8.3 Attenuation, Absorption & Intensity

What Attenuation Is Made Of

Attenuation is the progressive decrease in the amplitude and intensity of a sound wave as it propagates through a medium. It is measured in decibels (dB), a logarithmic relative scale — attenuation values say nothing about absolute intensity, only how much has been lost relative to the starting value. Three mechanisms contribute, and the exam expects you to rank them in soft tissue:

1. Absorption — acoustic energy is converted into heat as tissue particles rub and relax against the oscillating pressure wave. This is the dominant component in soft tissue, responsible for the great majority of attenuation. It is also the physical basis of thermal bioeffects and of therapeutic ultrasound, where focused absorption deliberately heats tissue.
2. Reflection — energy bounced back at impedance boundaries is removed from the forward-traveling beam. Specular reflection at large boundaries contributes modestly to attenuation in most tissue paths but dominates at bone and gas.
3. Scattering — redirection of energy in many directions by small or rough structures; individually weak, it adds up over long paths.

Because absorbed energy becomes heat, attenuation and patient heating are two faces of the same process — the reason the Thermal Index on screen rises when output increases, especially in fluid-free paths.

The Half-Decibel Rule and Worked Examples

For soft tissue, the rule of thumb to memorize is:

Attenuation ≈ 0.5 dB per cm per MHz (one way)

The total decibel loss is the product of three factors: 0.5 × path length (cm) × frequency (MHz). For imaging you must count the round trip, because the echo is attenuated coming back as well. Worked examples:

• 3 MHz probe, reflector at 6 cm: one-way path 6 cm → 0.5 × 6 × 3 = 9 dB; round trip (12 cm of travel) = 18 dB
• 5 MHz probe, reflector at 8 cm: one way = 0.5 × 8 × 5 = 20 dB; round trip = 40 dB
• 6 MHz probe, reflector at 10 cm: one way = 0.5 × 10 × 6 = 30 dB; round trip = 60 dB

That last figure shows why time gain compensation (TGC) exists: echoes from 10 cm arrive about 60 dB (a million-fold in power) weaker than superficial echoes, and the receiver must apply depth-dependent amplification to equalize brightness. TGC corrects for attenuation; overall gain simply amplifies everything.

Note the units embedded in the rule: 0.5 dB/cm/MHz means attenuation rises linearly with frequency and linearly with distance. The 0.5 dB value applies to average soft tissue; media differ markedly — lung attenuates enormously (air scatters and absorbs), bone attenuates strongly (~10–20 dB/cm/MHz), while simple fluid (urine, amniotic fluid, simple cysts) attenuates almost nothing. Fluid's negligible attenuation produces the classic posterior acoustic enhancement (through-transmission) behind cysts — the TGC ramps up gain with depth as if tissue were present, but the beam emerged from the fluid nearly intact, so tissues behind the cyst are over-amplified and appear brighter.

Half-Value Layer

The half-value layer (HVL), also called the half-power distance or penetration depth, is the depth of tissue that reduces intensity to one-half of its original value — a 3 dB loss. From the attenuation rule, HVL = 3 dB ÷ (0.5 × frequency). In soft tissue:

The inverse relationship is the whole story: doubling frequency halves the half-value layer. A 10 MHz beam has already lost half its intensity after 6 mm of soft tissue, which is why high-frequency probes are reserved for thyroid, breast, scrotum, and vascular structures within a few centimeters of the skin.

Penetration Versus Resolution: The Central Trade-off

Attenuation physics forces the fundamental compromise of probe selection. Axial resolution improves with higher frequency (shorter wavelength means finer detail), but penetration worsens with higher frequency (0.5 dB/cm/MHz means faster energy loss). The practical rules:

• Deep or large patients (abdominal aorta at 12 cm, the retroperitoneum, obstetric imaging in a high-BMI patient) → choose a lower frequency (2–5 MHz) and accept coarser resolution
• Superficial structures (thyroid nodules, testes, carotid walls) → choose a higher frequency (7–15 MHz) and accept shallow reach
• Broadband transducers let you tune frequency within an exam; dropping the frequency is the first move when the far field of a deep structure drops out, before reaching for more output power

Intensity and Distance

Intensity is power per unit area, reported in W/cm² or mW/cm², and it declines with distance for two distinct reasons: attenuation (absorption, reflection, scattering, as above) and, in the far field, beam divergence spreading the same power over a larger area. Because intensity ∝ amplitude², a 3 dB loss halves intensity while reducing amplitude to only about 71% of its starting value — halving amplitude (to 50%) corresponds to a 6 dB loss and quarters the intensity. Keeping decibel-amplitude-intensity conversions straight is a reliable exam discriminator: −3 dB = intensity halved; −6 dB = amplitude halved. Intensity parameters such as SPTA (spatial peak–temporal average) intensity are the quantities the FDA limits and the AIUM's output display indices estimate, tying the physics of this section directly to the ALARA (As Low As Reasonably Achievable) principle that governs safe scanning.