Week 10 Ultrasound Bioeffects: Thermal Effects

Ultrasound Bioeffects: Thermal Effects

What are Ultrasound Bioeffects?

• Undesirable changes in cells and tissue due to the interaction of ultrasound mechanical energy with biological tissue.

• Divided into thermal and non-thermal effects.

• Important due to widespread use of ultrasound in pregnancy and potential harm to rapidly developing cells in embryos, fetuses, and neonates.

Parameters Describing Ultrasound Beam Strength

• Amplitude, power, and intensity are used to describe the strength of an ultrasound beam.

• Amplitude:

  • Measure of a wave’s magnitude of oscillation; the maximum disturbance in the medium.

  • Measured in units of pressure: MPa (megapascals).

• Power:

  • The rate at which energy is generated and transferred by the acoustic wave per unit of time.

  • Measured in watts (W) and milliwatts (mW).

  • Proportional to the square of the amplitude: If power doubles, amplitude quadruples.

• Intensity:

  • The rate at which energy passes through a unit area.

  • Important for discussing bioeffects and safety.

  • Average intensity equals the power of an ultrasound beam (mW) divided by the cross-sectional area of the beam (cm²).

  • Units: mW/cm².

  • If beam power increases, intensity increases, and vice versa.

Maximum Intensity of Ultrasound Beam

• Located at the focus (narrowest part of the beam).

• Intensity varies from point to point across the beam (spatial considerations).

  • Spatial Peak Intensity (ISP): Highest intensity, found at the center of the beam.

  • Spatial Average Intensity (ISA): Average intensity across the beam.

Ultrasound Beam Intensity in Pulsed Ultrasound

• Intensity varies over time (temporal considerations).

  • Temporal Peak Intensity (ITP): Highest intensity found in a pulse; not averaged over time.

  • Pulse Average Intensity (IPA): Intensity averaged over the pulse duration; lies between ITP and ITA.

  • Temporal Average Intensity (ITA): Takes into account the dead time between pulses and is averaged over the pulse repetition period (PRP); it is the lowest value.

Thermal Bioeffects

• Ultrasound is attenuated as it propagates through tissue via reflection, absorption, scatter, and refraction.

• Predominant cause of attenuation is absorption.

• Absorbed energy converts into heat, producing a temperature rise.

• The rate of energy absorption depends on:

  • Attenuation coefficient of the tissue.

  • Operating frequency.

  • Power/intensity of the ultrasound beam.

  • Length of operating time (exposure time).

Factors Affecting Thermal Bioeffects

• Thermal effect is highest in tissues with a high absorption coefficient (e.g., bone) and low in those with little absorption (e.g., amniotic fluid).

• The potential of temperature rise depends on the tissue's thermal characteristics to dissipate heat.

• Highly vascular organs dissipate heat more readily than bones.

• Higher operating frequencies lead to higher absorption and potential for heat due to increased frictional losses.

• Heat can also be produced at the transducer surface and transferred to the tissue, which is significant for transvaginal probes during the first eight weeks post-conception.

Tissue Effects in Thermal Bioeffect

• Excessive temperature increase can result in toxic effects.

• Biological effects depend on exposure duration, tissue type, cellular proliferation rate, and regeneration potential.

• Age and developmental stage are important for fetal and neonatal safety.

• Temperature increases of several degrees Celsius above the normal core range can occur naturally.

• The probability of adverse biological effects increases with the duration of the temperature rise.

• Severe effects have been observed during organogenesis in animals.

• Fetal temperature elevations of 4°C above normal for 5 minutes or more can induce severe developmental defects.

• Thermally induced congenital anomalies have been observed in various animal species.

• In current clinical practice, such exposures are unlikely with commercially available equipment.

• Transducer self-heating significantly contributes to temperature rise in tissues close to the transducer.

• Exposures during the first trimester should be restricted to the lowest outputs consistent with obtaining the necessary diagnostic information.

• Adult tissues are more tolerant of temperature increases than fetal and neonatal tissues.

Other factors affecting Thermal Bioeffects

• Intensity can be changed by power output, scan depth, and mode of operation (e.g., color and Doppler modes).

• Power and color Doppler involve beam scanning, so heating potential is between B-mode and spectral pulsed Doppler.

• Temperature elevations generally increase from B-mode to color-Doppler to spectral-Doppler applications.

• For identical exposure conditions, thermal bioeffects increase with dwell time during examination.

