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 mattersIncreasing kVp also increases beam quantity to a certain extent: because the electrons with more energy are more efficient at removing K-shell electrons. Intensity ∝ kVp2CR 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!

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 

Exposure Latitude 

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Caption: : 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

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, PACSCR 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 readoutLaser Image Reader: Scanning mechanism to extract the latent image from the plate. Transmitted to network for review/reporting etc Through the PACS systemPhosphor 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

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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 plateConductor 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 recordedBacking 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. 

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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 MatrixBit depth - A descriptor of the grey scale value attributed to each picture elementPixels are the smallest component of a matrixThe 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 bitThe 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 systemLarger 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 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.

CR vs DR

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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

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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)

Blurring and spatial resolution

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Caption: : 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

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Caption: : 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

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 image receptor

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

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

CR vs DDR

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Caption: : 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.

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

Charge Coupled Devices (CCD)

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