Ultrasound physicsby

Dr/ Dina Metwaly

• WHAT DO YOU UNDERSTAND ABOUT ULTRASOUND ?

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• An ultrasound is machine that uses high frequency sound waves and their echoes to help determine the size, shape and depth of an abnormality.

• It allow various organs in the body to be examined right in the doctor's office or clinic.

Bats make high-pitched chirps which are too high for humans to hear. This is called ultrasound

Like normal sound, ultrasound echoes off objects

The bat hears the echoes and works out what caused them

• Dolphins also navigate with ultrasound

• Submarines use a similar method called sonar

Physical principles of sound wave

• What is a wave?

We all know what we mean by waves and the term is used in everyday speech to describe a crime waves, waving goodbye, waves of nausea, etc. - which share common features, eg. with all waves some quantity changes (with time or distance or both).

• A wave is a repeating disturbance or movement that transfers energy through matter or space

• Mechanical waves are waves which require a medium.

• A medium is a form of matter through which the wave travels (such as water, air, glass, etc.)

• Waves such as light, x-rays, and other forms of radiation do not require a medium.

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They are two kinds of mechanical waves

• Transverse

In a transverse wave the matter in the wave moves up and down at a right angle to the direction of the wave

• Longitudinal Waves (Compression Waves)

In a longitudinal wave the matter in the wave moves back and forth parallel to the direction of the wave

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Sound wave

• Sound is a compressional (longitudinal) wave which travels through the air through a series of compression and rarefaction.

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Compressional

Longitudinal wave

On a compressional wave the area squeezed together is called the compression. The areas spread out are called the rarefaction.

The wavelength is the distance from the center of one compression to the center of the next compression.

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The parts of a waveTransverse wave The crest is the highest point

on a transverse wave. The trough is the lowest point on a transverse wave.

The rest position of the wave is called the node or nodal line.

The wavelength is the distance from one point on the wave to the next corresponding adjacent point.

WAVE TERMINOLOGYThe characteristics of a sound wave can be described by the

following parameters: 1. Wavelength

2. Frequency (f)

3. Period (T)

4. Amplitude (A)

5. Velocity (c)

6. Power (W)

7. Intensity

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Wavelength and frequency

• Wavelength is a measure of distance, so the units for wavelength are always distance units, such as meter, centimeters, millimeters, etc.

• Frequency is the number of waves that pass through a point in one second.

• The unit for frequency is waves per second or Hertz (Hz). One Hz = One wave per second.

• Wavelength and frequency are inversely related

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Wavelength and frequency

• The higher the frequency the shorter is the wavelength the better the resolving of small structures but lesser the penetration.

Period • Period is the time it takes for one full wavelength to pass a certain

point.

• Frequency is waves per second.

• Period is seconds per wave.

Tf

periodfrequency

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Amplitude• The amplitude of a wave is directly related to the energy of a

wave.

• The amplitude of a transverse wave is determined by the height of the crest or depth of the trough

• Sound travels through different media at different speeds (e.g., sound travels faster through water than it does through air)

• The speed of sound through a material depends on both the density and the compressibility of the material.

1. Compressibilityvelocity is inversely proportionalliquids and solids propagate sound more rapidly

than gases (easily compressed)2. Density

denser materials have greater inertia - so, decreased velocity

• Wave speed is calculated as the product of a waves frequency and wavelength.

Sound Propagation(Velocity)

Note

Although gases have low density, they have very high compressibility, leading to relatively low speed of sound compared to liquids and solids.

The speed of light in water is lower than that in air, but the speed sound in water more than in air.

Power (W)

• the rate at which work is done or the rate of flow of energy through a given area. In diagnostic ultrasound energy is contained within the beam, so the power is the rate of flow of energy through the cross-sectional area of the beam. Power is expressed in Watts.

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Sound intensity• Sound intensity is the energy that the sound wave possesses expressed in

power per unit area (mWatts/cm2 ). • To differentiate between intensity & power :

The example of sunlight shining on wood shavings is often used to illustrate this phenomenon ! Sunlight does not usually burn wood shavings but when the sunlight is concentrated into a small area (increased in intensity) by a magnifying glass, the wood shavings can be burnt. So, power remains the same, intensity is increased and an effect is produced (increased heat).

• The greater the intensity of sound the farther the sound will travel and the louder the sound will appear.

• Loudness is very closely related to intensity. • Loudness is the human perception of the sound intensity. • The unit for loudness is decibels.

Acoustic impedance • The acoustic impedance of a medium is the impedance

(similar to resistance) the material offers against the passage of the sound wave through it and depends on the density and compressibility of the medium.

• The greater the change in the acoustic impedance, the greater the proportion of the ultrasound that is reflected.

