physics of utz

7/30/2019 PHYSICS OF utz

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BY:

LOIDA R. AVESTRUZ-HORA ,MD


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OBJECTIVE

To provide an overview of the fundamentals of

acoustics, the physics of ultrasound imaging andultrasound instrumentation with emphasis on pointsmost relevant to clinical practice.


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DEFINITION OF TERMS1. COMPRESSION-amechanical deformation

induced by an external force ,with resultant

increase in the pressure of the medium.2. RAREFACTION: occurs after compression event

,as backward motion reverses the force.

3. Wavelength-distance between

COMPRESSIONS or RAREFACTIONS, orbetween any two points that repeat on thesinusoidal wave of pressure amplitude.( )


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DEFINITION OF TERMS4. Period-time duration to complete a single wave

cycle .( cycles/sec.)

5. Frequency- number of times the wave oscillatesthrough a cycle each second.

6. Sound-is transmitted as series of alternatingpressure waves producing compression and

rarefaction of the conducting medium


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DEFINITION OF TERMS7. Hertz(Hz)-the unit of acoustic frequency where 1 Hz

= 1 cycle per second.

8. Piezoelectric effect-the conversion of electricalenergy into mechanical (sound) energy by physicaldeformation of the crystaline structure.

* A Piezoelectric material (often a crystal or

ceramic) is the functional component of thetransducer.


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INTRODUCTION Infrasound less than 15 cycles/sec(Hz)

Normal range of hearing 15 Hz and 20 kHz(audible acoustic spectrum).

Ultrasound high frequency range above 20 kHz.

Medical diagnostic or therapeutic ultrasound

uses frequency range of 2 - 10MHz.* withspecialized applications up to 50 MHz.


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BASIC ACOUSTIC PRINCIPLES1. PROPAGATION OF SOUND

- brief energy pulses applied continued back and

forth motion

series of compressions/rarefactionspropagated through the medium/tissuemediumis necessary for mechanical energy transfer (soundpropagation).

-

for energy propagation to occur , as a wave front inthe direction of energy travel as Longitudinal wave.


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-eventually ,an acoustic image is formed fromnumerous pulses of ultrasound reflected from tissuesinterfaces back to the receiver.

* The resolution of the ultrasound image and theattenuation of the ultrasound beam energy depend

on theWavelength and Frequency.


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BASIC ACOUSTIC PRINCIPLES2. ACOUSTIC IMPEDANCE ( Z=pc )- p- density and c-speed of sound

- gives rise to differences in transmission and reflectionof ultrasound energy ,which is the basis for pulse echoimaging

-acoustic interfaces-junction of tissues:

Large acoustic impedance differences (tissue &air/bone interface)reflects.

Smaller differences (muscle & fatinterface)reflects only part.


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BASIC ACOUSTIC PRINCIPLES3. REFLECTION-determined by size and surface feature of the interface.

-ultrasound energy at a boundary between two tissuesoccurs ,because of the differences in the acousticimpedances of the two tissues.

-Perpendicular sound beam sound bounce back tothe transducer

Non-perpendicular - ultrasound energy is reflectedat an angle equal to the incident angle.


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BASIC ACOUSTIC PRINCIPLES4. REFRACTION- in direction of propagation/ transmitted

ultrasound energy at a tissue boundary when thebeam is not perpendicular to the boundary.

-Ultrasound frequency does not changewhenpropagating into the next tissue,but a in thespeed of sound wave.misregistration of astructure.

-increasing scan angle perpendicular to theinterfaceminimize the artifact.


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BASIC ACOUSTIC PRINCIPLES5. SCATTERING

Most organs have tissue characteristic structurethat gives rise to a defined scatter signature toprovide diagnostic information

Hyperechoic (higher scatter amplitude)

Hypoechoic (lower scatter amplitude)* both denote #of scatterers ,acoustic impedance

and size of scatterers.


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BASIC ACOUSTIC PRINCIPLESCont..Scattering

a. Specular reflector- smooth boundary between

two media ,where the dimension are much largerthan the wavelenght of the ultrasound energyreturn echoes Td >W (-) frequency

b. Diffuse reflectors-smaller Td interface(within

solid organsechoes are scattered in alldirection. Td


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BASIC ACOUSTIC PRINCIPLES6.ATTENUATION-sound passes through tissueloses energy by

absorption(heating), reflection andscattering.

-determines efficiency of ultrasound to penetratespecific tissue.

-higher frequency attenuated more rapidly thanlower frequency.


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INSTRUMENTATIONS

1. TRANSMITTER OR PULSER-energizes the transducer-pulsed ultrasound-commonly used

-control output voltagecontrols rate of pulses emittedconsistent with diagnostic problem.

-spaced ultrasound pulses.


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INSTRUMENTATIONS2. TRANSDUCER- ceramic elements with electromechanical properties

electric energy to mechanical energy and viceversa using Piezoelectric materials.

