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Resident Physics Lectures

Resident Physics Lectures. Ultrasound Basics Principles. George David, M.S. Associate Professor of Radiology. Ultrasound Transducer. Acts as both speaker & microphone Emits very short sound pulse Listens a very long time for returning echoes Can only do one at a time. Microphone

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Resident Physics Lectures

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  1. Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology

  2. Ultrasound Transducer • Acts as both speaker & microphone • Emits very short sound pulse • Listens a very long time for returning echoes • Can only do one at a time Microphone receives echoes Speaker transmits sound pulses

  3. Piezoelectric Principle • Voltage generated when certain materials are deformed by pressure • Reverse also true! • Some materials change dimensions when voltage applied • dimensional change causes pressure change • when voltage polarity reversed, so is dimensional change V

  4. US Transducer Operation • alternating voltage (AC) applied to piezoelectric element • Causes • alternating dimensional changes • alternating pressure changes • pressure propagates as sound wave

  5. Ultrasound Basics • What does your scanner know about the sound echoes it hears? I’m a scanner, Jim, not a magician. Sound Acme Ultra- Sound Co. Echo

  6. What does your scanner know about echoed sound? How loud is the echo? • inferred from intensity of electrical pulse from transducer

  7. What does your scanner know about echoed sound? What was the time delay between sound broadcast and the echo?

  8. What else does your scanner know about echoed sound? The sound’s pitch or frequency

  9. What Does Your Scanner Assume about Echoes(or how the scanner can lie to you) • Sound travels at 1540 m/s everywhere in body • average speed of sound in soft tissue • Sound travels in straight lines in direction transmitted • Sound attenuated equally by everything in body • (0.5 dB/cm/MHz, soft tissue average)

  10. Luckily These Are Close Enough to Truth To Give Us Images • Sound travels at 1540 m/s everywhere in body • average speed of sound in soft tissue • Sound travels in straight lines in direction transmitted • Sound attenuated equally by everything in body • (0.5 dB/cm/MHz, soft tissue average)

  11. Dot Placement on Image • Dot position ideally indicates source of echo • scanner has no way of knowing exact location • Infers location from echo ?

  12. Dot Placement on Image • Scanner aims sound when transmitting • echo assumed to originate from direction of scanner’s sound transmission • ain’t necessarily so ?

  13. Positioning Dot • Dot positioned along assumed line • Position on assumed line calculated based upon • speed of sound • time delay between sound transmission & echo ?

  14. Distance of Echo from Transducer • Time delay accurately measured by scanner distance = time delay X speed of sound distance

  15. What is the Speed of Sound? distance = time delay X speed of sound • scanner assumes speed of sound is that of soft tissue • 1.54 mm/msec • 1540 m/sec • 13 usec required for echo object 1 cm from transducer (2 cm round trip) 13 msec 1 cm Handy rule of thumb

  16. So the scanner assumes the wrong speed? • Luckily, the speed of sound is almost the same for most body parts • Sometimes • soft tissue ==> 1.54 mm / msec • fat ==> 1.44 mm / msec • brain ==> 1.51 mm / msec • liver, kidney ==> 1.56 mm / msec • muscle ==> 1.57 mm / msec ?

  17. Gray Shade of Echo • Ultrasound is gray shade modality • Gray shade should indicate echogeneity of object ? ?

  18. How does scanner know what gray shade to assign an echo? • Based upon intensity (volume, loudness) of echo ? ?

  19. Gray Shade • Loud echo = bright dot • Soft echo = dim dot Loud Soft

  20. Complication • Deep echoes are softer (lower volume) than surface echoes. Loud Soft ?

  21. Gray Shade of Echo • Correction needed to compensate for sound attenuation with distance • Otherwise dots close to transducer would be brighter

  22. Echo’s Gray Shade • Gray Shade determined by • Measured echo strength • accurate • Calculated attenuation Who am I? Charles Lane

  23. Attenuation Correction Tissue Attenuation Coefficient (dB / cm / MHz) • Fat 0.6 • Brain 0.6 • Liver 0.5 • Kidney 0.9 • Muscle 1.0 • Heart 1.1 • scanner assumes entire body has attenuation of soft tissue • actual attenuation varies widely in body

  24. Ultrasound Display • One sound pulse produces • one image scan line • one series of gray shade dots in a line • Multiple pulses • two dimensional image obtained by moving direction in which sound transmitted

  25. How Do We Move the Beam? • Electronically • Phased Arrays

  26. Sound Wave Definition? • Sound is a Wave • Wave is a propagating (traveling) variation in a “wave variable” • “An elephant is big, gray, and looks like an elephant.”

