General Physics
Pulsed Wave Operation
Padmini Arora
Ultrasound
Continuous Wave:
➔ Wave that’s “on” for indefinite amount
of time with constant amplitude
➔ Also referred to as undamped wave
➔ Cannot create anatomical images
Pulsed Wave:
➔ To create anatomical images in diagnostic ultrasound
➔ Pulses separated by gap in time
➔ Pulses = a few cycles of ultrasound
➔ Also called damped wave
➔ Pulse is like an individual train car
➔ Pulsed wave is like the train
➔ Transmit time: talking time OR “on” time
➔ Receive time: listening time OR “off” time
Pulsed Ultrasound
Parameters for Pulsed Ultrasound besides the
other sound wave parameters from Ch. #3:
1.
2.
3.
4.
5.
Pulse Duration
Spatial Pulse Length
Pulse Repetition Period
Pulse Repetition Frequency
Duty Factor
Pulse Duration [PD]
➔ Definition: Time it takes for one pulse to occur
[n]
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⟡ Period [time for one cycle] ∗ # of cycles in pulse
⟡ Single “on” time
Formula: PD [µs] = n ∗ T [µs]
Unit: microseconds, milliseconds or seconds
Determined by: sound source
Adjustability: not by operator
Typical Value: 0.3 - 2.0 µs
Sonographic pulse = usually 2 - 3 cycles long
Doppler pulses = typically 5 - 30 cycles long
Shorter pulses improve image quality
Direct relationship between PD & n
⟡ If # of cycles ⇈
⟡ If Period ⇈
⤏ PD ⇈
⤏ PD ⇈
➔ Remember: inverse relationship between period & frequency
⟡ PD = n x [1/frequency] OR n/frequency
⟡ If frequency ⇈
⤏ period [T] ⇊ ⤏ PD ⇊
Spatial Pulse Length [SPL]
➔ Definition: length of pulse from front to end
➔ Wavelength [distance of one cycle] ∗ # of cycles in pulse [n]
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Formula: SPL [mm] = n ∗ 𝝺 [mm]
Unit: any unit of distance [mm]
Determined by: sound source & medium
Adjustability: not by operator
Typical value: 0.1 - 1.0 mm
➔ Shorter pulses improve image quality
➔ Direct relationship between SPL & n
➔ Direct relationship between SPL & 𝝺
⟡ If # of cycles ⇈
⟡ If 𝝺 ⇈
⤏ SPL ⇈
⤏ SPL ⇈
➔ Remember: inverse relationship between 𝝺 & f
⟡ SPL = n [c/f] OR [nc/f]
⟡ If frequency ⇈
⤏ 𝝺 ⇊ ⤏ SPL ⇊
Pulse Repetition Period [PRP]
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Definition: time from start of one pulse to start of next pulse --- has no relation c̅ period
Formula: PRP = 1/PRF
Unit: any unit of time [ms]
Determined by: sound source
Adjustability: yes - by changing imaging depth
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⟡ shallow depth
short PRP/deeper
➔ Typical value: 100 µs - 1 ms
⤏ longer PRP
➔ Directly related to depth of view [max distance into body that US system images]
⤏ PRP ⇈
⟡ If depth of view ⇊ ⤏ PRP ⇊
⟡ If depth of view ⇈
➔ Two components of PRP:
✶ Transmit time [pulse duration]
✶ Receive time
➷ Operator can’t ▵ PD [transmit time]
➷ Depth of view ▵’s receiving time
Pulse Repetition Frequency [PRF]
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Definition: # of pulses occurring in a second --- has no relation c̅ frequency
Formula: PRF = 1/PRP
Unit: Hz or per second
Determined by: sound source
Adjustability: yes - by changing imaging depth
⟡ shallow depth
⤏ high PRF/deeper ⤏ lower PRF
➔ Typical value: 1,000 - 10,000 hertz
➔ Inversely related to depth of view [max distance into body that
US system images]
⤏ PRF ⇊
⟡ If depth of view ⇊ ⤏ PRF ⇈
⟡ If depth of view ⇈
➔ PRF & PRP = inverse relationship
⤏ PRF ⇊
⟡ If PRP ⇊ ⤏ PRF ⇈
⟡ If PRP ⇈
➔ Use complementary units c̅ PRP & PRF
Duty Factor [DF]
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Definition: fraction of time or percentage that pulsed US = “on”
Formula: DF = PD µs / PRP µs
Unit: it’s a fraction so it’s unitless
Determined by: sound source
Adjustability: yes - by changing imaging depth
⟡ shallow depth
⤏ high DF/deeper ⤏ low DF
➔ Typical value: 0.002 - 0.005 OR 0.2% - 0.5% in pulsed US
➔ Inversely related to depth of view [max distance into body that US system images]
⤏ PRP ⇈ ⤏ DF ⇊
⟡ If depth of view ⇊ ⤏ PRP ⇊ ⤏ DF ⇈
⟡ If depth of view ⇈
➔ Note: PD can’t be ▵ by operator
➔ PRF & PRP = inverse relationship
⤏ PRP ⇊ ⤏ DF ⇈
⟡ If PRF ⇊ ⤏ PRP ⇈ ⤏ DF ⇊
⟡ If PRF ⇈
➔ DF = unit less
➔ Maximum value = 1 or 100%
➔ Minimum value = 0 or 0%
Formulas
➙ PD [µs] = n ∗ T [µs]
➙ SPL [mm] = n ∗𝝺 [mm]
➙ PRP = 1/PRF
➙ PRF = 1/PRP
➙ DF = PD [µs] / PRP [µs]
✘ PRP, PRF & DF = can ▵ by changing depth of
view
✘ PRP = unrelated to period
✘ PRF = unrelated to frequency