Digital Imaging: Chapters 1-3
Types of X-Ray Imaging
Three main types of X-Ray imaging:
Conventional Radiology (CR)
Also referred to as "Old School," nearly obsolete.
Utilizes intensifying screens and chemicals to create images.
Processing occurs in a dark room.
Computed Radiography (CR)
Stands for Photostimulable Plates (PSP).
Digital Radiography (DR)
Further characterized as Direct and Indirect.
Can operate without cassettes (Cassetless).
Conventional Film/Screen Radiography
Film and intensifying screens (F/S)
Employs film either on one screen or between two screens.
Function: Screens emit light when struck by X-rays, exposing the film chemically.
Resulting images are viewed on a lightbox after processing.
Computed Radiography (CR)
Photostimulable Phosphor (PSP)
Previously known as CR; now more accurate to avoid confusion.
First introduced in the U.S. by Fuji Medical Systems in 1983; acceptance was slow.
Increased installations seen in the early 1990s.
A 1996 law reducing Medicare reimbursements starting in 2018 may decrease PSP use.
Equipment for PSP Image Capture
Components:
Existing radiographic equipment, PSP storage plates, PSP reader, Technologist QC workstation, and viewing stations (either printer or both).
Storage phosphor plates:
Function like intensifying screens.
Store x-ray energy for extended periods.
Active element: Barium Fluorohalide.
Differences between CCD and ADC
Charge Coupled Device (CCD)
Represents an upgrade where the Photomultiplier tube and Analog to Digital Converter are integrated.
Cannot contain both a CCD and a Photomultiplier tube.
Latent vs. Manifest Image
Latent Image: The initial image prior to processing.
Manifest Image: The finalized image that appears on the screen post-processing.
Process of Accessing the Latent Image: A focused laser light scans the latent image on the phosphor plate to release the image.
Latent Image Formation
Composed of Barium Fluorohalide with Europium activator.
Stored in a cassette where some light is emitted as x-rays strike it, forming the latent image.
When processed, a laser causes electrons to revert to their original state, emitting light, which the photomultiplier tube converts into an electrical signal.
This signal is sent through an ADC to create a digital image for technologist review.
Importance of Prompt Image Processing
Delay Effects: Waiting too long before scanning a PSP plate results in energy loss from the electrons and degradation of image quality.
Digital Radiography (DR)
Indirect DR Components
Two Types of Processing:
Thin Film Transistors (TFT)
Comprises three layers: Scintillation, Photodiode (converts light into electric charge), and TFT (a matrix of Detector Elements - DELs).
Charged Coupled Device (CCD)
Two layers: Scintillation and Sensor Chip.
Responsible for converting light photons directly into an electrical signal sent to the computer without TFT and Photodiode layers.
Direct DR Components
Direct Conversion (Non-Scintillation)
Utilizes Amorphous Silicon-based Semiconductor with TFT layers.
Comparison of Direct & Indirect DR
Direct Conversion: Non-Scintillation, Charged Coupling Device, Semiconductor.
Indirect Conversion: Involves Thin Flat Panels and converts X-ray energy indirectly.
Comparison of Imaging Systems
Conventional vs. Digital Systems:
Equipment:
Conventional F/S and PSP utilize standard x-ray rooms.
Digital systems may use flat panel detectors (FPD) that can replace existing table and wall settings or be portable.
Efficiency:
Efficiency ratings for both Conventional F/S and PSP are similar.
FPD is significantly more efficient with instantaneous image availability.
Post-Processing of Images
PACS (Picture Archival and Communication Systems)
Definition & Purpose:
A network of computers, servers, and archives for managing and storing digital images.
Accepts DICOM format images and acts as a file room, reading room, duplicator, and courier.
Facilitates on-demand image access for multiple users, electronic annotations, and specialized image processing.
Components of PACS
Diverse functions include:
Reading stations, physician review stations, specialty workstations, web access, quality control stations, and administrative systems.
DICOM (Digital Imaging and Communications in Medicine)
Definition: DICOM is the medical language specific to imaging modalities such as Sono, CT, MRI, Fluoro, and X-Ray.
Only imaging uses DICOM (ONLY RADIOLOGY)
RIS (Radiology Information System)
Definition: Where to find all information about patient or etc.
Detailed Mechanics of Digital Radiography (Pre-Processing)
Understanding Pixels
Definition of Pixel: "Picture Element" - the smallest component in digital images.
Pixel size influences the image spatial resolution (detail).
Smaller pixels = higher spatial resolution.
Pixel size changes depending on matrix size and the field of view (FOV).
Bit Depth
Definition: Refers to the number of bits per pixel.
Number of gray tones produced is equal to .
Example: An 8-bit pixel yields shades of gray.
