The main advantage of ultrasound is that certain structures can be observed without using radiation. However, ultrasound is energy and there are ultrasound safety regulations, because two bioeffects of ultrasound are heat and cavitation. Ultrasound is a mechanical energy in which a pressure wave travels through tissue. Reflection and scattering back to the transducer are used to form the image. As sound energy is transmitted through the tissue, some energy is reflected and some power is absorbed.
Possible physical effects with ultrasound:
The ultrasound safety is based on two indices, the mechanical index (MI) and the thermal index (TI). The WFUMB guidelines state that ultrasound that produces temperature rises of less than 1.5°C may be used without reservation. They also state that ultrasonic exposure causing temperature rises of greater than 4°C for over 5 min should be considered potentially hazardous. This leaves a wide range of temperature increases which are within the capability of diagnostic ultrasound equipment to produce and for which no time limits are recommended. However, it has not been determined that medical ultrasound causes any adverse reaction or deleterious effect.
The American Institute of Ultrasound in Medicine states that as of 1982, no independently confirmed significant biologic effects had been observed in mammalian tissue below (medical usage) 100mW/cm2.

See also Ultrasound Regulations and Ultrasound Radiation Force.

In physics, the absorbed dose is the ultrasonic power absorbed per unit of mass of an object, and is measured in watts per kilogram (W/kg). The absorption increases with ultrasound intensity and frequency.
The thermal index describes the potential for heating of the patient's tissue due to the application of energy.

See also Thermal Effect, Ultrasound Safety, Ultrasound Regulations.

Breast ultrasound (sonography or ultrasonography) it is an important tool in the characterization of breast lesions, detected with mammography or clinical breast examination. However, a breast sonogram is not approved by the U.S. Food and Drug Administration (FDA) as a screening tool for breast cancer and is used additional to a mammogram.
Ultrasound is useful in guiding needles for fine needle aspiration and core biopsies. Breast ultrasound has optimal contrast resolution, but it lacks the spatial resolution of conventional mammography and cannot provide as much detail as a mammogram image. In addition, ultrasound is unable to show tiny calcium deposits (microcalcifications) that are often early indications of breast cancer.

See also Biopsy, Interventional Ultrasound, Ultrasound Safety, Side Effect and Ultrasound Regulations.

A liver sonography is a diagnostic tool to image the liver and adjoining upper abdominal organs such as the gallbladder, spleen, and pancreas. Deeper structures such as liver and pancreas are imaged at a lower frequency 1-6 MHz with lower axial and lateral resolution but greater penetration. The diagnostic capabilities in this area can be limited by gas in the bowel scattering the sound waves.
The application of microbubbles may be useful for detection of liver lesions and for lesion characterization. Some microbubbles have a liver-specific post vascular phase where they appear to be taken up by the reticuloendothelial system (RES). Dynamic contrast enhanced scans in a similar way as with CT or MRI can be used to studying the arterial, venous and tissue phase.
After a bolus injection, early vascular enhancement is seen at around 30sec in arterialized lesions (e.g., hepatocellular carcinomas (HCC), focal nodular hyperplasia (FNH)). Later enhancement is typical of hemangiomas with gradually filling towards the center. In the late phase at around 90sec, HCCs appear as defects against the liver background. Most metastases are relatively hypovascular and so do not show much enhancement and are seen as signal voids in the different phases.
Either with an intermittent imaging technique or by continuous scanning in a nondestructive, low power mode, characteristic time patterns can be used to differentiate lesions.

See also Medical Imaging, B-Mode, High Intensity Focused Ultrasound, Ultrasound Safety and Contrast Medium.

Ultrasound is the ideal tool to examine children of all ages. It is fast, painless, uses no ionizing radiation, and does not require a baby to remain still for long periods. Real-time modes show movement of internal tissues and organs. Advanced ultrasound imaging techniques such as color Doppler, 4D ultrasound, harmonic imaging, and higher resolution, as well as the application of ultrasound contrast agents broaden the potential of ultrasound.
Pediatric [paediatric, Brit.] ultrasound can be used in all body regions and reduce the number of more invasive or radiating examinations that often additionally need sedation or intravenous iodinated contrast agents.

See also Fetal Ultrasound, Reflux Sonography, Ultrasound Safety, Abdominal Ultrasound and Pregnancy Ultrasound.