# Resident Physics Lectures

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```					Resident Physics Lectures
01:
Ultrasound
Basics
Principles

George David, M.S.
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
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           V
dimensional change
US Transducer Operation

• alternating voltage (AC) applied
to piezoelectric element
• Causes
alternating dimensional changes
alternating pressure changes

• pressure propagates as sound
wave
Ultrasound Basics

the sound echoes it hears?
I’m a scanner,
Jim, not a
magician.

Acme
Ultra-
Sound
Co.
How loud is the echo?

inferred from intensity of electrical pulse from transducer
What was the time delay
and the echo?
• Direction sound was
emitted
The sound’s pitch or
frequency
(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

• For each dot, scanner
must calculate
position
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
?
modality
echogeneity of object

?           ?
How does scanner know what
gray shade to assign an echo?

• Based upon intensity (volume,
loudness) of echo

?           ?

• Loud echo = bright dot
• Soft echo = dim dot
Complication

• Deep echoes are softer (lower
volume) than surface echoes.

• Correction needed to
compensate for sound
attenuation with distance
• Otherwise dots close to
transducer would be brighter
Depth Correction

Measured echo strength
» accurate
Calculated attenuation

Who am I?
Charles Lane
Attenuation Correction
• scanner assumes              Tissue   Attenuation Coefficient
(dB / cm / MHz)
entire body has
attenuation of soft          • Fat             0.6
tissue                       • Brain           0.6
actual attenuation varies
widely in body              • Liver           0.5
• Kidney          0.9
• Muscle          1.0
• Heart           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

```
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