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← Section I · Physical Principles, Instrumentation, Examination Principles
I.F

Cross-Sectional Scanning: Technical Principles and Instrumentation

19 cards

Notes

Beam structure

  • Ultrasound beam has three regions: near field (Fresnel zone) → focal zone (narrowest) → far field (Fraunhofer zone) where the beam diverges.
  • Lateral resolution is best in the focal zone, degrades in the far field as the beam widens.
  • Larger aperture → tighter focal beam (better lateral resolution).
  • Focusing (electronic or mechanical) narrows the beam at the depth of interest.

Resolution (types)

  • Axial (longitudinal) resolution - distance between two reflectors along the beam axis. Determined by spatial pulse length (SPL). Shorter SPL (higher f, fewer cycles) = better. Best of the three types.
  • Lateral resolution - distance perpendicular to beam. Determined by beam width; best at the focal zone.
  • Temporal resolution - related to frame rate. Better with narrow sector, shallow depth, fewer focal zones.
  • Contrast resolution - ability to distinguish tissues of different reflectivity.
  • Spatial resolution - line density; write-zoom increases it, read-zoom does not.

Frame rate trade-offs

  • Frame rate ↑ with narrow sector, shallow depth, single focal zone, low line density.
  • Real-time 2D: ~30–100 Hz.

Zoom

  • Write zoom - reacquires with more scan lines / pixels in the ROI → truly improves image resolution.
  • Read zoom - magnifies the acquired image (no resolution gain).

Imaging artifacts (major types)

ArtifactMechanismRecognition / fix
ReverberationMultiple back-and-forth reflections between two strong parallel reflectorsEqually spaced, parallel lines at ↓ intensity with depth. Common with prosthetic valves.
Comet-tailReverberation from small metallic/highly reflective objectSolid hyperechoic beam distal to object; lines are not equidistant. Change to harmonic imaging helps.
Ring-downReverberation with strong repetitive ringing of the crystalSimilar to comet-tail but different mechanism. Often gas particles.
Mirror imageStructure in front of a highly reflective surface duplicates deeperDuplicated structure equidistant beyond the reflector (violates straight-line assumption).
Side lobeOff-axis energy from array transducer strikes a strong reflectorImage appears displaced laterally from true location.
Beam widthStructure at the edge of the beam superimposes on the central imageE.g., aortic valve "in" LA, atheroma "in" aortic lumen. Adjust focal zone.
RefractionBeam bends at an interface with different propagation speeds (Snell's law)Lateral displacement or duplication (e.g., double AV in short axis).
Range ambiguityEchoes from deep structures return after next pulse firesIncrease depth so echo returns before next pulse.
Acoustic shadowingStrong reflector attenuates sound beyond itAnechoic zone distal to prosthetic valve, calcification.
Near-field clutter ("bang" artifact)High-amplitude ringing of crystal in near fieldHigher-frequency transducer, harmonic imaging, decrease depth.
Focal enhancement (banding)Horizontal band of echoes at focal zoneAdjust focal zone.
GhostingMultiple reflections on color DopplerColor extends beyond anatomic borders.
Propagation-speed errorUS speed in tissue differs from assumed 1540 m/sDistorts apparent depth (e.g., silastic ball of Starr-Edwards).

Artifact spatial relationship

  • More distant than object: reverberation (parallel motion) or mirror image (opposite motion).
  • Same distance as object: beam width or side lobe.

Doppler-specific artifacts

  • Mirror-image (crosstalk): symmetric spectrum on both sides of baseline; caused by high Doppler gain - reduce gain.
  • Range ambiguity in Doppler: signals from > one depth mixed (high-PRF Doppler).
  • Aliasing: wrap-around above Nyquist.
  • Beam-width Doppler artifact: overlap of signals from adjacent flows.

Harmonic imaging

  • Second-harmonic imaging uses reflections at 2× the transmitted frequency.
  • Reduces side lobes, grating lobes, reverberations, near-field clutter.
  • Improves lateral resolution 20–50%; worsens axial resolution 40–100%.

