Ultrasonic Nonlinear Imaging- Contrast Imaging. History 1968 Gramiak et al published observation of...

Ultrasonic Nonlinear Imaging-Contrast Imaging

History

• 1968 Gramiak et al published observation of echo signal from LV injection of indocyanine dye

• Subsequent research showed this phenomenon occurred with just about any liquid injected through small needle

More History

• First work was with free gas bubbles– bubbles didn’t last very long– size too big to go through lungs, needed intra-arterial

injection

• Late ‘80’s - early ‘90’s - development of numerous agents– more stable– smaller size

Motivation

• X-ray, CT, nuclear, and MR all need it.

• Enhance backscatter signal from blood– Blood signal typically 40dB below tissue

• Provide visualization of low velocity flow normally masked by tissue motion– measure of microvasculature important in many

disease states

Desired Properties

• Non-toxic/easily eliminated

• Able to be injected intravenously

• Small enough to pass through microcirculation

• Physically stable

• Acoustically active

Contrast Imaging

• Contrast agents are used to provide higher contrast. The three commonly seen contrast agents are backscatter, attenuation and sound velocity.

• Contrast agents could be solid particles, emulsion, gas bubbles, encapsulated gas, or liquid.

Contrast Imaging

• Primary clinical benefits:– Enhanced contrast resolution between normal

and diseased tissues.– Outline of vessels or heart chambers.– Tissue characterization by using tissue specific

agents.– Increasing blood flow signals.– Dynamic study using washout curve.

Example

Before injection

After injection

Harmonic imaging

Harmonic Doppler

More Examples

Bubbles & Physiology

Parenchymal phase at 90-120 seconds, can be up to 5 min

Arterial phase starts20-45 seconds after injection

Portovenous phaseat 45-90 seconds

High MI, Harmonic B-modeusing Levovist

Liver Metastases- primary Breast Ca

Pre Post

Tumor Detection

Coded Harmonic Angiousing Levovist

Tumor CharacterizationFocal Nodular Hyperplasia

Coded Harmonic Angiousing Levovist

Tumor Characterization

Hepatocellular Carcinoma (HCC)

Low MI Harmonicusing Sonovue*

* Images with non-approved agents for internal GE training only

Tumor Characterization

Low MI Harmonic - 2using Definity*

* Images with non-approved agents for internal GE training only

Early Phase Late Phase

Tumor Detection

High MI Fundamental Colorusing Levovist

Hemangioma

Pre Post

Tumor Detection/Characterization

High MI Harmonic Colorusing Levovist

Tumor Detection

Hemangioma?, Adenomatous Nodule?

Clinical Values (I)

• Tumor Detection• presence or absence of liver, kidney or pancreatic masses

• Tumor Characterization • avascular- cyst• hypovascular- metastasis, hemangioma• hypervascular- primary carcinoma, hypervascular met

• Others• enhances vessels for RAS, Carotid stenosis, TCD, etc• better visualization of thrombus (IVC, TIPS)• post ablation follow up• trauma assessment

Clinical Values (II)

• Endocardial border detection.

• Left ventricle (LV) function.

• Valvular regurgitation quantification.

• LV flow patterns.

• Perfusion area of coronary artery.

• Assessment of surgery for ventricular septal defect.

Clinical Values (III)

• Liver tumor enhancement.

• Uro-dynamics and kidney functions.

• Tubal function and placenta perfusion.

• Transcranial Doppler enhancement.

• LV pressure measurements.

Current Contrast Agents

• Aerosomes (ImaRx, Tucson, AZ) • Albunex (MBI, San Diego, CA)• BY963 (Byk Gulden, Konstanz, Germany)• Echovist (Schering, Berlin, Germany)• EchoGen (Sonus, Bothell, WA) • DMP115 (DuPont-Merck, N. Billerica, MA) • Imagent US (Alliance, San Diego, CA) • Levovist (Schering, Berlin, Germany)• NC100-100 (Nycomed, Oslo, Norway)

Current Contrast Agents (Cont.)

• Optison (MBI, San Diego, CA) approved in US for cardiac• Oralex (MBI, San Diego, CA)• PESDA (Univ of Nebraska, Omaha, NE)• SonoRx (Bracco, Princeton, NJ) US approved oral agent • Sonovist (Schering, Berlin, Germany)• Sonovue (Bracco, Milan, Italy)• ST68 (Drexel Univ, Philadelphia, PA)• Quantison (Andaris, Nottingham, UK)• Quantison Depot (Andaris, Nottingham, UK)• Many more,…

Contrast Mechanisms

• Strong backscattering produced by air bubbles.

• The backscatter increases roughly linearly with the number of micro-bubbles.

• A bubble in liquid acts as a harmonic oscillator. Acoustic resonance provides the major echo enhancement. In addition, strong harmonics are produced.

Contrast Mechanisms

• Acoustic attenuation of soft tissues is typically represented by a constant (e.g., 0.5dB/cm/MHz).

• Since contrast agents significantly change the scattering properties, attenuation measurements can also be used for contrast enhancement.

Contrast Mechanisms

• Sound velocity is primarily determined by density and compressibility. Apparently, micro-bubble based contrast agents alter sound velocity.

• Contrast enhancement based on sound velocity variations is still academic.

Contrast Mechanisms

• Micro-bubbles produce strong harmonics when insonified near the resonance frequency.

• If such harmonics are stronger than tissue harmonics, contrast can be improved.

• Second harmonic signal is most useful due to limited transducer and system bandwidth.

(Encapsulated) Gas Bubbles

Bubble Characteristics

• Size

RBC6–8 µm

Microbubble2–8 µm

• Gas– use high molecular weight, less soluble gas

• Shell for stabilization– tune for desired acoustic properties

Ultrasound-Induced Encapsulated Microbubble Phenomena

• Oscillation

• Translation

• Coalescence

• Fragmentation

• Sonic cracking

• Jetting

• ,…

Optical Measurements


100 Mframes/s camera

Examples

Pressure Dependence of Expansion

MI = 0.089 MI = 0.15

MI = 0.39MI = 0.25

Variations in Bubbles Reaction

Bubble Oscillation

Ultrasound-Induced Oscillation

• Moderate: Alternate expansions and contractions with the same amplitude and duration at low driving pressures (stable cavitation).

• Violent: At higher pressures, greater bubble expansion amplitude than contraction amplitude, and relatively slow expansion followed rapid contraction (inertial or transient cavitation).

• Cavitation threshold: Above which the bubble’s maximum radius is larger than twice the equilibrium radius.

Modeling• Strength of backscatter signal depends on

difference in acoustic properties between two materials...

d2 4 m

m

p m

p m( ) =

c V f

)

2 +

p2

4

23

(cos

p = particle m = surrounding medium

For a particle of volume V in homogenous medium

= compressibility = mass density

Modeling• Now need to include shell effects...

RR + 3

2R

R

RRR - P(t)2

Keff tot0

3

1

For a shell encapsulated gas bubble of instantaneous radius R:

eff ~ elasticity of shell = density of surrounding media

tot = total damping coefficient P(t) = incident acoustic energy

Accurate only at low pressures

Simulations

Simulations

Free Encapsulated

Simulations

Measurements


MI = 0.09

MI = 0.67

Translation

Translation

• Resulted from primary radiation force (pressure gradient across the bubble surface).

• Maximal in contraction phase.

• Used for active targeting.

0adrad FFFF

Translation

Translation

• Secondary radiation force: The microbubbles translate toward each other (oscillating bubbles generate spatially varying pressure fields).

Coalescence

Coalescence

• Fusion of two or more bubbles.

• As bubbles expand, bubble surfaces flattens and thinning occurs.

• When critical thickness is reached (around 0.1 micron), bubbles rupture and merge with each other.

Coalescence

Fragmentation

Fragmentation

• Fission of a bubble into smaller bubbles.

3

2

:fragments ofNumber

3

237

3

1

tension)surface :(

nN

n

RR

Fragmentation

Sonic Cracking

Sonic Cracking

• Ultrasound induced formation of a shell defect causing gas to escape from the bubbles.

• Mechanism not yet known.

Jetting

Jetting

• During contraction near a boundary, collapse may be asymmetrical.

Potential Clinical Applications

Interference from Tissue Nonlinearities

Non-Linear Response

Contrast agentsTransmit freq.= fo

2fo

fo

Conventional Imaging: Receive freq. = fo

Harmonic Imaging: Receive freq. = 2fo

Various Contrast Modes

• High MI B-Mode, Harmonic - optimized for SAE harmonic imaging

• Low MI B-Mode, Harmonic 1 and 2 - optimized for nondestructive harmonic imaging

• High MI colorflow fundamental - optimized for SAE destruction effect

• High MI colorflow harmonic - optimized for SAE with reduced tissue flash artifact

Step 1:

Xmit2 5

Step 2:

Step 3:

2 5 74Suppress

Rcv2 74 10

Utilizes Encoding Technique

From ‘Coded Harmonics’ To

Suppress Fundamental Signal

Uses decoding techniques similar to B Flow to

separate tissue & contrast signal

Coded Harmonic AngioCoded Harmonic Angio

Step 4:

74SeparateContrastSignal

Contrast Agent

Tissue

Tissue and Contrast

Problem:

Blood echoes are very weak and

sometimes moving too slow for Doppler techniques

Detecting Blood Reflectors

BloodTissue

Noise

Solution: Inject contrast agents to enhance signal

Problem:




Agent

Noise

Tissue


Use codes to

1) detect harmonic return signal

Problem:




AgentTissue

Noise


Use codes to

1) detect harmonic return signal

2) Suppress tissue signal

Problem:




AgentTissue

Noise

Harmonic Interference• In contrast imaging, in which the tissue

harmonic signals are un-desirable, the amplitude of the propagating wave needs to minimized.

• Large apertures (smaller f-numbers) may be used.

• It was reported that tissue harmonic signal can be reduced by 3dB by doubling the aperture size.

Reduction of Interference from Tissue

• Harmonic cancellation system.• Sub-harmonic imaging.• Pulse-inversion Doppler (clutter).• Pulse-inversion fundamental imaging.

Harmonic Cancellation System

Aperture Size vs. Harmonic Generation

Harmonic Cancellation Using a Pre-biased Signal

Sub-Harmonic Imaging

Sub-Harmonic Imaging

• Tissue propagation does not generate significant sub-harmonic signals.

• Sub-harmonic signals may be generated with microbubbles in proper acoustic fields.

• Sub-harmonic imaging can thus be used for reduction of tissue nonlinear signals.

Sub-Harmonic Generation

0.6 MPa, 16 cycles

0.6 MPa, 64 cycles


0.23 MPa, Occurence

0.53 MPa, Growth

1.17 MPa, Saturation

Pulse Inversion Doppler (for Clutter Reduction)

Pulse Inversion Doppler

Pulse Inversion Doppler (Linear)

Pulse Inversion Doppler (Non-Linear)

Pulse Inversion Doppler

Pulse Inversion Fundamental Imaging

Effects of Transmission Phase

Pulse Inversion Fundamental Imaging

Ultrasonic Nonlinear Imaging- Contrast Imaging. History 1968 Gramiak et al published observation of...

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