This technical report provides background information for users of IEC 62359 to understand the relative merits of several of the potential replacements for the thermal index (TI) as described in IEC 60601-2-37 and IEC 62359. The report discusses: – parameters related to thermal aspects of diagnostic ultrasonic fields; – methods for the determination of an exposure parameter relating to temperature rise in theoretical tissue-equivalent models, resulting from absorption of ultrasound. The report is intended to be used by: – those involved in the development and maintenance of IEC 62359; – manufacturers of medical electrical equipment for risk assessment; – health care regulatory authorities, test houses and other organizations responsible for implementing standards for medical electrical equipment.