Improved cortical-layer specificity of vascular space occupancy fMRI with slab inversion relative to spin-echo BOLD at 9.4 T

Abstract

Cerebral blood volume (CBV)-weighted endogenous functional contrast can be obtained by the vascular space occupancy (VASO) technique. VASO relies on nonselective inversion for nulling blood signals, but the implementation of VASO at magnetic fields higher than 3 T is difficult due to similar T 1 values of tissue and blood water and a stronger counteracting blood oxygen leveldependent (BOLD) effect. To improve functional CBV sensitivity, we proposed to use VASO with slab-selective inversion (SI-VASO). Computer simulations showed that the SI-VASO approach significantly increases functional sensitivity compared to the original VASO in a stronger magnetic field with a shorter repetition time. To examine layer-dependent specificity, SI-VASO and spin-echo BOLD (SE-BOLD) functional magnetic resonance imaging (fMRI) experiments were performed on isoflurane-anesthetized cats during visual stimulation at 9.4 T. Unlike simultaneously acquired SE-BOLD signal, the SI-VASO signal peaked at 0.9 mm from the surface of the cortex and was localized to the middle cortical layer. The full-width at half maximal response across the cortex was narrower for SI-VASO than for SE-BOLD (1.7 mm vs. 2.5 mm, respectively), suggesting that SI-VASO is better localized to neuronally active sites than SE-BOLD fMRI. The magnitude of the SI-VASO change in the middle cortex was −1.45% with our experimental parameters, corresponding to a relative CBV change of ∼8% when baseline CBV was assumed to be 5% in a two-compartment model. The signal response profile across the cortex, the calculated CBV change, and the time course of SI-VASO fMRI were similar to those of previously obtained CBV-weighted fMRI with contrast agent in the same animal model, suggesting that SI-VASO measures predominately functional CBV responses.

Detecting resting-state brain activity by spontaneous cerebral blood volume fluctuations using whole brain vascular space occupancy imaging

“…Under such condition, the BOLD signals still extended outside of the gray matter areas. These results demonstrated the better spatial specificity of VASO contrast than that of BOLD, which is in agreement with the previous observations in the animal model (Jin and Kim, 2008).…”

“…However, BOLD signal depends on multiple physiological processes (Davis et al, 1998) and therefore is not straightforward to interpret. Furthermore, BOLD signal has relatively poor spatial localization, compared to CBV-and CBF-based signals (Duong et al, 2001;Jin and Kim, 2008). As such, imaging methods based on a lessconfounding physiological parameter, such as CBV or CBF, are needed to overcome the drawbacks of BOLD.…”

“…ECN could appear as unilateral or bilateral) compared to sensory networks and DMN, suggesting that high sensitivity is needed to detect these cognitive networks robustly. The differences in network patterns between the VASO and BOLD might owe to the different sensitivity and specificity of the two fMRI techniques, but remain to be verified, for example, using animal models (Jin and Kim, 2008).…”

“…Evoked fMRI experiments on cat brains showed that VASO signal was primarily located in the middle layers of the cortex (closer to neural cells), whereas BOLD signal based on a gradient-echo sequence was more on the surface layer (closer to blood vessels) (Jin and Kim, 2008), suggesting that VASO has better spatial localization than BOLD. Compared to ASL techniques, which usually need a pair of images to obtain CBF, a single-shot VASO imaging may provide higher temporal resolution.…”

“…Recently, arterial spin-labeling (ASL) perfusion imaging has been used to investigate resting-state brain activities Viviani et al, 2011;Zou et al, 2009), demonstrating the viability of characterizing intrinsic brain activities with CBF contrast, though low temporal resolution, low contrast-to-noise ratio, and contamination of BOLD signal remain challenging in these techniques. Like CBF, CBV could be another important, single physiological parameter to characterize brain activities with improved spatial specificity (Jin and Kim, 2008), but so far, there is no CBV-based restingstate fMRI study reported in humans yet.…”