• Large difference in acoustic impedance between soft tissue and bone, or between soft tissue and air, and such interfaces will produce large reflections.

Interaction of ultrasound beam with

tissuesreflectionAttenuation:• As sound travels through tissue, there is loss of signal

due to: • Absorption • Scattering • refraction

• It affects both the transmitted pulse and received echo so the effect is doubled.

• Higher frequencies being attenuated more quickly than lower frequencies.

• This is why higher ultrasound frequencies penetrate tissue less effectively than lower ultrasound frequencies and can only be used for imaging superficial structures

Refraction:

• Is the bending of a wave as it passes from one medium to another as it hits the interface at an oblique angle.

Diffraction:

• diffraction occurs when passing through a small

opening, they diffract and spread out as they pass through the hole.

Scatter: • Sound wave dispersed in all

directions

Absorption:

• Parts of the sound wave energy are absorbed by conversion to heat energy in the material, while the rest is transmitted through. The level of energy converted to heat energy depends on the sound absorbing properties of

the material.

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Reflection• When a wave bounces off an object back towards the probe

and changes direction, this is reflection.• The amount of sound wave reflection is dependent on the

“resistance”(impedance) of the substance it is trying to pass through

• ↑Resistance = ↑Reflection back to probe = ↑ Energy detected by probe

= Whiter image • ↓ Resistance = ↓Reflection back to probe =↓Energy detected

by probe

= Darker image

High resistance (WHITE on the ultrasound) • Bone/Stone • Liver/Spleen/Kidney

Low resistance (BLACK on the ultrasound) • Blood/Urine

False images caused by unique interactions between the sound wave and structures in the body

• Shadowing • Enhancement

Posterior acoustic shadowing: • Ultrasound wave hits a substance that causes

near total reflection • Everything behind the blocking structure

appears black (since no energy is getting through)

• Common causes • Bone ,gallstones, kidney stones,

calcification Posterior acoustic enhancement:• Ultrasound waves pass through an area of

low resistance with little attenuation (ie little loss of energy)

• As it hits a denser substance behind it, the energy is dispersed and “lights up” the deeper tissues

• Common causes Cyst, Gallbladder, Bladder

Ovarian cyst

Gallbladder stone

CONTINUOUS WAVE ULTRASOUND

• Sound that is continuously transmitted is termed continuous wave (CW) sound. We cannot image using CW ultrasound, though it is often employed for Doppler studies.

Ultrasound ProductionTransducer contains piezoelectric elements/crystals which

produce the ultrasound pulses.These elements convert electrical energy into a mechanical

ultrasound wave and reversely.Electrical------------ crystals---------------------sound

crystals

Electrical sound

Piezoelectric Effect

Definition: The principle of converting energy by applying pressure to a

crystal.

The reverse of the piezoelectric effect converts the energy back to its original form Based upon the :pulse-echo principle

– Electricity into sound = pulse– Sound into electricity = echo

Pulse–Echo Technique

• Ultrasound scanhead produces “pulses” of ultrasound waves – These waves travel within the body and interact with various organs – The reflected waves return to the scanhead and are processed by the ultrasound machine – An image which represents these reflections is formed on the monitor

• The instrument generating and receiving those pulses of sound is the transducer, which is a critical component of the system due to its profound effect on image quality.

Parameters of Pulsed Sound

• pulsed sound, or pulsed wave (PW) sound, has several specific parameters include

1. the pulse repetition frequency,

2. pulse repetition period,

3. pulse duration,

4. Duty Factor

5. and spatial pulse length (SPL).

Pulse Repetition Frequency

• Remember that frequency is defined as the number of cycles of sound produced in 1 second. The number of pulses of sound produced in 1 second is called the pulse repetition frequency (PRF).

• Frequency and PRF are not the same. • In diagnostic imaging, the PRF has typical values between 1000 and

10,000 Hz (1 to 10 KHz)

Relationship between imaging depth and pulserepetition frequency:

• the deeper the area of interest, the lower the PRF .As the imaging depth increases, the PRF decreases, and as the depth decreases, the PRF increases

depth PRF

depth PRF

Pulse Repetition Period• Definition: Pulse repetition period is the time from the start of one

pulse to the start of the next pulse. It includes one pulse duration and one “listening time.”

• The relationship that PRP has to PRF is similar to the relationship between period and frequency (inversely)

• when PRP increases, the PRF decreases and vice versa• Typical Values In clinical imaging, the PR period ranges from 100

microseconds to 1ms.

• As imaging depth increases, PR period increases. As imaging depth decreases, PR period decreases.

