Venous blood effects in spin-echo fMRI of human brain

Towards functional spin-echo BOLD line-scanning in humans at 7T

OBJECTIVE

Neurons cluster into sub-millimeter spatial structures and neural activity occurs at millisecond resolutions; hence, ultimately, high spatial and high temporal resolutions are required for functional MRI. In this work, we implemented a spin-echo line-scanning (SELINE) sequence to use in high spatial and temporal resolution fMRI.

MATERIALS AND METHODS

A line is formed by simply rotating the spin-echo refocusing gradient to a plane perpendicular to the excited slice and by removing the phase-encoding gradient. This technique promises a combination of high spatial and temporal resolution (250 μm, 500 ms) and microvascular specificity of functional responses. We compared SELINE data to a corresponding gradient-echo version (GELINE).

RESULTS

We demonstrate that SELINE showed much-improved line selection (i.e. a sharper line profile) compared to GELINE, albeit at the cost of a significant drop in functional sensitivity.

DISCUSSION

This low functional sensitivity needs to be addressed before SELINE can be applied for neuroscientific purposes.

Metabolism of hyperpolarised [1-13 C]pyruvate in awake and anaesthetised rat brains

The use of hyperpolarised ¹³ C pyruvate for nononcological neurological applications has not been widespread so far, possibly due to delivery issues limiting the visibility of metabolites. First proof-of-concept results have indicated that metabolism can be detected in human brain, and this may supersede the results obtained in preclinical settings. One major difference between the experimental setups is that preclinical MRI/MRS routinely uses anaesthesia, which alters both haemodynamics and metabolism. Here, we used hyperpolarised [1-¹³ C]pyruvate to compare brain metabolism in awake rats and under isoflurane, urethane or medetomidine anaesthesia. Spectroscopic [1-¹³ C]pyruvate time courses measured sequentially showed that pyruvate-to-bicarbonate and pyruvate-to-lactate labelling rates were lower in isoflurane animals than awake animals. An increased bicarbonate-to-lactate ratio was observed in the medetomidine group compared with other groups. The study shows that hyperpolarised [1-¹³ C]pyruvate experiments can be performed in awake rats, thus avoiding anaesthesia-related issues. The results suggest that haemodynamics probably dominate the observed pyruvate-to-metabolite labelling rates and area-under-time course ratios of referenced to pyruvate. On the other hand, the results obtained with medetomidine suggest that the ratios are also modulated by the underlying cerebral metabolism. However, the ratios between intracellular metabolites were unchanged in awake compared with isoflurane-anaesthetised rats.

Assessing motor, visual and language function using a single 5-minute fMRI paradigm: three birds with one stone

Clinical functional Magnetic Resonance Imaging (fMRI) requires inferences on localization of major brain functions at the individual subject level. We hypothesized that a single "triple use" task would satisfy sensitivity and reliability requirements for successfully assessing the motor, visual and language domain in this context. This was tested here by the application in a group of healthy adults, assessing sensitivity and reliability at the individual subject level, separately for each domain.Our "triple use" task consisted of 2 conditions (condition 1, assessing motor and visual domain, and condition 2, assessing the language domain), serving mutually as active/control. We included 20 healthy adult subjects. Random effect analyses showed activation in primary motor, visual and language regions, as expected. Less expected regions were activated both for the motor and visual domains. Further, reliability of primary activation patterns was very high across individual subjects, with activation seen in 70-100% of subjects in primary motor, visual, and left-lateralized language regions.These findings suggest the "triple use" task to be reliable at the individual subject's level to assess motor, visual and language domains in the clinical fMRI context. Benefits of such an approach include shortening of acquisition time, simplicity of the task for each domain, and using a visual stimulus. Following establishment of reliability in adults, the task may also be a valuable addition in the pediatric clinical fMRI context, where each of these factors is of high relevance.

A critical assessment of data quality and venous effects in sub-millimeter fMRI

Advances in hardware, pulse sequences, and reconstruction techniques have made it possible to perform functional magnetic resonance imaging (fMRI) at sub-millimeter resolution while maintaining high spatial coverage and acceptable signal-to-noise ratio. Here, we examine whether sub-millimeter fMRI can be used as a routine method for obtaining accurate measurements of fine-scale local neural activity. We conducted fMRI in human visual cortex during a simple event-related visual experiment (7 T, gradient-echo EPI, 0.8-mm isotropic voxels, 2.2-s sampling rate, 84 slices), and developed analysis and visualization tools to assess the quality of the data. Our results fall along three lines of inquiry. First, we find that the acquired fMRI images, combined with appropriate surface-based processing, provide reliable and accurate measurements of fine-scale blood oxygenation level dependent (BOLD) activity patterns. Second, we show that the highly folded structure of cortex causes substantial biases on spatial resolution and data visualization. Third, we examine the well-recognized issue of venous contributions to fMRI signals. In a systematic assessment of large sections of cortex measured at a fine scale, we show that time-averaged T2*-weighted EPI intensity is a simple, robust marker of venous effects. These venous effects are unevenly distributed across cortex, are more pronounced in gyri and outer cortical depths, and are, to a certain degree, in consistent locations across subjects relative to cortical folding. Furthermore, we show that these venous effects are strongly correlated with BOLD responses evoked by the experiment. We conclude that sub-millimeter fMRI can provide robust information about fine-scale BOLD activity patterns, but special care must be exercised in visualizing and interpreting these patterns, especially with regards to the confounding influence of the brain's vasculature. To help translate these methodological findings to neuroscience research, we provide practical suggestions for both high-resolution and standard-resolution fMRI studies.

Imaging at ultrahigh magnetic fields: History, challenges, and solutions

Following early efforts in applying nuclear magnetic resonance (NMR) spectroscopy to study biological processes in intact systems, and particularly since the introduction of 4 T human scanners circa 1990, rapid progress was made in imaging and spectroscopy studies of humans at 4 T and animal models at 9.4 T, leading to the introduction of 7 T and higher magnetic fields for human investigation at about the turn of the century. Work conducted on these platforms has provided numerous technological solutions to challenges posed at these ultrahigh fields, and demonstrated the existence of significant advantages in signal-to-noise ratio and biological information content. Primary difference from lower fields is the deviation from the near field regime at the radiofrequencies (RF) corresponding to hydrogen resonance conditions. At such ultrahigh fields, the RF is characterized by attenuated traveling waves in the human body, which leads to image non-uniformities for a given sample-coil configuration because of destructive and constructive interferences. These non-uniformities were initially considered detrimental to progress of imaging at high field strengths. However, they are advantageous for parallel imaging in signal reception and transmission, two critical technologies that account, to a large extend, for the success of ultrahigh fields. With these technologies and improvements in instrumentation and imaging methods, today ultrahigh fields have provided unprecedented gains in imaging of brain function and anatomy, and started to make inroads into investigation of the human torso and extremities. As extensive as they are, these gains still constitute a prelude to what is to come given the increasingly larger effort committed to ultrahigh field research and development of ever better instrumentation and techniques.

What is feasible with imaging human brain function and connectivity using functional magnetic resonance imaging

When we consider all of the methods we employ to detect brain function, from electrophysiology to optical techniques to functional magnetic resonance imaging (fMRI), we do not really have a 'golden technique' that meets all of the needs for studying the brain. We have methods, each of which has significant limitations but provide often complimentary information. Clearly, there are many questions that need to be answered about fMRI, which unlike other methods, allows us to study the human brain. However, there are also extraordinary accomplishments or demonstration of the feasibility of reaching new and previously unexpected scales of function in the human brain. This article reviews some of the work we have pursued, often with extensive collaborations with other co-workers, towards understanding the underlying mechanisms of the methodology, defining its limitations, and developing solutions to advance it. No doubt, our knowledge of human brain function has vastly expanded since the introduction of fMRI. However, methods and instrumentation in this dynamic field have evolved to a state that discoveries about the human brain based on fMRI principles, together with information garnered at a much finer spatial and temporal scale through other methods, are poised to significantly accelerate in the next decade.This article is part of the themed issue 'Interpreting BOLD: a dialogue between cognitive and cellular neuroscience'.

