ultrasound basics

1. BASICS OF
ULTRASOUND
PHYSICS
DR PRAJWITH . K . J. RAI

2. INTRODUCTION
 Rapid progress in the field of medical ultrasound has been achieved in the late 1940’s
and early 1950’s.
 Ultrasound imaging makes use of sound waves.
 Medical ultrasound makes use of sound waves of frequencies from 10,00,000 to
20,00,000 cycles per sec.

3. SOUND WAVES
 Sound beam is similar to an X-ray beam in that both transmit energy.
 X-rays can travel through vacum
 while sound waves need a medium for propagation.
 They travel in waveform.

4.  Sound waves are longitudinal waves .
 Ultrasound pulses are transmitted in the form of longitudinal waves ie the
motion of particles in the medium is parallel to direction of wave
propagation.
 Velocity of sound is independent of frequency & depends primarily on
physical make up of the material through which sound is being
transmitted.

5.  Important characteristics of transmitting medium are
 COMPRESSIBILITY &
 DENSITY
GAS LIQUID SOLID

6. Compressibility
 velosity α-1 Compressibility
SLOWER FASTER FASTEST

7. DENSITY
 Dense materials are those which are composed of heavy particles, which
have more inertia.
 It is difficult to either start movement in them or stop movement once they
begin to move.
 Since propagation of sound involves rhythmic particulate motion , the
heavy particles in dense molecules cannot transmit sound at faster
velocities compared to less dense materials .

8. ultrasonic frequency
 In ultrasonic frequency range, the velocity of sound is constant in any
particular medium.
 If the frequency increases then the wave length must decrease, because
wavelength and frequency are inversely proportional to each other.
 λ α-1 γ

9. Intensity
 Intensity of sound refers to loudness of sound.
 The greater the amplitude of oscillation of particles , the more intense
the sound.

10. PULSE ECHO PRINCIPLE
 Ultrasound imaging is based on this principle.
 Electricity into sound = PULSE
 Sound into electricity = ECHO

13. TRANSDUCERS
 Ultrasound transducers are used to convert an electric signal into
ultrasonic energy that can be transmitted into tissues & to convert
ultrasound energy reflected back from the tissues into an electric
signal.
 The heart of the transducer is a piezoelectric crystal.

14.  The heart of the transducer is a piezoelectric crystal ( lead zirconate
titanate).
 Piezoelectric materialsare those in which there is a change in physical
dimensions ( only few microns), on application of an electric field.
 They consist of multiple dipoles arranged in a geometric pattern.
 On application of electric field these dipoles realign themselves, thus
changing the dimensions of the crystal.

15. TRANSDUCERS
 In a transducer the piezoelectric crystal is placed between two electrodes
which behave as capacitors.
 The voltage between them produces an electric field which causes
change in shape of piezoelectric crystal.
 If the voltage is applied in multiple short bursts, the crystal vibrates and
generates sound waves.
 The backing block dampens the sound waves immediately in order to
prime the crystal for the returning echoes from patients body.


16. TRANSDUCERS
 An ultrasound transducer is maximally sensitive to a particular frequency
depending on the sthicknes of the piezoelectric crystal. That particular
frequency is called it’s resonant frequency.

17. TRANSDUCERS
 The returning echoes from the patient which carry information strike the
crystal which again vibrates and thereby induces a voltage between the
electrodes, which is amplified and is used to produce the signal.

18. CHARACTERISTICS OF ULTRASOUND
BEAM
 Intensity of the ultrasound beam varies along the length of the beam.
 The beam has a natural tendency to diverge.
 The parallel component is called the near zone or ‘Fresnel Zone’.
 Diverging portion of the beam is called far zone or ‘Fraunhoffer Zone’.
 Fresnal zone is longer with
1. larger transducers and
2. high frequency sound.

20. HIGH FREQUENCY BEAM
 Advantages
 better depth resolution.
 longer fresnel zone.
 Major drawback–
 Less depth penetration, (since tissue absorption increases with increasing
frequency.)

21. INTERACTIONS BETWEEN
ULTRASOUND AND MATTER
1. Reflection
2. Refraction
3. Absorption

22.  Diagnostic images are produced by reflected portion of the beam.
 The percentage of beam reflected at tissue interfaces depends on
 Tissue’s acoustic impedance
 Beam’s angle of incidence.
 Bending of light as it travels from one medium to another is called
REFRACTION.
 Refraction cause artifacts.
 ABSORPTION of ultrasound is due to frictional forces that oppose the
movement of particles .As the wave travels through the medium. This
absorbed energy is converted to heat.

24. ULTRASOUND DISPLAY
 A mode – Amplitude mode. Echoes are displayed as spikes and height of
spikes depends on returning echoes intensity. Usually used in
ophthalmology.
 TM mode – Time motion mode. Used in echocardiography.
 B mode – Brightness mode. Brightness of image depends on strength of
returning echoes.

25. TISSUE HARMONIC IMAGING
 THI is based on phenomenon of non-linear distortion of an acoustic
signal as it travels through the body.
 We send in waves of a particular frequency. Harmonic waves are generated
within the tissues & build up with depth to a point of maximum intensity
before they decrease due to attenuation.

26.  Current technology uses only the second harmonic for imaging. Harmonic
imaging is especially useful in obese patients.

28. Advantages of THI
 Lesions are clearer & better defined.
 - Use of higher frequencies improves resolution.
 - Helps differentiate cysts from hypoechoic solid masses.
 - Better clarity of contents.
 - It is superior to conventional USG in visualization of lesions containing
highly reflective tissues like fat, calcium & air.

29. KNOBOLOGY
1. Gain - Controls the degree of echo amplification or brightness of image.
2. Zoom – Enlarges the image.
3. Time Gain Compensation - Attempts to compensate for acoustic loss by
absorption, reflection & scatter & to show structures of same acoustic
strength with the same brightness no matter what the depth.
4. Dynamic Range - Refers to range of intensities from the
largest to the smallest echo that a system can display
5. Calipers – Used fro measurements.
6. Depth

30. ARTEFACTS
 Artifacts related to instrumental problems.
 Artifacts caused by technique
 Artifacts caused by sound tissue interactions

31. Artifacts related to instrumental
problems.
 Artifactual Noise
 Calibration Artifact
 Main Bang Artifact
 Veiling Artifact
 Side Lobe artifact.

32. Artifacts caused by technique
 Noise
 TGC problems

33. Artifacts caused by sound tissue
interactions
 Artifacts from strongly reflective structures
 Enhancement
 Reverberation Artifact
 Mirror image artifact
 Comet tail artifact.