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Basic Ultrasound Physics

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Kirk Spencer MD

The Physics of Echo •   Sound

– Audible range 20Khz – Medical ultrasound Megahertz range – Advantages of imaging with ultrasound

•   Directed as a beam •   Tomographic •   Reflected from small objects •   Non-ionizing

– Disadvantages •   Propagates poorly through air •   Penetration poor (attenuation)

The Physics of Echo λ = wavelength = v/f v= velocity f = frequency cycle

•   Velocity of sound α density and temperature 1,540 m/sec soft tissue

•   Frequency 3.5 MHz (1-7 MHz) •   λ = (1,540 m/sec)/(3.5 MHz) = 0.44 mm

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Scattering –  echoes originating from

relatively small, weakly reflective, irregularly shaped objects

–  object size > λ/4 –  not angle dependant –  inefficient

The Physics of Echo

Reflection –  echoes originating from

relatively large, regularly shaped objects with smooth surfaces

–  objects large α wavelength –  angle dependant –  valve, endocardium,

pericardium

The Physics of Echo

Is there pericardial calcification? The Physics of Echo

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Resolution: •   Lateral resolution - the ability to

resolve objects side by side •   Structures must be separated by

more than the width of the ultrasound beam to be distinguished as separate –  Transducer size (larger

better) –  Frequency (higher better) –  Focusing –  Gain (lower better)

The Physics of Echo

Beam width

The Physics of Echo

Low gain

Medium gain

High gain

Resolution: •   Axial Resolution - Axial resolution

is the ability to resolve objects that lie along the path of the ultrasound beam

•   Related to frequency of transducer and pulse duration

•   In practical terms, axial resolution is roughly twice the wavelength

The Physics of Echo

Frequency Wavelength 2.2 MHz 0.68 mm 3.5 MHz 0.43 mm 5.0 MHz 0.3 mm

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Axial resolution vs penetration •   ↑ frequency leads to better

resolution •   Penetration ∝ wavelength (1/

frequency) – More scattering (more reflection by

smaller scatterers) – More attenuation

The Physics of Echo

Reso

lutio

n

Pene

tratio

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Attenuation – Loss of ultrasound energy as it passes

through tissue (scattering and absorption) half-power (cm)

•  Water 380 •  Blood 15 •  Soft tissue 1-5 •  Muscle 0.6-1 •  Bone 0.2-0.6 •  Air 0.08

The Physics of Echo

The Physics of Echo Resolution vs penetration •   ↑ frequency leads to better resolution •   Penetration ∝ wavelength (1/frequency)

Use the highest frequency probe that gives you an adequate image

2.5 Mhz - 3.5 MHz

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Permanently-polarized material such as quartz (SiO2) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. These materials are piezoelectric, and this phenomenon is known as the piezoelectric effect.

The Physics of Echo

•   Conversely, an applied electric field can cause a piezoelectric material to change dimensions. This phenomenon is known as electrostriction, or the reverse piezoelectric effect

•   This shape deformation creates ultrasound waveforms

The Physics of Echo

Scanning – Mechanical transducers

•   Rotating multiple elements, or a single element and set of acoustic mirrors to generate the sweeping beam for 2D imaging

– Electronic / array transducers •   Have the ability to be steered by

sequentially stimulating each element. This feature creates the sector scan by rapidly steering the beam from left to right to give the two dimensional cross sectional image.

The Physics of Echo

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Electronic / array transducers Linear array •   Sequential firing or array elements

moves beam linearly •   Require large acoustic window •   Creates a linear or rectangular

shaped scan plane

The Physics of Echo

Electronic / array transducers Phased array •   Phased control of array firing

controls beam direction (and thus scan line)

•   Creates a sector or pie shaped scan plane.

The Physics of Echo

Imaging –  Electrical stimulate piezoelectric

crystal which sends ultrasound pulse

–  Transducer then “listens” for

returning ultrasound signals –  Transducer “listens” 99 percent

of time, which increases sensitivity

The Physics of Echo

1-2 µsec

0.4 µsec

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Modes:

•   A Mode - amplitude mode. Where the signals are displayed as spikes that are dependent on the amplitude of the returning sound energy.

•   B Mode - brightness mode. Where the signals are displayed as various points whose brightness depends on the amplitude of the returning sound energy.

The Physics of Echo

Modes:

•   M Mode - motion mode. The application of B-mode and a strip chart recorder allows visualization of the structures as a function of depth and time.

The Physics of Echo

Modes: •   2D Mode - 2 dimensional

mode. The spatially oriented B-mode where structures are seen as a function of depth and width. The beam is rapidly swept back and forth to create a cross section of the imaged structures.

The Physics of Echo

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The Physics of Echo

Shadowing The loss of information

behind an object because the sound energy was reflected back by the object such that no signal passes beyond it

Bone, metal valve, air

The Physics of Echo Artifacts - sidelobe Ultrasound reflections off

real objects, but from the ultrasound beam sidelobes, not the central beam

Occurs because ultrasound beam has width to it

Worse when gain is high

Is there a catheter in RA?

The Physics of Echo

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The Physics of Echo Artifacts - reverberations •   Multipath artifacts •   Sound bounces back

and forth between two interfaces. This prolongs the time of flight, producing an artifact deep to the interface.

Artifacts - reverberation •   Results from ultrasound strikes a target composed of

several highly reflective interfaces •   Appear as relatively parallel irregular bright lines

extending from the structure

Artifacts - reverberation •   Appears as a linear

brightness in the direction of the sound beam and deep to a strong reflector

•   Results from multiple back and forth reflections

•   Appear as relatively parallel irregular bright lines extending from the structure

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The Physics of Echo

•   Harmonics •   Mechanical Index •   Compression •   Focus •   Transducer frequency •   Gain •   Grayscale / power Doppler

•   Depth •   Triggering •   Frame rate •   PRF •   Packet size •   Post-processing

The Physics of Echo Overall gain •   Increases the intensity of received echoes •   Makes image brighter

The Physics of Echo

Depth •   Use the least depth that fits the structure of

interest on the screen

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Nonlinear distortion of ultrasound

Frequency

Am

plitu

de

Frequency

Am

plitu

de Fundamental

Fundamental

Harmonics

The Physics of Echo

The Physics of Echo

Harmonic imaging Lateral resolution

–  Smaller harmonic beam width

Clutter reduction –  Sidelobe levels decrease

with increasing harmonic number

Near field artifact reduction

Am

plitu

de

Frequency MHz

Fund Harm

On-Axis Reflectors

Off-Axis Reflectors

The Physics of Echo Doppler Effect •   Christian Johann Doppler 1842 •   If a source of sound is stationary, the wavelength

and frequency of sound emanating from the source are constant

•   If a source of sound is moving toward you , it’s wavelength is decreasing (frequency increasing)

•   If a source of sound is moving away from you , it’s wavelength is increasing (frequency decreasing )

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The Physics of Echo

The Physics of Echo

Dependence on angle between scatter and incident

ultrasound beam

The Physics of Echo Doppler

echocardiography Continuous wave

–  Separate transmit and receive transducer

– Continuously receiving – No maximal velocity

limit – Range is ambiguous

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The Physics of Echo Doppler echocardiography Continuous wave

The Physics of Echo Doppler echocardiography Pulsed wave •   Range gated Doppler •   Inability to detect high

frequency Doppler shifts •   Inability to detect high

velocities

The Physics of Echo

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The Physics of Echo

Doppler echocardiography Color Doppler •   Multiple pulsed Doppler

samples along each scan line

The Physics of Echo Doppler echocardiography Color Doppler •   Velocities colored coded •   Blue- away, Red - toward