Pulsed Waves

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Pulsed Waves
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Topics

• PW Defined
• PRF
• PRP
• Distance Equation
• Roundtrip Effect
• PD
• Duty Factor
• Intensity
• Spatial Intensity
• Temporal Intensity
• Intensity Instruments

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Pulsed Wave Ultrasound

• The transducer used to produce continuous wave
  (CW) US is busy transmitting sound and has no
  time to listen for a returning echo. Therefore
  it cannot produce an image.
• The transducer used to produced pulsed wave (PW)
  US is designed to transmit sound pulses then
  pauses to listen for a returning echo.
• Imaging requires a transmitter a receiver

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Pulsed Wave Ultrasound

• a few cycles of US followed by a gap of time with
  no US

The gap (pause) is used by the transducer to
listen for returning echoes (receiving time)
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Pulses generate scan lines
Each sound pulse represents one scan
line. Multiple scan lines create a single frame
or image.
Pulse 1
Pulse 2
Pulse 3
Time
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RECALL!

• Frequency
• Wavelength
• Period
• PW uses these more

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Pulse Repetition Frequency (PRF)

• of pulses in 1 sec.
• Dx. US has a few thousand pulses per second PRF
  is expressed in kHz.

1000 ?s
What is the PRF of this pulsed wave?
2 kHz
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Pulse Repetition Period (PRP)

• Time from the beginning of one pulse to the
  beginning of the next pulse expressed in ?s
• Determined by the system

time
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If PRF ?, then PRP ?
time
1
PRP - 500 ?s
time
2
PRP - 250 ?s
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PRF PRP are reciprocals

• PRF (kHz) X PRP (ms) 1
• PRF 1/PRP
• and
• PRP 1/PRF
• Determine the PRF in the previous slide

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Just a little diversion

• How long does it take you to travel 150 miles if
  you drive at 50 mph?
• Distance speed X time or
• Distance speed time
• How long does it take sound to travel 1 cm. into
  the body?

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Distance Equation

• Distance speed time
• 1 cm 1540 m/sec.
• 1 cm
• m 1/154000 sec.
• .0000065 sec. 6.5 µsec.

1 sec. 1540 m
1 100
1 sec. 1540 m
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Lets KICK IT UP A NOTCH!

• How long in time does it take for the sound echo
  to travel 1 cm. up through the body to return to
  the transducer?
• How long does it take a pulse of sound to
  complete a round trip through 1 cm. of ST?
• This is called the Roundtrip Effect

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• Dont you think the US system may get confused
  where an echo should show up on your monitor if
  it sends another pulse before the deep echo
  returns???
• So what can a sonographer do to help?

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• Tell the machine how long to wait to listen for
  the deep echo to return by adjusting the depth
  control to the deepest level you want to hear
  the echo from.
• Adjusting the depth is actually adjusting the
  pulse repetition period.

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Pulse Duration (PD)

• Time for 1 pulse to occur
• AKA transmit time
• Dx US 2 or 3 cycles/pulse
• Units - ?s
• PD range .5 - 3 ?s
• Doppler US 5-20 cycles/pulse
• Shorter pulses ? the quality of the images

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Pulse Duration - PD
time
PULSE DURATION (2 ms)
Pulse duration (ms) cycles/pulse x period
(ms) Pulse duration (ms) cycles/pulse ? ƒ
(kHz)
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If PRF ?, then PRP ? Pulse Duration ?
PD
time
PRP - 500?s
PD
time
PRP - 250?s
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Duty Factor (Duty Cycle)

• Fraction (or ) of time that pulsed US is
  transmitting (on) always between 0 and 1
  (0-100)
• 0 indicates that no pulse occurring
• 1 indicates that the pulse is on all of the
  time, meaning that it is not PW US but rather
  CW US

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Duty Factor Ranges

• 2D (B-mode) .01 - 1.0
• Doppler US .05 - 5.0

What implications do you think this might have?
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• pulse duration (?s)
• pulse repetition period (?s)

Duty factor
PRP 1000 ?s
time
PD 500 ?s
DF 500/1000 .5 or the pulse is on 50
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What is the PRF?
PRP 1000 ?s
time
PD 500 ?s
1 kHz
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What is the PD if the DF is .25 ?
500 ?s
PRP 2000 ?s
time
What is the PRF?
.5 kHZ
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Spatial Pulse Length (SPL)

• Length of space a pulse occupies is measured in
  mm.
• Shorter SPL improves image resolution

SPL
length
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SPL cycles in pulse X ? (mm)

• ? SPL ? with ? wavelength
• ? cycles/pulse
• ? frequency

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• For all of you who have ever told me that you
  like AP better than Physics . . .
• We have just completed the anatomy of a wave
  next will be its physiology

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Up another notch

• Lets add someIntensity!

