Chapter 4: Pulsed Waves

In Diagnostic medical ultrasound, we __________ use a sound wave that is continuous.

CANNOT

Pulsed Waves

• Anatomical imaging and Doppler calculations used to measure blood flow, relying on turning the sound wave OFF and listening to the returning echos

• The ultrasound system then keeps track of all the echoes, and the elaborate information in those echoes, which will then be represented in an image

Continuous Wave

• two crystals in the transducer

  • one constantly transmits (voltage continuously applied to crystal)

  • one constantly receives

• Echoes from every single depth (range ambiguity) are constantly received which means the machine cannot decipher depth to place echo (chaotic) so it CANNOT make an image

• Used in Doppler

• Not used in imaging

Pulsed Ultrasound

• A sound wave with multiple short bursts of sound, each followed by a resting period of no sound

• A set of burst of acoustic energy (voltage applied to the crystal intermittently)

• Each pulse of sound has multiple cycles all traveling as one

• NOT on continuously

• Requires one crystal that both transmits and receives.

Why Pulsed Waves?

• sound waves cannot be continuous in imaging because THEY MUST HAVE A BEGINNING AND AN END

• Sound must pause for a time of listening to receive information from the sound wave

Pulsed Wave Definition

Any modality which turns the transmitter on and off periodically to reduce range ambiguity

• PW Doppler, Color Doppler, 2D imaging, 3D imaging, and M-Mode

Acoustic Line

a single sound beam transmitted in a specific direction and the associated echoes

• the word beam is more accurate here because line makes you think of just one dimension

Receive Line

• the returning echoes registered by the system from a single direction over the time between the transmit event and the time until the next transmit occurs, as dictated by imaging depth

Display Line

(or image line) is the data displayed on the screen that corresponds to a single (or multiple) acoustic lines

Line

The general term used to refer to a beam

An Image

picture generated by sequentially transmitting many acoustic lines

• also called frame, scan, or scan region

Frame Time

time required to transmit multiple beams to create an image of the desired region

Frame Rate

reciprocal of frame time, frame frequency

Sampling Rate

the frequency at which signals are detected or viewed

Parameters to Describe PULSED sound waves

1. Pulse Duration

2. Spatial Pulse Length

3. Pulse Repetition Period

4. Pulse Repetition Frequency

5. Duty Factor

Pulse Duration (def, units, values, determined, adjustable)

• the actual time from the start of one pulse to the end of that pulse

  • duration = time

• The ON TIME of a pulsed wave

• UNITS: time (microseconds, ms, seconds)

• VALUES: 0.3-2.0 microseconds

• DETERMINED: by sound source ONLY

• ADJUSTABLE?: No

Pulse Duration Equation

PD = #cycles x period

PD = #cycles / frequency

• directly related/proportional to:

  • number of cycles in a pulse

  • period

• Inversely related/proportional to:

  • frequency

Pulse Duration Continued

• not all pulses are the same

• PD varies depending on modality (2D, Doppler, etc)

• The # of cycles in the pulse is part of determining the pulse duration

  • depends on the imaging modality (like 2D vs. Doppler)

• In 2D imaging:

  • SHORTER pulses = BETTER image quality

• PW Doppler requires a longer pulse (on time)

In pulse duration, Long duration = ???

• many cycles per pulse

• long periods per cycle

In pulse duration, Short duration = ???

• few cycles per pulse

• short periods per cycle

• shorter PD = greater image accuracy

If a pulse contains 10 cycles and the period of 0.2 microseconds, what is the PD?

2 microseconds

If a pulse contains 8 cycles and the frequency is 2MHz, what is the PD?

4 microseconds

Spatial Pulse Length (def, units, values, determined, adjustable)

• length or distance of a pulse

• UNITS: distance (mm)

• VALUES: 0.1-1.0 mm

• DETERMINED: by sound source AND medium (like wavelength)

• ADJUSTABLE: No

Spatial Pulse Length Equation

Spatial Pulse Length = #cycles x wavelength

• Directly proportional to:

  • number of cycles in a pulse

  • wavelength

• Inversely proportional to

  • frequency

***SPATIAL PULSE LENGTH AND PULSE DURATION MEASURE THE SAME THING IN DIFFERENT UNITS*******

In spatial pulse length, long pulses = ????

• many cycles

• long wavelengths per cycle

In spatial pulse length, short pulses = ????

• few cycles per pulse

• short wavelengths per cycle

• shorter pulses = greater image accuracy

Calculate SPL for 5MHz sound in soft tissue, 5 cycles per pulse

1.55 mm/pulse

Pulse Repetition Period (PRP) (def, units, values, determined, adjustable)

• time from start of one pulse to the start of the next (includes listening time)

• UNITS: time, microseconds

• VALUES: 100 microseconds - 1 ms

  • PRP is often given in sec, so you have to multiply by 1000 to get to the units of microseconds to calculate

• DETERMINED: by sound source ONLY

• ADJUSTABLE: Yes, when depth is adjusted, PRP is altered (only alters listening time, PD stays the same)

PRP Relationship

• Shallow Depth Imaging

  • time from one pulse to the next is short

• Deep Imaging

  • time from one pulse to the next is long

• As Depth increases, PRP increases

2 Components of PRP

1. Transmit time - one time (blue)

2. Receive time - off time (black)

Pulse Repetition Frequency (PRF) (def, units, values, determined, adjustable)

• number of pulses transmitted per second

• UNITS: Hz or per second

• VALUES: 1,000 - 10,000 Hz (or pulses per second)

• DETERMINED: by sound source ONLY

• ADJUSTABLE?: Yes, when depth is adjusted, PRF is altered

PRF Relationship

• shallow depth of imaging

  • more pulses are sent, as listening time decreases

  • less time to wait to send the next pulse

• deep imaging

  • less pulses are sent as listening time is longer

  • longer time to wait to send the next pulse

• As depth increases, PRF decreases

PRF and PRP Relationship

• inversely related

• reciprocals

• PRP = 1/PRF

• PRF = 1/PRP

• PRP x PRF = 1

Why is PRF so important?

PRF determines how quickly we can create an image

Duty Factor (def, units, value, determined, adjustable)

• Percentage or fraction of time a pulse is being transmitted

• the “on” time of a pulsed wave

• UNITS: none, %

• VALUES: in ultrasound, 0.2% is typical

  • about 500 times longer listening time than transmit time

• DETERMINED: by sound source ONLY

• ADJUSTABLE?: Yes, changing depth changes the DF
  
  

red line =

blue line =

red = PRP

blue = on time / transmit time / PD

Duty Factor Equation

DF = (PD / PRP) x 100

• max value = 100% or a factor of 1

  • in continuous sound waves

  • no listening “off” time

  • no anatomical image can be created with continuous wave

• min value = 0%

  • occurs when transducer is silent

  • ex: when we hit the freeze button

If you work 2 days out of 7 days, you work ____% of the time

29%

Duty Factor Relationship

• DF is proportional to the PD

• DF is inversely proportional to PRP

• DF = (PD/PRP) x 100

What happens to the DF if PRP increases?

DF decreases

What happens to the DF is PD increases?

DF increases

What happens to the DF if PRF increases?

DF increases (PRP and PRF are reciprocals)

What happens to the duty factor if imaging depth increases

DF decreases