Interaction of Ultrasound With Matter

INTERACTION OF ULTRASOUND WITH MATTER

J.J. John

Objectives

• At the end of this lecture, students should be able to Describe :
• How an ultrasound beam propagates through tissues
• Attenuation of ultrasound beam
• The interactions processes

Practical implications of each interaction process

Introduction

In ultrasonography, a beam of ultrasound must be directed into the tissues of the subject over a selected area of interest.

The ultrasonic energy will then interact with the tissues along its path.

The interaction processes are influenced by the characteristics of the ultrasound wave, as well as the physical properties of the tissues through which the beam passes.

So different types of tissues will respond differently to an ultrasound beam.

Terminologies

• Acoustic impendance
• Acoustic mismatch
• Acoustic boundary/interface

Attenuation

Acoustic impedance (z)

Acoustic impedance is a measure of the resistance of the particles of the medium to mechanical vibrations.

This resistance increases in proportion to the density of the medium, and the velocity of ultrasound in the medium.

• Mathematically:
• Also known as the characteristic acoustic impedance of a medium.

It differ from one medium to the other.

Acoustic impedance (z)

Acoustic boundaries

Positions within tissue where the values of acoustic impedance change are very important in ultrasound interactions.

These positions are called acoustic boundaries, or tissue interfaces.

For example, urine in the bladder will have an acoustic impedance value which differs from that of the bladder wall, hence their common interface constitutes an acoustic boundary.

URINE

Acoustic boundaries

The unique features of diagnostic ultrasound as an imaging modality are determined by the nature and distribution of the multitude of acoustic boundaries within the tissues of the body.

And the extent to which an acoustic boundary affects a beam of ultrasound incident upon it will depend on the magnitude of the difference between the acoustic impedance values of the two structures on either side of the boundary.

Acoustic mismatch

Acoustic mismatch can be defined as the difference in acoustic impedance (Z) values of the two media forming the boundary.

• For example an acoustic mismatch for:
• Muscle/fat boundary= (1.7-1.38)= 0.32
• Bone/fat boundary= (7.80-1.38)= 6.42
• Soft tissue/air = (1.63-0.0004)= 1.6296

Soft tissue/water = (1.63-1.48)= 0.15

The Interaction Processes

• Reflection
• Specular reflection
• Non specuar reflection (Scattering)
• Refraction

Absorption

Reflection of ultrasound

This is the most important single interaction process in ultrasound image formation.

When a beam of ultrasound strikes an acoustic boundary, some of the beam energy is transmitted across the boundary, while some is redirected backwards(reflected).

Depending on the size of the boundary relative to that of the ultrasound beam, or on irregularities of shape on the surface of the reflector. Two types of reflection can occur, these are:

• Specular reflections

Non-specular/Diffuse reflections (Scattering)

Specular reflections

Specular reflections occur when the boundary is smooth and larger than the beam dimensions.

For this type of reflection, a simple law similar to that governing the reflection of light is obeyed Angle of incidence (i) = angle of reflection (r) The reflected beam is referred to as the echo and the probability that an echo will go back to the transducer and be detected increases as the angles i and r decrease.

Diffuse/Non-specular Reflections (Scattering)

This is the reflection of ultrasound in many direction Occurs when the reflecting interface is irregular in shape, and its dimensions are smaller than the diameter of the ultrasound beam The direction of scatter does not obey the simple law of reflection, but depends on the relative sizes of the scattering target and the ultrasound beam diameter The dimensions of the interface should be about one wavelength of the ultrasound beam or less for scattering to occur

Non-specular Reflections (Scattering)

Since the wavelengths for typical diagnostic beams are 1 mm or less. and Within the organs, there are many structures which have dimensions of less than 1 mm, so scattered ultrasound provides much useful information about the internal texture of organs.

Scattered echoes are much weaker than specularly reflected echo but the high sensitivity of modern ultrasound equipment makes it possible to utilize information from scattered ultrasound for imaging Scattering shows very strong frequency dependence, increasing rapidly as the frequency of ultrasound is increased.

The intensity of an echo

• The intensity of an echo due to specular reflection depends on the
• Angle of incidence

Acoustic mismatch

The proportion of beam intensity that is reflected from the interface represented by the reflection coefficient which is given as ratio of the intensity of a reflected beam to that of incidence beam (Ir/Ii) For a given intensity at the source, echo sizes will vary in proportion to this ratio

Angle of incidence and Echo intensity

The most useful specular reflection takes place when the ultrasound beam strikes a reflector at 90° to the surface of the boundary. This is referred to as normal incidence.

On the Normal incidence, angles i and r are equal to zero, and the echo goes straight back with a high probability of being picked up by the transducer

Reflections

IMAGE A

The diaphragm is a large and relatively smooth surface that reflects sound like a mirror reflects light. Thus sound striking the diaphragm at nearly a 90-degree angle is reflected directly back to the transducer, resulting in a strong echo.

Sound striking the diaphragm obliquely is reflected away from the transducer, and an echo is not displayed (yellow arrow).

IMAGE B

n contrast to the diaphragm, the liver parenchyma consists of acoustic interfaces that are small compared to the wavelength of sound used for imaging. These interfaces scatter sound in all directions, and only a portion of the energy returns to the transducer to produce the image.

Acoustic mismatch and Echo intensity

As previously seen an acoustic mismatch is the difference in acoustic impedance (Z) values of the two media forming the boundary.

This difference in Z-value at an interface determines the intensity of the reflection coefficient.

