Computed Radiography/Digital Radiographic Technique Notes

History of Digital Imaging

First Electronic Digital Computer

Dr. John Atanasoff and Clifford Berry developed the first electronic computer. It could perform 500 additions or 350 multiplications in one second.

1st Generation Computers (1946-1959)

  • Introduced in 1946.
  • Used vacuum tubes.
  • Very large and slow.
  • Electronic vacuum tubes replaced electric relays, enabling stored program ideas of John Von Neumann.
  • Could multiply two ten-digit numbers 40 times per second.

2nd Generation Computers (1959-1965)

  • Used transistors instead of vacuum tubes.
  • Higher capacity of internal storage.
  • Vacuum tubes were replaced by individually packed transistors.

3rd Generation Computers (1965-1972)

  • Used integrated circuits with transistors and other electronic elements fused onto a chip.
  • Operating systems served as the user interface to computing.
  • Increased speed and efficiency.

4th Generation Computers (1972-1980)

  • Microprocessors were introduced, utilizing Very Large Scale Integration (VLSI).
  • Invention of the microprocessor by Intel.
  • A multipurpose programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and provides results as output.

5th Generation Computers

  • Much faster than fourth-generation computers.
  • Smaller and provide fast results compared to other generations.
  • Portable and accessible at any time.
  • Improvements in semiconductor technology and artificial intelligence.

6th Generation Computers

  • Different from other generations in terms of size, speed, and tasks performed.

Computer: 2 Principle Parts

  1. Hardware
  2. Software

Hardware

Physical components of the system, including input and output devices such as:

  • Screen
  • Monitor
  • Keyboard
  • Speaker
  • Microphone
  • Mouse

Software

Comprised of computer programs that tell the hardware what to do and how to manipulate and store data.

Computer Language

The computer operates in the binary system.

  • Binary system: A system in which information can be expressed by combinations of the digits 0 and 1.

Operating System

Instructions that organize the course of data through the computer to solve a problem.

  • Common operating systems: Mac-OS, Unix, and Windows.

Application Programs

Written by computer manufacturers or software manufacturers.

  • Examples: iTunes, Excel, Word, etc.
  • Other tasks: complete income tax forms, print mailing lists, reconstruct images from X-ray transmission patterns.

Central Processing Unit (CPU)

Element that allows the computer to manipulate data and carry out software instructions.

Analog System

Records a continuous series of gray shades on the receptor.

  • 3 components:
    • Cassette
    • Intensifying screen
    • Film

Conventional Radiography (Film/Screen)

X-rays interact with the phosphor of the screen and are converted into light photons.

  • These light photons interact with the film’s emulsion crystals by transferring information into them.
  • Once processed, an image is seen.

CR vs. DR

  • CR/PSP (Computed Radiography/Photostimulable Phosphor): Indirect, meaning the radiographer must move the detector (image plate) between image acquisition and display.
  • DR (Digital Radiography): Direct, meaning the detector and reader are a permanent part of the table or wall; therefore, a cassette is not needed.

Comparison of Conventional, PSP, and FPD

See Table 1-1 for a comparison of Conventional Radiography, PSP, and FPD considering factors such as imaging room, ease of use, latent image formation, processing, exposure response, image contrast, density, scatter radiation, and noise.

Digital Imaging

Any imaging acquisition process that produces an electronic image that can be viewed or manipulated on a computer.

Large Dynamic Range

The Image Receptor (IR) responds to a wide range of exposure values to create diagnostic images.

Digital Imaging System

  • Do not produce shades of gray (like analog) but produce 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)

  • Old term: Computed Radiography (CR)
  • PSP is a digital acquisition modality that uses storage phosphor plates to produce radiographic images.
PSP (Cont’d)
  • After the PSP is exposed, the plate is taken to a reader to process or create the image.
  • It is called “indirect digital” because the image goes onto the plate, into the reader, and then image acquisition/displaying occurs.
PSP Reader Functions
  • It records a wide range of exposures.
  • The data recognition program searches for anatomy recorded on the imaging plate by finding the collimation edges and then eliminates scatter outside the collimation.
  • Proper centering is also important to get correct or diagnostic images.

