Overview
This lecture covers temporal resolution in ultrasound imaging, focusing on factors that affect how accurately moving structures are displayed and how to optimize settings for best image quality.
Introduction to Temporal Resolution
- Temporal resolution is the ability of an ultrasound machine to accurately display moving objects.
- High temporal resolution shows smooth, real-time movement; low temporal resolution results in a laggy, jumpy image.
- Real-time imaging is achieved by displaying rapid sequences of still frames, similar to a movie or flip book.
Frame Rate Fundamentals
- Frame rate is the number of images (frames) produced per second, measured in hertz (Hz).
- Human eyes need a frame rate of at least 30 Hz to perceive motion as fluid.
- High frame rates improve temporal resolution; low frame rates degrade it.
- Frame rate is inversely related to the time needed to create a frame (Tframe): Frame rate × Tframe = 1.
Factors Affecting Frame Rate
- Three main factors: propagation speed in tissue (fixed), imaging depth, and number of pulses per frame.
- Increased imaging depth increases Tframe and decreases frame rate.
- More pulses per frame also increase Tframe and decrease frame rate.
Calculations and Formulas
- Tframe = Number of pulses per frame × Pulse Repetition Period (PRP).
- Frame rate (Hz) = 1 / Tframe (in seconds).
- PRP increases with imaging depth (PRP = Depth × 13 µs/cm).
- Doubling depth or number of pulses doubles Tframe and halves frame rate.
Controlling Number of Pulses per Frame
- More foci per scan line increase pulses per frame, worsening temporal resolution but improving lateral resolution.
- Wider sector size increases scan lines and pulses, reducing temporal resolution.
- Higher line density (more scan lines per angle) increases pulses, improving spatial resolution but reducing temporal resolution.
Image Quality Trade-offs
- Shallow imaging, single focus, narrow sectors, and low line density improve frame rate and temporal resolution.
- Deep imaging, multifocus, wide sectors, and high line density improve spatial/lateral resolution but worsen temporal resolution.
- Sonographers must balance between optimal still image (spatial/lateral resolution) and moving image (temporal resolution).
Key Terms & Definitions
- Temporal Resolution — ability to display moving objects in correct position in real-time.
- Frame Rate — number of frames produced per second (Hz).
- Tframe — time to create one image frame.
- Pulse Repetition Period (PRP) — time for one pulse to travel to maximum depth and return.
- Spatial Resolution — ability to distinguish small details in an image.
- Lateral Resolution — ability to distinguish two objects side by side.
- Line Density — number of scan lines per angle in the image sector.
Action Items / Next Steps
- Complete workbook activities on temporal resolution.
- Review and answer the open-ended Nerd Check questions.