IN THE NAME OF ALLAH THECOMPASSIONATE THE

MERCIFUL

Ultrasound Imaging (Obstetrical )I will discuss the following aspects. Please scroll down and start

reading.•Introduction to sound and ultrasound•ultrasound probe•Fundamentals of Ultrasound•Introduction to wave•Frequency, Wavelength, Resolution, and Depth•Beam focusing•Sending and receiving ultrasound•Interaction of ultrasound with body tissues•Imaging by ultrasound•Doppler Ultrasound•3D Ultrasound

What is sound? Sound (1)- a disturbance in pressure that propagates

through a compressible medium.Sound (2) the auditory sensation produced by transient

or oscillatory pressuresacting on the ear.

The first definition is what we mean by sound in these notes. The second definition is what is

meant by sound in everyday speech.

The human ear can hear between frequencies of about 20 Hz to 20,000 Hz.

An ultrasound element acts

like a bat!

Bats!

bats are gifted with a system of locating things with sound. First they

emit sound.

Like normal sound, ultrasound echoes off

objects

Bats navigate using ultrasound

1.65m

Bats: Navigating with ultrasound

• If a bat hears an echo 0.01 second after it makes a chirp, how far away is the object?

• Clue 1: the speed of sound in air is 330 ms

• Clue 2: The speed of sound equals the distance travelled divided by the time taken

• Answer: distance = speed x time

• Put the numbers in:

distance = 330 x 0.01 = 3.3 m

• But this is the distance from the bat to the object and back again, so the distance to the object is 1.65 m.

We can also use ultrasound to look inside

the body…

Generation of Ultrasound Waves

There is a special material called a “piezo electric crystal”. This material has a very

special property. When a voltage is applied to an piezo electric crystal it expands. When the voltage is removed, it contracts back into its

original thickness.

Receiving Ultrasound

1-When a piezo electric crystal is compressed, it generates a voltage

2-The crystal then generates a voltage that corresponds to the

intensity of the ultrasound wave that hits it.

obstacle

The ultrasound machine then very quickly switches to a listening

mode by monitoring the voltage across the piezo electric crystal.

The above examples show only one crystal for clarity. In reality, ultrasound probes are

composed of a large number of individual piezo electric crystals. The information gathered from

the crystals are processed by a computer to display the images on a screen.

piezo electric crystals

ultrasound probe

Fundamentals of Ultrasound

This section covers some basic notation and terminology used in acoustics, and

some of thefundamental physical principles.

What is a wave?

• A wave is any disturbance that transmits energy through matter or empty space.

• Waves can be Transverse, Longitudinal• Some waves combine both transverse and

longitudinal motions

TRANSVERSE WAVES• Transverse Waves are waves in which the

particles vibrate perpendicularly to the direction the wave is traveling.

• Transverse waves are made up of crests and troughs.

• Water waves, waves on a rope, and electromagnetic waves are examples of transverse waves.

• http://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html

Transverse Waves

direction the wave

longitudinal WAVES• Compression Waves are waves in which the

particles vibrate back and forth along the path that the waves moves.

• longitudinal waves are also known as Compression waves.

• Compression/longitudinal waves are made up of compressions and rarefactions.

• Waves on a spring are longitudinal waves.

• http://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html

Longitudinal Waves

direction the wave

Ultrasound waves are longitudinal, compressional waves, that can be periodic or pulsed,propagate at roughly 1500 m/s in water or biological tissue, can leave the medium unchanged(diagnostic ultrasound), but at higher intensities can also change it (therapeutic ultrasound).

Is soft tissue solid or liquid?Should we treat biological tissue as liquid or solid, at least, as far as ultrasound is concerned?Some tissues are obviously solid - bones, for instance - but what about soft tissues such asskin or muscle?1- they are very malleable and consist largely of water .2- One of the differences between a solid and a liquid is that a solid has rigidity and can support

a shear force.

What is shear force?

• Shear Force : A good example of shear force is seen with a simple scissors. The two handles put force in different directions on the pin that holds the two parts together. The force applied to the pin is called shear force.