Multi-phase passband balanced SSFP fMRI with 50ms sampling rate at 7Tesla enables high precision in resolving 100ms neuronal events

Passband balanced steady state free precession (b-SSFP) fMRI employs the flat portion of the SSFP off-resonance response to obtain microscopic susceptibility changes elicited by changes in blood oxygenation following enhancement in neuronal activity. This technique can reduce geometric distortion and signal dropout while maintaining rapid acquisition and high signal-to-noise ratio (SNR) compared with traditional fMRI techniques. In the study, we developed a novel multi-phase passband b-SSFP fMRI technique that can achieve a spatial resolution of a few mm³ and a high temporal sampling rate of 50ms per slice at 7Tesla. This technique was further applied for an event-related (ER) fMRI paradigm. As a comparison, gradient-echo echo-planar imaging (GE-EPI) with similar spatial resolution and temporal sampling rate was carried out for the same ER-fMRI experiment. Experiments with visual cortex stimulation were carried out at 7Tesla to demonstrate whether the multi-phase b-SSFP technique and GE-EPI are able to differentiate temporal delays in hemodynamic response function (HRF) separated by 100ms in stimulus onset. Consistent with ERP results, the upslope of the HRF of both techniques can differentiate 100ms delay in stimulus onset, with the former showing a lower level of intersubject variability. The present study demonstrated that the multi-phase passband b-SSFP fMRI technique can be applied for resolving neuronal events on the order of 100ms at ultrahigh magnetic fields.

Vessel-specific quantification of blood oxygenation with T2-relaxation-under-phase-contrast MRI

PURPOSE

Measurement of venous oxygenation (Yv) is a critical step toward quantitative assessment of brain oxygen metabolism, a key index in many brain disorders. The present study aims to develop a noninvasive, rapid, and reproducible method to measure Yv in a vessel-specific manner.

THEORY

The method, T2-Relaxation-Under-Phase-Contrast MRI, utilizes complex subtraction of phase-contrast to isolate pure blood signal, applies nonslice-selective T2-preparation to measure T2, and converts T2 to oxygenation using a calibration plot.

METHODS

Following feasibility demonstration, several technical aspects were examined, including validation with an established global Yv technique, test-retest reproducibility, sensitivity to detect oxygenation changes due to hypoxia and caffeine challenges, applicability of echo-planar-imaging (EPI) acquisition to shorten scan duration, and ability to study veins with a caliber of 1-2 mm.

RESULTS

T2-Relaxation-Under-Phase-Contrast was able to simultaneously measure Yv in all major veins in the brain, including sagittal sinus, straight sinus, great vein, and internal cerebral vein. T2-Relaxation-Under-Phase-Contrast results showed an excellent agreement with the reference technique, high sensitivity to oxygenation changes, and test-retest variability of 3.5 ± 1.0%. The use of segmented-EPI was able to reduce the scan duration to 1.5 minutes. It was also feasible to study pial veins and deep veins.

CONCLUSION

T2-Relaxation-Under-Phase-Contrast MRI is a promising technique for vessel-specific oxygenation measurement.

Data collection and analysis strategies for phMRI

Although functional MRI traditionally has been applied mainly to study changes in task-induced brain function, evolving acquisition methodologies and improved knowledge of signal mechanisms have increased the utility of this method for studying responses to pharmacological stimuli, a technique often dubbed "phMRI". The proliferation of higher magnetic field strengths and the use of exogenous contrast agent have boosted detection power, a critical factor for successful phMRI due to the restricted ability to average multiple stimuli within subjects. Receptor-based models of neurovascular coupling, including explicit pharmacological models incorporating receptor densities and affinities and data-driven models that incorporate weak biophysical constraints, have demonstrated compelling descriptions of phMRI signal induced by dopaminergic stimuli. This report describes phMRI acquisition and analysis methodologies, with an emphasis on data-driven analyses. As an example application, statistically efficient data-driven regressors were used to describe the biphasic response to the mu-opioid agonist remifentanil, and antagonism using dopaminergic and GABAergic ligands revealed modulation of the mesolimbic pathway. Results illustrate the power of phMRI as well as our incomplete understanding of mechanisms underlying the signal. Future directions are discussed for phMRI acquisitions in human studies, for evolving analysis methodologies, and for interpretative studies using the new generation of simultaneous PET/MRI scanners. This article is part of the Special Issue Section entitled 'Neuroimaging in Neuropharmacology'.

The physics of functional magnetic resonance imaging (fMRI)

Functional magnetic resonance imaging (fMRI) is a methodology for detecting dynamic patterns of activity in the working human brain. Although the initial discoveries that led to fMRI are only about 20 years old, this new field has revolutionized the study of brain function. The ability to detect changes in brain activity has a biophysical basis in the magnetic properties of deoxyhemoglobin, and a physiological basis in the way blood flow increases more than oxygen metabolism when local neural activity increases. These effects translate to a subtle increase in the local magnetic resonance signal, the blood oxygenation level dependent (BOLD) effect, when neural activity increases. With current techniques, this pattern of activation can be measured with resolution approaching 1 mm(3) spatially and 1 s temporally. This review focuses on the physical basis of the BOLD effect, the imaging methods used to measure it, the possible origins of the physiological effects that produce a mismatch of blood flow and oxygen metabolism during neural activation, and the mathematical models that have been developed to understand the measured signals. An overarching theme is the growing field of quantitative fMRI, in which other MRI methods are combined with BOLD methods and analyzed within a theoretical modeling framework to derive quantitative estimates of oxygen metabolism and other physiological variables. That goal is the current challenge for fMRI: to move fMRI from a mapping tool to a quantitative probe of brain physiology.

Noninvasive functional imaging of cerebral blood volume with vascular-space-occupancy (VASO) MRI

Functional MRI (fMRI) based on changes in cerebral blood volume (CBV) can probe directly vasodilatation and vasoconstriction during brain activation or physiologic challenges, and can provide important insights into the mechanism of blood oxygenation level-dependent (BOLD) signal changes. At present, the most widely used CBV fMRI technique in humans is called vascular-space-occupancy (VASO) MRI, and this article provides a technical review of this method. VASO MRI utilizes T1 differences between blood and tissue to distinguish between these two compartments within a voxel, and employs a blood-nulling inversion recovery sequence to yield an MR signal proportional to 1 - CBV. As such, vasodilatation will result in a VASO signal decrease and vasoconstriction will have the reverse effect. The VASO technique can be performed dynamically with a temporal resolution comparable with several other fMRI methods, such as BOLD or arterial spin labeling (ASL), and is particularly powerful when conducted in conjunction with these complementary techniques. The pulse sequence and imaging parameters of VASO can be optimized such that the signal change is predominantly of CBV origin, but careful considerations should be taken to minimize other contributions, such as those from the BOLD effect, cerebral blood flow (CBF) and cerebrospinal fluid (CSF). The sensitivity of the VASO technique is the primary disadvantage when compared with BOLD, but this technique is increasingly demonstrating its utility in neuroscientific and clinical applications.

