Cerebral blood flow and BOLD responses to a memory encoding task: a comparison between healthy young and elderly adults

Assessment of Alzheimer's disease risk with functional magnetic resonance imaging: an arterial spin labeling study

Functional magnetic resonance imaging (fMRI) of older adults at risk for Alzheimer's disease (AD) by virtue of their cognitive (i.e., mild cognitive impairment [MCI]) and/or genetic (i.e., apolipoprotein E [APOE] ε4 allele) status demonstrate divergent brain response patterns during memory encoding across studies. Using arterial spin labeling MRI, we examined the influence of AD risk on resting cerebral blood flow (CBF) as well as the CBF and blood oxygenation level dependent (BOLD) signal response to memory encoding in the medial temporal lobes (MTL) in 45 older adults (29 cognitively normal [14 APOE ε4 carriers and 15 noncarriers]; 16 MCI [8 APOE ε4 carriers, 8 noncarriers]). Risk groups were comparable in terms of mean age, years of education, gender distribution, and vascular risk burden. Individuals at genetic risk for AD by virtue of the APOE ε4 allele demonstrated increased MTL resting state CBF relative to ε4 noncarriers, whereas individuals characterized as MCI showed decreased MTL resting state CBF relative to their cognitively normal peers. For percent change CBF, there was a trend toward a cognitive status by genotype interaction. In the cognitively normal group, there was no difference in percent change CBF based on APOE genotype. In contrast, in the MCI group, APOE ε4 carriers demonstrated significantly greater percent change in CBF relative to ε4 noncarriers. No group differences were found for BOLD response. Findings suggest that abnormal resting state CBF and CBF response to memory encoding may be early indicators of brain dysfunction in individuals at risk for developing AD.

Template-based approach for detecting motor task activation-related hyperperfusion in pulsed ASL data

Arterial spin labeling (ASL) permits the noninvasive measurement of quantitative values of cerebral blood flow (CBF) and is thus well adapted to study inter- and intrasubject perfusion variations whether at rest or during an fMRI task. In this study, a template approach to detect brain activation as a CBF difference between resting and activated groups was compared with a standard generalized linear model (GLM) analysis. A basal perfusion template of PICORE-Q2TIPS ASL images acquired at 3T from a group of 25 healthy subjects (mean age 31.6 ± 8.3 years) was created. The second group of 12 healthy subjects (mean age 28.6 ± 2.7 years) performed a block-design motor task. The template was compared with the mean activated image of the second group both at the individual and at the group level to extract activation maps. The results obtained using a GLM analysis of the whole sequence was used as ground truth for comparison. The influences of spatial normalization using DARTEL registration and of correction of partial volume effects (PVE) in the construction of the template were assessed. Results showed that a basal perfusion template can detect activation-related hyperperfusion in motor areas. The true positive ratio was increased by 2.5% using PVE-correction and by 3.2% using PVE-correction with DARTEL registration. On average, the group comparison presented a 2.2% higher true positive ratio than the one-to-many comparison.

Inverse correspondence between hippocampal perfusion and verbal memory performance in older adults

Understanding physiological changes that precede irreversible tissue damage in age-related pathology is central to optimizing treatments that may prevent, or delay, cognitive decline. Cerebral perfusion is a tightly regulated physiological property, coupled to tissue metabolism and function, and abnormal (both elevated and reduced) hippocampal perfusion has been reported in a range of cognitive disorders. However, the size and location of the hippocampus complicates perfusion quantification, as many perfusion techniques acquire data with spatial resolution on the order of or beyond the size of the hippocampus, and are thus suboptimal in this region (especially in the presence of hippocampal atrophy and reduced flow scenarios). Here, the relationship between hippocampal perfusion and atrophy as a function of memory performance was examined in cognitively normal healthy older adults (n = 20; age=67 ± 7 yr) with varying genetic risk for dementia using a custom arterial spin labeling acquisition and analysis procedure. When controlling for hippocampal volume, it was found that hippocampal perfusion correlated inversely (P = 0.04) with memory performance despite absent hippocampal tissue atrophy or white matter disease. The hippocampal flow asymmetry (left hippocampus perfusion-right hippocampus perfusion) was significantly (P = 0.04) increased in APOE-ϵ4 carriers relative to noncarriers. These findings demonstrate that perfusion correlates more strongly than tissue volume with memory performance in cognitively normal older adults, and furthermore that an inverse trend between these two parameters suggests that elevation of neuronal activity, possibly mediated by neuroinflammation and/or excitation/inhibition imbalance, may be closely associated with minor changes in memory performance.

