Resident Physics Lectures
Ultrasound
Basics
Principles
George David, M.S.
Associate Professor of Radiology
Ultrasound Transducer
 Acts as both speaker & microphone
 Emits very short sound pulse
 Listens a very long time for returning echoes
 Can only do one at a time
Speaker
Microphone
transmits sound pulses
receives echoes
Piezoelectric Principle
 Voltage generated when certain materials are
deformed by pressure
 Reverse also true!
 Some materials change dimensions when
voltage applied

dimensional change causes pressure change
 when voltage polarity reversed, so is
dimensional change
V
US Transducer Operation
 alternating voltage (AC) applied to
piezoelectric element
 Causes
 alternating dimensional changes
 alternating pressure changes
 pressure propagates as sound wave
Ultrasound Basics
 What does your scanner know about
the sound echoes it hears?
I’m a scanner,
Jim, not a
magician.
Acme
UltraSound
Co.
What does your scanner know
about echoed sound?
How loud is the echo?
inferred from intensity of electrical pulse from transducer
What does your scanner know
about echoed sound?
What was the time delay between
sound broadcast and the echo?
What else does your scanner know
about echoed sound?
The sound’s pitch or frequency
What Does Your Scanner Assume
about Echoes
(or how the scanner can lie to you)
 Sound travels at 1540 m/s
everywhere in body
 average speed of sound in soft tissue
 Sound travels in straight
lines in direction
transmitted
 Sound attenuated equally
by everything in body
 (0.5 dB/cm/MHz, soft tissue average)
Luckily These Are Close Enough to Truth To Give
Us Images
 Sound travels at 1540 m/s
everywhere in body
 average speed of sound in soft tissue
 Sound travels in straight
lines in direction
transmitted
 Sound attenuated equally
by everything in body
 (0.5 dB/cm/MHz, soft tissue average)
Dot Placement on Image
 Dot position ideally indicates
source of echo
 scanner has no way of knowing
exact location
 Infers location from echo
?
Dot Placement on Image
 Scanner aims sound when
transmitting
 echo assumed to originate from
direction of scanner’s sound
transmission
 ain’t necessarily so
?
Positioning Dot
 Dot positioned along assumed line
 Position on assumed line calculated based upon
 speed of sound
 time delay between sound transmission & echo
?
Distance of Echo from Transducer
 Time delay accurately measured by scanner
distance = time delay X speed of sound
distance
distance =
time delay X speed of sound
What is the Speed of Sound?
 scanner assumes speed of sound is that of soft tissue
 1.54 mm/msec
 1540 m/sec
 13 usec required for echo object 1 cm from transducer (2
cm round trip)
13 msec
1 cm
So the scanner assumes the wrong
speed?
 Sometimes
•Luckily, the speed of
sound is almost the
same for most body
parts
soft tissue ==> 1.54 mm / msec
fat ==> 1.44 mm / msec
brain ==> 1.51 mm / msec
liver, kidney ==> 1.56 mm / msec
muscle ==> 1.57 mm / msec
?
Gray Shade of Echo
 Ultrasound is gray shade modality
 Gray shade should indicate
echogeneity of object
?
?
How does scanner know what gray
shade to assign an echo?
 Based upon intensity (volume, loudness) of
echo
?
?
Gray Shade
 Loud echo = bright dot
 Soft echo = dim dot
Complication
 Deep echoes are softer (lower volume) than
surface echoes.
Gray Shade of Echo
 Correction needed to compensate for sound
attenuation with distance
 Otherwise dots close to transducer would be
brighter
Echo’s Gray Shade
 Gray Shade determined by
 Measured echo strength

accurate
 Calculated attenuation
Who am I?
Charles Lane
Attenuation Correction
 scanner assumes entire
body has attenuation of
soft tissue
 actual attenuation
varies widely in body
Tissue
Attenuation Coefficient
(dB / cm / MHz)
• Fat
• Brain
• Liver
• Kidney
• Muscle
• Heart
0.6
0.6
0.5
0.9
1.0
1.1
Ultrasound Display
 One sound pulse produces
 one image scan line

