Ultrasound Safety and Quality Assurance

Ultrasound is non-ionizing radiation, unlike X-rays.
Ultrasound is considered generally safe when used prudently by trained staff.
Concerns about ultrasound safety include:
- Increasing number of ultrasound scans
- Wider range of clinical applications
- Modern diagnostic equipment sophistication
Ultrasound involves energy exposure, potentially initiating biological effects.
In 1917, Paul Langevin observed the immediate death of fish near a high-intensity ultrasound beam.
Wood and Loomis (1927) published research on the physical and biological effects of high-frequency ultrasound.
High frequency and intensity of ultrasound can cause high localized temperature.
Ultrasound can cause cavitation, especially at high intensities and low frequencies.
Cavitation is the formation of bubbles in liquid, producing shear stress, heat, and possible cell damage.
Thermal effects of ultrasound are related to heating and the resulting changes in tissue.
The tissue within the focus of the beam will heat up more rapidly.
The amount of absorbed energy depends on tissue composition and ultrasound frequency.
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Not all transducer energy is converted into ultrasound energy; some is converted into heat.
Transducer surface temperatures are limited, to less than 50°C in air, and less than 43°C in contact with the patient..
Some tissues, like those of a developing fetus, are particularly sensitive to thermal effects.
A rise in temperature of 1.5°C above normal physiological levels in a diagnostic setting is usually acceptable.
Significant temperature rises over 4°C can be hazardous to the fetus/embryo.
Non-thermal effects include cavitation, involving the formation and collapse of bubbles.
Stable cavitation is associated with repetitive oscillation around the equilibrium bubble radius.
Transient cavitation (inertial) involves bubble expansion beyond the equilibrium radius and rapid collapse, often associated with a shockwave and generation of free radicals that induce further biochemical reactions in the biological tissue or cell.
Contrast agents can be ruptured at relatively low acoustic pressures, releasing free gas bubbles and leading to inertial cavitation, potentially causing micro-vascular damage.
Lung capillary bleeding has been observed in mammals exposed to high acoustic pressures.
Exposure controls such as Output Display Standards (ODS) are used in most diagnostic ultrasound machines to control potential for heating due to high-intensity ultrasound.
The ODS standard displays a thermal index (TI) and a mechanical index (MI) to help standardize diagnostic scans and alert users to potential risks.
The FDA removed application specific output limits, but retained a maximum overall limit of 720 mW/cm².
TI must be displayed with each ultrasound image and is a figure for the maximum temperature rise at that focus, while MI is a figure that indicates risk of cavitation.
ALARA principle means keeping ultrasound parameters as low as reasonably achievable while ensuring adequate diagnostic images.
Users should regularly adjust machine controls to maintain low levels of acoustic parameters.
Epidemiological studies have not shown a link between ultrasound exposure and adverse outcomes in humans, but studies continue to be performed.

Quality Assurance

Quality assurance (QA) programs monitor ultrasound system aspects that might change or deteriorate over time.
QA programs monitor aspects that will impact on clinical efficacy to ensure the ultrasound equipment delivers accurate and complete images.
Baseline and routine testing of new and upgraded equipment for diagnostic purposes, is carried out by Medical Physics. - Ultrasound physicists (often in the medical physics department or clinical engineering) do the baseline testing and also oversee the routine test and quality checks.
QA includes user-performed tests at 1–4-week intervals to assess ongoing function.
Yearly or semi-annually, routine quality assurance tests are undertaken through the Medical Physics Department or company service engineers.
TETO test objects are used to assess imaging performance, measure imaging parameters.
They mimic soft tissue, containing targets that may be used to evaluate specific parameters of interest to ensure image quality across the entire scope of clinical use, not just across one individual test.
Various types of Doppler phantoms are utilized evaluate Doppler imaging performance.
Flow phantoms, that are filled with fluids designed to mimic blood flow patterns, are used to confirm imaging parameters against expectations.
String phantoms contain a moving object, such as a string that moves through the imaging equipment, enabling assessment of Doppler principles at work.
The material used to develop these phantoms have expected physical attributes such as speed of sound, attenuation coefficient, etc that should match known values for biological tissues.
In addition to the physical attributes, a string phantom must also match the biological properties or behaviour or tissues, such as those that reflect blood and fluid movement.
The user tests of the equipment should check and verify equipment function, image quality, and measure accuracy.