Diagnostic ultrasound systems now come with displays meant to warn the system operator when
there may be a risk to the patient (or fetus) from the heat or mechanical effects caused by ultrasound. The
system displays numbers that provide crude measures of the risk. The Thermal Index (TI) is an estimate of
risk from heat, and the Mechanical Index (MI) is an estimate of risk from the nonthermal effects of
ultrasound. Manufacturers began incorporating these displays into ultrasound systems in order to meet the
US government’s 1991 new regulations allowing them to increase ultrasound system outputs. If they used
the Index displays, they could increase outputs. When the MI is above 0.5 or the TI is above 1.0, the NCRP
recommends that the risks of ultrasound be weighed against the benefits.