Ultrasound Beam | Ultrasound Physics | Radiology Physics Course #15

Summary

This educational video script delves into the intricacies of ultrasound beams, explaining the principles that govern their shape and behavior. It distinguishes between single element and multi-element transducers, highlighting how they affect beam properties like focal point and divergence angle. The script explores Huygen's principle to illustrate wavelet interference and beam convergence, and discusses how adjusting transducer diameter and ultrasound frequency impacts the near field distance and divergence. It also touches on side lobes and grating lobes, offering insights into optimizing ultrasound imaging for better resolution and penetration.

Takeaways

• 🌟 Ultrasound probes are categorized into single element transducers and multi-element transducer arrays, each affecting the ultrasound beam differently.
• 📡 The ultrasound beam shape is influenced by properties such as the transducer type, diameter, and frequency, which can be manipulated to optimize image quality.
• 🎯 The ultrasound beam naturally converges to a focal point within the tissue, known as the near field or Fresnel zone, before diverging in the far field or Fraunhofer zone.
• 🔍 The focal distance, or the distance from the transducer to the focal point, can be adjusted on ultrasound machines to affect image resolution and depth.
• 📏 The near field distance is calculated based on the transducer's diameter and the wavelength of the ultrasound wave, with larger diameters and higher frequencies resulting in a deeper focal point.
• 📉 The divergence angle of the ultrasound beam is determined by the transducer's diameter and the frequency of the ultrasound wave, with larger diameters and higher frequencies reducing divergence.
• 🛠 Huygen's principle explains how the ultrasound beam converges and diverges, as each point on the wavefront can be considered a source of spherical wavelets that interfere constructively or destructively.
• 📋 The width of the ultrasound beam at the transducer face is crucial for image resolution, with the beam width doubling at the focal point in the near field.
• 🌐 Side lobes and grating lobes are secondary ultrasound phenomena that can affect image quality, and can be mitigated by adjusting transducer design and wave properties.
• 🔧 In multi-element transducer arrays, the effective diameter used for calculating beam properties is the combined diameter of the active elements, allowing for deeper focal points and less divergence.

Q & A

What are the two broad categories of transducers mentioned in the script?

The two broad categories of transducers mentioned are single element transducers and multi-element transducer arrays.

What is the term used to describe the region where the ultrasound beam narrows down to its most intense point?

The region where the ultrasound beam narrows down to its most intense point is known as the focal point.

What is the near field or Fresnel zone in the context of an ultrasound beam?

The near field or Fresnel zone is the distance from the transducer to the narrowest part of the beam, where it converges to the focal point.

How is the far field or Fraunhofer zone different from the near field in an ultrasound beam?

The far field or Fraunhofer zone is the region after the near field where the ultrasound beam starts to diverge and continues until it is fully attenuated, unlike the near field which is the converging region up to the focal point.

What is Huygen's principle as it relates to the ultrasound beam?

Huygen's principle states that a wave can be separated into an infinite amount of small discrete wavelets, which can act independently as small sources of wave energy, affecting how the ultrasound beam converges and diverges.

How does the diameter of the transducer affect the near field distance?

As the diameter of the transducer element increases, the near field distance also increases, meaning the focal point is deeper within the tissue.

What is the relationship between the frequency of the ultrasound wave and the near field distance?

Higher frequency ultrasound waves result in a deeper focal point, thus increasing the near field distance.

What is the formula used to calculate the near field distance in soft tissue?

The near field distance in soft tissue is calculated as the diameter in millimeters squared times the frequency in megahertz over four times the speed of sound in soft tissue.

How does the divergence angle of an ultrasound beam relate to the diameter and frequency of the transducer?

The divergence angle decreases as the diameter of the transducer element or the frequency of the ultrasound probe increases, resulting in less beam divergence.

What are side lobes in the context of an ultrasound beam and how can they be reduced?

Side lobes are secondary ultrasound waves that propagate in the forward direction and can interfere with the primary beam. They can be reduced by dampening the ultrasound wave, reducing the quality factor, narrowing transducer elements to less than half the wavelength, or reducing the amplitude of peripheral waves.

What is a grating lobe and how does it differ from a side lobe?

A grating lobe is a type of wave produced by a multi-element transducer array due to the interference of waves along the array. It differs from a side lobe as it generally occurs more in transducer arrays and is less likely to interfere with the primary image.

