Hiện tương vật lý và âm thanh trong siêu âm Ultrasound Physics and Instrumentation

Ultrasound Waves • Imaging with reflected acoustic signal • Short ultrasound pulse of specific frequency is used for imaging Ultrasound machine Acoustic signal Acoustic signal generat

Ultrasound Physics and Instrumentation

Ravi Managuli, PhD, RDMS

Ultrasound from a Radiologists perspective

!   Have a good understanding of ultrasound and what it is capable of

Ultrasound Machine

Ultrasound Machine Features

!   For a given transducer and an application

!   System is optimized to give acceptable results for

an average patient

!   Main optimization sonographer performs

!   BW

!   TGC à Adjust for depth of penetration

!   Compound imaging à Enhance shadows

!   Harmonic imaging à Enhance anechoic area

!   Advanced image processing à Better contrast

!   Gain à enhance certain structures

!   Color Doppler

!   Optimization is very very difficult!!

Cons Ultrasound

!   It is generally not useful in imaging bony structures

!   The ultrasound waves cannot penetrate bone well so brain

imaging is not useful with Sonography

!   It can be hard to visualize large patients and pockets of gas can cause

distortion

!   Ultrasound exams can also be invasive and uncomfortable

Thus understanding of ultrasound physics becomes quiet critical : More so

in ultrasound imaging

Topics

!   Ultrasound wave characteristics

!   Ultrasound interaction with the media

© UW and Renée Dickinson, MS

Sources of information on

Ultrasound

! Bushberg Chatper 16

!   AAPM/RSNA web modules

!   Basic US Imaging and Display

!   Image Quality – Artifacts – Doppler

!   US – Concepts and Transducers

!   AAPM/RSNA Physics Tutorials

!   This course website

!   AAPM/RSNA Physics Curriculum – Module 15 Outline

Ultrasound Waves

•  Imaging with reflected acoustic signal

•  Short ultrasound pulse of specific frequency is used for imaging

Ultrasound machine Acoustic signal

Acoustic signal generator and receiver (transducer)

Tissue

Ultrasound Waves

! Two types of ultrasound waves

! Classified based upon how they move

and how they transfer energy

! Transverse wave

! Consists of oscillations occurring perpendicular to the direction of energy transfer

! Longitudinal waves

! Particles move to and fro and parallel

to the wave

the ultrasound images

Transverse

Longitudinal

Longitudinal Wave : Sound

Movement of particles and propagation of wave are in the same direction

Movement of particle Propagation of wave

Ultrasound Waves

!   This wave:

!   Is generated by the transducer

!   Is affected by the medium in which it is travelling

!   Characteristics this wave control

!   Axial, lateral resolution

!   Is responsible for harmonic imaging

!   Is responsible for signal to noise ratio in an image

Short pulse transmitted by the ultrasound transducer That propagate into the body

1.  Frequency, Period, Wavelength, Velocity

2.  Power, Energy, Intensity

3.  Pulse duration, pulse length, Bandwidth

!   Period (1/f) [seconds] – time duration of one wave cycle

!   Reciprocal of frequency : 1 microsec to 0.05 microsec

Ultrasound: Characteristics

!   Wavelength (λ) [mm or μm] – distance between compression and

refraction

!   0.1 to 1.5 mm

!   Speed (velocity) of sound [m/sec]

!   Depends upon the density and stiffness of the medium

!   ~1400 m/s (fat) to ~1500 m/s (most soft tissues) to ~1700m/s (muscle,

Relationships : Wave Parameters

Period = 1/Frequency

Wavelength = Speed * Period

Distance = Speed * time

Electromagnetic spectrum

Ultrasound: Characteristics

!   Duration : Number of cycles in a pulse

!   Typically 2 to 3 cycles for BW-mode

!   About 8 to 10 cycles for color-mode

!   Pulse length (mm)

!   Length of the pulse : Wave length * number of cycles

!   For BW : 0.1 * 3 = 0 6mm

!   Controls axial resolution

!   Speed (velocity) of sound [m/sec]

!   Low ~1400 m/s (fat)

!   Middle ~1500 m/s (most soft tissues)

!   High ~1700m/s (muscle, cartilage, tendon)

][][[m/sec]

c

][1

]

[[sec]

][[m/sec]

c

Hz f

m

Hz f

m period

Speed, Frequency (period),

and wave length are all related by:

Ultrasound Physics

Frequency range ( f ) : 1Mhz – 15Mhz

Human Hearing Range: ~20hz - 20, 000hz Wavelength range ( l ) : 1.5 mm – 0.1mm

Relationships : Wave Parameters

Period = 1/Frequency

Wavelength = Speed * Period

Distance = Speed * time

Magnitude parameter

Magnitude of Sound

!   Difference between maximum positive value and average

!   Units of pressure (pascal),

!   Typical values in clinical imaging : 1 MPa to 3 Mpa

Power ∞ Amplitude2

Display Beamformer

Pulsed Wave

!   Pulse wave sequence is:

1.  Transmit sound wave

2.  Wait for signal to come back

3.  Receive the signal

!   How long do we wait?

