Computed Radiography/Digital Radiographic Technique Notes

History of Digital Imaging

  • First Electronic Digital Computer:
    • Developed by Dr. John Atanasoff and Clifford Berry.
    • Could perform 500 addition or 350 multiplication operations in one second.
  • 1st Generation Computers (1946-1959):
    • Used vacuum tubes.
    • Very large and slow.
  • 2nd Generation Computers (1959-1965):
    • Replaced vacuum tubes with individually packed transistors.
    • Higher capacity of internal storage.
  • 3rd Generation Computers (1965-1972):
    • Used integrated circuits with transistors and other electronic elements fused onto a chip.
    • Operating systems provided user interface.
    • Increased speed and efficiency.
  • 4th Generation Computers (1972-1980):
    • Microprocessors were invented by Intel (VLSI).
    • Multipurpose programmable device accepting digital data, processing it, and providing results.
  • 5th Generation Computers:
    • Much faster and smaller than fourth generation.
    • Portable with faster results.
    • Improvements in semiconductor technology and artificial intelligence.
  • 6th Generation Computers:
    • Different in terms of size, speed, and tasks performed.

Computer Components

  • Hardware: Physical components, including input and output devices.
  • Software: Computer programs instructing the hardware.
  • Computer Language: Operates in the binary system (0 and 1).
  • Operating System: Instructions organizing data flow (e.g., Mac-OS, UNIX, Windows).
  • Application Programs: Software for tasks like completing tax forms or image reconstruction (e.g., iTunes, Excel, Word).
  • Central Processing Unit (CPU): Manipulates data and carries out software instructions.

Analog vs. Digital Systems

  • Analog System:
    • Records a continuous series of grays.
    • Components: cassette, intensifying screen, film.
  • Conventional Radiography (Film/Screen):
    • X-rays interact with the screen's phosphor, converting them into light photons.
    • Light photons interact with film emulsion crystals, transferring information.
    • Image is seen once processed.
  • Digital Imaging: Acquisition process producing an electronic image viewable/manipulable on a computer.

CR vs. DR

  • CR/PSP (Computed Radiography/Photostimulable Phosphor):
    • Indirect; detector (image plate) must be moved between acquisition and display.
  • DR (Digital Radiography):
    • Direct; detector and reader are a permanent part of the table/wall; no cassette is needed.

Digital Imaging

  • Any imaging acquisition process that produces an electronic image that can be viewed or manipulated on a computer.
  • Large dynamic range; the IR responds to a wide range of exposure values to create diagnostic images.
  • Does not produce shades of gray (like analog) but produces individual discrete values.
  • Information is converted into binary language (0,1).

Types of Digital Image Receptors

  • Photostimulable Phosphor (PSP)
  • Flat Panel with Thin-Film Transistor (TFT)
  • Charge-Coupled Device (CCD)
  • Complementary Metal Oxide Semiconductor (CMOS)

Photostimulable Phosphor (PSP) / Computed Radiography (CR)

  • Uses storage phosphor plates to produce radiographic images.
  • Process:
    • Plate is exposed.
    • Taken to a reader to process/create the image.
    • Called indirect digital because the image goes onto the plate, then into a reader for acquisition/displaying.

PSP Reader Functions

  • Records wide range of exposures.
  • Data recognition program searches for anatomy by finding the collimation edges and eliminating scatter.
  • Proper centering is important for correct images.

4 Steps of Creating an Image with PSP Imaging

  • 1. Metastable State (Expose)
  • 2. Stimulate
  • 3. Read
  • 4. Erase

Metastable State

  • X-ray beam exposes the PSP, transferring energy that excites electrons into the metastable state.
  • 50% of electrons return to the ground state immediately; the rest return over time.
  • Imaging Plate (IP) must be read soon after exposure, or the latent image will fade.

Stimulate

  • Focused beam of infrared light directed at the PSP.
  • S∝IoriginalIntensityS \propto \frac{I_{original}}{Intensity}
  • As beam penetrates, it spreads.

Read

  • Laser beam causes electrons to return to the ground state.
  • The light signal emitted after stimulation is detected and measured.

Erase

  • Residual metastable electrons are moved to the ground state by intense light before reuse.

