Chapter 4 Describing Pulsed Waves PDF

Chapter 4 DESCRIBING PULSED Waves  Describing Pulsed Waves In addition to the seven parameters that describe continuous waves, new terms and their definitions must be added to our vocabulary  Pulsed Ultrasound In diagnostic ultrasound, continuous wave soun...

Chapter 4 DESCRIBING PULSED Waves  Describing Pulsed Waves In addition to the seven parameters that describe continuous waves, new terms and their definitions must be added to our vocabulary  Pulsed Ultrasound In diagnostic ultrasound, continuous wave sound cannot create anatomic images. imaging systems produce short bursts or pulses of acoustic energy to create every picture of anatomy. Transmitting / “on time” The transducer is sending a pulse Receiving time / “off time” (when the transducer is not sending a pulse) 3 It is waiting for the reflection  WHAT IS PULSED SOUND? A pulse of ultrasound is a collection of cycles that travel together. A pulse must have a beginning and an end. Although a pulse is made up of individual cycles, the entire pulse moves as a single unit. A pulse is similar to a train. A train consist of individuals cars, but travels as a single entity. Similarly, a pulse consists of individual cycles, but propagates as a single unit  PULSED ULTRASOUND HAS 2 COMPONENTS Talking or TRANSMIT “ON”time TIME The transducer is sending the pulse OFF Listening or O RECEIVE TIME “OFF” time N When the transducer is not sending a pulse. It is waiting for the reflection  a PULSE is a PULSE is a PULSE…  We CANNOT change the characteristics of the actual pulse!  The characteristics of the PULSE are determined by the SOURCE (machine/transducer). PARAMETERS DESCRIBE PULSED SOUND PULSE DURATION SPATIAL PULSE PULSE REPETITION LENGTH PERIOD PULSE DUTY REPETITION FACTOR FREQUENCY  PULSE DURATION  The TIME from the START of the PULSE to the END of the PULSE PULSE DURATION DETERMINED BY THE PULSE SOURCE – DURATION CANNOT BE Pulse duration ADJUSTED (µs) = # cycles x  SPATIAL PULSE LENGTH The LENGTH or DISTANCE that a PULSE occupies in space. SPL IS DIRECTLY PROPORTIONAL DETERMINED TO WAVELENGTH (LONGER BY THE WAVELENGTH, LONGER SPL) SOURCE & MEDIUM – CANNOT BE ADJUSTED (THE LENGTH OF ONE CYCLE) Spatial pulse length (THE LENGTH OF THE ENTIRE (mm) = # cycles x PULSE)  PULSE REPETITION PERIOD The TIME from the START of one pulse to the START of the next pulse.  PULSE REPETITION PERIOD DETERMINED BY THE SOURCE YES! CAN BE ADJUSTED YES! USE YOUR DEPTH BUTTON TO ADJUST YOUR PRP!  PULSE REPETITION PERIOD & DEPTH When we INCREASE the DEPTH, we are INCREASING the TIME that it takes for a sound beam to return. INCREASED DECREASED DEPTH DEPTH  PRP IS DIRECTLY RELATED TO DEPTH When you INCREASE the DEPTH, PRP INCREASES Because you are increasing listening time.  Shallower Deeper imaging imaging requires requires LESS MORE LISTENING LISTENING TIME TIME Sonographer changes the LISTENING TIME not the PULSE; with a change in DEPTH! SHALLOW IMAGING LESS WAITING TIME FOR THE REFLECTION BEFORE ANOTHER PULSE IS SENT FROM THE TRANSDUCER  DEEP IMAGING MORE WAITING TIME FOR THE REFLECTION BEFORE ANOTHER PULSE IS SENT FROM THE TRANSDUCER  PULSE REPETITION FREQUENCY Number of PULSES that an ultrasound system transmits into the body EACH SECOND. Let’s break it down… – PULSE REPETITION = Number of Pulses – FREQUENCY = Cycles occurring in one second – Meaning…How often the PULSE can be REPEATED!  