SonographyAcademy

Curriculum · Ultrasound Physics & Instrumentation

Pulsed Ultrasound & Axial Resolution

Pulse duration, spatial pulse length, PRF/PRP, duty factor, and how spatial pulse length sets axial resolution (SPL/2).

~30 min · level: foundation · SPI draft — pending clinical review

Learning objectives

  • Define pulse duration, spatial pulse length, PRF, PRP, and duty factor with their equations.
  • Explain why diagnostic imaging is pulsed and how CW differs (duty factor = 100%).
  • Derive and apply axial resolution = SPL/2 and identify what improves it.

Diagnostic imaging is pulsed: the transducer transmits a brief pulse, then listens for echoes before transmitting again. The listening interval is how the system assigns depth. This introduces parameters absent from continuous-wave sound.

ParameterDefinitionEquationAdjustable?

Pulse duration (PD)

Time the pulse is 'on'

PD=n T=n/fPD = n\,T = n/f

No (damping-set)

Spatial pulse length (SPL)

Physical length of the pulse

SPL=n λSPL = n\,\lambda

No

Pulse repetition frequency (PRF)

Pulses per second

set by depth

Indirectly (depth)

Pulse repetition period (PRP)

Time between pulse starts

PRP=1/PRFPRP = 1/PRF

Indirectly

Duty factor (DF)

Fraction of time transmitting

DF=PD/PRP=PD⋅PRFDF = PD/PRP = PD \cdot PRF

No

Pulsed-ultrasound parameters.

Here nn is the number of cycles in the pulse. Imaging pulses are short (2–3 cycles, heavy damping) so PD≈0.5PD \approx 0.5–3 μs3\,\mu s. Deeper imaging forces a lower PRF (you must wait longer for distant echoes), which has knock-on effects for frame rate (Lesson 6) and Doppler aliasing.

Axial resolution (also longitudinal / range / depth resolution) is the minimum separation along the beam axis at which two reflectors yield distinct echoes:

Axial resolution=SPL2=n λ2\text{Axial resolution} = \frac{SPL}{2} = \frac{n\,\lambda}{2}
The ÷2 is because the pulse travels down and back (round trip).

Two interfaces closer than half the spatial pulse length produce overlapping echoes that merge. Therefore a shorter pulse → better (smaller-number) axial resolution, achieved by (1) higher frequency (shorter λ\lambda) and (2) fewer cycles / heavier backing damping. Axial resolution is independent of depth — the pulse keeps its length as it travels.

Worked example — axial resolution

A 5 MHz probe emits a 2-cycle pulse in soft tissue. Estimate the axial resolution.

Solution.

λ=1.54/5=0.308 mm\lambda = 1.54/5 = 0.308\text{ mm}; SPL=nλ=2(0.308)=0.616 mmSPL = n\lambda = 2(0.308) = 0.616\text{ mm}; axial resolution =SPL/2=0.308 mm≈0.31 mm= SPL/2 = 0.308\text{ mm} \approx 0.31\text{ mm}. Raising the frequency or shortening the pulse would improve (reduce) this number.

Key takeaways

  • Axial resolution equals SPL/2 = n*lambda/2, with the divide-by-2 reflecting the pulse's down-and-back round trip.
  • A shorter pulse gives better (smaller-number) axial resolution, achieved by higher frequency (shorter wavelength) and fewer cycles or heavier backing damping; more cycles lengthen the pulse and worsen it.
  • Axial resolution is independent of depth because the pulse keeps its length as it travels.
  • Duty factor = PD/PRP = PD*PRF; B-mode runs at DF < 1% (mostly listening) while CW Doppler has DF = 100%, which is exactly why CW has no depth/range resolution.
  • Deeper imaging forces a lower PRF because the system must wait longer for distant echoes (PRP = 1/PRF).

Check your understanding

Registry-style items with worked rationales.

1Axial resolution is best described as:recall

2Which change would IMPROVE axial resolution?analysis

3Continuous-wave operation is characterized by a duty factor of:recall

Go deeper — trusted free resources

Hand-picked, verified links to authoritative open resources. Opens in a new tab.

Radiopaedia
Axial resolution (ultrasound)

Concise reference defining axial resolution as half the spatial pulse length and explaining how frequency, wavelength, and pulse length determine resolving power along the beam.

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Radiopaedia
Pulse repetition frequency

Covers PRF, pulse repetition period, and pulse duration, including the formula PRF = 1/PRP and how PRF relates to duty factor and Doppler artifacts.

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StatPearls
Ultrasound Physics and Instrumentation

Free foundational chapter tying together pulsed ultrasound, spatial pulse length, duty factor, and axial versus lateral resolution for the physics basics.

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Radiopaedia
Radiopaedia articles search: spatial pulse length

Always-resolving search across Radiopaedia articles to explore spatial pulse length, wavelength, and how pulse geometry sets axial resolution.

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References

  1. Edelman SK. Understanding Ultrasound Physics. 4th ed. ESP Inc.; 2012.
  2. Kremkau FW. Sonography Principles and Instruments. 9th ed. Elsevier; 2016.
  3. Ultrasound Physics and Instrumentation. StatPearls, NCBI Bookshelf.