Sound in Medicine

Sound

• Longitudinal wave & require medium

cf) Transverse waves: EM wave– RF waves, Light, X-ray

Sound Speed

• Depedent on medium property

– Different from particle movement speed

• c(m/sec)= (B/)1/2

– B: bulk modulus, kg/msec2

measure of stiffness

– : density: kg/m3

• cair(m/sec)=(313.3+0.606T[C])

Medium Speed

air(0C) 313

air(20C) 343

water 1482

sea 1522

copper 5010

glass 5640

steel 5960

Sound Frquency

• Wavelength of 1 KHz sound at air(20C) and water– air,1KHz=343/1000=0.34m, water,1KHz=1282/1000=1.48m,

• Number of condensation(rerefaction) coming per second

– f=c/, =c/f

Sound Pressure & Intensity

• P: Pressure amplitude

– Varaition caused by paticle density of medium

• Intensity=P2

– Loudness of sound

• Decibel

– Relative sound intensity

– =10log(I/Io)

• Io:1.0×10-12 W/m2

(weakest intensity level can hear)

Soulnd Intensity Level

• By sound level meter

Intensity I (W/m2)

Intensity Level(dB)

Threshold of hearing 1.0×10–12 0

Rustling leaves 1.0×10–11 10

Whisper 1.0×10–10 20

Normalconversation 1.0×10–5 70

Inside car in city traffic 1.0×10–4 80

Car without muffler 1.0×10–2 100

Live rock concert 1.0 120

Threshold of pain 10 130

Doppler Effect

• By moving source

– Change in wavelength and frequency

– When the source is moving, the wavelength in front of the source becomes smaller, while the wavelength behind the source becomes larger.

Frequency Change

• Moving toward

– ’= - vsT

• vs : speed of the source

• T: 1/fs

– fo= v/(’) = v/(-vsT)

= fs /(1- vs /v)

• Moving away

– fo= fs /(1 +vs /v)

NEXRAD

• Next Generation Weather Radar System

– Doppler weather radar system used by the National Weather Service

– Dramatically improved early warning of severe storms

Ultrasound

• Sound waves: Require medium

• Frequency above human hearing– Audible range: 20Hz ~ 20kHz

– Infrasound: Less than 20Hz

– Ultrasound: Higher than 20KHz (cf: supersonic)

• Medical use: 2MHz~10MHz

• 1912 : Titanic 호의 추적• Sonar(Sound Navigating

and Ranging ) : World War II

• 1940 ~ 1950: Medical Application: Transducer, Ultrasound beam: Display의 개발: Start to use in obstetrics

Use of Ultrasound

Using Echo

Echo in mountains Fishfinder

Ultrasound Imaging

• Transducer echo pulse (body) reflection transducer processing display

• Less harmful than ionizing radiation

- Preferred for obstetric patient

- Not suitable for lung & bone

Unique Energy Mode

Microscope

Endoscope

Radiography

Ultrasound

MRI

PETCT

SPECT

Gamma Camera

Visual Image

OphthalmoscopeThermograph

RF IR Visible X-ray γ-ray

Not in EM wave

EM wave

1st Ultrasound Scanner

• Somagram: In water-bath, pulse echo, 2MHz

– In 1952 by Douglass Howry

– B/W image on scope

Ultrasound Imaging System

Imaging Systems

Interaction with Matter

• Reflection: at tissue boundary

– Due to the difference in acoustic impedance

• Refraction

– Change in direction of transmission

• Scattering: cause beam diffuse

– By reflection and refraction

• Absorption

– Energy loss by converting into heat

• Attenuation

– Absorption + scattering

Propagation Speed

• c(m/sec)= f =c/f

– f: cycles/sec, :wave length

• c(m/sec)= (B/)1/2

– B: bulk modulus, kg/msec2

measure of stiffness

– : density: kg/m3

• 2MHz in soft tissue

– =c/f=1540/2106

=0.77mm

MaterialDensity (kg/m3 )

Speed (m/sec)

air 1.2 330

lung 300 600

fat 924 1450

water 1000 1480

soft tissue 1050 1540

kidney 1041 1565

blood 1058 1560

liver 1061 1555

muscle 1068 1600

bone 1912 4080

PZT 7500 4000

Acoustic Impedance

• Z(kg/m2sec)= c

– 1 rayls = 1 kg/m2sec

• Determines reflection

– Small Z difference

small reflection

– Large Z difference

large reflection

MaterialDensity (kg/m3 )

