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Digital IRs, Modulation Transfer Function (MTF) - Coggle Diagram

- - - - The active layer contains the Photostimulable Storage Phosphors (PSP)
        
        Stores the absorbed energy from x-ray radiation exposure (latent image)
      - Composed of Barium Fluorohalide Crystals Doped (Laced) with Europium (activator)
        
        Fluorohalide
        
        Possesses a unique property called stimulated phosphorescence
        
        This is what gives the CR the ability to store a latent image over time, then release it later upon restimulation
      - Sensitive to scatter radiation
      - Types of Phosphor
        
        Turbid Phosphor (Top Image)
        
        Phosphor crystals are distributed throughout a phosphor layer
        
        Needle/Columnar Phosphor (Bottom Image)
        
        Phosphor crystals are arranged as needles or columns
    - - Grounds plate to reduce static
      - Absorbs light to increase sharpness
    - - Rigid base to coat the other layers onto
    - - Prevents light from erasing the IP
    - - Protects the back of the IP
  - - - Minimizes build-up of static electricity and cushions the plate
  - - - Electrons in the Phosphor layer are excited to a higher energy
        
        As electron energy drops, some energy is immediately released as visible light (fluorescence) and the rest is stored in the phosphor
        
        Electrons "trapped" at higher energy levels (F-Centers) in the phosphor form the latent image
        
        The stored energy will be released when the phosphor is stimulated with an intense infrared laser light (stimulated luminescence) in the cassette reader
        
        F-Centers
        
        Metastable defects in the CR Phosphor
        
        A result of europium doping (lacing) of the pure crystal structure of the barium fluorohalide compound
        
        Like "holes" that can trap electrons at higher energy levels
        
        Filled Energy Bands
        
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        Valence Band
        
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        Conduction Band
        
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        F-Center is the space between Conduction and Valence bands
        
        "Energy Band" or "Energy Level"
        
        Electrons that are excited enough to overcome the binding energy and leave the atom (ionization)
        
        The freed electrons become trapped at a higher energy state in the "F-Centers"
        
        Only a small amount of electrons are trapped
        
        To release the trapped electrons and form the latent image the barium fluorohalide crystals are excited again by scanning the IP with a helium-neon red laser beam
        
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  - - - To move the IP through the scanning process
    - - Collecting Optics
      - Filters
      - Red Laser
    - - Amplifies the small amount of blue-violet light restimulated by the red laser
      - When light strikes the input side of the PMT it increases the energy state of the photocathode
        
        During this process, the elections are accelerated and generate more energy as their velocity is increased
      - Sends to
    - - Changes the analog electrical (frequency) signal into digital (numeric data)
        
        The image data is processed by the computer and displayed on a monitor
        
        Any remaining latent image is erased by a high intensity light and then the PSP is ready for use
      - ADC samples analog signal from the PMT and turns it into numerical digital data
        
        Sampling Frequency
        
        How often the analog signal is reproduced in its discrete digitized form
        
        Increasing Sampling Frequency
        
        Decreases Sampling Pitch
        
        Decreases Pixel Size
        
        Increases Image Resolution
        
        Determined by
        
        Fixed Sampling Frequency
        
        Some Fix the sampling frequency to maintain fixed spatial resolution
        
        Matrix Size will vary for a different IP size
        
        A larger IP size will have a larger matrix
        
        The Pixel Size will stay the same
        
        Fixed Matrix
        
        Some fix the matrix size and vary the sampling frequency (resolution will vary)
        
        A larger IP Size will result in a Larger Pixel Size
        
        Decreased Spatial Resolution
        
        Sampling Pitch
        
        The distance between sampling points
        
        Increased Pitch = Decreased Image Resolution
        
        Determined by
        
        Nyquist Frequency
        
        Determines level of Spatial Resolution for an IR
        
        States that for a given spatial frequency, or resolution, the signal must be sampled at twice the desired spatial frequency
        
        Determines Pixel Pitch
  - - - 1. The exposed cassette is put into the reader
      - 2. The IP is scanned with a red laser. The stored energy in the F-centers is released as violet-blue light
      - 3. The light (analog signal) is collected and amplified by the photomultiplier tube (PMT)
      - 4. The analog data is sent from the PMT to the analog-digital-converter (ADC)
      - 5. The ADC changes the analog signal into digital data
      - 6. The data is processed by the computer and displayed on a monitor
      - 7. The IP is erased with a bright light
- - - - 1st Layer - X-Ray converter
      - 2nd Layer - Thin-film transistor (TFT) array
      - 3rd Layer - Glass Substrate
      - Detector Element (DEL)
        
        X-ray sensitive (Gray section)
        
