Physics: Chapter 7

PRF & maximum imaging depth (example) -For example: -When the depth of view is set to 7.7cm, PRF is 10,000Hz -When the depth of view is set to 15.4cm, PRF is 5,000Hz

-For example: -When the depth of view is set to 7.7cm, PRF is 10,000Hz (77,000/7.7 = 10,000) -When the depth of view is set to 15.4cm, PRF is 5,000Hz (77,000/15.4 = 5000) -So you just plug it in the formula (the 77,000 will always be divided by the imaging depth!)

Time-of-flight relation to depth...

•The time-of-flight is directly related to how deep a sound pulse travels. Greater distances prolong the time-of-flight ; lesser distances shorten the time-of-flight.

•The maximum imaging depth (depth of view) during an ultrasound exam is 10cm. The sonographer exam is 10cm. The sonographer adjusts the imaging depth to 20cm. What happens to our PRP? a.) It is unchanged b.) It is halved c.) It is doubled d.) It is 20 times longer.

First off, lets think about our relationship. How are depth of view & PRP related? Directly. So with our depth of view doubling, our PRP is going to double! So "C"!

•The maximum imaging depth (depth of view) during an ultrasound exam is 10cm. The sonographer exam is 10cm. The sonographer adjusts the imaging depth to 20cm. What happens to our PRF? a.) It is unchanged b.) It is halved c.) It is doubled d.) It is 20 times longer.

First off, lets think about our relationship. How are depth of view and PRF related? Inversely! With our imaging depth doubling, our PRF is going to be halved. So "B"!

The 13 microsecond rule (Chart)

NEED to KNOW the chart!! Look back at powerpoint print-off

•A sound wave is created by a transducer, reflects off an object and returns to the transducer. The depth of the reflector is 10cm in soft tissue. What is the go-return time? a.) 13 µs b.) 1.3µs c.) 65µs d.) 130µs

Our problem is giving us the distance, 10cm. We know for every 13 microseconds, the object creating the reflection is 1cm deeper in the body! So we multiply 10 x 13 to get 130! So "D"!

Go-return time When a reflector is twice as deep as another reflector, the pulse's time-of-flight is _____________ for the deeper reflector. In other words, the time of flight will be increased by a factor of ____. Depth (mm) =

When a reflector is twice as deep as another reflector, the pulse's time-of-flight is doubled for the deeper reflector. In other words, the time of flight will be increased by a factor of 2. Depth (mm) = 1.54mm/µs x go-return-time (µs) _____________________________________________ 2

The 13 microsecond rule On the exam, pay attention to the subtle differences in the terms depth & total distance. •Example: •A sound wave is created by a transducer, reflects off an object, and returns to the transducer. The go-return time is 26 µs. What is the depth of the reflector? •A.)1cm B.) 2cm •C.) 3cm D.) 4cm

•B - the reflector depth is 2cm! (This is just TO the reflector!) (Because for 13 microseconds, the object creating the reflection is 1cm deeper in the body!)

The 13 microsecond rule On the exam, pay attention to the subtle differences in the terms depth & total distance. •Example: •A sound wave is created by a transducer, reflects off an object, and returns to the transducer. The go-return time is 26 µs. What is the total distance of the reflector? •A.)1cm B.) 2cm •C.) 3cm D.) 4cm

•D - the total distance traveled (to the reflector and back) is TWICE the depth of the reflector.

The 13 microsecond rule (example)

•For example: - A pulse's time-of-flight is 13µs when a reflector is 1cm deep. -A pulse's time-of-flight is 26µs when a reflector is 2cm deep. -A pulse's time-of-flight is 39µs when a reflector is 3cm deep. Since a pulse travels to the reflector and back to the transducer, the TOTAL distance that a pulse travels is twice the reflector depth.

PRP & maximum imaging depth •For example: When the depth of view is set to 10cm, PRP is 130 µs •When the depth of view is set to 5cm, PRP is 65µs

•For example: When the depth of view is set to 10cm, PRP is 130 µs (10 x 13µs) •When the depth of view is set to 5cm, PRP is 65µs (5 x 13µs) •You just plug it into the formula!

Time-of-flight (definition and superficial vs. deep reflectors)

•This is the elapsed time from pulse creation to pulse reception. •When a reflector is located superficially, the time from pulse creation to reception is brief because it isn't very far. Whereas trips to and from deeper reflectors require more time. This is the basis for measuring distances in diagnostic imaging.

The 13-microsecond rule**

•This rule always applies when sound travels through soft tissue. For every 13 µs of go-return time the object creating the reflection is 1 cm deeper in the body

Range equation

•To create an anatomic image, a sound pulse must travel to a reflector located in the body and return to the transducer. The travel time of this journey allows us to accurately position the reflector.

Go-return time

•Ultrasound system's computers are programmed with the average speed of sound in soft tissue. Since the average speed of soft tissue Is known, the time-of-flight and the distance that the pulse travels are directly related.

Time-of-flight or go-return time

•Ultrasound systems determine reflector depth by measuring a pulse's time-of-flight with a very accurate stopwatch. • So in other words, time-of-flight aka go-return and aka round-trip time, is the time needed for a pulse to travel to and from the transducer and the reflector.

PRP & maximum imaging depth •When the depth of view is shallow remember, our PRP is _________. When the depth of view is deep we have a _______________ PRP. •PRP (µs) =

•When the depth of view is shallow remember, our PRP is short. When the depth of view is deep we have a longer PRP. •PRP (µs) = imaging depth x 13µs/cm

PRF & maximum imaging depth •When the depth of view is shallow, PRF is _______ and when depth of view is deep, PRF is ______. •Mathematically in soft tissue: •PRF (Hz) =

•When the depth of view is shallow, PRF is high and when depth of view is deep, PRF is low. •Mathematically in soft tissue: •PRF (Hz) = 77,000 cm/s -------------------- Imaging depth (cm)

PRP & maximum imaging depth

•When we adjust the system's maximum imaging depth, the PRP is altered. •Recall, PRP is the time from the start of one pulse to the start of the next pulse. Therefore, the PRP is the go-return time of a sound pulse between the transducer and the bottom of the image. The 13-microsecond rule applies.