Functional arterial spin labeling: Optimal sequence duration for motor activation mapping in clinical practice

PURPOSE

To determine the minimal optimal functional arterial spin labeling (fASL) sequence duration allowing steady and reproducible motor activation mapping.

MATERIALS AND METHODS

Three magnetic resonance imaging (MRI) sessions including fASL and blood oxygenation level-dependent (BOLD) functional MRI (fMRI) sequences were performed on 12 healthy subjects at 3T with a 32-channel coil. The raw 7-minute fASL sequence was truncated to obtain six fASL sequences with durations ranging from 1-6 minutes. All the resulting fASL activations were compared between themselves and with both the 7-minute fASL and BOLD activations. Quantitative parameters assessed activation location (activated volume, barycenter, and distance between barycenters), activation quantification (activation-related cerebral blood flow), and intraindividual reproducibility across fMRI sessions. The statistical analysis was based on analysis of variance (ANOVA) and Tukey's multiple comparisons.

RESULTS

Four-minute fASL achieved steady location and quantification of activation with the activated volume corresponding to 81% of the 7-minute fASL volume and a barycenter located 1.2 mm from the 7-minute fASL barycenter and 3.0 mm from the BOLD fMRI barycenter. Four-minute fASL reproducibility was high and statistically equivalent to 7-minute values.

CONCLUSION

A 4-minute fASL sequence is thus a reliable tool for motor activation mapping and suitable for use in clinical practice.

Quantitative fMRI and oxidative neuroenergetics

The discovery of functional magnetic resonance imaging (fMRI) has greatly impacted neuroscience. The blood oxygenation level-dependent (BOLD) signal, using deoxyhemoglobin as an endogenous paramagnetic contrast agent, exposes regions of interest in task-based and resting-state paradigms. However the BOLD contrast is at best a partial measure of neuronal activity, because the functional maps obtained by differencing or correlations ignore the total neuronal activity in the baseline state. Here we describe how studies of brain energy metabolism at Yale, especially with (13)C magnetic resonance spectroscopy and related techniques, contributed to development of quantitative functional brain imaging with fMRI by providing a reliable measurement of baseline energy. This narrative takes us on a journey, from molecules to mind, with illuminating insights about neuronal-glial activities in relation to energy demand of synaptic activity. These results, along with key contributions from laboratories worldwide, comprise the energetic basis for quantitative interpretation of fMRI data.

Diffusion modulation of the fMRI signal: early investigations on the origin of the BOLD signal

The early 1990s was a very special period for functional MRI (fMRI). Many original concepts were formed during that period which helped set up the foundations for modern neuroimaging development. I was fortunate to be in graduate school at the time. I was even more fortunate to be enrolled in one of the pioneer groups in fMRI at the Medical College of Wisconsin, and witnessed some of the early fMRI experiments taking place in the lab. Under the daily influence and steady guidance by the extraordinarily talented researchers there, I also began my own work on the contrast mechanisms of fMRI. In particular, I was developing diffusion weighted strategies to investigate the origin of the functional signal using blood oxygenation level dependent (BOLD) contrast. Our results, that there was significant BOLD signal in large veins and their vicinities at low field strengths (e.g. 1.5T), played an immediate role in moving fMRI applications to higher fields (3T and above) where small vessels (e.g. capillaries) contribute more significantly to improve the neuronal specificity of the BOLD signal. This manuscript gathers some of my own recollections concerning this particular development.