        Contains its own
        
        Thin Film Transistor (TFT)
        
        Control data readout
        
        Sends collected signal off to processing
        
        Made up of a matrix of detector elements (DELs)
        
        Entire TFT array is overlaid with a scintillation layer
        
        The Scintillation Layer
        
        produces visible light following absorption of radiation
        
        Structured Scintillator Phosphors (Preferred)
        
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        Unstructured Scintillator Phosphors
        
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        Converts photons into an electrical signal (latent image) and stores it until readout of the electrical signal
        
        Sends out its own electrical signal in sequence
        
        The electrical signal is sent to the ADC for digital conversion
        
        Capacitor
        
        Stores collected signal
        
        Crisscrossed by wires called Data and Gate Lines
        
        Gate Lines
        
        Controlled by Address Driver
        
        Controls the order in which each DEL sends its signal to the ADC
        
        Each DEL corresponds to a single pixel in the image matrix
        
        Fill Factor
        
        The percentage of the DEL surface that is sensitive to x-rays (or light)
        
        Smaller DELs
        
        Lower Fill Factor
        
        Radiation exposure must be increased to maintain adequate IR exposure
        
        Higher Image Resolution
        
        Bigger Pixel
        
        Same Size TFT & Capacitor
        
        Improved Spatial Resolution
    - - The visible light is converted into electrical charges/signal (latent image) by photodetectors
  - - - Converts exit radiation directly into electrical charges
- - - - 1 Bit
        
        8 Bits =
        
        1 Byte
        
        2 Bytes =
        
        A Word
- - - - FOV
        
        W/ Fixed FOV
        
        Increased Matrix Size = Decreased Pixel Size
        
        Decreasing FOV
        
        Given Matrix
        
        Decrease Pixel Size
        
        Increased Spatial Resolution
      - Spatial Resolution
        
        Limited by Pixel Size
        
        Better with a Larger Image Matrix
        
        Image Sharpness
        
        Spatial Frequency
        
        Masured by the numbers of line pairs per millimeter (lp/mm)
        
        Line Pairs (lp/mm)
        
        Human eye can discern up to 5 lp/mm
        
        Increasing line pairs per mm = increased spatial frequency
        
        A imaging system that can resolve more lp/mm has an increased spatial resolution
        
        Resolution test patterns
        
        Large objects
        
        Lower Spatial Frequency
        
        Small objects
        
        Higher Spatial Frequency
        
        Based on the number of cycles per unit of time
        
        High-Resolution Monitors are required to maximize spatial resolution
    - - Assigning Numerical Values to each pixel
        
        Numbers are determined by the amount of attenuation
    - - Number of gray shades that can be represented by a pixel
        
        Gray tone (brightness level)
      - Excessive Bit Depth
        
        Slows down image processing time
        
        Requires more storage space for increased data size
      - Insufficient Bit Depth
        
        Loss of image details
        
        Prevents full post processing capabilities for the image
      - Expressed as
        
        2^n
        
        n = bits
        
        1-Bit Depth
        
        2^1 = 2 Shades of Gray
        
        8-Bit Depth
        
        2^8 = 256 Shades of gray
      - Increased Bit Depth
        
        Improves # of shades of gray
        
        Contrast Resolution
        
        Smallest exposure change (Signal Difference) that can be captured by an IR
        
        Limited by Dynamic Range & Bit Depth of IR
        
        Dynamic Range
        
        The range of exposures that may be captured by an IR
        
        Confused as
        
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        Digital Imaging has a wide dynamic range
    - - The range of pixel values actually present in a displayed image
    - - The number of pixels per mm
      - Increased Pixel Density = Increased Image Resolution
    - - Measured in microns (µm)
      - The space from the center of a pixel to the center of the adjacent pixel
      - Equal to Pixel Size
        
        Smaller Pixels = Decreased Pitch = Increased Image Resolution
  - - - Decrease Pixel Size
      - Increased Spatial Resolution
    - - Greater # of Smaller individual Pixels
        
        Increased Spatial Resolution
  - - - The mathematical algorithm used to change an image from the spatial location domain to the spatial frequency domain
- - - - The Type of material used in the IR to capture radiation impacts the DQE
    - - As Spatial Frequency increases, the IR is less efficient
    - - Directly proportional to DQE
- - - - Caused by the electronic components of the system
    - - Occurs when not enough photons reach the IR to create a latent image
    - - Caused by Compton interactions within the patient's body
    - - Describes the contrast resolution compared with the "noise" in an image
      - Higher CNR increases the visibility of anatomic tissues
    - - affects the SNR, but also affects contrast
      - Tissue with low subject contrast will not be well visualized
      - As Noise increases, Contrast decreases