Outlines

00:00

🔍 Understanding the Ultrasound Beam Shape

This paragraph introduces the concept of the ultrasound beam and its properties. It explains the difference between single-element and multi-element transducer arrays and their effect on the beam shape. It discusses how the ultrasound beam naturally converges to a focal point in a region known as the near field or Fresnel Zone, before diverging at a set angle in the far field or Fraunhofer Zone. The text also highlights how adjusting the focal distance can change the beam's properties for different imaging purposes.

05:00

📏 Factors Influencing Near Field Distance

This section focuses on the factors that influence the near field distance of an ultrasound beam, including the diameter of the transducer element and the frequency of the wave. It describes the formula for calculating the near field distance and explains how increasing either the diameter or the frequency of the transducer element extends the depth of the near field. Additionally, it clarifies the common misconception that higher frequency leads to faster attenuation, emphasizing instead that it affects the shape of the beam and the depth of the focal point.

10:02

🔬 Exploring Side Lobes and Grating Lobes

This paragraph explores secondary ultrasound phenomena like side lobes and grating lobes, which can interfere with the quality of the ultrasound image. Side lobes are created due to changes in the height of the ultrasound crystals, while grating lobes result from interference patterns in multi-element transducer arrays. The text discusses various methods to minimize these unwanted effects, such as reducing the amplitude of peripheral waves, narrowing the transducer elements, and damping the ultrasound waves to lower the quality factor.

15:03

📡 Techniques for Beam Focusing and Steering

This section covers the manipulation of ultrasound beams using phased arrays to steer and focus the beam for clearer imaging. It hints at future discussions on techniques like spatial compounding that help produce crisper ultrasound images. The paragraph also promotes additional study resources, including a linked question bank that contains curated past exam questions, ideal for self-assessment and knowledge testing.

Keywords

💡Ultrasound beam

The ultrasound beam refers to the path and spread of the ultrasonic waves emitted from a transducer. In the video, it is described as naturally converging to a focal point, which is the narrowest and most intense part of the beam. The shape and properties of the ultrasound beam are crucial for image quality in ultrasound imaging, as they determine the resolution and the depth to which tissues can be effectively imaged. The script discusses how the beam's shape can be manipulated by changing the transducer's properties.

💡Transducer

A transducer is a device that converts one form of energy into another, and in the context of ultrasound, it converts electrical energy into mechanical vibrations (ultrasound waves) and vice versa. The video distinguishes between single element transducers and multi-element transducer arrays, which are capable of more complex beam manipulation. The properties of the transducer, such as its diameter and the frequency of the waves it generates, directly affect the ultrasound beam's characteristics.

💡Focal point

The focal point is the point where the ultrasound beam is at its narrowest and most focused. It is a critical concept in the video as it represents the region of highest resolution in the ultrasound image. The script explains that the focal point can be manipulated by adjusting the focal distance, which is the distance from the transducer to the focal point. This manipulation is important for optimizing image quality at different depths within the body.

💡Near field

The near field, also known as the Fresnel zone, is the region of the ultrasound beam from the transducer to the focal point. It is where the beam converges and is most tightly focused. The video script discusses how the near field distance can be calculated and how it is influenced by the transducer's diameter and the frequency of the ultrasound waves. Understanding the near field is essential for predicting the beam's behavior and its interaction with tissues.

💡Far field

The far field, also referred to as the Fraunhofer zone, is the region beyond the focal point where the ultrasound beam begins to diverge. The video emphasizes that the far field is infinite, extending until the beam is fully attenuated. The divergence angle in the far field is important as it affects the amount of information that can be retrieved from deeper tissues. The script explains how the far field's characteristics can be influenced by the transducer's design and the ultrasound wave's properties.

💡Divergence angle

The divergence angle is the angle at which the ultrasound beam spreads out after it passes the focal point. It is a key parameter in the video that affects the beam's coverage area and the quality of the ultrasound image. The script provides a formula to calculate the divergence angle based on the transducer's diameter and the wavelength of the ultrasound wave. A smaller divergence angle, which can be achieved with larger transducer diameters or higher frequencies, results in a more focused beam and better image quality.

💡Huygen's principle

Huygen's principle is a fundamental concept in wave theory, mentioned in the video to explain how an ultrasound wave can be considered as a collection of smaller wavelets that interfere constructively to form the main beam. This principle is used to understand how the ultrasound beam converges to a focal point and then diverges in the far field. The script uses Huygen's principle to illustrate the behavior of the ultrasound beam and the factors that influence its shape.

💡Side lobes

Side lobes are secondary beams that are produced alongside the primary ultrasound beam. They can interfere with the primary beam and potentially cause artifacts in the ultrasound image. The video script discusses how side lobes are generated due to the geometry of the transducer elements and how they can be minimized by adjusting the transducer's design or the ultrasound wave's properties. Understanding and managing side lobes is important for improving the clarity and accuracy of ultrasound imaging.