  27. Sound Wave Variable • Examples • pressure (force / area) • density (mass / volume) • temperature • Also called acoustic variable Sound is a propagating (moving) variation in a “wave variable”

  28. Energy & Power 75 Watt Light Bulbs rated in power! • Power • rate of energy use • Units: watts or milliwatts • Energy = Power X Time • Units: kilowatt-hours Power Energy Electric Bill 300 KW-hr. 75 Watts for 4 hours or 150 Watts for 2 hours Electricity billed in energy!

  29. Intensity • Intensity of Sound Beam intensity = power / cross sectional area

  30. Sound Wave Variation • Freeze time • Measure some acoustic variable as a function of position Pressure Density Temperature Acoustic Variable Value Position

  31. MORE • Make multiple measurements of an acoustic variable an instant apart • Results would look the same but appear to move in space 1 Instant #1 Instant #2 2

  32. MORE • Track acoustic variable at one position over time

  33. Sound Waves • Waves transmit energy • Waves do not transmit matter • “Crowd wave” at sports event • people’s elevation varies with time • variation in elevation moves around stadium • people do not move around stadium

  34. Transverse Waves • Particle moves perpendicular to wave travel • Water ripple • surface height varies with time • peak height moves outward • water does not move outward

  35. Compression (Longitudinal) Waves • Particle motion parallel to direction of wave travel 1 1 Motion ofIndividual Coil 2 2 Wave Travel

  36. Medium • Material through which wave moves • Medium not required for all wave types • no medium required for electromagnetic waves • radio • x-rays • infrared • ultraviolet • medium is required for sound • sound does not travel through vacuum Talk louder! I can’t hear you.

  37. Sound Waves • Information may be encoded in wave energy • radio • TV • ultrasound • audible sound

  38. Sound Frequency # of complete variations (cycles) of an acoustic variable per unit time • Units cycles per second 1 Hz = 1 cycle per second 1 kHz = 1000 cycles per second 1 MHz = 1,000,000 cycles per second • Human hearing range 20 - 20,000 Hz

  39. Sound Frequency • Ultrasound definition > 20,000 Hz • not audible to humans • dog whistles are in this range • Clinical ultrasound frequency range 1 - 10 MHz 1,000,000 - 10,000,000 Hz

  40. Period • time between a point in one cycle & the same point in the next cycle • time of single cycle • Units • time per cycle (sometimes expressed only as time; cycle implied) Magnitude of acoustic variable period time

  41. Period 1 Period = ------------------- Frequency • as frequency increases, period decreases • if frequency in Hz, period in seconds/cycle

  42. Period Period = 1 / Frequency • if frequency in kHz, period in msec/cycle • if frequency in MHz, period in msec/cycle 1 kHz frequency ==> 1 msec period 1 MHz frequency ==> 1 msec period

  43. Reciprocal Units

  44. Sound Period & Frequency are determined only by the sound source. They are independent of medium. Who am I? Burt Mustin

  45. Propagation Speed • Speed only a function of medium • Speed virtually constant with respect to frequency over clinical range • Speed depends on medium’s • Density (mass per unit volume) • more dense ==> lower speed • Stiffness (or bulk modulus; opposite of elasticity or compressibility) • more stiffness ==> higher speed • “same letter, same effect”

  46. Wavelength • distance in space over which single cycle occurs OR • distance between a given point in a cycle & corresponding point in next cycle • imagine freezing time, measuring between corresponding points in space between adjacent cycles

  47. Wavelength Units • length per cycle • sometimes just length; cycle implied • usually in millimeters or fractions of a millimeter for clinical ultrasound

  48. Wavelength Equation Speed = Wavelength X Frequency [ c = l X n ](dist./time)(dist./cycle) (cycles/time) • As frequency increases, wavelength decreases • because speed is constant

  49. Wavelength Speed = Wavelength X Frequency c = l X n (dist./time)(dist./cycle) (cycles/time) mm/msec mm/cycle MHzCalculate Wavelength for 5 MHz sound in soft tissue Wavelength = 1.54 mm/msec / 5 MHz 5 MHz = 5,000,000 cycles / sec = 5 cycles / msec Wavelength = 1.54 / 5 = 0.31 mm / cycle

  50. Wavelength is a function of both the sound source and the medium! Who am I? John Fiedler

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