Speed (m/sec)

Z (106

rayls)

air 1.2 330 0.0004

lung 300 600 0.18

fat 924 1450 1.34

water 1000 1480 1.48

soft tissue 1050 1540 1.62

kidney 1041 1565 1.63

blood 1058 1560 1.65

liver 1061 1555 1.65

muscle 1068 1600 1.71

bone 1912 4080 7.8

PZT 7500 4000 30.0

Reflection

• At boundary interface

• Due to the difference in acoustic impedance

• Reflection coefficient

– Fraction of reflected pressure• Rp= Pr/Pi =(Z2 – Z1)/(Z2 + Z1)

– Fraction of reflected intensity • RI= Ir/Ii =(Z2 – Z1)

2/(Z2 + Z1)2

• Transmission coefficient

– TI =1- RI

R

I

T

Z1 Z2

Reflections between tissues• RI,(Fat Muscle)=[(1.71-1.34)/(1.71+1.34)]2 =0.015

• RI,(Muscle Air)= [(1.71-0.0004)/(1.71+0.0004)]2 =0.999

– Impossible to image beyond lung

– Need coupling gel to avoid air gap between transducer and skin

Tissue interface Intensity Reflected Intensity Transmitted

Liver-Kidney 0.003 % 99.7 %

Liver-Fat 1.1 % 98.9 %

Fat-Muscle 1.5 % 98.5 %

Muscle-Bone 41.0 % 59.0 %

Muscle-Lung 65.0 % 35.0 %

Muscle-Air 99.9 % 0.001 %

Transducer• Pulse generation

– Electrical energy

Mechanical energy

• Detection– Mechanical energy

Electrical energy

• Piezoelectric Material– Dipole molecular arrangement

– Heating over Curie temperature & slowly cooling while applying voltage

– Lead-zirconate–titanate(PZT)

Generation & Detection

Pulse GenerationElectrical energy change in dipole arrangement pressure(ultrasound)

Pulse DetectionUltrasound (pressure) change in dipole arrangement electrical signal

Image Data Acquisition

• Ultrasound production Propagation Interaction Receiving Processing Display

Pulse Echo • Pulse Repetition Frequency: 2~4kHz

– Determine imaging depth• 2KHz 1540(m/sec)500(usec)/2=38.5cm

– Pulse duration: 1~2 usec, 0.2~0.4% duty cycle• 3 cycles of transducer frequency

• Most of time for signal receiving: more than 99.5%

Receiver

to compensate the attenuation by depth

To reduce dynamic range

Smoothed single pulse

Rejection of low level noise and clutter

Display Modes

• A-mode

– Amplitude mode

– Midline of brain

• B-mode

– Brightness mode

– Amplitude

brightness of point

• M-mode

– Motion mode: time variation of B-mode signal

– Excellent temporal resolution of a single line

2-D Image display• Scan Converter

– Create 2-D image from B-mode signal

• Articulating Arm & static display

– Linear, sector, compound scanning

– Display storage: several seconds per image

Ultrasound Images

•M(Motion) mode •B(Brightness) mode

3-D Imaging

• Linearly

• Freeform motion – external localizer

• Rocking

• Rotation

• Rendering: – Maximum Intensity

Projection

3D Surface Display

Doppler Ultrasound

• Doppler shift

– Frequency shift by moving reflector

– Blood cells

• fd=2fiv cos()/c

– fi: incident frequency

– v: blood velocity

– v= fdc/2fivcos()

– Usually in audible range• Convert into sound for

positioning

Doppler Images

• Power Doppler•Color Doppler

A flow map based on the integrated power of the Doppler spectrum

A flow map based on the mean Doppler frequency

Measurements• Distance, Area, Volume

• Higher resolution in beam direction

• Fetal age determination by measuring fetal head diameter

19weeks 6daysby diameter

18weeks 4daysby circumference

Spectral Interpretation

• Blood flow: laminar, blunt, turbulent

– Different spectrum

– Vessel wall property, size, shape, flow rate

• Velocity distribution along time

– Of selected area

– Stenosis

Turbulent flow

extended velocity

profile

Bioeffect

• At higher intensities

– Thermal effect• Heat production

• Dissipation by blood

– Mechanical effect• Mechanical movement

on particles

• Negative pressure induce bubble formation cavitation

• Remarkably safe in diagnostic range

– Deleterious bioeffect not reported

Attenuation

• Absorption of intensity, convert into heat

• Attenuation coefficient: (dB/cm)