Localization of the hand motor area by arterial spin labeling and blood oxygen level-dependent functional magnetic resonance imaging

The new clinically available arterial spin labeling (ASL) perfusion imaging sequences present some advantages relatively to the commonly used blood oxygen level-dependent (BOLD) method for functional brain studies using magnetic resonance imaging (MRI). In particular, regional cerebral blood flow (CBF) changes are thought to be more directly related with neuronal activation. In this study, we aimed to investigate the accuracy of the functional localization of the hand motor area obtained by simultaneous CBF and BOLD contrasts provided by ASL functional MRI (fMRI) and compare it with a standard BOLD fMRI protocol. For this purpose, we measured the distance between the center of gravity of the activation clusters obtained with each contrast (CBF, BOLD(ASL), and Standard BOLD) and 11 positions defined on a well-established anatomical landmark of the hand motor area (the omega in the axial plane of the precentral gyrus). We found that CBF measurements were significantly closer to the anatomical landmark than the ones obtained using either simultaneous BOLD(ASL) or standard BOLD contrasts. Moreover, we also observed reduced intersubject variability of the functional localization, as well as percent signal change, for CBF relative to both BOLD contrast measurements. In conclusion, our results add further evidence in support to the notion that CBF provides a more accurate localization of motor activation than BOLD contrast, indicating that ASL may be an appropriate technique for clinical fMRI studies.

Biophysical model estimation of neurovascular parameters in a rat model of healthy aging

Neuronal, vascular and metabolic factors result in a deterioration of the cerebral hemodynamic response with age. The interpretation of neuroimaging studies in the context of aging is rendered difficult due to the challenge in untangling the composite effect of these modifications. In this work we integrate multimodal optical imaging in biophysical models to investigate vascular and metabolic changes occurring in aging. Multispectral intrinsic optical imaging of an animal model of healthy aging, the LOU/c rat, is used in combination with somatosensory stimulation to study the modifications of the hemodynamic response with increasing age. Results are fitted with three macroscopic biophysical models to extract parameters, providing a phenomenological description of vascular and metabolic changes. Our results show that 1) biophysical parameters are estimable from multimodal data and 2) parameter estimates in this population change with aging.

Cerebrovascular hemodynamic correlates of aging in the Lou/c rat: a model of healthy aging

The LOU/c rat is an inbred strain considered a model of healthy aging. It exhibits a longer free disease lifespan and a low adiposity throughout life. While this animal model has been shown to maintain eating behavior and neuroendocrine, metabolic and cognitive functions with age, no study has yet investigated vascular correlates in this model of healthy aging. In the present work, multispectral optical imaging was used to investigate the hemodynamic response in the somatosensory cortex of LOU/c rats following forepaw stimulation in three age groups, 4, 24 and 40months. Results indicate reduced hemodynamic responses in the contralateral somatosensory cortex between young (4months) and older groups following stimulation. This decrease was associated with an increase in the spatial extent of activation. The ipsilateral response did not change with aging leading to decreased laterality. Estimations of the relative change in the local cerebral metabolic rate of oxygen during stimulation based on multimodal data showed no significant change with age. The exponent describing the relation between blood volume and blood flow changes, Grubb's parameter, did display a significant change with age which may suggest vessel compliance modifications. This work finds its relevance in recent findings underlying the importance of vascular changes with aging and its impact on neurodegenerative disease.

