Ultrasound Physics & Machine Setting

 

Ultrasound Physics & Machine Settings: A Comprehensive Guide

Ultrasound is an indispensable diagnostic tool in modern medicine, especially in obstetrics, gynecology, cardiology, and emergency care. Mastery of ultrasound physics and machine settings is crucial for accurate image acquisition and interpretation. This blog aims to provide a detailed understanding of the physics behind ultrasound and the optimal use of machine settings for high-quality imaging.

I. Basic Principles of Ultrasound Physics

1. Sound Waves

Ultrasound uses high-frequency sound waves, typically ranging from 2 to 15 MHz, beyond the audible range for humans. These waves are produced by piezoelectric crystals in the transducer that vibrate when an electric current is applied.

2. Propagation of Sound

Sound waves travel through different tissues at varying speeds:

  • Air: 330 m/s (poor conductor)

  • Fat: ~1450 m/s

  • Soft Tissue: ~1540 m/s (standard reference)

  • Bone: ~4080 m/s

The average speed of ultrasound in soft tissue is assumed to be 1540 m/s, which is critical in calculating depth and image resolution.

3. Reflection, Refraction, and Attenuation

  • Reflection: When sound hits a boundary between two tissues with different acoustic impedances, part of it reflects back.

  • Refraction: Bending of the wave at tissue interfaces.

  • Attenuation: Loss of signal strength due to absorption and scattering as it travels deeper.




4. Frequency vs. Resolution & Penetration

  • High Frequency (7–15 MHz): Better resolution, lower penetration – ideal for superficial structures (e.g., breast, thyroid).

  • Low Frequency (2–5 MHz): Greater penetration, lower resolution – used for deeper organs (e.g., abdomen, obstetrics).

II. Key Ultrasound Machine Settings

1. Gain

  • Definition: Controls the overall brightness of the image by amplifying the returning echoes.

  • Adjustment Tip: Too much gain makes the image too bright; too little makes it too dark. Balance is key.




2. Time Gain Compensation (TGC)

  • Function: Adjusts gain at different depths to compensate for attenuation.

  • Use: Helps ensure uniform brightness across the entire image.

3. Depth

  • Purpose: Controls how deep into the tissue the image is displayed.

  • Optimization: Set depth so the area of interest is centered and occupies most of the screen.

4. Focus

  • Definition: The point where the ultrasound beam is narrowest and resolution is best.

  • Setting: Place the focal zone at or just below the structure of interest.

5. Frequency Selection

  • Use: Some transducers allow switching frequencies.

  • Tip: Choose the highest frequency that still penetrates adequately for your target organ.

6. Dynamic Range (Compression)

  • Explanation: Adjusts contrast by controlling the range of grey shades.

  • Higher Dynamic Range: More shades of grey, softer image.

  • Lower Dynamic Range: Higher contrast image, but may lose detail.

7. Zoom

  • Types:

    • Write Zoom: Higher resolution as data is acquired at a focused area.

    • Read Zoom: Post-processing zoom, doesn’t improve resolution.

Understanding common artifacts helps in correct image interpretation:

  • Reverberation: Multiple reflections creating parallel lines.

  • Shadowing: From dense structures like bone or stones.

  • Enhancement: Increased brightness behind fluid-filled structures.

  • Mirror Image: Duplication of structures near reflective surfaces.

  • Edge Artifact: Refraction at curved surfaces causing shadow lines.

IV. Image Optimization Tips

  • Always begin with a general scan at lower gain and increase gradually.

  • Use TGC sliders to equalize brightness from superficial to deep tissues.

  • Adjust focus and frequency to match your structure of interest.

  • Practice using harmonic imaging and speckle reduction filters for clearer views.

  • Keep the transducer perpendicular to the structure to maximize reflection.

V. Safety and Bioeffects

Ultrasound is generally safe, but users must be aware of the following:

  • Thermal Index (TI) and Mechanical Index (MI) guide safe use.

  • Keep exposure time as short as necessary.

  • Use ALARA principle (As Low As Reasonably Achievable) to minimize risk.

Conclusion

A solid grasp of ultrasound physics and machine settings is fundamental for producing diagnostic-quality images. Whether you are a radiologist, gynecologist, emergency physician, or a trainee, spending time on image optimization and understanding the core physics principles enhances both patient safety and diagnostic confidence.

Further Learning

  • Workshops & Hands-on Training: Institutes like HaleQad Ultrasound Training Solutions offer structured courses.

  • Recommended Textbooks:

    • Ultrasound Physics and Instrumentation by Kremkau

    • Diagnostic Ultrasound by Carol M. Rumack

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