Physics - waves in tissues and tissue properties

speeds in different tissues, acoustic properties and impedance,

What is the speed of ultrasound propagation in soft tissue?

1540 m/s

What is the speed of ultrasound propagation in bone? cortical bone (outer) and trabecular bone?

4400 m/s

3500 - 4400 m/s

2000- 2400 m/s

What is the speed of ultrasound propagation in air?

330 m/s

What happens at tissue boundaries ?

some of the pulse can be echoed back to the receiver

What do we need in order for an echo to form

sound waves propagated across different material boundaries produces an echo. such as the boundary between air and muscle, muscle and soft tissue

every time theres a difference in tissue properties we get an echo

what materials reflect ultrasound waves almost 100%

air and bone and metal

How does fat effect ultrasound?

subcutaneous fat causes phase abberation which causes the beam to disperse rather than be a focused line

Ultrasound waves refract in fat, causing artefact

What angle should the ultrasound beam hit the tissue

as close to 90º as possible (0º to the normal)

perpendicular to the tissue so that most of the reflections are received at the transducer

What is impedance

a materials resistance to ultrasound waves

How does impedance of tissues relate to echoes

echoes (reflections) are formed at the boundary between two tissues as long as they have different impedance

everytime theres a change in tissue impedance an echo is reflcted

what is the formula for impedance?

Z = p X c

Z: impedance

p = tissue density

c= constant for speed of sound through that tissue

How does impedance relate to tissues and materials?

each tissue has a unique impedance

What are reflections?

Name the 2 types of reflections?

• Specular reflections (mirror like):

• Scatter / backscatter

what are specular reflections?

• occur at tissue boundaries where there is a change in impedance

• reflections off larger (>2 wavelengths in diameter) and smooth structures

• strong signals

What are specular reflectors?

tissues or structures which are large enough (> 2 wavelenghts) and smooth enough to produce specular reflections

How big do structures need to be to produce specular reflections?

> 2 wavelengths

Examples of specular reflectors?

• valves

• walls: but not the myocardial contents

• major vessel walls

• pericardial sac

• the connective/ structural/ vascular parts of tissue

Where are specular reflections produced?

at tissue boundaries where there is a change in acoustic properties such as impedance. Echoes are produced.

How can you visually tell what the strength of an echo is?

the grey level it is assigned

stronger echoes are assigned brighter greys

• i.e reflections off specular reflectors will appear bright (light greys/ whites)

weaker echoes are assigned darker greys

• (backscatter) off smaller or rougher structures like fluid will appear as darker.

How is grey level effected by the strength of the echo?

the brighter the grey the stronger the echo

the darker the grey the weaker the echo

Why does blood appear as darker than tissue on an echo?

Blood is a non-homogenous medium consist of blood cells which are < the wavelength of the ultrasound wave so produce backscatter reflections which are weaker echoes than spectral reflections and are assigned darker grey.

Blood a non-homogenous medium consisting of red and white blood cells suspended in plasma which have similar acoustic properties so there’s less impedance at the boundaries and therefore less of the ultrasound is reflected.

Why is blood dark on echo?

1. Blood is a non-homogenous medium consisting of different cell types: red and white blood cells suspended in plasma. These two tissue types have similar acoustic properties, so little difference in impedance reduces the amount of ultrasound reflected at the tissue boundaries.

What is backscatter reflection?

• when ultrasound waves strikes a structure which is < the wavelength of the beam or a rough structure

• this causes many reflections in many different directions which have smaller amplitudes and weaker signals compared to specular reflections

• The transducer receives various reflections from different directions of different amplitudes and strengths which appear as speckled structures on the image

• The reflections are not dependent on the angle of incidence however, unlike with specular reflections

What happens to the majority of scattered reflections?

Absorbed or reflected by other tissues but not received by tranducer

Much of the signal is attenuated due to absorbtion or more scattering

Only a small fraction of the incoming beam is reflected back at the transducer

List some tissues which give backscatter reflections?