Quantification of Perfusion Changes during a Motor Task Using Arterial Spin Labeling

Arterial spin labeling (ASL) is a non-invasive MRI technique that allows the quantitative measurement of perfusion, (regional cerebral blood flow (rCBF)). The ASL techniques use the labeling of the blood, by inverting or saturating the spins of water molecules of the blood supplying the imaged region. When reaching the capillary bed, these will be exchanged with tissue water giving rise to a perfusion-weighted signal. The subtraction of control (without label) from labeled images yields a signal difference that directly reflects the local perfusion. Being a non-invasive method, it can be repeated as many times as needed allowing the brain perfusion variation quantification associated with endogenous and exogenous stimuli. In this study, the authors have evaluated the CBF variation induced by the neural activity during a common motor task. The study was conducted on a Siemens Verio 3T system using a 12-channel head coil and a pulsed ASL Q2TIPS-PICORE sequence with a GE-EPI readout. The sequences were driven in 3D PACE mode for prospective motion correction. Fifteen healthy volunteers were studied using a simple motor task consisting in sequential thumb-digit opposition. Two different functional ASL protocols were used: #1 one perfusion scan was obtained during rest and another one during an equal period of motor task (total scan time ~8 min) (TI1 = 700 ms, TI1s = 1600 ms, TI2 =1800 ms; 91 Interleaved tag and control volumes were acquired; TR/TE = 2500/25 ms and flip angle = 90°; nine contiguous axial slices of 8 mm thickness acquired in-line with the AC-PC axis, positioned from the vertex of the brain to the top of cerebellum; FOV = 256 × 256 mm(2); matrix 64 × 64; gap between the labeling slab and the proximal 18.8 mm) and #2 a block design alternating five 25s periods of motor task with five 25s periods of rest (total scan time ~4 min) (TI1 = 700 ms, TI1s = 1600 ms, TI2 = 1800 ms; 101 interleaved tag and control volumes were acquired; TR/TE = 2500/11 ms and flip angle = 90°; nine contiguous axial slices of 6 mm thickness acquired in-line with the AC-PC axis, positioned from the vertex of the brain to the top of cerebellum; FOV = 256 × 256 mm(2) ; matrix 64 × 64; gap between the labeling slab and the proximal 18.8 mm). The post-processing was performed using FSL (www.fmrib.ox.uk/fsl). The mean CBF values obtained for protocols #1 / #2 were: CBFrest = 61.0 / 69.4 ml/100g/min; CBFactivation = 104.8 / 109.9 ml/100g/min; and CBFvariation = CBFactivation - CBFrest = 43.7 / 40.5 ml/100g/min. The relative perfusion changes during activation [defined as CBFvariation / CBFrest (%)] were 73±6 % and 62±7 % (mean±SE) for protocols #1 and #2, respectively. These results show that both activation vs rest and block design functional protocols were capable to detect consistent variations in perfusion associated with a simple motor task. However, the block design has the advantages of requiring shorter acquisitions, directly comparing rest and activation conditions and allowing the acquisition of simultaneous Blood oxygenation level dependent (BOLD) contrast information, while still providing comparable results with the more conventional activation vs rest protocol. In conclusion, our results indicate that a block design ASL-BOLD protocol may be a preferable approach for the evaluation of perfusion changes to endogenous stimuli.

The search for neuroimaging biomarkers of Alzheimer's disease with advanced MRI techniques

The aim of this review is to examine the recent literature on using advanced magnetic resonance imaging (MRI) techniques for finding neuroimaging biomarkers that are sensitive to the detection of risks for Alzheimer's disease (AD). Since structural MRI techniques, such as brain structural volumetry and voxel-based morphometry (VBM), have been widely used for AD studies and extensively reviewed, we will only briefly touch on the topics of volumetry and morphometry. The focus of the current review is about the more recent developments in the search for AD neuroimaging biomarkers with functional MRI (fMRI), resting-state functional connectivity MRI (fcMRI), diffusion tensor imaging (DTI), arterial spin-labeling (ASL), and magnetic resonance spectroscopy (MRS).