!   Depends upon maximum depth we are interested in

!   What is the wait time for for 3 cm in soft tissue?

!   39 micro sec (13 micro sec /cm)

Pulsed wave (PW) Parameter

!   Pulsed Wave Parameters

!   Pulse Duration

!   Pulse Length

!   Pulse Repetition Period

!   Pulse Repetition Frequency

!   Duty Factor

Spatial pulse length Spatial pulse duration

Pulsed Wave (PW) Parameters

Pulsed repetition period (PRP)

Pulse repetition frequency (PRF)

Duty factor

Spatial pulse length

Pulsed Duration

Ability to resolve Structure

Resolution : Pulse Length

!   Axial resolution : Ability to resolve two structures closely placed along

the longitudinal direction

!   Perpendicular to the beam

Axial Resolution

Reflected signal Transmitted signal

Axial Resolution

Spatial pulse length

Cannot resolve : Reflections overlap

Resolve

Resolution of the system

Pulse Length

!   If objects are spatial pulse length/2, they can be separated

!   Axial resolution is equal = spatial pulse length/2

!   Pulse length = Distance occupied by pulse

!   Wavelength * number of cycles

!   Pulse duration

!   Time occupied by the pulse

!   Duration of the pulse?

!   Typical values?

!   Typical values : 0.3 to 2.0 µsec

Spatial pulse length

Pulseduration = Pulse length

Speed

Spatial Pulse Length

!   Duration : Number of cycles in a pulse

!   Typically 2 to 3 cycles for BW-mode

!   About 8 to 10 cycles for color-mode

!   Pulse length (mm)

!   Length of the pulse : Wave length * number of cycles

!   For BW : (wavelength typically is 0.3mm for 5 MHz)

!   (0.3/2) * 2 = 0 3 mm à Resolution

Pulsed Wave Ultrasound

!   Transmit AND Receive capability

!   Pulsing voltage to crystals, produces pulses of sound energy :

!   During on time transducer is transmitting

!   Transducer listens to the reflecting signal : off time

!   During off time transducer is listening

!   Depends upon depth Controlled by transducer

What is pule repetition period? What is pule repetition frequency?

on off

on

Pulse Repetition Period (PRP)

!   PRP : Time from beginning of one pulse, to the beginning of the next pulse

!   On + off time

!   Waiting for the echo to return before sending another pulse

!   Depends upon the depth

!   130 μs for 10cm depth

!   Typical PRP values : 100 microsecond to millisecond

!   Off time is about 100 to 1000 times larger than on time duration

Pulse Repetition Frequency

(PRF)

!   Reciprocal of pulse repetition period

!   # of transmitted pulses per second

!   Typical value is 1 kHz to 10 kHz

!   Controlled by DEPTH

1s 0.5s

PRF

!   Deep imaging

!   Shallow imaging

PRP (=1/PRF)

!   PRP (or PRF) plays in important role for determining the frame rate (frames/s)

!   Each vector is PRF apart

!   For 10 cm depth, 300 vectors

!   Frame time = 130 (microsec)* 300 = 39 ms

!   Frame rate = 1/39 = 25 frames/s

Transmitting Very Much?

!   Percentage or fraction of time that the system is transmitting the pulse

!   In clinical setting for PW, typical values are 0.2% to 0.5%

100

× +

=

PRP

ion

PulseDurat t

t

t DF

of on on

ton

toff PRP

Toff : Listening time

Typical values

For pulsed wave : 0.2% or 0.5%

For example, for

1cm depth PRP = 13µs, ton = 0.02 µ s and toff = 12.98µs

2 cm depth PRP = 26µs, ton = 0.02 µ s and toff = 25.98µs

That is why ultrasound are considered safe:

