Unit 7: Calculating Reflector Depth

Section 7.1: Creating the Image

Machines send a pulse out and wait for its return to create a scanline.
A whole image is created by multiple scanlines.
The time to create a whole image is based on maximum imaging depth and speed of sound.
Sound interacts with reflectors, returning some energy to the transducer while some is scattered or absorbed.
If sound attenuates before reaching max depth, the far field is black.
Echoes return to the transducer, for example:
- 1st echo at 13 microseconds displayed at 1 cm depth.
- 2nd echo at 39 microseconds, 3rd at 52 microseconds.
Pulse Repetition Period (PRP) = pulse time + waiting time.

PRP: time for pulse creation + waiting for echo return.
Pulse Repetition Frequency (PRF): number of pulses per second.
Shallow imaging: less off-time, higher PRF, shorter PRP.
Deep imaging: more waiting time, lower PRF, longer PRP.
PRP and PRF are reciprocals: PRP x PRF = 1.
Example for calculating PRP using propagation speed (1.54 mm/μs):
- Distance = Speed x Time; Time = Distance/Speed.
- Round-trip for 10 cm depth: 130 μs.

PRP and PRF help machine determine echo return time and initiate new pulse.
Machine uses a "big stopwatch" to time and map the body.
Reflector depth calculated using go-return time and speed (1.54 mm/μs).
13 Microsecond Rule: 1 cm depth = 13 μs round-trip.
Calculation Example:
- Reflector at 12 cm depth has a go-return time of 156 μs.
- 156 μs / 13 = 12 cm depth.
Total distance traveled = 2x depth (out and back).

Calculating PRP and PRF:
- PRP = 13 μs x max depth (cm).
- PRF = 77,000 cm/s / max depth (cm).
Example: 5 MHz transducer, depth 5 cm:
- PRP = 65 μs.
- PRF = 15,400 Hz.
Changing transducer frequency does not affect PRP and PRF, only max depth.

Activities include filling in charts using formulas.
Bonus exercises involve complex calculations for various ultrasound parameters.
Important questions:
- Calculation differences between reflector depth and total travel distance.