• Temperature rise near bone is significantly greater than in soft tissues and increases with ossification development throughout gestation.

• Acoustic beams insonating ossifying fetal bone deserve special attention due to their proximity to developing tissues like the brain or spinal cord.

Output Display Standards (ODS)

• World Federation for Ultrasound in Medicine and Biology (WFUMB) concluded that a temperature rise of no more than 1.5°C may be used clinically without reservation.

• Exposures elevating embryonic and fetal temperatures higher than 4°C for 5 minutes or more should be considered potentially hazardous.

• Ultrasound system acoustic power output limits are set by the US Food and Drug Administration (FDA).

• In 1991, the FDA decided that ultrasound equipment should provide an on-screen display relating to acoustic output to inform users of potential risks for biological effects.

• This requires users to understand the potential for biological effects and minimize exposure times.

Output Display Standards (ODS) cont.

• In 1992, the American Institute of Ultrasound in Medicine (AIUM) and the National Electrical Manufacturers Association (NEMA) defined a set of output display standards (ODS).

• For each ultrasound examination, a real-time on-screen display indicates the risk of producing biological effects.

• ODS for thermal effects is shown by Thermal Index (TI).

Thermal Index (TI)

• Defined by AIUM/NEMA as the ratio of the acoustic power produced by the transducer (W) to the power required to raise the temperature in tissue by 1°C (W/°C).

• TI = \frac{Acoustic\,Power\,(W)}{Power\,to\,raise\,temp\,by\,1°C\,(W/°C)}

• Ultrasound systems are programmed with an algorithm that calculates the TI.

• A thermal index of 1 indicates the acoustic power achieving a temperature increase of 1°C.

• A thermal index of 2 has doubled power but would not necessarily indicate a peak temperature rise of 2°C.

Different Thermal Indices

• TIS (Thermal Index Soft tissue): Applied when scanning through a soft-tissue structure.

• TIB (Thermal Index Bone): Applies if bone is at or near the focus of the transducer: increased potential for heating.

  • Most appropriate for fetal ultrasound during the second or third trimester pregnancy scan where the highest temperature increase would be expected occurring at the soft tissue/bone interface.

• TIC (Thermal Index Cranial bone): Applied when scanning cranial bone: bone is at or very near the skin surface.

TI Recommendations

• Adult:

  • TI < 2: No adverse effects seen in adults up to 50 hours.

  • TI > 2: Risk rises with the rise in temperature and length of exposure time.

• Prenatal:

  • First trimester: TI < 0.5.

  • TI > 0.5 to 1: Scanning times limited to 30 minutes.

  • TI > 2.5: Scan time less than 1 minute postnatal.

  • TI < 2.0: Scan time can be extended.

  • TI 2 to 6: Scan time less than 30 minutes.

  • TI of > 4 for 5 minutes: Has the potential to induce severe developmental defects in an embryo.

  • TI > 6: Scan time less than 1 minute.

The ALARA Principle

• ALARA = As Low As Reasonably Achievable.

• Ultrasound exposures during examinations should be as low as reasonably achievable.

• During a diagnostic ultrasound examination, the output power and exposure time should be kept to a minimum.

• The examination should not be prolonged unnecessarily.

• Sonographers should regularly check the TIS, TIB, and/or TIC values and adjust controls while scanning.

• Sonographers should be aware that changes in scanning modes (grayscale to color and Doppler) can alter the output power and hence the TIs.

• Always use the appropriate pre-set and transducer frequency for the area being examined.

• Scan should be performed by a trained operator and for a clinical indication.

• Entertainment scans should be avoided, especially in the first trimester.

In Summary

• Ultrasound bioeffects are undesirable changes in cells and tissue due to ultrasound mechanical energy interaction.

• Amplitude, power, and intensity are parameters describing ultrasound beam strength.

• The main cause of ultrasound attenuation is absorption.

• Absorbed energy converts into heat, producing a temperature rise.

• Fetal temperature elevations of 4°C above normal for 5 minutes or more can induce severe developmental defects.

• Thermal Index (TI) is an output display standard (ODS) defined as the ratio of acoustic power produced by the transducer (W) to the power required to raise the temperature in tissue by 1°C (W/°C).

• Three classifications of thermal indices: TIS, TIB, and TIC.

• The ALARA principle should always be exercised during ultrasound examinations, especially in obstetric scanning.