7.2 Frame Rate, Line Density, Dynamic Range, Harmonics & Compounding

Frame Rate and Temporal Resolution

Frame rate — the number of complete images displayed per second, in hertz — determines temporal resolution: the ability to represent motion accurately in time. Frame rate is not set directly; it is a consequence of four operator controls, and the exam expects you to identify all four:

1. Depth — the go-return time is about 13 microseconds per centimeter, so deeper imaging lowers the maximum pulse repetition frequency (PRF) and therefore the frame rate. Halve the depth and you can roughly double the frame rate.
2. Line density — the number of scan lines per frame. More lines placed closer together improve lateral detail, but each line costs pulses, so increasing line density decreases frame rate.
3. Sector width — narrowing the sector (the angular or linear width of the image) means fewer lines are needed per frame, which increases frame rate.
4. Number of focal zones — each additional transmit focus repeats pulses along every line, roughly dividing the frame rate by the number of zones.

A worked example: at 10 cm depth the maximum PRF is about 7700 Hz. With 120 lines per frame and a single focal zone, the theoretical frame rate is 7700 / 120, or about 64 frames per second. Double the depth to 20 cm and it falls to about 32 fps; add a second focal zone and it drops again to about 16 fps. Fast-moving anatomy — the adult heart, an active fetus — demands high frame rates, so the sonographer narrows the sector, reduces depth, and uses a single focus. Static organs such as the liver and thyroid tolerate the slower frame rates that come with high line density and multiple focal zones.

Dynamic Range and Compression

Dynamic range is the ratio, in decibels, between the largest and smallest signals a system can process. Returning echoes span roughly 100 to 120 dB, but the scan converter handles only about 40 to 60 dB and a monitor renders only about 20 to 30 dB as visibly distinct gray shades, so the receiver applies compression to squeeze the echo range down to what can actually be stored and displayed. The operator's dynamic range control chooses how much of that compressed range is shown:

• Wide (high) dynamic range gives many shades of gray — a low-contrast, smooth, soft-looking image that preserves subtle parenchymal texture differences.
• Narrow (low) dynamic range gives few shades of gray — a high-contrast, nearly bistable (black-and-white) image that makes borders and caliper measurements crisp but discards subtle low-level information.

Compression is a receiver (pre-processing) function applied before the data reach the scan converter, so the dynamic range you scanned with is baked into the stored image. Unlike gray maps, B-color, or read magnification, it cannot be changed after freeze — you must re-scan. The ARRT outline mirrors this split, listing dynamic range and compression with the receiver and postprocessing separately.

Tissue Harmonic Imaging (THI)

Because sound travels slightly faster through compressed tissue than through rarefied tissue, the transmitted pulse distorts progressively as it propagates, generating harmonic frequencies — integer multiples of the fundamental frequency. Tissue harmonic imaging exploits this: the system transmits at the fundamental frequency (for example, 2 MHz) and listens only at twice that frequency (4 MHz, the second harmonic).

Two consequences matter clinically and on the exam:

• Harmonic signals are generated within the tissue and are nearly absent in the near field, where the beam has not yet distorted enough. THI therefore markedly reduces near-field artifacts — reverberation, clutter, side-lobe echoes, and the haze that fills superficial fluid structures.
• Harmonic beams have lower side lobes, so contrast resolution and border definition improve, especially in technically difficult patients such as those with large body habitus.

The trade-off is that harmonic echoes are weaker than fundamental echoes, so penetration decreases somewhat and THI is less helpful for very deep targets. On most systems THI is toggled with a single control and is enabled by default in abdominal presets; it is frequently combined with spatial compounding, since the two technologies attack different artifacts.

Spatial Compounding

Spatial compounding (compound imaging) steers the beam at multiple angles — commonly three to nine different steering directions — and averages the resulting frames in real time. Because speckle is angle-dependent while real reflectors are not, the averaging cancels much of the speckle pattern: tissue looks smoother and the margins of masses and organ capsules appear better defined. Compounding also reduces other angle-dependent artifacts and can partially fill in areas behind superficial shadowing.

The costs are equally testable. Because several frames are acquired to produce each displayed frame, frame rate and temporal resolution decrease. More subtly, since posterior acoustic shadowing is itself angle-dependent, compounding can erase shadowing — and the diagnostic information it carries. A classic exam scenario: when deciding whether a gallbladder stone casts a clean shadow, turn spatial compounding off, or a real shadow may be averaged away. The same caution applies to posterior enhancement behind cysts.