Functional MRI in human subjects with gradient-echo and spin-echo EPI at 9.4 T

PURPOSE

The increased signal-to-noise ratio and blood oxygen level dependent signal at ultra-high field can only help to boost the resolution in functional MRI studies if the spatial specificity of the activation signal is improved. At a field strength of 9.4 T, both gradient-echo and spin-echo based echo-planar imaging were implemented and applied to investigate the specificity of human functional MRI. A finger tapping paradigm was used to acquire functional MRI data with scan parameters similar to standard neuroscientific applications.

METHODS

Spatial resolution, echo, and readout times were varied to determine their influence on the distribution of the blood oxygen level dependent signal. High-resolution co-localized images were used to classify the signal according to its origin in veins or tissue.

RESULTS

High-quality activation maps were obtained with both sequences. Signal contributions from tissue were found to be smaller or slightly larger than from veins. Gradient-echo echo-planar imaging yielded lower ratios of micro-/macro-vascular signals of around 0.6 than spin-echo based functional MRI, where this ratio varied between 0.75 and 1.02, with higher values for larger echo and shorter readout time.

CONCLUSION

This study demonstrates the feasibility of human functional MRI at 9.4 T with high spatial specificity. Although venous contributions could not be entirely suppressed, venous effects in spin-echo echo-planar imaging are significantly reduced compared with gradient-echo echo-planar imaging.

Vascular space occupancy MRI during breathholding at 3 Tesla

PURPOSE

To evaluate the vasodilatory response of normal human brain and meningiomas under repeated breathholding challenges using vascular space occupancy (VASO) MRI at 3 Tesla (T).

MATERIALS AND METHODS

Five normal volunteers and five patients with meningiomas were recruited for this study. For the normal group, VASO MRI during repeated breathholds of different duration (5 to 30 s) was acquired. Patients performed a 15-s breathhold paradigm for VASO MRI. The maximum signal change and full-width at half-maximum (FWHM) were determined by curve fitting.

RESULTS

Significant VASO signal decreases in the gray matter could be detected for a breathhold period as short as 5 s. The fractional activation volume vs. breathhold duration reached a plateau around 34.21 ± 3.39% at 15 s. In the patient group, there were significant VASO signal decreases in normal gray matters and also in small areas of three large-sized meningiomas.

CONCLUSION

The 3T VASO MRI detected significant signal decreases in the gray matter, but not in the white matter, during short periods of breathholding. The fractional activation volume reached the plateau at 15-s breathhold, which is recommended for clinical application.

Spatial location and strength of BOLD activation in high-spatial-resolution fMRI of the motor cortex: a comparison of spin echo and gradient echo fMRI at 7 T

The increased blood oxygenation level-dependent contrast-to-noise ratio at ultrahigh field (7 T) has been exploited in a comparison of the spatial location and strength of activation in high-resolution (1.5  mm isotropic) gradient echo (GE) and spin echo (SE), echo planar imaging data acquired during the execution of a simple motor task in five subjects. SE data were acquired at six echo times from 30 to 55  ms. Excellent fat suppression was achieved in the SE echo planar images using slice-selective gradient reversal. Threshold-free cluster enhancement was used to define regions of interest (ROIs) containing voxels showing significant stimulus-locked signal changes from the GE and average SE data. These were used to compare the signal changes and spatial locations of activated regions in SE and GE data. T(2) and T(2)* values were measured, with means of 48.3 ± 1.1  ms and 36.5 ± 3.4  ms in the SE ROI. In addition, we identified a dark band in SE images of the motor cortex corresponding to a region in which T(2) and T(2)* were significantly lower than in the surrounding grey matter. The fractional SE signal change in the ROI was found to vary linearly as a function of TE, with a slope that was dependent on the particular ROI assessed: the mean ΔR(2) value was found to be 0.85 ± 0.11  s(-1) for the SE ROI and -0.37 ± 0.05  s(-1) for the GE ROI. The fractional signal change relative to the shortest TE revealed that the largest signal change occurred at a TE of 45  ms outside of the dark band. At this TE, the ratio of the fractional signal change in GE and SE data was found to be 0.48 ± 0.05. Phase maps produced from high-resolution GE images spanning the right motor cortex were used to identify veins. The GE ROI was found to contain 18% more voxels overlying the venous mask than the SE ROI.

The BOLD post-stimulus undershoot, one of the most debated issues in fMRI

This paper provides a brief overview of how we got involved in fMRI work and of our efforts to elucidate the mechanisms underlying BOLD signal changes. The phenomenon discussed here in particular is the post-stimulus undershoot (PSU), the interpretation of which has captivated many fMRI scientists and is still under debate to date. This controversy is caused both by the convoluted physiological origin of the BOLD effect, which allows many possible explanations, and the lack of comprehensive data in the early years. BOLD effects reflect changes in cerebral blood flow (CBF), volume (CBV), metabolic rate of oxygen (CMRO(2)), and hematocrit fraction (Hct). However, the size of such effects is modulated by vascular origin such as intravascular, extravascular, macro and microvascular, venular and capillary, the relative contributions of which depend not only on the spatial resolution of the measurements, but also on stimulus duration, on magnetic field strength and on whether spin echo (SE) or gradient echo (GRE) detection is used. The two most dominant explanations of the PSU have been delayed vascular compliance (first venular, later arteriolar, and recently capillary) and sustained increases in CMRO(2), while post-activation reduction in CBF is a distant third. MRI has the capability to independently measure CBF and arteriolar, venous, and total CBV contributions in humans and animals, which has been of great assistance in improving the understanding of BOLD phenomena. Using currently available MRI and optical data, we conclude that the predominant PSU origin is a sustained increase in CMRO(2). However, some contributions from delayed vascular compliance are likely, and small CBF undershoot contributions that are difficult to detect with current arterial spin labeling technology can also not be excluded. The relative contribution of these different processes, which are not mutually exclusive and can act together, is likely to vary with stimulus duration and type.

Using manganese-enhanced MRI to understand BOLD

The 1990s were designated "The Decade of the Brain" by U.S. Congress, perhaps in great anticipation of the impact that functional neuroimaging techniques would have on advancing our understanding of how the brain is functionally organized. While it is impossible to overestimate the impact of functional MRI in neuroscience, many aspects of the blood oxygenation level-dependent (BOLD) contrast remain poorly understood, in great part due to the complex relationship between neural activity and hemodynamic changes. To better understand such relationship, it is important to probe neural activity independently. Manganese-enhanced MRI (MEMRI), when used to monitor neural activity, is a technique that uses the divalent manganese ion, Mn(2+), as a surrogate measure of calcium influx. A major advantage of using Mn(2+) as a functional marker is that the contrast obtained is directly related to the accumulation of the ion in excitable cells in an activity dependent manner. As such, the contrast in MEMRI is more directly related to neural activity then hemodynamic-based fMRI techniques. In the present work, the early conceptualization of MEMRI is reviewed, and the comparative experiments that have helped provide a better understanding of the spatial specificity of BOLD signal changes in the cortex is discussed.

Advances in High-Field BOLD fMRI

This review article examines the current state of BOLD fMRI at a high magnetic field strength of 7 Tesla. The following aspects are covered: a short description of the BOLD contrast, spatial and temporal resolution, BOLD sensitivity, localization and spatial specificity, technical challenges as well as an outlook on future developments are given. It is shown that the main technical challenges of performing BOLD fMRI at high magnetic field strengths-namely development of array coils, imaging sequences and parallel imaging reconstruction-have been solved successfully. The combination of these developments has lead to the availability of high-resolution BOLD fMRI protocols that are able to cover the whole brain with a repetition time (TR) shorter than 3 s. The structural information available from these high-resolution fMRI images itself is already very detailed, which helps to co-localize structure and function. Potential future applications include whole-brain connectivity analysis on a laminar resolution and single subject examinations.

