7.1 Power, Gain, TGC, Depth & Focal Zone

Nearly a third of the scored items on the ARRT Sonography exam come from Image Production, and within that domain the Image Formation outline (2.A) lists the exact controls covered in this chapter. The exam rarely asks what a control does in isolation; it asks what happens to the image, the frame rate, or patient exposure when you change it.

Output Power: The ALARA-Last Control

Output power (also called acoustic power or transmit power) controls the amplitude of the pulses the transducer sends into the patient. Raising output power increases the intensity of the transmitted sound, which makes returning echoes stronger and therefore the image brighter. It is the only brightness control that also increases patient exposure, so it is governed by ALARA — As Low As Reasonably Achievable. The monitor displays two safety indices tied directly to output power: the mechanical index (MI), which estimates the likelihood of cavitation-related bioeffects, and the thermal index (TI), which estimates potential tissue heating. Increasing output power raises both indices.

The correct optimization sequence is an exam favorite: if the image is too dark overall, first increase receiver gain; only increase output power when gain and TGC adjustments are insufficient. Output power is usually adjusted in decibels — remember that a change of +3 dB doubles the intensity, and −3 dB halves it.

Receiver Gain: Amplification Without Exposure

Gain is applied in the receiver, after the echoes return. It electronically amplifies the electrical signals so weak echoes become visible. Because gain acts on returning signals, it changes brightness with zero change in patient exposure — this is why ALARA logic prefers it. Overall gain amplifies echoes from all depths equally. Over-gaining makes the image uniformly too bright, fills in anechoic structures (a simple cyst starts to show low-level internal echoes), and blooms strong reflectors; under-gaining drops out real low-level echoes such as weak scattering from soft-tissue parenchyma.

Time-Gain Compensation (TGC/DGC)

Sound attenuates as it travels — approximately 0.5 dB per centimeter per megahertz, one way. A 5 MHz beam loses about 2.5 dB/cm going in, and the echo loses another 2.5 dB/cm coming out: roughly 5 dB of round-trip loss per centimeter of depth. Without compensation, deeper structures would always look darker than identical shallow structures. Time-gain compensation (TGC), also called depth-gain compensation (DGC), selectively amplifies echoes that arrive later (from greater depths) more than echoes that arrive early.

On the console, each TGC slider corresponds to a depth zone. Sliding a control to the right increases gain for that depth; sliding left decreases it. Typical consoles provide a near gain control (near-field suppression), a bank of depth sliders that together form the compensation slope, and sometimes a separate far gain. The goal is a uniform brightness profile for uniform tissue: the liver at 4 cm should look the same shade of gray as the liver at 12 cm.

Common misuse traps the exam tests:

• Using overall gain instead of TGC when only one part of the image is wrong — overall gain lifts everything, so the near field becomes too bright while the far field is corrected.
• A visible horizontal band of brightness or darkness across the image means one slider sits far out of line with its neighbors.
• Over-compensating the far field so deep cysts fill in and look solid — always recheck fluid-filled areas after a TGC change.

Depth Setting

Depth determines how much of the patient is displayed and is one of the most consequential frame-rate controls. The system must wait for the round trip of each pulse before firing the next: sound takes about 13 microseconds per centimeter of depth (the go-return time at the assumed soft-tissue speed of 1540 m/s). Doubling the displayed depth halves the maximum pulse repetition frequency (PRF), which directly lowers the frame rate. For example, at 10 cm depth the maximum PRF is about 1540 m/s divided by (2 x 0.10 m), or roughly 7700 pulses per second; at 20 cm it falls to about 3850. Best practice: set depth so the region of interest fills the display with only a small margin below it. Excessive depth wastes frame rate and shrinks anatomy on screen, while insufficient depth clips deeper structures and can hide posterior features such as acoustic shadowing or enhancement.

Focal Zone Placement

The transmit focal zone is the depth at which the beam is electronically narrowed to its smallest width. Lateral resolution is best at the focus, because lateral resolution depends on beam width. The placement rule: put the focal zone at or just below the region of interest, keeping the entire target within the useful focal region. A single focal zone is standard for moving structures. Adding multiple focal zones narrows the beam over a greater depth range, improving lateral resolution through the field, but each additional zone requires extra transmit pulses along every scan line, so the frame rate drops roughly in proportion to the number of zones. Multiple focal zones suit slow-moving or static anatomy (thyroid, liver) and are inappropriate for the beating heart or an active fetus.