Improved spatial localization of post-stimulus BOLD undershoot relative to positive BOLD

Abstract

The negative blood oxygenation level-dependent (BOLD) signal following the cessation of stimulation (post-stimulus BOLD undershoot) is observed in functional magnetic resonance imaging (fMRI) studies. However, its spatial characteristics are unknown. To investigate this, gradient-echo BOLD fMRI in response to visual stimulus was obtained in isoflurane-anesthetized cats at 9.4 T. Since the middle cortical layer (layer 4) is known to have the highest metabolic and cerebral blood volume (CBV) responses, images were obtained to view the cortical cross-section. Robust post-stimulus BOLD undershoot was observed in all studies, and lasted longer than 30 s after the cessation of 40-60 s stimulation. The magnitude of post-stimulus BOLD undershoot was linearly dependent on echo time with little intercept when extrapolating to TE = 0, indicating that the T2* change is the major cause of the BOLD undershoot. The post-stimulus BOLD undershoot was observed within the cortex and near the surface of the cortex, while the prolonged CBV elevation was observed only at the middle of the cortex. Within the cortex, the largest post-stimulus undershoot was detected at the middle of the cortex, similar to the CBV increase during the stimulation period. Our findings demonstrate that, even though there is significant contribution from pial vessel signals, the post-stimulus undershoot BOLD signal is useful to improve the spatial localization of fMRI to active cortical sites.

Figure 1

TE-dependence of BOLD fMRI signals. T₁-weighted anatomical image (A) and conventional positive functional t-value map (B) of one animal are shown. The cortical surface is outlined in green contours and white matter boundaries are delineated by black contours from anatomic image. These were overlaid on images in Figs. 1B, 2 and 3. A cyan arrow in (B) indicates signal intensity loss due to susceptibility-induced dephasing around the large surface veins. The statistical t-value threshold was set to 2.0 with a minimum cluster size of 4 pixels (p < 0.01). D: dorsal, L: lateral. (C) Average time courses of five animals were plotted for four different TEs. A red bar under the time courses indicates the 60 sec visual stimulation period. (D) The percentage signal change vs. TE for positive BOLD and post-stimulus BOLD undershoot is shown in individual animals (n = 5). Additionally, linearly fitted lines are also drawn.

Figure 2

BOLD maps vs. post-stimulus BOLD undershoot maps. t-value maps of positive BOLD (left) and post-stimulus BOLD undershoot (right) were obtained from TE-averaged data in two cats. Images acquired during a stimulation period except initial 8 sec and during 12-36 sec after cessation of stimulation were used for positive and post-stimulus BOLD maps, respectively. To minimize the influence of different CNRs for positive BOLD and post-stimulus BOLD undershoot, 1000 pixels with highest t values were chosen by adjusting the threshold individually. D: dorsal, L: lateral.

Figure 3

Time-dependent subtraction maps of conventional positive BOLD and post-stimulus BOLD signals. Functional subtraction maps are absolute changes from the pre-stimulus baseline level. Maps were calculated during the different time periods (indicated by the numbers left of the maps) after onset (left column) and cessation (right column) of stimulation. Red arrows indicate the middle of the cortex. Clearly, highest post-stimulus BOLD undershoot is observed in middle of the cortex, as well as the cortical surface (dark pixels in F-I). Green contour: cortical surface; black contour: white matter boundary; D: dorsal, L: lateral. The unit of gray scale bars is the multiple of the standard deviation within the noise area outside the brain.

Figure 4

Time-dependent subtraction maps of BOLD vs. CBV-weighted fMRI. BOLD (left column) and CBV-weighted (right column) subtraction maps are shown as absolute signal changes from pre-stimulus baseline levels. Maps were calculated during the steady-state stimulation condition (12 s - 40 sec after the onset of 40-s long stimulation, the red bar indicates stimulation) (A and E), an initial post-stimulus period (44-52 sec) (B and F), and two later post-stimulus periods (56-64 s and 68-76 s after the termination of stimulation) (C and G, D and H, respectively). Bright pixels in BOLD indicate positive signal changes, while those in CBV-weighted fMRI indicate a decrease in MRI signal and consequently an increase in CBV. The unit of gray scale bars is the multiple of the standard deviation within the noise area outside the brain. Red arrows indicate the middle of the cortex. Clearly, post-stimulus BOLD undershoot was observed (C and D). Green contour: cortical surface; yellow contour: white matter boundary; D: dorsal, L: lateral.

Figure 5

Average profiles of BOLD and CBV-weighted fMRI signals across the cortex. Cortical depth dependent profile was generated for each animal from the quadrangular ROIs in area 18 (e.g., two red quadrangular ROIs in inset image), then data was averaged across all animals. Post-stimulus data was obtained from 12-36 s after the cessation of stimulation, while stimulation data was obtained from entire stimulation period except initial 8 seconds. TE-average data are shown in A, while high-resolution BOLD and CBV data during stimulation and post-stimulation periods are shown in B and C, respectively. The surface of the cortex is at zero, with cortical depth represented by increasing distances. Approximate location of cortical layers was determined by relative distances of those layers in area 18 (Payne and Peters, 2002) and is differentiated by colored bands. Negative change in CBV-weighted fMRI indicates an increase in CBV. The middle of the cortex has the highest stimulation-induced CBV increase as well as highest post-stimulus BOLD undershoot. For clarity, only one side of the error bars (SEM) is shown.