CHAPTER 3: PHYSICS (BASICS)
OF ULTRASOUND

Velocity

Speed at which sound wave travel through a medium

Determined by the density & stiffness of media

slowest in air or gas

fastest in solid

The average speed of ultrasound in human body is 1540 m/s

Amplitude

Defines the brightness of the image

Strength/intensity of sound wave at any given time

decrease with increasing of depth

Higher the amplitude, the brighter is the image and lower the amplitude the darker is the image

How the ultrasound machine
make an image

Electrical energy converts to sound wave (mechanical vibrations)

The sound waves are reflected by the tissue in the specimen

Reflected sound waves are converted back to electrical energy, later on will produce an image

Interactions of Ultrasound
with Tissue

Reflection

Transmission

Attenuation

Scattering

Goal of an Ultrasound System

The ultimate goal of any ultrasound system is to make like tissues look alike and unlike tissues look different

To accomplish the ultimate
goal of an ultrasound system

Acoustic impedance

Resolving capability of the system

Axial/lateral resolution

Contrast resolution

Temporal resolution

Spatial resolution

Beam formation: send & receive

Processing power: ability to capture, preserve & display the information for further analysis

What determines how far
ultrasound waves can travel ?

The frequency of the transducer

  • the higher the frequency, the less it can penetrate
  • the lower the frequency, the deeper it can penetrate

** attenuation is directly related to frequency

How is an Image Formed
on the Monitor ?

Amplitude of each reflected wave is represented as a dot

The position of the dot represents the depth from which the echo is received

The brightness of the dot represents the strength of the returning echo

These dots are combined to form a complete image

Doppler in Ultrasound

used to evaluate & quantify the blood flow

  • Transducer is the sound source & sound receiver
  • Flow is in motion relative to the transducer
  • Therefore, doppler will produce an audible signal as well as the graphical representation of the flow known as spectral waveform

What defines a good
Doppler Display

no background noise

clean window/envelope in normal flow state

clear audible signal

accurate display of velocities