Unit 13: Temporal Resolution

Introduction to Temporal Resolution

• Temporal resolution is the machine’s ability to accurately display moving objects.
• Reflectors can be physically moving within the body or the sonographer can be moving the transducer.
• Real-time imaging is a concept relating to this.

13.1 Real-time Imaging

• Early Ultrasound Imaging:
  • Static scanning involved physically moving a transducer to create scan lines for a single frame.
  • Articulating arm was used to move the transducer, creating one image at a time.
• Modern Real-time Imaging:
  • Automated scan line generation allows seamless real-time imaging.
  • Machine sends pulses, waits for echoes, and processes images quickly, giving the appearance of real-time.
• Analogy: Similar to movie frames played rapidly to create a perception of continuous motion.

13.2 Temporal Resolution

• Defines accuracy in displaying moving objects in their correct position.
• Frame Rate:
  • High frame rates (e.g., 60 fps) give seamless, movie-like images.
  • Low frame rates (below 30 fps) make motion appear jumpy.
  • Human eye needs at least 30 fps to perceive continuous motion.

13.3 Frame Rate

• Definition:
  • Number of frames produced per second, measured in Hertz (Hz).
  • High frame rate improves temporal resolution.
• Factors affecting Frame Rate:
  1. Sound Speed in Medium:
     • Constant at 1540 m/s in soft tissue.
  2. Depth of Imaging:
     • Greater depth increases time, reducing frame rate.
  3. Number of Pulses per Picture:
     • More pulses increase the time to create an image, reducing frame rate.
• Relation to Time (T frame):
  • Frame rate and time to make one frame (T frame) are reciprocals.
  • High frame rate = Shorter T frame.

Calculating Frame Rate and T frame

• Example Calculations:
  • T frame = Number of pulses x Pulse Repetition Period (PRP).
  • Frame rate = 1 / T frame.
• Effect of Imaging Depth:
  • Doubling depth doubles T frame and halves frame rate.

Impact of Pulses on Frame Rate

• Factors Increasing Pulse Number:
  • Multi-focus settings increase pulses per scan line.
  • Wider sector size requires more scan lines.
  • Higher line density increases scan lines per degree.
• Trade-offs:
  • Multi-focus improves lateral resolution but reduces temporal resolution.
  • Narrowing sector reduces scan lines, improving temporal resolution.

Section 13.4 Image Quality

• Sonographer’s Choices:
  • Decisions affect temporal resolution and other image qualities.
• Improving Temporal Resolution:
  • Shallow imaging and single focus improve frame rate.
  • Narrow sector and low line density also enhance temporal resolution.
• Trade-offs in Resolution:
  • Balancing temporal vs. lateral and spatial resolution.
  • Consider clinical needs: better still images or moving images.

Conclusion

• Key factors: depth, pulse number, and adjustments for optimal resolution.
• Understanding trade-offs helps in achieving desired imaging quality.
• Engage with workbook activities and discussion questions for deeper understanding.