Church Charles C
National Center for Physical Acoustics, University of Mississippi, University, MS 38677, USA.
Ultrasound Med Biol. 2007 Sep;33(9):1489-94. doi: 10.1016/j.ultrasmedbio.2007.03.004. Epub 2007 May 23.
The thermal index (TI) displayed on the screens of most modern diagnostic ultrasound machines is linearly proportional to the absorbed power or, equivalently, to the in-situ intensity or temperature rise. Users are instructed to interpret the TI as a "relative indication of bioeffect risk." The thermal dose is a well-known empirical relationship between the temperature T of a biological system and the time t needed for that temperature to induce a deleterious effect. For any two temperatures, T1 and T2, and the corresponding times t1 and t2, required to produce the same level of effect, this general relation holds: t1/t2=RT2-T1, where R is the thermal normalization constant. Hence, it is experimentally determined that the rate of induction, or risk, of a thermal effect increases exponentially with temperature. Because exponential relationships are not intuitive to many users, there is a significant potential for underestimation of the thermal risk associated with exposure to diagnostic ultrasound. To better quantify this risk and thereby make the displayed information more useful, the current linear display of the calculated value of the thermal index, i.e., of TIcur, should be altered to an exponential form based on the thermal dose and representing the excess risk associated with the exposure: TInew=(RTIcur-1)/(R-1). This expression has the advantage that for the usual choice of R=4 for T<or=43 degrees C, TInew approximately TIcur in the range most often seen onscreen, i.e., TIcur<1.2, minimizing any confusion during a transition from TIcur to TInew. For the relatively rare but potentially much more serious circumstances when TIcur>3.5, the displayed TInew>>TIcur, consistent with empirical observations of the likelihood of harm. Additional advantages also obtain.
大多数现代诊断超声仪屏幕上显示的热指数（TI）与吸收功率成正比，或者等效地，与原位强度或温度升高成正比。用户被指导将TI解释为“生物效应风险的相对指标”。热剂量是生物系统温度T与该温度产生有害效应所需时间t之间的一种众所周知的经验关系。对于产生相同效应水平所需的任意两个温度T1和T2以及相应的时间t1和t2，这个一般关系成立：t1/t2 = RT2 - T1，其中R是热归一化常数。因此，实验确定热效应的诱导率或风险随温度呈指数增加。由于指数关系对许多用户来说不直观，与诊断超声暴露相关的热风险有被显著低估的可能性。为了更好地量化这种风险并使显示的信息更有用，热指数计算值（即TIcur）的当前线性显示应改为基于热剂量的指数形式，并表示与暴露相关的额外风险：TInew = (RTIcur - 1)/(R - 1)。这个表达式的优点是，对于T≤43摄氏度时通常选择的R = 4，在屏幕上最常看到的范围内，即TIcur < 1.2时，TInew约等于TIcur，从而在从TIcur过渡到TInew期间将混淆最小化。对于相对罕见但可能更严重的情况，即TIcur > 3.5时，显示的TInew >> TIcur，这与危害可能性的经验观察结果一致。还能获得其他优点。