CR Reader

  • Image is scanned into a digital format.
  • Cassette (Photostimulable Phosphor Plate (PSP)) can then be cleared and reused for future scans.

PSP Imaging Plate Layers

  • Protective layer
  • Phosphor layer (active layer, traps electrons during exposure)
  • Light reflective layer
  • Conductive layer (electroconductive layer; prevents static electricity)
  • Support layer
  • Backing layer (light shield layer; prevents light from erasing data).
  • Barcode label

Definitions

  • Photoconductor & Photodiode (Both Detect Light):

    • Material used to absorb X-rays & emit an electric charge.
    • As light or X-ray photons are absorbed, the energy of the incoming photon excites electrons and produces an electrical charge.
    • No amplifier required.
    • Examples: Amorphous selenium, cesium iodide, or amorphous silicon.
    • It is a device used to detect electromagnetic radiation.
    • Solid-state diode that converts light into an electric current in only one direction.
    • Typically uses an amplifier to detect low levels of light.
    • Example: Amorphous Silicon

  • Scintillator & Non-Scintillator:

    • Scintillator: Phosphor that glows when hit with high energy photons.
      • Flat Panel Type: (Indirect capture)
      • Amorphous silicon or Cesium Iodide (A.K.A. Photoconductive )
      • Converts X-rays → light → electrons by a photoconductive layer typically amorphous silicon →electrons collected by the TFT then converted to an electric signal image

  • Non-Scintillator:

    • Flat panel type: (Direct capture)
      • Amorphous selenium (A.K.A. Photoconductor)
      • Convert X-rays → electrons by amorphous selenium → changes to an electronic signal image because of the TFT (no light produced).

  • Thin Film Transistor (TFT):

    • Collects electrons emitted from either amorphous selenium or amorphous silicon.
    • Electronic switch allowing charges to collect at each individual pixel rapidly making them turn on & off much faster.

  • Used in liquid crystal display (LCD)

Flat Panel Detector (FPD)

  • May have cassettes or be cassette-less.
  • Two distinct methods of image acquisition:
    • Indirect Capture (Scintillator Based)
    • Direct Capture (Non-Scintillator Based)

Indirect Capture (Scintillator Based)

  • Scintillator (phosphor) converts X-rays→ light→ electrical signals→ X-ray image.
  • CCD or TFT converts light into electrical signals.
  • Phosphors: Amorphous Silicon or Cesium Iodide may be used.

Direct Capture (Non-Scintillator Based)

  • A.K.A. – Photoconductor Based
  • Converts X-rays→ electronic signal to digital image (no light emitted here).
  • TFT collects electronic signal & sent for processing.
  • Phosphor: Amorphous Selenium

CCD (Charge Coupled Device)

  • Highly sensitive photon detector that transfers photons into an electric charge in the chip.
  • Requires a scintillator to produce light (either cesium iodide or gadolinium oxysulfide).
  • Indirect form of image capture: light → hits the CCD chip → electric signal.
  • Then sent to a ADC (analog-to-digital converter) to make a digital image.
  • Good for low-dose imaging since it responds to low light levels.

Complementary Metal Oxide Semiconductor (CMOS)

  • Special type of memory chip that uses a lithium or rechargeable battery.
  • Uses a scintillator.
  • Process: X-ray → light & stored in capacitors (which stores electrical charges).
  • Each pixel has its own amplifier that is switched on & off by circuitry within the pixel converting light photons → electrical charges.
  • Then sent to a ADC (analog-to-digital converter) to make a digital image.

CCD vs. CMOS

  • CCD: High quality, less noise, better quality, resolution, and sensitivity, more power used.
  • CMOS: More susceptible to noise, light sensitivity is lower, uses very little power, cheaper.

Picture Archiving and Communication Systems (PACS)

  • A networked group of computers, servers, and archives used to manage digital images.
  • Serves as the file room, reading room, duplicator, and courier.
  • Made up of many parts: reading stations, physician review stations, web-access, technologist quality control stations, administrative stations, archive systems.

Imaging Chain

  • Patient demographics, identification markers, exposure factors selection, various speed systems, viewing preparations, sending images to the radiologist.

Patient Demographics

  • Include patient name, patient identification number, facility name, date of birth, and examination date.

Digital Image Capture

  • Digital image capture is linear, which uses all X-ray photons and uses computer software to adapt the diagnostic range.
  • So digital, high kVp and low mAs will not compromise image quality.

