[MRI and cognition]

Many groups are trying out functional MRI (fMRI) to study normal and diseased brain function. FMRI is a technique that images intrinsic blood signal changes accompanying variations of cerebral blood flow (CBF), cerebral blood volume (CBV) and oxygenation during mental activity. The main advantages which account for fMRI's increasing popularity are its time resolution (less than 1 second), a fine spatial resolution (on the scale of 1 mm), and the ability to acquire, within the same individual, repeated, multiple scans in a non-invasive manner. While the basic design of most studies centers on comparison of baseline to an active (test) condition, as the Positron Emission Tomography (PET), fMRI allows an important departure from the single scan per condition study-design, into more flexible paradigms of multiple scans over varying time intervals and multiple conditions. Although the technique is only a few years old, a large volume of experimental data have already been published. In this presentation we would like to provide potential users with a short overview of the basic principles and the main applications in cognitive neuroscience, as well as potential clinical applications.

Latencies in fMRI time-series: effect of slice acquisition order and perception

In BOLD fMRI a detailed analysis of the MRI signal time course sometimes shows time differences between different activated regions. Some researchers have suggested that these latencies could be used to infer the temporal order of activation of these cortical regions. Several effects must be considered, however, before interpreting these latencies. The effect of a slice-dependent time shift (SDTS) with multi-slice acquisitions, for instance, may be important for regions located on different slices. After correction for this SDTS effect the time dispersion between activated regions is significantly decreased and the correlation between the MRI signal time course and the stimulation paradigm is improved. Another effect to consider is the latency which may exist between perception and stimulus presentation. It is shown that the control of perception can be achieved using a finger-spanning technique during the fMRI acquisition. The use of this perception profile rather than an arbitrary waveform derived from the paradigm proves to be a powerful alternative to fMRI data processing, especially with chemical senses studies, when return to baseline is not always correlated to stimulus suppression. This approach should also be relevant to other kinds of stimulation tasks, as a realistic way of monitoring the actual task performance, which may depend on attention, adaptation, fatigue or even variability of stimulus presentation.

Frequency-dependent changes of regional cerebral blood flow during finger movements: functional MRI compared to PET

To evaluate the effect of the repetition rate of a simple movement on the magnitude of neuronal recruitment in the primary sensorimotor cortex, we used a blood flow-sensitive, echo planar functional magnetic resonance imaging (fMRI) sequence in six normal volunteers. Three of the volunteers also had [15O]water positron emission tomography (PET) studies using the same paradigm. Previous PET studies had shown an increase in regional CBF (rCBF) with movement frequencies up to 2 Hz and then a plateau of regional cerebral blood flow (rCBF) at faster frequencies. To evaluate the extent of the activation, the correlation coefficient (cc) of the Fourier-transformed time-signal intensity change with the Fourier-transformed reference function was calculated pixel by pixel. The degree of activation was measured as the signal percent change of each region of interest with a cc > 0.5. The left primary sensorimotor cortex was constantly activated at 1, 1.5, 2, and 4 Hz, while there was only inconsistent activation at 0.25 and 0.5 Hz. Percent change in signal intensity linearly increased from 1 to 4 Hz. Area of activation increased up to 2 Hz and showed a tendency to decrease at higher frequencies. Individual analysis of PET data showed activation in the same location as that revealed by fMRI. The combination of progressively increasing signal intensity with an area that increases to 2 Hz and declines at faster frequencies explains the PET finding of plateau of rCBF at the faster frequencies. Functional magnetic resonance imaging shows similar results to PET, but is better able to dissociate area and magnitude of change.

Noninvasive assessment of language dominance in children and adolescents with functional MRI: a preliminary study

BACKGROUND

Assessment of language organization is crucial in patients considered for epilepsy surgery. In children, the current techniques, intra-carotid amobarbital test (IAT) for language dominance, and cortical electrostimulation mapping (ESM), are invasive and risky. Functional magnetic resonance imaging (fMRI) is an alternative method for noninvasive functional mapping, through the detection of the hemodynamic changes associated with neuronal activation. We used fMRI, to assess language dominance in children with partial epilepsy.

METHODS

Eleven right handed children and adolescents performed a word generation task during fMRI acquisition focused on the frontal lobes. Areas where the signal time course correlated with the test paradigm (r = 0.7) were considered activated. Extent and magnitude of signal changes were used to calculate asymmetry indices. Seven patients had IAT, ESM, or surgery outcome available for comparison.