Alcohol effects on cerebral blood flow in subjects with low and high responses to alcohol

BACKGROUND

Although there are multiple indications that alcohol can alter many physiological brain functions, including cerebral blood flow (CBF), studies of the latter have generally used small- or modest-sized samples. Few investigations have yet evaluated how CBF changes after alcohol relate to subsets of subjects with elevated alcoholism risks, such as those with lower levels of response (LR) to alcohol. This study used arterial spin labeling (ASL) after alcohol administration to evaluate a large sample of healthy young men and women with low and high alcohol responses, and, thus, varying risks for alcohol use disorders (AUD).

METHODS

Healthy young adult social drinkers with low and high LR (N=88, 50% women) matched on demography and drinking histories were imaged with whole-brain resting ASL ~1 hour after ingesting ~3 drinks of ethanol and after a placebo beverage (i.e., 178 ASL sessions). The relationships of CBF changes from placebo to alcohol for subjects with low and high LR were evaluated.

RESULTS

CBF increased after alcohol when compared to placebo in 5 frontal brain regions. Despite identical blood alcohol concentrations, these increases with alcohol were less prominent in individuals who required more drinks to experience alcohol-related effects (i.e., had a lower LR to alcohol). The LR group differences remained significant after covarying for recent drinking quantities.

CONCLUSIONS

The results confirm that alcohol intake is associated with acute increases in CBF, particularly in frontal regions. Less intense CBF changes were seen in subjects with a genetically influenced characteristic, a low LR to alcohol, that relates to the future risk of heavy drinking and alcohol problems.

Learning Shapes the Representation of Visual Categories in the Aging Human Brain

The ability to make categorical decisions and interpret sensory experiences is critical for survival and interactions across the lifespan. However, little is known about the human brain mechanisms that mediate the learning and representation of visual categories in aging. Here we combine behavioral measurements and fMRI measurements to investigate the neural processes that mediate flexible category learning in the aging human brain. Our findings show that training changes the decision criterion (i.e., categorical boundary) that young and older observers use for making categorical judgments. Comparing the behavioral choices of human observers with those of a pattern classifier based upon multivoxel fMRI signals, we demonstrate learning-dependent changes in similar cortical areas for young and older adults. In particular, we show that neural signals in occipito-temporal and posterior parietal regions change through learning to reflect the perceived visual categories. Information in these areas about the perceived visual categories is preserved in aging, whereas information content is compromised in more anterior parietal and frontal circuits. Thus, these findings provide novel evidence for flexible category learning in aging that shapes the neural representations of visual categories to reflect the observers' behavioral judgments.

fMRI congruous word repetition effects reflect memory variability in normal elderly

Neural circuits mediating repetition effect for semantically congruous words on functional MRI were investigated in seventeen normal elderly (mean age=70). Participants determined if written words were semantically congruent (50% probability) with spoken statements. Subsequent cued-recall revealed robust explicit memory only for congruous items (83% versus 8% for incongruous). Event-related BOLD responses to New>Old congruous words were found in the left>right cingulate and fusiform gyri, left parahippocampal cortex, middle and inferior frontal gyri (IFG). A group with above-median subsequent recall had markedly more widespread BOLD responses than a Low-Recall subgroup, with larger responses in the left medial temporal lobe (LMTL), IFG, and bilateral cingulate gyri. The magnitude of LMTL activation (New-Old) correlated with subsequent cued-recall, while the spatial extent of LMTL activation (New>Old) correlated with recall and recognition. Both magnitude and spatial extent of left fusiform activation correlated with subsequent recall/recognition. A neural circuit of left-hemisphere brain regions, many identified as P600 generators by invasive electrophysiological studies, was activated by New>Old congruous words, likely mediating successful verbal encoding.

Hippocampal activity during the transverse patterning task declines with cognitive competence but not with age

Background

The hippocampus is a brain region that is particularly affected by age-related morphological changes. It is generally assumed that a loss in hippocampal volume results in functional deficits that contribute to age-related cognitive decline. In a combined cross-sectional behavioural and magnetoencephalography (MEG) study we investigated whether hippocampal-associated neural current flow during a transverse patterning task - which requires learning relational associations between stimuli - correlates with age and whether it is modulated by cognitive competence.