Imagine gluing the palm of your hand to a table and then trying to push your

hand along the table top. You can move your hand a bit as the skin deforms but it will soon

reach a point where you can't push it any further (without tearing the skin). Your skin will

be supporting a shear force - it seems to behave like a solid.

• Remove your hand from the table and your skin will return to the same shape it was originally - it is an elastic solid. This suggests we should treat soft tissue as an elastic solid.

In ultrasound imaging, soft tissue is usually modelled as a fluid.

Why do we treat soft tissue as a fluid when it is actually an elastic solid? The pragmaticreason is that this approximation has proven to be reasonably accurate and useful over halfa century of ultrasound studies. Another motivation is that wave propagation in fluids ismuch simpler to visualise and model mathematically than wave propagation in solids. Thereason why we get away with it, though, is because treating tissue as a fluid is equivalent toignoring shear waves, and there are good reasons why shear waves can usually be neglectedin ultrasound imaging

Wavelength, frequency and wave speed

• Wavelength, (lambda)• A wavelength is the distance between any

point on a wave to an identical point on the next wave.

• A wave with a shorter wavelength carries more energy than a wave with a longer wavelength does.

typically on a 0.1-1 mm scale for medical ultrasound.

Frequency

• Frequency is the number of waves produced in a given amount of time. Medicalultrasound typically uses frequencies from 1-15 MHz.

Wave speed, c.

• Wave Speed is the speed at which a wave travels.

• Sounds waves travel faster in a medium if the temperature is increased.

• Sound travels at about 340 m/s in air and 1500 m/s in water.

• v f• (Speed = frequency x wavelength)

Ultrasound IntensityOne property of propagating waves is that they transfer energy from one

point to another without the transfer of matter. In acoustics, this flow of energy is called the acoustic intensity.

Instantaneous acoustic intensity, I(x,t), in a time-varying acoustic field, is a vector

defined asI(x,t) = pa(x,t).ua(x,t) [J/s/m2 = W/m2]where pa is the acoustic pressure and ua the acoustic particle

velocity. (Is this a plausible definition of intensity? Recall that, in general terms, pressure = force per unit area, velocity= distance per unit time and `work done' = force X distance. Acoustic intensity is a measure of power per unit area = work done per unit time per unit area = pressure X velocity.)

Safety limits

Use (Intensity)max (mW/cm2)

Cardiac 430

Peripheral vessels 720

Opthalmic 17

Abdominal 94

Fetal 94

Maximum ultrasound intensities recommended by the US Food and Drug Administration (FDA) for various diagnostic applications.

Reflection, Refraction and ScatteringAcoustic impedance:Materials in which the density, 0 , and sound speed c0 are constant. If this were the case in soft tissue then ultrasound imaging would not work. There need to changes in the sound speed or the density in order for the ultrasound waves to be reflected. More precisely, the characteristicacoustic impedance of the material, 0 c0, must vary between different tissue types.characteristic acoustic impedance, Z = 0 c0

Reflection and transmission coefficients

Boundary conditions When a wave reaches a boundary, part will be reflected and part

transmitted. These three parts (incident wave, reflected wave, transmitted wave) must obey

two boundary conditions:(1) Continuity of pressure The acoustic pressure must be

the same on both sides of theboundary. There must be no net force.

(2) Continuity of normal particle velocity The particle velocities normal to the boundaries

must be equal. The fluid must stay in contact.

Normal incidence pressure reflection and transmission coefficients

When an acoustic pressure wave with amplitude pi is normally incident on an interface (a change in characteristic acoustic impedance), a wave with amplitude pr will be reflected and another wave, with amplitude pt will be transmitted. These wave amplitudes define

the pressure reflection and transmission coefficients, R and T:

Oblique incidence pressure reflection and transmission coefficients

Z1Z2

θi θr

θt

Incident wave Reflected wave

Transmitted wave

Refraction

• When a wave moves from one medium to another, the wave’s speed and wavelength changes. As a result, the wave bends and travels in a new direction.

Refraction (snell’s law)

• When a wave moves from one medium to another, the wave’s speed and wavelength changes. As a result, the wave bends and travels in a new direction.