one series of gray shade dots in a
line
 Multiple pulses
 two dimensional image
obtained by moving direction
in which sound transmitted
How Do We Move the Beam?
 Electronically
 Phased Arrays
Sound Wave Definition?
 Sound is a Wave
 Wave is a propagating (traveling)
variation in a “wave variable”
 “An elephant is big, gray, and looks
like an elephant.”
Sound Wave Variable
 Examples
 pressure (force / area)
 density (mass / volume)
 temperature
 Also called acoustic variable
Sound is a propagating (moving)
variation in a “wave variable”
Energy & Power
 Power
 rate of energy use
 Units: watts or milliwatts
 Energy = Power X Time
 Units: kilowatt-hours
Electric
Bill
Light Bulbs rated in power!
300 KW-hr.
Electricity billed in energy!
Intensity
 Intensity of Sound Beam
intensity = power / cross sectional area
Sound Wave Variation
 Freeze time
 Measure some acoustic variable as a function of
position
Pressure
Density
Temperature
Acoustic
Variable
Value
Position
MORE
 Make multiple measurements of an acoustic
variable an instant apart
 Results would look the same but appear to move in
space
1
2
MORE
 Track acoustic variable
at one position over
time
Sound Waves
 Waves transmit energy
 Waves do not transmit matter
 “Crowd wave” at sports event
 people’s elevation varies with time
 variation in elevation moves around stadium

people do not move around stadium
Transverse Waves
 Particle moves perpendicular to wave travel
 Water ripple
 surface height varies with time
 peak height moves outward

water does not move outward
Compression (Longitudinal)
Waves
 Particle motion parallel to direction of
wave travel
1
1
Motion of
Individual Coil
2
2
Wave Travel
Medium
 Material through which wave moves
 Medium not required for all wave types
 no medium required for electromagnetic waves




radio
x-rays
infrared
ultraviolet
 medium is required for sound

sound does not travel through vacuum
Talk louder!
I can’t hear
you.
Sound Waves
 Information may be encoded in wave energy
 radio
 TV
 ultrasound
 audible sound
Sound Frequency
# of complete variations (cycles) of an acoustic
variable per unit time
 Units
cycles per second
1 Hz = 1 cycle per second
1 kHz = 1000 cycles per second
1 MHz = 1,000,000 cycles per second
 Human hearing range
20 - 20,000 Hz
Sound Frequency
 Ultrasound definition
> 20,000 Hz
 not audible to humans

dog whistles are in this range
 Clinical ultrasound frequency range
1 - 10 MHz
1,000,000 - 10,000,000 Hz
Period
 time between a point in one cycle & the
same point in the next cycle
 time of single cycle
 Units
 time per cycle (sometimes expressed
only as time; cycle implied)
Magnitude
of acoustic
variable
period
time
Period
1
Period = ------------------Frequency
 as frequency increases, period decreases
 if frequency in Hz, period in seconds/cycle
Period
Period = 1 / Frequency
 if frequency in kHz, period in msec/cycle
 if frequency in MHz, period in msec/cycle
1 kHz frequency ==> 1 msec period
1 MHz frequency ==> 1 msec period
Reciprocal Units
Frequency
Units
Period Units
Hz (cycles/sec)
seconds/cycle
kHz (thousands
of cycles/sec)
msec/cycle
MHz (millions of
cycles/sec)
msec/cycle
Sound Period &
Frequency are
determined only by the
sound source. They are
independent of medium.
Who am I?
Burt Mustin
Propagation Speed
 Speed only a function of medium
 Speed virtually constant with respect to frequency
over clinical range
 Speed depends on medium’s
 Density (mass per unit volume)

more dense ==> lower speed
 Stiffness (or bulk modulus; opposite of elasticity or
compressibility)

more stiffness ==> higher speed
 “same letter, same effect”
Wavelength
 distance in space over which single cycle
occurs
OR
 distance between a given point in a cycle &
corresponding point in next cycle
 imagine freezing time, measuring between
corresponding points in space between
adjacent cycles
Wavelength Units
 length per cycle
 sometimes just length; cycle implied
 usually in millimeters or fractions of a millimeter for
clinical ultrasound
Wavelength Equation
Speed = Wavelength X Frequency
[c=lXn]
(dist./time)
(dist./cycle)
(cycles/time)
 As frequency increases, wavelength
decreases
 because speed is constant
Wavelength
Speed = Wavelength X Frequency
c=lXn
(dist./time)
mm/msec
(dist./cycle)
mm/cycle
(cycles/time)
MHz
Calculate Wavelength for 5 MHz sound
in soft tissue
Wavelength = 1.54 mm/msec / 5 MHz
5 MHz = 5,000,000 cycles / sec = 5 cycles / msec
Wavelength = 1.54 / 5 = 0.31 mm / cycle
Wavelength is a
function of both the
sound source and the
medium!
Who am I?
John Fiedler
Pulsed Sound
 For imaging ultrasound, sound is
 Not continuous
 Pulsed on & off
 On Cycle (speak)
 Transducer produces short duration sound
 Off Cycle (listen)
 Transducer receives echoes
 Very long duration
ON
OFF
ON
(not to scale)
OFF
Pulse Cycle
 Consists of
 short sound transmission
 long silence period or dead time
 echoes received during silence
 same transducer used for
 transmitting sound
 receiving echoes
sound
silence
sound
Pulsed Sound Example
 ringing telephone
 ringing tone switched
on & off
 Phone rings with a
particular pitch