Results

Better performance in several tests of verbal memory, verbal fluency and executive function was indeed associated with higher hippocampal neural activity. Age, however, was not related to the strength of hippocampal neural activity: elderly participants responded slower than younger individuals but on average produced the same neural mass activity.

Conclusions

Our results suggest that in non-pathological aging, hippocampal neural activity does not decrease with age but is rather related to cognitive competence.

Increasing measurement accuracy of age-related BOLD signal change: minimizing vascular contributions by resting-state-fluctuation-of-amplitude scaling

In this report we demonstrate a hemodynamic scaling method with resting-state fluctuation of amplitude (RSFA) in healthy adult younger and older subject groups. We show that RSFA correlated with breath hold (BH) responses throughout the brain in groups of younger and older subjects which RSFA and BH performed comparably in accounting for age-related hemodynamic coupling changes, and yielded more veridical estimates of age-related differences in task-related neural activity. BOLD data from younger and older adults performing motor and cognitive tasks were scaled using RSFA and BH related signal changes. Scaling with RSFA and BH reduced the skew of the BOLD response amplitude distribution in each subject and reduced mean BOLD amplitude and variability in both age groups. Statistically significant differences in intrasubject amplitude variation across regions of activated cortex, and intersubject amplitude variation in regions of activated cortex were observed between younger and older subject groups. Intra- and intersubject variability differences were mitigated after scaling. RSFA, though similar to BH in minimizing skew in the unscaled BOLD amplitude distribution, attenuated the neural activity-related BOLD amplitude significantly less than BH. The amplitude and spatial extent of group activation were lower in the older than in the younger group before and after scaling. After accounting for vascular variability differences through scaling, age-related decreases in activation volume were observed during the motor and cognitive tasks. The results suggest that RSFA-scaled data yield age-related neural activity differences during task performance with negligible effects from non-neural (i.e., vascular) sources.

Quantitative analysis of arterial spin labeling FMRI data using a general linear model

Arterial spin labeling techniques can yield quantitative measures of perfusion by fitting a kinetic model to difference images (tagged-control). Because of the noisy nature of the difference images investigators typically average a large number of tagged versus control difference measurements over long periods of time. This averaging requires that the perfusion signal be at a steady state and not at the transitions between active and baseline states in order to quantitatively estimate activation induced perfusion. This can be an impediment for functional magnetic resonance imaging task experiments. In this work, we introduce a general linear model (GLM) that specifies Blood Oxygenation Level Dependent (BOLD) effects and arterial spin labeling modulation effects and translate them into meaningful, quantitative measures of perfusion by using standard tracer kinetic models. We show that there is a strong association between the perfusion values using our GLM method and the traditional subtraction method, but that our GLM method is more robust to noise.

Increased functional brain response during word retrieval in cognitively intact older adults at genetic risk for Alzheimer's disease

Recent language studies in aging and dementia provide two complementary lines of evidence that: (1) measures of semantic knowledge and word-finding ability show declines comparable to those of episodic memory, and greater impairment than executive function measures, during the prodromal period of Alzheimer's disease and (2) cognitively intact older adult carriers of the apolipoprotein E (APOE) epsilon4 allele also demonstrate poorer object naming than their low-risk peers. Given that possible changes in the neural substrates of word retrieval (e.g., Broca's area and fusiform gyrus) in at-risk adults may signal impending cognitive decline and serve as a prodromal marker of AD, we examined whether APOE epsilon4 carriers exhibit changes in brain response in regions subserving word retrieval and semantic knowledge. Eleven cognitively intact APOE epsilon4 older adults and 11 age, education, and family history of AD-matched APOE epsilon3 adults named aloud photographs of animals, tools, and vehicles during event-related fMRI. Results showed that, in the face of equivalent naming accuracy, APOE epsilon4 adults demonstrated more widespread brain response with greater signal change in the left fusiform gyrus, bilateral medial prefrontal cortex, and right perisylvian cortex. Findings are discussed in the context of possible compensatory mechanisms invoked to maintain performance in those at genetic risk for AD.