• c : sound velocity

c1

c2

c1 > c2

c2

c1 < c2

c1

This phenomenon causes artifacts in medical echo image.

Scattering and diffraction

- Scattering refers to the reflection of sound from surfaces or heterogeneities in a

medium. It is quite a general term and includes reflection and diffraction.

- Diffraction is usually used to refer to the `leakage' of sound into `shadow zones'.

Diffraction is the reason you can hear someone talking in the next room even though

you can't see them; the sound waves `bend' around the corners more than light waves do

as they have a much longer wavelength. (Dif fraction is quite different from refraction

and the two should not be confused.)

More about transducer• With only air behind the crystal, ultrasound transmitted back into the

cylinder from the crystal is reflected from the cylinder’s opposite end. • The reflected ultrasound reinforces the ultrasound propagated in the

forward direction from the transducer. • This reverberation of ultrasound in the transducer itself contributes

energy to the ultrasound beam (i.e., it increases the efficiency). • It also extends the time over which the ultrasound pulse is produced.• Extension of the pulse duration (decreases bandwidth, increases Q) is no

problem in some clinical uses of ultrasound such as continuous wave applications.

• However, most ultrasound imaging applications utilize short pulses of ultrasound, and suppression of ultrasound reverberation is desirable.

• Backing of transducer with an absorbing material (tungsten powder embedded in epoxy resin) reduces reflections from back, causes damping at resonance frequency– Reduces the efficiency of the transducer– Increases Bandwidth (lowers Q)

Fresnel (or near) zone & Fraunhofer (or far) zone

• Plane wave– Line sound source, infinite length– No diffusion attenuation

Sound source

Fresnel (or near) zone & Fraunhofer (or far) zone

• Spherical wave– Point sound source– Diffuse sound field

Point source

Fresnel (or near) zone & Fraunhofer (or far) zone

• Practical condition –ultrasonic element-– Finite element size (about 0.3mm)– Not plane wave, not spherical wave

D

Near field(Fresnel zone) Far field

(Fraunhofer zone)

Plane wave Spherical wave

4D2

l: wavelength = 0.437mmD:diameter = 0.3mm

Fresnel zone= 0.052mm

NFL for 2 MHz(l=0.77 mm)

Diameter NFL

1 cm 3.2 cm

2 cm 13 cm

4 cm 52 cm

NFL for 4 MHz(l=0.385 mm)

Diameter NFL

1 cm 6.4 cm

2 cm 26 cm

4 cm 104 cm

If the diameter doubles, NFL increases by 4.If the frequency doubles, NFL doubles.If the diameter doubles, NFL increases by 4.

Divergence in far field

• (The ‘sin’ is a function of the angle)• Larger diameter diverges less• Higher frequency (smaller wavelength) diverges

less

What is the divergence angle for a 2 cm diameter, 3 MHz transducer?

o1 75.1036.0sin

036.020

51.02.1sin

51.0/000,000,3

/000,540,1

/000,000,3

/1540

2.1sin

mm

mm

mms

smm

s

sm

f

cd

Focusing, Methods

• Focusing reduces the beam width in the focal zone

• Methods– Lens– Curved element– Electronic

Transducers can be designed to produce either a focused or non-focused beam, as shown in the

following figure. A focused beam is desirable for most imaging applications because it produces pulses

with a small diameter which in turn gives better visibility of detail in the image. The best detail will be

obtained for structures within the focal zone. The distance between the transducer and the focal zone is

the focal depth.

focusing technique

• Acoustic Lens

Ultrasonicelement

Acoustic lenssound velocity : c1

Human bodySound velocity : c2

c1 < c2

Focal point

wavefrontWeak point : a fixed focus

The Principle of Electronic Focusing with an Array Transducer

• Focusing is achieved by not applying the electrical pulses to all of the transducer elements simultaneously. The pulse to each element is passed through an electronic delay. Now let's observe the sequence in which the transducer elements are pulsed in the figure above. The outermost element (annular) or elements (linear) will be pulsed first. This produces ultrasound that begins to move away from the transducer. The other elements are then pulsed in sequence, working toward the center of the array. The centermost element will receive the last pulse. The pulses from the individual elements combine in a constructive manner to create a curved composite pulse, which will converge on a focal point at some specific distance (depth) from the transducer.

