Display Modes – A-Mode, B-Mode, M-Mode (CVT 1610C)

Display Modes Overview

• Ultrasound imaging systems commonly offer three foundational display modes:
  • A-Mode – Amplitude Mode
  • B-Mode – Brightness (2-D) Mode
  • M-Mode – Motion Mode
• Each mode plots returning echo data on specific axes, extracting different clinical information (depth, amplitude, motion) from the same acoustic signals.

A-Mode (Amplitude Mode)

• Principle: Plots reflector strength as vertical spikes while the sampling dot travels horizontally across the screen.
• Axes
  • $X$-axis = Depth (calculated from time-of-flight)
  • $Y$-axis = Amplitude / Reflector strength
  • $Z$-axis = None (image is a 2-D graph only)
• Signal pathway
  • Transducer emits a single sound pulse.
  • A dot moves horizontally at a constant speed.
  • Upon echo reception, the dot deflects vertically; the height of the deflection is proportional to echo amplitude.

Key Characteristics of A-Mode

• Depth is derived from the echo’s round-trip time ("time-of-flight" concept).
• Highly accurate for pinpointing exact reflector depth.
• Strong echoes → tall spikes (bright reflectors, e.g., bone)
• Weak echoes → short spikes (low-impedance interfaces, e.g., soft tissue)
• Historically used in ophthalmology and non-destructive testing; less common in general imaging today.

B-Mode (Brightness Mode)

• Principle: Converts echo amplitude into grayscale brightness of dots, building a two-dimensional image.
• Appearance:
  • Stronger echoes → white dots
  • Weaker echoes → lighter gray dots
• Continuously updated to create real-time 2-D images ("2-D Mode").

Brightness Mode Signal Process

• A pulse is emitted while the display’s cursor (invisible) sweeps across the screen.
• On echo return, the system brightens the cursor’s position, converting it from invisible to visible.
• Repetition of many lines forms a full frame.

Axes in B-Mode

• $X$-axis = Depth (again via time-of-flight; governed by the 13 µs/cm rule in soft tissue)
• $Z$-axis = Amplitude (encoded as pixel brightness)
• $Y$-axis = Not explicitly displayed; instead, successive beam lines stacked together create the 2-D image.

Clinical Mentions

• Example organs captured in lecture slides: kidney, liver.

M-Mode (Motion Mode)

• Principle: Tracks motion of structures (e.g., cardiac valves) along a single ultrasound line over time.
• Setup:
  • System fires pulses along one fixed line.
  • Returned B-mode line is recorded on photosensitive paper or digital display that scrolls at a constant speed.

Axes in M-Mode

• $X$-axis = Time (paper / screen moves horizontally)
• $Y$-axis = Depth (from time-of-flight)
• $Z$-axis = None (brightness not separately encoded; motion creates trace)

Performance Note

• Requires very high sampling rate, equal to the system’s Pulse Repetition Frequency (PRF), to capture rapid tissue movement accurately.

Display Mode Axes Comparison

Additional References

• Edelman, S. Understanding Ultrasound Physics, 3rd ed., 2007 (primary textbook cited in the lecture).