PULSE REPETITION FREQUENCY THE MORE SHALLOW YOU THE DEEPER YOU ARE THE ARE THE MORE FREQUENT LESS FREQUENT THE THE PULSE CAN BE PULSE CAN BE REPEATED REPEATED (LESS LISTENING TIME) (LONGER LISTENING TIME)  PULSE REPETITION FREQUENCY DETERMINED BY THE CAN PRF BE SOURCE ADJUSTED? PRF is unrelated to frequency. It is related to depth of view. USE YOUR DEPTH BUTTON TO ADJUST YOUR PRF reported in units of HZ, or per Second)  RELATIONSHIP OF PRP & PRF - Summary of PRP and PRF Relationship: - PRP and PRF are reciprocals: PRP (sec) x PRF (Hz) = 1 - They are inversely related: PRP (sec) = 1/ PRF (Hz) PRF (Hz) = 1/ PRP (sec)  DUTY FACTOR Duty Factor is the percentage or fraction of time that the ultrasound machine is producing a pulse or transmitting sound It is also called duty cycle % of time the transduce Maximum value = 1.0 or 100 % (CW) is sending a pulse Minimum value = 0.0OFF or 0% (machine off) 0% Units of measurements: unitless Duty factor (%) = pulse duration (sec) x 100 PRP (sec) Clinical imaging duty factor ranges = 0.002 to 0.005 ( 0.2 % to 0.5 % )  DUTY FACTOR The PERCENTAGE Duty factor= PD x The 100 or FRACTION of = time of the PRP Duty factor pulse time that the is a percentage system transmits The time from the a pulse. start of one pulse to the start of the next pulse PRP THE MORE SHALLOWER YOU ARE, THE HIGHER THE DUTY FACTOR Question……….? What is the duty factor of a wave that has a pulse repetition period of 30 microseconds and a pulse duration of 0.3 microseconds? A. 0.03 B. 0.90 C. 30.3 D. 0.01  DUTY FACTOR DETERMINED BY THE SOURCE CAN IT BE ADJUSTED YES! USE YOUR DEPTH BUTTON TO ADJUST YOUR DUTY FACTOR!  DUTY FACTOR DUTY FACTOR IS DIRECTLY RELATED TO PRF PRF DUTY FACTOR DEEP SHALLOW DUTY FACTOR IS INVERSELY RELATED TO DEPTH  Questions Which of the following will increase the Duty factor when using a pulsed wave probe? a.Increase the depth b.Decrease the depth c.Increase the frequency d.Decrease the frequency What is the duty factor if the pulse duration is 2 microseconds and the PRP IS 1 millisecond e.2% f. 0.5% g.0.2% h.1.0% ANSWER FOR THE PREVIOUS QUESTIONS DUTY FACTOR =PD/PRP micro -6 DF=0.000002s/0.001 milli -3 DF=0.002X100%=0.2% Which of the following terms does not belong with the others? A.Increased depth of view B.Decreased duty factor C.Increased pulse repetition period D.Decreased spatial pulse length The frequency of continuous acoustic wave is 5 Mhz. The wave is then pulsed with a duty factor of 0.1. What is the new frequency? A. 0.5 B. 0.5 Mhz C. 5 Mhz D.10 Mhz What are the units of pulse repetition frequency (PRF)? A.Seconds B.1/seconds C.mm/μs D.Seconds-2 True or False?. The sonographer alters the duty cycle when adjusting the maximum imaging depth of a scan. Two pulses are created by different transducers. Both waves have the same frequency. The pulse containing more cycles has a _________ A.Lower pulse repetition frequency B.Shorter pulse duration C.Longer pulse duration D.Longer pulse repetition period Which of these four pulses with PRFs listed below has the shortest pulse PRP? a. 12 kHz b. 6,000 Hz c. 20 kHz d. 1 kHz  What happens to the pulse repetition period if the sonographer decreases the maximum imaging depth achieved in an ultrasound scan? A. Increases B. Decreases C. Remains the same D.Cannot be determined Using a specific transducer, what happens to the spatial pulse length as the sonographer increases the maximum imaging depth? A.Increases B.Decreases C.Remains the same D.Cannot be determined