Overview
This lecture reviews the concepts of compression and dynamic range in ultrasound imaging, focusing on their roles in signal processing and image quality.
Compression and Dynamic Range: Concepts
- Compression and dynamic range are closely related; dynamic range is the outcome of compression.
- Dynamic range refers to the number of signal levels a system can display, measured in decibels (dB).
- Compression reduces the dynamic range so system components can process signals accurately.
Rules of Compression
- The largest signal must remain the largest after compression.
- The smallest signal must remain the smallest after compression.
- The choices (range) between the largest and smallest are reduced, condensing the range of signal strengths.
- Proper compression preserves signal hierarchy and introduces no errors.
First Compression (System Component Processing)
- First compression is automatic and not user-controlled.
- It occurs as signals move between system components (e.g., transducer, receiver, archive).
- Transducer has the widest dynamic range (~120 dB); archive has the narrowest (10–30 dB).
- Signals outside component limits are compressed to fit within the component's range.
Second Compression (Image Display)
- Second compression is controlled by the sonographer using the "compression" or "dynamic range" knob.
- Adjusting dynamic range changes the number of gray shades used to display signals.
- High (wide) dynamic range: more grays, low-contrast, washed-out images.
- Low (narrow) dynamic range: fewer grays, high-contrast, more black-and-white images.
Clinical Applications and Examples
- High dynamic range is useful for detecting low-level echoes but may hide subtle tissue differences.
- Low dynamic range enhances contrast but can hide weak echoes, possibly missing pathology.
- Sonographers must balance dynamic range setting based on diagnostic needs.
Key Terms & Definitions
- Dynamic Range — The span between the largest and smallest signal a system can process or display, measured in decibels (dB).
- Compression — The process of reducing the dynamic range so system components or displays can handle signals without error.
- First Compression — Automatic, system-based reduction of dynamic range between components.
- Second Compression — User-controlled adjustment of grayscale mapping for image display.
- Contrast Resolution — The ability to distinguish between differences in echo intensity on an image.
Action Items / Next Steps
- Experiment with dynamic range settings during scanning to observe effects on image contrast and detail.
- Complete workbook activities and open-ended "nerd check" questions related to this unit.