Ultrasound Physics

1. Ultrasound Physics • Sound is a mechanical, longitudinal wave that travels in a straight line • Sound requires a medium through which to travel • Ultrasound is a mechanical, longitudinal wave with a frequency exceeding the upper limit of human hearing, which is 20,000 Hz or 20 kHz. • Medical Ultrasound 2MHz to 16MHz

2. Amplitude Basic Ultrasound Physics oscillations/sec = frequency - expressed in Hertz (Hz)

3. ULTRASOUND – How is it produced? Produced by passing an electrical current through a piezoelectrical (material that expands and contracts with current) crystal

4. Ultrasound Production • Transducer produces ultrasound pulses (transmit 1% of the time) • These elements convert electrical energy into a mechanical ultrasound wave • Reflected echoes return to the scanhead which converts the ultrasound wave into an electrical signal

5. 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 Low Frequency 3 MHz High Frequency 12 MHz

6. Image Formation Electrical signal produces ‘dots’ on the screen • Brightness of the dots is proportional to the strength of the returning echoes • Location of the dots is determined by travel time. The velocity in tissue is assumed constant at 1540m/sec Distance = Velocity Time

7. Transducer Medium 1 Medium 2 Medium 3 Interactions of Ultrasound with Tissue • Acoustic impedance (AI) is dependent on the density of the material in which sound is propagated • - the greater the impedance the denser the material. • Reflections comes from the interface of different AI’s • greater  of the AI = more signal reflected • works both ways (send and receive directions)

8. Interaction of Ultrasound with Tissue • Greater the AI, greater the returned signal • largest difference is solid-gas interface • we don’t like gas or air • we don’t like bone for the same reason • GEL!! • Sound is attenuated as it goes deeper into the body

9. Interactions of Ultrasound with Tissue • Reflection • Refraction • Transmission • Attenuation

10. transducer Interactions of Ultrasound with Tissue • Reflection • The ultrasound reflects off tissue and returns to the transducer, the amount of reflection depends on differences in acoustic impedance • The ultrasound image is formed from reflected echoes

11. Refraction reflective refraction Scattered echoes Incident Angle of incidence = angle of reflection

12. transducer Interactions of Ultrasound with Tissue Transmission • Some of the ultrasound waves continue deeper into the body • These waves will reflect from deeper tissue structures

13. Interactions of Ultrasound with Tissue 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

14. Reflected Echo’s • Strong Reflections = White dots Diaphragm, tendons, bone ‘Hyperechoic’

15. Reflected Echo’s Weaker Reflections = Grey dots • Most solid organs, • thick fluid – ‘isoechoic’

16. Reflected Echo’s • No Reflections = Black dots • Fluid within a cyst, urine, blood ‘Hypoechoic’ or echofree

17. 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

18. Ultrasound Beam Profile • Beam comes out as a slice • Beam Profile • Approx. 1 mm thick • Depth displayed – user controlled • Image produced is “2D” • tomographic slice • assumes no thickness • You control the aim 1mm

19. Accomplishing this goal depends upon... • Resolving capability of the system • axial/lateral resolution • spatial resolution • contrast resolution • temporal resolution • Processing Power • ability to capture, preserve and display the information

20. Types of Resolution • Temporal Resolution • the ability to accurately locate the position of moving structures at particular instants in time • also known as frame rate

21. Types of Resolution • Contrast Resolution • the ability to resolve two adjacent objects of similar intensity/reflective properties as separate objects - dependant on the dynamic range

22. Liver metastases

23. Ultrasound Applications Visualisation Tool: Nerves, soft tissue masses Vessels - assessment of position, size, patency Ultrasound Guided Procedures in real time – dynamic imaging; central venous access, nerve blocks

24. Imaging Know your anatomy – Skin, muscle, tendons, nerves and vessels Recognise normal appearances – compare sides!

25. Skin, subcutaneous tissue Epidermis Loose connective tissue and subcutaneous fat is hypoechoic Muscle interface Muscle fibres interface Bone

26. Transverse scan – Internal Jugular Vein and Common Carotid Artery

27. Summary • Imaging tool – Must have the knowledge to understand how the image is formed • Dynamic technique • Acquisition and interpretation dependant upon the skills of the operator.

28. Acoustic Impedance • The velocity of sound in a tissue and tissue density = determine acoustic impedance • Most soft tissues = 1400-1600m/sec • Bone = 4080, Air = 330 • Sound will not penetrate – gets reflected or absorbed • Travel time – dot depth

29. Attenuation • Absorption = energy is captured by the tissue then converted to heat • Reflection = occurs at interfaces between tissues of different acoustic properties • Scattering = beam hits irregular interface – beam gets scattered

30. Ultrasound Terminology • Never use dense, opaque, lucent • Anechoic • No returning echoes= black (acellular fluid) • Echogenic • Regarding fluid--some shade of grey d/t returning echoes • Relative terms • Comparison to normal echogenicity of the same organ or other structure • Hypoechoic, isoechoic, hyperechoic • Spleen should be hyperechoic to liver