ultrasound sarah gillies sarah.gillies@ekht.nhs.uk

UltrasoundUltrasound

Sarah GilliesSarah Gillies

Sarah.gillies@ekht.nhs.ukSarah.gillies@ekht.nhs.uk

Sound: What is it?Sound: What is it?- A longitudinal, ‘pressure’ wave- A longitudinal, ‘pressure’ wave

• Wavelength, Wavelength, – length of one complete cycle of a wave (m)

• Period, Period, TT – the time required to complete a full cycle

• Frequency, Frequency, ff – the number of cycles per second = 1/T

• Amplitude, Amplitude, AA – maximum pressure displacement

• Speed, cSpeed, c – distance travelled by a given point on the wave in a given interval of time (m/s)

UltrasoundUltrasound

Ultrasound is acoustic (sound) energy in the form of waves which have a frequency frequency

above the human hearing range. above the human hearing range.

Human hearing frequency range:The highest frequency that the human ear can detect is approximately 20 thousand cycles per second - 20,000 Hz.20,000 Hz.

Diagnostic ultrasound frequency range:Typical diagnostic ultrasound scanners operate in the frequency range of 2 to 18 2 to 18 megahertz (MHz)megahertz (MHz)

Speed of SoundSpeed of SoundSpeed, c = Speed, c = distancedistance = = λλ = = f f λλ

• Wave speed, c, depends on the density and compressibility of the medium:

• A denser medium → slower speed

Harder materials are more difficult to compress, this means the material impedes the formation of compressions and rarefactions.

Vs

c f

Speed of sound in the body.Speed of sound in the body.

Medium Speed (m/s)

Air 330Water 1480Fat 1460Liver 1570Muscle 1600

Soft tissues (average) 1540

The speed of sound in a given medium remains fixed. Therefore…

FrequencyFrequency

High Frequency:High Frequency:

Low Frequency:Low Frequency:

if decreases …. f increases (and vice versa)

Double the frequency half the wavelengthDouble the frequency half the wavelength

SoundSoundSound needs a medium to travel throughSound needs a medium to travel through

In diagnostic ultrasound, this medium is In diagnostic ultrasound, this medium is provided by using……?provided by using……?

Gel !Gel !

Using sound-waves to create an Using sound-waves to create an image.image.

Transducer placed onto gel Transducer placed onto gel applied to skin.applied to skin.

Short bursts of ultrasound are Short bursts of ultrasound are sent into the patient.sent into the patient.

As the pulses travel into the body they are As the pulses travel into the body they are reflected and scattered, generating echoes.reflected and scattered, generating echoes.

Some echoes travel back to the transducer where Some echoes travel back to the transducer where they are detected-these echoes are used to form a they are detected-these echoes are used to form a B-Mode image.B-Mode image.

Echo RangingEcho RangingTo display each echo in a position that To display each echo in a position that corresponds correctly to the target corresponds correctly to the target detected, the ultrasound system needs to detected, the ultrasound system needs to know:know:

The distance of the target from the transducerThe distance of the target from the transducer

The direction of the target from the transducer The direction of the target from the transducer

The range of the target from the transducer The range of the target from the transducer is measured using theis measured using the pulse echo pulse echo

principle.principle.

Pulse Echo PrinciplePulse Echo Principle The same principle is used in echo sounding equipment in boats to The same principle is used in echo sounding equipment in boats to

measure the depth of water:measure the depth of water:

Transducer transmits pulse of ultrasound, it travels through water to Transducer transmits pulse of ultrasound, it travels through water to bottom of seabed, a reflection/echo is produced and is detected on its bottom of seabed, a reflection/echo is produced and is detected on its return.return.

Pulse Echo PrinciplePulse Echo Principle t = 0 t = d/c t = 2d/ct = 0 t = d/c t = 2d/c

To measure send & return time a clock is started as the pulse is sent (t = 0)

If the speed of sound in water is c and the depth d, then the pulse reaches the seabed at time t = d/c

The returning echo also travels at speed c and takes a further time d/c to reach the transducer where it is detected.

Therefore the echo arrives back at a total of t = 2d/c

d = ct/2

Therefore system calculates distance by measuring arrival time t of an echo assuming

a fixed value for the speed of sound

Depth is calculated from the time of transmission of pulse to reception of echo,

taking into account the speed of sound.

The B-Mode ImageThe B-Mode Image

Cyst in breast tissue

A sonographic study of valves in a patient's heart

Creation of an imageCreation of an image The B-mode image is formed from a a large number of B-mode The B-mode image is formed from a a large number of B-mode

scan lines where each line is produced by a pulse-echo sequence.scan lines where each line is produced by a pulse-echo sequence.

