradient – a faster film has a steeper gradient, historically because the crystals which made up the film were bigger, requiring less exposure to oxidise to silver. A slower film therefore had smaller crystals requiring more exposure per unit time to generate the same degree of oxidation

Folie 3

• Film screen vs CR
• CR
• and DR are digital imaging modalities. Both CR and DR are relative newcomers to
• x-ray imaging
• Film-screen
• uses a cassette similar to the cassette used in CR
• Film-screen
• uses a processor with chemicals where as CR uses a digital processor
• In
• CR the film and intensifying screen is replaced with a phosphor plate
• CR equipment:
• Cassette
• and phosphor plate, Laser
• Image Reader, PACS
• CR Advantages
• –A
• digital image is generated
• –Ability
• to retrofit to existing radiography equipment
• –Mobile
• radiography is easily accomplished
• –Excellent
• image quality
• –Initially
• less expensive than DR
• CR disadvantages
• –Still
• have to use an imaging plate
• –No
• real time saving benefit over traditional radiography
• –Need
• to purchase an imaging reader
• Imaging Plate
• Average
• life cycle over 10000 uses
• Two
• year warranty
• Average
• cost £ 1000 each
• Flexible
• Durable
• High
• Absorption efficiency - high
• % of incoming x-rays effectively contribute to information capture, therefore
• enabling x-ray dose reduction
• Smooth
• Shiny surface
• Correct
• & Incorrect sides? TUBE side
• 35 x 43 cm, 35 x 35 cm, 24 x 30 cm, 18 x 24 cm, 15 x 30 cm
• Storage: Upright ( NOT flat) to prevent pressure on the Phosphor plate. Must not be left unprotected in the x-ray room, sensitive to scatter.
• Cleaning: Every TWO weeks and erased. If not there is an increased risk of becoming stuck in the digitizer.
• Cassette Erasure –Erased ONCE a week at least, UHL it is done first thing every day. Reduce risk of ghost image.

Phosphor Plate

• Phosphor plate: Photo-stimulable storage phosphor plate, that stores the
• latent image, is a re-usable detector.
• Two
• part process:
• 1.converting incoming x-rays to light
• photons - image capture
• 2.converting light into electrical signal - image readout
• Laser Image Reader: Scanning mechanism to
• extract the latent image from the plate.
• Transmitted to network for
• review/reporting etc
• Through the PACS system
• Phosphor
• plate is analogous to film – it is sensitive to x-ray irradiation, whereas film
• is not; BUT to demonstrate what happens during an x-ray exposure instead of an intensifying screen – a scintilator
• which converts x-ray to visible light which exposes a film, the light is
• trapped within the plate to be read out later
• 

Folie 5

• 
• Protective layer - thin
• & transparent, tough
• and clear. Protects the phosphor layer during handling
• from damage.
• Phosphor layer - active
• component, barium fluorohalide
• crystals doped with europium, contains
• colour centres or F centres, which trap electrons during an exposure.
• Support
• layer - semi
• rigid, give
• strength to the plate
• Conductor
• layer - absorbs
• and removes static electricity
• Light
• shield layer - reflective
• layer, sends
• light in forward direction when released in the reader, ensuring as much light
• as possible is recorded
• Backing
• layer - protects the back of the cassette.

Latent Image Acquisition

• •Exit
• photons are absorbed by the atoms of the phosphor and europium
• •This
• is by the photoelectric effect
• Phosphor plate - image capture
• The absorbed energy excites the electrons
• They are raised to a higher energy state
• Here they become stored/trapped in colour (F) centres
• This is what forms the latent image
• The number of electrons trapped in the F centres is directly related to X-ray beam intensity
• This energy ( latent image ) can be stored for several hours
• However the image will lose about 25%of its energy in 7-8 hours
• The latent image is processed by loading the cassette into an image reader.
• Image reader unit (digitiser)
• The cassette is placed into the reader
• unit
• The imaging plate is extracted and
• scanned
• The image plate is scanned with a finely
• focussed helium-neon laser
• beam.
• The laser releases the trapped electrons
• and they return to a lower energy state.
• This releases energy as visible light.

