Biological Effects of Diagnostic Ultrasound

Introduction

Two main effects:
- Thermal (heating) effect
- Mechanical effect (tissue distortion)

Caused by energy transfer from the wave being absorbed, resulting in local heating.
Beam Characteristics Affecting Thermal Effects:
- Frequency: Higher frequency leads to increased attenuation and absorption, resulting in less penetration and more local heating.
- Intensity: Greater intensity results in a greater potential for heating.
- Mode: Continuous or pulsed, real-time scanning, or Doppler mode affects heating potential.
  - M-mode and Doppler use repeated pulses along a single beam path, increasing heating potential.
  - Pulsed wave Doppler can use up to 28 pulses along a single path and longer pulse lengths, posing a high heating risk, especially in fetal and obstetric applications.
- Time: Longer exposure increases heating potential.
Tissue Characteristics Affecting Thermal Effects:
- Bone interfaces absorb a higher percentage of the beam, increasing heating potential.
- Highly vascularized tissue minimizes thermal effects via heat transfer by moving blood cells.
Under normal conditions, temperature rises in tissue may be as little as 0.1 degrees Celsius, which is well below hazardous levels.
Normal daily body temperature changes can exceed 1 degree Celsius.
No harmful effects have been reported with current B-mode equipment.
Doppler modes have higher average intensities and longer exposure times, requiring caution.

Used to determine the potential for a 1 degree Celsius temperature increase.
Displayed on all machines.
It is the ratio between the ultrasound unit's power and the power required to raise the temperature by 1 degree Celsius.
Calculated under three specific conditions:
- TIS (Thermal Index Soft Tissue): Used when soft tissue is insonated, particularly in embryo/fetus examinations (up to 8 weeks post conception) where there is no ossified bone.
- TIB (Thermal Index Bone): Recommended for all other applications to avoid constantly switching between TIS and TIB and provides a safety factor because TIB values are greater than or equal to TIS values.
- TIC (Thermal Index Cranial): Used when scanning through surface bone, such as in neonatal transcranial applications.
A thermal index of 1 suggests that the machine settings have the potential to raise tissue temperature by 1 degree Celsius.
Vascularity and time also play a significant role.

British Medical Ultrasound Society recommends a thermal index value of less than 0.7 is optimal for obstetric scanning.
Time limits are suggested for values over 0.7, such as a 1 hour limit for values between 0.7 and 1.

Ultrasound physically distorts tissue to a small degree as it is transmitted.
Cavitation: Small gas bubbles come out of solution.
Microbubbles: Dissolved gases can come out of solution, producing microbubbles proportional to the beam's wavelength.
Resonance: Microbubbles expand and contract rapidly, potentially causing shear stress to cell membranes and large molecules.
High Intensities: Microbubbles may grow rapidly and collapse, producing shock waves that can disrupt cell membranes.
High intensity is needed for cavitation; however, it is possible to produce cavitation with the available diagnostic ultrasound equipment.

A measure of the potential for cavitation to occur.
Formula: MI = \frac{\text{peak rarefactional pressure}}{\sqrt{\text{transducer frequency}}}
Higher acoustic power at a given frequency increases peak pressure and the MI number, therefore increasing cavitation potential.
A value below 0.7 is considered safe.
FDA limits the MI of diagnostic imaging to 1.9.
Animal studies using high-powered ultrasound (high MI levels) have shown bioeffects like fetal growth retardation and fetal death; however, none of these biological effects have been demonstrated with current diagnostic pulsed ultrasound machines with MI values up to 1.9.
No reproducible studies have shown harmful effects in humans using diagnostic ultrasound.

American Institute of Ultrasound Medicine and the World Federation of Ultrasound Medicine and Biology affirm the clinical safety of diagnostic ultrasound when used prudently.
Biological effects, like localized pulmonary capillary bleeding in mammalian studies, have been noted at diagnostically relevant exposures, but their clinical significance is not yet known.
Diagnostic ultrasound should only be performed by trained, competent personnel, and devices must be appropriately maintained.

Higher intensity and longer irradiation time increase the likelihood of effects.
At very low intensities, time of irradiation does not increase the risk.
Diagnostic ultrasounds use low-intensity values, well below the level where effects may occur.

Diagnostic ultrasound equipment used for B-mode imaging operates at outputs that do not cause harmful temperature rises and is not contraindicated on thermal grounds.
Some Doppler diagnostic equipment has the potential to produce biologically significant temperature rises at bone-soft tissue interfaces in unperfused tissue.
The effect of elevated temperatures may be minimized by keeping the beam's time passing through any point in tissue as short as possible.
Use the lowest available power setting to obtain the diagnostic information desired.

Medical ultrasound imaging should only be used for medical diagnosis.
Equipment should only be used by trained personnel aware of thermal and mechanical bioeffects.
Examination time and output levels should be minimized while achieving diagnostic results.
The active transducer should not rest on the skin when not in use.