Basic of Basic of UltrasoundUltrasound

Basic of Basic of UltrasoundUltrasound

Dr.Shamim RimaDr.Shamim RimaMBBS,DMU,FCGPMBBS,DMU,FCGP

M.Phil (Radiology & M.Phil (Radiology & Imaging)Imaging)

Dr.Shamim RimaDr.Shamim RimaMBBS,DMU,FCGPMBBS,DMU,FCGP

M.Phil (Radiology & M.Phil (Radiology & Imaging)Imaging)

• Sound • Wave parameter• Velocity of sound• Pulse ultrasound

parameter• Pizoelectric effect

TOPIC

• The frequency range of sound above 20kHz is known as ultrasound.

• These waves, inaudible to humans, can be transmitted in beams and used to scan the tissues of the body.

• Medical uses of US ranges normally from 2MHz to 10MHz.

What is ultrasound ?

Periodic motion causes

pressure waves in

surrounding physical

media. In the diagram,

when the piston is shoved

forward it compresses the

medium. The compression

travels through the

medium. As the piston

moves back and forth, it

creates more

compressions that travel

through the medium like

cars down a highway. The

more quickly the piston

moves back and forth, the

closer one compression is

to the next one.

• Ultrasound is produced through the conversion of electrical energy into mechanical energy, and is detected by the reverse process, by converting mechanical energy into electrical energy.

• The transducer is a device that is both a transmitter and receiver of the ultrasound signal and it serves a dual role in pulse echo imaging.

• The transducer contains a special type of crystal with in it named piezoelectric crystal.

• In a electric field the alignment of dipole with in the crystal changes, which in turn causes the crystal to change the shape.

• If the voltage is applied in a sudden burst the crystal vibrates and generates sound.

• Ultrasound can be directed as a beam.• Ultrasound obeys the laws of reflection and refraction.• Ultrasound is reflected by objects of small size.

Advantages

Disadvantages • Ultrasound propagates poorly through a gaseous medium. • The amount of ultrasound reflected depends on the acoustic mismatch.

Nature of sound

• Sound waves are longitudinal in nature, and require a material medium ( solid, liquid or gas) for their transmission; they will not pass through a vacuum.

• Sound must be generated mechanically by an oscillating body of matter.

• The velocity of sound depends on nature of medium.

• The velocity of sound is determined by the rate at which the force is transmitted from one molecule to another.

• It moves by producing band of compression and rarefaction motion of the molecule of conducting particle.

• The length of the wave is the distance between two adjacent bands of compression and rarefaction.

• The velocity of transmission is independent of frequency.

• The velocity of transmission depends upon physical make up of the matter through which the sound is being transmitted.

• It transmitted rapidly through less compressible material and slowly in gases.

• The intensity of ultrasound varies longitudinally along the length of the beam.

Properties of ultrasound

• Ultrasound obeys the wave equation.

• Velocity = frequency x wave length.

• Velocity= meter/ sec.• Frequency= Hz (hertz= 1 cycle/ sec ).• Wave= meter

Frequency

• Number of cycles per second.

• Sound waves with frequencies ranging about 15Hz to 20,000 Hz are audible to the normal human ear.

• Medically useful ultrasound involves frequencies of 1 to 10 MHz.

• The duration of the pulse is about 1 microsecond and the pulses repeated about 1000 times per second.

• The higher the frequency, the longer the will be cylindrical segment or near field ( fresnel zone).

• At the same time. The far field ( fraunhofer zone) becomes less divergent at higher frequencies.

• The best lateral resolution exists at the junction of the near and far fields ( ie, ability to display two closely spaced points in the same plane, as two separate images).

• Depth resolution improves at higher frequencies (points closely spaced in depth, displayed as two separate images).

• As frequency is increased, greater absorption of sound energy occures in the tissues, weakening the beam intensity.

• The higher the frequency, the longer the will be cylindrical segment or near field ( fresnel zone).

• At the same time. The far field ( fraunhofer zone) becomes less divergent at higher frequencies.

• The best lateral resolution exists at the junction of the near and far fields ( ie, ability to display two closely spaced points in the same plane, as two separate images).

• Depth resolution improves at higher frequencies (points closely spaced in depth, displayed as two separate images).

• As frequency is increased, greater absorption of sound energy occures in the tissues, weakening the beam intensity.