focusing technique

• Electronic focus (transmission)Array of ultrasonic Element

Delay circuit

Focal point

Desired focal length by control of delay circuit

focusing technique

• Electronic focus (receiving)Array of ultrasonic Element

Point scatterer

delay

+

High S/NThe same principle as radar

scanning techniques - grouping -

Elementarray

linear convex linear annular

Control of beam direction

Switched array methodPhased array

methodmechanical

scan linear Offset sector sector

Probe form linear convex sector

Region of image

thyroid, breastAbdominal

regionheart

scanning techniques…

• Performed with transducer arrays– multiple elements inside transducer

assembly arranged in either• a line (linear array)

• concentric circles (annular array)

Linear Array Scanning

• Two techniques for activating groups of linear transducers– Switched Arrays

• activate all elements in group at same time– Phased Arrays

• Activate group elements at slightly different times

• impose timing delays between activations of elements in group

Linear Switched Arrays

• Elements energized as groups– group acts like one large

transducer• Groups moved up & down

through elements– same effect as manually

translating– very fast scanning possible

(several times per second)• results in real time image

Linear Switched Arrays

Linear Phased Array

• Groups of elements energized– same as with switched arrays

• voltage pulse applied to all elements of a group

BUT• elements not all pulsed at

same time

1

2

Linear Phased Array• timing variations allow beam

to be – shaped– steered– focused

Above arrows indicate timing variations.By activating bottom element first & top last, beam directed upward

Beam steered upward

Linear Phased Array

Above arrows indicate timing variations.By activating top element first & bottom last, beam directed downward

Beam steered downward

By changing timing variations between pulses, beam can be scanned from top to bottom

Linear Phased Array

Above arrows indicate timing variations.By activating top & bottom elements earlier than center ones, beam is focused

Beam is focused

Focus

Linear Phased ArrayFocus

Focal point can be moved toward or away from transducer by altering timing variations between outer elements & center

scanning techniques - grouping -

Elementarray

linear convex linear annular

Control of beam direction

Switched array methodPhased array

methodmechanical

scan linear Offset sector sector

Probe form linear convex sector

Region of image

thyroid, breastAbdominal

regionheart

scanning techniques…

Control of beam direction : phased array Scanning : sector

Heart image

scanning techniques…

Control of beam direction : switched array Scanning : linear

Thyroid image

scanning techniques…

Control of beam direction : switched array Scanning : offset sector

Liver image

Doppler ultrasound

Doppler ultrasound is based upon the Doppler Effect. When the object reflecting the ultrasound waves is

moving, it changes the frequency of the echoes, creating a higher frequency if it is moving toward the probe and a lower frequency if it is moving away from

the probe. How much the frequency is changed depends upon how fast the object is moving. Doppler ultrasound measures the change in frequency of the

echoes to calculate how fast an object is moving. Doppler ultrasound has been used mostly to measure

the rate of blood flow through the heart and major arteries.

A Doppler flow meter measures the speed of red

blood cells.

Imaging by ultrasound

When we look at things with our eyes, there are various ways in

which we “look “

At times, we might choose to look only straight ahead like when we read a notice on a wall.

Or we might look horizontally when scanning the seaOr we might scan the whole area, up and down, left and right, in many dimensions when absorbing scenery such as

the one below in Sri Lanka.

In a similar way, there are many different ways a ultrasound probe can “look “ at �things. These ways are called “modes

A mode (Amplitude mode)

B mode (Brightness mode) including real time, 2 dimensional, B mode

M mode (Motion mode)

A Mode Scanning•The A mode is the

simplest form of ultrasound imaging and is

not frequently used. •One use of the A scan is

to measure length. For an example,

ophthalmologists can use it to measure the diameter

of the eye ball.

Imagine that the red circle below is the eye ball and you want to

measure the diameter of it.

•An ultrasound machine scanning in “A scan” mode can be used. The probe is placed on one end of the eye ball.•An ultrasound wave is sent from the probe and at the same instance, a line from the left of the screen starts to be drawn. This line moves horizontally measuring time.