💡Grating lobe

A grating lobe is a type of side lobe that occurs when using a multi-element transducer array. It is a result of the interference of waves from the individual transducer elements and can lead to artifacts in the ultrasound image. The video script explains that grating lobes are more pronounced in transducer arrays and can be reduced by optimizing the design of the transducer elements. Managing grating lobes is part of the broader effort to enhance the quality of ultrasound images.

💡Spatial compounding

Spatial compounding is a technique mentioned in the video that improves ultrasound image quality by combining images taken from different angles. This technique helps to reduce speckle noise and enhance the visualization of tissue structures. The script suggests that spatial compounding will be discussed in more detail in a future talk, indicating its importance in advanced ultrasound imaging techniques.

Highlights

Overview of the various ultrasound probes and transducer types.

Introduction to the concept of the ultrasound beam and its shape.

Explanation of single element transducers and multi-element transducer arrays.

Understanding the near field or Fresnel Zone and its relation to the focal point.

Definition of the far field or Fraunhofer Zone and its infinite divergence.

Introduction to Huygen's principle and its application in ultrasound wave formation.

Relationship between the diameter and frequency of the transducer and the near field distance.

Impact of changing transducer diameter and frequency on the ultrasound beam's convergence and divergence.

Explanation of side lobes and how they can affect ultrasound imaging quality.

Methods to reduce side lobes: damping, reducing transducer element width, and adjusting amplitude.

Introduction to grating lobes and their occurrence in multi-element transducer arrays.

Discussion of the primary ultrasound beam, side lobes, and grating lobes in different contexts.

Use of multi-element transducer arrays to manipulate near field distance and beam divergence.

Explanation of phased array transducer manipulation techniques for ultrasound beam focusing and steering.

Preview of future topics: focusing mechanisms, spatial compounding, and ultrasound resolution.