To summariseTo summarise

• Correct depth = reasonable high frequency = reasonably short wave length = reasonably good resolution.

• Too much depth = low frequency = long wavelength = poor resolution

• Too little depth = Wont see structures of interest !

• Correct depth = reasonable high frequency = reasonably short wave length = reasonably good resolution.

• Too much depth = low frequency = long wavelength = poor resolution

• Too little depth = Wont see structures of interest !

• Correct depth = reasonable high frequency = reasonably short wave length = reasonably good resolution.

• Too much depth = low frequency = long wavelength = poor resolution

• Too little depth = Wont see structures of interest !

• Correct depth = reasonable high frequency = reasonably short wave length = reasonably good resolution.

• Too much depth = low frequency = long wavelength = poor resolution

• Too little depth = Wont see structures of interest !

• The term velocity refers to speed in a given direction.

• If direction is along a straight line, velocity and speed are the same.

• Velocity = distance / time.

• The of sound and ultrasound in particular, depends on the density (gm/cm3) and the compressibility of the conducting medium.

• The term velocity refers to speed in a given direction.

• If direction is along a straight line, velocity and speed are the same.

• Velocity = distance / time.

• The of sound and ultrasound in particular, depends on the density (gm/cm3) and the compressibility of the conducting medium.

RefractionRefraction

• The waves pass through the tissues at different speeds.

• Various soft tissues are essentially liquids, velocity of US is 1540 m/s.

• Rapid conduction occur in bone.• Slowest conduction occur in gas.

• The waves pass through the tissues at different speeds.

• Various soft tissues are essentially liquids, velocity of US is 1540 m/s.

• Rapid conduction occur in bone.• Slowest conduction occur in gas.

Wavelength

• The length of a single cycle of the ultrasound wave.

• It is inversely proportional to the frequency and determines the resolution of the scanner.

• The length of a single cycle of the ultrasound wave.

• It is inversely proportional to the frequency and determines the resolution of the scanner.

Interaction between ultrasound and matter

Types:

Reflection Refraction Absorption.

Reflection • In ultrasound the reflected portion of the

beam produces the image. Transmitted sound contributes nothing to image formation but transmission must be strong enough to produce echo at deeper level.

• The percentage of the beam reflected at tissue

interfaces depend on- 1. The tissue acoustic impedance2. The beams angle of incidence.

The tissue acoustic impedance

• The impedance of a material is the product of its density and velocity of sound in the material.

• The velocity of sound in tissue is fairly constant over a wide range of frequency , so a substance’s acoustic impedance is constant.

• As sound waves pass from one tissue plane to another, the amount of reflection is determind by the difference in the impedance of the two tissues.

• The greater the difference, the greater the percentage reflected.

The beams angle of incidence

• The amount of reflection is determined by the angle of incidence between the sound beam and the reflecting surface.

• The higher the angle of incidence, the less the amount of reflected sound.

• In medical ultrasound, in which the same transducer both transmit and receive ultrasound, almost no reflected sound will be detected if the ultrasound strikes the patient’s surface at an angle of more than 30 from perpendicular.

• When sound passes from one medium to another its frequency remain constant but its wavelength changes to accommodate a new velocity in the second medium.

• When the sound beam strikes the second medium at an angle, the change in wavelength necessitates a change in direction.

• The beam finally emerges in the second medium with a different wavelength and direction.

• This bending of waves as they pass from one medium to another is called refraction.

Refraction

Snell’s law

• Sin Θ1 / SinΘ2 = V1 / V2 .

• Sin Θ1 = Angle of incidence SinΘ 2= Angle of transmission V1 = Velocity of sound for incident

medium V2 = Velocity of sound for transmitting

medium

ABSORPTION

• The term absorption refers to the conversion of ultrasound or ultrasonic to thermal energy.

• Absorption of ultrasound in fluids is a results of frictional forces that oppose the motion of the particles in the medium. The energy removed from the ultrasound beam is converted into heat.

• Three factor determine the amount of absorption:1. The frequency of the sound.2. The viscosity of the conducting medium.3. The relaxation time of the medium.

• The frequency of sound effects the amount of absorption produced by the viscosity of a material.

• The higher the frequency, the more its motion is affected by the drag of a viscous material.

• Frequency also affects the amount of absorption produced by the relaxation time.