– 0.5dB/cm/MHz• 2cm by 5MHz pulse 250.5dB/cm/MHz=5dB

• Half value thickness

– 50% attenuation thickness

– Frequency dependent• smaller for higher frequency

Resolution & Attenuation

• Resolution (depth)

– Depends on : shorter higher resolution

• Attenuation

– Increase with frequency

– I P2

– Relative intensity(dB)=10log(I2/I1)= 20log(P2/P1)

• Thick tissue (abdomen): 3.5M~5.0MHz

• Thin tissue (thyroid, breast): 7.5M~10.0MHz

Matching Layer

• Large impedance difference between transducer and patient

– RI,(PZT tissue)=[(30-1.62)/(30+1.62)]2 =80.6% TI,=19.4%

• Matching layer: Zm=(Z1Z2)1/2

– RI(PZT ML)=[(30-7)/(30+7)]2 =38.6% T1=61.4%

– RI(ML tissue)=[(7-1.62)/(7+1.62)]2=38.9% T2=61.1%

– Ttotal= T1T2 =37.5%

– Multiple matching layers

• ¼ of resonance frequency

• Also acoustic coupling gel

TGC & Log Compression

Time Gain Control Logarithmic Compression

Mechanical Scanning

• By wobbling of single element transducer

• By mechanical rotation of transducers

• Also dynamic scanning with real-time

Electronic scanning

• Linear array: 256~512– ~20 elements

– Overall size: 6~8cm

– Wide FOV near transducer

– Uniform pixel size

• Phased array: 64~256

– Time delay of all elements

– Overall size: 3~6cm

Frame Rate & Spatial Sampling

• Minimum time for 1 line scan

– Tline=D(cm)2/1540(m/sec)=D13usec

• One image with N lines

• Frame rate

– FR=1/(NTline)=77,000/ND

– Line density : by FOV• Spatial sampling

– Trade off: • FR FOV, N, D

Spatial Resolution

• Axial resolution: depth resolution

• Lateral resolution: azimuthal resolution

• Elevational resolution: slice thickness

Axial Resolution

• Resolution in beam direction

• ½ of spatial pulse length

– SPL: No. of cycles per pulses• Usually 3 cycles

• High resolution

– Short SPL• Require heavy damping

– Higher frequency

• Not depend on depth

Resolution of 5MHz transducer- SPL=30.31mm=0.93mm Axial resolution=0.47mm

Lateral resolution

• Perpendicular to beam direction

• Depends on beam diameter– Maximum resolution at

focal point• ½ of transducer diameter

– 2~5mm typically

• Also depth dependent

• Can control by phase array

Beam Properties

• By overlap of individual waveform

• Near field

– Converging

– d2/4

• Far field

– Diverging

– Sin=1.22(/d)

Near Field

• Near field length

= d2/4

= d2f(MHz)/(41.54mm)

– Increase with diameter

– Increase with frequency

• Focal transducer

– Curved elements or acoustic

lens

– For narrow beam width and

shorter focal length

– Focal zone: Less than 2Minimum width

Elevational Resolution

• Perpendicular to image

plane

• Determined by transducer

element height

• Worst resolution– Partial volume averaging effect

• Elevational focusing– Minimize slice thickness by controlling time delay

– Limit access due to bulky transducer volume

– Full 2D array Uniform resolution for 3D imaging

Special Transducers

• Array transducers

• Intracavity transducers

– Inside-out mapping

• Intravascular transducer

Contrast Agents

• For vascular imaging & perfusion imaging

• Encapsulated micro-bubbles containing gases

– Air, Nitrogen, insoluble gases

– Diffuse and propagate into targeting site

• Micro-bubble act like point source

– Diffuse ultrasound and generate harmonics

• Need harmonic imaging for selective imaging

– Input frequency of beam: f0– Receiving frequency: 2f0