Timothy Q. Duong

High-resolution mapping of iso- orientation columns by fMRI

Blood-oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is an important tool for localizing brain functions in vivo. However, the ability of BOLD fMRI to map cortical columnar structures is highly controversial, as the ultimate functional specificity of BOLD remains unknown. Here we report a biphasic BOLD response to visual stimulation in the primary visual cortex of cats. In functional imaging, the initial BOLD signal decrease accurately labeled individual iso-orientation columns. In contrast, the delayed positive BOLD changes indicated the pattern of overall activation in the visual cortex, but were less suited to discriminate active from inactive columns.

Localized cerebral blood flow response at submillimeter columnar resolution

Functional magnetic resonance imaging (fMRI) has been widely used for imaging brain functions. However, the extent of the fMRI hemodynamic response around the active sites, at submillimeter resolution, remains poorly understood and controversial. With the use of perfusion-based fMRI, we evaluated the hemodynamic response in the cat visual cortex after orientation-specific stimuli. Activation maps obtained by using cerebral blood flow fMRI measurements were predominantly devoid of large draining vein contamination and reproducible at columnar resolution. Stimulus-specific cerebral blood flow responses were spatially localized to individual cortical columns, and columnar layouts were resolved. The periodic spacing of orientation columnar structures was estimated to be 1.1 ± 0.2 mm (n = 14 orientations, five animals), consistent with previous findings. The estimated cerebral blood flow response at full width at half-maximum was 470 μm under single-stimulus conditions without differential subtraction. These results suggest that hemodynamic-based fMRI can indeed be used to map individual functional columns if large-vessel contributions can be minimized or eliminated.

Microvascular BOLD contribution at 4 and 7 T in the human brain: Gradient-echo and spin-echo fMRI with suppression of blood effects

The BOLD signal consists of an intravascular (IV) and an extravascular (EV) component from both small and large vessels. Their relative contributions are dependent on field strength, imaging technique, and echo time. The IV and EV contributions were investigated in the human visual cortex at 4 and 7 T using spin‐echo and gradient‐echo BOLD fMRI with and without suppression of blood effects. Spin‐echo acquisition suppresses EV BOLD from large veins and reflects predominantly blood T2 changes and EV BOLD signal from small blood vessels. At a short echo time (32 ms), diffusion gradient‐based suppression of blood signals resulted in a 75% and 20% decrease in spin‐echo BOLD changes at 4 T and 7 T, respectively. However, at echo times (55–65 ms) approximating tissue T2 typically used for optimal BOLD contrast, these gradients had much smaller effects at both fields, consistent with the decreasing blood T2 with increasing field strength. Gradient‐echo BOLD percent changes, with relatively long echo times at both fields, were virtually unaffected by gradients that attenuated the blood contribution because the EV BOLD surrounding both large and small vessels dominated. These results suggest that spin‐echo BOLD fMRI at 4 and 7 T, with TE approximating tissue T2, significantly reduces nonspecific mapping signals from large vessels and significantly accentuates microvasculature contributions. Magn Reson Med 49:1019–1027, 2003.

Functional MRI of calcium-dependent synaptic activity: cross correlation with CBF and BOLD measurements.

Spatial specificities of the calcium‐dependent synaptic activity, hemodynamic‐based blood oxygenation level‐dependent (BOLD) and cerebral blood flow (CBF) fMRI were quantitatively compared in the same animals. Calcium‐dependent synaptic activity was imaged by exploiting the manganese ion (Mn++) as a calcium analog and an MRI contrast agent at 9.4 T. Following forepaw stimulation in α‐chloralose anesthetized rat, water T1 of the contralateral forepaw somatosensory cortex (SI) was focally and markedly reduced from 1.99 ± 0.03 sec to 1.30 ± 0.18 sec (mean ± SD, N = 7), resulting from the preferential intracellular Mn++ accumulation. Based on an in vitro calibration, the estimated contralateral somatosensory cortex [Mn++] was ∼100μM, which was 2–5‐fold higher than the neighboring tissue and the ipsilateral SI. Regions with the highest calcium activities were localized around cortical layer IV. Stimulus‐induced BOLD and CBF changes were 3.4 ± 1.6% and 98 ± 33%, respectively. The T1 synaptic activity maps extended along the cortex, whereas the hemodynamic‐based activation maps extended radially along the vessels. Spatial overlaps among the synaptic activity, BOLD, and CBF activation maps showed excellent co‐registrations. The center‐of‐mass offsets between any two activation maps were less than 200 μm, suggesting that hemodynamic‐based fMRI techniques (at least at high field) can be used to accurately map the spatial loci of synaptic activity. Magn Reson Med 43:383–392, 2000.

