As far as patient safety is concerned, whenever ultrasound is used in diagnosis or therapy, considerations must address not only general aspects such as electrical and mechanical safety, but also two essential biological effects that ultrasound waves have on human tissue. Firstly, the heating of tissue caused by the absorption of ultrasonic waves, and secondly, cavitation caused by high levels of negative sound pressure. Safety requirements, safety concepts and risk management differ depending on which of the three medical applications of ultrasound on patients a device is designed to perform.
1. Ultrasound diagnosis
Ultrasound scanning for diagnostic purposes is used for imaging and measuring certain parameters including blood flow rate and bone density. In this type of application, biological effects should be avoided as far as possible. To ensure this, the relevant standard requires diagnostic ultrasound devices to display certain parameters that inform the person performing the scan of the current "thermal index" (risk of tissue heating) and "mechanical index" (risk of cavitation) values.
Calculation of the thermal index differs depending on the field of application and the imaging mode and differentiates between scanning modes (ultrasonic signal follows different acoustic paths) and nonscanning modes (ultrasonic signal follows the same acoustic path). In view of the increasing number of combined imaging methods, i.e. methods that combine a variety of modes including scanning and nonscanning modes, the calculation rule used so far has been replaced by a new formula. This formula now differentiates between two types of heating caused by combined modes, i.e. heating near the transducer and deep tissue heat, and thus provides more precise assessment of the thermal effect. This change in the method of calculation of the thermal index is also accepted by the U.S. Food and Drug Administration (FDA) for registration of diagnostic ultrasound equipment in the US market.
When we look at the harmonised standard EN 60601-2-37 for ultrasonic diagnostic equipment, this change in detail is easily overlooked since the formula for the calculation of index values is not included in the standard itself. The formula is only included in the IEC 62359 standard referenced by the harmonised EN 60601-2-37 standard. As soon as the reporting format of the new index values is published in an amendment to EN 60601-2-37, this changed method of calculation will have to be applied by all manufacturers and the index values will have to be re-calculated. Where necessary, the measurements on which calculations are based may also have to be repeated. In this case, manufacturers will also have to review and, if necessary, revise their risk management.
2. Ultrasound physiotherapy
Ultrasonic physiotherapy produces mild heat deep in the tissue, stimulating healing and regeneration. The applicable limit values for the sound-field characteristics restrict the biological effect, ensuring that neither cavitation nor critical tissue heating can take place. The EN 60601-2-5 standard applies to ultrasound physiotherapy. To date, the edition from 2000 is still listed as the applicable harmonised standard. In the meantime, the standard has been revised and updated by the IEC. IEC 60601-2-5:2009 now matches the third edition of the basic standard IEC 60601-1.
The new edition includes guidelines on maintenance and new requirements regarding the high-voltage test. In addition, measurement of the transducer surface temperature rise has been modified to allow for simulated use. Regarding the measurement of ultrasound parameters, the standard now references the 2nd edition of the 61689:2007 standard. The most important change in ultrasound-related parameters is the definition of the effective radiating area, which will affect the calculation of effective intensity. It is expected that this new edition of the IEC 60601-2-5 will also be harmonised under the MDD and will then, as EN 60601-2-5, become relevant in the European conformity assessment procedure.
3. Ultrasound surgery
The use of ultrasound in surgery enables targeted destruction of cells and cell areas through intense (local) heating. The energy levels used are therefore much higher than in ultrasound diagnosis or physiotherapy. There is no specific part 2-standard for the ultrasonic scalpel, which uses ultrasonic energy to simultaneously cut through and coagulate tissue. The basic standard in conjunction with appropriate risk management must be applied to this area of application.
High intensity therapeutic ultrasound (HITU) is another ultrasound surgery technique that has been on the rise in recent years. In this method cells are destroyed as the result of being heated by high-intensity ultrasonic fields. This form of noninvasive surgery is applied, for example, to ensure less traumatic surgery or when no other methods are available, for example in the treatment of certain types of brain tumors. A new part 2 standard of the 60601 series addressing these high-intensity uses is currently under development and available as a draft version at the International Electrotechnical Commission (IEC).
Since high-intensity therapeutic ultrasound (HITU) is used for the targeted destruction of tissue, limiting the power output is not expedient in this case. However, the standard will define relevant parameters and establish criteria which manufacturers must use as the basis for risk assessment of this type of equipment. Manufacturers should keep up to date with developments in standardisation in this sector to ensure they can anticipate the applicable requirements for devices and technical dossiers. The IEC 62555 standard that addresses output power measurement for high-intensity therapeutic ultrasound transducers and systems is under preparation.