RESULTS

fMRI language dominance always agreed with IAT (6 cases) and ESM (1 case), showing left dominance in six and bilateral language in one. fMRI demonstrated left dominance in three additional children, and right dominance in one with early onset of left temporal epilepsy. Four children whose initial studies were equivocal due to noncompliance or motion artifacts were restudied successfully.

CONCLUSIONS

fMRI can be used to assess language lateralization noninvasively in children. It has the potential to replace current functional mapping techniques in patients, and to provide important data on brain development.

The b matrix in diffusion tensor echo-planar imaging

In diffusion tensor imaging (DTI) an effective diffusion tensor in each voxel is measured by using a set of diffusion-weighted images (DWIs) in which diffusion gradients are applied in a multiplicity of oblique directions. However, to estimate the diffusion tensor accurately, one must account for the effects of all imaging and diffusion gradient pulses on each signal echo, which are embodied in the b matrix. For DTI to be practical clinically, one must also acquire DWIs rapidly and free of motion artifacts, which is now possible with diffusion-weighted echo-planar imaging (DW-EPI). An analytical expression for the b matrix of a general DW-EPI pulse sequence is presented and then validated experimentally by measuring the diffusion tensor in an isotropic phantom whose diffusivity is already known. The b matrix is written in a convenient tabular form as a sum of individual pair-wise contributions arising from gradient pulses applied along parallel and perpendicular directions. While the contributions from readout and phase-encode gradient pulse trains are predicted to have a negligible effect on the echo, the contributions from other imaging and diffusion gradient pulses applied in both parallel and orthogonal directions are shown to be significant in our sequence. In general, one must understand and account for the multiplicity of interactions between gradient pulses and the echo signal to ensure that diffusion tensor imaging is quantitative.

Double-sampled echo-planar imaging at 3 tesla

A persistent artifact in the images acquired by the echo-planar imaging (EPI) method is the Nyquist or N/2 ghost which interferes with the image and reduces the signal-to-noise ratio (SNR). The Nyquist ghost is the result of the time-reversal asymmetry between the even and odd echoes. To eliminate this artifact, the authors present a double-sampled EPI (DSEPI) method in which echoes from each even and odd echo pair are equally phase encoded. The even and odd echoes are separately reconstructed into two distinct images which are then added together. The DSEPI method has been applied to human brain at 3.0 T and shown to be a simple and effective way to eliminate the Nyquist ghost and restore image SNR loss.

Contribution of Sinerem used as blood-pool contrast agent: detection of cerebral blood volume changes during apnea in the rabbit

The authors suggest that ultra-small paramagnetic iron oxide (USPIO) particles used as blood pool contrast agents may increase the sensitivity of midfield MRI (i.e., less than 1.5 Tesla) to physiological variations in cerebral blood volume. This hypothesis was tested on a rabbit model of apnea which increases pCO2 and cerebral blood volume. Using Sinerern as the USPIO at a blood concentration of 60 mumol Iron/kg body weight, an 8% T2*-weighted signal decrease could be observed at 1.0 T with 25-33% increase in pCO2. Comparatively, in the absence of USPIO, T2*-weighted signal dropped only 4% during apnea and after mild hyperoxygenation beforehand, due to increased deoxyhemoglobin content. These preliminary data suggest that USPIOs could play an important role in functional MRI at midfield strength, by sensitizing the signal to cerebral blood volume changes.

Functional MRI of the brain principles, applications and limitations

MRI now allows noninvasive monitoring of brain function with a combined spatial and temporal resolution never achieved by other imaging modalities. Among several methods proposed to evaluate changes in blood volume, flow or oxygenation during mental activity, the most successful is based on the sensitivity of MRI to magnetic effects induced by the modulation of the oxygenation status of hemoglobin (oxy/deoxyhemoglobin) which results from local variations in blood flow. In the brain cortex, such variations may be induced by task activation or by cognitive processes, such as language or mental imagery. Typically, MRI signal is increased by a few percents when brain is activated due to sharp increase in oxygen supply (blood flow). Brain activation maps obtained with MRI using various task paradigms agreed well with previous PET results. However MRI permits direct correlation of function with underlying anatomy within a single imaging modality and repetitive studies on the same individuals. These studies suggest that MRI may be the method of choice in conjunction with other functional techniques, to study mental and cognitive processes underlying the function of the human brain. Clinically, potential applications include presurgical mapping, recovery monitoring of stroke or head injuries, exploration of seizure disorders or monitoring of the effects of neuropharmaceuticals.

