chapter 2
Pulsed Ultrasound
In diagnostic ultrasound imaging, short bursts, or pulses, of
acoustic energy are used to create anatomic images.
Continuous wave sound cannot create anatomic images.
Definition
Analogy
Two Components
of Pulsed Ultrasound
A pulse is a collection of cycles that travel together.
Imagine a pulse as a train: although there are individual
cycles (cars) that make up the pulse (train), the pulse
moves as one.
•
the cycles (“on” or “transmit” time), and
•
the dead time (“off” or “receive” time)
Five Additional Parameters
of Pulsed Ultrasound
Pulse Duration
ls e
l Pu
a
i
t
Spa ength
L
Pulse Repetition Period
Pulse Repetition Frequency
Duty Factor
Note:
Edelman · Ultrasound Physics
Parameters describe the features of sound
Pulsed Ultrasound · 9
Pulse Duration
Definition
Units
Determined By
The time from the start of a pulse to the end of that pulse,
the actual time that the pulse is “on”.
seconds—any unit of time
Sound source
Pulse duration is determined by multiplying the number of
cycles in the pulse and the period of each cycle.
Changed by Sonographer
No, does not change when sonographer alters imaging depth.
Pulse duration is a characteristic of each transducer.
Typical Values
In clinical imaging, pulse duration ranges from 0.5 to 3 secs.
In clinical imaging, a pulse is comprised of 2–4 cycles.
Remember
A pulse is a pulse is a pulse
pulse
duration
time
10 · Pulsed Ultrasound
ESP, Inc.© 2013
Spatial Pulse Length
Definition
Units
Determined By
Changed by Sonographer
Example
Typical Values
Note
The length or distance that a pulse occupies in space. The
distance from the start to the end of one pulse.
mm—any unit of distance
Both the source and the medium
No
For SPL, think of a train (the pulse), made up of cars
(individual cycles). The overall length of our imaginary
train from the front of the locomotive to the end of the
caboose.
0.1 to 1mm.
SPL determines axial resolution (image accuracy) .
Shorter pulses create images of greater accuracy.
spatial pulse
length
distance
Edelman · Ultrasound Physics
Pulsed Ultrasound · 11
Pulse Repetition Period
Definition
HINT:
Units
Determined By
Changed by Sonographer
Pulse repetition period is the time from the start of one pulse
to the start of the next pulse. It includes one pulse
duration and one “listening time.”
PRP is determined only by depth of view.
ms—any units of time
Sound source
Yes
The operator changes only the “listening time” (when
adjusting the depth of view), never the pulse duration.
Typical Values
Note:
In clinical imaging, the PR period ranges from 100 seconds
to 1ms.
In reality, the listening time is hundreds of times longer than
the talking time (pulse duration).
Deeper Imaging
Time
Shallower Imaging
Time
Hint: Tap your fingers.
Remember
A pulse is a pulse is a pulse!
As imaging depth increases, PR period increases.
As imaging depth decreases, PR period decreases.
12 · Pulsed Ultrasound
The sonographer changes
the
listening time between pulses.
ESP, Inc.© 2013
PRF - (Pulse Repetition Frequency)
Definition
PRF is the number of pulses that occur in one second.
PRF is the reciprocal of pulse repetition period.
HINT:
Units
Determined By
Changed by Sonographer
Typical values
Notes
Shallow image, higher PRF.
PRF is determined by depth of view, PRF is entirely unrelated
to frequency.
Hertz, hz, per second
Sound source
Yes
In clinical imaging, from 1,000–10,000Hz (1-10kHz)
•
The PRF depends only upon imaging depth. It is unrelated
to the transducer frequency.
•
As imaging depth increases, PRF decreases (inverse
relationship).
•
In most cases, only one pulse of US travels in the body at
one time. Thus, as imaging depths changes, PRF changes.
Deeper image, lower PRF.
Since depth of view is changed by the sonographer, the
sonographer changes the PRF.
Similarly, the operator also adjusts the pulse repetition
period.
Time
Relationship
Edelman · Ultrasound Physics
•
Pulse repetition period and pulse repetition frequency are
reciprocals. Therefore, pulse repetition period also
depends upon imaging depth.
Pulsed Ultrasound · 13
Duty Factor
Definition
The percentage or fraction of time that the system
transmits sound. Important when discussing intensities.
Units
» 1.0 or 100% for continuous wave sound Unitless!
» typically less than 1%Minimum = 0.0 or 0%
Determined By
Changed by Sonographer
Typical values
Sound source
Yes
From 0.1% to 1% (little talking, lots of listening).
As we know, the operator adjusts imaging depth and thereby
determines the pulse repetition period. Thus, the operator
indirectly changes duty factor while altering depth of view.
Note
Important Concept
CW sound cannot create anatomical images.
Terms That Have the Same Meaning
•
shallow imaging
•
short pulse repetition period
•
high PRF
•
high duty factor
Parameters that Describe Both Pulsed and
Continuous Waves
14 · Pulsed Ultrasound
Period
Frequency
Wavelength
Propagation Speed
ESP, Inc.© 2013
Parameters of Pulsed Waves
Parameters
Basic Units
Units
Determined By
Typical Values
pulse duration
time
seconds, time
sound source
0.5–3.0 sec
pulse repetition period
time
mseconds, time
sound source
0.1–1.0 msec
pulse repetition frequency
1/time
1/sec, Hz
sound source
1–10 kHz
spatial pulse length
distance
mm
source & medium
0.1–1.0 mm
duty factor
none
none
sound Source
0.001–0.01
By adjusting the imaging depth, the operator changes the pulse repetition period,
pulse repetition frequency, and duty factor.
The pulse duration and spatial pulse length are characteristics of the pulse itself and are
inherent in the design of the transducer system. They are not changed by sonographer.
