Ultrasound is a version of the sonar used to “see” submarines under water. A medical ultrasound for fetal imaging has a computer, an electronic display and a transducer. The transducer sends short, inaudible, high frequency sound waves into the body. These sound waves interact with the body’s tissues and return echoes back to the transducer. The computer analyzes the echoes and converts it to a display.

Sound is vibration. The intensity, or power, of the vibrations can do damage to soft tissue. (65) Ultrasounds used in therapy create heat through the intense vibrations. Imaging ultrasounds are know as diagnostic ultrasounds and are built to operate at a much lower level of intensity than therapeutic ultrasound; however, they also create heat.

The FDA has set limits for intensity based on the area of the body to be viewed. There are short-form designations for the limits:

TI – Thermal Index is a calculated estimate of temperature increase at the focus.
TIB – Thermal Index Bone is TI for a focus near bone after a pulse has passed through soft tissue.
TIC – Thermal Index Cranial is TI when the pulse only passes through a thin layer of soft tissue before encountering bone.
MI – Mechanical index is a measure of pressure fluctuations within the ultrasound pulse and is a representation of the mechanical energy of the pulse.

Acoustic output is in Watts per centimeter squared (W/cm2) and is ISPPA3 in equations where I is intensity, SPPA means spatial-peak pulse-average and the 3 refers to a factoring for the liquid environment. The maximum intensity averaged over the pulse time period is ISPTA3 with the units milliwatts per centimeter squared (mW/cm2). SPTA means spatial-peak temporal-average and the 3 again refers to the liquid environment.

The depth of ultrasound penetration is described in terms of the half-value depth for the specific ultrasound frequency. The half-value depth is the distance at which 50% of the ultrasound energy has been dissipated. The depth of penetration is related to the frequency of the ultrasound wave. Higher frequencies have a shorter depth of penetration. Lower frequencies have a longer depth of penetration.

The FDA publishes “Guidance for Industry and FDA Staff – Information for Manufacturers Seeking Marketing Clearance of Diagnostic Ultrasound Systems and Transducers.”

The FDA guidelines have the limits shown in the table. The spacial-peak temporal-average used to increase cell formation in the healing of broken bones is 1/3 of the limit for fetal imaging.

Regulations limit the maximum capability of newer devices to 30% over the outputs in the table.  There are still devices in service that were not required to have this maximum limit.

David A. Toms, MD FR CPC of Canada has developed a website with a great deal of information on the technology of ultrasound (