-Two functions:

1. Converts electric energy to acousticpulses.

2. Receives reflected echoes converting weakpressure changes into electric signal.


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INSTRUMENTATIONS-Elements:1. Piezoelectric crystals-natural and synthetic

natural= quartz crystal

synthetic = lead-zirconate-titanate(PZT).

2. Damping Block /Backing materials-soften sound

-reduce pulse length (2-3cycles)better axial

resolution.3. Specialized transducer coatings and coupling gels-efficient energy transfer(transducer to body).


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INSTRUMENTATIONS 4. Matching Layer

provides interface between the transducer

element and the tissue> minimizes the acousticimpedance differences between the transducer andthe patient.


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INSTRUMENTATIONS

3. RECEIVER AND PROCESSOR

-detect and amplify the back scattered energy andmanipulate the reflected signals for display.

-provides means for compensatory differences inechostrength due to attenuation.

-compression and maping of data.


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INSTRUMENTATIONS4. IMAGE DISPLAY( echo display modes)

A) A-mode AMPLITUDE)- display of the processed

information from the receiver versus time.B) M-mode (MOTION)-uses B-mode info/ display

echoes from moving organs/ organ tissue.

C) B-mode ( BRIGHTNESS) electronic conversion of

the A-mode and A-line into brightness-modulateddots on a display screen.


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INSTRUMENTATIONS

D. Real time, gray scale B-mode displaygenerate

2D image-greatest intensitywhite

-absence of signalblack

-intermediate intensityshadows of gray

-real time -2D images at rate of 15-60 frames /sec.

Assesses anatomy and motion. Effect is dynamic.


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INSTRUMENTATIONS

TRANSDUCERS USED:

1) Linear Array Transducer-rectangular image

-OB, small parts and peripheral vascular examination.

-larger filled of view(FOV)

-large surface area


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INSTRUMENTATIONS

2) Sector scanners

- either mechanical or electronic steering- small surface area

- examination where access is limited

- superimposes multiple views of the anatomy in asingle plane.


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INSTRUMENTATIONS-transducer frequency selection:1) Superficial vessels and organs (thyroid, breast,

testicles & intraoperative application): 1-3 cm. from

the surface- 7.5 to 10 MHz

2) Deeper structures (abdomen or pelvis): more than12 to 15 cm. from the surface

- 2.25 to 3.5 MHz


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INSTRUMENTATONS

4. IMAGE DATA ACQUISIION

Image formation requires knowledge of ultrasoundproduction ,propagation and interactions

Images are created using pulse echo method

Pulse transmit into the patient> partial reflectionsfrom tissue interfaces > create echoes> return totranducer

Image formation> hardware components>data


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INSTRUMENTATIONS5. IMAGE STORAGE-video display are standardized and match to hard

copy device.

-image display determined by brightness andcontrast settings (system gain settings and TGCadjustment).

-permanent storage-transparencies printed or filmby optical or laser cameras and printers, as well ason videotape. Digital storage.


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IMAGE QUALITY The key determinants of the quality of an

ultrasound image include its spatial, contrast andtemporal resolution, and freedom from certainartifacts

1) Spatial resolution-ability to differentiate 2 closely

situated object as distinct structures.


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IMAGE QUALITYa. axial resolution-refers to the ability to discern two

closely spaced objects in the direction of the beam.

b. lateral resolution-plane perpendicular to the beamand parallel to the transducer. Determined by thewidth of the ultrasound beam.

c. elevation/azimuth resolution-slice thickness is

perpendicular to the plane of the image and to thetransducer.


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IMAGE QUALITY 2. CONTRAST RESOLUTION

- reflections that delineate tissue boundaries andinternal architectures

- density and size of scatterers>specific texture

3. ARTIFACTS

- arise from incorrect display of anatomy or noise

during imaging- machine and operator related


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IMAGING PITFALLS1) Artifacts-structures not actually presenta. reverberation

b. refractionc. side lobes

d. shadowing

e. multipath artifact


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IMAGING PITFALLS2) Improper adjustment of system gain and time gaincompensation settings(strength of sound)

3) Inadequate penetration

4) Poor resolution5) Careless selection of transducer frequency and lack

of attention to the focal characteristics of the beam


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BIOLOGIC EFFECTS/MECHANISM

OF ACTION1. THERMAL EFFECTS-molecular agitation and

relaxation stresses.

2. CAVITATION-tiny bubbles of gas or cavities willform.

3. VISCOUS STRESSES-tissue interface(differenttissue viscosities)viscous

stress(boundary)microstreamingdisruptsmembrane and cells(interface).


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BIOLOGIC EFFECTS

The biologic effects is not the real issue, theknowledge and skill of the users are majordeterminants of the risk-to-benefit implication of theuse of ultrasound in a specific clinical situation.


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