Resident Physics Lectures
01:
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 sound echoes?
• Direction sound was
emitted
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)
Ultrasound Display
• B-scan (“Brightness” Mode)
Image
series of gray shade dots
• For each dot, scanner
must calculate
position
Gray shade
Images from Echos
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
Depth Correction
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
Moving the Sound Beam
• electronically
phased or pulsed transducer arrays
Focus
Arrows indicate timing
variations.
Activating top & bottom
elements earlier than
center ones focuses
beam.
Scan Patterns
• Linear
beam translated
» moved sideways
produces rectangular image
• sector
beam pivoted
produces pie-shaped image
Th’ Th’ Th’ Th’
That’s All Folks