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Biomedical subjects

R B Buxton

Publications and source records attributed to R B Buxton.

At least 19 recordsLinked to original sources
PubMed2001

T(1) and T(2) selective method for improved SNR in CSF-attenuated imaging: T(2)-FLAIR.

We present here a method for improving SNR in CSF-attenuated imaging relative to the standard technique of using an inversion pulse and imaging at the null point of CSF. In this new method the inversion pulse is replaced with a 90(x)-180(y)-90(x) preparation sequence that provides T(1) and T(2) selectivity. This allows the tissue magnetization to recover more rapidly, allows for the use of shorter TR values, and reduces T(1) weighting. Magn Reson Med 45:529-532, 2001.

Blood Volume↗
PubMed2001

Nonlinear temporal dynamics of the cerebral blood flow response.

The linearity of the cerebral perfusion response relative to stimulus duration is an important consideration in the characterization of the relationship between regional cerebral blood flow (CBF), cerebral metabolism, and the blood oxygenation level dependent (BOLD) signal. It is also a critical component in the design and analysis of functional neuroimaging studies. To study the linearity of the CBF response to different duration stimuli, the perfusion response in primary motor and visual cortices was measured during stimulation using an arterial spin labeling technique with magnetic resonance imaging (MRI) that allows simultaneous measurement of CBF and BOLD changes. In each study, the perfusion response was measured for stimuli lasting 2, 6, and 18 sec. The CBF response was found in general to be nonlinearly related to stimulus duration, although the strength of nonlinearity varied between the motor and visual cortices. In contrast, the BOLD response was found to be strongly nonlinear in both regions studied, in agreement with previous findings. The observed nonlinearities are consistent with a model with a nonlinear step from stimulus to neural activity, a linear step from neural activity to CBF change, and a nonlinear step from CBF change to BOLD signal change.

Cerebrovascular Circulation↗
PubMed2001

Estimation of respiration-induced noise fluctuations from undersampled multislice fMRI data.

Functional MRI time series data are known to be contaminated by highly structured noise due to physiological fluctuations. Significant components of this noise are at frequencies greater than those critically sampled in standard multislice imaging protocols and are therefore aliased into the activation spectrum, compromising the estimation of functional activations and the determination of their significance. However, in this work it is demonstrated that unaliased noise information is available in multislice data, and can be used to estimate and reduce noise due to high-frequency respiratory-related fluctuations. Magn Reson Med 45:635-644, 2001. Published 2001 Wiley-Liss, Inc.

Heart↗
PubMed2001

Detection power, estimation efficiency, and predictability in event-related fMRI.

Experimental designs for event-related functional magnetic resonance imaging can be characterized by both their detection power, a measure of the ability to detect an activation, and their estimation efficiency, a measure of the ability to estimate the shape of the hemodynamic response. Randomized designs offer maximum estimation efficiency but poor detection power, while block designs offer good detection power at the cost of minimum estimation efficiency. Periodic single-trial designs are poor by both criteria. We present here a theoretical model of the relation between estimation efficiency and detection power and show that the observed trade-off between efficiency and power is fundamental. Using the model, we explore the properties of semirandom designs that offer intermediate trade-offs between efficiency and power. These designs can simultaneously achieve the estimation efficiency of randomized designs and the detection power of block designs at the cost of increasing the length of an experiment by less than a factor of 2. Experimental designs can also be characterized by their predictability, a measure of the ability to circumvent confounds such as habituation and anticipation. We examine the relation between detection power, estimation efficiency, and predictability and show that small increases in predictability can offer significant gains in detection power with only a minor decrease in estimation efficiency.

Artifacts↗
PubMed2001

Putting spatial attention on the map: timing and localization of stimulus selection processes in striate and extrastriate visual areas.

This study investigated the cortical mechanisms of visual-spatial attention in a task where subjects discriminated patterned targets in one visual field at a time. Functional magnetic imaging (fMRI) was used to localize attention-related changes in neural activity within specific retinotopic visual areas, while recordings of event-related brain potentials (ERPs) traced the time course of these changes. The earliest ERP components enhanced by attention occurred in the time range 70-130 ms post-stimulus onset, and their neural generators were estimated to lie in the dorsal and ventral extrastriate visual cortex. The anatomical areas activated by attention corresponded closely to those showing increased neural activity during passive visual stimulation. Enhanced neural activity was also observed in the primary visual cortex (area V1) with fMRI, but ERP recordings indicated that the initial sensory response at 50-90 ms that was localized to V1 was not modulated by attention. Modeling of ERP sources over an extended time range showed that attended stimuli elicited a long-latency (160-260 ms) negativity that was attributed to the dipolar source in area V1. This finding is in line with hypotheses that V1 activity may be modulated by delayed, reentrant feedback from higher visual areas.

