Computed Radiography/Digital Radiographic Technique Notes

Computer: Principle Parts

  • Hardware

  • Software

Hardware and Software

  • Hardware: Physical components of the system including input and output devices.

  • Software: Computer programs that tell the hardware what to do and how to manipulate and store data.

Computer Language and Systems

  • Binary System: 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 OS: Mac-OS, Unix, and Windows.

  • Application Programs: Written by computer or software manufacturers for tasks like completing income tax forms or reconstructing 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 on the receptor.

  • Components: cassette, intensifying screen, and film.

Conventional Radiography (Film/Screen)

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

  • Light photons interact with the film’s emulsion crystals, transferring information.

  • Once processed, an image is seen.

CR and DR Differences

  • CR/PSP: Indirect; the radiographer must move the detector between image acquisition and display.

  • DR: Direct; the detector and reader are a permanent part of the table or 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.

  • Does not produce shades of gray (like analog) but 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.

  • After exposure, the plate is taken to a “reader” to process or create the image.

  • "Indirect digital" because the image goes onto the plate, into the reader, and then image acquisition/displaying occurs.

PSP Reader Functions

  • Records a wide range of exposures.

  • Data recognition program searches for anatomy recorded on the imaging plate by finding collimation edges.

  • Proper centering is important.

PSP Imaging Steps

  • Metastable State (Expose)

  • Stimulate

  • Read

  • Erase

PSP Imaging Steps Details

  • Metastable State (Expose): X-ray beam exposure excites electrons into a metastable state; IP must be read soon after exposure to prevent latent image fade.

  • Stimulate: Focused beam of infrared light is directed to the PSP.

  • Read: Laser beam causes electrons to return to the ground state; the emitted light signal is detected and measured.

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

CR Reader

  • The image is scanned into a digital format.

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

PSP Imaging Plate Layers

  • Protective layer

  • Phosphor layer

  • Light reflective layer

  • Conductive layer

  • Support layer

  • Backing layer

  • Light shielding layer

  • Barcode label

PSP Imaging Plate Layers Functions

  • Backing Layer (Light Shield Layer): Prevents 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 (Electroconductive Layer): Facilitates transportation through the scanner/reader and prevents image artifacts resulting from static electricity.

  • Reflective Layer: Helps send the light forward in the IR.

  • Phosphor Layer: "Active layer" that "traps" electrons during exposure; plates are made of barium fluorohalide with europium as an activator.

  • Protective Layer: Thin, tough, clear plastic that protects the phosphor layer.

Photoconductor & Photodiode

  • Both detect light.

  • Photoconductor: material used to absorb x-rays and emit an electric charge, e.g., amorphous selenium, cesium iodide, or amorphous silicon. No amplifier required.

  • Photodiode: solid-state diode that converts light into an electric current in only one direction, typically using an amplifier; e.g., amorphous silicon.

Scintillator & Non-Scintillator

  • Scintillator: Phosphor that glows when hit with high-energy photons.

    • Flat panel type (indirect capture) using amorphous silicon or cesium iodide.

    • Converts X-rays to light to electrons by a photoconductive layer and TFTs.

  • Non-Scintillator: Flat panel type (direct capture) using amorphous selenium.

    • Converts X-rays to electrons by amorphous selenium; TFT changes to electronic signal image (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.

  • Used in liquid crystal display (LCD).

  • Converts light to electrical charges.

  • Collects released electrons in detector elements (DEL).

Flat Panel Detector (FPD)

  • May have cassettes or be cassette-less.

  • Two methods of image acquisition:

    • Indirect Capture (Scintillator based)

    • Direct Capture (Non-Scintillator based)

Image Capture Methods

  • Indirect Capture (Scintillator Based): Scintillator converts X-rays to light to electrical signals to 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): Converts X-rays to electronic signal to digital image (no light emitted); TFT collects electronic signal and 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).

  • Light hits the CCD chip to electric signal; then sent to 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 to light and 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 to 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

    • Uses very little power

    • Cheaper

Picture Archiving and Communication Systems (PACS)

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

  • Serves as the file room, reading room, duplicator, and courier.

  • Software is generally the same at most facilities, but the components are specific to individual facilities.

  • Made up of reading stations, physician review stations, web-access, technologist quality control stations, administrative stations, archive systems, other interfaces.

Imaging Chain

  • Patient demographics

  • Identification markers

  • Exposure factors selection

  • Various speed systems

  • Viewing preparations

  • Sending images to the radiologist

Imaging Chain Details

  • Patient Demographics: Include patient name, ID number, facility name, date of birth, and examination date.

  • Identification Markers: Marker should be used at all times on all images.

  • Exposure Factor Selection:

    • Digital image capture is linear, using all X-ray photons and computer software to adapt the diagnostic range; high kVp and low mAs.

  • Various Speed System: Speed class refers to PSP’s ability to capture the image using certain exposure factors.

  • Viewing Preparations: All relevant information must be attached to the digital file, including position indicators or image acquisition markers.

  • Sending Images to the Radiologist: Images are reviewed at a workstation and sent to PACS.

Digital Imaging Elements

  • Bit: A single unit of data and the smallest increment of data on a computer.

  • Byte: Made up of 8 bits.

  • Pixels (Picture Element): Smallest element in a digital image; more pixels = better image resolution

  • Pixel Size: Directly related to the 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 (e.g., pixel depth of 8 =28== 2^8 = 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: feedback to the technologist regarding the estimated exposure on the image receptor

  • Dynamic Range: Number of gray shades that an imaging system can reproduce; 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.

  • Spatial Resolution: Ability to render small objects on the image (image detail).

Spatial Resolution Details

  • 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 and converting contrast values.

  • Noise: Any type of signal interference in a digital image. radiographic noise occurs during the acquisition of the image, equipment noise comes from noise in the detector elements

  • Exposure Latitude: Amount of error that can be made in exposure factor choice and still result in the capture of a quality image; the higher the dynamic range of 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; Amorphous selenium detectors have the highest DQE

Digital Radiography Image Sampling

  • Imaging Sampling: Amount of information gathered from pixel storage.

  • Histogram: Represents an image’s gray scale.

  • Aliasing: Loss of digital information because of a fluctuation signal.

  • Automatic Rescaling: Occurs due to the computer trying to fix an image where the exposure is too low/high. Too little exposure = quantum mottle, too much exposure = loss of contrast.

  • Dose Creep: 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 for each pixel.

Basic Functions of Processing System

  • Image Manipulation:

    • Window Level: Controls how bright or dark the screen image is.

    • Window Width: Controls the ratio of black and white, or contrast; wider window width = lower contrast.

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

  • Image Annotation: Software function that allows text or markers to be digitally added to an image.

  • Imaging Stitching: "Sewing" together multiple images to form one continuous image.

  • Background Removal or Shuttering: Blacken out the white collimation borders.

  • Veil Glare: Excess light that enters the eye due to unexposed borders around the collimation.

  • Magnification:

    • Magnifying Glass: Magnifying a certain area of the image.

    • Zoom: Allows magnification of the entire image.

Image Management

  • Patient Demographic Input: Information regarding patient age, medical record number, ordering physician, date of birth, etc.

  • Manual Send: Computer function that allows images to be sent to specified reading stations.

  • Archive Query: Function that allows retrieval of images from PACS.