Educational Course - Imaging Ultrasound I

J. Brian Fowlkes, PhD* University of Michigan

Department of Radiology and Biomedical Engineering

*Equipment support from GE Medical and Toshiba Medical fowlkes@umich.edu

Triplex Mode

Gas Molecules in a Sound Wave

λ  - Spatial f - Temporal

Wave Propagation

Animation from Dr. Dan Russell, Kettering University"

Relationships Velocity-Frequency-Wavelength

c = Sound Velocity"f = Frequency"λ = Wavelength"

c = f λ

Speed of Sound

c (m/s)

Refraction"c2 < c1"

Propagation

Reflection"

Reflection and Refraction

θi = θr "

sin θi/ sin θt = c1/c2 (Snellʼs Law)"

Reflection and Transmission •  The reflection coefficient is

R = [(Z2-Z1)/(Z2+Z1)]2

•  The transmission coefficient is

T = (4Z2Z1)/(Z2+Z1)2

where Z1 and Z2 are the impedances of the two media.

Specular Reflection Noise or Structure?"

Speckle"

Electronic"

Speckle

Scatter dia << λ"

λ

Image Statistics

SNR=I/σ"

CNR=(Io−Ib)/(σo2+σb

2)1/2"

where averages are taken "over an ROI"

Attenuation (from Absorption and Scatter)

I = Ioe-2µd"

Depth"d"

Inte

nsity"

ATTENUATION COEFFICIENT

7.5 2.5 5.0

Lateral Resolution"Approximately equal to the beamwidth W"

W"

φ If φ < 50ο"Wf = λF/D""= λ(F#)"

D"

F"

I/Q Data

•  Baseband result for a specified carrier frequency

•  I - In phase

• Q - quadrature (shifted 900)

I = A(t)cos(ωt)cos(ω0t)

Q = A(t)cos(ωt)sin(ω0t)

€

BB = I + jQ€

I = (A(t) /2)[cos(ωt +ω0t) + cos(ωt −ω0t)]

€

Q = (A(t) /2)[sin(ωt +ω0t) + sin(ωt −ω0t)]

Env = I2 +Q2

RF Data

RF Data

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Time (microsec)

Am

plit

ude

(a.u

.)

Series1

A-mode

Phased Array Beam Steering

Transducer

Traditional B-mode Imaging

Transducer

Compound Imaging

Compound Imaging

Speckle Reduction"

Compound Imaging

Normal B-Mode

Example in Breast Imaging Wave Fronts

Animation courtesy of Dr. Dan Russell, Kettering University"

Stationary Sound Source" Source moving with vsource < vsound ( Mach 0.7 )"

Wave Fronts

Animation courtesy of Dr. Dan Russell, Kettering University"

Stationary Sound Source"Source moving with vsource < vsound ( Mach 0.7 )"

Doppler Equation

fD = 2f cosθ vo/c"f = Center frequency of

transmitted ultrasound"

θ = Angle of motion with respect to sound propagation"

vo = Velocity of blood"c = Sound speed"

€

cos(ω0t)cos((ω0 +ωD )t) =

12cos(2ω0 +ωD ) +

12cos(ωD )

Sum and Difference Frequencies"

Transmit" Receive"

ω0 = Transmit frequency"ωD = Doppler shift frequency"t = Time"

X"

Spectrum of Doppler Po

wer"

Frequency"

Wall Filter"

0 Fblood"

Spectral Data Display

Pow

er"

Frequency"

Time"

Scanner Display

Freq

uenc

y"

Time"Power"

Freq

uenc

y"

Time"Power"

Doppler Spectra

Hepatic Artery Response"

Portal Vein Response"

Δ t

Pulse 1

Pulse 2

Color Flow Velocity Estimation (Time Domain Correlation)

Spectrum of Bandpass

Pow

er"

Frequency"

Power Doppler Image

Doppler"Signal Power"

Power Doppler Image Why flow detected at poles?

Spectral Broadening Spectrum of Bandpass

Pow

er"

Frequency"

Mean Freq. = 0"Integrated Power ≠ 0

0"

Color Flow Image

k1 k2

θ

Transducer

True Velocity Imaging

x

yAxial"

Lateral"

Vy"

Vx"

V"

orthogonal flow

steer left flow steer right flow

12.5 cm

-12.5 cm

Integration of Doppler Velocity Vectors

imaging angle (lateral)

sweep angle (elevational)

C-plane - Torus surface (doughnut)

Pump setting [mL/s]"

Mea

sure

d flo

w [m

L/s]"

95% confidence interval

two realizations:

X = aperture ⊥ tube

O = aperture ‖ tube

line fit:

Y = (1.09±0.06) • X  - (0.36±0.52)

Results - varying flow

Ramnarine et al. Cardiovasc Ultrasound. 2003; 1: 17.

Tissue Doppler Imaging

Ramnarine et al. Cardiovasc Ultrasound. 2003; 1: 17.

