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Pulsed Waves

Pulsed Waves. Topics. PW Defined PRF PRP Distance Equation Roundtrip Effect PD Duty Factor Intensity Spatial Intensity Temporal Intensity Intensity Instruments. Pulsed Wave Ultrasound.

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Pulsed Waves

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  1. Pulsed Waves

  2. Topics • PW Defined • PRF • PRP • Distance Equation • Roundtrip Effect • PD • Duty Factor • Intensity • Spatial Intensity • Temporal Intensity • Intensity Instruments

  3. Pulsed Wave Ultrasound The transducer used to produce continuous wave (CW) US is busy transmitting sound and has no time to “listen” for a returning echo. Therefore it cannot produce an image. The transducer used to produced pulsed wave (PW) US is designed to transmit sound pulses & then pauses to “listen” for a returning echo. Imaging requires a transmitter & a receiver

  4. Pulsed Wave Ultrasound • a few cycles of US followed by a gap of time with no US The gap (pause) is used by the transducer to listen for returning echoes (receivingtime)

  5. Pulses generate scan lines Each sound “pulse” represents one scan line. Multiple scan lines create a single frame or image. Pulse 1 Pulse 2 Pulse 3 Time

  6. RECALL! • Frequency • Wavelength • Period PW uses these + more

  7. Pulse Repetition Frequency (PRF) • # of pulses in 1 sec. • Dx. US has a few thousand pulses per second; PRF is expressed in kHz. 1000 s What is the PRF of this pulsed wave? 2 kHz

  8. Pulse Repetition Period (PRP) • Time from the beginning of one pulse to the beginning of the next pulse; expressed in s • Determined by the system time

  9. If PRF , then PRP  time 1 PRP - 500 s time 2 PRP - 250 s

  10. PRF & PRP are reciprocals PRF (kHz) X PRP (ms) = 1 • PRF = 1/PRP and PRP = 1/PRF Determine the PRF in the previous slide

  11. Just a little diversion How long does it take you to travel 150 miles if you drive at 50 mph? Distance = speed X time or Distance ÷ speed = time How long does it take sound to travel 1 cm. into the body?

  12. Distance Equation Distance ÷ speed = time 1 cm ÷ 1540 m/sec. = 1 cm * = m * = 1/154000 sec. .0000065 sec. = 6.5 μsec. 1 sec. 1540 m 1 100 1 sec. 1540 m

  13. Let’s KICK IT UP A NOTCH! How long in time does it take for the sound echo to travel 1 cm. up through the body to return to the transducer? How long does it take a pulse of sound to complete a round trip through 1 cm. of ST? This is called the Roundtrip Effect

  14. Don’t you think the US system may get confused where an echo should show up on your monitor if it sends another pulse before the deep echo returns??? So what can a sonographer do to help?

  15. Tell the machine how long to wait to listen for the deep echo to return by adjusting the depth control to the deepest level you want to ‘hear’ the echo from. Adjusting the depth is actually adjusting the pulse repetition period.

  16. Pulse Duration (PD) • Time for 1 pulse to occur • AKA – transmit time • Dx US: 2 or 3 cycles/pulse • Units - s • PD range: .5 - 3 s • Doppler US: 5-20 cycles/pulse • Shorter pulses  the quality of the images

  17. Pulse Duration - PD time PULSE DURATION (2 ms) Pulse duration (ms) = # cycles/pulse x period (ms) Pulse duration (ms) = # cycles/pulse  ƒ (kHz)

  18. If PRF , then PRP  & Pulse Duration  PD time PRP - 500s PD time PRP - 250s

  19. Duty Factor (Duty Cycle) • Fraction (or %) of time that pulsed US is transmitting (on); always between 0 and 1 (0-100%) • 0 indicates that no pulse occurring • 1 indicates that the pulse is on all of the time, meaning that it is not PW US but rather CW US

  20. Duty Factor Ranges 2D (B-mode) .01 - 1.0 % Doppler US .05 - 5.0 % What implications do you think this might have?

  21. Duty factor = pulse duration (s) pulse repetition period (s) PRP 1000 s time PD 500 s DF = 500/1000 = .5 or the pulse is on 50%

  22. What is the PRF? PRP 1000 s time PD 500 s 1 kHz

  23. What is the PD if the DF is .25 ? 500 s PRP 2000 s time What is the PRF? .5 kHZ

  24. Spatial Pulse Length (SPL) • Length of space a pulse occupies; is measured in mm. • Shorter SPL improves image resolution SPL length

  25. SPL 3  = 4 mm X X SPL  = 2.5 mm XX  = 2.5 mm 2 SPL X X SPL = # cycles in pulse X (mm)  SPL  with:  wavelength  # cycles/pulse  frequency

  26. For all of you who have ever told me that you like A&P better than Physics . . . We have just completed the anatomy of a wave; next will be its physiology

