Beam Former, Pulser & Output Power
Key Takeaways
- The pulser generates the high-voltage electrical pulse that drives the transducer elements via the converse piezoelectric effect, setting the transmitted pulse's amplitude.
- Increasing output (acoustic) power raises both the intensity delivered to tissue and the risk of thermal and mechanical bioeffects.
- Transmit beam forming applies fixed firing delays across elements to focus and steer the outgoing beam; receive beam forming applies continuously updated delays to dynamically focus returning echoes.
- Apodization tapers the drive amplitude across the active aperture's elements to reduce side lobes and grating lobes.
- Each independently controlled element or element group connects to the beam former through its own channel, carrying its own delay and amplitude control.
The Pulser: Creating the Transmit Pulse
The pulser is the electronic component that generates the short, high-voltage electrical pulses sent to the transducer's active elements. Through the converse piezoelectric effect, each voltage pulse drives the PZT crystal to vibrate mechanically, producing the ultrasound pulse that leaves the transducer face and travels into the patient. The amplitude of the pulser's driving voltage sets the amplitude of the transmitted acoustic pulse — and therefore sets the output power (also called acoustic power) delivered into the patient.
⚠ Output Power Raises Intensity AND Bioeffect Risk
This is one of the most heavily tested relationships in this chapter. Output power is a transmit-side control: raising it increases the pulser's driving voltage, which increases the transmitted pulse's amplitude, which increases the intensity (power per unit area, W/cm²) delivered into tissue. Higher intensity means greater potential for tissue heating (thermal bioeffect) and for cavitation (mechanical bioeffect) — covered fully with the Thermal Index and Mechanical Index in Chapter 11.
Output power is the only major console control that changes the acoustic energy actually delivered to the patient. Every other brightness-related control downstream of the pulser — overall gain, TGC, dynamic range — only changes how the machine amplifies or displays echoes it has already received; none of them alter what was transmitted. This distinction is the foundation of ALARA practice: keep output power As Low As Reasonably Achievable, and use gain rather than power to brighten a dim image.
The Beam Former: Directing and Shaping the Beam
The beam former is the electronic circuitry that controls the precise timing, relative amplitude, and channel assignment of every pulse sent to — and every echo received from — the array elements. On modern digital scanners nearly all electronic focusing, steering, and aperture shaping happens inside the beam former, sitting between the pulser (transmit) and the receiver (receive).
Transmit Beam Forming vs. Receive Beam Forming
| Stage | What the beam former does | Purpose |
|---|---|---|
| Transmit beam forming | Applies precisely timed firing delays across the elements so each element's wavefront arrives in phase at a chosen focal point or steering angle | Electronic focusing and steering of the outgoing beam |
| Receive beam forming | Applies continuously changing (dynamic) delays to the returning echo signal from each element, re-timing them so echoes originating from the same depth are summed in phase | Dynamic receive focusing that tracks a moving focus as echoes arrive from increasing depth |
Because receive delays can be updated continuously as echoes return, receive focusing can track essentially the entire depth of the image in real time. Transmit focus, by contrast, is fixed the instant a pulse fires, which is why sonographers can select multiple discrete transmit focal zones (at the cost of frame rate) but cannot dynamically refocus the outgoing beam mid-pulse.
Apodization
Apodization is the deliberate shading, or weighting, of drive amplitude (and/or receive sensitivity) across the elements of the active aperture, so elements near the edge of the group contribute less than elements near the center. Rather than driving every active element with equal voltage, apodization tapers the amplitude toward the aperture's edges. This taper reduces the amplitude of side lobes and grating lobes — unwanted secondary beams that travel off the main beam axis and can generate artifactual echoes from reflectors outside the intended scan line. Apodization therefore improves image contrast resolution and reduces certain artifacts without changing the main beam's focusing.
Channels
Each independently controlled element (or small group of elements) within the active aperture connects to the beam former through its own channel — a dedicated electronic pathway carrying its own transmit delay, drive amplitude, and, on receive, amplification and digitization. The number of channels a scanner supports limits how many elements can be independently time-delayed and summed together. A system with more channels can generally provide:
- Finer, more precise control over focusing and steering
- Smoother apodization shading across the aperture
- Support for larger active apertures and for 2D matrix arrays, which require independent control of elements in two dimensions and are the hardware basis for real-time 3D/4D imaging (Chapter 8)
Putting It Together: The Front End of the Imaging Chain
The pulser and beam former together form the "front end" of the pulse-echo imaging chain, upstream of the receiver. In sequence: the beam former calculates the correct delay pattern for the chosen focus/steering angle → the pulser fires each channel's element at its assigned delayed time and apodization-shaded voltage → the resulting individual wavefronts combine in the patient to form one focused, steered ultrasound beam → returning echoes are picked up independently by each element/channel → the beam former's receive side re-delays and sums these channel signals into a single, dynamically focused radiofrequency (RF) line, which then passes to the receiver for amplification (Section 7.2).
Key exam distinction: Output power is a transmit control that changes what leaves the transducer, and therefore changes patient acoustic exposure. Beam-former functions — transmit/receive focusing, steering, apodization, and channel count — shape image quality and beam geometry, but by themselves they do not change the acoustic energy delivered to the patient the way output power does. Confusing "more channels/better focusing" with "more power/more bioeffect risk" is a classic SPI distractor pattern.
Which control, when increased, raises the amplitude of the transmitted pulse and therefore increases both echo intensity in tissue and the risk of bioeffects?
What is apodization?