Tissue Doppler Imaging

Speckle Tracking

•  Tissue Motion Quantification (TMQ)

•  Time Motion Annular Displacement (TMAD)

Speckle Reduction

•  Post processing of image data

•  May be performed on pre or post envelop (B-mode)

•  Can be thought of as filtering but algorithms can be complex

Simple Example Phantom Result on Scanner

No Speckle Reduction"

Phantom Result on Scanner

Too Much? "Speckle Reduction"

Speckle Reduction in Liver

Optimization

•  Can be applied to b-mode or Doppler

•  Relies on commonly known imaging behavior

•  Allows for quick adjustment

•  Use in conjunction with presets

Digital Encoding

•  Uses longer pulse sequences

•  Relies on code recognition (pulse compression)

•  Allow for extraction of low amplitude signals

Coded Sequences

1 1 0 1 0 0 1 1 1 1 0 0 1 0 1 1 0 "

Pulsed B-mode" Coded B-mode"Possible Advantages-Coded

Sequences

•  Improved detection of weak signals

•  Multichannel processing

•  Applications to harmonic imaging

Zone Sonography Rapid Zone Acquisition • Acquires an image frame ~10X faster • Akin to photography

Channel Domain Memory • Retains transducer element data • Stores a “Virtual Patient”

Channel Domain Processing • Enables iterative processing • Leverages Moore’s Law

Rapid Zone Acquisition: Acoustic Currency

time time

Acoustic Currency For Advanced Modes

• Line-by-line echo acquisition • Sequential processing of scan

lines • Image formation tied to sound

speed

• Echo data acquired from zones • SW processing of entire echo

data set • Image formation tied to

computer speed

Zero Acoustic Currency Left 90+% of Acoustic Currency Available

-1

0

1

2

3

Aco

ustic

Pre

ssur

e (M

Pa)

Distance (or Time)

Wavelength(or Period)

p+, pr

p–, pc

For the record...

•  Nonlinear acoustic propagation has been known for many years!!

•  Ultrasound contrast agents research led to increased nonlinear acoustics research

•  Physicians began commenting on image quality improvements in harmonic imaging without contrast

•  Analysis pointed towards tissue nonlinearity

t'"

u"

c=c0+βu!c>c0!

c<c0!

c=c0!c=c0! Propagation speed changes with particle velocity u!

t' =t-z/ c0!Retarded time!

Nonlinear Propagation!

Circular source"p0=0.45 MPa"MI=1.2"

Axial waveforms"KZK simulation"

Nonlinear propagation in tissue!

focus"

time waveforms" spectra"

Harmonic bandwidth considerations!

best axial resolution"

best harmonic separation"

WAVEFORM " SPECTRUM"

KZK simulation of waveform at the focus"

Measurements in water!P3-2 phased array!

MI=0.3!z=d=10cm!

Beam patterns!

Pulse inversion

Circular source"p0=0.45 MPa"MI=1.2"

Odd harmonics"sin(nωt+nπ)=-sin(nωt)"

Even harmonics"sin(nωt+nπ)=sin(nωt)"

Motion!addressed with more than 2 pulses"

time waveforms" spectra" Conventional Processing

Acoustic!field!

After!processing!

Receive filters

Tran

sduc

er

Tissue

Tran

sduc

er

Tissue

Transmitted signal

Fundamental energy

Harmonic energy

Harmonic Processing

Acoustic!field!

After!processing!

Receive filters

Tran

sduc

er

Tissue

Tran

sduc

er

Tissue

Transmitted signal

After!processing!

Harmonic Processing

Acoustic!field!

Tran

sduc

er Shallow fat layers

Tran

sduc

er Shallow fat layers

Clean image

Distorted sound beam

Receive filters

Transmitted signal

Conventional imaging! Tissue Harmonic Imaging!

Clinical examples

Conventional imaging Tissue Harmonic Imaging

Clinical benefits of THI

Clinical Benefits

•  Cardiology – Reduced overall clutter level –  Improved endocardial visualization – Difficult to image patients addressed

•  Radiology – Reduced haze / clutter –  Improved contrast resolution –  Improved border delineation – Difficult to image patients addressed

Perfluorocarbon Microbubbles

Insert Your Favorite Carbonated Beverage

Rasor Associates, US Patent Number 4,442,843

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Harder Drive - Subharmonic

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25 dB Increase in"Second Harmonic"

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Linear Scattering"

+"

Nonlinear Scattering"

Power Pulse Inversion Assume breathing motion of 2 cm/s This is 20 um per firing, ie. 4.8° phase shift @ 1 MHz

5°

10°

_

+

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5°

5°

Echo 1"

Echo 2"

Echo 3"2X"

Microvascular Imaging

Source: M Bruce, M Averkiou, K Tiemann, S Lohmaier, J Powers, K Beach "Vascular flow and perfusion imaging with ultrasound contrast agents” Ultrasound in Med. & Biol., Vol. 30, No. 6, pp. 735–743, 2004

1800"3600"

Microvascular Imaging

Source: M Bruce, M Averkiou, K Tiemann, S Lohmaier, J Powers, K Beach "Vascular flow and perfusion imaging with ultrasound contrast agents” Ultrasound in Med. & Biol., Vol. 30, No. 6, pp. 735–743, 2004

• Hemangioma. Both images with ultrasound contrast agent. (A) conventional imaging (B) Pulse inversion imaging (a)

Source: (a) Averkiou M., Powers J., Skyba D., Bruce M., and Jensen S. "Ultrasound Contrast Imaging Research. " Ultrasound Quarterly Vol. 19, No. 1, pp. 27-37 (2003)

One at a Time, Please

Source: Fuminori Moriyasu, M.D., Ph.D., Department of Gastroenterology & Hepatology, Tokyo Medical University, Japan"

Hold It!!!

Source: Fuminori Moriyasu, M.D., Ph.D., Department of Gastroenterology & Hepatology, Tokyo Medical University, Japan"

Fraunhofer Institute"

Developments to Watch

•  Compound Imaging

•  Advanced Image Postprocessing

•  Automation

•  Pulse Sequencing – Coded Sequences – Harmonics