  27. Up another notch Let’s add someIntensity!

  28. Intensity • Important when considering bioeffects • Changed by the operator using the output power control to change the wave amplitude • Directly related to power; if power is doubled, the intensity doubles • Proportional to wave amplitude squared(I  A2)

  29. Sound beams are not uniform… They vary in intensity depending on the location & time where the measurement is taken in the beam • Intensity: usually highest in the center; weakest in the periphery • Intensity varies with time in pulsed US • Intensity is not constant within the pulse

  30. Intensity is usually highest in the center & weakest in the periphery

  31. Intensity varies with time in PW US

  32. Intensity is not constant within the pulse

  33. Because Intensity varies: Specific terms are used to describe variation in intensities associated with clinical ultrasound: Spatial - refers to a location or space Temporal - refers to time Peak - maximum value Average - mean value

  34. Spatial Intensities Center of the sound beam is more intense than the edges Spatial peak intensity Isp - Maximum beam’s intensity Spatial average intensity Isa - Ave of beam’s intensity

  35. Spatial Intensities • Spatial Peak (SP) –greatest intensity in the sound field; usually at the center • Spatial Average (SA) –average intensity in the sound beam field • SP/SA Factor (BUC - Beam Uniformity Coefficient) – describes the distribution of a beam in space • Must be  1 • Relates to space (distance) as duty factor relates to time

  36. Temporal Intensities Temporal Peak (TP) - greatest intensity in the pulse as it passes by. Since it doesn’t include the pulse’s ‘off’ time, it is always  the average. Only in CW (w/ constant amplitudes) is the TP =TA. Temporal Average (TA) - average intensity across the PRP (includes when the pulse is ‘on’ and ‘off’). Only in CW is TP =TA because there is no ‘off’ time.

  37. Pulse Average (PA) - average intensity over the PD (time when the pulse is ‘on’). Only describes a PW US; CW US doesn’t have pulses to average. Pulse Average Temporal Peak Intensity Temporal Average Time

  38. Why is the TA lower than the PA? TA averages the intensity over the entire PRP; so when the transducer is receiving (listening for the returning echo) the intensity is minimal thus lowering the average (liken this to a low test score that brings down your grade average)

  39. PA & TA are related to the duty factor TA = PA X DF Recall: DF = PD X PRF If the duty factor is “1” (100%), then we are describing a continuous wave. What are we describing when the duty factor is “0” ?

  40. HOWEVER, Physicists like to measure the beam over a certain area & over a certain amount of time. SO…. By combining the spatial & temporal values, 6 intensities can be measured

  41. Intensities can be converted… to another intensity by using the duty factor or the SP/SA factor. SPTP SPTA SPPA SATP SATA SAPA

  42. Example A wave’s SPPA intensity was measured at 400 mW/cm2 & a duty factor of 25%. What is its SPTA? Recall that: SPTA = SPPA x Duty Factor SPTA = 400 mW/Cm2 x .25 SPTA = 100 mW/cm2

  43. Example A wave’s SPPA intensity was measured at 400 mW/cm2, a DF of 25% & SP/SA of 5. What is its SATA? Recall that: SATA = SPTA (SP/SA) SATA = 100 mW/cm2 5 SATA = 20 mW/cm2

  44. Ranking the 7 ways to measure intensity in order from largest to smallest: • SPTP – highest intensity measurement used in Dx. US • Im – ave. intensity measured in most intense half cycle (similar in value to SPTP) • SPPA(used only for PW US) • SPTA – used to measure biological effects • SATP • SAPA(used only for PW US) • SATA – lowest intensity measurement used in Dx. US

  45. Determining the Sound BeamIntensity Various methods are performed by the manufacturer or a physicist to determine the intensity of the US beam (requires special equipment)

  46. Equipment to Determine the Intensity of a Sound Beam Radiation force balance or scale determines the intensity or power of the sound beam by measuring the force the sound beam exerts on the balance or scale.

  47. Equipment to Determine the Intensity of a Sound Beam Hydrophone system: a hydrophone and probe; are placed in a water bath in the field of the emitted sound beam. The output is calibrated to indicate pressure or intensity.

  48. What ways can a sonographer decrease a patient’s chance for bioeffects from US?

  49. Formulas to remember! c =  x   x t (period) = 1 DF = PD/PRP

  50. ƒ = # cycles/sec T = time for 1cycle ƒ x T = 1 ƒ = 1/T T = 1/ƒ c = ƒ x λ λ = distance of 1 cycle PRF = # pulses/sec PRP = 1 pulse + dead time (s) PRF (kHz) x PRP (s) = 1 PRF = 1/PRP PRP = 1/PRF PRP = PD/DF PD = time for 1 pulse [on] (s) PD = # cycles in pulse (n) x T PD = n / ƒ (kHz) PD = DF x PRP DF = PD/PRP = on  (on + off) DF = PD x PRF SPL = n x λ

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