sound frequency
sound
silence
sound
Parameters
Sound
 frequency
 period
 wavelength
 propagation speed
Pulse
• pulse repetition
frequency
• pulse repetition
period
• pulse duration
• duty factor
• spatial pulse
length
• cycles per pulse
Pulse Repetition Frequency
 # of sound pulses per unit time
 # of times ultrasound beam turned on & off per
unit time
 independent of sound frequency
 determined by source
 clinical range (typical values)
 1 - 10 KHz
Pulse Repetition Period
 time from beginning of one pulse until
beginning of next
 time between corresponding points of adjacent
pulses
Pulse Repetition Period
Pulse Repetition Period
 Pulse repetition period is reciprocal of
pulse repetition frequency
PRF = 1 / PRP
 as pulse repetition frequency increases, pulse repetition
period decreases
 units
 time per pulse cycle (sometimes simplified to just time)
 pulse repetition period & frequency
determined by source
Pulsed Sound
 Pulse repetition frequency & period independent sound
frequency & period
Same Frequency
Higher Pulse
Repetition Frequency
Higher Frequency
Same Pulse
Repetition Frequency
Pulse Duration
 Length of time for each sound pulse
 one pulse cycle =
 one sound pulse
and
one period of silence
 Pulse duration independent of
duration of silence
Pulse Duration
Pulse Duration
 units
 time per pulse (time/pulse)
 equation
pulse duration = Period X # cycles per pulse
(time/pulse) (cycles/pulse) (time/cycle)
Pulse Duration
Period
Pulse Duration
Longer Pulse Duration
Same frequency; pulse repetition frequency,
period, & pulse repetition period
Shorter Pulse Duration
Pulse Duration
Pulse duration is
a controlled by
the sound
source, whatever
that means.
Duty Factor
 Fraction of time sound generated
 Determined by source
 Units
 none (unitless)
 Equations
Duty Factor = Pulse Duration / Pulse Repetition Period
Duty Factor = Pulse Duration X Pulse Repetition Freq.
Pulse Duration
Pulse Repetition Period
Spatial Pulse Length
 distance in space traveled by ultrasound
during one pulse
H.......E.......Y
HEY
Spatial Pulse Length
Spatial Pulse Length
Spat. Pulse Length = # cycles per pulse X wavelength
(dist. / pulse)
(cycles / pulse)
(dist. / cycle)
 depends on source & medium
 as wavelength increases, spatial pulse length
increases
Wavelength
Calculate SPL for 5 MHz sound in
soft tissue, 5 cycles per pulse
(Wavelength=0.31 mm/cycle)
Spat. Pulse Length = # cycles per pulse X wavelength
SPL = 0.31 mm / cycle X 5 cycles / pulse = 1.55 mm / pulse
Spatial Pulse Length
Spat. Pulse Length = # cycles per pulse X wavelength
Wavelength = Speed / Frequency
 as # cycles per pulse increases, spatial pulse
length increases
 as frequency increases, wavelength decreases &
spatial pulse length decreases
 speed stays constant
Why is Spatial Pulse Length
Important
Spat. Pulse Length = # cycles per pulse X wavelength
Wavelength = Speed / Frequency
Spatial pulse
length determines
axial resolution
Acoustic
Impedance
 Definition
Acoustic Impedance = Density X Prop. Speed
(rayls)
(kg/m3)
 increases with higher
 Density
 Stiffness
 propagation speed
 independent of frequency
(m/sec)
Acoustic Impedance of Soft
Tissue
 Density:
 1000 kg/m3
 Propagation speed:
 1540 m/sec
Acoustic Impedance = Density X Prop. Speed
(rayls)
(kg/m3)
(m/sec)
1000 kg/m3 X 1540 m/sec = 1,540,000 rayls
Why is Acoustic Impedance
Important?
 Definition
Acoustic Impedance = Density X Prop. Speed
(rayls)
(kg/m3)
(m/sec)
 Differences in acoustic impedance determine
fraction of intensity echoed at an interface