The NPL (National Physical Laboratory) from England put online a guida to power measurement of medical ultrasonic equipment (based on IEC 61161) . It´s very detailed and will surely help people making these kind of measurements.

Link: http://www.npl.co.uk/server.php?show=ConWebDoc.2282

The navigation is a littleweird (each one of : Introduction | Radiation Force Balances | Measurement | Additional Information | Further Reading has various pages)