Spin-Echo fMRI in Humans Using High Spatial Resolutions and High Magnetic Fields

The Hahn spin‐echo (HSE)‐based BOLD effect at high magnetic fields is expected to provide functional images that originate exclusively from the microvasculature. The blood contribution that dominates HSE BOLD contrast at low magnetic fields (e.g., 1.5 T), and degrades specificity, is highly attenuated at high fields because the apparent T2 of venous blood in an HSE experiment decreases quadratically with increasing magnetic field. In contrast, the HSE BOLD contrast is believed to arise from the microvasculature and increase supralinearly with the magnetic field strength. In this work we report the results of detailed and quantitative evaluations of HSE BOLD signal changes for functional imaging in the human visual cortex at 4 and 7 T. This study used high spatial resolution, afforded by the increased signal‐to‐noise ratio (SNR) of higher field strengths and surface coils, to avoid partial volume effects (PVEs), and demonstrated increased contrast‐to‐noise ratio (CNR) and spatial specificity at the higher field strengths. The HSE BOLD signal changes induced by visual stimulation were predominantly linearly dependent on the echo time (TE). They increased in magnitude almost quadratically in going from 4 to 7 T when the blood contribution was suppressed using Stejskal‐Tanner gradients that suppress signals from the blood due to its inhomogeneous flow and higher diffusion constant relative to tissue. The HSE signal changes at 7 T were modeled accurately using a vascular volume of 1.5%, in agreement with the capillary volume of gray matter. Furthermore, high‐resolution acquisitions indicate that CNR increased with voxel sizes < 1 mm3 due to diminishing white matter or cerebrospinal fluid‐space vs. gray matter PVEs. It was concluded that the high‐field HSE functional MRI (fMRI) signals originated largely from the capillaries, and that the magnitude of the signal changes associated with brain function reached sufficiently high levels at 7 T to make it a useful approach for mapping on the millimeter to submillimeter spatial scale. Magn Reson Med 49:655–664, 2003.

Regional cerebral blood flow and BOLD responses in conscious and anesthetized rats under basal and hypercapnic conditions: implications for functional MRI studies

Anesthetics, widely used in magnetic resonance imaging (MRI) studies to avoid movement artifacts, could have profound effects on cerebral blood flow (CBF) and cerebrovascular coupling relative to the awake condition. Quantitative CBF and tissue oxygenation (blood oxygen level–dependent [BOLD]) were measured, using the continuous arterial-spin-labeling technique with echo-planar-imaging acquisition, in awake and anesthetized (2% isoflurane) rats under basal and hypercapnic conditions. All basal blood gases were within physiologic ranges. Blood pressure, respiration, and heart rates were within physiologic ranges in the awake condition but were depressed under anesthesia (P < 0.05). Regional CBF was heterogeneous with whole-brain CBF values of 0.86 ± 0.25 and 1.27 ± 0.29 mL · g–1 · min–1 under awake and anesthetized conditions, respectively. Surprisingly, CBF was markedly higher (20% to 70% across different brain conditions) under isoflurane-anesthetized condition compared with the awake state (P < 0.01). Hypercapnia decreased pH, and increased Pco2 and Po2. During 5% CO2 challenge, under awake and anesthetized conditions, respectively, CBF increased 51 ± 11% and 25 ± 4%, and BOLD increased 7.3 ± 0.7% and 5.4 ± 0.4%. During 10% CO2 challenge, CBF increased 158 ± 28% and 47 ± 11%, and BOLD increased 12.5 ± 0.9% and 7.2 ± 0.5%. Since CBF and BOLD responses were substantially higher under awake condition whereas blood gases were not statistically different, it was concluded that cerebrovascular reactivity was suppressed by anesthetics. This study also shows that perfusion and perfusion-based functional MRI can be performed in awake animals.

Relative changes of cerebral arterial and venous blood volumes during increased cerebral blood flow: implications for BOLD fMRI.

Measurement of cerebral arterial and venous blood volumes during increased cerebral blood flow can provide important information regarding hemodynamic regulation under normal, pathological, and neuronally active conditions. In particular, the change in venous blood volume induced by neural activity is one critical component of the blood oxygenation level‐dependent (BOLD) signal because BOLD contrast is dependent only on venous blood, not arterial blood. Thus, relative venous and arterial blood volume (rCBV) and cerebral blood flow (rCBF) in α‐chlorolase‐anesthetized rats under hypercapnia were measured by novel diffusion‐weighted 19F NMR following an i.v. administration of intravascular tracer, perfluorocarbons, and continuous arterial spin labeling methods, respectively. The relationship between rCBF and total rCBV during hypercapnia was rCBV(total) = rCBF0.40, which is consistent with previous PET measurement in monkeys. This relationship can be linearized in a CBF range of 50–130 ml/100 g/min as ΔrCBV(total)/ ΔrCBF = 0.31 where ΔrCBV and ΔrCBF represent rCBV and rCBF changes. The average arterial volume fraction was 0.25 at a basal condition with CBF of ∼60 ml/100 g/min and increased up to 0.4 during hypercapnia. The change in venous rCBV was 2‐fold smaller than that of total rCBV (ΔrCBV(vein)/ΔrCBF = 0.15), while the arterial rCBV change was 2.5 times larger than that of total rCBV (ΔrCBV(artery)/ΔrCBF = 0.79). These NMR results were confirmed by vessel diameter measurements with in vivo videomicroscopy. The absolute venous blood volume change contributes up to 36% of the total blood volume change during hypercapnia. Our findings provide a quantitative physiological model of BOLD contrast. Magn Reson Med 45:791–800, 2001.

