研究显示:fNIRS采用二波段组合信噪比更高
信噪比是指一个设备或系统中信号与噪声的比例。如在音频放大器中,我们希望的是该放大器除了放大信号外,不应该添加任何其它额外的东西。因此,信噪比应该越高越好。
利用近红外光测量血红蛋白浓度长度(本研究中为血红蛋白浓度与光程长度的乘积)的信噪比(signal-to-noise ratio ,SNR)取决于波长组合和分析方法。
虽然当入射光功率增加时,信噪比会增加,但出于安全标准的考虑,光功率应该受到限制。近红外脑功能成像技术(fNIRS)就是利用600-900nm近红外光进行脑功能成像的,那在光功率一定的情况下,当波段组合和信噪比究竟有什么关系呢?
日本学者Tsukasa FUNANE等人通过研究二波段、三波段和四波段组合的信噪比回答了这个问题,结果发现,在这三种设置中,二波段信噪比更高。
该研究通过各种公式和算法,比较了650-900nm、695-830nm区间范围内,多个波长组合中,两波段、三波段和四波段的信噪比。
Table 1 和 Table 2分别为两个区间范围内理论上的最小误差波长组合。
下图显示了695至830nm范围内,当(a)780和830nm和(b)695和830nm两个波长固定而另一个波长(水平轴)改变时,氧合、脱氧和总血红蛋白浓度长度的估计误差。
图(b)中的氧合和脱氧血红蛋白的误差比图(a)中的小,而图(a)和图(b)中的总血红蛋白的误差大致相同。这是因为695nm和830nm的波长组合比780nm和830nm的波长组合对氧合和脱氧血红蛋白的信噪比更好。
当只用两波段时,误差分别为δ(△CoxyL)=11.61、δ(△CdeoxyL)=6.02和δ(△CtotalL)=7.78 mM·mm;
当使用三波段时(其中两个波段为780和830nm),误差分别为δ(△CoxyL)=16.22-34.63、δ(△CdeoxyL)=8.97-32.83和δ(△CtotalL)=9.60-11.66mM·mm;
当三波段中的两个波段为695和830nm时,误差分别为δ(△CoxyL)=13.13-17.42、δ(△CdeoxyL)=6.64-9.02和δ(△CtotalL)=9.88-13.70mM·mm。
需要注意的重要一点是,信噪比总是随着额外波长的增加而降低,主要原因是由于各波长的光功率降低,导致光源的信噪比变差。
该研究结果表明,波长组合对fNIRS采集信噪比有影响,当波长两端在650-900nm范围内时,CoxyL和CdeoxyL的信噪比最大。
当总入射功率一定时,使用较少波长时血红蛋白浓度长度变化的信噪比更高,应适当控制各波长的测量误差。
Tsukasa FUNANE, Hirokazu ATSUMORI, Hiroki SATO,et al.Relationship between Wavelength Combination and Signal-to-Noise Ratio in Measuring Hemoglobin Concentrations Using Visible or Near-Infrared Light[J].OPTICAL REVIEW Vol.(2009) 442–448.以上内容由依瑞德集团医学中心整理,有不足之处请指正,转载请注明出处。脑科学与神经调控依瑞德集团旗下学术公众号,致力于分享国内外脑科学前沿技术、研究成果及行业动态,并带你玩转TMS、tDCS、fNIRS等最新脑检测和神经调控技术
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