Chapter 8 digital imaging
Chapter 8: Digital Imaging
Learning Objectives
Key Terms in Digital Imaging: Define the essential terminology related to digital imaging technologies and processes.
Equipment for Digital Imaging: List all necessary equipment required for executing digital imaging procedures effectively.
Computed Radiography (CR) System: Explain the workings and components of the CR digital imaging system.
Digital Radiography (DR) System: Explain the practical operation and features of the DR digital imaging system.
Comparison of CR and DR: Compare and contrast the CR and DR systems, highlighting their differences and similarities.
Exposure Technique Charts: Recognize the significance of utilizing exposure technique charts in digital imaging to ensure safe and accurate imaging.
Image Processing: Describe the processes involved in the processing and postprocessing stages of a digital image.
Picture Archival and Communication System (PACS): Explain the function of PACS in image storage and management.
DICOM and DICOM Grayscale: Define DICOM and the associated grayscale standard utilized in medical imaging contexts.
Digital Imaging Overview
Image Generation:
Images are produced, processed, viewed, and stored using computer technology.
Computer networks facilitate remote image viewing outside medical facilities.
Types of Digital Imaging:
Computed Radiography (CR)
Digital Radiography (DR)
Computed Radiography (CR)
Image Acquisition: Images are obtained using CR cassettes that contain photostimulable phosphor plates.
CR Systems: Include a dedicated CR reader essential for processing the images.
Digital Radiography (DR)
Cassette-less System:
DR is termed as “cassette-less” because it incorporates a flat-panel detector integrated within the x-ray table or an upright wall unit.
Images are ready for immediate viewing, typically within seconds.
Flat-panel Detectors:
Indirect Conversion: X-rays are converted to light before being transformed into an electric signal.
Direct Conversion: X-rays are transformed directly into an electric charge without the intermediate light step.
Indirect Conversion Detectors
Scintillator (Cesium Iodide): Converts X-rays into visible light.
Thin Flat Panel Technology: Includes photodiodes (e.g., amorphous silicon) that convert light into an electrical signal.
Thin Film Transistor (TFT): Acts like a switch to relay electrical signals from the detector to the image processor using an analog-to-digital converter.
Charged Coupled Device (CCD): Converts light photons into an electrical signal based on the photoelectric effect.
Complementary Metal Oxide Semiconductor (CMOS): Converts light into a digital signal using photosensitive pixels integrated with on-chip circuitry.
Direct Conversion Detectors
Non-scintillation Technology: Employs semiconductors (e.g., amorphous selenium) to convert X-ray photons directly into electric charges with the aid of TFT.
Matrix and Pixel
Pixel (Picture Element): The smallest unit in a digital image that forms the foundation of digital imaging.
Matrix: A configuration of pixels that collectively forms an image.
Matrix Size: Refers to the total count of pixels in a defined matrix, influencing overall image resolution.
Spatial Resolution
Definition: The capability of an imaging system to differentiate between closely located objects within an image.
Pixel Characteristics:
Pixel Size: Impacts image resolution directly.
Pixel Pitch: The distance between adjacent pixels; smaller distances yield better resolution.
Detector Elements (DEL): A matrix of pixels in the detector that collect electrical charges.
DEL Size: Influences the spatial resolution of the image.
Fill Factor: Percentage of a pixel area that is responsive to the image signal (whether light photons or electric charge).
Matrix Size and Sampling Frequency are additional factors affecting resolution.
Image Signal (Exposure-related)
Distinction Capability: The ability of the digital system to recognize different intensity levels within an image.
Dynamic Range: Reflects the detector’s capability to respond sensitively to various radiation levels.
Signal-to-Noise Ratio (SNR):
Measures the system's efficiency in translating X-ray input into a quality radiographic image.
Signal: Useful information.
Noise: Non-useful information amidst the signal.
Quantum Noise (Quantum Mottle): Occurs when there are insufficient photons reaching the detectors to produce a high-quality image.
Contrast-to-Noise Ratio (CNR): Assesses image quality based on contrast rather than raw signal data.
Contrast Resolution (Equipment Related)
Definition: The capacity to identify variations in intensity in an image.
Bit Depth: The number of bits assigned per pixel, determining the range of grayscale representation.
Modulation Transfer Function (MTF): Measures the contrast at a particular spatial frequency in an image.
Detective Quantum Efficiency (DQE): Quantifies the overall signal and noise performance of an imaging system.
Digital Processor Functions
Window Level: Controls the overall image density, affecting the lightness or darkness of an image.
Window Width: Manipulates the contrast displayed in the image.
Brightness: Overall luminescence levels in an image are adjusted during processing.
Shuttering: Techniques used to blacken non-image areas (like collimation borders) in an image.
Image Stitching: Combining multiple images to create a single cohesive image.
Image Annotation: Enables text to be added directly onto an image for clarity and documentation.
Edge Enhancement: A processing method to sharpen image edges, resulting in increased contrast.
Image Processing and Display
Raw Data Processing:
Analog-to-Digital Converter (ADC): Converts analog information into digital values for processing.
Quantization: The process of deriving a pixelated digital image by sampling a continuous tone image resulting in a matrix filled with grayscale values.
Corrections Applied:
Rescaling: Applied when the X-ray exposure is inadequate or excessive to produce a quality image.
Flat Fielding: A technique to correct uniformity issues in shading across images.
Dead Pixel Correction: Software methods to fill in missing data using surrounding pixel information.
Smoothing: Averaging the frequency of each pixel with adjacent pixels to reduce noise.
Data for Display
Look-Up Table (LUT): Utilized to apply desired contrast to displayed images.
Values of Interest (VOI): Establishes the range of histogram data that should be shown in the final image display.
Exposure Technique Charts
The introduction of digital imaging allows manipulation of image contrast and density, but it does not replace the necessity for traditional technique charts.
Technique Chart Utilization:
To choose the correct mA, kVp, and exposure time to minimize patient exposure.
Practice ALARA (As Low As Reasonably Achievable) principles.
Ethical consideration: It is unacceptable to set exposure levels high intentionally to avoid image repeats.
Exposure Indicators
Denote the quantity of exposure absorbed by the phosphors utilized in particular digital imaging systems.
Histogram: A graphical representation that maintains a record of exposure values captured by the imaging receptor.
Postprocessing
Health Level-7: Represents a communication standard for all medical information systems utilized within a hospital or clinic setup.
DICOM: The standard that ensures all medical imaging devices speak a common language.
DICOM Grayscale: Defines the response of a display system in such a way that the observer's perception of image contrast remains consistent across different pixel values throughout the image.
EMR (Electronic Medical Records): Represents the digital record of medical and clinical data within individual doctor offices.
EHR (Electronic Health Records): A system that aggregates data across multiple providers.
RIS (Radiology Information System): Manages radiological data.
HIS (Hospital Information System): Manages broader hospital data.
Picture Archival and Communication System (PACS)
PACS Role: Utilized for managing and storing images effectively.
PACS Network Components: