Unit 5: Reflections

Reflection

the process of echo formation, offering information necessary to display images and Doppler in DMU

What two processes produce echoes for DMU?

Reflection and Scattering

Reflection

provides contour/capsule of the organs

Scattering

provides texture of tissues (organ parenchyma)

Specular Reflection

responsible for bright appearance of fibrous structures; typically reflection AND transmission occur

Specular Reflector must have an interface that is…

larger than wavelength, smooth

Specular Reflection can be…

normal or oblique

For Normal Incidence:

if Z₁ ≅ Z₂, most of the energy transmits

if Z₁ ≠ Z_2, most of the energy will be reflected

For Oblique Incidence (no DMU interest):

angel of reflection (θ_r) = angle of incidence (θ_i)

Anisotropy

effect that makes tendon appear bright when it runs at 90^o to the ultrasound beam, but is dark when angle is changed

Why does anisotropy exist?

at smooth boundaries, the θ_r = θ_i, thus the probe will only receive the reflected sound if the beam strikes the surface at a right angle

Specular Reflection Requirements

Z mismatch, Z₁ ≠ Z₂

-the larger Z mismatch, the stronger the echoes generated by reflector

Perpendicular Incidence

-angle-dependent, non-frequency dependent

Acoustic Impedance (Z)

-determines the amount of reflection at an interface

-if Z₁ = Z₂ the whole incident beam is transmitted —> I_T = I_I

Z equation

Z = ρ x c

Reflection Coefficient (R)

the fraction of the incident or initial wave that will be sent back to the transducer as an echo

R Equation

R = [(Z₁ - Z₂) / (Z₁ + Z₂)]²

R% Equation

R = [(Z₁ - Z₂) / (Z₁ + Z₂)]² × 100

Transmission Coefficient

the fraction of the incident sound that will be transmitted farther in the tissue to bring more information

T Equation

T = 1- R

T = 1- [(Z₁ - Z₂) / (Z₁ + Z₂)]²

T% Equation

T% = 100 - R%

T% = 100 - [(Z₁ - Z₂) / (Z₁ + Z₂)]² × 100

Stronger echoes mean what?

Not much more penetration

Strongly Attenuating Structures

Air: shadowing from reflection

Solids: shadowing from absorption

-DMU capitalizes on weak echoes that are amplified, allowing beam to travel deeper

Coupling Gel

how we compensate for attenuation; displaces air between the transducer and skin

-has Z closer to soft tissue (matching) —> increased intensity of transmitted ultrasound

R & T

% of the original intensity\

-given I_o we can calculate:

-Intensity Reflected (I_R)

-Intensity Transmitted (I_T)

Scattering

gives rise to characteristic texture (echo texture) of the image seen within soft tissue

2 Types of Scattering

Rayleigh Scattering and Diffuse Reflection

Rayleigh Scattering

-small amounts of energy are absorbed and retransmitted in all directions

-occurs at small boundaries that exist within tissue (<λ)

Diffuse Reflection

produced by reflectors with rough surface and dimensions > λ

Backscatter

echo that returns to transducer on same path of incident

-usually weak

-non-angle dependent, frequency dependent

Speckle

noise that degrades image, contributes to attenuation

Contrast Agents

-enhances scattering

-micro bubbles = Rayleigh Scatterers

-used in conjunction with harmonics —>

Contrast Harmonic Imaging

Properties of Contrast Agents

-easy administration

-nontoxic

-small enough to pass through capillaries

-echogenic with low attentuation

Elimination of Microbubbles

gas eliminated through lungs while shell components (metabolized) filtered by kidney and eliminated by liver

Clinical Applications

Contrast Harmonic Imaging can be used to:

-enhance LV opacification and endocardial border detection

-improve lesion and Doppler detection

Enhancement of Sub-Optimal Echocardiograms

up to 20% of echocardiograms are sub optimal —> 2 of 6 myocardial segments of LV can’t be visualized

-contrast may reduce need for subsequent imaging

Detection of Focal Liver Lesions

liver is most common site for meastatic spread of cancer

-can be isoechoic to liver parenchyma

CHI increases increases contrast between lesion and normal parenchyma because micro bubbles accumulate in normal tissue

Renal Perfusion

aids in diagnosis of renal artery stenosis and urinary obstruction

Artifacts

-High Intensity Transient Signals

-Blooming

High Intensity Transient Signals

sharp spikes of strong echoes on Doppler spectrum caused by micro bubble bursts

Blooming

-presence of color or power Doppler signal outside vessels

-caused by overload of Doppler signal detection from strong signals and multiple re-reflections between adjacent micro bubbles

-fix by reducing color gain or increasing PRF to decrease sensitivity