Resident Physics LecturesResident Physics Lectures

01:01:

Ultrasound Ultrasound Basics Basics PrinciplesPrinciples

George David, M.S.Associate Professor of Radiology

Ultrasound TransducerUltrasound Transducer

Speakertransmits sound pulses

Microphonereceives echoes

• Acts as both speaker & microphone Emits very short sound pulse Listens a very long time for returning echoes

• Can only do one at a time

Piezoelectric PrinciplePiezoelectric Principle

• Voltage generated when certain materials are deformed by pressure

• Reverse also true! Some materials change dimensions when

voltage applied» dimensional change causes pressure change

when voltage polarity reversed, so is dimensional change

V

US Transducer OperationUS Transducer Operation

• alternating voltage (AC) applied to piezoelectric element

• Causes alternating dimensional changes alternating pressure changes

• pressure propagates as sound wave

Ultrasound BasicsUltrasound Basics

• What does your scanner know about the sound echoes it hears?

AcmeUltra-Sound

Co.

I’m a scanner, Jim,

not a magician.

What does your scanner know about echoed sound?

What does your scanner know about echoed sound?

How loud is the echo?

inferred from intensity of electrical pulse from transducer

What does your scanner know about echoed sound?

What does your scanner know about echoed sound?

What was the time delay between sound broadcast

and the echo?

What else does your scanner know about sound echoes?

What else does your scanner know about sound echoes?

• Direction sound was emitted

What else does your scanner know about echoed sound?

What else does your scanner know about echoed sound?

The sound’s pitch or frequency

What Does Your Scanner Assume about Echoes(or how the scanner can lie to you)

What Does Your Scanner Assume about Echoes(or how the scanner can lie to you)

• Sound travels at 1540 m/s everywhere in body average speed of sound in soft tissue

• Sound travels in straight lines in direction transmitted

• Sound attenuated equally by everything in body (0.5 dB/cm/MHz, soft tissue average)

Luckily These Are Close Enough to Truth To Give Us Images

Luckily These Are Close Enough to Truth To Give Us Images

• Sound travels at 1540 m/s everywhere in body average speed of sound in soft tissue

• Sound travels in straight lines in direction transmitted

• Sound attenuated equally by everything in body (0.5 dB/cm/MHz, soft tissue average)

Ultrasound DisplayUltrasound Display

• B-scanB-scan (“Brightness” Mode) Image series of gray shade dots

• For each dot, scannermust calculate position Gray shade

Images from EchosImages from Echos

Dot Placement on ImageDot Placement on Image

• Dot position ideally indicates source of echo

• scanner has no way of knowing exact location Infers location from echo

?

Dot Placement on ImageDot Placement on Image

• Scanner aims sound when transmitting

• echo assumed to originate from direction of scanner’s sound transmission

• ain’t necessarily so

?

Positioning DotPositioning Dot

• Dot positioned along assumed line

• Position on assumed line calculated based upon speed of sound time delay between sound transmission & echo

?

Distance of Echo from Transducer

Distance of Echo from Transducer

• Time delay accurately measured by scanner

distance = time delay X speed of sound

distance

What is the Speed of Sound?What is the Speed of Sound?

• scanner assumes speed of sound is that of soft tissue 1.54 mm/sec 1540 m/sec 13 usec required for echo object 1 cm from transducer

(2 cm round trip)

distance = time delay X speed of sound

1 cm13 sec

Handy rule

of thumb

So the scanner assumes the wrong speed?

So the scanner assumes the wrong speed?

• Sometimes

?

soft tissue ==> 1.54 mm / sec

fat ==> 1.44 mm / sec

brain ==> 1.51 mm / sec

liver, kidney ==> 1.56 mm / sec

muscle ==> 1.57 mm / sec

•Luckily, the speed of sound is almost the same for most body parts

Gray Shade of EchoGray Shade of Echo

• Ultrasound is gray shade modality

• Gray shade should indicate echogeneity of object

? ?

How does scanner know what gray shade to assign an echo?How does scanner know what gray shade to assign an echo?

• Based upon intensity (volume, loudness) of echo

? ?

Gray ShadeGray Shade

• Loud echo = bright dot

• Soft echo = dim dot

ComplicationComplication

• Deep echoes are softer (lower volume) than surface echoes.

Gray Shade of EchoGray Shade of Echo

• Correction needed to compensate for sound attenuation with distance

• Otherwise dots close to transducer would be brighter

Depth CorrectionDepth Correction

                        

Echo’s Gray ShadeEcho’s Gray Shade

• Gray Shade determined by Measured echo strength

» accurate

Calculated attenuation

Charles LaneWho am I?

Attenuation CorrectionAttenuation Correction

• scanner assumes entire body has attenuation of soft tissue actual attenuation varies

widely in body

• Fat 0.6

• Brain 0.6

• Liver 0.5

• Kidney 0.9

• Muscle 1.0

• Heart 1.1

Tissue Attenuation Coefficient (dB / cm / MHz)

Ultrasound DisplayUltrasound Display

• One sound pulse produces one image scan line

» one series of gray shade dots in a line

• Multiple pulses two dimensional image

obtained by moving direction in which sound transmitted

Moving the Sound BeamMoving the Sound Beam

• electronically phased or pulsed transducer arrays

Arrows indicate timing variations.Activating top & bottom elements earlier than center ones focuses beam.

Focus

Scan PatternsScan Patterns

• LinearLinearbeam translated

» moved sideways

produces rectangular image

• sectorsectorbeam pivoted produces pie-shaped image

Th’ Th’ Th’ Th’ That’s All

Folks