Upload
jojolilimomo
View
218
Download
0
Embed Size (px)
Citation preview
7/30/2019 PHYSICS OF utz
1/53
BY:
LOIDA R. AVESTRUZ-HORA ,MD
7/30/2019 PHYSICS OF utz
2/53
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.
7/30/2019 PHYSICS OF utz
3/53
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.( )
7/30/2019 PHYSICS OF utz
4/53
7/30/2019 PHYSICS OF utz
5/53
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
7/30/2019 PHYSICS OF utz
6/53
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.
7/30/2019 PHYSICS OF utz
7/53
7/30/2019 PHYSICS OF utz
8/53
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.
7/30/2019 PHYSICS OF utz
9/53
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.
7/30/2019 PHYSICS OF utz
10/53
-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.
7/30/2019 PHYSICS OF utz
11/53
7/30/2019 PHYSICS OF utz
12/53
7/30/2019 PHYSICS OF utz
13/53
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.
7/30/2019 PHYSICS OF utz
14/53
7/30/2019 PHYSICS OF utz
15/53
7/30/2019 PHYSICS OF utz
16/53
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.
7/30/2019 PHYSICS OF utz
17/53
7/30/2019 PHYSICS OF utz
18/53
7/30/2019 PHYSICS OF utz
19/53
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.
7/30/2019 PHYSICS OF utz
20/53
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.
7/30/2019 PHYSICS OF utz
21/53
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
7/30/2019 PHYSICS OF utz
22/53
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.
7/30/2019 PHYSICS OF utz
23/53
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.
7/30/2019 PHYSICS OF utz
24/53
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.
7/30/2019 PHYSICS OF utz
25/53
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).
7/30/2019 PHYSICS OF utz
26/53
INSTRUMENTATIONS 4. Matching Layer
provides interface between the transducer
element and the tissue> minimizes the acousticimpedance differences between the transducer andthe patient.
7/30/2019 PHYSICS OF utz
27/53
7/30/2019 PHYSICS OF utz
28/53
7/30/2019 PHYSICS OF utz
29/53
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.
7/30/2019 PHYSICS OF utz
30/53
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.
7/30/2019 PHYSICS OF utz
31/53
7/30/2019 PHYSICS OF utz
32/53
7/30/2019 PHYSICS OF utz
33/53
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.
7/30/2019 PHYSICS OF utz
34/53
7/30/2019 PHYSICS OF utz
35/53
7/30/2019 PHYSICS OF utz
36/53
INSTRUMENTATIONS
TRANSDUCERS USED:
1) Linear Array Transducer-rectangular image
-OB, small parts and peripheral vascular examination.
-larger filled of view(FOV)
-large surface area
7/30/2019 PHYSICS OF utz
37/53
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.
7/30/2019 PHYSICS OF utz
38/53
7/30/2019 PHYSICS OF utz
39/53
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
7/30/2019 PHYSICS OF utz
40/53
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
7/30/2019 PHYSICS OF utz
41/53
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.
7/30/2019 PHYSICS OF utz
42/53
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.
7/30/2019 PHYSICS OF utz
43/53
7/30/2019 PHYSICS OF utz
44/53
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.
7/30/2019 PHYSICS OF utz
45/53
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
7/30/2019 PHYSICS OF utz
46/53
IMAGING PITFALLS1) Artifacts-structures not actually presenta. reverberation
b. refractionc. side lobes
d. shadowing
e. multipath artifact
7/30/2019 PHYSICS OF utz
47/53
7/30/2019 PHYSICS OF utz
48/53
7/30/2019 PHYSICS OF utz
49/53
7/30/2019 PHYSICS OF utz
50/53
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
7/30/2019 PHYSICS OF utz
51/53
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).
7/30/2019 PHYSICS OF utz
52/53
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.
7/30/2019 PHYSICS OF utz
53/53