Specific Elements to Digital Imaging

  • Bit: A single unit of data; the smallest increment of data on a computer.
  • Byte: Made up of 8 bits.
  • Pixels: Smallest element in a digital image.
    • More pixels = better image resolution.
  • Pixel Size: Directly related to amount of spatial resolution; smaller pixel = greater detail.
  • Pixel Pitch: The distance from the center of one pixel to the center of the next.
  • Bit Depth: Number of bits stored per pixel; defines shades of gray available for each pixel.
    • Example: Pixel Depth: 8 28=2562^8 = 256 shades of Gray
  • Matrix: Square arrangement of numbers in columns & rows that correspond to discrete pixel values.
  • Field of View (FOV): Body part of an image; larger FOV = more area imaged.
  • Exposure Index: Provides feedback to the technologist regarding the estimated exposure on the image receptor.
  • Dynamic Range: Number of gray shades that an imaging system can reproduce.
  • Post-processing can restore necessary constrast; identified by the bit depth of each pixel.

Image Quality Characteristics

  • Brightness: The amount of light transmitted by the monitor.
  • Contrast Resolution: The ability to image adjacent similar tissues; enhanced by dynamic range and post-processing.
  • Spatial Resolution: Ability to render small objects on the image (image detail).
    • Described by the quantity “Spatial Frequency”
      • Measured by LP/MM; thinner Phosphor layer = Higher Resolution
    • DR - Is determined principally by pixel size
    • CR - Phosphor Layer Thickness & Pixel Size determines resolution in PSP systems.
  • Spatial Frequency: Quantifies how close lines can be to each other and still be visibly resolved, expressed in line pair per millimeter (LP/MM).
  • Modulation Transfer Function (MTF): Ability of a system to record available spatial frequencies.
  • Noise: Any type of signal interference in a digital image.
  • Exposure Latitude: Amount of error that can be made in exposure factor choice and still result in the capture of a quality image.
  • Detective Quantum Efficiency (DQE): Measurement of how efficiently a system converts an X-ray input signal into a useful output image.
    • Amorphous Selenium Detectors have the highest DQE
      • DR Displayed Immediately

Digital Radiography Image Sampling

  • Amount of information gathered from pixel storage.
  • The size of the signal is determined and a value is placed on each pixel.

Histogram

  • Represents an image’s gray scale.
  • Low energy = Wider histogram.
  • High energy = Narrow histogram.

Aliasing

  • Loss of digital information because of a fluctuation signal

Automatic Rescaling

  • Occurs when exposure is greater or less than the optimal amount to produce a diagnostic image.
  • Too little exposure = Quantum Mottle; too much exposure = Loss of Contrast

Dose Creep

  • Increasing amount of dose per examination over a period of time that occurs when a tech sets technical factors, particularly mAs, higher than necessary to avoid exposure errors.

Look-Up Table (LUT)

  • Data stored in the computer that is used to substitute new values for each pixel during the processing.
  • The image will have the appropriate appearance in brightness (density) and contrast

Basic Functions of Processing System

Image Manipulation

  • Window Level: Controls how bright or dark the screen image is; increase level = increase brightness.
  • Window Width: Controls the ratio of black and white, or contrast; wider the window width = lower the contrast.

Image Orientation

  • Refers to the way anatomy is oriented on the imaging plate.

Image Annotation

  • A software function that allows text or markers to be digitally added to an image

Imaging Stitching

  • When area of interest is too large to fit on one cassette
  • Process of "sewing" together multiple images to form one continuous image

Background Removal or Shuttering

  • Used to blacken out the white collimation borders in a digital image, effectively eliminating veil glare.

Veil Glare

  • Excess light that enters the eye due to unexposed borders around the collimation which causes temporary white light blindness.

Magnification

  • Magnifying glass- Magnifying a certain area of the image
  • Zoom – Allows magnification of the entire image

Patient Demographic Input

  • Information regarding patient age, medical record number, ordering physician, date of birth, etc..
  • This information should be input or linked via barcode label scan before the start of the examination and before the processing phase.

Archive Query

  • Function that allows retrieval of images from the PACS based on date of examination, patient name or number, examination number, pathologic condition, or anatomic area.