Abstract
Frequency-domain near-infrared spectro-imaging offers significant advantages over the continuous-wave method in human brain applications. However, the drawback of existing instruments is a low signal-to-noise ratio for measured phase and modulation depth changes caused by cerebral activation. In this paper we show that in the case of the geometry specific for the activated area in the human brain, the SNR can be significantly improved by increasing the modulation frequency. We present the results of two studies: one performed experimentally using a subnanosecond pulsed light source and a spherical absorbing inhomogeneity immersed in a highly scattering solution, and the other performed numerically using Monte Carlo simulations of light transport in an MRI based digital phantom of the adult human head. We show that changes caused by the absorbing inhomogeneity in both phase and modulation depth increase with frequency and reach maximum values at frequencies between 400 and 1400 MHz, depending on the particular source-detector distance. We also show that for the human head geometry an increase of the modulation frequency from 100 to 500 MHz can increase the phase SNR 2-3 times, and the modulation depth SNR up to 10 times.
Highlights
Near-infrared biomedical spectro-imaging is a rapidly developing method, which offers significant advantages such as high biochemical specificity, high temporal resolution, and relatively low cost [1]
We show that for the human head geometry an increase of the modulation frequency from 100 to 500 MHz can increase the phase signalto-noise ratio (SNR) 23 times, and the modulation depth SNR up to 10 times
We show that changes caused by the absorbing inhomogeneity, in both phase and modulation depth (MD) increase with frequency and reach their maximum values between 400 and 1400 MHz depending on the source-detector distance
Summary
Near-infrared biomedical spectro-imaging is a rapidly developing method, which offers significant advantages such as high biochemical specificity, high temporal resolution, and relatively low cost [1]. It has been shown that a advantageous technique for imaging functional cerebral activation is frequency-domain spectro-imaging [2,3,4,5,6], which employs near-infrared light harmonically modulated at a frequency of 100 MHz or above. The reason for such a superiority of the frequency-domain method over the continuous-wave method is that it provides much more information, which is encoded in three parameters of the photon density wave: the intensity (DC), the modulation amplitude (AC), and the phase. Some phase SNR improvement at higher frequencies can be seen in Fig. 12 (f) in Ref. [7], the authors concluded that the highest SNR for detecting absorbing inhomogeneities in the slab geometry was at zero frequency
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