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Basic Ultrasound Physics
Tavakoli. M.B,
Isfahan University of Medical
Sciences, School of Medicine
Department of Medical Physics and
Medical Engineering
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Main topics
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Basic Physics
Mechanical Phenomena
Sound wave
Spectrum
Velocity of sound
Transmission through a barrier
Attenuation
Production and detection
Ultrasonic field in front of a transducer
Resolution
Ultrasonic systems
Clinical Uses
Biological effects
Basic Ultrasound Physics continue
• Sound Wave: Is a type of mechanical energy that is
transmitted through medium.
• Propagation: Sound wave propagate through
deformation of the elastic medium
• Sound wave spectrum: Is divided into three region
of Inferasound (f<20Hz); Sound (f=20 to 20000Hz)
and Ultrasound (f>20kHz)
• Wave equation:
A=A0sin (ωt+θ)
A=amplitude; A0=Maximum amplitude;
θ=initial phase and ω =2пf
f=1/T
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Basic Ultrasound Physics continue
• Types of sound wave:
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Longitudinal
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Shear wave
• Compressibility: The fractional decrease in volume
when pressure applied to the material.
• B=Bulk modulus=-stress/strain
• The reciprocal of compressibility is bulk modulus
• B(bulk modulus)=1/ K (compressibility of medium)
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Sound velocity
• Sound velocity
• c(m./sec)=f(1/sec or Hz)λ(m)
• Sound velocity depends on
compressibility (K)
• c=1/(Kρ)1/2=(B/ ρ)1/2
In materials with higher
compressibility velocity of sound is
less and Vic versa
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 Propagation velocity = C
C
B


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K
B = modulus of elasticity
ρ = density of rest
K=Comperesibility
C =λf
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Acoustic Impedance
 Z is the acoustic impedance
 Acoustic Pressure: P=Zv
 It can be show that
B
Z   Z   C   .B
C
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Reflection
• According to Snell laws
• 1-Incident and reflected
angles are equal
αi = α r
• 2-the relation between
incident and transmission
angles is:
Sinαi/ Sinαt=ci/ct
• 3-All of the incident reflection
and transmitted rays are in
the same plane
 Z cos  i  Z1 cos  t
I
A
R  r  ( r ) 2   2
Ii
At
 Z 2 cos  i  Z1 cos  t
• Energy transmission and
reflection percentages are:
T
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


2
It
A
4Z1Z 2 cos  i cos  t
 ( t )2 
Ii
Ai
Z 2 cos i  Z1 cos t 2
For specular reflection:
Amplitude reflection coefficient r :
r
I r Pr Z 2  Z1
 
I i Pi Z 2  Z1
Energy transmitted and reflection coefficient t : •
T
It
 1 R2 
Ii
4Z1Z 2
Z 2  Z1 2
Pt
Z
2Z 2
 t 2  1 R 
Pi
Z1
Z 2  Z1
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The continuity condition is:
R+T=1
 Typical values for diagnostic ultrasound:
 Ultrasound
f > 20KHz
 f : 1 to 10 MHz
 λ : 1.5 to 0.15 mm in muscle
 J (Acoustic Intensity)< 100
mW
2
cm
 Acoustic Pressure P<0.57 bar
 Particle velocity v<3.5 cm/s
 Elongation ξ < 2*10-6
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 Particle acceleration < 7*104 g
usually 1 to 10
mW
cm2
 Physical effects
 Reflection
 Refraction
 Diffraction
 Scatter
 Absorption
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Attenuation
• Intensity(Watt/cm2)
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The special unit for these coefficient is
the Neper (NP) per centimeter.
The attenuation coefficient at 1 MHZ for
various tissue types are different.
Level (db)= 10 log ( AMax )2  ( I max )
A
Level (db)= 20
I
A
log ( Max ) 10log( I max )
A
I
dB=8.868np
Ultrasound production and
detection
• Use pizoelectric
• Natural-Quartz
• Artificial-PVDF, PZT…
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- crystal with coated electrodes on each side
- Backing material
- Matching layer
- Housing and insulator
- Ground electrode
- Connector
- Resolution
Near field:
D2 F D2 a2
NFD 


4C
4 
Far field:
  1.22  1.22c
Sin  0.61
a
D
Df
Actual intensity distribution
- Focused transducer with an
acoustic lens.
- Focused transducer with a
curved piezoelectric.
• Side lobes are secondary projections of ultrasonic energy
that radiate away from the main ultrasound beam.
• The intensity of side lobes is normally 60 to 100 dB below
that of the main ultrasound beam, which usually does not pose
significant problems.
 Ultrasonic technique
 Pulse echo
 Real time
 A-scan
 B-scan
 M-mode
 Doppler
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A-scan
Main part
 Clock

pulse repetition frequency (PRF)
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