The Skinny II
all determined by sound source
a pulse is a pulse
is a pulse
change with
depth of
view
pulse duration
e
uls
p
l
tia
spa ngth
le
determined by both
source & medium
pulse repetition period
pulse repetition frequency
duty factor
Edelman · Ultrasound Physics
Pulsed Ultrasound · 15
Review 1—Pulsed Waves
1.
____________ is the time from the start of a
pulse to the end of that pulse.
2.
____________ is the time from the start of a
pulse to the start of the next pulse.
3.
Pulse repetition frequency is the reciprocal of ___.
4.
What is the duty factor of the following 4 signals?
(To determine a duty factor, use one transmit and
receive times, then determine the ratio.)
a
Answers
1.
Pulse Duration is the time from the start of a pulse
to the end of that pulse.
2.
Pulse Repetition Period is the time from the start
of a pulse to the start of the next pulse.
3.
Pulse repetition frequency is the reciprocal of Pulse
Repetition Period.
4.
a.
b.
c.
d.
b
0.5
0.0
0.333
1.0
c
d
5.
Which of the patterns in question #5 indicate a
system with a superficial imaging depth?
6.
Which of the patterns in question #5 indicate a
system with the deepest imaging depth?
7.
Which two patterns in #4 identify an ultrasound
system that cannot create an image of anatomy?
8.
By changing the imaging depth, which of the
following parameters does the operator also
change? (more than one may be correct)
a.
b.
c.
d.
e.
f.
9.
pulse repetition frequency
duty factor
propagation speed
pulse repetition period
amplitude
spatial pulse length
5.
Choice A has the shallowest imaging depth
because the pulse repetition period is the shortest.
The pulse repetition frequency is the highest.
6.
Choice C has the deepest imaging depth because
the pulse repetition period is the longest. The
pulse repetition frequency is the lowest.
7.
System D cannot perform imaging because it
produces a continuous ultrasound beam. Only
pulsed ultrasound systems can create an image of
anatomy. Additionally, system B cannot perform
imaging because it does not produce acoustic
waves.
8.
a.pulse repetition frequency
b.duty factor
d. pulse repetition period
9.
The propagation speed for pulsed and continuous
US is the same and depends only upon the
medium through which the sound travels. The new
propagation speed is exactly the same as the old
propagation speed, 1.8km/sec.
The speed of a 5MHz continuous wave is
1.8km/sec. The wave is then pulsed with a duty
factor of 0.5. What is the new propagation speed?
16 · Pulsed Ultrasound
ESP, Inc.© 2013
Review 2—Pulsed Waves
1.
2.
Which of the following best describes line A?
a. frequency
b. pulse repetition period
c. period
d. pulse duration
e. duty factor
f. amplitude
5.
Which of the following best describes line B?
6.
a.
b.
c.
d.
e.
f.
3.
a.
b.
c.
d.
e.
f.
4.
7.
frequency
pulse repetition period
period
pulse duration
duty factor
amplitude
frequency
peak-to peak amplitude
period
pulse duration
duty factor
amplitude
Which of the following best describes line A?
a.
b.
c.
d.
e.
f.
8.
frequency
pulse repetition period
period
listening time
peak-to peak amplitude
amplitude
Which of the following best describes line F?
a.
b.
c.
d.
e.
f.
frequency
pulse repetition period
period
pulse duration
duty factor
amplitude
Which of the following best describes line D?
a.
b.
c.
d.
e.
f.
a.
b.
c.
d.
e.
f.
frequency
pulse repetition period
period
pulse duration
duty factor
amplitude
Which of the following best describes line C?
Which of the following best describes line E?
frequency
pulse repetition period
period
spatial pulse length
duty factor
amplitude
Which of the following best describes line D?
a.
b.
c.
d.
e.
f.
frequency
pulse repetition period
wavelength
pulse duration
duty factor
amplitude
Answers
1. d 2. f 3. b 4. c 5. d 6. b 7. d 8. c
Edelman · Ultrasound Physics
Pulsed Ultrasound · 17
Review 3—Pulsed Waves
1.
2.
A sonographer adjusts the depth of view of an
ultrasound scan from 8cm to 16cm. What happens
to each of the following parameters? Do they
increase, decrease or remain the same?
a. period
b. frequency
c. wavelength
d. speed
e. intensity (initial)
f. pulse duration
g. PRF
h. duty factor
i. spatial pulse length
j. pulse repetition period
A sonographer is using a 3MHz transducer and
changes to a 6MHz transducer. The imaging depth
remains unchanged. What will happen to each of
the following parameters? Do they increase,
decrease or remain the same?
a.
b.
c.
d.
e.
f.
g.
3.
period
frequency
wavelength
speed
intensity (initial)
PRF
pulse repetition period
A sonographer is using a 3Mhz transducer and
increases the output power in order to visualize
structures that are positioned deeper in the
patient. No other controls are adjusted. What
happens to each of the following parameters. Do
they increase, decrease or remain the same?
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
Answers
1a.
1b.
1c.
1d.
1e.
1f.
1g.
1h.
1i.
1j.
2a.
2b.
2c.
2d.
2e.
2f.
2g.
3a.
3b.
3c.
3d.
3e.
3f.
3g.
3h.
3i.
3j.
remains same
remains same
remains same
remains same
remains same
remains same
decreases
decreases
remains same
increases
decreases
increases
decreases
remains same
remains same
remains same
remains same
remains same
remains same
remains same
remains same
increases
remains same
remains same
remains same
remains same
remains same
period
frequency
wavelength
speed
intensity (initial)
pulse duration
PRF
duty factor
spatial pulse length
pulse repetition period
18 · Pulsed Ultrasound
ESP, Inc.© 2013