Tiny piece of piezoelectric material on a needle connected to an oscilloscope that measures the acoustic pressure at specific locations within a sound beam

Also measures period, PRP, PRF, and PD

Must be calibrated prior to use

May also be constructed from a very thin membrane of piezoelectric plastic with a very small pressure sensitive area in the center of the membrane

A very small, but measurable, force on a target that a sound beam strikes

When the target is a balance or a flat, which acts as an extremely sensitive miniature postal scale, the measured force relates to the power in the beam

1. Calorimeter
2. Thermocouple
3. Liquid crystal

A thermos bottle filled with absorbing material that measures the total power in a sound beam via absorption

The sound beam's total power is calculated by measuring the temperature rise and the time of heating

A tiny electronic thermometer with a dab of absorbing material that is placed in a sound beam to measure temperature

Temperature rise is related to the power of the sound beam at the particular location where the device is positioned

Material that changes color based on temperature 

Sound beam strikes the material, energy is absorbed, and the change in temperature causes a change in the material's color, providing insight into the shape and strength of the sound beam

The benefits to the patient must outweigh the risks of the exam

Under controlled circumstances, bioeffects are beneficial

• In vitro bioeffects research is important
• In vitro bioeffects are real even though they may not apply to the clinical setting
• In vitro bioeffect research that claims direct clinical significance (without in vivo validation) should be viewed with caution

1. Mechanical approach
2. Empirical approach

Searches for a relationship between cause and effect

Step 1: Propose that a specific mechanism has the potential to produce bioeffects

Step 2: Perform a theoretical analysis to estimate the scope of the bioeffects at various exposure levels

Strengths: broad exposure range can be evaluated

Weaknesses: uncertainty about assumptions; are other mechanisms involved?; is the bioeffect clinically significant?

Searches for a relationship between exposure and response

Based on the acquisition and review of information from patients or animals exposed to ultrasound

Strengths: Biological significance is obvious; no need to understand mechanism

Weaknesses: Species differences may alter results; no need to understand mechanism

1. Thermal
2. Cavitation (nonthermal)

Bioeffects result from tissue temperature elevation

Rationale: As sound propagates in the body, energy is converted into heat; core temperature is regulated at 37°C, beyond which life processes may not function

A predictor of max temperature increase under most clinically relevant conditions in vivo

Reported in soft tissue (TIS), bone (TIB), and cranial bone (TIC)

Serious tissue damage occurs from prolonged and excessive elevation of tissue temperature

Tissue heating is related to the output characteristics of the transducer and the properties of the tissue

A combination of temperature and exposure time determine the likelihood of harmful bioeffects

Maximal heating is related tot he beam's SPTA intensity - current regulatory limit is 720 mW/cm², SPTA

No confirmed bioeffects have been reported for temperature elevations of up to 2°C above normal for exposures of less than 50 hours

Fetal and neonatal tissues appear less tolerant of tissue heating than adult tissues, though none at less than 39°C

Bone absorbs more acoustic energy than soft tissue; therefore, temperature rise in soft tissues near bone is significantly higher than in other locations

Theoretical models appear to correlate with experimental data even though: 

• the ultrasound beam is quite complex
• diagnoistic equipment is diverse
• tissue characteristics are different 

Exerted by a sound beam on tissues

Sheer stresses and streaming of fluids can distort or disturb biologic structures

The interaction of sound waves with microscopic, stabilized gas bubbles (gaseous nuclei) in the tissue

ALSO describes the creation of gaseous nuclei from dissolved gases in a fluid normally found in tissues

Stable vs Transient

Gaseous nuclei oscillate at lower mechanical index (MI) levels, but do not burst

Bubbles intercept and absorb much of the acoustic energy of a sound beam

Fluids surrounding the cells undergo microstreaming and the cells are exposed to shear stresses

Bubble-bursting that occurs at higher mechanical index (MI) levels

Produces colossal temperatures and shock waves (enormous pressures)

Highly localized and affect few cells, so not considered clinically important

Related to the likelihood of harmful bioeffects from cavitation

Related to peak rarefaction pressure and lower frequency

Greater likelihood with additional negative pressure and lower frequency

Lower = less cavitation, less pressure, higher frequency

Higher = more cavitation, more pressure, lower frequency

A branch of medicine associated with population studies

Empirical (exposure-response method) utilizing clinical surveys

Prospective vs Randomized

Often retrospective

Ambiguities may exist in the data, including justification for the exam, gestational age, # of scans, technique, exposure time

Risk factors other than exposure to ultrasound may precipitate a bad outcome in the fetus - poor nutrition, smoking, alcohol/drug abuse, etc

Forward looking studies

Establish a protocol, specific information is systematically obtained

Advantage: complete and accurate compilation of meaningful information is obtained

Studies in which patients are divided into one placebo group and one group that is exposed to ultrasound

Advantage: other risk factors that could negatively affect fetal outcome are present in both groups and can be accounted for

No confirmed harmful bioeffects from exposure to diagnostic ultrasound have ever been reported

It is possible that bioeffects may be identified in the future

The benefits to the patient must outweigh the risks

It is appropriate to use diagnostic ultrasound prudently to provide benefit to the patient

It is inappropriate to use diagnostic ultrasound in a non-medical setting for entertainment

No confirmed bioeffects on patients or sonographers have been found with the use of diagnostic ultrasound

Experience with diagnostic ultrasound may differ from research and training, due in part to longer research exams and greater exposure

When used without direct medical benefit tot he patient, the subject should be informed how the research study differs from standard diagnostic procedures

A cracked transducer housing presents the greatest risk of electrical shock

Image quality may be compromised with using damaged transducers