4 Steps of Creating an Image with PSP Imaging

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

When the X-ray beam exposes the PSP, there is energy transfer that excites the electrons into the metastable state (expose).

  • 50% of electrons return to the ground state immediately.
  • The other 50% return to the ground over time.
  • That is why the IP (imaging plate) must be read soon after exposure, or the latent image will fade.
Stimulate
  • A focused beam of infrared light is directed to the PSP.

I<em>beam∝I</em>originalI<em>{beam} \propto I</em>{original}

  • Where I<em>beamI<em>{beam} is beam intensity and I</em>originalI</em>{original} is original intensity.
  • As the 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

Before reuse, any residual metastable electrons are moved to the ground state by intense light.

CR Reader

The image is scanned into a digital format.

  • The cassette (photostimulable phosphor plate (PSP)) can then be cleared and reused for future scans.

PSP Imaging Plate

Layers include:

  • Protective layer
  • Phosphor layer
  • Light reflective layer
  • Conductive layer
  • Support layer
  • Backing layer
  • Light shielding layer
  • Barcode label
Layers of the PSP Imaging Plate
  • Backing Layer (a.k.a. Light Shield Layer)
    • Serves to prevent light from erasing image plate data.
    • Soft polymer that protects the back of the cassette.
  • Support Layer
    • Semi-rigid material that gives the imaging sheet strength.
    • Made of polyester.
  • Color Layer
    • Absorbs the stimulating light but reflects emitted light.
    • Found in newer plates.
  • Conductive Layer (a.k.a. – Electroconductive Layer)
    • Serves to facilitate transportation through the scanner/reader and prevent image artifacts resulting from static electricity.
  • Reflective Layer
    • Helps send the light forward in the IR.
  • Phosphor Layer
    • Known as the “active layer.”
    • It “traps” electrons during exposure.
    • Most plates today are made of barium fluorohalide with europium as an activator.
  • Protective Layer
    • Thin, tough, clear plastic that protects the phosphor layer.

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.
    • Scintillator – 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.
      • Note: TFT’s are used for both Scintillator & Non-Scintillator.
  • Non-Scintillator – Flat Panel Type: (Direct Capture)
    • Amorphous Selenium (A.k.a. Photoconductor)
      • Converts X-rays → Electrons by amorphous selenium → Changes to an electronic signal image because of the TFT (no light produced).
      • Note: TFT’s are used for both Scintillator & Non-Scintillator.

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).
    • Here it converts Light → Electrical Charges
    • Indirect Capture: Collects Light
    • Direct Capture: Collects Electronic Signal
  • Collects the released electrons in an area of the circuit assembly called Detector Elements (DEL).

Flat Panel Detector (FPD)

This system may have cassettes or be cassette-less.

  • 2 distinct methods of image acquisition:
    • A. Indirect Capture
    • B. Direct Capture
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 & is sent for processing
  • Phosphor: Amorphous selenium

CCD (Charge Couple 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).
  • This is considered an “indirect form of image capture.”
    • Light → Hits the CCD chip → Electric signal
  • Then sent to an 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 an 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 (100 times more)
  • Older & more developed technology
  • Pixel fill factor is better
CMOS
  • More susceptible to noise
  • Light sensitivity is lower (working on improving this problem)
  • Uses very little power
  • Cheaper

Sending Images to PACS

Picture Archiving and Communication Systems (PACS)

A networked group of computers, servers, and archives that can be used to manage digital images.

  • This system serves as: the file room, reading room, duplicator, and courier.
  • Software generally the same at most facilities, but the components are specific to individual facilities.
  • Examples: number of areas where images are interpreted, volume of patients, locations where images are viewed by physicians other than the radiologists.

PACS

Made up of many parts:

  • Reading stations
  • Physician review stations
  • Web-access
  • Technologist quality control stations
  • Administrative stations
  • Archive systems
  • Other interfaces in hospital & radiology systems

Imaging Chain

  • Patient demographics
  • Identification markers
  • Exposure factors selection
  • Various speed systems
  • Viewing preparations
  • Sending images to the radiologist

Patient Demographics

Include the following:

  • Patient name
  • Patient identification number
  • Name of facility
  • Date of birth
  • Examination date

Identification Markers

Marker should be used at all times on all images.