As the wave reaches the first wall of the eye, some of the ultrasound is reflected back into the probe. The returned wave is recorded on the line as a bump. The stronger is the

returned wave, higher the height of the bump. The height of the bump is called Amplitude which is what the “A” of “A scan” stands for

The time difference between the first bump and the second bump represents how long the ultrasound wave took to travel between the two walls. Longer the length, longer is

the time difference. The speed of ultrasound in the eye is known to be 1500 meters per second (yes, that is fast). So if you know the time difference (given by the interval

between the two bumps), you can calculate how far the wave traveled between the two walls of the eye, giving you the eyeball length.

B Mode Scanning

•In its simplest form, the B scan mode is very similar to the A scan mode. Just like the A scan, a wave of ultrasound is sent

out in a pencil like narrow path. And again like the A scan, the horizontal line represents the time since the wave was

released.

Again using the eye ball as an example, the probe is placed on one end. Like in the A scan, when the wave meets the first wall, a part of the wave is

reflected back into the probe. However, this time, instead of a bump, the strength of the returning wave is recorded by a bright dot. The brightness of the dot represents the strength of the returning wave. The brighter the dot,

the stronger is the returning wave. The letter “B” of “B scan” represents Brightness.z

The B scan in the form discussed doesn’t amount to much …. just a few dots of different brightness along a line. However, if a B scan is done at different levels of the object, you will get a two dimensional image on the screen as shown below. First a B scan is done at the top of the structure chosen, e.g.

the eye.

The first B scan line is kept on the screen. Then at a slightly different level, the B scan is repeated.

In this way, a two dimensional (2 D) image of the object is formed on the screen.

M Mode Scanning

• M stands for motion. This approach is used for the analysis of moving organs. It is based on A-mode data from a single ultrasound beam that are represented as function of time.

3D Ultrasound Imaging

http://www.3d-4d-ultrasounds.com/images/gallery/before-after.jpg

http://www.doctorscareclinic.com/html/ultrasound.html

• 3D ultrasound is a data set that contains a large number of 2D planes (B-mode images).

• This is analogous to assuming that a page of a book is one 2D plane, and the book itself is the entire data set.

• Once the Volume is acquired using a dedicated 3D probe you can “Walk” through the volume in a manner similar to leafing through the pages of a book, meaning you can walk through the various 2D planes that make up the entire volume.

• This is also known as translation and the planes are reconstructed using a computer.

3D from Conventional 2D Ultrasound

Volume Construction

Engine

Workstation

Volume Rendering

Engineji

k

(x,y,z)

2D Images

Position Data

US Probe

Tracking DeviceKane, Physics in Modern Medicine, CRC Press

Each US image represents one slice of the body and by taking therefore multiple cross sectional scans and putting them “side-by-side” you can

render a 3D image or you could view any one of the 2D slices.

3D imaging allows you to get a better look at the organ being examined and is best used for:

• Early detection of cancerous and benign tumors • examining the prostate gland for early detection

of tumors • looking for masses in the colon and rectum • detecting breast lesions for possible biopsies • Visualizing a fetus to assess its development,

especially for observing abnormal development of the face and limbs

• Visualizing blood flow in various organs or a fetus

Summary of how ultrasound imaging works

1. The ultrasound machine transmits high-frequency (1 to 5 megahertz) sound pulses into your body using a probe.

2. The sound waves travel into your body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone).

3. Some of the sound waves get reflected back to the probe, while some travel on further until they reach another boundary and get reflected.

4. The reflected waves are picked up by the probe and relayed to the machine.

5. The machine calculates the distance from the probe to the tissue or organ (boundaries) using the speed of sound in tissue (5,005 ft/s or1,540 m/s) and the time of the each echo's return (usually on the order of millionths of a second).

6. The machine displays the distances and intensities of the echoes on the screen, forming a two dimensional image like the one shown below.

Ultrasound image of a growing fetus (approximately 12 weeks old) inside a mother's uterus. This is a side view of the baby,

showing (right to left) the head, neck, torso and legs.