Recommended Pulse Repetition Frequency (PRF) Settings in Ultrasound
The recommended Pulse Repetition Frequency (PRF) settings in ultrasound vary by application, with temporal arteries requiring 2-7 kHz, extracranial supra-aortic arteries needing 3-8 kHz, and lower frequencies (0.3-3 MHz) being optimal for microbubble-based applications. 1
PRF Settings by Anatomical Region
Vascular Applications
- For temporal arteries, the recommended PRF range is 2-7 kHz 1
- For extracranial supra-aortic arteries, a PRF of 3-8 kHz is commonly applied 1
- When imaging blood vessels with color Doppler, the PRF should be optimized to avoid underfilling or overfilling of the vessel lumen 1
- Flow parameters should be adjusted to maximize sensitivity for detecting vessels with low-velocity flow in the bowel wall 1
Tissue-Specific Applications
- For microbubble-based applications, the optimal frequency range is between 0.3-3 MHz, which corresponds to the resonance frequencies of commercial microbubbles (1-5 μm in diameter) 1
- For trans-scleral drug delivery applications, lower PRF settings are more effective as they increase penetration depth 1
- For morbidly obese patients, lower frequency settings (2-3 MHz) on curved array transducers are recommended for adequate tissue penetration 2
Technical Considerations for PRF Selection
Frequency-Penetration Relationship
- Lower frequencies provide greater tissue penetration but reduced resolution, making them suitable for deeper structures and obese patients 2
- Higher frequencies (≥15 MHz for temporal arteries, 7-15 MHz for extracranial arteries) provide better resolution but limited penetration 1
- Ultra-high frequency ultrasound (30-100 MHz) is appropriate only for superficial structures like skin, small vessels, and musculoskeletal anatomy 3
Flow Velocity Considerations
- PRF settings below 1.3 kHz may result in aliasing effects, particularly with higher flow velocities 4
- For flow velocities below 20 cm/s, PRF and wall motion filter settings significantly influence power Doppler quantification 4
- The color box should be angled in longitudinal scans to avoid perpendicularity between sound waves and the artery 1
Optimizing PRF for Specific Applications
Drug Delivery Applications
- For focused ultrasound applications in drug delivery, reducing the PRF (e.g., from 1 Hz to 0.25 Hz) can increase harmonic emissions and delivery efficiency 5
- In microbubble-based applications, the PRF should be matched to the resonant frequency of the microbubbles being used 1
- For trans-scleral drug delivery, lower frequencies (40 kHz vs. 500 kHz) significantly improve penetration depth 1
Image Quality Optimization
- The bandwidth of the ultrasound system directly affects image resolution - wider bandwidth provides better resolution 6
- For optimal image quality, the focus should be set at the level of the structure being examined 1
- B-mode gain should be adjusted to avoid anechoic appearance of vessel walls 1
Common Pitfalls and Limitations
- Using inappropriate PRF settings can lead to either underfilling (missing low-flow vessels) or overfilling (artifactual color outside vessels) 1
- Higher frequency transducers (>5 MHz) produce inadequate images in obese patients due to limited penetration depth 2
- Aliasing can affect power Doppler quantification, especially when using PRF settings below 1.3 kHz 4
- Wall motion filter settings interact with PRF selection and significantly impact power Doppler signal intensity 4
By selecting the appropriate PRF for the specific clinical application and anatomical region, optimal ultrasound imaging can be achieved while balancing the trade-offs between resolution and penetration depth.