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Ultrasound Axial Resolution Overview

Sep 29, 2025

Overview

This lecture covers the concept of axial resolution in ultrasound imaging, focusing on the role of spatial pulse length and the factors that affect image quality and depth.

Introduction to Ultrasound Resolution

  • Ultrasound resolution refers to the ability to distinguish between two separate objects in an image.
  • The main types discussed are axial, lateral, elevational, and temporal resolution.

Axial Resolution

  • Axial resolution is the ability to differentiate two objects at different depths along the ultrasound beam’s path.
  • The limit of axial resolution occurs when two objects are so close together that they cannot be distinguished as separate.
  • It is measured along the longitudinal (depth) axis of the ultrasound image.

Spatial Pulse Length and Its Role

  • Spatial pulse length (SPL) is the physical length of an ultrasound pulse as it travels through tissue.
  • SPL = number of cycles in the pulse Ă— wavelength.
  • Axial resolution is determined by the spatial pulse length; shorter SPL means better axial resolution.

Examples of Axial Resolution

  • If two boundaries are at a distance equal to the SPL, they appear as separate objects.
  • If two boundaries are less than half an SPL apart, they merge and cannot be resolved.

Factors Affecting Axial Resolution

  • Axial resolution limit = ½ Ă— spatial pulse length.
  • Reducing the number of cycles in a pulse (using more dampening behind the piezoelectric crystal) shortens SPL and improves resolution.
  • Shorter wavelength (by using thinner piezoelectric material or higher frequency) also shortens SPL and improves resolution.
  • Axial resolution does not change with imaging depth, only with SPL.

Frequency and Axial Resolution Trade-offs

  • Higher frequency probes provide better axial resolution due to shorter wavelengths but have shallower imaging depth because of higher attenuation.

Key Terms & Definitions

  • Axial Resolution — Ability to distinguish two structures along the ultrasound beam’s path (depth).
  • Spatial Pulse Length (SPL) — The total physical length of an ultrasound pulse; calculated as cycles per pulse Ă— wavelength.
  • Pulse Duration — The time taken for an ultrasound pulse.
  • Dampening — Material behind the piezoelectric crystal that reduces the number of cycles per pulse.
  • Piezoelectric Material — The crystal in the ultrasound transducer producing the sound waves.
  • Frequency — Number of wave cycles per second; higher frequency leads to better resolution but less depth penetration.

Action Items / Next Steps

  • Review the concepts of SPL, frequency, and dampening for the next lecture.
  • Prepare for upcoming talks on lateral and elevational resolution.
  • (If studying for an exam) Practice related questions from a curated question bank as suggested.

Full transcript