visweswar: Ultrasonography

ULTRASONOGRAPHY - PRINCIPLES AND ITS APPLICATION IN VETERINARY PRACTICE

PRINCIPLES OF ULTRA SONOGRAPHY

Introduction

Sound wave echoes are used to create images called sonogram.

Medical sonography is the only diagnostic imaging modality that does not use electromagnetic radiation.

In 1957 – Ian Donald invented – scanner or diagnostic ultrasound.

History of Ultrasound:

In 1794- Lazzaro Spallanzani discovered echolocation in bats.

In 1880 – Discovery of piezo electricity by curie brothers.(Jacques and Pierre curie)

In 1915- Electrostatic transducers for ultrasonic submarine detector by P.Langevin.

1940- Piezoelectric ceremics

1969- Piezoelectric polymers

1994 – Micromachined electrostatic transducers by M.Haller and B.T.Khuri-Yakub.

What is ultrasound?

Sound waves of frequencies greater than audible to the human ear i.e greater than 20- 20,000 Hz. is called Ultrasound waves. Diagnostic ultrasound uses frequencies between 1 to 10 MHz
A sound wave travels in a pulse or a wave and when it is reflected back it becomes an Echo and this pulse-echo principle, is used for ultrasound imaging.
A transducer is a device that converts one form of energy to another. The piezoelectrical crystal(Lead Zirconate Titanium) in an ultrasound transducer generates a pulse. When this crystal is stimulated electrically it changes its shape and produces sound waves of a particular frequency.
For 1% of time, Peizo crystal acts as emitter and 99% of time as receiver of echoes.

How is ultrasound generated?

When a high voltage electrical current is applied crystals in the transducer are vibrated and this is called piezo electric effect.
A sound wave travels in a pulse and when it is reflected back it becomes an Echo.
It is this pulse-echo principle, which is used for ultrasound imaging. A pulse is generated by one or more piezoelectrical crystal in an ultrasound transducer.
When this crystal is stimulated electrically it changes its shape and produces sound waves of a particular frequency.
As the transducer is placed in close contact with the body surface through a coupling medium it undergoes continuous modification, which occurs through three processes those are absorption, reflection and scattering.
By means of the echo principle, an image can be produced on the display of the scanner which relates to the acoustic independence of tissues encountered by the ultrasound beam and the depth / distance of tissue interfaces.

PROPERTIES OF ULTRASOUND WAVES

Frequency, wavelength and velocity are parameters used to describe the sound waves.
Diagnostic ultrasound frequency ranges between 1 mega Hertz and 10 mega Hertz.
Wave length is the distance travelled by the sound in one cycle and is expressed in millimeters and is important for image resolution.
Velocity is the rate at which sound travels through an acoustic medium. As a rule it is greatest in solids, lower in liquids and lowest in gases.Medically sound waves travel fastest in bone and slowest in gas filled structures.
This causes a problem for diagnostic ultrasound machines, because they use the average velocity of sound in soft tissue 1540m/s.
Attenuation: Depth dependent decrease in the intensity of amplitude of sound wave in tissues.
Acoustic impedence: It is defined as the product of density of the medium and speed of travel of sound in it.
Gain: It is the amplification of electrical signals generated by returning echo, it is all equally radio opaque substances in the field appearing as similar objects.

Interaction of ultrasound with matter

Absorption: It occurs when the tissues absorb heat energy in the sound beam. Absorption process forms the basis of therapeutic ultrasound.
Reflection: The reflection gives rise to an echo and forms the basis for ultrasound scanning. Interfaces between tissues of different acoustic impedence give rise to different echoes. These echoes are converted by piezoelectric effect into electrical signals and displayed onto a oscilloscope screen.
Scattering: It occurs when the beam encounters an interface that is irregular and smaller than the sound beam. The portion of the beam than interacts with this interface is scattered in all directions. Since the scattering interfaces are small, only a small portion of the beam is involved. Once the echoes are converted into electrical signals, these are processed and transformed into a visual display of the measure of the amplitude of the echo. This is known as echo quantification.

Generation of images on the display

Two basic shapes of ultrasound images are encountered: images made in sector fashion are pie shaped and linear fashion are rectangular
Information is displayed regarding distance and amplitude.
Each echo position is represented as a dot on the screen. Thus a two dimensional image is generated.
The brightness of each dot is related to the amplitude of the reflection and is referred as a grey scale display.
Resolution is the ability of the ultrasound machines to distinguish echoes on the basis of time, space and strength.
- Axial resolution: Ability to differentiate two objects lying closely together in the direction of the beam.
- Lateral resolution: Ability to different the two objects lying side by side. The ultrasound beam is refracted when it enters a tissue of different acoustical density.
The image of refraction depends on the relative velocity of sound in the two tissues.
Depth of sound wave penetration varies inversely with frequency.
Higher frequency wave travels for lesser depth in tissues and vice versa.

DIFFERENT MODES OF ECHO DISPLAY

Different modes of echo display

Different modes of echo display are Brightness mode, B – mode B scan or grey scale used commonly for abdominal scanning and cardiac imaging.
Static B mode: Transducer is moved in the scanning plane by hand.
Real time B mode: Sound beam automatically and rapidly moves in the scan plane. In this method the image is continuously updated to allow movement.
Motion mode: M–mode mainly used for echocardiography.
A MODE or amplitude mode is simplest form of display. It displays two parameters of the echoes in the form of spikes, ie., distance from the transducer and the amplitude. The horizontal line shows the distance and the amplitude is depicted on the vertical line. It is used for ocular biometry.

Different types of basic probes

The pitch of a siren in a train changes with the proximity of the train movement, due to difference in the sound wave frequency, called Doppler shift, the principle used in imaging the direction and velocity of blood flow. It was first proposed by Johann Christian Andreas Doppler in 1842.
Four Doppler modes are used in medical ultra sonography.
They are Continous wave Doppler, pulsed wave Doppler colour Doppler and power Doppler.

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