Transcript Ultrasound Physics

Ultrasound Physics
Have no fear
Presentation by Alexis Palley MD
Department of Emergency Medicine
Cooper University Hospital
“How does it do that?”
Lecture Objectives:
 Review
basic physics vocabulary
 Explain the principles of sound waves
 Use ultrasound physics to explain how
images are produced
 Teach how to use these principles to help
your diagnostic abilities
Basics
 Sound
is energy traveling though matter
as a wave
 The wave travels by compressing and
rarefacting matter
 Depending on the matter- the wave will
travel at different velocities or directions
 U/S probes emit and receive the energy as
waves to form pictures
Physical Principles
Cycle

1 Cycle = 1 repetitive periodic oscillation
Cycle
Frequency
#
of cycles per second
 Measured in Hertz (Hz)
-Human Hearing 20 - 20,000 Hz
-Ultrasound > 20,000 Hz
-Diagnostic Ultrasound 2.5 to 10 MHz
(this is what we use!)
frequency
1 cycle in 1 second = 1Hz
1 second
= 1 Hertz
High Frequency
 High
frequency (5-10 MHz)
greater resolution
less penetration
 Shallow structures
vascular, abscess, t/v gyn,
testicular
Low Frequency
 Low
frequency (2-3.5 MHz)
greater penetration
less resolution
 Deep structures
Aorta, t/a gyn, card, gb, renal
Wavelength
 The
length of one complete cycle
 A measurable distance
Wavelength
Wavelength
Amplitude
 The
degree of variance from the norm
Amplitude
Producing an image
 Probe
emits a sound wave pulsemeasures the time from emission to return
of the echo
 Wave travels by displacing matter,
expanding and compressing adjacent
tissues
 It generates an ultrasonic wave that is
propagated, impeded, reflected, refracted,
or attenuated by the tissues it encounters
Producing an image
 Important
concepts in production of an U/S
image:
• Propagation velocity
• Acoustic impedance
• Reflection
• Refraction
• Attenuation
Propagation Velocity

Sound is energy transmitted through a medium Each medium has a constant
velocity of sound (c)
 Tissue’s resistance to compression

density or stiffness
Product of frequency (f) and wavelength (λ)
c=fλ
 Frequency and Wavelength therefore are
directly proportional- if the frequency increases
the wavelength must decrease.

Propagation Velocity
 Propagation
velocity
Increased by increasing stiffness
Reduced by increasing density
 Bone: 4,080 m/sec
 Air: 330 m/sec
 Soft Tissue Average: 1,540 m/sec
Impedance
 Acoustic
impedance (z) of a material is the
product of its density and propagation
velocity
Z= pc
 Differences in acoustic impedance create
reflective interfaces that echo the u/s
waves back at the probe
 Impedance mismatch = ΔZ
Acoustic Impedance
 Homogeneous
mediums reflect no sound
 acoustic interfaces create visual
boundaries between different tissues.
 Bone/tissue or air/tissue interfaces with
large Δz values reflect almost all the
sound
 Muscle/fat interfaces with smaller Δz
values reflect only part of the energy
Refraction

A change in direction of the sound wave as it
passes from one tissue to a tissue of higher or
lower sound velocity
 U/S scanners assume that an echo returns
along a straight path
 Distorts depth reading by the probe
 Minimize refraction by scanning perpendicular to
the interface that is causing the refraction
Reflection
 The
production of echoes at reflecting
interfaces between tissues of differing
physical properties.
 Specular
- large smooth surfaces
 Diffuse – small interfaces or nooks and
crannies
Specular Reflection
 Large
smooth interfaces (e.g. diaphragm,
bladder wall) reflect sound like a mirror
 Only the echoes returning to the machine
are displayed
 Specular reflectors will return echoes to
the machine only if the sound beam is
perpendicular to the interface
Diffuse Reflector
 Most
echoes that are imaged arise from
small interfaces within solid organs
 These interfaces may be smaller than the
wavelength of the sound
 The echoes produced scatter in all
directions
 These echoes form the characteristic
pattern of solid organs and other tissues
Reflectors
Specular
Diffuse
Attenuation
 The
intensity of sound waves diminish as
they travel through a medium
 In ideal systems sound pressure
(amplitude) is only reduced by the
spreading of waves
 In real systems some waves are scattered
and others are absorbed, or reflected
 This decrease in intensity (loss of
amplitude) is called attenuation.
The Machine
Ultrasound scanners
 Anatomy
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of a scanner:
Transmitter
Transducer
Receiver
Processor
Display
Storage
Transmitter
a
crystal makes energy into sound waves
and then receives sound waves and
converts to energy
 This is the Piezoelectric effect
 u/s machines use time elapsed with a
presumed velocity (1,540 m/s) to calculate
depth of tissue interface
 Image accuracy is therefore dependent on
accuracy of the presumed velocity.
Transducers
 Continuous



continuous alternating current
doppler or theraputic u/s
2 crystals –1 talks, 1 listens
 Pulsed


mode
mode
Diagnostic u/s
Crystal talks and then listens
Receiver
 Sound
waves hit and make voltage across
the crystal The receiver detects and amplifies these
voltages
 Compensates for attenuation
Signal Amplification

