9.3 Bioeffects & Output Display Measures

Thermal Bioeffects

Ultrasound loses energy to tissue primarily by absorption, and absorbed acoustic energy becomes heat. The temperature rise at any point depends on the local intensity, the tissue's absorption coefficient (which rises with frequency), the beam area and focusing, perfusion (blood flow carries heat away), and dwell time — how long the beam rests on one spot. Absorption in soft tissue is moderate, but bone absorbs far more strongly: its absorption coefficient is many times higher, and it converts a much larger fraction of incident sound into heat. Heating is therefore greatest in bone and in soft tissue immediately adjacent to bone — a particular concern once the fetal skeleton begins ossifying in the second trimester, and in neonatal cranial imaging. In gas-free soft tissue, diagnostic-level exposures produce rises generally well under 1°C; documented biologically significant heating begins with sustained elevations of several degrees.

The Thermal Index

The thermal index (TI) is the on-screen estimate of heating risk. Formally, it is the ratio of the acoustic power the system is actually emitting to the power that would be required to raise tissue temperature by 1°C under a conservative worst-case model. Reading a TI of 2.0 means the current settings could, under worst-case assumptions, produce up to about a 2°C rise — it is an estimate, not a thermometer. Three variants match the tissue model to the anatomy:

• TIS — thermal index, soft tissue: assumes only soft tissue in the beam. Appropriate for first-trimester obstetric scanning before significant ossification, and for abdominal and gynecologic imaging.
• TIB — thermal index, bone: assumes bone lies at or near the beam focus, where heating is worst. Appropriate for second- and third-trimester fetal imaging, where the ossified skeleton sits deep in the field.
• TIC — thermal index, cranial: assumes bone lies at or near the tissue surface immediately under the transducer. Appropriate for adult transcranial studies and neonatal head imaging, where the skull is superficial.

Most professional guidance suggests keeping TI as low as practicable, exercising extra caution when it exceeds about 1.0, and limiting exposure time at higher values.

Mechanical Bioeffects: Cavitation

The second mechanism is non-thermal. Cavitation is the interaction of the sound field with small gas bodies (microbubbles) in tissue or fluid, and it comes in two forms:

• Stable cavitation: the bubble oscillates in size rhythmically with the pressure cycles but persists without collapsing. The oscillation drives local fluid motion called microstreaming, which can shear nearby cell membranes. Stable cavitation is the mechanism deliberately exploited by ultrasound contrast agents, whose injected microbubbles resonate strongly at imaging frequencies.
• Inertial cavitation (historically called transient or collapse cavitation): the bubble grows during the low-pressure (rarefaction) half-cycles and then collapses violently during compression. Collapse generates localized temperatures of thousands of degrees, enormous pressures, shock waves, and chemically reactive free radicals — the potentially damaging event.

The mechanical index (MI) estimates the likelihood of inertial cavitation: MI equals the peak rarefactional pressure (in MPa, derated for tissue attenuation) divided by the square root of the center frequency (in MHz). A higher peak negative pressure and a lower frequency both raise MI — lower-frequency pulses give bubbles more time to grow during each rarefaction. MI is displayed on screen alongside TI; regulators cap MI at 1.9 for most applications, with much lower limits (0.23) for ophthalmic use.

Output Display Measures and SPTA

Federal regulation historically limited scanner output by intensity, and the variant that matters for bioeffects is the spatial peak, temporal average intensity (SPTA). The name describes the measurement exactly: find the point in the beam where intensity is highest (spatial peak), then average the intensity there over the full time the beam dwells, including the dead time between pulses (temporal average). Because SPTA captures both where heating concentrates and how continuously energy is delivered, it is the intensity most relevant to thermal bioeffects — the classic exam answer. Current FDA (Track 3) limits cap derated SPTA at 720 mW/cm² for most applications and 50 mW/cm² for ophthalmic imaging, alongside the MI ceilings of 1.9 and 0.23 noted above. The older 17 mW/cm² ophthalmic figure belongs to the superseded Track 1 regime and is a common stale-source trap.

Since 1992, systems approved under the output display standard (ODS) instead show MI and TI on screen in real time, shifting responsibility to the operator: the machine reports estimated risk, and the sonographer must watch the indices and act accordingly. This is why the ARRT expects fluency with the meaning of each displayed number rather than memorized intensity limits alone.

ALARA

ALARA — as low as reasonably achievable — is the governing safety philosophy of diagnostic ultrasound. Because no exposure can be proven perfectly risk-free, prudent practice keeps exposure at the minimum that still yields a diagnostic image. In concrete terms:

• Use the lowest output (transmit) power that answers the clinical question; brighten the image with receiver gain instead, which costs the patient nothing.
• Minimize dwell time: keep the probe moving and do not linger on one structure, especially at high TI/MI settings.
• Prefer the highest frequency that penetrates adequately when cavitation is a concern, and limit Doppler use — spectral and color Doppler use long dwell times and high pulse repetition, producing the highest TI values of any mode. This is why first-trimester Doppler of the fetus is discouraged unless clinically indicated.
• Take special care with obstetric (especially first-trimester embryonic) and neonatal imaging, where tissues are most sensitive.

AIUM Position and Epidemiologic Evidence

The American Institute of Ultrasound in Medicine (AIUM) has reviewed decades of laboratory and epidemiologic data. Its official statements conclude that no independently confirmed harmful bioeffects have been demonstrated in mammalian tissue at diagnostically relevant exposures that do not produce significant temperature elevation, and epidemiologic studies of humans exposed in utero have revealed no confirmed causal relationship between diagnostic ultrasound and adverse outcomes. The AIUM simultaneously emphasizes that research continues and prudent use under ALARA — including avoiding non-medical 'keepsake' imaging — remains the professional standard.