Transcripts

so we've looked now at some of the

ultrasound probes that are available to

us as well as the various different

transducer types now let's turn our

attention to the ultrasound beam itself

we're going to look at the shape of the

ultrasound beam and some of the

properties that will change the

ultrasound beam shape as well as later

on looking at the various parameters

that we can change in order to

manipulate that ultrasound beam shape

depending on the type of image we are

trying to create

now I mentioned in our previous talk

that we can broadly separate transducer

types into single element transducers

and multi-element transducer arrays

these transducer arrays have many

discrete transducer elements that can be

fired independently of one another

now when looking or learning about the

ultrasound beam and the properties of

the ultrasound beam it's useful to look

at a single element transducer

now these principles the principles that

apply to a single element transducer

will also apply to these multi-element

transducer arrays and at the end of this

talk I'll show you how we go about doing

that so let's look at an ultrasound beam

here and assume that this transducer is

a single element transducer array

we have our beam forming on the face of

that transducer and this width here this

diameter of our transducer is the

diameter of our ultrasound beam now that

ultrasound being more naturally

converged to a point known as our focal

point and that converging region is also

known as our near field or our fresnal

Zone this is a distance from our

transducer to the narrowest part of our

beam and we saw that when we looked at

intensity our beam Narrows down to a

point to the most intense point of our

being it then diverges at a set angle

known as the Divergence angle and this

is what's known as our far field now our

far field is infinite it goes until the

ultrasound beam is fully attenuated

there's no end to this far field Zone it

just diverges out as it hits into the

tissue now the far field is also known

as the frown Hopper Zone

now this distance from the transducer to

our focal point is also known as the

focal distance and you may have seen on

your ultrasound machines that you can

manipulate that focal distance and in

our next talk we're going to look at how

we can go about manipulating this

distance here

now the Region's slightly closer from

the focal point to our transducer and

slightly past that focal point is what's

known as our focal Zone and you'll see

that that focal zone is the region that

has our best resolution within our image

so why does the ultrasound beam converge

like this to our focal point before

diverging well whenever we create a wave

we can use the principle known as

huygen's principle now in the single

element transducer we think of that as

creating one solid wave that's heading

into tissues but hygen's principle

states that a wave can be separated into

an infinite amount of small discrete

wavelets and these wavelets can act

independently as small sources of wave

energy

now the outermost parts of our

ultrasound beam here will constructively

interfere with those innermost wavelets

and that interference continues until we

reach our focal point the end of our

near field Zone and the geometry of that

wave and the pattern of interference

will then cause the beam to diverge into

our far field

now when we look at the width of our

ultrasound beam here it should be the

same width as our single element

transducer this width or the diameter of

our ultrasound beam should be double

that of the diameter at the end of our

near field Zone at our focal point so

this width here is half of this width

here

now when we are looking at the shape of

an ultrasound beam we are generally

interested in the distance between the

transducer and our focal point or our

near field distance and we are

interested in the amount of Divergence

that happens after that near field

distance obviously the more Divergence

that happens the less information or the

less useful information that comes back

to our ultrasound probe so we want to

look at the factors that change that

near field distance as well as the

factors that change that Divergence that

Divergence angle and there are generally

two factors that change this the

diameter of our ultrasound probe and the

frequency of the wave that we are

generating

so let's start by having a look at our

near field distance now again our near

field is the distance between our

transducer and the focal point we can

calculate our near field by taking the

diameter of our transducer element and

squaring that value we can then divide

that value by four times the wavelength

of the wave we are propagating through

tissue

now we looked at previously wavelength

is determined both by the frequency of

the ultrasound probe and the tissue

through which the wave is traveling

through

that we can see that the wavelength is

the speed of sound divided by the

frequency this is a Formula that we've

seen over and over again in these talks

so we can plug this back into our

initial equation and see that the

diameter times the frequency over four

times the speed of sound in soft tissue

will give us our near field distance

another way to write this is the radius

of our transducer element half the

diameter squared over the wavelength of

our wave but I prefer to use this

equation here because once we've

substituted our speed of sound and our

frequency into this equation we can get

this equation at the bottom here our

near field distance in soft tissue is

equal to our diameter in millimeters

squared times our frequency in megahertz

over four times the speed of sound in

soft tissue now the speed of sound in

soft tissue I've written it as

millimeters per second here 1.54

millimeters per second to ensure that

all our units are the same now I have

very rarely seen this question asked

where you actually asked to calculate

specific values what is important to

understand here is what does changing

the diameter of our transducer element

do to our near field distance and what

does changing the frequency of our

ultrasound probe do to our near fuel

distance you can see that as we increase

the diameter of our transducer element

we increase that near field distance the

depth at which our focal point is within

the tissues the same perhaps

counter-intuitively happens for

frequency the higher the frequency the

deeper our focal point will be now

people often get this confused because

we think of high frequency as

attenuating early and that's true the

higher the frequency of the ultrasound

wave the faster that wave attenuates but

that has nothing to do with the shape of

the ultrasound beam heading into the

tissue the higher the frequency the

slower that convergence of the

ultrasound being based on that

interference of our wavelets in highgens

principle and the deeper our focal point

will be so the take-home Point here is

as we increase diameter of our

transducer elements and as we increase

the frequency of our wave we increase

the depth of our Neo field the depth of

our focal Zone

now the second thing we look at when

looking at the ultrasound beam geometry

is the Divergence angle how much that

beam diverges after it's reached the end

of our near field

now again this far field has no set

limit it's continuous until that beam is

completely attenuated

now we can use this formula here sine

Theta Theta being this angle here equals

1.22 times the wavelength divided by the

diameter of our transducer here again we

can substitute wavelength Here For Speed

over frequency

now speed and soft tissue is constant

1514 meters per second and this 1.22 is

a constant here we see that our diameter

