9.1 Receiver, Signal Processing & Analog-Digital Conversion

The Receiver's Job

After the transducer converts returning echoes into tiny electrical voltages, those signals pass to the receiver, the portion of the ultrasound system that prepares them for storage and display. Receiver processing is an example of amplitude processing: the information carried in the strength of each echo is extracted and reshaped, while the timing of each echo (which encodes reflector depth via the range equation, using the assumed soft-tissue propagation speed of 1540 m/s) has already been established. The five classic receiver functions, performed in a fixed sequence, are:

1. Amplification — uniform increase of all signal amplitudes, controlled by the operator with the gain (overall gain) control. Unlike the transmit power control, which raises acoustic output and exposes the patient to more energy, gain only amplifies signals after they return, so it carries no bioeffect cost.
2. Compensation — depth-dependent amplification that corrects for attenuation. Because soft tissue attenuates at roughly 0.5 dB per cm per MHz (one-way), an echo from 8 cm deep at 5 MHz has lost about 20 dB one way — 40 dB round trip — compared with a superficial echo. Compensation equalizes the display brightness of identical reflectors at different depths. The operator implements it with the time gain compensation (TGC) sliders, also called depth gain compensation (DGC); each slider adjusts gain for a particular depth band along the 13 µs/cm round-trip clock.
3. Compression — reduction of the range of signal amplitudes (dynamic range) so the display can present them, discussed in detail below. The operator controls this with the dynamic range control.
4. Demodulation — conversion of the high-frequency radiofrequency (RF) signal into a lower-frequency video signal. Demodulation removes the rapidly oscillating carrier and retains the signal's envelope (its outline of peaks), which is what actually encodes reflector strength and is what gets displayed.
5. Rejection — also called threshold or suppression. It eliminates low-level electronic noise and weak artifactual echoes by refusing to display signals below a set amplitude. Rejection cleans up the image but discards genuine weak echoes, so it cannot be undone later — those low-amplitude signals never reach memory.

A useful exam distinction: all five functions act on the signal before it is written to memory, so they are preprocessing. Once an image is frozen, the operator cannot recover rejected echoes or re-spread compressed amplitudes — only the stored numbers remain for postprocessing.

Dynamic Range and Compression

Dynamic range is the ratio of the largest to the smallest signal a component can handle, expressed in decibels (dB). The numbers are dramatic: the transducer can produce echo voltages spanning roughly 100 to 120 dB — a ratio of a million to one in amplitude. No monitor can display that; a typical display presents only about 20 to 30 dB as visibly distinct gray shades. Compression therefore squeezes the transducer's enormous dynamic range down to the roughly 40 to 60 dB handled by the scan converter and ultimately the display.

Because the decibel scale is logarithmic, equal dB steps represent equal ratios, not equal differences: every 6 dB in voltage (amplitude) terms represents a halving or doubling. A higher dynamic range setting on the scanner shows more shades of gray and a softer, lower-contrast image that reveals subtle tissue texture — preferred for organs like the liver. A lower setting produces a bistable, high-contrast image with mostly black and white, useful for echo-free fluid and sharp boundaries.

The Scan Converter: ADC and DAC

The scan converter is the system's memory and translator. Two converters live here, and the ARRT outline expects you to know both:

• The analog-to-digital converter (ADC) samples the processed analog voltage and assigns it a binary number for storage. The number of bits per pixel sets the available gray scale: 4 bits yields 2^4 = 16 shades, 6 bits yields 64, and 8 bits yields 256 shades, the standard for modern gray-scale imaging. The ADC must also sample fast enough to capture the signal faithfully; per the Nyquist criterion, the sampling rate must be at least twice the highest frequency present in the signal.
• The digital-to-analog converter (DAC) performs the reverse operation when the image is sent to an analog display device, converting the stored numbers back into a voltage. In modern fully digital flat-panel chains the image may remain digital to the screen, but the DAC remains the classic exam answer for 'what converts numbers back to an analog signal for display.'

Brightness and Contrast

Once echoes exist as numbers in memory, each number must be mapped to a shade of gray on the screen. Two display-level adjustments shape that mapping:

• Brightness shifts every stored number by the same constant — like adding +30 to all values — making the whole image uniformly lighter or darker without changing the relationships between pixels.
• Contrast changes the spread between stored numbers, steepening or flattening the slope of the mapping curve. High contrast pushes dim echoes toward black and bright echoes toward white; low contrast compresses everything into a narrow band of mid-grays.

Both adjustments alter only the display of already-stored data, so they are postprocessing: reversible, repeatable, and available after the image is frozen. This is the conceptual bridge to Section 9.2 — anything that changes stored numbers themselves was locked in at the receiver; anything that merely reinterprets them belongs to postprocessing.