SonographyAcademy

Curriculum · Ultrasound Physics & Instrumentation

Instrumentation & Image Formation

The pulse-echo signal path: the range equation (13 µs/cm), TGC vs gain vs output power, dynamic range/compression, pre- vs post-processing, the frame-rate tradeoff, harmonic imaging, and spatial compounding.

~35 min · level: intermediate · SPI draft — pending clinical review

Learning objectives

  • Apply the range equation and the 13-µs-per-cm rule.
  • Distinguish TGC, overall gain, and output power and their ALARA implications.
  • Explain dynamic range/compression and the pre- vs post-processing distinction.
  • Describe the depth–line-density–frame-rate tradeoff, tissue harmonic imaging, and spatial compounding.

The scanner converts echo arrival time → depth, assuming c=1540c = 1540 m/s:

depth=c t2⇒13 μs round-trip=1 cm\text{depth} = \frac{c\,t}{2} \quad\Rightarrow\quad \textbf{13 } \mu\text{s round-trip} = 1\text{ cm}
The ÷2 accounts for the round trip; speed errors (true c ≠ 1540) cause range artifacts.

Three controls are easily confused. Time-gain compensation (TGC) selectively amplifies deeper echoes to offset attenuation (uniform brightness with depth). Overall gain amplifies all received echoes uniformly. Output power increases the transmitted energy.

Dynamic range is the ratio of largest to smallest handled signal (20log⁡10(max⁡/min⁡)20\log_{10}(\max/\min), often 100–120 dB), and compression logarithmically squeezes it onto the display. A wide displayed dynamic range gives more grays → softer, lower-contrast image (subtle tissue, e.g., liver); a narrow range gives fewer grays → higher-contrast image (boundaries, stones).

Each scan line takes one round trip, so frame rate is constrained:

frame time≈(13 μs/cm)×depth×lines/frame\text{frame time} \approx (13\,\mu s/\text{cm}) \times \text{depth} \times \text{lines/frame}
Depth, line density, and frame rate cannot all be maximized.

Tissue harmonic imaging (THI) exploits nonlinear propagation: a high-amplitude pulse distorts as it travels (compressions outrun rarefactions), generating harmonics (2f02f_0, 3f03f_0…). The system transmits at f0f_0 and receives at 2f02f_0. Because harmonics build within tissue (negligible in the near field and in low-amplitude side lobes/clutter), THI reduces reverberation, clutter, and side-lobe artifact and improves lateral and contrast resolution — especially valuable in technically difficult patients. Spatial compounding averages frames from multiple steering angles, reducing speckle and angle-dependent dropout at the cost of frame rate.

Key takeaways

  • The scanner converts echo arrival time to depth as depth = c·t/2 assuming c = 1540 m/s, which gives the rule that 13 µs of round-trip time equals 1 cm of depth.
  • TGC selectively amplifies deeper echoes to offset attenuation, overall gain amplifies all received echoes uniformly, and output power increases transmitted energy; under ALARA raise gain before output power because gain adds no patient exposure (only noise) while output power raises exposure.
  • Dynamic range is 20·log10(max/min) (often 100–120 dB), and a wide displayed dynamic range gives more grays and a softer lower-contrast image (e.g., liver) while a narrow range gives fewer grays and higher contrast (e.g., boundaries, stones).
  • If a setting can be changed on a frozen image it is postprocessing (gray maps, B-color, read magnification); preprocessing applied before image-memory storage (TGC, compression, persistence, write magnification) cannot be undone without re-scanning.
  • Depth, line density, and frame rate cannot all be maximized: increasing depth lowers PRF and frame rate, and increasing line density, sector width, or focal zones lowers frame rate; THI transmits at f0 and receives at 2f0 to reduce reverberation/clutter/side lobes, while spatial compounding averages multiple steering angles to reduce speckle at the cost of frame rate.

Check your understanding

Registry-style items with worked rationales.

1An echo returns 65 µs after the pulse is transmitted. At what depth is the reflector placed?application

2To reduce patient exposure while keeping image brightness, the sonographer should first:application

3A control that can be adjusted on a frozen image is an example of:recall

4Increasing imaging depth will, all else equal:analysis

Go deeper — trusted free resources

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

StatPearls
Ultrasound Physics and Instrumentation

Comprehensive open-access chapter covering transducers, gain, time gain compensation, dynamic range, B-mode image formation, and processing controls.

Open →
StatPearls
Sonography Physical Principles and Instrumentation

Foundational chapter on how the ultrasound machine forms images, including gain, compression, dynamic range, harmonic imaging, and spatial compounding.

Open →
Radiopaedia
Time Gain Compensation

Concise reference explaining how TGC corrects for depth-dependent attenuation so equally echogenic tissues appear uniform across the image.

Open →
Radiopaedia
Tissue Harmonic Imaging

Explains how nonlinear wave propagation generates harmonics that improve resolution and signal-to-noise while reducing reverberation and side-lobe artifacts.

Open →

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.
  4. AIUM Practice Parameters and bioeffects statements.