On the numerically predicted spatial BOLD fMRI specificity for spin echo sequences

This work utilises general numerical magnetic resonance imaging MRI simulations to predict the spatial specificity of the blood oxygenation level-dependent (BOLD) functional MRI (fMRI) signal. A Monte Carlo simulation approach was utilized on a microvascular structure consisting of randomly oriented cylinders representing blood vessels. This framework was employed to numerically investigate the spatial specificity, defined as ratio of pial vessel to microvascular signal, of the spin echo BOLD fMRI signal as a function of field strength, echo time and tissue types [grey matter (GM) and cerebrospinal fluid (CSF), respectively]. Spatial specificity of spin echo BOLD fMRI signal was determined to increase with field strength up to 16 T and with maximal specificity for echo time shorter than tissue T(2). In addition, it was found that, for large pial vessels, the extravascular signal decay could not be described using the oversimplified but nevertheless commonly employed mono-exponential signal decay approximation (MEA). Consequently, a recently proposed model relying on the MEA deviates substantially from our results on the spatial specificity. A refinement of this model is proposed based on an available, more detailed signal description. Finally, the effect of CSF on the spatial specificity was investigated. While a large spatial specificity of the spin echo BOLD fMRI signal is observed if a pial vessel is surrounded by grey matter, this is greatly reduced for a pial vessel situated on a GM/CSF interface, rendering the suppression of pial vessels on the cortex surface unlikely.

Functional localization in the human brain: Gradient-Echo, Spin-Echo, and arterial spin-labeling fMRI compared with neuronavigated TMS

A spatial mismatch of up to 14 mm between optimal transcranial magnetic stimulation (TMS) site and functional magnetic resonance imaging (fMRI) signal has consistently been reported for the primary motor cortex. The underlying cause might be the effect of magnetic susceptibility around large draining veins in Gradient-Echo blood oxygenation level-dependent (GRE-BOLD) fMRI. We tested whether alternative fMRI sequences such as Spin-Echo (SE-BOLD) or Arterial Spin-Labeling (ASL) assessing cerebral blood flow (ASL-CBF) may localize neural activity closer to optimal TMS positions and primary motor cortex than GRE-BOLD. GRE-BOLD, SE-BOLD, and ASL-CBF signal changes during right thumb abductions were obtained from 15 healthy subjects at 3 Tesla. In 12 subjects, tissue at fMRI maxima was stimulated with neuronavigated TMS to compare motor-evoked potentials (MEPs). Euclidean distances between the fMRI center-of-gravity (CoG) and the TMS motor mapping CoG were calculated. Highest SE-BOLD and ASL-CBF signal changes were located in the anterior wall of the central sulcus [Brodmann Area 4 (BA4)], whereas highest GRE-BOLD signal changes were significantly closer to the gyral surface. TMS at GRE-BOLD maxima resulted in higher MEPs which might be attributed to significantly higher electric field strengths. TMS-CoGs were significantly anterior to fMRI-CoGs but distances were not statistically different across sequences. Our findings imply that spatial differences between fMRI and TMS are unlikely to be caused by spatial unspecificity of GRE-BOLD fMRI but might be attributed to other factors, e.g., interactions between TMS-induced electric field and neural tissue. Differences between techniques should be kept in mind when using fMRI coordinates as TMS (intervention) targets.

Contrast Response Functions for Single Gabor Patches: ROI-Based Analysis Over-Represents Low-Contrast Patches for GE BOLD

IMPORTANT FOR THE INTERPRETATION OF BOLD FMRI DATA IS A LINEAR RELATIONSHIP BETWEEN THE BOLD RESPONSE AND THE UNDERLYING NEURAL ACTIVITY: increased BOLD responses should reflect proportionate increases in the underlying neural activity. While previous studies have demonstrated a linear relationship between the peak amplitude of the BOLD response and neural activity in primary visual cortex (V1), these studies have used stimuli that excite large areas of cortex, and the linearity of the BOLD response has not been demonstrated when only a small patch of cortex is stimulated. The BOLD response to isolated Gabor patches of increasing contrast was measured with gradient echo (GE) BOLD and spin echo (SE) BOLD at 7 T. Our primary finding is notable spatial heterogeneity of the BOLD contrast response, particularly for the GE BOLD data, resulting in a more reliably linear relationship between BOLD data and estimated neural responses in the center of the cortical representations of the individual Gabor patches than near the edges. A control experiment with larger sinusoidal grating patches confirms that the observed sensitivity to voxel selection in the regions of interest-based analysis is unique to the small stimuli.

BOLD fMRI using a modified HASTE sequence

For more than a decade, turbo spin echo (TSE) pulse sequences have been suggested as an alternative to echo planar imaging (EPI) sequences for fMRI studies. Recent development in parallel imaging has renewed the interest in developing more robust TSE sequences for fMRI. In this study, a modified half Fourier acquisition single-shot TSE (mHASTE) sequence has been developed with a three-fold GRAPPA to improve temporal resolution as well as a preparation time to enhance BOLD sensitivity. Using a classical flashing checkerboard block design, the BOLD signal characteristics of this novel method have been systematically analyzed as a function of several sequence parameters and compared to those of gradient-echo and spin-echo EPI sequences. Experimental studies on visual cortex of five volunteers have provided evidence suggesting that mHASTE can be more sensitive to extra-vascular BOLD effects around microvascular networks, which leads to more accurate function localization. The studies also show that the activation cluster size in mHASTE increases with the refocusing RF flip angle and TE while decreasing with the echo number (n(center)) used to sample the k-space center. Compared to spin-echo EPI, mHASTE incurs an approximately 50% reduction in activation cluster size and an approximately 20% decrease in BOLD contrast. However a higher signal-to-noise ratio and a spatially more uniform temporal stability have been observed in mHASTE as compared to the EPI sequences when the scan times are held constant. With further refinement and optimization, mHASTE can become a viable alternative for fMRI in situations where the conventional EPI sequences are limited or prohibitive.

Quantification of venous blood signal contribution to BOLD functional activation in the auditory cortex at 3 T

Most modern techniques for functional magnetic resonance imaging (fMRI) rely on blood-oxygen-level-dependent (BOLD) contrast as the basic principle for detecting neuronal activation. However, the measured BOLD effect depends on a transfer function related to neurophysiological changes accompanying electrical neural activation. The spatial accuracy and extension of the region of interest are determined by vascular effect, which introduces incertitude on real neuronal activation maps. Our efforts have been directed towards the development of a new methodology that is capable of combining morphological, vascular and functional information; obtaining new insight regarding foci of activation; and distinguishing the nature of activation on a pixel-by-pixel basis. Six healthy volunteers were studied in a parametric auditory functional experiment at 3 T; activation maps were overlaid on a high-resolution brain venography obtained through a novel technique. The BOLD signal intensities of vascular and nonvascular activated voxels were analyzed and compared: it was shown that nonvascular active voxels have lower values for signal peak (P<10(-7)) and area (P<10(-8)) with respect to vascular voxels. The analysis showed how venous blood influenced the measured BOLD signals, supplying a technique to filter possible venous artifacts that potentially can lead to misinterpretation of fMRI results. This methodology, although validated in the auditory cortex activation, maintains a general applicability to any cortical fMRI study, as the basic concepts on which it relies on are not limited to this cortical region. The results obtained in this study can represent the basis for new methodologies and tools that are capable of adding further characterization to the BOLD signal properties.

