Output Display Standard

The Output Display Standard (ODS) consists of two unitless indices (TI and MI) that are displayed on the viewing monitor.  The two indices provide real-time information to the sonographer about the potential for bioeffects.  This information can then be used to help the operator implement the ALARA principle.

The TI is the Thermal Index.  It indicates the potential for a relative temperature increase at a specific point along the ultrasound beam. 

For example, a TI of “2” represents a higher temperature rise than a TI of “1” but does not necessarily represent a rise of 2°C. 

The TI is designed to make the operator aware of the possible temperature rise at a particular point in tissue.

There are three thermal indices:

• TIS provides information on the estimated temperature rise in soft tissue

• TIB gives information on the estimated temperature rise in bone. TIB in later obstetrics (at or after 10 weeks when bone ossification has occurred) (Mnemonic: TIB for big bony babies)

• TIC is used in transcranial Doppler

In general, we want to scan with the lowest TI value that will provide diagnostic information.  Typically, this is a TI of less than 0.7 for obstetrical scans or less than 1.5 for abdominal, cardiac, or vascular scans.  The AIUM has an important statement on the recommended scan times for specific TI values

The MI is the Mechanical Index.  It indicates the potential for mechanical effects, such as cavitation 

The MI can range up to 1.9 for all uses except ophthalmic, which has a maximum MI limit of 0.23.  Again, the index levels do not indicate that a biological effect is happening but provide the operator with info regarding the probability of a biological effect. 

By monitoring the ODS throughout the scan, the operator can more efficiently apply the ALARA principle.