Discrepancies between BOLD and flow dynamics in primary and supplementary motor areas: application of the balloon model to the interpretation of BOLD transients

The blood-oxygen-level-dependent (BOLD) signal measured in the brain with functional magnetic resonance imaging (fMRI) during an activation experiment often exhibits pronounced transients at the beginning and end of the stimulus. Such transients could be a reflection of transients in the underlying neural activity, or they could result from transients in cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO2), or cerebral blood volume (CBV). These transients were investigated using an arterial spin labeling (ASL) method that allows simultaneous measurements of BOLD and CBF responses. Responses to a finger-tapping task (40-s stimulus, 80-s rest) were measured in primary motor area (M1) and supplementary motor area (SMA) in five healthy volunteers. In SMA, the average BOLD response was pronounced near the beginning and end of the stimulus, while in M1, the BOLD response was nearly flat. However, CBF responses in the two regions were rather similar, and did not exhibit the same transient features as the BOLD response in SMA. Because this suggests a hemodynamic rather than a neural origin for the transients of the BOLD response in SMA, we used a generalization of the balloon model to test the degree of hemodynamic transients required to produce the measured curves. Both data sets could be approximated with modest differences in the shapes of the CMRO2 and CBV responses. This study illustrates the utility and the limitations of using theoretical models combined with ASL techniques to understand the dynamics of the BOLD response.

BOLD and perfusion response to finger-thumb apposition after acetazolamide administration: differential relationship to global perfusion

The authors studied the effects of altering global cerebral blood flow on both blood oxygen level-dependent (BOLD) response and perfusion response to finger-thumb apposition. A PICORE/QUIPSS II protocol was used to collect interleaved BOLD-weighted and perfusion-weighted images on eight finger-thumb apposition trials. Subjects were studied on a drug-free day and on a day when acetazolamide was administered between the second and third trials. After acetazolamide administration, resting cortical perfusion increased an average of 20% from preadministration levels, whereas the BOLD response to finger-thumb apposition decreased by an average of 35% in the S1M1 hand area. Contrary to predictions from the exhausted cerebrovascular reserve hypothesis and the oxygen limitation model, an effect of acetazolamide on cerebral blood flow response in the S1M1 hand area was not observed. Across the acetazolamide trials, BOLD response was inversely correlated with resting cortical perfusion for individual subject data. These results suggest that resting perfusion affects the magnitude of the BOLD response and is thus an important confounding factor in fMRI studies, and that the physiologic systems that increase cerebral blood flow in response to acetazolamide administration and systems that increase cerebral blood flow in response to altered neural activity appear to have additive effects.

Increased diffusion sensitivity with hyperechos

It is shown that the introduction of a 180 degrees refocusing pulse into a standard diffusion weighted stimulated echo sequence is equivalent to the simplest hyperecho sequence with identical diffusion weighting but equal or greater signal-to-noise (SNR) and thus equal or greater diffusion contrast. For high b-value imaging, the hyperecho sequence thus possesses the high diffusion contrast in the presence of small T(1)/T(2) ratios characteristic of stimulated echo sequences but with less than the 50% loss in SNR that is associated with the stimulated echo. For low b-value imaging, the hyperecho signal converges to that of the standard spin echo. The advantages of the two-pulse diffusion weighted hyperecho sequence are demonstrated theoretically. Experimental results are shown in the application to high angular resolution diffusion encoding (HARD) in normal human brain.

Alternative brain organization after prenatal cerebral injury: convergent fMRI and cognitive data

The current study presents both longitudinal behavioral data and functional activation data documenting the effects of early focal brain injury on the development of spatial analytic processing in two children, one with prenatal left hemisphere (LH) injury and one with right hemisphere (RH) injury. A substantial body of evidence has shown that adults and children with early, lateralized brain injury show evidence of spatial analytic deficits. LH injury compromises the ability to encode the parts of a spatial pattern, while RH injury impairs pattern integration. The two children described in this report show patterns of deficit consistent with the site of their injury. In the current study, their longitudinal behavioral data spanning the age range from preschool to adolescence are presented in conjunction with data from a functional magnetic resonance imaging (fMRI) study of spatial processing. The activation results provide evidence that alternative profiles of neural organization can arise following early focal brain injury, and document where in the brain spatial functions are carried out when regions that normally mediate them are damaged. In addition, the coupling of the activation with the behavioral data allows us to go beyond the simple mapping of functional sites, to ask questions about how those sites may have come to mediate the spatial functions.

Development of a clinical-scale method for generation of dendritic cells from PBMC for use in cancer immunotherapy

BACKGROUND

There is growing interest in the use of dendritic cells (DCs) for treatment of malignancy and infectious disease. Our goal was to develop a clinical scale method to prepare autologous DCs for cancer clinical trials.

METHODS

PBMC were collected from normal donors or cancer patients by automated leukapheresis, purified by counterflow centrifugal elutriation and placed into culture in polystyrene flasks at 1 x 10(6) cells/mL for 5-7 days at 37 degrees C, with 5% CO(2), with IL-4 and GM-CSF. Conditions investigated included media formulation, supplementation with heat in activated allogeneic AB serum or autologous plasma and time to harvest (Day 5 or Day 7). DCs were evaluated for morphology, quantitative yield, viability, phenotype and function, including mixed leukocyte response and recall response to tetanus toxoid and influenza virus.