Ultra-high field parallel imaging of the superior parietal lobule during mental maze solving

We used ultra-high field (7 T) fMRI and parallel imaging to scan the superior parietal lobule (SPL) of human subjects as they mentally traversed a maze path in one of four directions (up, down, left, right). A counterbalanced design for maze presentation and a quasi-isotropic voxel (1.46 x 1.46 x 2 mm thick) collection were implemented. Fifty-one percent of single voxels in the SPL were tuned to the direction of the maze path. Tuned voxels were distributed throughout the SPL, bilaterally. A nearest neighbor analysis revealed a "honeycomb" arrangement such that voxels tuned to a particular direction tended to occur in clusters. Three-dimensional (3D) directional clusters were identified in SPL as oriented centroids traversing the cortical depth. There were 13 same-direction clusters per hemisphere containing 22 voxels per cluster, on the average; the mean nearest-neighbor, same-direction intercluster distance was 9.4 mm. These results provide a much finer detail of the directional tuning in SPL, as compared to those obtained previously at 4 T (Gourtzelidis et al. Exp Brain Res 165:273-282, 2005). The more accurate estimates of quantitative clustering parameters in 3D brain space in this study were made possible by the higher signal-to-noise and contrast-to-noise ratios afforded by the higher magnetic field of 7 T as well as the quasi-isotropic design of voxel data collection.

Spin-echo MRI underestimates functional changes in microvascular cerebral blood plasma volume using exogenous contrast agent

While most functional MRI studies using exogenous contrast agent employ gradient-echo (GE) signal, spin echo (SE) imaging would represent an attractive alternative if its detection power were more comparable with GE imaging. This study demonstrates that SE methods systematically underestimate functional changes in microvascular cerebral blood plasma volume (CBV), so that SE detection power in brain tissue cannot match that provided by GE signal. Empirically, the in vivo response of SE-CBV was about 40% smaller than that of GE-CBV in rat brain at low basal values of CBV, a result that is consistent with physics predictions under the simplifying assumption of uniform vessel dilation. However, increasing values of basal CBV were associated with monotonically increasing mean vessel sizes and monotonically decreasing GE to SE ratios of functional changes in CBV (fCBV). This result suggests the presence of large but weakly reactive conduit vessels at high basal values of CBV. Hence, we conclude that GE imaging is the method of choice for functional MRI (fMRI) using exogenous contrast agent in most cases, although SE methods may represent a more spatially linear representation of underlying neural activity that becomes most apparent in regions with high basal CBV, such as the cortical surface.

CA3 NMDA receptors are required for experience-dependent shifts in hippocampal activity

The anatomical distribution of sensory-evoked activity recorded from the hippocampal long-axis can shift depending on prior experience. In accordance with Marr's computational model of hippocampal function, CA3 NMDA receptors have been hypothesized to mediate this experience-dependent shift in hippocampal activity. Here we tested this hypothesis by investigating genetically-modified mice in which CA3 NMDA receptors are selectively knocked-out (CA3-NR1 KO). First, we were required to develop an fMRI protocol that can record sensory-evoked activity from the mouse hippocampal long-axis. This goal was achieved in part by using a dedicated mouse scanner to image odor-evoked activity, and by using non-EPI (echo planer imaging) pulse sequences. As in humans, odors were found to evoke a ventral-predominant activation pattern in the mouse hippocampus. More importantly, odor-evoked activity shifted in an experience-dependent manner. Finally, we found that the experience-dependent shift in hippocampal long-axis activity is blocked in CA3-NR1 knock-out mice. These findings establish a cellular mechanism for the plasticity imaged in the hippocampal long-axis, suggesting how experience-dependent modifications of hippocampal activity can contribute to its mnemonic function.

Very high-resolution three-dimensional functional MRI of the human visual cortex with elimination of large venous vessels

We propose a very high-resolution, three-dimensional (3D) gradient-echo technique with a twofold parallel imaging acceleration using a specialized occipital receiver coil at 3 T to perform functional MRI (fMRI) of the visual cortex. This configuration makes it possible to acquire 3D fMRI data within a timescale compatible with a block design. Without further processing, the functional maps at an isotropic 3D resolution of 0.42 microL (0.75 mm voxel size) and near-isotropic resolution of 1.2 microL (1 mm voxel size) show very robust activation in visual areas, but with clear contamination from larger veins. As this technique allows direct identification of veins in the functional scan, it permits removal of their effect from the activation maps. In our study, elimination of veins qualitatively improves the spatial specificity of activation maps, while reducing the activated volume by about 25%. The proposed technique provides functional information at the resolution of anatomical scans, is localized to gray matter, and facilitates functional to anatomical co-registration because of minimal distortions.

Complex data analysis in high-resolution SSFP fMRI

In transition-band steady-state free precession (SSFP) functional MRI (fMRI), functional contrast originates from a bulk frequency shift induced by a deoxygenated hemoglobin concentration change in the activated brain regions. This frequency shift causes a magnitude and/or phase-signal change depending on the off-resonance distribution of a voxel in the balanced-SSFP (bSSFP) profile. However, in early low-resolution studies, only the magnitude signal activations were shown. In this paper the task-correlated phase-signal change is presented in a high-resolution (1 x 1 x 1 mm3) study. To include this phase activation in a functional analysis, a new complex domain data analysis method is proposed. The results show statistically significant phase-signal changes in a large number of voxels comparable to that of the magnitude-activated voxels. The complex-data analysis method successfully includes these phase activations in the activation map and thus provides wider coverage compared to magnitude-data analysis results.

Quantification of fMRI BOLD signal and volume applied to the somatosensory cortex

Functional magnetic resonance imaging based on blood-oxygenation-level-dependent (BOLD) signal variations is clinically used to investigate the impact of neurological disorders on brain function. Such disorders effect not only the localization but also the amplitude and extent of the BOLD signal. Statistical methods are useful to localize the BOLD signal but fail to quantify functional activity because they rely on arbitrary thresholds. This article presents a method that uses a priori defined VOI (volume of interest) and independently quantifies the mean BOLD signal and extent of the activated volume. The technique is based on the separation of the VOI signal difference distribution into a noise and an activation contribution. The technique does not require any threshold and is nearly independent of the preselected VOI size. The technique was verified in a test group of 17 subjects performing bilateral finger tapping. The results were compared with those of conventional analysis based on statistical tools. A standard imaging technique using FID-EPI (free induction decay echo-planar imaging, TR = 4000 ms, TE = 66 ms, 60 images activation, 60 images rest) was employed. The activated volume, V, and signal difference, deltaS, of the motor cortex were determined with an accuracy of sigma(V) = 17.1% and sigma(deltaS) = 3.6%, respectively. The activated volume of the left hemispheric motor area was significantly greater (P = 0.025) then in the right hemispheric, VL = 7.35 +/- 2.29 cm3 versus VR = 6.39 +/- 2.34 cm3. The result is consistent with the findings obtained by other techniques. On the other hand, the statistical methods did not yield any significant difference in activation between both hemispheres. The VOI-based method presented here is an additional tool to study the extent and amplitude of the BOLD signal.