Separating function from structure in perfusion imaging of the aging brain

The accuracy of cerebral blood flow (CBF) imaging in humans has been impeded by the partial volume effects (PVE), which are a consequence of the limited spatial resolution. Because of brain atrophy, PVE can be particularly problematic in imaging the elderly and can considerably overestimate the CBF difference with the young. The primary goal of this study was to separate the structural decline from the true CBF reduction in elderly. To this end, a PVE-correction algorithm was applied on the CBF images acquired with spin-echo EPI continuous arterial spin labeling MRI (voxel size = 3.4 x 3.4 x 8 mm(3)). Tissue-specific CBF images that were independent of voxels' tissue fractional volume were obtained in elderly (N = 30) and young (N = 26); mean age difference was 43 years. Globally, PVE-corrected gray matter CBF was 88.2 +/- 16.1 and 107.3 +/- 17.5 mL/100 g min(-1) in elderly and young, respectively. The largest PVE contribution was found in the frontal lobe and accounted for an additional 10% and 12% increase in the age-related CBF difference between men and women, respectively. The GM-to-WM CBF ratios were found to be on average 3.5 in elderly and 3.9 in young. Whole brain voxelwise comparisons showed marked CBF decrease in anterior cingulate (bilateral), caudate (bilateral), cingulate gyrus (bilateral), cuneus (left), inferior frontal gyrus (left), insula (left), middle frontal gyrus (left), precuneus (bilateral), prefrontal cortex (bilateral), and superior frontal gyrus (bilateral) in men and amygdala (bilateral), hypothalamus (left), hippocampus (bilateral), and middle frontal gyrus (right) in women.

APOE-4 genotype and neurophysiological vulnerability to Alzheimer's and cognitive aging

Many years before receiving a clinical diagnosis of Alzheimer's disease (AD), patients experience evidence of cognitive decline. Recent studies using a variety of brain imaging technologies have detected subtle changes in brain structure and function in normal adults with a genetic risk for AD; these brain changes have similar pathological features as AD, and some appear to be predictive of future cognitive decline. This review examines the most recent data on brain changes in genetic risk for AD and discusses the benefits and potential risks of detecting individuals at risk.

Effects of aging on cerebral blood flow, oxygen metabolism, and blood oxygenation level dependent responses to visual stimulation

Calibrated functional magnetic resonance imaging (fMRI) provides a noninvasive technique to assess functional metabolic changes associated with normal aging. We simultaneously measured both the magnitude of the blood oxygenation level dependent (BOLD) and cerebral blood flow (CBF) responses in the visual cortex for separate conditions of mild hypercapnia (5% CO(2)) and a simple checkerboard stimulus in healthy younger (n = 10, mean: 28-years-old) and older (n = 10, mean: 53-years-old) adults. From these data we derived baseline CBF, the BOLD scaling parameter M, the fractional change in the cerebral metabolic rate of oxygen consumption (CMRO(2)) with activation, and the coupling ratio n of the fractional changes in CBF and CMRO(2). For the functional activation paradigm, the magnitude of the BOLD response was significantly lower for the older group (0.57 +/- 0.07%) compared to the younger group (0.95 +/- 0.14%), despite the finding that the fractional CBF and CMRO(2) changes were similar for both groups. The weaker BOLD response for the older group was due to a reduction in the parameter M, which was significantly lower for older (4.6 +/- 0.4%) than younger subjects (6.5 +/- 0.8%), most likely reflecting a reduction in baseline CBF for older (41.7 +/- 4.8 mL/100 mL/min) compared to younger (59.6 +/- 9.1 mL/100 mL/min) subjects. In addition to these primary responses, for both groups the BOLD response exhibited a post-stimulus undershoot with no significant difference in this magnitude. However, the post-undershoot period of the CBF response was significantly greater for older compared to younger subjects. We conclude that when comparing two populations, the BOLD response can provide misleading reflections of underlying physiological changes. A calibrated approach provides a more quantitative reflection of underlying metabolic changes than the BOLD response alone.