Visualizing cortical activation during mental calculation with functional MRI

Cortical activation during arithmetic calculation (silent subtraction by sevens) was compared to that observed during a control condition for which subjects were required to count forward by ones. Nine normal subjects underwent 1.5-T functional magnetic resonance imaging while performing these tasks. All subjects showed bilateral premotor, posterior parietal, and prefrontal cortex activation during serial calculation. There was a large degree of individual variation in activation outside of these areas. These results confirm the role of posterior parietal cortex in arithmetic calculation and implicate other regions, including prefrontal cortex.

Imaging focal reperfusion injury following global ischemia with diffusion-weighted magnetic resonance imaging and 1H-magnetic resonance spectroscopy

The purpose of the study was to determine whether diffusion-weighted magnetic resonance imaging (DWI) could identify focal lesions that develop in ischemia-sensitive cerebral tissues during reperfusion following global brain ischemia. Localized 1H-Magnetic Resonance Spectroscopy (1H-MRS) measurements were also obtained to determine whether abnormal spectroscopic markers were associated with focal lesions and to define time correlations between DWI and metabolic changes. Brain diffusion-weighted magnetic resonance imaging measurements were made in a cat model of repetitive global cerebral ischemia and reperfusion. Five animals were exposed to three episodes of 10 min vascular occlusions at hourly intervals. Three animals were evaluated as controls. DWI, T2WI, and 1H-MRS data were acquired for up to 12 h. Transient focal DWI hyperintensity was detected in the hippocampus, basal ganglia, and cortical watershed areas. These focal abnormalities usually appeared during the final reperfusion and eventually spread to encompass all of the gray matter. Spectroscopic measurements demonstrated the expected elevation of the lactate signal intensity during vessel occlusion, which returned to normal during early reperfusion. A subsequent rise in the lactate signal occurred approximately 3-4 h after the beginning of the third reperfusion. This late lactate elevation occurred after focal hyperintensities were identified by DWI. No significant signal changes were seen in spectroscopic metabolites other than lactate. The study illustrates that DWI and 1H-MRS are sensitive to focal cerebral lesions that occur during reperfusion following global cerebral ischemia.

Molecular diffusion, tissue microdynamics and microstructure

Diffusion NMR is the only method available today that noninvasively provides information on molecular displacements over distances comparable to cell dimensions. This information can be used to infer tissue microstructure and microdynamics. However, data may be fairly difficult to interpret in biological tissues which differ markedly from the theoretical "infinite isotrope medium", as many factors may affect the NMR signal. The object of this paper is to analyze the expected effects of temperature, restriction, hindrance, membrane permeability, anisotropy and tissue inhomogeneity on the diffusion measurements. Powerful methods, such as q-space imaging, diffusion tensor imaging and diffusion spectroscopy of metabolites further enhance the specificity of the information obtained from diffusion NMR experiments.

Functional MRI during word generation, using conventional equipment: a potential tool for language localization in the clinical environment

OBJECTIVE

To test the accuracy of bilateral language mapping using a standard clinical magnetic resonance (MR) imaging device during word generation.

DESIGN

A study of normal volunteers.

SETTING

Volunteers from the Washington, DC, area.

PARTICIPANTS

Nine normal, right-handed, native English speakers (four women, five men, mean age 31 years).

INTERVENTIONS

During four MR acquisition periods, subjects would alternately rest and silently generate words. Sagittal MR images covered the middle and inferior frontal gyri, insulae, and part of the temporal and parietal lobes bilaterally.

MAIN OUTCOME MEASURES

(1) Anatomic maps of task-related signal changes obtained by comparing, in each voxel, the signal during word generation and rest periods, and (2) analysis of the time course of the signal.

RESULTS

Maximum responses were in the left hemisphere, mainly in the frontal lobe (Broca's area, premotor cortex, and dorsolateral prefrontal cortex) but also in posterior regions such as Wernicke's area. In agreement with previous studies, some degree of task-related changes was present in a subset of the corresponding regions in the right hemisphere.

CONCLUSION

Despite certain limitations, it is possible, using widely available MR equipment, to obtain results consistent with previous studies. The technique may have important implications for assessment of cognitive functions in patients with neurologic disorders in a clinical environment.

Applications of magnetic resonance imaging to the study of human brain function

Important questions relating to the coupling of local neuronal activity to the hemodynamic response measured using functional magnetic resonance imaging (fMRI), as well as issues concerning imaging sequence, paradigm design and data analysis processing, have been addressed during the past year. Initial fMRI studies identified visual, somatosensory, auditory and motor activation areas in the primary cortices. Current 'second generation' studies aim to identify changes in the fMRI signal associated with specific tasks and stimulus parameters. Dynamic aspects of brain processing for performing higher order cognitive functions, such as language, attention, mental imagery, and learning and memory, have also been explored.