Fast spin echo sequences for BOLD functional MRI

At higher field strengths, spin echo (SE) functional MRI (fMRI) is an attractive alternative to gradient echo (GE) as the increased weighting towards the microvasculature results in intrinsically better localization of the BOLD signal. Images are free of signal voids but the commonly used echo planar imaging (EPI) sampling scheme causes geometric distortions, and T₂* effects often contribute considerably to the signal changes measured upon brain activation. Multiply refocused SE sequences such as fast spin echo (FSE) are essentially artifact free but their application to fast fMRI is usually hindered due to high energy deposition, and long sampling times. In the work presented here, a combination of parallel imaging and partial Fourier acquisition is used to shorten FSE acquisition times to near those of conventional SE-EPI, permitting sampling of eight slices (matrix 64  ×  64) per second. Signal acquisition is preceded by a preparation experiment that aims at increasing the relative contribution of extravascular dynamic averaging to the BOLD signal. Comparisons are made with conventional SE-EPI using a visual stimulation paradigm. While the observed signal changes are approximately 30% lower, most likely due to the absence of T₂* contamination, activation size and t-scores are comparable for both methods, suggesting that HASTE fMRI is a viable alternative, particularly if distortion free images are required. Our data also indicate that the BOLD post-stimulus undershoot is most probably attributable to persistent elevated oxygen metabolism rather than to delayed vascular compliance.

Principles of magnetic resonance assessment of brain function

MRI has advanced to being one of the major tools for the assessment of brain function. This review article examines the basic principles that underpin these measurements. The main emphasis is on the characteristics and detection of blood oxygen level dependent (BOLD) contrast. In the first part of the article the relationship between BOLD, blood flow, blood oxygen, and the rate of metabolic consumption of oxygen is described. The four contrast mechanisms that contribute to the BOLD signal change, namely extravascular static and dynamic dephasing, intravascular T2-like changes, and the intravascular frequency offset effect are described in terms of their spatial localization and relative contributions to the BOLD signal. The current model of changes in blood flow being an indirect consequence of synaptic input to a region is presented. The second section of the article deals with the imaging characteristics of BOLD in terms of the attainable spatial resolution and linear system characteristics. In the third section, practical BOLD imaging is examined for choice of pulse sequence, resolution, echo time (TE), repetition time (TR), and flip angle. The final section touches on other MRI approaches that are relevant to cognitive neuroimaging, in particular the measurement of blood flow, blood volume, resting state fluctuations in the BOLD signal, and measures of connectivity using diffusion tensor imaging and fiber-tracking.

Cortical layer-dependent BOLD and CBV responses measured by spin-echo and gradient-echo fMRI: Insights into hemodynamic regulation

Spatial specificity of functional magnetic resonance imaging (fMRI) signals to sub-millimeter functional architecture remains controversial. To investigate this issue, high-resolution fMRI in response to visual stimulus was obtained in isoflurane-anesthetized cats at 9.4 T using conventional gradient-echo (GE) and spin-echo (SE) techniques; blood oxygenation-level dependent (BOLD) and cerebral blood volume (CBV)-weighted data were acquired without and with injection of 10 mg Fe/kg monocrystalline iron oxide nanoparticles (MION), respectively. Studies after MION injection at two SE times show that the T2' contribution to SE fMRI is minimal. GE and SE BOLD changes were spread across the cortical layers. GE and SE CBV-weighted fMRI responses peaked at the middle cortical layer, which has the highest experimentally-determined microvascular volume; full-width at half-maximum was <1.0 mm. Parenchymal sensitivity of GE CBV-weighted fMRI was approximately 3 times higher than that of SE CBV-weighted fMRI and approximately 1.5 times higher than that of BOLD fMRI. It is well known that GE CBV-weighted fMRI detects a volume change in vessels of all sizes, while SE CBV-weighted fMRI is heavily weighted toward microvascular changes. Peak CBV change of 10% at the middle of the cortex in GE measurements was 1.8 times higher than that in SE measurements, indicating that CBV changes occur predominantly for vasculature connecting the intracortical vessels and capillaries. Our data supports the notion of laminar-dependent CBV regulation at a sub-millimeter scale.

Complementary aspects of diffusion imaging and fMRI; I: structure and function

Studying the intersection of brain structure and function is an important aspect of modern neuroscience. The development of magnetic resonance imaging (MRI) over the last 25 years has provided new and powerful tools for the study of brain structure and function. Two tools in particular, diffusion imaging and functional MRI (fMRI), are playing increasingly important roles in elucidating the complementary aspects of brain structure and function. In this work, we review basic technical features of diffusion imaging and fMRI for studying the integrity of white matter structural components and for determining the location and extent of cortical activation in gray matter, respectively. We then review a growing body of literature in which the complementary aspects of diffusion imaging and fMRI, applied as separate examinations but analyzed in tandem, have been exploited to enhance our knowledge of brain structure and function.

Effects of oxygen saturation on BOLD and arterial spin labelling perfusion fMRI signals studied in a motor activation task

Effects of oxygen availability on blood oxygenation level dependent (BOLD) and arterial spin labelling (ASL) perfusion functional magnetic resonance imaging (fMRI) signal changes upon motor activation were studied. Mild hypoxic hypoxia was induced by reducing the inspired oxygen content (FIO(2)) to 12%, decreasing blood oxygen saturation (Y) from 0.99 +/- 0.01 to 0.85 +/- 0.03. The fMRI signal characteristics were determined during finger tapping. BOLD activation volume decreased as a function of declining Y in the brain structures involved in execution of the motor task, however, the BOLD signal increase in activated parenchyma was not influenced by Y. ASL fMRI showed that the baseline CBF of 61.8 +/- 3.6 ml/100 g/min was not affected by hypoxic hypoxia. Similar to the BOLD fMRI, the volume of motor cortex areas displaying increase in perfusion by ASL fMRI decreased, but the signal change due to perfusion increase was not influenced in hypoxia. The present fMRI results show distinct patterns of haemodynamic and metabolic responses in the brain to motor task between normoxia and hypoxia. On one hand, neither BOLD nor ASL fMRI signal changes are influenced by hypoxia during motor activation. On the other hand, hypoxia attenuates increase in both BOLD and perfusion fMRI signals upon finger tapping from the levels determined in normoxia. These observations indicate that haemodynamic and metabolic responses may be heterogeneous in brain during execution of motor functions in mild hypoxia.

Quantifying the spatial resolution of the gradient echo and spin echo BOLD response at 3 Tesla

The blood oxygen level dependent (BOLD) response, as measured with fMRI, offers good spatial resolution compared to other non-invasive neuroimaging methods. The use of a spin echo technique rather than the conventional gradient echo technique may further improve the resolution by refocusing static dephasing effects around the larger vessels, so sensitizing the signal to the microvasculature. In this work the width of the point spread function (PSF) of the BOLD response at a field strength of 3 Tesla is compared for these two approaches. A double echo EPI pulse sequence with simultaneous collection of gradient echo and spin echo signal allows a direct comparison of the techniques. Rotating multiple-wedge stimuli of different spatial frequencies are used to estimate the width of the BOLD response. Waves of activation are created on the surface of the visual cortex, which begin to overlap as the wedge separation decreases. The modulation of the BOLD response decreases with increasing spatial frequency in a manner dependent on its width. The spin echo response shows a 13% reduction in the width of the PSF, but at a cost of at least 3-fold reduction in contrast to noise ratio.