RESULTS

DCs with a typical immature phenotype (CD14-negative, CD1a-positive, mannose receptor-positive, CD80-positive, CD83-negative) were generated most consistently in RPMI 1640 supplemented with 10% allogeneic AB serum or 10% autologous plasma. Cell yield was higher at Day 5 than Day 7, without detectable differences in phenotype or function. In pediatric sarcoma patients, autologous DCs had enhanced function compared with monocytes from which they were generated. In this patient group, starting with 8.0 +/- 3.7 x 10(8) fresh or cryopreserved autologous monocytes, DC yield was 2.1 +/- 1.0 x 10(8) cells, or 29% of the starting monocyte number.

DISCUSSION

In the optimized clinical-scale method, purified peripheral monocytes are cultured for 5 days in flasks at 1 x 10(6) cells/mL in RPMI 1640, 10% allogeneic AB serum or autologous plasma, IL-4 and GM-CSF. This method avoids the use of FBS and results in immature DCs suitable for clinical trials.

Encoding activity in the medial temporal lobe examined with anatomically constrained fMRI analysis

Functional neuroimaging studies have produced a sizable number of observations of increased activity in the human medial temporal lobe (MTL) during encoding of novel memories. The studies have suggested possible functional specialization within the anatomical components of the MTL (hippocampus and the entorhinal, perirhinal, and parahippocampal cortical areas). Neuroimaging studies have just begun to link anatomical regions to specific functions. To address functional specialization hypothesis, a method is described for using high-resolution structural information from magnetic resonance imaging MRI to constrain the analysis of functional magnetic resonance imaging (fMRI) data, for independent assessment of functional activity change in each component of the MTL. With this method, increased activity was detected throughout the MTL in a group of participants (n = 5) who encoded novel pictures. A separate group (n = 5) who encoded words exhibited lower-levels of evoked activity. Laterality effects were found reflecting increased right hemisphere activity during picture encoding (parahippocampal cortex) and increased left hemisphere activity during word encoding (posterior hippocampus and parahippocampal cortex). Neither condition provided evidence for greater activity in the posterior hippocampus than in the anterior hippocampus during encoding, although the greatest increases in activity were observed in the parahippocampal cortex. The anatomically driven methodology is shown to provide detailed comparison of levels of activity change across specific brain areas and to provide increased sensitivity to functional change in each region of the MTL.

Comparing the brain areas supporting nondeclarative categorization and recognition memory

Brain areas associated with both nondeclarative categorization and recognition memory were identified and contrasted using functional magnetic resonance imaging (fMRI) of healthy volunteers. Activity during dot-pattern categorization and recognition were compared with a control task (counting dots) in two separate groups of participants (n=5 each). The network of areas associated with nondeclarative categorization was found to include bilateral inferior prefrontal and parietal cortical areas that have been implicated in several other studies of categorization. During recognition, increased activity was found in posterior visual areas, the precuneus, posterior cingulate and right prefrontal cortex. Using the common control condition as a reference, recognition and categorization were contrasted and recognition was found to evoke more activity in posterior early visual cortex, the precuneus, right medial temporal lobe and right dorso-lateral prefrontal cortex. Previous research has implicated changes in visual representation in learning a category of dot-pattern by comparing activity evoked by categorical and non-categorical stimuli. The current findings support those results and additionally identify brain areas active during categorization that are involved in expressing this category knowledge.

Functional MRI of global and local processing in children

Functional magnetic resonance imaging was used to examine developmental change in hemispheric biases for globally and locally directed analysis of hierarchical forms. In a previous reaction time (RT) study, which presented hierarchical stimuli to the visual hemifields, children 7 to 14 years of age demonstrated an emerging pattern of hemispheric differences. Initially children analyzed local elements more slowly, without a strongly lateralized advantage for local or global level processing. With age, children's development was marked by a left hemisphere advantage for local level processing that resembled an adult's and a trend toward a right hemisphere advantage for global. In the current study, 20 children 12 to 14 years old were imaged during attend-global and attend-local conditions to determine whether the developmental change in cognitive measures corresponded to a change in distribution of functional activation. Children formed two groups based on their RT performance, immature-bilateral (IB) or mature-lateralized (ML). The volume of task-related activation within lateral temporo-occipital regions of interest was compared for global and local conditions between the two groups. The IB children showed greater activation overall for local level processing, comparable activation across the two hemispheres for the global condition, and a trend of right greater than left hemisphere activation for local. In contrast, the ML children displayed right greater than left hemisphere activation during global analysis and the opposite during local processing. Importantly these patterns of functional activation mirror the profiles of RT performance. Together they demonstrate a shift from undifferentiated, bilateral processing toward hemispheric lateralization.