In a typical linear array transducer, the beam is stepped across the In a typical linear array transducer, the beam is stepped across the transducer array producing an image line of echoes which are transducer array producing an image line of echoes which are displayed on-screen as bright spots. displayed on-screen as bright spots.

How do we create the sound How do we create the sound waves for diagnostic ultrasound?waves for diagnostic ultrasound?

Sound generally comes from a vibrating device.Sound generally comes from a vibrating device.

The source of this vibration for diagnostic ultrasound The source of this vibration for diagnostic ultrasound comes in the form of a small wafer of comes in the form of a small wafer of piezoelectric piezoelectric materialmaterial which vibrates a millions of times per second. which vibrates a millions of times per second.

The piezoelectric material The piezoelectric material (approx (approx 128 individual piezoelectric128 individual piezoelectric

elements in a transducer)elements in a transducer) is the is the main component within the main component within the ultrasound transducer.ultrasound transducer.

PiezoelectricsPiezoelectrics Convert electrical Convert electrical

energy into mechanical energy into mechanical energy (sound) to energy (sound) to produce ultrasound.produce ultrasound.

Mechanical energy into Mechanical energy into electrical energy for electrical energy for ultrasound detection.ultrasound detection.

It does this by It does this by physical deformation physical deformation of the crystal/ceramic of the crystal/ceramic

structure.structure.

TransducersTransducersLinear array:Linear array: linear probelinear probe

Linear array:Linear array: curved probecurved probe

Intra-cavity probe

Phased array

B-mode imagesB-mode imagesGall Stones

Gall Stones

Twin Pregnancy at 10 weeks

Twin Pregnancy at 10 weeks

Breast cystsBreast cysts

The Doppler The Doppler EffectEffect

The doppler effect is the change in The doppler effect is the change in the observed frequency of the sound the observed frequency of the sound

wave wave ((ffrr)) compared to the emitted compared to the emitted

frequency frequency ((fftt)) which occurs due to the which occurs due to the

relative motion between the observer relative motion between the observer and the source.and the source.

Doppler exampleDoppler example The sound the driver hears will The sound the driver hears will

remain the same.remain the same.

The observer located in front of The observer located in front of the car will hear a higher-pitched the car will hear a higher-pitched noise. Why?noise. Why?

Because the sound waves Because the sound waves compress as the vehicle compress as the vehicle approaches the observer located approaches the observer located in front. This increases the in front. This increases the frequency of the wave, and the frequency of the wave, and the pitch of the vroom rises. pitch of the vroom rises.

The observer located behind the The observer located behind the car will hear a lower-pitched noise car will hear a lower-pitched noise because the sound waves stretch because the sound waves stretch out as the car recedes. This out as the car recedes. This decreases the frequency of the decreases the frequency of the wave, and the pitch of the vroom wave, and the pitch of the vroom falls.falls.

Using doppler in medical Using doppler in medical ultrasoundultrasound

In a Doppler ultrasound In a Doppler ultrasound examination, sound waves of examination, sound waves of a certain frequency are a certain frequency are transmitted into i.e. the transmitted into i.e. the heart.heart.

The sound waves bounce off The sound waves bounce off blood cells moving through blood cells moving through the heart and blood the heart and blood vessels. vessels.

The movement of these The movement of these cells, either toward or away cells, either toward or away from the transmitted waves, from the transmitted waves, result in a frequency shift result in a frequency shift that can be measured. that can be measured.

The diagram shows a Doppler The diagram shows a Doppler transducer placed on the skin transducer placed on the skin and aimed at an angle, θ, and aimed at an angle, θ, towards a blood vessel, which towards a blood vessel, which contains blood flowing with a contains blood flowing with a velocity of u m/s, at any velocity of u m/s, at any instant. instant.

The transducer emits The transducer emits ultrasound waves of ultrasound waves of frequency, fo, and echoes frequency, fo, and echoes generated by moving reflectors generated by moving reflectors in the blood, e.g. red blood in the blood, e.g. red blood cells, have a frequency, fr. cells, have a frequency, fr.

The difference between these The difference between these two frequencies, Δf, is related two frequencies, Δf, is related to the velocity of the flowing to the velocity of the flowing reflectors through the following reflectors through the following equation: equation:

A higher-frequency Doppler signal is obtained if the

beam is aligned more to the direction of flow.

In the diagram, beam (A) is more aligned than (B) and produces higher-frequency Doppler signals.

The beam/flow angle at (C) is almost 90° and there is a very poor Doppler signal.

The flow at (D) is away from the beam and there is a negative signal.

Effect of the Doppler angle in the sonogram.

The Doppler EffectThe Doppler Effect

Technological AdvancesTechnological Advances3D imaging

Portable scanners

PACS

Picture Archiving &

Communications System