Heterogeneous oxygen extraction in the visual cortex during activation in mild hypoxic hypoxia revealed by quantitative functional magnetic resonance imaging

Functional magnetic resonance imaging (fMRI) techniques were used to study haemodynamic and metabolic responses in human visual cortex during varying arterial blood oxygen saturation levels (Y(sat), determined by pulse-oximeter) and stimulation with contrast-reversing checkerboards. The visual-evoked potential amplitude remained constant at lowered Y(sat) of 0.82+/-0.03. Similarly, fMRI cerebral blood flow (CBF) responses were unchanged during reduced Y(sat). In contrast, visual cortex volume displaying blood oxygen level-dependent (BOLD) fMRI response decreased as a function of Y(sat), but the BOLD signal change of 3.6%+/-1.4% was constant. Oxygen extraction ratio (OER) during visual activation showed values of 0.26+/-0.03 for normal Y(sat). At lowered Y(sat), two OER patterns were observed. Firstly, a reduced OER of 0.14+/-0.03 in the visual cortex structures showing BOLD in hypoxia was observed. Secondly, signs of much higher OER in other parts of visual cortex were obtained. T2*-weighted magnetic resonance imaging revealed signal increases by 0.8%+/-0.4% with visual activation during lowered Y(sat) in the visual cortex structures, which showed BOLD of 3.6% in magnitude under normoxia. Because the CBF response in the visual cortex was quantitatively similar during stimulation in normoxia and hypoxia, attenuated T2*-weighted signal increase in parts of visual cortex indicated high OER during visual activation in hypoxia, which was close to that encountered in the resting brain. These spatially localised regions of tissue oxygen extraction and metabolism argue for dissociation between CBF and BOLD fMRI signals in mild hypoxia. The findings point to heterogeneity with regard to oxygen requirement and its coupling to the haemodynamic response in the brain.

T1rho contrast in functional magnetic resonance imaging

The application of T1 in the rotating frame (T1rho) to functional MRI in humans was studied at 3 T. Increases in neural activity increased parenchymal T1rho. Modeling suggested that cerebral blood volume mediated this increase. A pulse sequence named spin-locked echo planar imaging (SLEPI) that produces both T1rho and T2* contrast was developed and used in a visual functional MRI (fMRI)experiment. Spin-locked contrast significantly augments the T2* blood oxygen level-dependent (BOLD) contrast in this sequence. The total functional contrast generated by the SLEPI sequence (1.31%) was 54% larger than the contrast (0.85%) obtained from a conventional gradient-echo EPI sequence using echo times of 30 ms. Analysis of image SNR revealed that the spin-locked preparation period of the sequence produced negligible signal loss from static dephasing effects. The SLEPI sequence appears to be an attractive alternative to conventional BOLD fMRI, particularly when long echo times are undesirable, such as when studying prefrontal cortex or ventral regions, where static susceptibility gradients often degrade T2*-weighted images.

Logarithmic transformation for high-field BOLD fMRI data

Parametric statistical analyses of BOLD fMRI data often assume that the data are normally distributed, the variance is independent of the mean, and the effects are additive. We evaluated the fulfilment of these conditions on BOLD fMRI data acquired at 4 T from the whole brain while 15 subjects fixated a spot, looked at a geometrical shape, and copied it using a joystick. We performed a detailed analysis of the data to assess (a) their frequency distribution (i.e. how close it was to a normal distribution), (b) the dependence of the standard deviation (SD) on the mean, and (c) the dependence of the response on the preceding baseline. The data showed a strong departure from normality (being skewed to the right and hyperkurtotic), a strong linear dependence of the SD on the mean, and a proportional response over the baseline. These results suggest the need for a logarithmic transformation. Indeed, the log transformation reduced the skewness and kurtosis of the distribution, stabilized the variance, and made the effect additive, i.e. independent of the baseline. We conclude that high-field BOLD fMRI data need to be log-transformed before parametric statistical analyses are applied.

Comparison of fMRI activation as measured with gradient- and spin-echo EPI during visual perception

In this study, we compared fMRI activation measured with gradient- and spin-echo-based fMRI during visual perception of faces, which is mediated by neural activation within a distributed cortical network. With both fMRI techniques, bilateral activation was observed in multiple regions including the inferior occipital gyrus, fusiform gyrus, superior temporal sulcus, amygdala, inferior frontal gyrus, and orbitofrontal cortex. When compared with the gradient-echo sequence, activation measured with the spin-echo sequence was significantly reduced. This decrease was manifested by smaller cluster size, lower statistical significance, smaller amplitude of the fMRI signal, and smaller number of subjects who showed activation in all face-responsive regions. In orbitofrontal cortex, a region prone to susceptibility-related signal dephasing, the spin-echo acquisition considerably restored the signal, but did not reveal stronger activation when compared with the gradient-echo acquisition. Our data indicate that optimized GE sequences that reduce susceptibility artefacts are sufficient to detect activation in regions such as the orbitofrontal cortex.

Signal and noise characteristics of Hahn SE and GE BOLD fMRI at 7 T in humans

At very high magnetic fields, GE BOLD fMRI is expected to contain nonspecific contributions and behave differently than HSE fMRI data. Similarly, the two approaches can conceivably suffer from different contributions to temporal instabilities in a times series that ultimately determine the contrast-to-noise ratio (CNR). We investigate the signal and signal fluctuation characteristics in GE and HSE fMRI data with the imaging parameters separately optimized for each contrast at 7 T. In HSE fMRI, activation-induced fractional signal change (DeltaS/S) decreased rapidly, and the ratio of standard deviations of image-to-image fluctuations due to physiological processes (sigmaPhys) to thermal noise (sigmaTherm) remained constant with increasing voxel volume. In contrast, DeltaS/S as well as volume of activated voxels was virtually independent of voxel size for GE BOLD, and sigma(Phys)/sigmaTherm increased with increasing voxel size. The ratio of BOLD signal changes (GE/HSE) was much closer to 1 in tissue areas compared to vessel areas. These observations led to the conclusions that the spatial extent of the activation-induced DeltaS/S was much broader in the GE data, and that the physiological processes that give rise to the temporal fluctuations lost coherence over millimeter distances in HSE compared to GE fMRI data. While further studies are needed to characterize it fully, sigmaPhys in HSE data was clearly different than in GE data. It was concluded that HSE imaging yields a significantly reduced amount of nonspecific signals compared to GE imaging, and, would be the method of choice (over GE) for high-resolution applications in humans.

Combination of BOLD-fMRI and VEP recordings for spin-echo MRI detection of primary magnetic effects caused by neuronal currents

In the present paper, for the first time, the feasibility to detect primary magnetic field changes caused by neuronal activity in vivo by spin-echo (SE) magnetic resonance imaging (MRI) is investigated. The detection of effects more directly linked to brain activity than secondary hemodynamic-metabolic changes would enable the study of brain function with improved specificity. However, the detection of neuronal currents by MRI is hampered by such accompanying hemodynamic changes. Therefore, SE image acquisition, rather than gradient-echo (GE) image acquisition, was preferred in the present work since the detection of primary neuronal and not blood oxygenation level-dependent (BOLD)-related effects may be facilitated by this approach. First of all, a precise spatiotemporal synchronization of image acquisition with the neuronal event had to be performed to avoid refocusing of the dephasing phenomenon during the course of the SE sequence. At this aim, we propose the combined use of visual evoked potential (VEP) recordings and BOLD-fMRI measurements prior to SE MRI scanning. Moreover, we exemplify by theory and experimentation how the control of artefactual signal changes due to BOLD and movement effects may be further improved by the experimental design. Finally, results from a pilot study using the proposed combination of VEP recordings and MRI techniques are reported, suggesting the feasibility of this method.

