Sound and Ultrasound in Medicine (PHR 177)Course

1. Sound and Ultrasound in Medicine(PHR 177)Course Prof. Dr. Moustafa. M. Mohamed Vice Dean Faculty of Allied Medical Science Pharos University Alexandria Dr. Yasser Khedr Department of Medical Biophysics Pharos University

2. Interactions of Ultrasound withTissue • Reflection • Refraction • Transmission • Attenuation

3. Reflection – The production of echoes at reflecting interfaces between tissues of differing physical properties. – The ultrasound reflects of tissue and returns to the transducer, the amount of reflection depends on differences in acoustic impedance – The ultrasound image is formed from reflected echoes

4. Refraction • A change in direction of the sound wave as it passes from one tissue to a tissue of higher or lower sound velocity • U/S scanners assume that an echo returns along a straight path

5. Transmission – Some of the ultrasound waves continue deeper into the body – These waves will reflect from deeper tissue structures

6. Attenuation • The intensity of sound waves diminish as they travel through a medium • In ideal systems sound pressure (amplitude) is only reduced by the spreading of waves • In real systems some waves are scattered and others are absorbed, or reflected • This decrease in intensity (loss of amplitude) is called attenuation.

7. Attenuation Defined - the deeper the wave travels in the body, the weaker it becomes -3 processes: reflection, absorption, refraction – Air (lung)> bone > muscle > soft tissue>blood > water

8. Producing an image • Important concepts in production of an ultrasound image: • Propagation velocity. • Acoustic impedance. • Reflection • Refraction • Attenuation

9. Propagation Velocity • Sound is energy transmitted through a medium- • Each medium has a constant velocity of sound (c) • Product of frequency (f) and wavelength (λ) c = fλ • Frequency and Wavelength therefore are directly proportional- if the frequency increases the wavelength must decrease.

10. Impedance • Acoustic impedance (z) of a material is the product of its density and propagation velocity Z= pc • Differences in acoustic impedance create reflective interfaces that echo the u/s wavesback at the probe • Impedance mismatch = ΔZ

11. Common Interface Reflection Factors • air/tissue interfaces reflect (99.9%) of the incident ultrasound beam, thus • imaging through lungs (air) is generally not possible • gel is applied between the transducer (PZT) and the skin to displace the air and • minimize large reflections (80%) that would interfere with ultrasound transmission • into the patient • bone/tissue interfaces also reflect substantial fractions (30%) of the incident intensity • the lack of transmissions beyond these interfaces results in an area void of echoes • called shadowing • in imaging the abdomen, organs such as the kidney, pancreas, spleen and liver are • composed of sub-regions that contain many scattering sites, the strongest echoes • which results in ultrasound images • organs that contain fluids such as the bladder, cysts, and blood vessels have no • internal structure and almost no echoes (i.e.: show black on images)

12. Benefits and Risks of Ultrasound • Benefits (Non-invasive, No use of Radiation, Widely Available and cheaper than CT, Real-time and MRI • Risks • development of heat - tissues or water absorb the ultrasound energy which increases their temperature locally • formation of bubbles (cavitations) – when dissolved gases come out of solution due to local heat caused by ultrasound