1. COMPARISON OF OPTICAL AND fMRI MEASURES OF NEUROVASCULAR COUPLING
E. Maclin, C. Brumback, B. Gordon, M. Pearson, Y. Lee, G. Gratton & M. Fabiani
Beckman Institute and Psychology Dept., University of Illinois at Urbana-Champaign
Introduction:
Hemodynamic function is known to vary with aging and fitness, but the details of these changes, particularly in the brain, are currently poorly understood. Functional MRI (fMRI) uses the Blood Oxygenation Level Dependent (BOLD) signal to detect task-specific regional hemodynamic changes. The BOLD signal, however, conflates several hemodynamic parameters, and provides no direct information about the underlying neuronal function. This makes it difficult to test specific hypothesis regarding changes in neurovascular coupling associated with aging. Optical imaging of both slow (hemodynamic, NIRS; Toronov, et al., 2003) and fast (neuronal, EROS; Gratton & Fabiani, 2001) signals can provide quantitative measures of oxy- and deoxy-hemoglobin concentration along with direct measurements of neuronal activity (Gratton et al., 2001). This combination of features, along with the relatively high temporal resolution of optical measures, may be useful in clarifying the effects of aging and fitness on the hemodynamic resources of the brain.
We investigated neurovascular coupling by comparing fast optical (EROS) activity to both slow optical responses (NIRS) and the fMRI BOLD response in the same subjects.
Methods:
Participants: Fifty subjects comprised 3 groups; young (age 21 to 28, mean=22.4, n=16), old with high VO2 max (age 65 to 80, mean=70.9, n=19; VO2 score=31.2), and old with low VO2 max (age 66 to 81, mean=73.7, n=15, VO2 score=18.1).
Stimuli and procedures: A high-contrast black & white checkerboard was contrast reversed at 1, 2, 4, 6 and 8 Hz. Stimuli were presented in 20-second blocks alternating with 20 seconds of blank screen. Optical and fMRI data were recorded in separate sessions.
Optical recording: Optical responses were recorded from 160 channels using an ISS Imagent frequency domain oximeter with sources operating at 690 and 830 nm. Phase, DC and AC data were recorded at 62.5 Hz. The integrated fast optical (EROS) response was estimated from the phase data at 830 nm by averaging after each checkerboard reversal, and multiplying the amplitude of the oscillations by the stimulation frequency. The slow optical response (NIRS) was estimated by averaging the DC response across blocks, calculating oxy- and deoxy-hemoglobin concentration changes computed with respect to the baseline.
fMRI recording: The BOLD fMRI response was measured with a Siemens 3T MRI using EPI, and the mean percent change was calculated. All responses were estimated in a 2x2x2 cm region of interest at the posterior extreme of the calcarine fissure. The five stimulation frequencies were kept separate for all analyses.
Within Subject Effects
Stimulus frequency had similar effects on all hemodynamic measures as well as on integrated fast activity (amplitude of EROS response x stimulus frequency).
Hemodynamic Measures
Correlation with
F x EROS
NIRS: [HbO2]
.962
NIRS: [Hb]
-.497
fMRI: BOLD
.907
Neurovascular coupling was measured as the relationship between the integrated fast response (EROS) and the various hemodynamic measures. Coupling was strongest for the BOLD and oxy-Hb measures, and considerably weaker for deoxy-Hb.
Between Subject Effects
Conclusions:
The similarity of the effect of stimulus frequency on all of our hemodynamic measures demonstrates the validity of these measures. The differences between measures of oxy- and deoxy-hemoglobin across subject groups illustrates the potential for optical measures to provide additional information about neurovascular coupling beyond that provided by the fMRI BOLD signal. The optical signal discriminates changes in oxy- and deoxy-hemoglobin, whereas the BOLD signal reflects changes in deoxy-hemoglobin. Differences in these measures may reflect changes in the relationship of blood volume and blood flow which have different effects on [Hb] and [HbO2].
This project was supported by an NIA grant to M. Fabiani and a NIBIB grant to G. Gratton.
The BOLD and deoxy-Hb signals showed a similar (albeit inverse) relationship across subject groups. In contrast, the oxy-Hb response of the high-VO2 older group was greater than the BOLD and deoxy-Hb responses.