Unit 16: Compression and Dynamic Range

Introduction

• Compression & Dynamic Range: Used interchangeably but have specific meanings.
• Dynamic Range: Result of compression.
• Review of concepts previously discussed in relation to receiver functions and contrast resolution.

Section 16.1: Compression

• Two stages in compression.
  • First Compression: Automatic, not controlled by the sonographer.
  • Second Compression: Controlled by the sonographer.
• Three Rules of Compression:
  1. Largest signal remains largest.
  2. Smallest signal remains smallest.
  3. The range of signal choices is reduced without errors.

First Compression

• Automatic Process: Ensures no errors in image processing.
• Dynamic Range: Decreases as information is processed.
  • Transducer: Highest dynamic range (~120 dB).
  • Archive: Lowest dynamic range (10-30 dB).
• Ensures strong/weak signal hierarchy is maintained.
• Compression prevents loss of information.

Second Compression

• Controlled by Sonographer:
  • Adjusts gray scale spread to display reflector strengths.
  • High dynamic range: More grays, washed-out images.
  • Low dynamic range: Less grays, high contrast.

Examples of Compression

• Bathroom Scale Analogy:
  • Real weight vs. digital display range.
  • Compression maps real weights into scale's range.
• Age and Height Analogy:
  • Range of ages and corresponding height changes.
  • Compression reduced height difference while maintaining age range.

Application in Ultrasound Systems

• Transducer & Receiver:
  • Transducer handles a broad dynamic range (120 dB).
  • Receiver processes a reduced range (e.g. 70 dB).
  • Compression aligns signals to the receiver's dynamic range.

Dynamic Range Adjustments by Sonographer

• High Dynamic Range: More grays, low contrast, often results in washed-out images.
• Low Dynamic Range: Higher contrast, uses fewer grays, better differentiation.
• Clinical Considerations:
  • Vascular/Echo Imaging: Low dynamic range preferred for clear black/white contrast.
  • Tissue Imaging: High dynamic range can hide subtle differences.

Conclusion

• Key Takeaways:
  • Maintain hierarchy of signal strengths.
  • Adjust dynamic range for optimal imaging based on diagnostic needs.
  • Practice adjusting settings to understand the impact on image quality.
• Encouragement to experiment with dynamic range settings in clinical practice for better understanding and preparation for exams.