Functional magnetic resonance imaging of the brain

This conference reviewed the potential scope of application of recently developed techniques for functional magnetic resonance imaging (fMRI) of the brain. The most successful technique is based on the sensitivity of magnetic resonance imaging (MRI) to magnetic effects caused by the modulation of the oxygenation state of hemoglobin, which is induced by local variations in blood flow during task activation. Typically, the MRI signal increases by a few percentage points during brain activation because blood flow and oxygen supply sharply increase. Brain activation images with excellent combined spatial and temporal resolution have been obtained noninvasively using visual, sensorimotor, or auditory stimuli, or during higher-order cognitive processes such as language or mental imagery. Although sensitive to misregistration artifacts and macroscopic vessels, MRI permits both the direct correlation of function with underlying anatomy and repeated studies on the same person. It may become the method of choice for studies of mental and cognitive processes, presurgical mapping, monitoring recovery from stroke or head injuries, exploration of seizure disorders, or monitoring the effects of neuropharmaceuticals.

Anomalous transverse relaxation in 1H spectroscopy in human brain at 4 Tesla

Longitudinal (T1) and apparent transverse relaxation times (T2) of choline-containing compounds (Cho), creatine/phosphocreatine (Cr/PCr), and N-acetyl aspartate (NAA) were measured in vivo in human brain at 4 Tesla. Measurements were performed using a water suppressed stimulated echo pulse sequence with complete outside volume presaturation to improve volume localization at short echo times. T1-values of Cho (1.2 +/- 0.1 s), Cr (1.6 +/- 0.3 s), and NAA (1.6 +/- 0.2 s) at 4 Tesla in occipital brain were only slightly larger than those reported in the literature at 1.5 Tesla. Thus, TR will not adversely affect the expected enhancement of signal-to-noise at 4 Tesla. Surprisingly, apparent T2-values of Cho (142 +/- 34 ms), Cr (140 +/- 13 ms), and NAA (185 +/- 24 ms) at 4 Tesla were significantly smaller than those at 1.5 Tesla and further decreased when increasing the mixing interval TM. Potential contributing factors, such as diffusion in local susceptibility related gradients, dipolar relaxation due to intracellular paramagnetic substances and motion effects are discussed. The results suggest that short echo time spectroscopy is advantageous to maintain signal to noise at 4 Tesla.

Diffusion, perfusion and functional magnetic resonance imaging

Recent developments in the use of Magnetic Resonance Imaging (MRI) to measure and image molecular diffusion and blood microcirculation (perfusion) hold significant promise in the noninvasive evaluation of normal brain function and functional disorders. Molecular diffusion is the result of spontaneous random motion that involves all molecules and probes molecular motion at microscopic level. Using diffusion MRI, information on tissue geometry and compartmentation effects can be obtained. Diffusion MRI has been used to map myelin fiber orientation in brain with high accuracy. Diffusion MRI is also the only imaging modality which shows brain ischemia at a very early stage, even before T1w or T2w MR images become abnormal, offering great promises in the management of stroke patients. Also, diffusion imaging may be used to monitor tissue temperature changes noninvasively during hyperthermia or laser surgery. On the other hand, MRI can provide information on tissue perfusion. Several methods have been proposed, some of them including tracers or contrast agents. The most successful approach for brain function studies, however, is based on the sensitivity of MRI to magnetic effects induced by changes in the oxygenation status of hemoglobin (deoxyhemoglobin). These effects have already been used to characterize hematomas. These effects may also be exploited to detect small modulation in red blood cell oxygen content related to local variations in blood flow and oxygen consumption in tissues. In the brain cortex, such variations may be induced by external stimuli or internal cognitive processes. Capillary blood deoxyhemoglobin thus acts as a natural endogeneous contrast agent.(ABSTRACT TRUNCATED AT 250 WORDS)

High speed 1H spectroscopic imaging in human brain by echo planar spatial-spectral encoding

We introduce a fast and robust spatial-spectral encoding method, which enables acquisition of high resolution short echo time (13 ms) proton spectroscopic images from human brain with acquisition times as short as 64 s when using surface coils. The encoding scheme, which was implemented on a clinical 1.5 Tesla whole body scanner, is a modification of an echo-planar spectroscopic imaging method originally proposed by Mansfield Magn. Reson. Med. 1, 370-386 (1984), and utilizes a series of read-out gradients to simultaneously encode spatial and spectral information. Superficial lipid signals are suppressed by a novel double outer volume suppression along the contours of the brain. The spectral resolution and the signal-to-noise per unit time and unit volume from resonances such as N-acetyl aspartate, choline, creatine, and inositol are comparable with those obtained with conventional methods. The short encoding time of this technique enhances the flexibility of in vivo spectroscopic imaging by reducing motion artifacts and allowing acquisition of multiple data sets with different parameter settings.