Cerebral perfusion and oxygenation differences in Alzheimer's disease risk

Functional MRI has demonstrated differences in response to memory performance based on risk for Alzheimer's disease (AD). The current study compared blood oxygen level dependent (BOLD) functional MRI response with arterial spin labeling (ASL) perfusion response during an associative encoding task and resting perfusion signal in different risk groups for AD. Thirteen individuals with a positive family history of AD and at least one copy of the apolipoprotien E epsilon4 (APOE4) gene (high risk) were compared to ten individuals without these risk factors (low risk). In the medial temporal lobes (MTLs) the high risk group had an elevated level of resting perfusion, and demonstrated decreased fractional BOLD and perfusion responses to the encoding task. However, there was no difference in the absolute cerebral blood flow during the task. These data demonstrate that individuals with increased risk for Alzheimer's disease have elevated MTL resting cerebral blood flow, which significantly influences apparent differences in BOLD activations. BOLD activations should be interpreted with caution, and do not necessarily reflect differences in neuronal activation.

Calibrated fMRI in the medial temporal lobe during a memory-encoding task

Prior measures of the blood oxygenation level-dependent (BOLD) and cerebral blood flow (CBF) responses to a memory-encoding task within the medial temporal lobe have suggested that the coupling between functional changes in CBF and changes in the cerebral metabolic rate of oxygen (CMRO(2)) may be tighter in the medial temporal lobe as compared to the primary sensory areas. In this study, we used a calibrated functional magnetic resonance imaging (fMRI) approach to directly estimate memory-encoding-related changes in CMRO(2) and to assess the coupling between CBF and CMRO(2) in the medial temporal lobe. The CBF-CMRO(2) coupling ratio was estimated using a linear fit to the flow and metabolism changes observed across subjects. In addition, we examined the effect of region-of-interest (ROI) selection on the estimates. In response to the memory-encoding task, CMRO(2) increased by 23.1+/-8.8% to 25.3+/-5.7% (depending upon ROI), with an estimated CBF-CMRO(2) coupling ratio of 1.66+/-0.07 to 1.75+/-0.16. There was not a significant effect of ROI selection on either the CMRO(2) or coupling ratio estimates. The observed coupling ratios were significantly lower than the values (2 to 4.5) that have been reported in previous calibrated fMRI studies of the visual and motor cortices. In addition, the estimated coupling ratio was found to be less sensitive to the calibration procedure for functional responses in the medial temporal lobe as compared to the primary sensory areas.

Differential age effects on cerebral blood flow and BOLD response to encoding: associations with cognition and stroke risk

Changes in the cerebrovascular system due to age or disease can significantly alter the blood-oxygenation-level-dependent (BOLD) signal and complicate its interpretation. The simultaneous acquisition of arterial spin labeling (ASL) and BOLD data represents a useful technique to more fully characterize the neurovascular underpinnings of functional brain response to cognition. We conducted a functional magnetic resonance imaging (FMRI) study of episodic memory encoding to investigate whether age is related to cerebral blood flow (CBF) and BOLD response in the medial temporal lobe (MTL). Results demonstrated a significant reduction in resting-state CBF in older compared to young adults. Conversely, older adults showed significantly increased CBF but not BOLD response in the MTL during picture encoding relative to young adults. Correlations between CBF response and cognition were demonstrated whereas associations with BOLD were not observed. Stroke risk was associated with both CBF and BOLD response. Results suggest that aging effects on CBF and BOLD responses to encoding are dissociable and that cerebrovascular alterations contribute to findings of age-related differences.