Exposure Factor Selection

  • Film/Screen – Save images in a logarithmic way
  • 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.

Various Speed System

Speed Class- Refers to PSP’s ability to capture the image using certain exposure factors.

  • 1st system – speed class of 200

Viewing Preparations

Digital System- All relevant information must be attached to the digital file; including position indicators (supine, PA, etc.) or image acquisition markers (portable or cross-table).

Sending Images to the Radiologist

Images are reviewed at a workstation and then sent to PACS.

  • Here, a list is generated for the radiologist to access the exams to read.

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 (Picture Element)

Smallest element in a digital image.

  • A digital image is separated into pixels by the binary language.
  • More pixels = better image resolution.

Pixel Size

Size of pixel is directly related to the amount of spatial resolution in an image.

  • Smaller pixel = greater detail

Pixel Pitch

Pixel pitch is 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
    • 256 shades of gray

Matrix

Square arrangement of numbers in columns & rows that correspond to discrete pixel values.

  • Each box corresponds to a specific location of the patient’s tissue.
  • Typical range is from about 512
    mides 512 to 1024
    mides 1024.
  • As the digital image matrix size increases, the pixel size decreases.
  • The numeric value in each pixel corresponds to the intensity of the beam absorbed by a particular area of the digital receptor that gets converted on the monitor to digital values of “gray.”

Field of View (FOV)

Body part of an image.

  • Larger FOV = more area imaged
  • Changes in FOV will not affect the size of the matrix, but changes in the matrix will affect pixel size.
    • Reason: FOV remains the same, pixel size decreases to fit into the matrix.

Exposure Index

The exposure index is currently a method by which digital radiography manufacturers provide 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 contrast.
  • Even though some systems may be able to produce more shades of gray for certain techniques, the human eye has limitations to only 30 shades of gray.
  • Is identified by the bit depth of each pixel.

Image Quality Characteristics

Brightness

The amount of light transmitted by the monitor as well as light reflected off the monitor that will affect the image appearance on a display monitor.

  • Brightness can be controlled by monitor functions & post-processing functions

Contrast Resolution

Is the ability to image adjacent similar tissues.

  • Refers to the ability of the digital system to display changes in grayscale values.
  • Example: Higher contrast resolution = more shades of gray may be demonstrated = ability to differentiate between small differences in densities
  • Is enhanced by dynamic range and post-processing

Spatial Resolution

Ability to render small objects on the image (image detail).

  • Is described by the quantity “spatial frequency”.
  • Measured by lp/mm.
  • DR - is determined principally by pixel size.
  • CR-phosphor layer thickness & pixel size determines resolution in PSP systems.
    • Thinner phosphor layer = higher resolution

Spatial Frequency

Quantifies how close lines can be to each other and still be visibly resolved.

  • It refers to the line pairs
  • One line pair = a line and the interspace width
  • Is expressed in line pairs per millimeter (lp/mm)
  • As spatial frequency becomes larger, the objects become smaller
  • High spatial frequency = better resolution

Modulation Transfer Function (MTF)

Ability of a system to record available spatial frequencies.

  • Responsible for converting contrast values of different-sized objects into contrast intensity levels in the image.
  • Used to measure how accurately the lens can reproduce detail from an object to an image.

Noise

Any type of signal interference in a digital image

  • Signal-to-noise ratio (SNR) limits contrast resolution
  • Radiographic noise = occurs during the acquisition of the image
  • Equipment noise = comes from noise in the detector elements and non-uniform detector responses (this has to do with the manufacturer, technology, and detector quality)

Exposure Latitude

Amount of error that can be made in exposure factor choice and still result in the capture of a quality image

  • Latitude is dependent on the image detector; the higher the dynamic range of the detector, the more values can be detected

Detective Quantum Efficiency (DQE)

Measurement of how efficiently a system converts an X-ray input signal into a useful output image

  • It’s a measurement of the percentage of X-rays that are absorbed when they hit the detector
  • PSP systems have a better ability to convert incoming X-rays to