Ultrasound : Interaction with

the Medium

1.  Frequency, Period, Wavelength, Intensity

Display Beamformer

Reflection

!   Reflection occurs when the dimension

of the boundary is more than few

wavelength of the sound

Diaphragm Renal capsule Kidney surface

!   DIFFUSE

!   When the interface is not smooth

!   Backscatter signals are not strong

!   Occurs within the tissue

!   Even if no normal incidence still reflection is detected

SPECULAR

o  Occurs when the interface is smooth

o  Mainly occurs at the tissue boundary

o  Normal incidence is necessary for

detection

o  Strong signal

Scattering

!   Apart from specular and diffuse reflection, another form of

interaction is

!   Scattering

!   Random reflection in many direction

!   Dimensions of targets are less than wavelength

!   Blood cells are 6 to 8 micro meter

!   Wavelength are typically 308 micrometer

!   Thus blood is a scattered

!   Higher the frequency (f=c/wavelength)

!   Higher the scattering

Reflections from blood cells are mainly due to scattering

Rayleigh scattering

!   Special form of scattering

!   Occurs when the structures dimensions are much less than the

wavelength

!   Redirects sound equally in all direction

!   Interaction between ultrasound and Red blood cells is Rayleigh scattering

!   Higher the frequency larger will be the Rayleigh scattering

!   When frequency doubles, Rayleigh scattering increases by 16x

4

f R∞

Interaction :Reflection and Scattering

Sound in all direction

Rayleigh scattering Objects are smaller than wavelength

Much weaker signal

Absorption

!   As the ultrasound propagate

!   It transfers energy to the medium

!   Contributes MOST to Attenuation

!   This transfer of energy oscillates the medium

!   Generates friction

!   Increase in frequency à Increase in heat generation

!   Gets converted into heat

!   Loss of ultrasound energy

!   Thus as the sound propagate they attenuate due to

!   Reflection, Scattering, and absorption (Heat generated)

Factors in Attenuation

!   Three factors contributing to the attenuation

!   Reflection , Scattering , Absorption

All these three are related to Frequency and

depth of penetration:

!   Attenuation coefficient!

!   In Soft tissue attenuation and frequency are directly related

!   Attenuation coefficient in soft tissue = 0.5 dB/cm/MHz

!   For 5 MHz, 1 cm, attenuation is 2.5 dB

!   For 5 MHz, 10cm, attenuation is 25 dB

Half Value Layer (depth)

!   Half-value layer thickness

!   Distance at which the intensity is ½ the original intensity

!  3dB attenuation

!   Units are centimeter

Half value layer is thin:

For tissues that attenuates the sound highest : Lung or bone

Half value layer is thick:

For tissues that attenuates the sound least : fluids

Media with high attenuation

Attenuation

Attenuation in Various Tissues Absorption, Scattering, Reflection

!   Air-absorption

!   Bone-absorption

!   Lung-absorption, scatter

!   Muscle-absorption

!   Soft Tissue- all three

!   Fat-absorb & scatter

!   Biologic Fluids-<ST

!   Water-no absorption

Take Away Points

!   The higher the frequency the greater the attenuation

coefficient and attenuation rate…

!   The longer the distance, the greater the attenuation,

loss of sound energy from original

!   There is not much sound energy left by the time it

returns to the transducer

Incident, Reflected and Transmission of

Ultrasound

Incident, Reflected and

Acoustic (Characteristic) Impedance

!   How much of ultrasound gets reflected depends upon the Characteristic (acoustic)

Impedance

!   Measured in RAYLES

!   Higher is the difference in the impedance between two media

!   Larger is the reflection

c

Intensity reflection coefficient

(IRC)

Transmission

!   Intensity transmission coefficient

!   Percentage of intensity that passes through the medium

How much is

Itransmitted? How much is Ireflected?

Reflected Transmitted Incident

Conservation of Energy

!   At boundary, energy is conserved

2

1 2

1 2

z

z I

I

i r

Intensity transmission coefficient (ITC) = X 100 = 100% - IRC

i

t

I I

Bushberg Table 16-3 Acoustic

impedance (Z) for selected tissues

!   Little reflection at the interface

!   Example: soft tissue to air-filled lung – large ΔZ, beam almost entirely reflected

!   …whereas if Z1 ~ Z2, then only minor reflections occur

Refraction

!   Refraction of beam occurs only with

!   Oblique incidence and

!   C1 ≠ C2

!   With these two conditions, sound will not travel in straight line

!   Soft tissue – muscle interface

!   Muscle – blood interface

! Snells law : Relates angle of incidence to angle of transmission

Speed in the Body

For simplicity : 1540 m/s is used while forming the image

Refract small degree at soft-tissue fat interface

Refract greater extent between soft tissue bone interface

1

2

) sin(

Refraction : Many artifacts

speed required