Adolescent↗
PubMed2000

Altered brain response to verbal learning following sleep deprivation.

The effects of sleep deprivation on the neural substrates of cognition are poorly understood. Here we used functional magnetic resonance imaging to measure the effects of 35 hours of sleep deprivation on cerebral activation during verbal learning in normal young volunteers. On the basis of a previous hypothesis, we predicted that the prefrontal cortex (PFC) would be less responsive to cognitive demands following sleep deprivation. Contrary to our expectations, however, the PFC was more responsive after one night of sleep deprivation than after normal sleep. Increased subjective sleepiness in sleep-deprived subjects correlated significantly with activation of the PFC. The temporal lobe was activated after normal sleep but not after sleep deprivation; in contrast, the parietal lobes were not activated after normal sleep but were activated after sleep deprivation. Although sleep deprivation significantly impaired free recall compared with the rested state, better free recall in sleep-deprived subjects was associated with greater parietal lobe activation. These findings show that there are dynamic, compensatory changes in cerebral activation during verbal learning after sleep deprivation and implicate the PFC and parietal lobes in this compensation.

Adult↗
PubMed1999

Sleep deprivation-induced reduction in cortical functional response to serial subtraction.

Thirteen normal volunteers were studied with fMRI during arithmetic performance after a normal night of sleep and following sleep deprivation (SD). Aims included determining whether the prefrontal cortex (PFC) and the parietal lobe arithmetic areas are vulnerable to the effects of SD. After a normal night of sleep, activation localized to the bilateral PFC, parietal lobes and premotor areas. Following SD, activity in these regions decreased markedly, especially in the PFC. Performance also dropped. Data from the serial subtraction task are consistent with Horne's PFC vulnerability hypothesis but, based on this and other studies, we suggest the localized, functional effects of SD in the brain may vary, in part, with the specific cognitive task.

Adult↗
PubMed1999

Dynamic imaging of perfusion in human skeletal muscle during exercise with arterial spin labeling.

MR images acquired by using an arterial spin-labeling technique showed spatial and temporal variations of perfusion in the skeletal muscle of exercising humans. Perfusion measurements made during plantar flexion exercise in normal volunteers were consistent with those obtained by traditional techniques reported in the literature. Spatial heterogeneity of perfusion values clearly delineated the various muscle groups within the lower leg. These results are interpreted in terms of a quantitative model for the perfusion signal in muscle. This method can provide a useful tool in the study of muscle physiology. Magn Reson Med 42:258-267, 1999. Published 1999 Wiley-Liss, Inc.

Arteries↗
PubMed1999

Brain activation and pupil response during covert performance of the Stroop Color Word task.

Patterns of brain activation associated with covert performance of the Stroop Color-Word task were studied in young, healthy, adult volunteers using blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI). Comparisons of the incongruous Stroop condition were made with both color naming and word reading baselines. Areas of the left and right anterior cingulate, the right precuneus, and the left pars opercularis displayed larger BOLD signal responses during the incongruous Stroop condition than during baseline conditions. Activation of BOLD signals in these areas was highly repeatable. In a second experiment, pupil diameter was used to assess cognitive load in 7 individuals studied during overt and covert performance of both Stroop and color naming conditions. Cognitive load was similar in overt and covert response conditions. Results from the BOLD study indicate that brain regions participating in selective visual attention and in the selection of motor programs involved in speech were activated more by the Stroop task than by the baseline tasks. The neural substrate involved in the resolution of the perceptual and motor conflicts elicited by the Stroop Color-Word task does not appear to be a single brain region. Rather, a network of brain regions is implicated, with separate regions within this system supporting distinct functions.

Adult↗
PubMed1999

Involvement of striate and extrastriate visual cortical areas in spatial attention.

We investigated the cortical mechanisms of visual-spatial attention while subjects discriminated patterned targets within distractor arrays. Functional magnetic resonance imaging (fMRI) was used to map the boundaries of retinotopic visual areas and to localize attention-related changes in neural activity within several of those areas, including primary visual (striate) cortex. Event-related potentials (ERPs) and modeling of their neural sources, however, indicated that the initial sensory input to striate cortex at 50-55 milliseconds after the stimulus was not modulated by attention. The earliest facilitation of attended signals was observed in extrastriate visual areas, at 70-75 milliseconds. We hypothesize that the striate cortex modulation found with fMRI may represent a delayed, re-entrant feedback from higher visual areas or a sustained biasing of striate cortical neurons during attention. ERP recordings provide critical temporal information for analyzing the functional neuroanatomy of visual attention.

