1 / 30

Ultrasound Physics

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.

neorah
Download Presentation

Ultrasound Physics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related