(SA) The spatial average intensity is the acoustic power within the beam, divided by the beam area.

The acoustic power of sound and ultrasound is the energy delivered per unit of time. The power is measured in Watt (W) and is proportional to the square of the amplitude.
1 W = 1 joule/second.

See also Directivity Index, Spatial Average Intensity, and Source Level.

Power in ultrasound describes the energy generated by a wave or in a signal per unit of time. The power is measured in watts (W) and is proportional to the square of the amplitude.

See also Spatial Average Intensity, Ultrasound Radiation Force and Time Average Intensity.

(I-SPTA) Spatial peak time averaged intensity is the measure most associated with temperature rise.

See also Thermal Effect, and Time Average Intensity.

Fetal ultrasound is a safe and non-invasive imaging technique used to visualize and monitor the development of a fetus during pregnancy. It employs high-frequency sound waves to create detailed images of the baby, the placenta, and the uterus. Fetal ultrasound provides valuable information about the baby's growth, organ development, and overall well-being. It is commonly used to determine gestational age, assess fetal anatomy, detect abnormalities, and monitor fetal movements and heart rate. This essential tool enables healthcare professionals to ensure the optimal health of both the mother and the baby throughout the pregnancy.
The FDA (Food and Drug Administration) has established regulations governing ultrasound usage, including specific guidelines for fetal ultrasound examinations. These regulations permit an eight-fold increase in ultrasound intensity for fetal scans. They place considerably responsibility on the user to understand the output measurements, the mechanical index (MI), the thermal index (TI) and to use them in their scanning. The primary safety concern in prenatal diagnostic imaging is temperature rise. It is known that hyperthermia is teratogenic. The efforts of investigators have concentrated on defining the temperature increases and exposure times which may give rise to biological effects and on determining the ultrasound levels which might, in turn, lead to those temperature rises.
In fetal ultrasound, the highest temperature increase would be expected to occur at bone and the thermal index with bone at/near the focus (TIB) would give the 'worst case' conditions. The mechanical index and thermal index must be displayed if the ultrasound system is capable of exceeding an index of 1. The displayed indices are based on the manufacturer's experimental and modeled data. However, an independent study has demonstrated significant discrepancies over declared spatial peak time averaged intensity (I-SPTA) output of up to 400%.

See also ALARA Principle, Pregnancy Ultrasound and Doppler Fluximetry in Pregnancy.