Partitioning of physiological noise signals in the brain with concurrent near-infrared spectroscopy and fMRI

Abstract

The blood-oxygen level dependent (BOLD) signals measured by functional magnetic resonance imaging (fMRI) are contaminated with noise from various physiological processes, such as spontaneous low-frequency oscillations (LFOs), respiration, and cardiac pulsation. These processes are coupled to the BOLD signal by different mechanisms, and represent variations with very different frequency content; however, because of the low sampling rate of fMRI, these signals are generally not separable by frequency, as the cardiac and respiratory waveforms alias into the LFO band. In this study, we investigated the spatial and temporal characteristics of the individual noise processes by conducting concurrent near-infrared spectroscopy (NIRS) and fMRI studies on six subjects during a resting state acquisition. Three time series corresponding to LFO, respiration, and cardiac pulsation were extracted by frequency from the NIRS signal (which has sufficient temporal resolution to critically sample the cardiac waveform) and used as regressors in a BOLD fMRI analysis. Our results suggest that LFO and cardiac signals modulate the BOLD signal independently through the circulatory system. The spatiotemporal evolution of the LFO signal in the BOLD data correlates with the global cerebral blood flow. Near-infrared spectroscopy can be used to partition these contributing factors and independently determine their contribution to the BOLD signal.

Figure 1

Temporal traces (and their power spectra) of the near-infrared spectroscopy (NIRS) data collected from one source-detector pair (3 cm) on subject 3. (A) Temporal traces of Δ[HbO] (gray) and Δ[Hb] (black) calculated from the NIRS recording. (B) Enlarged section of (A) indicated by the black block. (C) Spectra from Δ[HbO] (gray) and Δ[Hb] (black).

Figure 2

(A) Power spectra and temporal traces of Δ[HbO] collected on subject 3. (B) Very low-frequency oscillation (VLF) component of Δ[HbO] is represented in spectral domain (left), in time domain (middle). Enlarged section of the temporal trace (indicated by the gray area) with resampled data marked with circles (right). Similar plots are shown for low-frequency oscillation (LFO) (C), respiration (D), cardiac pulsation, (E) components of Δ[HbO].

Figure 3

The thresholded z-statistic maps obtained by using resampled Δ[HbO] regressors of certain time shifts (indicated by the number below each z-statistic map) for subject 3. The z-statistic maps in panel A corresponded to the Δ[HbO] regressors in the spectral range of low-frequency oscillation (LFO), while that in panel B corresponded to the Δ[HbO] regressors in the cardiac range. The maps were arranged by their corresponding time shifts to assess the dynamic patterns.

Figure 4

Comparison between the blood vessel map and maximum z-statistic results obtained from subject 3. (A) The blood vessel map obtained from phase contrast magnetic resonance imaging (MRI). (B–D) The z-statistic maps of the brain as results of using near-infrared spectroscopy (NIRS) Δ[HbO] of different spectral bands, low-frequency oscillation (LFO) in (B), respiration in (C), and heartbeat in (D) as regressors. Images are z-statistic map overlaid on the high-resolution scan transformed into standard (MNI152) space. The left hemisphere of the brain corresponds to the right side of the image.

Figure 5

Averaged blood vessel map and averaged maximum z-statistic results in various frequency bands for comparison. Data were obtained from six subjects, except in (C), where the averaging was performed on the three subjects whose respiration components were visible. (A) The averaged blood vessel map obtained from phase contrast magnetic resonance imaging (MRI). (B–D) The averaged maximum z-statistic maps of the brain as results of using near-infrared spectroscopy (NIRS) Δ[HbO] of different spectral bands, low-frequency oscillation (LFO) in (B), respiration in (C), and cardiac pulsation in (D) as regressors. Images are averaged z-statistic maps overlaid on the averaged high resolution scan transformed into standard (MNI152) space. The left hemisphere of the brain corresponds to the right side of the image.

Figure 6

Averaged maximum z-statistic maps of six subjects using near-infrared spectroscopy (NIRS) low-frequency oscillation (LFO) Δ[HbO] regressors show activations in regions contained in familiar resting state patterns in (A, B). Images are averaged z-statistic maps overlaid on the averaged high-resolution anatomic scan transformed into standard (MNI152) space. The left hemisphere of the brain corresponds to the right side of the image.