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