• In liquids, which have low viscosity, very little absorption takes place.

• In soft tissues where viscosity is higher and a medium amount of absorption occurs.

• In bone shows high absorption of ultrasound.

• The frequency of sound effects the amount of absorption produced by the viscosity of a material.

• The higher the frequency, the more its motion is affected by the drag of a viscous material.

• Frequency also affects the amount of absorption produced by the relaxation time.

• In liquids, which have low viscosity, very little absorption takes place.

• In soft tissues where viscosity is higher and a medium amount of absorption occurs.

• In bone shows high absorption of ultrasound.

• The relaxation time is the time that it takes for a molecule to return to its original position after it has been displaced.

• The relaxation time is constant for any

particular material.

• A molecule with a longer relaxation time may not return completely before a second compression wave arrives.

Transducer

Pulse Generator

Amplification

Scan Generator

Scan Converter

Image Processor

Display

THE ULTRASOUND IMAGING SYSTEM

The Principal Functional Components of an Ultrasound Imaging System

Modes of ultrasound display

• The ultrasound images is an electronic representation of data generated from returning echoes and displayed on a TV monitor.

• The image is assembled, one bit at a time. Each retuning echo generates one bit of data, and many bits together form the electronic image.

The modes of ultrasound display are as follows

• A mode• M mode• TM mode• B mode

• A mode• M mode• TM mode• B mode

A mode

• In the A mode, echoes are displayed as spikes projecting from a baseline. The base line identifies the central aqxis of the beam.

• Spike height is proportional to echo intensity, with strong echoes producing large spikes.

A mode is used in:

• Ophthalmology• Echoencephalogra

phy• Echocardiography

• Ophthalmology• Echoencephalogra

phy• Echocardiography

M mode

• When an image is displayed as dots instead of spikes is known as M mode.

• Depth is proportional to time.

• When an image is displayed as dots instead of spikes is known as M mode.

• Depth is proportional to time.

TM modeTM mode

• TM mode is similar to the A mode, except that the echoes are recorded as dots instead of the spikes and the TM mode used to study moving parts.

• TM mode is one dimensional image composed of dots.

• Depth is proportional to the height of the dots.

• Use : echocardiography.

• TM mode is similar to the A mode, except that the echoes are recorded as dots instead of the spikes and the TM mode used to study moving parts.

• TM mode is one dimensional image composed of dots.

• Depth is proportional to the height of the dots.

• Use : echocardiography.

B modeB mode

• The B mode produces a picture of a slice of tissue. When image is produce as a picture of a slice of tissue and result is obtained by analyzing brightness of its different parts.

• Echo depth is determined by the time delay, as in A mode. In B-mode scanning, computed contact scanning is necessary.

• Two types of B mode display –

* Gray scale image

* Real time image.

• The B mode produces a picture of a slice of tissue. When image is produce as a picture of a slice of tissue and result is obtained by analyzing brightness of its different parts.

• Echo depth is determined by the time delay, as in A mode. In B-mode scanning, computed contact scanning is necessary.

• Two types of B mode display –

* Gray scale image

* Real time image.

• Certain material (Lead Zirconate Titanate) are such that the application of an electric field causes a change in their physical dimensions, and vice versa. This is called piezoelectric effect.

• Piezoelectric materials are made up of innumerable dipoles arranged in geometric pattern.

Pizoelectric effect

• An electric dipole is a distorted molecule that appears to have a positive charge on one end and a negative charge on the other.

• The positive and negative ends are arranged so that an electric field will cause them to realign, thus changing the dimension of the crystal.

• If the voltage is applied in a sudden burst, or pulse, the crystal vibrates like a cymbal that has been struck a sharp blow and generates sound waves.

• As the sound pulse passes through the body, echoes reflect back towards the transducer from each tissue interface.

• These echoes carry energy and causing a physical compression on the crystal element in the transducer.

• This compression forces the tiny dipoles to change their orientation, which induces a voltage between the electrodes.

• The voltage is amplified and serves as ultrasonic signal for display on a monitor.

• The compression forces and associated voltage are responsible for the name piezoelectricity, which means ‘pressure’ electricity.

• The voltage is amplified and serves as ultrasonic signal for display on a monitor.

• The compression forces and associated voltage are responsible for the name piezoelectricity, which means ‘pressure’ electricity.