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Medical Imaging Ultrasound

Medical Imaging Ultrasound. Edwin L. Dove 1412 SC edwin-dove@uiowa.edu 335-5635. 3D Reconstruction. Why Ultrasound in Cardiology?. Portable, relatively cheap Non-ionizing During the echocardiogram, it is possible for the cardiologist to: Watch the heart’s motion – in 2D real-time

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Medical Imaging Ultrasound

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  1. Medical ImagingUltrasound Edwin L. Dove 1412 SC edwin-dove@uiowa.edu 335-5635

  2. 3D Reconstruction

  3. Why Ultrasound in Cardiology? • Portable, relatively cheap • Non-ionizing • During the echocardiogram, it is possible for the cardiologist to: • Watch the heart’s motion – in 2D real-time • Ascertain if the valves are opening and closing properly, and view any abnormalities • Determine the size of the heart chambers and major vessels • Measure the thickness of the heart walls • Calculate standard metrics of health/disease • e.g., Volume, EF, SV, CO • Dynamic evaluation of abnormalities

  4. Sinusoidal pressure source

  5. p pressure applied in z-direction density  viscosity Quantitative Description

  6. Speed of Sound in Tissue • The speed of sound in a human tissue depends on the average density  (kg·m3) and the compressibility K (m2·N-1) of the tissue.

  7. Sound Velocity for Various Tissues

  8. Tissue Characteristics • Engineers and scientists working in ultrasound have found that a convenient way of expressing relevant tissue properties is to use characteristic (or acoustic) impedance Z (kg·m-2 ·s-1)

  9. Pressure Generation • Piezoelectric crystal • ‘piezo’ means pressure, so piezoelectric means • pressure generated when electric field is applied • electric energy generated when pressure is applied

  10. Charged Piezoelectric Molecules Highly simplified effect of E field

  11. Piezoelectric Effect

  12. Piezoelectric Principle

  13. Vibrating element

  14. Transducer Design

  15. Transducer

  16. Reflectance and Refraction Snells’ Law (Assumes i = r)

  17. Reflectivity At normal incidence,i = t = 0 and

  18. Reflectivity for Various Tissues

  19. Specular Reflection • The first, specular echoes, originate from relatively large, strongly reflective, regularly shaped objects with smooth surfaces. These reflections are angle dependent, and are described by reflectivity equation . This type of reflection is called specular reflection.

  20. Scattered Reflection • The second type of echoes are scattered that originate from small, weakly reflective, irregularly shaped objects, and are less angle-dependent and less intense. The mathematical treatment of non-specular reflection (sometimes called “speckle”) involves the Rayleigh probability density function. This type of reflection, however, sometimes dominates medical images, as you will see in the laboratory demonstrations.

  21. Circuit for Generating Sharp Pulses

  22. Pressure Radiated by Sharp Pulse

  23. Ultrasound Principle

  24. Echoes from Internal Organ

  25. Attenuation • Most engineers and scientists working in the ultrasound characterize attenuation as the “half-value layer,” or the “half-power distance.” These terms refer to the distance that ultrasound will travel in a particular tissue before its amplitude or energy is attenuated to half its original value.

  26. Attenuation • Divergence of the wavefront • Elastic reflection of wave energy • Elastic scattering of wave energy • Absorption of wave energy

  27. Ultrasound Attenuation

  28. Attenuation in Tissue • Ultrasound energy can travel in water 380 cm before its power decreases to half of its original value. Attenuation is greater in soft tissue, and even greater in muscle. Thus, a thick muscled chest wall will offer a significant obstacle to the transmission of ultrasound. Non-muscle tissue such as fat does not attenuate acoustic energy as much. The half-power distance for bone is still less than muscle, which explains why bone is such a barrier to ultrasound. Air and lung tissue have extremely short half-power distances and represent severe obstacles to the transmission of acoustic energy.

  29. Attenuation • As a general rule, the attenuation coefficient is doubled when the frequency is doubled.

  30. Pressure Radiated by Sharp Pulse

  31. Beam Forming • Ultrasound beam can be shaped with lenses • Ultrasound transducers (and other antennae) emit energy in three fields • Near field (Fresnel region) • Focused field • Far field (Fraunhofer region)

  32. Directing Ultrasound with Lens

  33. A lens will focus the beam to a small spot according to the equation Beam Focusing

  34. Linear Array

  35. Types of Probes

  36. Modern Electronic Beam Direction

  37. Beam Direction (Listening)

  38. Wavefronts Add to Form Acoustic Beam

  39. Phased Linear Array

  40. A-mode Ultrasound Amplitude of reflected signal vs. time

  41. A-mode

  42. M-mode Ultrasound

  43. M-mode

  44. B-mode Ultrasound

  45. Fan forming

  46. B-mode Example

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