Overview
This lecture covers how to calculate reflector depth in ultrasound imaging, focusing on pulse timing, formulas for pulse repetition period/frequency, and the 13-microsecond rule for depth determination.
Creating the Image & Pulse Timing
- Each outgoing pulse creates one scan line; a new pulse is sent once echoes from the last reach max imaging depth.
- The transducer waits for returning echoes, processes them, and then sends out the next pulse.
- If the pulse attenuates (weakens) before max depth, the far field appears black.
- The time from pulse creation to echo return is called Pulse Repetition Period (PRP).
PRP and PRF
- PRP is the total time from sending one pulse to sending the next (pulse plus wait time).
- Pulse Repetition Frequency (PRF) is the number of pulses sent per second.
- PRP and PRF are reciprocals: PRP × PRF = 1.
- Increasing imaging depth increases PRP (more wait time) and decreases PRF.
Calculating PRP & PRF
- Sound in soft tissue travels at 1.54 mm/μs.
- PRP = 13 μs × max imaging depth in cm.
- PRF = 77,000 cm/s ÷ max imaging depth (in cm).
- PRP and PRF depend only on max depth, not transducer frequency.
Practice Examples
- To find PRP: multiply imaging depth (cm) by 13 μs.
- To find PRF: divide 77,000 by imaging depth (cm).
- Depth ↑ → PRP ↑, PRF ↓.
Bonus Calculations (Period, Wavelength, Etc.)
- Period = 1 ÷ frequency (MHz).
- Wavelength = propagation speed ÷ frequency.
- Pulse Duration = number of cycles × period.
- Spatial Pulse Length = cycles × wavelength.
- Duty Factor = (pulse duration ÷ PRP) × 100%.
The 13-Microsecond Rule & Depth Calculation
- Each 13 μs of go-return time equals 1 cm reflector depth.
- Depth (cm) = go-return time (μs) ÷ 13.
- Formula: depth = (propagation speed × go-return time) ÷ 2.
- For total round-trip distance, do not divide by 2; for depth, always divide by 2.
Key Terms & Definitions
- Pulse Repetition Period (PRP) — Time from one pulse to the next, including wait time (μs).
- Pulse Repetition Frequency (PRF) — Number of pulses sent per second (Hz or kHz).
- 13-Microsecond Rule — Each 13 μs of echo time corresponds to 1 cm of depth in soft tissue.
- Go-return time (time of flight) — Total round-trip time for pulse and echo.
- Propagation speed — Rate sound travels in tissue (1.54 mm/μs).
Action Items / Next Steps
- Complete practice charts in the workbook using the PRP and PRF formulas.
- Use the 13-microsecond rule in practice problems to calculate reflector depth.
- Review and answer open-ended “nerd check” questions for self-assessment.