Effects of hypoxia, hyperoxia, and hypercapnia on baseline and stimulus-evoked BOLD, CBF, and CMRO2 in spontaneously breathing animals

Functional magnetic resonance imaging (fMRI) was used to investigate the effects of inspired hypoxic, hyperoxic, and hypercapnic gases on baseline and stimulus-evoked changes in blood oxygenation level-dependent (BOLD) signals, cerebral blood flow (CBF), and the cerebral metabolic rate of oxygen (CMRO2) in spontaneously breathing rats under isoflurane anesthesia. Each animal was subjected to a baseline period of six inspired gas conditions (9% O2, 12% O2, 21% O2, 100% O2, 5% CO2, and 10% CO2) followed by a superimposed period of forepaw stimulation. Significant stimulus-evoked fMRI responses were found in the primary somatosensory cortices. Relative fMRI responses to forepaw stimulation varied across gas conditions and were dependent on baseline physiology, whereas absolute fMRI responses were similar across moderate gas conditions (12% O2, 21% O2 100% O2, and 5% CO2) and were relatively independent of baseline physiology. Consistent with data obtained using well-established techniques, baseline and stimulus-evoked CMRO2 were invariant across moderate physiological perturbations thereby supporting a CMRO2-fMRI technique for non-invasive CMRO2 measurement. However, under 9% O2 and 10% CO2, stimulus-evoked CBF and BOLD were substantially reduced and the CMRO2 formalism appeared invalid, likely due to attenuated neurovascular coupling and/or a failure of the model under extreme physiological perturbations. These findings demonstrate that absolute fMRI measurements help distinguish neural from non-neural contributions to the fMRI signals and may lend a more accurate measure of brain activity during states of altered basal physiology. Moreover, since numerous pharmacologic agents, pathophysiological states, and psychiatric conditions alter baseline physiology independent of neural activity, these results have implications for neuroimaging studies using relative fMRI changes to map brain activity.

Spatiotemporal dynamics of the BOLD fMRI signals: Toward mapping submillimeter cortical columns using the early negative response

The existence of the early‐negative blood‐oxygenation‐level‐dependent (BOLD) response is controversial and its practical utility for mapping brain functions with columnar spatial specificity remains questionable. To address these issues, gradient‐echo BOLD fMRI studies were performed at 4.7 T and 9.4 T using the well‐established orientation column model in the cat visual cortex. A robust transient early‐negative BOLD response was consistently observed in anesthetized cat (‐0.35 ± 0.09%, mean ± SD, n = 8 at 2.9 ± 0.5 sec poststimulus onset for 4.7 T, TE = 31 ms; ‐0.29 ± 0.10%, n = 4 at 3.0 ± 0.8 sec poststimulus onset for 9.4 T, TE = 12 ms). In addition to its temporal evolution, the BOLD response also evolved dynamically in the spatial domain. The initially spatially localized early‐negative signal appeared to dynamically drain from the active sites toward large vessels, followed by a wave of the delayed positive signal, which exhibited similar spatiotemporal dynamics. Only the early‐negative BOLD response within 2 sec of the stimulus onset (not the entire dip) yielded columnar layouts without differential subtraction. The functional maps of two orthogonal orientations using the first 2‐sec dip were indeed complementary. On the other hand, the delayed positive BOLD response appeared diffused and extended beyond the active sites. It was thus less suitable to resolve columnar layouts. These results have implications for the design and interpretation of the BOLD fMRI at columnar resolution. Magn Reson Med 44:231–242, 2000.

Procedure for minimizing stress for fMRI studies in conscious rats

Functional magnetic resonance imaging (fMRI) in conscious animals is evolving as a critical tool for neuroscientists. The present study explored the effectiveness of an acclimation procedure in minimizing the stress experienced by the animal as assessed by alterations in physiological parameters including heart rate, respiratory rate, and serum corticosterone levels. Results confirm that as the stress of the protocol is minimized, there is a significant decrease in head movements and enhancement in data quality. The feasibility of improving the quality of fMRI data acquired in alert rats by utilizing a relatively simple technique is presented.