Infographics / Ultrasound Physics

Real-time B-mode frame rate, line density and depth

A timing strip shows one frame built from N scan lines, each lasting a pulse-echo round-trip time of 2d over c to depth d, so frame time is N times 2d over c and frame rate equals PRF divided by N, which equals c over (2 d N). A curve below plots maximum frame rate against imaging depth for a fixed number of lines: it falls as 1 over depth. A 100-line curve stays above the flicker band across the shown depths, while a 200-line curve drops into the flicker band (below about 20 frames per second) at greater depth. Deeper field, more lines or higher line density, wider field of view, and more focal zones all lower the frame rate.

Real-time ultrasound frame rate: set by round-trip time per scan line and lines per frame A timing strip shows one frame built from N scan lines, each lasting a pulse-echo round-trip time of 2d over c to depth d, so frame time is N times 2d over c and frame rate equals PRF divided by N, which equals c over (2 d N). A curve below plots maximum frame rate against imaging depth for a fixed number of lines: it falls as 1 over depth. A 100-line curve stays above the flicker band across the shown depths, while a 200-line curve drops into the flicker band (below about 20 frames per second) at greater depth. Deeper field, more lines or higher line density, wider field of view, and more focal zones all lower the frame rate. Frame rate is set by round-trip time per scan line multiplied by the number of lines Round-trip time per line = 2d / c (c fixed, 1540 m/s in tissue) one frame = N scan lines (frame time = N × 2d/c) 2d/c 2d/c 2d/c 2d/c 2d/c … × N each line = one pulse–echo round trip to depth d, lasting 2d/c (must wait for the echo) frame rate ≈ PRF / N = c / (2 × d × N) Deeper field or more lines → lower maximum frame rate 120 100 80 60 40 20 0 max frame rate (fps) 0 10 20 30 imaging depth (cm) flicker / jerky motion (< ~20 fps) 100 lines 200 lines 15 cm, 100 lines ≈ 51 fps What lowers frame rate ▾ greater imaging depth (longer 2d/c per line) ▾ more lines / higher line density ▾ wider field of view (needs more lines) ▾ more focal zones (extra pulses per line) PRF is capped by depth (PRFmax = c/2d); aim for ≥20–25 fps to keep motion smooth. Each of depth, line density and focal zones trades against frame rate — the depth–resolution–frame-rate compromise.
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