Analysis and design of perfusion-based event-related fMRI experiments

Perfusion-based functional magnetic resonance imaging (fMRI) using arterial spin labeling (ASL) methods has the potential to provide better localization of the functional signal to the sites of neural activity compared to blood oxygenation level-dependent (BOLD) contrast fMRI. At present, experiments using ASL have been limited to simple block and periodic single-trial designs. We present here an adaptation of the general linear model to perfusion-based fMRI that enables the design and analysis of more complicated designs, such as random and semirandom event-related designs. Formulas for the least-squares estimate of the perfusion response and the F statistic for the detection of a response are derived. Exact expressions and useful approximations for detection power and estimation efficiency are presented, and it is shown that the trade-off between power and efficiency for perfusion experiments is similar to that previously observed for BOLD experiments. The least-squares estimate is compared with an estimate formed from the running subtraction of tag and control images. The running subtraction estimate is shown to be approximately equal to a temporally low-pass-filtered version of the least-squares estimate. Numerical simulations and results from ASL experiments are used to support the theoretical findings.

Design and development of the European American values scale for Asian Americans

Existing instruments for measuring Asian American acculturation emphasize behavior acculturation to the exclusion of value acculturation. Most are based on the assumption that acquisition of European American behavior occurs simultaneously with the loss of Asian behavior. With the advent of the Asian Values Scale (AVS; B.S.K. Kim, D.R. Atkinson, & P.H. Yang, 1999), it is now possible to assess adherence to Asian cultural values. This article describes the development of a scale that can be used to measure Asian American adherence to European American values. The current scale, combined with the AVS, can be used to independently measure Asian American acculturation to European American values and enculturation in Asian values.

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.

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.

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.

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.

Turbo ASL: arterial spin labeling with higher SNR and temporal resolution

A modified pulsed arterial spin labeling (ASL) technique is introduced here that has both higher temporal resolution and higher SNR per unit time than existing ASL techniques. In this technique, the time TI between the application of the tag and image acquisition is longer than the repetition time TR, allowing for the use of greatly reduced TR values without a significant decrease in the amplitude of the ASL signal. This improves both the temporal resolution and the sensitivity of ASL for functional brain mapping.

Characterization of cerebral blood oxygenation and flow changes during prolonged brain activation

The behavior of cerebral blood flow and oxygenation during prolonged brain activation was studied using magnetic resonance imaging (MRI) sensitized to flow and oxygenation changes, as well as positron emission tomography sensitized to flow. Neuronal habituation effects and hemodynamic changes were evaluated across tasks and cortical regions. Nine types of activation stimuli or tasks, including motor activation, vibrotactile stimulation, and several types of visual stimulation, were used. Both flow and oxygenation were evaluated in separate time course series as well as simultaneously using two different MRI methods. In most cases, the activation-induced increase in flow and oxygenation remained elevated for the entire stimulation duration. These results suggest that both flow rate and oxygenation consumption rate remain constant during the entire time that primary cortical neurons are activated by a task or a stimulus.

Comparison of simultaneously measured perfusion and BOLD signal increases during brain activation with T(1)-based tissue identification

Perfusion and blood oxygenation level-dependent (BOLD) signals were simultaneously measured during a finger-tapping task at 3T using QUIPSS II with thin-slice TI(1) periodic saturation, a modified pulsed arterial spin labeling technique that provides quantitative measurement of perfusion. Perfusion and BOLD signal changes due to motor activation were obtained and correlated with the T(1) values estimated from echo-planar imaging (EPI)-based T(1) maps on a voxel-by-voxel basis. The peak perfusion signal occurs in voxels with a T(1) of brain parenchyma while the peak BOLD signal occurs in voxels with a T(1) characteristic of blood and cerebrospinal fluid. The locations of the peak signals of functional BOLD and perfusion only partially overlap on the order of 40%. Perfusion activation maps will likely represent the sites of neuronal activity better than do BOLD activation maps. Magn Reson Med 44:137-143, 2000.

Antibiotic-labeled probes and microvolume fluorimetry for the rapid detection of bacterial contamination in platelet components: a preliminary report

BACKGROUND

Approximately 1 platelet in 2000 components is bacterially contaminated. Most commonly, contaminating organisms are gram positive skin saprophytes (such as Staphylococcus sp. or Bacillus sp.). A novel approach to the rapid diagnosis of gram positive contamination by the use of a fluorescence-labeled antibiotic probe with affinity for the gram positive cell was investigated.

STUDY DESIGN AND METHODS

Two isolates of Staphylococcus epidermidis were inoculated into bags of Day 0 platelets. Quantitative cultures along with a semi-automated screening assay on a microvolume fluorimeter employing a fluorescence-conjugated vancomycin probe was performed for each day of storage. In addition, serial dilutions of the bacteria were added to sterile platelets to achieve a range spanning 10(1) to 10(8) CFUs per mL.

RESULTS

All samples with a bacterial contamination of > or =10(5) CFU per mL were detected. Sterile samples were nonreactive. The entire procedure requires three pipetting steps and took less than 1 hour to perform.

CONCLUSION

These preliminary results with the use of fluorescence-labeled antibiotics as probes combined with microvolume fluorimetry for the rapid detection of bacterial contamination of platelet components suggest that this is a promising approach. Further studies with additional organisms and alternative conjugates, bacteria, and antibiotics are underway.

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.

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.