Ultrasound Physics. History: Ultrasound 2/13/2019. Ultrasound

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1 Ultrasound Physics History: Ultrasound Ultrasound 1942: Dr. Karl Theodore Dussik transmission ultrasound investigation of the brain : Holmes and Howry subject submerged in water tank to achieve good acoustic coupling, pioneers of B-mode imaging 1958: Dr. Ian Donald incorporating ultrasound into OB/GYN field of medicine 1

2 History: Ultrasound History: Ultrasound 2

3 Ultrasound Physics Ultrasound is a longitudinal wave compression and rarefaction Speed of sound in tissue approximately 1540 m/sec. 3

4 but the waves are actually a 3D phenomenon 4

5 but the waves are actually a 3D phenomenon 3D wave equation 3D Laplacian Operator 5

6 typically reduce down to one spatial direction at a time plane wave wave equation so what kind of functions might work? 6

7 so what kind of functions might work? Relationship between wavelength, speed of sound, and frequency. Example suppose a steady-state sinusoidal wave with frequency 2 MHz is traveling in the +z direction in the liver. What is its wavelength? 7

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9 Refraction When a wave passes from one medium to another the frequency is constant, and since c changes, then so must the wavelength Reflection Coefficient - Intensity Previous for pressure/amplitude, often easier to deal with intensity recall: 9

10 Example suppose a plane wave traveling through fat is incident upon the liver with normal incidence, what fraction of acoustic intensity is reflected back from the interface? Attenuation 10

11 Attenuation Example suppose a 5 MHz acoustic pulse travels from a transducer through 2 cm of fat, then encounters and interface with the liver at normal incidence. At what time interval after the transmitted pulse will the reflected pulse arrive back at the transducer? Taking both attenuation and reflection into account, what will be the amplitude loss in db of the returning waveform? 11

12 Scattering Size of targets/scatterers much smaller than acoustic wavelength Specular Reflections echoes originating from relatively large, regularly-shaped objects with smooth surfaces. Scattered Reflections echoes originating from small weakly reflective, irregularly shaped objects 12

13 Scattering Size of targets/scatterers much smaller than acoustic wavelength Each target acts like a spherical wave source, converting a fraction of the incident plane wave energy into spherical wave. Non-linear Propagation Generally assume sinusoidal input results in sinusoidal output, but not exactly true Speed of sound for a given material depends on the acoustic pressure that is present distorts the acoustic waveform as it propagates. 13

14 Tissue Harmonic Imaging Doppler Effect ht/dopplershift.html For pulse-echo imaging 14

15 Example suppose it is known that a 5-MHz transducer axis makes an angel of 30 degrees relative to the direction of motion of blood in a vessel. If the Doppler frequency is measured to be +500Hz, what is the velocity of the blood? Towards or away from the transducer? Ultrasound Physics: Beam Patterns and Focusing Simple Field Pattern Model 15

16 Ultrasound Physics: Beam Patterns and Focusing Transducer arrays can produce side-lobes that cause artifacts Ultrasound Physics: Beam Patterns and Focusing Focusing shape the beam into a narrower shape than what s from a flat plane 1) Curved crystal 2) Apply a lens 3) Electronic focusing using arrays 16