Adult↗
PubMed1999

Mapping the physiological parameters of articular cartilage with magnetic resonance imaging.

The ability of magnetic resonance imaging (MRI) to visualize the spatial distribution of parameters related to the physiological and structural properties of tissues makes it an ideal tool for the study of articular cartilage. A variety of ingenious MRI methods have been devised to probe the complex composition and biochemistry of normal and degenerate articular cartilage. In this article we review the current status of this research and pose some questions concerning the future directions of articular cartilage research and clinical applications.

Cartilage Diseases↗
PubMed1999

Quantitative perfusion imaging using arterial spin labeling.

Arterial spine labeling (ASL) techniques have matured to the point that they can provide robust quantitative multislice measurements of cerebral blood flow (CBF) under most circumstances. These techniques provide better spatial and temporal resolution than positron-emission tomography (PET) and are entirely noninvasive, requiring no injections or radiation. The most obvious clinical application is in the evaluation of acute stroke, in which the primary pathology is a lack of CBF, precisely the quantity that is measured directly by ASL. The one major technical challenge that currently prevents more general application in the brain is the sensitivity to abnormally long transit delays.

Arteries↗
PubMed1998

Probabilistic analysis of functional magnetic resonance imaging data.

Probability theory is applied to the analysis of fMRI data. The posterior distribution of the parameters is shown to incorporate all the information available from the data, the hypotheses, and the prior information. Under appropriate simplifying conditions, the theory reduces to the standard statistical test, including the general linear model. The theory is particularly suited to handle the spatial variations in the noise present in fMRI, allowing the comparison of activated voxels that have different, and unknown, noise. The theory also explicitly includes prior information, which is shown to be critical in the attainment of reliable activation maps.

Humans↗
PubMed1998

Correction of off resonance-related distortion in echo-planar imaging using EPI-based field maps.

We present, here, a simple method for measurement and correction of off-resonance related geometric distortion in echo-planar imaging (EPI). This method uses high signal-to-noise ratio (SNR) EPI-based field maps, rapidly acquired using a series of gradient recalled images collected across a range of TE values. This field map is distorted in the same manner as the EPI images to be unwarped, providing a direct look-up table for the correct location of each pixel of data. This method adds very little scan time and is robust and easy to implement.

Algorithms↗
PubMed1998

Quantitative imaging of perfusion using a single subtraction (QUIPSS and QUIPSS II).

In the pulsed arterial spin labeling (ASL) techniques EPISTAR, PICORE, and FAIR, subtraction of two images in which inflowing blood is first tagged and then not tagged yields a qualitative map of perfusion. An important reason this map is not quantitative is that there is a spatially varying delay in the transit of blood from the tagging region to the imaging slice that cannot be measured from a single subtraction. We introduce here two modifications of pulsed ASL (QUIPSS and QUIPSS II) that avoid this problem by applying additional saturation pulses to control the time duration of the tagged bolus, rendering the technique relatively insensitive to transit delays and improving the quantitation of perfusion.

Brain↗
PubMed1998

Dynamics of blood flow and oxygenation changes during brain activation: the balloon model.

A biomechanical model is presented for the dynamic changes in deoxyhemoglobin content during brain activation. The model incorporates the conflicting effects of dynamic changes in both blood oxygenation and blood volume. Calculations based on the model show pronounced transients in the deoxyhemoglobin content and the blood oxygenation level dependent (BOLD) signal measured with functional MRI, including initial dips and overshoots and a prolonged poststimulus undershoot of the BOLD signal. Furthermore, these transient effects can occur in the presence of tight coupling of cerebral blood flow and oxygen metabolism throughout the activation period. An initial test of the model against experimental measurements of flow and BOLD changes during a finger-tapping task showed good agreement.

Blood Glucose↗
PubMed1998

A theoretical and experimental comparison of continuous and pulsed arterial spin labeling techniques for quantitative perfusion imaging.

Under ideal conditions, continuous arterial spin labeling (ASL) techniques are higher in SNR than pulsed ASL techniques by a factor of e. Presented here is a direct theoretical and experimental comparison of continuous ASL and pulsed ASL, using versions of both that are amenable to multislice imaging and insensitive to variations in transit times (continuous ASL with a delay before imaging, and QUIPSS II (Quantitative Imaging of Perfusion Using a Single Subtraction-second version)). Perfusion image quality for comparable imaging time was nearly identical for both single-slice and multislice imaging. The measured raw signal was approximately 25% higher with continuous ASL, but the SNR per unit time was identical.

Artifacts↗