Digitiser

• 
• The
• varying electrical signals are analogue, converted by an ADC to a digital image.
• Once
• the plate is read it is erased to remove all trace of the latent image
• By
• exposing the plate with intense white light to release any trapped electrons.
• Imaging
• plates are sensitive to `Scatter radiation` and should be erased once a week to
• prevent background signal build up. The readers have an erase mode
• The scanning of the plate results in a
• continuous pattern of light intensities sent to a photomultiplier tube (which
• is a photo detector)
• Photomultiplier tube
• collects, amplifies and coverts the light to an electrical signal
• The electrical signal is proportional to
• the range of energies stored in the image plate.

Matrix

• A
• digital image is made up of a 2D array of numbers called a Matrix
• Bit depth - A
• descriptor of the grey scale value attributed to each picture element
• Pixels are the smallest component of a matrix
• The density of each Pixel ( picture
• element) has to be a whole number. Each pixel has a bit depth or a number of
• bits which controls the brightness of the pixel – 4bit, 8 bit, 10 bit or 12 bit
• The greater the bit depth the more the
• precise digitalization of the analogue signal, and the greater the various
• shades of grey
• This is turn improves its contrast
• resolution - a previous slide suggested
• that 1024 discrete shades of grey can be displayed - but we should consider actually how shades of
• grey can the eye distinguish under optimum image display conditions?
• It
• is now established that the human eye can discriminate 'between 700 and 900
• simultaneous shades of gray for
• the available luminance range of current medical displays and in optimal
• conditions.' It is presently felt that it is of 'no use to simultaneously
• display more than 10 bits of gray (1,024 gray shades) because this already exceeds the
• capabilities of the human visual system
• Larger the matrix size, greater the
• number of pixels, better the image quality

What next

• Now we have an image, what happens next?
• PACS – Picture Archiving and
• Communication System
• Images are captured by a “ Image Server
• ”. PACS does not generate any images, simply handles the data.
• The Server is the primary point of entry. It
• receives digital signals from all radiology modalities, CT, MR, NM, C-arm, DSA,
• digitisers…
• The language of medical digital imaging is
• called “DICOM”.
• DICOM allows various components to communicate
• with each other.
• In PACS these images are captured as data
• frames and routed through a server to digital display monitor.
• For more information please see resources
• on blackboard

CR vs DR

• CR imaging screens are ‘doped’ usually
• Barium Fluorohalide;
• with a small quantity of Europium.
• The addition of Eu
• causes defects within the crystal lattice which allows electrons to become
• trapped more efficiently (F-Centre)
• Incident x-ray is absorbed by the BaFH:Eu
• phosphor – which oxidises the Eu atoms, the electrons liberated by this
• process become trapped in a higher energy metastable
• state
• This is the ‘latent’
• image
• The number of trapped electrons is
• proportional to the intensity of the incident x-ray beam
• Flat panel: Caesium Iodide or Selenium
• detector ( FPD)
• Absorbs x-rays directly and converts into
• a digital signal in 4 - 8 seconds
• Indirect flat panel detectors use an intensifying screen or scintillator to convert x-rays to visible light, and then to a photodetector to produce a visible image
• Direct flat panel detectors use incident
• x-rays to generate free electrons which generate the ‘latent’ image directly.
• Selenium is commonly used as a
• photoconductor
• A large voltage difference is applied
• across the array at the same time, which causes the electrons to move to their
• detectors.

Indirect Digital Radiography

• 
• Caesium Iodide doped detectors are used as indirect flat panel detectors – there is a intermediate step in the generation of a digital signal.
• Incident x-ray photons interact via photoelectric absorption
• This leads to scintillation
• The light is detected by a photodiode
• The intensity of the light is directly
• proportional to the number of incident x-ray photons

Blurring and spatial resolution

• 
• a)Internal
• reflection and diffusion processes
• b) the electron can wander about ionising
• as it goes and losing energy, or
• c)Can be absorbed by another atom leading
• to another ionisation type event, with corresponding re-emission of light. This
• would simulate x-ray incidence additional to the primary point
• d)How can you tell which is which – You
• cant, since the number of interactions is a phenomenal amount (259 * 6.023
• x10exp23) ions present in 1 mole of CsI
• e)Approx ten times the number of stars in the known
• universe (1 x 10 exp 24)
• 
• Bildunterschrift: d) Wide range of intensity e) Narrower range, but decreased amplitude –so less light available f) So structured phosphor is the way to go