Hypercapnic normalization of BOLD fMRI: comparison across field strengths and pulse sequences

The blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signal response to neural stimulation is influenced by many factors that are unrelated to the stimulus. These factors are physiological, such as the resting venous cerebral blood volume (CBV(v)) and vessel size, as well as experimental, such as pulse sequence and static magnetic field strength (B(0)). Thus, it is difficult to compare task-induced fMRI signals across subjects, field strengths, and pulse sequences. This problem can be overcome by normalizing the neural activity-induced BOLD fMRI response by a global hypercapnia-induced BOLD signal. To demonstrate the effectiveness of the BOLD normalization approach, gradient-echo BOLD fMRI at 1.5, 4, and 7 T and spin-echo BOLD fMRI at 4 T were performed in human subjects. For neural stimulation, subjects performed sequential finger movements at 2 Hz, while for global stimulation, subjects breathed a 5% CO(2) gas mixture. Under all conditions, voxels containing primarily large veins and those containing primarily active tissue (i.e., capillaries and small veins) showed distinguishable behavior after hypercapnic normalization. This allowed functional activity to be more accurately localized and quantified based on changes in venous blood oxygenation alone. The normalized BOLD signal induced by the motor task was consistent across different magnetic fields and pulse sequences, and corresponded well with cerebral blood flow measurements. Our data suggest that the hypercapnic normalization approach can improve the spatial specificity and interpretation of BOLD signals, allowing comparison of BOLD signals across subjects, field strengths, and pulse sequences. A theoretical framework for this method is provided.

Acute cerebral ischemia in rats studied by Carr-Purcell spin-echo magnetic resonance imaging: assessment of blood oxygenation level-dependent and tissue effects on the transverse relaxation

Acute cerebral ischemia has been shown to be associated with an enhanced transverse relaxation rate in rat brain parenchyma, chiefly due to the blood oxygenation level-dependent (BOLD) effect. In this study, Carr-Purcell R(2) (CP R(2)), acquired both with short and long time intervals between centers of adiabatic pi-pulses (tau(CP)), was used to assess the contributions of BOLD and tissue effects to the transverse relaxation in two brain ischemia models of rat at 4.7 T. R(1rho) and diffusion MR images were also acquired in the same animals. During the first minutes of global ischemia, the long tau(CP) R(2) in brain parenchyma increased, whereas the short tau(CP) R(2) was unchanged. Based on the simulations, and using constraints of intravascular BOLD effect on parenchymal R(2), the former observation was ascribed to be due to susceptibility changes arising in the extravascular compartment. R(1rho) declined almost immediately after the onset of focal cerebral ischemia, and further declined during the evolution of ischemic damage. Interestingly, short tau(CP) CP R(2) started to decline after some 20 min of focal ischemia and declined over a time course similar to that of R(1rho), indicating that it may be an MRI marker for irreversible tissue changes in cerebral ischemia. The present results show that CP R(2) MRI can reveal both tissue- and blood-derived contrast changes in acute cerebral ischemia.

Assessment of cerebral hemodynamics and oxygen extraction using dynamic susceptibility contrast and spin echo blood oxygenation level-dependent magnetic resonance imaging: applications to carotid stenosis patients

Blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) has been recently used to quantify cerebral blood volume (CBV) and oxygen extraction ratio (OER). In the present study, we have exploited the intravascular BOLD model to assess gray matter (GM) OER at hemispheric level using parenchymal T(2) and CBV data at 1.5 T, obtained by single spin echo and dynamic susceptibility contrast (DSC) perfusion MRI, respectively. An OER of 0.40 +/- 0.07 was determined in gray matter for control subjects. A group of carotid stenosis (CS) patients (n = 22) was examined by multiparametric MRI. The degree of CS was determined by contrast agent-enhanced magnetic resonance angiography. Within the group, eight cases with <70% narrowing of a carotid lumen, nine cases with 70-99%, and five cases with complete occlusion of either carotid arteries were found. DSC MRI revealed abnormalities in 14 patients in dynamic parameters of perfusion images. These included four cases with elevated hemispheric gray matter CBV ipsilateral to the stenosis, above 2 SD of the level determined in control subjects. These four patients showed large variation in the degree of stenosis. We also found three cases with ipsilateral gray matter CBV below 2 SD of the control value, two of these with >70% stenosis. Gray matter OER ipsilateral to the stenosis was above 2 SD of the control range in eight CS patients, three of these showing also high CBV. Use of the present approach to determine OER for the assessment of hemodynamic adaptations in CS patients is discussed in the light of documented hemodynamic adaptations to carotid stenosis.

High field human imaging

This review article examines the state of knowledge regarding human imaging using MRI at high main magnetic field strengths. The article starts with a summary of the technical issues associated with magnetic field strengths in the range of 3-8 T, including magnet characteristics and the properties of radiofrequency magnetic fields, with special reference to sensitivity, power deposition, and homogeneity. The published data on tissue-water relaxation times in the brain is tabulated and the implications for contrast and pulse sequence implementation is elucidated. The behavior of the major fast imaging sequences, fast low angle shot (FLASH), rapid acquisition with relaxation enhancement (RARE), and echo planar imaging (EPI), is examined in this context. A number of anatomical images from 3 T systems are presented as examples. Particular attention is given to various forms of vascular imaging, namely, time of flight angiography, venography, and arterial spin labeling. The most complex changes in contrast with main magnetic field strength are in activation studies utilizing the blood oxygen level dependent mechanism, which are examined in detail. Improvements in spatial specificity are emphasized, particularly in conjunction with spin-echo imaging. The article concludes with a discussion of the current status and the potential impact of technical developments such as parallel imaging.

Functional magnetic resonance imaging based on changes in vascular space occupancy

During brain activation, local control of oxygen delivery is facilitated through microvascular dilatation and constriction. A new functional MRI (fMRI) methodology is reported that is sensitive to these microvascular adjustments. This contrast is accomplished by eliminating the blood signal in a manner that is independent of blood oxygenation and flow. As a consequence, changes in cerebral blood volume (CBV) can be assessed through changes in the remaining extravascular water signal (i.e., that of parenchymal tissue) without need for exogenous contrast agents or any other invasive procedures. The feasibility of this vascular space occupancy (VASO)-dependent functional MRI (fMRI) approach is demonstrated for visual stimulation, breath-hold (hypercapnia), and hyperventilation (hypocapnia). During visual stimulation and breath-hold, the VASO signal shows an inverse correlation with the stimulus paradigm, consistent with local vasodilatation. This effect is reversed during hyperventilation. Comparison of the hemodynamic responses of VASO-fMRI, cerebral blood flow (CBF)-based fMRI, and blood oxygenation level-dependent (BOLD) fMRI indicates both arteriolar and venular temporal characteristics in VASO. The effect of changes in water exchange rate and partial volume contamination with CSF were calculated to be negligible. At the commonly-used fMRI resolution of 3.75 x 3.75 x 5 mm(3), the contrast-to-noise-ratio (CNR) of VASO-fMRI was comparable to that of CBF-based fMRI, but a factor of 3 lower than for BOLD-fMRI. Arguments supporting a better gray matter localization for the VASO-fMRI approach compared to BOLD are provided.

Influence of baseline hematocrit on between-subject BOLD signal change using gradient echo and asymmetric spin echo EPI

The dependence of BOLD signal change (BSC) on baseline hematocrit is in the process of being characterized, primarily using conventional Gradient Echo (GE) echo planar imaging (EPI). We describe the first empiric exploration of this relationship using, in addition to GE, Spin Echo (SE) and two Asymmetric Spin Echo EPI sequences (ASE10 and ASE20), which are less susceptible to large vessel noise. Motor cortex BSC was measured (N = 17) and regressed against hematocrit and hemoglobin concentration using linear and non-linear functions. GE measurements of BSC yielded a positive linear relationship (r(2) = 0.240, p = 0.0459) whereas a positive non-linear relationship was observed using ASE10 (r(2) = 0.571, p = 0.0146). Results suggest that between-subjects BSC is significantly dependent on baseline hematocrit. The nature of dependence, and implications for quantitative studies vary with the vessel size selectivity of the imaging sequence, and with the effect of hematocrit on blood viscosity in the imaged vessels.