Three-dimensional echo-planar MR spectroscopic imaging at short echo times in the human brain

PURPOSE

To demonstrate the feasibility of three-dimensional echo-planar spectroscopic imaging (EPSI) at short echo time (13 msec) with a conventional clinical imager in the human brain.

MATERIALS AND METHODS

Periodic inversions of a readout gradient were used during data acquisition to simultaneously encode chemical shift and one spatial dimension in one excitation. Aliasing artifacts were avoided with a modified acquisition-and-processing method based on oversampling. A double outer-volume suppression technique that adapts to the ovoid brain shape was used to strongly reduce extracranial lipid resonances.

RESULTS

Three-dimensional spatial encoding in vivo of eight sections with 32 x 32 voxels each (0.75 cm3) was performed in 34 minutes with four signal averages. The spectral resolution and signal-to-noise ratio (S/N) of resonances of inositol, choline, creatine, glutamate and glutamine, and N-acetyl aspartate were consistent with those previously recorded with conventional phase encoding.

CONCLUSION

EPSI substantially reduces acquisition time for three-dimensional spatial encoding and yields a spectral quality similar to that obtained with conventional techniques without affecting the S/N per unit time and unit volume.

Magnetic resonance imaging functional activation of left frontal cortex during covert word production

Six normal volunteers underwent 4-T functional magnetic resonance imaging while performing a covert letter fluency task. An echo planar imaging sequence was utilized to detect activation based on deoxyhemoglobin contrast. All 6 subjects showed consistent activation in the frontal operculum and premotor and primary motor cortices. Activation was also detected in the supramarginal gyrus and the posterior part of the superior temporal gyrus. These results show that magnetic resonance functional neuroimaging can be used to investigate cerebral activity noninvasively during performance of complex cognitive tasks.

Activation of human primary visual cortex during visual recall: a magnetic resonance imaging study

The degree to which the process involved in visual perception and visual imagery share a common neuroanatomical substrate is unclear. Physiological evidence for localization of visual imagery early in the visual pathways would have important bearing on current theories of visual processing. A magnetic resonance imaging technique sensitive to regional changes in blood oxygenation was used to obtain functional activation maps in the human visual cortex. During recall of a visual stimulus, focal increases in signal related to changes in blood flow were detected in V1 and V2 cortex in five of seven subjects. These experiments show that the same areas of the early visual cortex that are excited by visual stimulation are also activated during mental representation of the same stimulus. Some of the processes used in topographically mapped cortical areas during visual perception may also be utilized during visual recall.

Human brain: proton diffusion MR spectroscopy

Diffusion of brain metabolites was measured in 10 healthy volunteers by using localized proton diffusion magnetic resonance (MR) spectroscopy. Measurements were conducted with a clinical MR imager by using a stimulated-echo pulse sequence (3,000/60 [repetition time msec/echo time msec], 200-msec mixing time) with additional outside-volume suppression. Motion artifacts due to macroscopic brain movements were compensated by means of peripheral cardiac gating and separate collection of individual spectroscopic acquisitions into a two-dimensional data matrix. Phase errors due to macroscopic motion were subsequently corrected in individual data traces prior to spectral averaging. Mean (+/- 1 standard deviation) apparent diffusion coefficients of choline-containing compounds ([0.13 +/- 0.03] x 10(-3) mm2/sec), creatine and phosphocreatine ([0.15 +/- 0.03] x 10(-3) mm2/sec), and N-acetyl aspartate ([0.18 +/- 0.02] x 10(-3) mm2/sec) were substantially smaller than that of water and were consistent with recently published data obtained in anesthetized and paralyzed animals. Adequate diffusion sensitivity for metabolites in the human brain can be obtained with clinical whole-body imagers despite macroscopic head and brain movements.

Is water diffusion restricted in human brain white matter? An echo-planar NMR imaging study

Water diffusion is extremely anisotropic in brain white matter, depending on myelin fiber orientation. A suggested, but unproven cause of anisotropy is that, in the direction transverse to the myelin fibers, axonal water diffusion is prevented by the presence of the myelin sheath. We found, however, using an ultra-fast nuclear magnetic resonance imaging technique, that the dependence of the diffusion coefficient on the diffusion time does not support the model of water restriction by impenetrable barriers. These results, which were obtained non-invasively in vivo in the human brain, imply that water diffuses with a measurable rate across myelin fibers.