Echo sounding uses ultrasound waves directed into the body. The waves pass through tissues and are partially reflected at boundaries, allowing an internal image to be obtained. Higher frequency transducers provide better resolution but penetrate less deeply, while lower frequency penetrates deeper but with lower resolution. The fraction of ultrasound intensity reflected at a boundary depends on the difference in acoustic impedance of the tissues, with a larger difference resulting in more reflection. In this example, the fraction of intensity reflected at the boundary between bone and muscle is calculated using their acoustic impedances.

1. ECHO SOUNDING
RASHIK CHAND

2. Principle
• Ultrasound waves
directed into the
body
• (2 and 18 MHz)
Step 1
• Waves pass through
various tissues
• Partially reflected at
each boundary
• Speed changes
Step 2 • Reflected waves
detected
• Internal image of
body obtained
Step 3

3. Frequency vs. Resolution
• The frequency also affects the QUALITY of the ultrasound
image
The HIGHER the frequency, the BETTER the
resolution
The LOWER the frequency, the LESS the resolution
• A 12 MHz transducer has very good resolution, but cannot
penetrate very deep into the body
• A 3 MHz transducer can penetrate deep into the body, but
the resolution is not as good as the 12 MHz

4. - Fraction of
incident intensity
that is reflected
- depends on
acoustic
impedance (Z)

5. Z = ρ*c
where, ρ is the density in kgm-3
c is speed of light in ms-1
ACOUSTIC IMPEDANCE

6. - At the boundary between two materials,
greater the difference in acoustic
impedance, greater the reflected fraction
of ultrasound wave
CALCULATING REFLECTED INTENSITIES

7. Intensity reflection coefficient (α)
FOR NORMAL INCIDENCE (I.E. ANGLE OF INCIDENCE = O⁰)
- A BIG CHANGE IN ACOUSTIC IMPEDANCE GIVES A LARGE
FRACTION OF REFLECTED INTENSITY

8. Q. A beam of ultrasound is normally
incident on the boundary between
muscle and bone. The acoustic
impedance of the bones of an average
adult is 6.40*106 kgm-2s-1 and that of
the muscle is 1.71*106 kgm-2s-1.
Determine the fraction of its intensity
which is reflected.