and our frequency are now the

denominator in this equation as our

diameter increases this angle gets

smaller as our frequency increases this

angle gets smaller so the wider the

diameter of our transducer element the

less Divergence there is and the higher

the frequency of our ultrasound probe

the less Divergence there is so

increasing frequency and increasing

diameter prevents that beam from

diverging too much we get more

information coming back towards our

ultrasound probe because those returning

Echoes are in line with the ultrasound

probe the lower the frequency or the

smaller the diameter of our ultrasound

transducer the more Divergence there is

the less information that we'll get back

from our far field here again this is

some that we don't necessarily need to

calculate we more need to understand

that the diameter and the frequency

affect that far field geometry

so we've looked at two things our near

field and our beam Divergence and we've

seen how diameter and frequency affect

both the distance of that near field as

well as the Divergence of that far field

beam

now both of these factors refer to the

primary ultrasound beam that is being

propagated within tissues there are two

separate ultrasound phenomena that I

want to touch on just so that you're

aware of them the first is what's known

as side lobes here we can see our

primary ultrasound beam here in blue

being propagated into tissues we also

get what is called a side lobe where an

ultrasound wave is propagated in the

forward Direction and can contribute to

some of the signal that returns towards

our front do so

now when we looked at the piezoelectric

effect we saw that the shape of the

crystals change depending on the

location of this Titanium or zirconium

atom

you can see that this change results in

the crystal Getting Thinner but not only

does the crystal get thinner it also

gets taller there are two Dimensions

that are changing here I've represented

this with these boxes here we have our

Crystal changing from a cube into a

rhomboid we see the thickness of the

crystal is changing and as that

thickness change that allows for

propagation of our ultrasound wave in

the primary beam but a second thing is

changing the height of our crystals is

also changing and not only the height in

the y-axis but also the width in the

z-axis is changing into the slide here

we are getting radial expansion of our

ultrasound transducer elements so not

only is that thickness changing but also

the crystal is changing in diameter here

and it's this change this changing

height of our ultrasound crystals that

causes a separate wave or side load to

be propagated into the tissue

now there are a couple of ways that we

can reduce the amount of cycle that is

created in this tissue

continuous ultrasound waves high quality

Factor ultrasound waves have greater

side lobes waves with a low quality

factor a high dampened wave will have

less side lobes so the more we dampen

our transducer elements the lower our

quality Factor the less these side lobes

will come into effect

we can also reduce the amplitude of the

waves that we create on the peripheries

of our transducer element if this is a

multi-element transducer array we can

decrease the intensity of the waves that

we create on the edge here in order to

prevent these side loads from

interfering with our primary image and

the last thing we can do is reduce the

width of our individual transducer

elements in our multi-element transducer

array we can reduce the width of those

single transducer elements to less than

half the wavelength of our ultrasound

beam that we are propagating and when

that width is less than half we get

reduction in the amount of side lobe

that is created so to reduce the amount

of side load produced by our ultrasound

machine we can firstly dampen our

ultrasound wave reduce the quality

Factor we can narrow our transducer

elements to less than half the

wavelength of our ultrasound beam and we

can reduce the amplitude of these

peripheral waves that we create here

now there's another type of wave that is

produced or field from our ultrasound

transducer it interferes less with the

image that we are creating and this is

what's known as the grating lobe if we

were to take a single element transducer

we would get very little grating lobes

produced it's called a grating though

because if we were to place a grate in

front of the single element transducer

we would get interference of those waves

that would cause these grating lobes to

be produced now effectively what is

happening here is we are creating a

multi-element transducer array so these

grating lobes generally occur more in

these transducer arrays and this is

again a function of hydrogen's principle

where the points along a wave can be

seen as single wavelets and the

interference of those waves will cause a

creating load to be produced out the

side

so we've looked at the primary beam the

side lobes as well as the equating lobes

and we've seen that changing the

diameter of our transducer as well as

the frequency of the ultrasound wave

changes both the near field and the

amount of Divergence in that far field

now when we were looking at this primary

ultrasound beam we were looking at a

single element transducer but we've seen

that in our multi-element transducer

arrays we have small elements that can

be fired off independent of the other

elements now when calculating either our

Divergence angle or our near field

distance and using this diameter and

frequency we can take the diameter of

the transducers that we fire off when

creating the wave in here a linear

sequential transducer array we fire off

multiple elements at the same time in

this example we are firing of three

transducer elements when calculating the

diameter here or when using the diameter

to calculate our Divergence and our near

field distance we take the diameter of

the elements combined the diameter of

all three of these transducer elements

now that's the reason we use multiple

transducer elements instead of one

individually allowing us to increase

that near field distance enough that we

can actually image tissues if we were to

fire off one of these elements at a time

our focal point would be far too shallow

within our tissues

firing multiple transducer elements at

once allows us to increase the depth of

our near field as well as decreasing the

amount of Divergence within our beam

we can then shift that one transduced

element at a time in order to keep our

lateral resolution which we're going to

look at in a future talk the take-home

Point here is that as we increase our

diameter and our frequency we increase

our near field distance our focal point

as well as decreasing the amount of

Divergence within our wave

so we can see that changing that

diameter and changing that frequency

will change our focal point within our

ultrasound beam and we've seen that in a

phased array we can manipulate that

ultrasound beam in order to steer that

beam through tissues so in the next talk

we're going to look at the various

mechanisms that allow us to change that

Focus as well as steer that beam as well

as looking at spatial compounding which

allows us to get a crispr ultrasound

image after that we can then go on to

look at ultrasound resolution again if

you are studying for an exam I have

linked a question Bank in the

description below curated multiple past

paper questions and answered them in

video format and it's a great way to

test your knowledge see where your gaps

are within your knowledge so that's

something you think you'd find helpful

go and check that link out otherwise

I'll see you in the next talk where we

look at focusing steering and spatial

compounding until then goodbye everybody