Matrix Structure
Definition: A square arrangement of pixels in rows and columns, corresponding to pixel values and specific image areas.
Size determines pixel dimensions. For example,:
A 10 × 12 and 14 × 17 PSP cassette with both having a 512 × 512 matrix:
The 10 × 12 will have smaller pixels.
Effect of Matrix Size on Resolution
Increasing the matrix size for a constant FOV results in:
Greater pixel count.
Smaller pixel dimensions improving image quality.
Examples of matrix sizes include:
(A) 64 × 64, (B) 215 × 215, (C) 1024 × 1024, (D) 2048 × 2048.
Field of View (FOV)
Definition: The area of the body part imaged, reflecting collimation size.
Larger FOV = larger imaged area.
Relationship with Matrix:
Changing the FOV does not affect the matrix size.
Adjusting the matrix affects pixel size.
Relationship Summary: FOV, Matrix Size, and Pixel Size
Swapping FOV and matrix sizes affects pixel dimensions. For instance:
Increasing FOV without matrix adjustment increases pixel size leading to decreased spatial resolution.
Exposure Indicators and Standardization
Exposure Index Definition
Refers to the exposure received by the Image Receptor (IR), not the patient.
Key for balancing exposure factors while minimizing patient exposure.
Variability in numerical representation among manufacturers complicates standardization.
Important Terms in Exposure Standardization
Air Kerma: Measurement of radiation energy per unit mass in air, reported in J/kg or Gy.
Standardized Radiation Exposure (KSTD): Represents standard exposure for a specific imaging system, under conditions simulating patient tissue.
Indicated Equivalent Air Kerma (KIND): Measures radiation incident on the IR, derived from pixel values during processing, visually compared to KSTD for exposure analysis.
Target Equivalent Air Kerma Value (KTGT): Specifically set optimal exposure values by body part and imaging view.
Deviation Index (DI): Difference between actual (KIND) and target (KTGT) exposures, helps detect under or overexposure conditions.
Dynamic Range: Indicates the detector's ability to respond to varying levels of exposure.
Characteristics of Digital Images
Key Characteristics
Brightness: Appearance of image lightness/darkness (display quality).
Contrast Resolution: Ability to portray subtle gray changes; relies on pixel bit depth.
Spatial Resolution: Capability to show tiny object details; enhanced by smaller pixel size.
Noise: Often results from low mAs levels affecting the image's quality.
Contrast Variations
Long Scale Contrast (Low Contrast): Multiple shades, smooth appearance, lower differences.
Short Scale Contrast (High Contrast): Fewer shades, pronounced black/white differences, increased tissue differentiation.
Adjustments to kVp can shift the contrast scale.
Image Quality Assessment Parameters
Detective Quantum Efficiency (DQE): Efficiency of image conversion from X-ray input.
Modular Transfer Function (MTF): Reflects system performance in transferring contrast across different spatial frequencies.
Signal-to-Noise Ratio (SNR): Measurement of net information compared to background noise; high SNR indicates clearer images.
Contrast-to-Noise Ratio (CNR): Assesses the visibility of structures despite noise presence.
Post-Processing Image Techniques
Histogram Interaction
Function: Recognizes anatomical data, searches for collimation edges while eliminating scatter & incorrect data collections.
Histogram characteristics determine brightness, and exposure impacts the histogram's width and shape.
Automatic Rescaling and Look-Up Table (LUT)
Automatic Rescaling: Balances output image brightness/contrast regardless of exposure amount, though not a substitute for proper exposure techniques.
Look-Up Table (LUT):
A histogram detailing luminance values utilized to correct brightness and contrast in images, influencing overall image output quality.
Spatial Frequency Filtering Techniques
Edge Enhancement (High-Pass Filtering): Increases visibility of structural details but risks high-frequency noise.
Smoothing (Low-Pass Filtering): Reduces noise via averaging pixel values, enhancing small structures at potential cost of larger details.
Image Manipulation Techniques
Window Level and Width Controls
Window Level: Adjusts image brightness.
Window Width: Determines contrast ratio; quick manipulation facilitated by mouse movement controls.
Background Management (Shutter) Techniques
Veil Glare: Excess light causing temporary blindness, mitigated through proper shuttering methods.
Image Stitching and Annotation Methods
Stitching: Combines multiple images for larger anatomical areas, enhancing processing capabilities.
Annotation: Critical for added non-standard identification information; should not substitute anatomical markers.
Magnification Techniques for Detailed Imaging
Types:
Magnifying glass method: isolates small anatomy portions.
Zoom method: enlarges entire images facilitating selective viewing.
Patient Identification and Demographics Input
Critical for digital imaging: Incorrect identification hampers retrieval.
Required demographics include name, facility, patient ID, DOB, and exam date, tracked for accuracy and accountability before processing.