-Blood / red blood cells

-myocardium cells (within walls)

-the parenchyma of tissues: the functional parts of tissue responsible for specialised tasks (the bulk of the inner mass of tissues)

What type of reflections does blood produce?

Backscatter

this is where there are lots of small particles so sound waves are reflected off in different directions to the main beam

Specifically Rayleigh scatters which are equal in all directionsn

What are Rayleigh Scatters?

Specific type of backscatter refleciton produced when ultrasound hits red blood cells.

Reflections are equal in all directions.

how much of ultrasound is reflected when soft tissue meets muscle

about 1%

how much of ultrasound is reflected when soft tissue meets gas

99.9%

how much of ultrasound is reflected when soft tissue meets bone?

40-50%

of these boundaries, which will have the most reflection ?

• when soft tissue meets bone?

• when soft tissue meets gas?

• when soft tissue meet muscle?

the greater the difference in tissues impedance the greater the reflections

greatest reflection when soft tissue meets air.

What is the purpose of ultrasound gel?

• a coupling agent

• medium between the transducer and the skin, to enable the ultrasound waves to enter the skin and not be reflected by air.

air reflects 100% of ultrasound waves

What acoustic properties should ultrasound gel have?

similar impedance to soft tissue to minimise reflections

How does depth affect the attenuation?

there is greater attenuation at depth

What happens if the ultrasound signal bends

Refraction of a sound wave occurs if it travels between tissues with different propagation speeds. As the incident pulse or returning echo strikes an interface of different density or elasticity and therefore a different propagation speed, the direction of the wave changes according to Snell’s law

What causes an ultrasound beam to bend (refract)?

if the beam travels through tissues with different speedd of sound constants.

Can ultrasound machines account for refraction?

no. US machines assume that all reflections travel in a straight path so refraction will appear as artefact. things are plotted in the wrong position so cannot reallty out-optimise this effect.

what tissue is particualry susceptible to producing refractions?

subcutaneous fat

at what angle does refraction not occur'?

when the beam is 90 degrees to the surface

visually what can refraction make images look like?

blurry

Which law governs refraction? S

Snell’s law

What is Snell’s law for refraction?

how do we calculate the angle/ index of refraction?

[may have to do some maths rearranging, not sure if acc need to be able to calculate this].

C1 / C2. = sinθ 1 / sinθ2

or C1 x sinθ1 = C2 X sinθ2

What is diffraction?

wave spreads out over larger area? amplitude decreases

What is scatter?

scatter is a type of artefact.

scatter occurs when ultrasound waves reflect off small particles in different directions. the image produced not producing an image of a solid sturtcure as it is, rather the constituent parts. so the image we see is the contents of the tissue but not exactly how it is structured.

-the reflections

How can we distinguish scatter from normal reflections?

scatter pattern will change if the probe moves. whereas reflections off solid items will not change pattern.

Lecture Q’s: What is the purpose of ultrasound gel?

Gel is a coupling agent which means it enables ultrasound waves to propagate from the transducer through the skin of the patients and into the patient.

Air reflects 99.9% of ultrasound waves so gel is needed to eliminate air between the transuducer and probe so that the ultrasound isn’t reflected before entering the patient.

Lecture Q’s: what is a suitable characteristic impedance of gel?

• similar acoustic properties to soft tissue

• similar impedance to soft tissue to minimise reflections when the ultrasound beams travel across the boundary between gel and skin

Lecture Q’s: what are the problems of scanning near ribs and lung?

• Ribs (bone) reflect a large % of US so most US will not be transmitted deeper than and will appear bright on image

• Lungs filled with air also highly reflective so

Lecture Q’s: List all the causes of lost ultrasound intensity ?

• reflection

• backscatter

• refraction

• absorption

• attenuation (just depth)

Lecture Q’s: what is the diameter of a RBC