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Pulsed Waves

Topics. PW DefinedPRFPRPDistance EquationRoundtrip EffectPDDuty FactorIntensitySpatial IntensityTemporal IntensityIntensity Instruments. 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 e

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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. a few cycles of US followed by a gap of time with no US Pulsed Wave Ultrasound

    5. Pulses generate scan lines

    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.

    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

    9. If PRF ?, then PRP ?

    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.

    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

    18. If PRF ?, then PRP ? & Pulse Duration ?

    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 %

    21. pulse duration (?s) pulse repetition period (?s)

    22. What is the PRF?

    23. What is the PD if the DF is .25 ?

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

    25. ? SPL ? with: ? wavelength ? # cycles/pulse ? frequency SPL = # cycles in pulse X ? (mm)

    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. Let’s add some Intensity! Up another notch

    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. Intensity: usually highest in the center; weakest in the periphery Intensity varies with time in pulsed US Intensity is not constant within the pulse Sound beams are not uniform…

    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

    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.

    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” ?

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

    42. Example A wave’s SPPA intensity was measured at 400 mW/cm2 & a duty factor of 25%. What is its SPTA?

    43. Recall that: SATA = SPTA ?(SP/SA) SATA = 100 mW/cm2 ? 5 SATA = 20 mW/cm2 Example

    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 Beam Intensity 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

    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

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