Lubbers Jaap, Hekkenberg Rob T, Bezemer Robert A
Department of BioMedical Engineering (AO), University of Groningen, Groningen, The Netherlands.
Ultrasound Med Biol. 2003 May;29(5):755-64. doi: 10.1016/s0301-5629(02)00790-1.
Activities in the standardisation of medical ultrasonic imaging and monitoring devices have been undertaken to ensure the safe application and to provide the user with tools to perform risk-to-benefit analysis. Internationally, agreement has not been reached to implement a system based on a physical quantity such as temperature rise. Therefore, recently, the IEC Standard 60601-2-37 has adopted the thermal index (TI) as a safety parameter. TI indicates conditions that are more likely than others to produce thermal effects. The models used to calculate TI are based on the equilibrium temperature rise for a stationary transducer. Ultrasound (US) is often applied during a short time at one place. To characterise the situation before equilibrium is reached, this study proposes a new parameter: TT (time to threshold). TT indicates the time after which a threshold temperature rise is exceeded. TT is calculated for some tissue models to illustrate the meaning of TT. TT indicates how long one piece of tissue can be insonated safely, provided the safe threshold is known. Using TT, higher intensities can be applied safely during a limited time. This paper illustrates the effects of short insonation times and the influence of the beam diameter. Questions that are critical to the introduction of TT as a safety parameter are discussed.
为确保医学超声成像和监测设备的安全应用，并为用户提供进行风险效益分析的工具，已开展了相关标准化活动。在国际上，尚未就实施基于诸如温度升高之类物理量的系统达成一致意见。因此，最近，国际电工委员会（IEC）标准60601-2-37采用了热指数（TI）作为安全参数。TI表示比其他情况更有可能产生热效应的条件。用于计算TI的模型基于固定换能器的平衡温度升高。超声（US）通常在短时间内在一个部位施加。为了描述达到平衡之前的情况，本研究提出了一个新参数：TT（达到阈值的时间）。TT表示超过阈值温度升高之后的时间。针对一些组织模型计算了TT，以说明TT的含义。如果已知安全阈值，TT表示一块组织可以安全地进行超声照射的时间。使用TT，可以在有限的时间内安全地施加更高的强度。本文阐述了短时间超声照射的影响以及束直径的影响。讨论了将TT作为安全参数引入时至关重要的问题。