Pulse Duration • Definition: The time from the start of a pulse to the end of that pulse. (the

actual time that the pulse is “on”(transmitted)).• Units: seconds• Determined By: Sound source , Pulse duration is determined by

multiplying the number of cycles in the pulse and the period of each cycle.

• Changed by Sonographer : No, does not change when sonographer alters imaging depth. Pulse duration is a characteristic of each transducer.

• Typical Values In clinical imaging : pulse duration ranges from 0.5 to 3 secs.

Spatial Pulse Length• Definition: The length or distance that a pulse occupies in space.

The distance from the start to the end of one pulse. • Units: mm—• Determined By : Both the source and the medium• Changed by Sonographer : No • Typical Values: 0.1 to 1mm. • Note: SPL determines axial resolution (image accuracy) . Shorter

pulses create images of greater accuracy.

Duty Factor

• Definition: The percentage of time that the system transmits sound. • Units» 1.0 or 100% for continuous wave sound Unitless! » typically

less than 1%Minimum = 0.0 or 0%• Determined By : Sound source• Changed by Sonographer: Yes • Typical values: From 0.1% to 1% (little talking, lots of listening).• As we know, the operator adjusts imaging depth and thereby

determines the pulse repetition period. Thus, the operator indirectly changes duty factor while altering depth of view.

Parameters that Describe Both Pulsed and Continuous Waves

• Period• Frequency • Wavelength • Propagation Speed

Parameters of Pulsed WavesParameters Basic Units Units Determined By Typical Values

pulse duration time Microseconds time sound source 0.5–3.0 micro sec

pulse repetition period

time mseconds, time soundsource 0.1–1.0msec

pulse repetition frequency

1/time 1/sec, Hz sound source 1–10 kHz

spatial pulse length

distance mm source & medium

0.1–1.0mm

duty factor none none sound Source 0.001–0.01

By adjusting the imaging depth, the operator changes the pulse repetition period, pulse repetition frequency, and duty factor. The pulse duration and spatial pulse length are characteristics of the pulse itself and are inherent in the design of the transducer system. They are not changed by sonographer.

Components of ultrasound machine

1. Transducer 2. CPU3. Display4. Key board / cursor5. Disc storage device.6. Printer

Components

Transducer

• Instrument which converts one form of energy to other

• The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy.

Electrical Energy Mechanical Energy

SELECTION OF TRANSDUCER

• Superficial vessels and organs within 1 to 3cms depth and intra operative imaging –

• 7.5 to 15 Hz

• Deeper structures in abdomen and pelvis within 12 to 15cms –

• 2.25 to 3.5Hz

Transducer - Parts• A simple single-element,

plane-piston source transducer has major components including the – Piezoelectric

material, Sensor electrodes,

– Insulated layer, Backing block,

– Acoustic insulator Insulating cover, and Transducer housing.

Piezoelectric Element• The active element is basically a piece of polarized material - a

piezoelectric ceramic sandwiched between electrodes

• The piezoelectric element converts electrical signals into

mechanical vibrations (transmit mode) and mechanical vibrations

into electrical signals (receive mode).

• Natural – Quartz • Artificial

– most of USG materials– ferroelectrics

• barium titanate • PZT (lead zirconate titanate)

Piezoelectric materials

How piezoelectricity works

1. Normally, the charges in a piezoelectric crystal are exactly balanced, even if they're not symmetrically arranged.

2. The effects of the charges exactly cancel out, leaving no net charge on the crystal faces. exactly cancel one another out.)

3. If you squeeze the crystal, you force the charges out of balance.

4. Now the effects of the charges no longer cancel one another out and net positive and negative charges appear on opposite crystal faces. By squeezing the crystal, you've produced a voltage across its opposite faces—and that's piezoelectricity!

Piezoelectric Effect

Piezoelectric crystal – how thick?

• The thickness is determined by the desired frequency of the transducer

• Piezoelectric crystals are cut to a thickness that is 1/2 the desired radiated wavelength

• Heating the crystal above this temp reduces its usefulness. So, transducers should not be autoclaved.

Backing/Damping Block

• The rear face of the piezoelectric crystal material is usually supported by a backing material.

• It absorbs backward direction of ultrasound energy, attenuates stray US signals from housing and dampens the transducer vibration) to create an ultrasound pulse with a short spatial pulse length, which is necessary to preserve detail along the beam axis (axial resolution).

Couplant• Material that facilitates the

transmission of ultrasonic energy from the transducer into the test specimen.

• Necessary to overcome the acoustic impedance mismatch between air and solids.

• The best coupling agents are

1. water-soluble gels, which are commercially available.

2. Water is suitable for very short examinations.

3. Disinfectant fluids can also be used for short coupling of the transducer during guided punctures.

4. Oil has the disadvantage of dissolving rubber or plastic parts of the transducer.