Chapter 7: Range Equation

What is time-of-flight or go-return time

The elapsed time from pulse creation to pulse reception

When is there a short or long go-return/time-of-flight time

Shorter: when the reflector is more superficial

Longer: reflector is deeper into the body

What is the average speed of sound in soft tissue

1.54mm/µs

How is time-of-flight and distance that the pulse travels related

they are directly related.

when reflector is twice as deep in the body, the time-of-flight is twice as long.

Equation for calculating reflector depth

Depth (mm) = (1.54 mm/µs × go-return time (µs)) / 2

The pulse travels to the reflector and back, so the total travel distance is divided by 2.

What is the 13 microsecond rule and when does it apply

for every 13µs of go-return time, the object/reflector creating the reflection is 1cm deep.

this applies when sound travels through soft tissue

if a reflected pulse returns to the transducer after 26µs is was sent, how deep is the reflector

2cm.

26µs / 13µs/cm = 2cm

Why is the total distance traveled by an ultrasound pulse twice the value of the reflector depth

The pulse travels from the transducer to the reflector and then returns back to the transducer (travels the length of the depth twice)

What is pulse repetition period (PRP)

the time it takes from the start of one pulse to the start of the next

Knowing what PRP is, what is it in regards to when a pulse it sent/recieved

PRP is the go-return time of a sound pulse between the transducer and the bottom of the image (deepest part of depth)

How does max image depth affect PRP

shallow depth has short PRP

deep depth has long PRP

Equation for calculating PRP in soft tissue

PRP (µs) = imaging depth (cm) × 13 µs/cm

If max imaging depth is 10cm, what is the PRP

130µs.

10 × 13 = 130µs

How is PRP and maximum imaging depth related

they are directly related.

increasing depth increases the time needed until next pulse is sent

What happens to PRP is max imaging depth is doubled

PRP doubles since they are directly related

What is pulse repetition frequency (PRF)

number of pulses produced by the transducer each second

What is the speed of sound in soft tissue

1540m/s or 154000cm/s

At what depth can sound travel to and from through soft tissue in 1 second

77000cm. or it can travel 7.7cm 10000 times.

(77000cm may be a single round trip or divided into many thousands of trips)

How does max image depth affect PRF

shallow depth has high PRF

deep depth has low PRF

Equation for calculating PRF in soft tissue

PRF (Hz) = 77,000 / imaging depth (cm)

If maximum imaging depth is 7.7 cm, what is the PRF

10,000 Hz.

77,000 / 7.7 = 10,000 Hz

If maximum imaging depth is 15.4 cm, what is the PRF

5,000 Hz.

77,000 ÷ 15.4 = 5,000 Hz.

How are PRF and max imaging depth related

they are inversely related.

increasing imagine depth decreases PRF.

What happens to PRF if maximum imaging depth is doubled

PRF is cut in half because PRF is inversely related to maximum imaging depth

A sonographer increases the maximum imaging depth from 10 cm to 20 cm. What happens to PRP and PRF

PRP doubles and PRF is halved because PRP is directly related to depth, while PRF is inversely related to depth