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Physics of Ultrasound And why you should know about it… Misha Bhat, MD AEPC basic echo course 2018

Outline � Understanding basic concepts of sound � How ultrasound is generated, captured

and interpreted �  Possibilities of ultrasound och technologies �  Limitations

� Understanding how to optimize your images and image what you want

� Hopefully you wont fall asleep by the end of the talk

Physics of Sound � Sound is made by longitudinal waves of

compression and rarefaction

V=1540 m/s in soft tissue

compression

rarefaction

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Definition � Ultra = “beyond” “above” � Sonus = “noise”

� Sound with frequencies higher than what is audible to human ear (20 kHz)

� Medical US is 2-15 MHz

https://sofiasounds.weebly.com/ultrasonic-sound-and-infrasonic-sound.html

bbc.co.uk http://zaprilepite.biodiversity.bg/

How does it work?

� Piezoelectric crystals � Change shape with

current

� Application of alternating current

http://people.bath.ac.uk/rjm64/Site/present%20applications.html www.makeagif.com

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How does it work? � Piezoelectric crystals

� Alternating current � Cycles of deformation

and rarefaction – acoustic wave

� Acoustic wave is transmitted from probe and into the tissue @1540 m/s

http://people.bath.ac.uk/rjm64/Site/present%20applications.html www.makeagif.com

Reflection � Results in sound returning to the transducer

� At interfaces between tissues esp if different density; myocardium and blood pool

� Diaphragm and mediastinal surface are specular (mirror-like) reflectors

� Almost all is reflected off bone – no penetration beyond

� Most reflection if perpendicular. If parallel to the surface, less reflection, which may cause false “droupout”.

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False drop-out in atrial septum

Scatter � As sound wave hits smaller

particles (RBC, tissues), the waves scatter.

� A smaller portion will bounce back towards the transducer (back scatter) and is used for generating image.

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Refraction � Occurs at interface

between tissues � Depending on angle

and mismatch of acoustic impedance

� Sound waves are instead refracted

Attenuation �  Loss of energy with distance

�  Loss to heat �  Dispersion of signals

�  Higher frequencies have greater attenuation

�  Lower frequencies have better penetration (reach deeper structures)

�  Signals from deeper structures are weaker:

�  Modern machines will have attentuation correction

�  Otherwise manually correct by your TGC

From sound to pictures � Transducer sends out sound and

then listens � Returning waves from reflection

and backscatter cause deformation of piezoelectric crystals – generate electric signals

� Processed to generate a image

� Each transmission-listening cycle is called pulse echo

� 100 microseconds (5 MHz)/pulse echo

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Returning signal is encoded on a gray scale More returning signal is coded brighter Less signal is coded darker

Lai, Mertens, Cohen, Geva – Echocardiography in Pediatric and Congenital heart disease

www.emergencypedia.com

From linear signal to image: M-mode

�  Probe in same position over time allows us to see motion and change in thickness over time in one cross section/beam

�  “Motion” or M-mode �  Use to measure dimensions over the cardiac

cycle, function, valvular movement

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2D image/B-mode

Phased Array Transducers � Sequentially

activating the crystal allows form form beams in different directions

� Allows wide area of imaging with smaller probe size

� Arranged as a fan with scan lines

�  If arranged as a matrix – 3D imaging

Ok but what about image quality

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� Resolution is the ability to see two near objects as separate

� Direct relationship between higher frequency and greater resolution

� However higher frequency has poor penetration

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A quick note on trade-off

DEPTH VS RESOLUTION

Resolution types in ultrasound

� Lateral � Axial � Elevational

TEMPORAL

SPATIAL

Lateral resolution

Better with � Higher frequency �  In focal zone � Narrower beam width � Decreased depth � Larger probe

(limitations of size)

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Axial Resolution

� Directly related to higher frequency

� Attenuation may limit depth

Elevational resolution

� Depends on beam thickness

�  Inferior to lateral resolution due to multiple reasons which cannot be modified

� Less of a problem in matrix array transducers

Temporal resolution �  How many times a second

the entire image refreshes (frame rate - Hz)

�  Number of scan lines/beams + distance traveled

�  Movement is smoother, allows you to follow movement and structures

�  Regular TV = 25Hz

�  Frame increases with: �  Decreased depth �  Narrowing sector width �  Lower line density �  Single focal point �  Parallel beam forming �  M-mode>2D>3D

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Summary � Ultrasound can help image heart and

other structures � Only part of the ultrasound is reflected

back, rest is lost � Resolution is key feature and linked to

frequency used � Higher frequency gives better lateral and

axial resolution but at tradeoff of depth � Temporal resolution important to study

movement of the heart