Cards

  • basicI.F-001
    Name the three regions of an ultrasound beam.
    → Near field (Fresnel zone), focal zone (narrowest, best lateral resolution), and far field (Fraunhofer zone, beam diverges).
  • basicI.F-002
    Where is lateral resolution best? Where is it worst?
    → Best at the focal zone. Worst in the far field where the beam widens.
  • basicI.F-003
    Which is best: axial, lateral, or temporal resolution?
    → Axial resolution — determined by spatial pulse length. Improved by higher-frequency transducers (shorter wavelength).
  • basicI.F-004
    Write-zoom vs read-zoom — which one improves resolution?
    → Write zoom (increases line density and pixels in the ROI). Read zoom just magnifies without adding resolution.
  • basicI.F-005
    Describe reverberation artifact and give a common clinical example.
    → Multiple back-and-forth reflections between two strong parallel reflectors produce equally spaced, parallel lines of decreasing intensity beyond the object. Common with prosthetic valves.
  • basicI.F-006
    How does a comet-tail artifact differ from a reverberation artifact?
    → Comet-tail also arises from a highly reflective (often metallic) object but appears as a solid hyperechoic beam distal to the object — lines are not equidistant. Both violate the time-of-flight assumption.
  • basicI.F-007
    What is a mirror-image artifact?
    → A structure placed in front of a highly reflective interface produces a duplicated copy at a deeper position (behind the reflector). The transducer misinterprets the multi-hop return path as a longer straight path.
  • basicI.F-008
    What is a side-lobe artifact?
    → Some ultrasound energy is emitted off the central beam axis; if it hits a strong reflector, the machine assumes it originated on-axis and displays the echo at the wrong lateral location.
  • basicI.F-009
    What is a beam-width artifact and give an example.
    → When a structure at the edge of the beam is superimposed on the central image (occurs distal to the focal zone). E.g. aortic valve appearing 'in' the LA, or atheroma 'in' the aortic lumen. Adjust focal zone.
  • basicI.F-010
    What law governs ultrasound refraction, and what causes it?
    → Snell's law. Refraction occurs when the beam crosses an interface obliquely between media with different propagation speeds — the beam bends, displacing structures laterally.
  • basicI.F-011
    How do you fix a range-ambiguity artifact?
    → Increase the imaging depth. This lowers the PRF so echoes from the deepest structures return before the next pulse fires.
  • basicI.F-012
    What is acoustic shadowing? Common source?
    → An anechoic (black) region distal to a strong reflector that attenuates the beam. Classic sources: mechanical prosthetic valves, calcifications, pacemaker wires.
  • basicI.F-013
    Which artifacts appear at the SAME distance from the transducer as the true object?
    → Beam-width and side-lobe artifacts. Reverberation appears further than the object with parallel motion; mirror-image appears further with opposite motion.
  • basicI.F-014
    Which imaging mode helps reduce side lobes, grating lobes, reverberation, and near-field clutter?
    → Second-harmonic imaging.
  • basicI.F-015
    Mirror-image artifact in spectral Doppler — what causes it and how do you fix it?
    → Caused by excessive Doppler gain; a symmetric, lower-intensity signal appears on the opposite side of the baseline (crosstalk). Fix: decrease gain/power output, optimize angle.
  • basicI.F-016
    What is 'ghosting' on color Doppler?
    → Color signals extending beyond anatomic borders due to multiple reflections — often from moving structures or prosthetic material.
  • basicI.F-017
    Explain propagation-speed-error artifact using the Starr-Edwards silastic ball.
    → US travels slower through the silastic ball than the assumed 1540 m/s, so distances appear longer than they are — the ball is displayed as oval rather than round.
  • basicI.F-018
    How can you identify Type A vs Type B artifact around the ascending aorta on TEE?
    → Type A artifact: 2× as far from the transducer as from the posterior aortic wall (reverberation from the posterior aortic wall). Type B artifact: 2× distance from right PA posterior wall as from posterior aortic wall. Both move parallel to the aortic wall (no independent motion, unlike a true intimal flap).
  • basicI.F-019
    How does frame rate change when you narrow the sector?
    → Frame rate increases (fewer scan lines per frame). Deeper depth, wider sector, and more focal zones all decrease frame rate.