Principles of
Ultrasound Physics
(Basics)
Abel Girma (RR)

Objectives :
▪ After completing this presentation, the learner will be able
to:
1. Explain the fundamental principle used in sonographic
imaging
2. Describe the image formats used in sonography
3. Describe how echoes are generated
4. List the primary components of a sonographic transducer
5. List the primary components of a sonographic instrument
6. Describe the Doppler effect
7. List the ways in which Doppler information is

Introduction:
Medical imaging with ultrasound is called sonog

What is Sound ?
• Mechanical and Longitudinal
waves wave that can transfer a
distance using a media.
• Cannot travel throughVacuum.

What is
Ultrasound?
• Ultrasound is amechanical,longitudinal wave with a
frequency exceeding the upper limit of human hearing,
which is 20,000 Hz or 20 kHz.
• T
ypically at 2 – 20 Mhz.

Characteristics of Sound

Veloci
ty
• Speed at which a sound wave travels through a
medium(cm/sec)
• Determined by density and stiffness of media
– Slowest in air/gas
– Fastest in solids
• Average speed ofultrasound in body
is 1540m/sec

Frequency
• Number of cycles per second
• Units are Hertz
• Ultrasound imaging frequency range 2-20Mhz
• Higher the frequency Lower the penetration and
Higher the resolution.

Wavelength… Distance over which one cycle
occurs

Wave Equation..
• Given a constant velocity
,as frequency
increases wavelength decreases
V = ƒ λ

Amplitude
Defines the Brightness of
the image
Irrespective ofthe Freq theAmp
remains constant
The Higher the Amp the brighter the
image and the lower the more

Acoustic
Impedance (Z)
• The product of the tissue’s density and the sound velocity
within the tissue
• Amplitude of returning echo is proportional to the difference in
acoustic impedance between the two tissues
• Velocities:
– Soft tissues = 1400-1600m/sec
– Bone = 4080
– Air = 330
• Thus, when an ultrasound beam encounters two regions of
very different acoustic impedances, the beam is reflected or
absorbed
– Cannot penetrate
– Example:soft tissue – bone interface

Gain ….

How ultrasound Works…
• Ultrasound transducer produces “pulses”of
ultrasound waves
• These waves travel within the body and interactwith
various tissues
• The reflected waves return to the transducer and are processed by
the ultrasound machine
• An image which represents these reflections is formed on the

Pulse- Echo Principle

Ultrasound ?

Interactions of Ultrasound
with tissue
• Reflectio
n
• Transmis
sion
• Attenuat
ion

Reflectio
n
–
–
• O ccurs at a boundary between 2 adjacent tissues or
media
• The amount of reflection depends on differences in
acoustic impedance (z) between media
• The ultrasound image is formed from reflected echoes
Transdu
cer
Z =Density x
Velocity

Scatteri
ng
•
•
•
Redirection of sound in several directions
Caused by interaction with small reflector
or rough surface O nly portion of sound
wave returns to transducer

• N ot all the sound wave is reflected,some
continues deeper into the body
• These waves will reflect from deeper
tissue structures
Transdu
cer
Transmiss
ion

• The deeper the wave travels in the body
,
the weaker it becomes
• The amplitude ofthe wave decreases
with increasing depth
Attenuat
ion

Attenuation…

Ultrasound range ..

Refraction:

Refraction in action …

Echogenicity (caused by
Reflection)
Anechoi
c
Hypoechoic
Hyp
erechoic

Time Gain Compensation (Post
Processing)
• The Returned signal is amplified for better
imaging at depths.
TGC too low far field TGC
corrected

Knobology…

Probe Orientation…

Imaging Planes
Transverse or Axial
Longitudinal or Sagittal
Coronal

Transducers…

Reflection

Doppler…
▪ Apparent change in received frequency due to a
relative motion between a sound source and
sound receiver
– SoundTOWARD receiver = increased frequency
– Sound AWAY from receiver = Decreased frequency

Cos of 0 degree is 1 and cos of 90 degree is
0

Doppler information is presented in d/t forms
• audible,
• Color-Doppler,
• Spectral-Doppler forms.
Types of Doppler:
CW (Continuous Wave)
PW (PulsedWave)

What is Colour Doppler?
▪ Utilizes pulse-echo Doppler flow principles to generate
a color image
▪ This image is superimposed on the 2D image
▪ The red and blue display provides information
regarding DIRECTION andVELOCITY of flow
▪ Regardless of colour
,the top of the bar represents flow
coming towards the transducer and the bottom of the
bar represents flow awayfrom the transducer

Ultrasound artifact