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Intensity

• Important when considering bioeffects
• Changed by the operator using the output power
  control to change the wave amplitude
• Directly related to power if power is doubled,
  the intensity doubles
• Proportional to wave amplitude squared (I ? A2)

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Sound beams are not uniform
They vary in intensity depending on the location
time where the measurement is taken in the beam

• Intensity usually highest in the center
  weakest in the periphery
• Intensity varies with time in pulsed US
• Intensity is not constant within the pulse

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Intensity is usually highest in the center
weakest in the periphery
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Intensity varies with time in PW US
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Intensity is not constant within the pulse
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Because Intensity varies

• Specific terms are used to describe variation in
  intensities associated with clinical ultrasound
• Spatial - refers to a location or space
• Temporal - refers to time
• Peak - maximum value
• Average - mean value

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Spatial Intensities

• Center of the sound beam is more intense than the
  edges

Spatial peak intensity Isp - Maximum beams
intensity
Spatial average intensity Isa - Ave of beams
intensity
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Spatial Intensities

• Spatial Peak (SP) greatest intensity in the
  sound field usually at the center
• Spatial Average (SA) average intensity in the
  sound beam field
• SP/SA Factor (BUC - Beam Uniformity Coefficient)
  describes the distribution of a beam in space
• Must be ? 1
• Relates to space (distance) as duty factor
  relates to time

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Temporal Intensities

• Temporal Peak (TP) - greatest intensity in the
  pulse as it passes by. Since it doesnt include
  the pulses off time, it is always ? the
  average. Only in CW (w/ constant amplitudes) is
  the TP TA.
• Temporal Average (TA) - average intensity across
  the PRP (includes when the pulse is on and
  off). Only in CW is TP TA because there is no
  off time.

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Pulse Average (PA)
- average intensity over the PD (time when the
pulse is on). Only describes a PW US CW US
doesnt have pulses to average.
Pulse Average
Temporal Peak
Intensity
Temporal Average
Time
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Why is the TA lower than the PA?

• TA averages the intensity over the entire PRP so
  when the transducer is receiving (listening for
  the returning echo) the intensity is minimal thus
  lowering the average (liken this to a low test
  score that brings down your grade average)

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• PA TA are related to the duty factor
• TA PA X DF Recall DF PD X PRF
• If the duty factor is 1 (100), then we are
  describing a continuous wave.
•  
• What are we describing when the duty factor is
  0 ?

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HOWEVER, Physicists like to measure the beam
over a certain area over a certain amount of
time. SO. By combining the spatial temporal
values, 6 intensities can be measured
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Intensities can be converted

• to another intensity by using the duty factor
  or the SP/SA factor.
• SPTP SPTA SPPA
• SATP SATA SAPA

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Example

• A waves SPPA intensity was measured at 400
  mW/cm2 a duty factor of 25.
• What is its SPTA?

Recall that SPTA SPPA x Duty Factor SPTA 400
mW/Cm2 x .25 SPTA 100 mW/cm2
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Example
A waves SPPA intensity was measured at 400
mW/cm2, a DF of 25 SP/SA of 5. What is its
SATA?

• Recall that
• SATA SPTA ?(SP/SA)
• SATA 100 mW/cm2 ? 5
• SATA 20 mW/cm2

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Ranking the 7 ways to measure intensity in order
from largest to smallest

1. SPTP highest intensity measurement used
   in Dx. US
2. Im ave. intensity measured in most
   intense half cycle (similar in value
   to SPTP)
3. SPPA (used only for PW US)
4. SPTA used to measure biological effects
5. SATP
6. SAPA (used only for PW US)
7. SATA lowest intensity measurement used
   in Dx. US

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Determining the Sound Beam Intensity

• Various methods are performed by the
  manufacturer or a physicist to determine the
  intensity of the US beam (requires special
  equipment)

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Equipment to Determine the Intensity of a Sound
Beam

• Radiation force balance or scale determines the
  intensity or power of the sound beam by
  measuring the force the sound beam exerts on the
  balance or scale.

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Equipment to Determine the Intensity of a Sound
Beam
Hydrophone system a hydrophone and probe are
placed in a water bath in the field of the
emitted sound beam. The output is calibrated to
indicate pressure or intensity.
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• What ways can a sonographer decrease a patients
  chance for bioeffects from US?

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Formulas to remember!

• c ? x ?
• ? x t (period) 1
• DF PD/PRP

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ƒ cycles/sec T time for 1 cycle ƒ x T
1 ƒ 1/T T 1/ƒ c ƒ x ? ? distance of 1
cycle