Chapter_008+Digital+Imaging

Chapter 8: Digital Imaging

Copyright Notice

  • Copyright © 2017 by Elsevier, Inc. All rights reserved.

Learning Objectives

  • Define the key terms used in digital imaging
  • List the equipment needed to perform digital imaging
  • Explain the computed radiography (CR) digital system
  • Explain the digital radiography (DR) system
  • Compare CR and DR digital systems
  • Recognize the importance of using exposure technique charts with digital imaging
  • Describe the processing and postprocessing of a digital image
  • Explain what a picture archival and communications system (PACS) is and how it is used
  • Explain the DICOM and DICOM grayscale

Digital Imaging Overview

  • Digital imaging involves the production, processing, viewing, and storage of images on a computer.
  • Computer networks enhance the capability of image viewing from locations outside the medical facility.
  • Types of digital imaging:
    • Computed Radiography (CR)
    • Digital Radiography (DR)

Computed Radiography (CR)

  • CR utilizes cassettes filled with photostimulable phosphor plates to obtain images.
  • A CR system comprises:
    • CR reader for image processing.

Digital Radiography (DR)

  • DR is referred to as “cassette-less” because the flat-panel detector is integrated into the x-ray table or the upright wall unit.
  • Images in DR are ready for viewing within seconds.
  • Two types of flat-panel detectors:
    • Indirect-conversion: Converts x-rays to light.
    • Direct-conversion: Converts x-rays directly to an electric charge.

Indirect-conversion Detectors

  • Features:
    • Scintillator (cesium iodide): Converts X-rays into visible light.
    • Photodiode (amorphous silicon): Converts light into an electric signal.
    • Thin Flat Panel Transistor (TFT): Acts like a switch that sends electrical signal to the image processor using an analog-to-digital converter.
    • Charged Couple Device (CCD): Converts light photons into electrical signals.
    • Complementary Metal Oxide Semiconductor (CMOS): Converts light into a digital signal utilizing photosensitive pixels along with on-chip circuitry.

Direct-conversion Detectors

  • Non-scintillation method:
    • Semiconductor (amorphous selenium): Directly converts X-ray photons into electric charge.
    • Thin Flat Panel Transistor (TFT): Functions similarly as in indirect-conversion, manages the signal processing using an analog-to-digital converter.

Matrix and Pixel

  • Pixel (picture element): The fundamental building block of digital images.
  • Matrix: A collection of pixels which together forms the image.
    • Matrix size: Total number of pixels determines the resolution.

Spatial Resolution

  • Defined as the ability of an imaging system to differentiate between two nearby objects.
  • Related characteristics include:
    • Pixel size: A measure of resolution.
    • Pixel pitch: The distance between pixels in an image; smaller distance results in enhanced image resolution.
    • Detector Elements (DEL): Structure of matrix pixels that collect electric charge and send a signal.
    • DEL size: This determines the resolution of the image.
    • Fill factor: Percentage of pixel area sensitive to the image signal (electric charge or light photons).
    • Matrix Size and Sampling Frequency: Essential components related to capturing the image.

Image Signal (Exposure Related)

  • Refers to the system's capacity to distinguish differences in intensity in an image.
  • Dynamic Range: Shows how the detector responds to varying levels of radiation.
  • Signal to Noise Ratio (SNR): Measures the capacity of the digital system to convert the x-ray input electric signal into a useful radiographic image; defined as:
    • Signal: Useful information.
    • Noise: Non-useful information.
  • Quantum noise (quantum mottle): Occurs when there aren’t enough photons reaching the detectors, compromising image quality.
  • Contrast to Noise Ratio (CNR): A measure of image quality based on contrast instead of raw signal.

Contrast Resolution (Equipment Related)

  • Ability to discern differences in intensity within an image.
  • Bit depth: Simultaneously defines the number of bits per pixel, which determines grayscale.
  • Modulation Transfer Function (MTF): Measures contrast at a specific spatial frequency.
  • Detective quantum efficiency (DQE): Metrics quantifying the combined impact of signal and noise performance of the imaging apparatus.

Digital Processor Functions

  • Processes fundamental aspects of digital images:
    • Window Level: Controls density in the image.
    • Window Width: Controls contrast of the image.
    • Brightness: Overall lightness or darkness of the image.
    • Shuttering: Blackens out white collimation borders.
    • Image Stitching: Compiles multiple images into one.
    • Image Annotation: Allows addition of textual information to the image.
    • Edge Enhancement: A technique enhancing image sharpness and contrast.

Image Processing and Display

  • Raw Data Processing:
    • Analog-to-Digital Converter (ADC): Converts stored analog information into digital values.
    • Quantization: Transforms a continuous tone image into a digital image by sampling to form a matrix of grayscale values.
  • Corrections Initiated During Processing:
    • Rescaling: Adjusts exposure to suit image requirements.
    • Flat Fielding: Corrects shading nonuniformity in images.
    • Dead Pixel Correction: Uses surrounding pixels to fill in lost information.
    • Smoothing: Averages frequency of each pixel with its neighbors to minimize noise.

Data for Display

  • Look Up Table (LUT): Process employed to ensure desired contrast in images.
  • Values of Interest (VOI): Identifies the relevant range of histogram data that should be depicted in the final image.

Exposure Technique Charts

  • Vital for image manipulation regarding contrast and density, although they do not diminish the necessity for technique charts.
  • Usage focuses on selecting parameters like mA, kVp, and time to prevent unnecessary patient exposure.
  • Emphasizes practice of ALARA (As Low As Reasonably Achievable).
  • Misuse of settings for high exposure to avoid repeat imaging is considered unethical.

Exposure Indicators

  • Metrics indicating how much exposure