A large change of Z value at an interface makes the reflection coefficient large and therefore gives rise to a large echo, whereas small changes of Z produce small echoes

PERCENT REFLECTION OF ULTRASONIC ENERGY AT VARIOUS BOUNDARIES

Acoustically homogeneous media

No echoes are seen within compartments in which the Z-value remains constant.

The reflection coefficient is zero throughout such compartments and therefore there are no acoustic interfaces within.

In clinical ultrasound, acoustic homogeneity is observed in liquid cavities as clear, echo-free zones

Negative role of gas

When a very large proportion of the ultrasonic energy incident upon a boundary is reflected, the residual intensity becomes too low for interrogation of structures lying beyond the boundary.

The near total reflection of ultrasound at boundaries between gas and other materials makes gas a barrier to the transmission of ultrasound

How can we overcome this??

Coupling gel

It is necessary to apply suitable gel or oil between the transducer surface and the patient's skin in order to exclude air.

Transducer matching layer

The front face of the transducer, between the piezoelectric crystal and the skin, has a matching layer made of a material of suitable Z and thickness This serves to reduce the acoustic mismatch between the crystal material and soft tissue.

The Z-value of the matching layer is chosen to lie between that of the crystal and soft tissue. This reduces the reflection of ultrasound that would be experienced at the crystal/soft tissue interface in the absence of the matching layer Thus, improving the transmission of ultrasound into the patient, and of the returning echo into the transducer.

Refraction of ultrasound

Refraction of ultrasound

Refraction is a change of beam direction at a boundary between two media in which ultrasound travels at different velocities.

It is caused by a change of wavelength as the ultrasound crosses from the first medium to the second while the beam frequency remains unchanged.

Significance of Refraction in ultrasonography

Unlike reflection, refraction does not contribute usefully to the process of image formation.

However, the deviation of ultrasonic energy into new directions contributes to loss of beam intensity.

• Refraction may also be the cause of artefacts.
• Duplicate artefact

Edge shadowing artefact

The image above shows a Axial transabdominal image of the uterus with a small gestational sac (A) and what appears to be a second sac (B) due to refraction artifact.

Absorption of ultrasound

Absorption

Absorption is the process by which energy in the ultrasound beam is transferred to the propagating medium, where it is transformed into a different form of energy, mostly heat.

• The medium is said to absorb energy from the beam.
• The extent of absorption in a medium is affected by three main variables. These are:
• (i) the viscosity of the medium
• (ii) the relaxation time of the medium

(iii) the beam frequency.

Effect of Viscosity on ultrasound absorption

Viscosity is a measure of the frictional forces between particles of the medium as they move past one another.

• The greater these frictional forces the more heat generated by the vibrating particles.

Therefore, absorption of ultrasound increases with increasing viscosity.

Effect of Relaxation time on ultrasound absorption

Relaxation time is a measure of the time taken by medium particles to revert to their original mean positions within the medium following displacement by an ultrasound pulse.

• Its value is characteristic of the medium.

When the relaxation time is short:

Vibrating particles are able to revert to their original positions before the next disturbing pulse

Effect of Relaxation time on ultrasound absorption cont..

When the relaxation time is long:

The next pulse may encounter the particles on route before they are fully relaxed and The new compression and the particles may then be moving in opposing directions, thus resulting in additional dissipation of energy from the beam.

Therefore, the longer the relaxation time of a medium, the higher the absorption of ultrasound.

Effect of beam frequency on ultrasound absorption

The frequency of vibrations affects the amount of heat generated through both the viscous drag and the relaxation process.

On Viscocity:

Higher frequency means that medium particles move past each other at an increased rate, thus generating more frictional heat.

On Relaxation time:

Increased frequency reduces the probability that, following an ultrasonic pulse, vibrating particles will have reverted to their equilibrium positions before the next disturbance, thereby increasing energy absorption as the new wave moves in opposition to the relaxing particles We conclude that Absorption of ultrasound increases with increasing beam frequency.

Significance of absorption in ultrasonography

The significance of absorption is that energy is removed from the beam, leaving less energy available for examination of tissues lying beyond the absorbing medium.

When there is total absorption of beam energy, no Diagnostic information can be obtained in the shadow of the absorber.

The absorbing object is said to cast an acoustic shadow behind it.

Among materials of biological interest, bone absorbs ultrasound much more strongly than the soft tissues.

Significance of absorption in ultrasonography

A stone in Gallbladder (Gallbladder caliculus/Cholelithiasis) reflect and absorbs all the US causes a shadow behind it (post. acoustic shadowing)

Attenuation of ultrasound in tissue

The interaction processes discussed in this chapter have the effect of progressively diminishing the intensity of the ultrasound beam as it passes through the tissues.

The tissues are said to attenuate the ultrasound beam.

Attenuation refers to the total propagation losses that result in a progressively reduction of the beam intensity.

These losses include those due to reflection, scattering, refraction, and absorption

Ultrasonic half value thickness (HVT)

The attenuation in a specified medium may be quantified in terms of the ultrasonic half value thickness (HVT).

The HVT of a beam of ultrasound in a specified medium is the distance within that medium which reduces the intensity of the beam to one half of its original value

ULTRASONIC HALF VALUE THICKNESSES FOR DIFFERENT MATERIALS AT

FREQUENCIES OF 2MHz AND 5MHz

Acoustic windows

Acoustic windows are media or materials which allow ultrasound to pass readily through them.

Liquid cavities within the body, containing bile, urine, amniotic fluid, cerebrospinal fluid, and so on, are examples of accoustic windows

Acoustic barriers

These are materials or media that impedes the transmission of ultrasonic energy For Example Bone absorbs heavily, while gas boundaries reflect almost totally. fat also impedes the transmission of ultrasonic energy, for a variety of complex reasons.

focus.