Solutions

• Have a thin phosphor plate (0.5 to 2 mm thick)
• Have detectors parallel to the incident
• x-ray
• Each crystal needle is very fine between
• 5 and 20 µm in width
• The edges of the crystals act as light
• guides, the detector can either be on top of the crystals or below

Direct Digital Radiography

• Selenium is a metalloid material,
• exhibiting both insulator and conductor properties depending on it’s state – it
• has several allotropes (cf
• sulphur or carbon).
• The
• selenium-based technology uses an amorphous selenium-coated
• thin-film-transistor (TFT) array to capture and convert X-ray energy directly
• into digital signals.
• Incident X-rays directly generate electron-hole
• pairs in the selenium layer
• via photoelectric absorption.
• These charges are collected by individual
• storage capacitors associated with each detector element for readout by
• customised electronics within the array.
• A Bias voltage is applied across the detector
• structure during the exposure
• When the exposure is terminated the
• individual detector elements are read, which generates the x-ray image
• 
• Bildunterschrift: g) Low Potential, electrons can ‘easily’ accumulate on adjacent capacitor plates h) Thicker photoconductor means more electron/hole pairs generated, but still low potential so see above i) High field constrains electrons to discrete capacitor plates, thick photoconductor ensures plenty on electron/hole pairs

DDR image receptor

• Software
• converts to appropriate grey scale pixels according to voltage (proportional
• to intensity of incident x-ray photons)
• Higher
• spatial
• resolution - mammography applications down to 50 microns
• Direct
• conversion of x-rays to an electrical charge
• Can
• have in excess of 7 million detectors on a 40cm x 40cm plate.
• Images
• are available within 3-8 seconds
• DDR Advantages
• No processing time and almost immediate
• image acquisition
• Excellent image quality
• Pre readout info attributes appropriate
• image algorithm
• Immediate transfer to PACS / ER / Review
• terminal
• DDR Disadvantages
• More expensive than CR
• The imaging sensor is more expensive to
• replace than an imaging plate or cassette (of the order of ~ £20,000 each c/w
• ~£1-2000 each for 35 x 43 CR plate)
• Cannot usually
• retrofit to existing x-ray equipment – but some companies offer stand alone
• solutions

CR vs DDR

• Dose
• Slightly
• reduced with DR – depending on whose literature you read, upto 50%
• dose saving
• Resolution
• CR and DR are similar, DR is better for very
• high resolution applications
• Speed
• / throughput
• DDR
• is faster as there is no intermediate steps before next image acquisition
• Can
• use ‘tomosynthesis’ –
• repeated acquisition in plane at different projection angle to enhance lesion
• detection – lower dose than standard multi-planar projection
• Cost
• CR is
• cheaper than DDR
• 
• Bildunterschrift: Amorphous selenium–based direct conversion DR systems. (a) Drawing illustrates a selenium drum–based system. A rotating selenium-dotted drum with a positive electrical surface charge is exposed to x-rays. Alteration of the charge pattern of the drum surface is proportional to the incident x-rays. The charge pattern is then converted into a digital image by an analog-to-digital (A/D) converter. (b) Drawing illustrates a selenium-based flat-panel detector system. Incident x-ray energy is directly converted into electrical charges within the fixed photo-conductor layer and read out by a linked TFT array beneath the detective layer.

Charge Coupled Devices (CCD)

• These are variants to flat panel
• detectors – used in cameras, video cameras, telescopes, and some mobile
• phones*.
• Electrons are liberated through the same
• process as CR, but using visible light instead of x-ray photons.
• The principle difference is that the
• detector is itself an integrated circuit made of crystalline silicon.
• The pixel electronics are etched onto the
• surface of the chip.
• The images produced in this way are of
• high quality.
• They are used in video fluoroscopy and
• cine radiography applications, and for very small field radiography such as
• dental applications.
• Quip about CMOS detectors and infra red
• detector (remote control)
• cheaper cameras use CMOS detector chips – lower resolution lower cost
• CCD
• cameras – higher resolution, higher cost
•