TGC (time gain
compensation)

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Manual control
Selective enhancement or
suppression of sectors of
the image
enhance deep and
suppress superficial
*blinders

Gain

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
Manual control
Affects all parts of the
image equally
Seen as a change in
“brightness” of the images
on the entire screen
*glasses
Changing the TGC
Changing the Gain
Displays
 B-mode


Real time gray scale, 2D
Flip book- 15-60 images per second
 M-mode

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Echo amplitude and position of moving
targets
Valves, vessels, chambers
“B” Mode
“M” Mode
Image properties
 Echogenicity-
amount of energy reflected
back from tissue interface

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Hyperechoic - greatest intensity - white
Anechoic - no signal - black
Hypoechoic – Intermediate - shades of gray
Hyperechoic
Hypoechoic
Anechoic
Image Resolution
 Image
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quality is dependent on
Axial Resolution
Lateral Resolution
Focal Zone
Probe Selection
Frequency Selection
Recognition of Artifacts
Axial Resolution
 Ability
to differentiate two objects along the
long axis of the ultrasound beam
 Determined by the pulse length
• Product of wavelength λ and # of cycles in pulse
• Decreases as frequency f increases
 Higher
frequencies produce better
resolution
Axial Resolution

5 MHz transducer

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Wavelength 0.308mm
Pulse of 3 cycles
Pulse length
approximately 1mm
Maximum resolution
distance of two objects
= 1 mm

10 MHz transducer

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Wavelength 0.15mm
Pulse of 3 cycles
Pulse length
approximately 0.5mm
Maximum resolution
distance of two objects
= 0.5mm
Axial Resolution
screen
body
Lateral Resolution
 The
ultrasound beam is made up of
multiple individual beams
 The individual beams are fused to appear
as one beam
 The distances between the single beams
determines the lateral resolution
Lateral resolution
 Ability
to differentiate objects along an axis
perpendicular to the ultrasound beam
 Dependent on the width of the ultrasound
beam, which can be controlled by focusing
the beam
 Dependent on the distance between the
objects
Lateral Resolution
screen
body
Focal Zone

Objects within the focal zone
Focal zone

Objects outside of focal zone
Focal zone
Probe options

Linear Array

Curved Array
Ultrasound Artifacts
 Can
be falsely interpreted as real
pathology
 May obscure pathology
 Important to understand and appreciate
Ultrasound Artifacts
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Acoustic enhancement
Acoustic shadowing
Lateral cystic shadowing (edge artifact)
Wide beam artifact
Side lobe artifact
Reverberation artifact
Gain artifact
Contact artifact
Acoustic Enhancement
 Opposite
of acoustic shadowing
 Better ultrasound transmission allows
enhancement of the ultrasound signal
distal to that region
Acoustic Enhancement
Acoustic Shadowing
 Occurs
distal to any highly reflective or
highly attenuating surface
 Important diagnostic clue seen in a large
number of medical conditions
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Biliary stones
Renal stones
Tissue calcifications
Acoustic Shadowing
 Shadow
may be more prominent than the
object causing it
 Failure to visualize the source of a shadow
is usually caused by the object being
outside the plane of the ultrasound beam
Acoustic Shadowing
Acoustic Shadowing
Lateral Cystic Shadowing
 A type
of refraction artifact
 Can be falsely interpreted as an acoustic
shadow (similar to gallstone)
Lateral Cystic Shadowing
X
Beam-Width Artifact
 Gas
bubbles in the duodenum can
simulate a gall stone
 Does not assume a dependent posture
 Do not conform precisely to the walls of
the gallbladder
Beam-Width Artifact
Beam-width artifact
Gas in the duodenum
simulating stones
Side Lobe Artifact
 More
than one ultrasound beam is
generated at the transducer head
 The beams other than the central axis
beam are referred to as side lobes
 Side lobes are of low intensity
Side Lobe Artifact

Occasionally cause
artifacts
 The artifact by be
obviated by
alternating the angle
of the transducer
head
Side Lobe Artifact
Reverberation Artifacts
 Several
types
 Caused by the echo bouncing back and
forth between two or more highly reflective
surfaces
Reverberation Artifacts
 On
the monitor parallel bands of
reverberation echoes are seen
 This causes a “comet-tail” pattern
 Common reflective layers



Abdominal wall
Foreign bodies
Gas
Reverberation Artifacts
Reverberation Artifacts
Gain Artifact
Contact artifact

Caused by poor probepatient interface
Take homes!
review
 u/s
uses waves echoed off reflective
interfaces to display the structures of the
body works by “talking and listening”
 Must be careful when interpreting images
 We have more control than we thought!
Think before you ultrasound!
Think before you ultrasound!
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Choose the right frequency probe –
frequency = shallow & detailed
Hold the probe perpendicular to the organ wall
being studied
Adjust the depth, tgc and gain to help your
image
Artifact may be affecting your image
Use knowledge of physical properties of tissues
to help with positioning- ie. use bladder for
acoustic window to ta u/s.
Thank you
Any questions?