4.2 Beam Configuration: Near Field, Far Field & Focus

Once the pulse leaves the crystal, its shape in space determines how finely the system can separate structures lying side by side. Beam configuration — the near and far zones, the focal zone, and the beam profile — is classic ARRT physics material, and the near-zone length formula in particular rewards memorizing with real numbers.

Two Regions of the Beam

For an unfocused disc-shaped element, sound wavelets from every point of the face interfere with one another. From the face out to the point where the beam naturally converges lies the near zone, also called the Fresnel zone or near field. Beyond that point the beam spreads progressively wider in the far zone, the Fraunhofer zone or far field. The boundary sits at exactly one near zone length, where the unfocused beam has narrowed to about half the transducer diameter — the narrowest point an unfocused beam ever reaches.

Near Zone Length Formula

NZL = r² ÷ λ = D² ÷ (4λ) = D² × f ÷ (4c)

where r is the element radius, D the aperture diameter, λ the wavelength, f the frequency, and c the 1,540 m/s soft-tissue propagation speed. Worked example: a 5 MHz, 10 mm diameter element has a wavelength of 1,540 ÷ 5,000,000 = 0.308 mm, so NZL = 10² ÷ (4 × 0.308) ≈ 81 mm.

The dependencies are exact exam fodder: near zone length increases with higher frequency (shorter wavelength) and increases with the square of the aperture — doubling the diameter quadruples the near zone. A small-footprint, low-frequency probe has a short near zone, so its beam begins diverging close to the patient.

Beam Diameter, Divergence, and Lateral Resolution

In the far zone the beam diverges at an angle given by sin θ = 0.61λ ÷ r (about 1.22λ ÷ D). Larger apertures and higher frequencies produce less divergence, keeping the beam tighter at depth. Beam diameter matters because it is the lateral resolution: two reflectors lying side by side at the same depth are displayed as separate only if the beam is narrower than their separation. Lateral resolution therefore varies with depth — best (smallest numerical value) at the focus or end of the near zone, and steadily worse as the beam diverges in the far field. This is why a subtle 2 mm liver lesion can vanish simply because it sits in a wide part of the beam.

The Focal Zone and Focusing Methods

Focusing narrows the beam at a chosen focal depth, and the focal zone is the region around the focus where the beam stays acceptably narrow — the depth range of best lateral resolution. Three focusing methods are tested:

1. Curved (concave) crystal — an internal, fixed focus; the element's curvature makes the beam converge at a depth built into the probe.
2. Acoustic lens — also internal and fixed; a curved lens refracts sound the way an optical lens bends light. Lenses also provide the fixed elevational (slice-thickness) focus of array probes.
3. Electronic phasing — used by array transducers; the elements are pulsed with tiny time delays so their wavelets arrive simultaneously only at the desired depth. The focus is adjustable by the sonographer, and on receive the system applies dynamic focusing, continuously retuning delays as echoes arrive from increasing depths.

Two rules about electronic focusing appear repeatedly on exams. First, multiple transmit focal zones improve lateral resolution over a greater depth range, but each additional zone requires extra pulses along every scan line, so frame rate and temporal resolution fall. Second, focusing can only move the narrowest point closer than the natural end of the near zone — a beam cannot be focused deeper than its near zone length.

Beam Profile

Intensity is not uniform across the beam. A plot of intensity versus distance from the central axis — the beam profile — shows maximum intensity along the center (the main lobe) with weaker intensity toward the edges, plus small off-axis side lobes that can generate artifactual echoes. Along the axis, intensity also varies: it peaks near the end of the near zone for an unfocused beam, or at the focal depth for a focused beam, then falls off with depth. This nonuniformity is one reason gain and TGC settings must be judged against depth, not set once and forgotten.

Putting It Together: Frequency, Aperture, and Resolution

Higher frequency and larger aperture both lengthen the near zone, reduce far-field divergence, and narrow the focus — all of which improve lateral resolution. The penalty is that higher frequency attenuates faster in tissue, limiting penetration. Transducer design is therefore a compromise: a large-aperture, high-frequency probe gives superb lateral resolution superficially, while deep abdominal or cardiac work accepts a wider beam from a smaller, lower-frequency aperture in exchange for reaching the target at all. Note also that focusing improves lateral resolution only within the focal zone. It is not free beyond it: a focused beam converges to a narrow waist at the focus and then diverges more rapidly than an unfocused beam of the same aperture, so lateral resolution deep to the focal zone is actually worse. That is exactly why a focus parked too shallow degrades the far field. This is why the sonographer places the focal zone marker at or just below the structure of interest — positioning the focus correctly on the image is a direct, practical application of everything in this section.