Ultrasonic velocity
Depth of investigation
Number of line per image
Filter
 Transmitter
 Receiver
 TGC+Amplifier
 Radio Amp.
 Video Amp.
 Time generator
 Processing
 B-scan
 One dimension
 Two dimension
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Commercial system
Mechanical
 Linear
 Sector
Electrical
 Linear
 Sector
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Mechanical sector scanner
Advantages
 Simple
 Cheap
 Acceptable resolution
Disadvantage
 Noise
 Mechanical fracture
 Reverberation
 Sector field
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Electronic Scanners
 Linear
Typical values:
 Number of elements 60-120
 Elements width (b) 1-4 λ
 Frequency 3.5-7MHz
 Length of ceramic (L) 2-11.5 cm
 Scanning length (image width) 2-10 cm
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Sector
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Electronic focusing
M-Mode
ioo •
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nbbnnb •
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Doppler ultrasound
jdffgg •
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 Velocity determination
2vf
fD 
cos 
C
 Optimum frequency for doppler is f0=90/R
R = soft tissue distance from target
 Suitable frequency=2 MHz for deep
5-7 MHz for superficial
 Doppler examination
 continues wave
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 pulse wave
 Color flow imaging
 Duplex scanners
 spectral Analysis
 Power spectrum
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Doppler systems
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Ultrasonic probes
tkyk •
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Clinical application
• Cardiology
Shortcut to Doppler_mitral_valve.lnk
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Endocarinology •
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Gasteroenterology
ghjhgjgj •
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Obstetric and Gynaechology
bdzz •
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Urology
hkhh •
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Ophtalemology
Malignant melanoma of the iris at
pupil margin.
Pigmented lesion of the iris in the
region of the angle demonstrating
extension to the ciliary body.
Ciliary body melanoma.
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sdfs •
Quality controls (Q.A)
 Axial resolution
 Lateral resolution
 Penetration depth and sensitivity
 Dynamic range
 Accuracy
 Hand copy performance
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Biological effects
• 1-Thermal
• 2-Mecanical
• 3-Cavitation
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Biological effects
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The Ear
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Structure
External ear
Canal length=2.4cm
Tympanic membrane diameter=7mm
Thickness=0.1mm
Vibration amplitude=a few A
Middle ear
Three small bones: of malleus, incus, stepes and oval window
Amplification by the bones=1.5
by the membrane area=20
Internal ear
Three semicircular canal for balance
Chochlea
Prelymph, Vestibular chamber and endolymph
Hearing level and ear sensitivity
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Threshold of hearing=10-12W/m2
Hearing level is determined by dB
Ear sensitivity depend on frequency
Max sensitivity is about 2-3kHz
Resonant frequency of ear canal 3300Hz
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Resonant frequency of middle ear 700-1500Hz
High frequency is sensed by initial portion of inner ear
Low frequency is sensed by end portion
Resolution of ear is very high (about 3Hz)
Ear can differentiate different sound level
Ear can determined source position
• Hearing loss
• 1-Conduction loss
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Can be treated
• 2-nerve loss
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Can not be treated
Audiometry
• Determination of threshold of hearing level
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High frequency electricity
• Mainly used in therapy
• Biological effects
• The main effective factors are frequency
and current
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Types of current
• AC current
• DC current
• Production of AC current
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Types of AC current
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1-Contineus damping ac current
2-Noncountinous damping ac current
3-Non-countineous current
4-Countineous current
Heat production
• 1-Capacitor applicator
• 2-Coil applicator
• 3-Shortwave (10 to 100MHz)
• 4-Microwave (2450 and 4329.2MHz)
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Types of capacitor applicators
• 1-conterplannar (chest)
• 2-Co-plannar (vertebra column)
• 3-Crossfire (sinus)
• 4-Monopolar (Mostly for surgery)
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Physiological effects of high frequency
currents
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1-Increase blood flow
2-Increase metabolism
3-Effect of nervous stimulation
4-Effects of muscle
5-Reduce blood pressure
7-Increase endocrine
Diathermy
• Use of electric current to increase temperature
• The frequency used (10-100MHz and mainly 27MHz
• The effects are:
• Stimulation with less than 0.01s period can not be
sensed=> f above 100KHz can not make shock
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Forbidden case for diathermy
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1-bleeding
2-therombosis
3-pregnancy
4-inability to sense temperature
5-Tumers and radiotherapy
6-Mental impaired
Electric surgery (electrocuthery)
• 1-Coagulation
• 2-Vaporization
• 3-Discication
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Types of electrodes
• Usually needs two electrodes of active and
null or inactive
• Active types:
• Inactive types
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