Magnetodendrimers allow endosomal magnetic labeling and in vivo tracking of stem cells

Magnetic resonance (MR) tracking of magnetically labeled stem and progenitor cells is an emerging technology, leading to an urgent need for magnetic probes that can make cells highly magnetic during their normal expansion in culture. We have developed magnetodendrimers as a versatile class of magnetic tags that can efficiently label mammalian cells, including human neural stem cells (NSCs) and mesenchymal stem cells (MSCs), through a nonspecific membrane adsorption process with subsequent intracellular (non-nuclear) localization in endosomes. The superparamagnetic iron oxide nanocomposites have been optimized to exhibit superior magnetic properties and to induce sufficient MR cell contrast at incubated doses as low as 1 microg iron/ml culture medium. When containing between 9 and 14 pg iron/cell, labeled cells exhibit an ex vivo nuclear magnetic resonance (NMR) relaxation rate (1/T2) as high as 24-39 s-1/mM iron. Labeled cells are unaffected in their viability and proliferating capacity, and labeled human NSCs differentiate normally into neurons. Furthermore, we show here that NSC-derived (and LacZ-transfected), magnetically labeled oligodendroglial progenitors can be readily detected in vivo at least as long as six weeks after transplantation, with an excellent correlation between the obtained MR contrast and staining for beta-galactosidase expression. The availability of magnetodendrimers opens up the possibility of MR tracking of a wide variety of (stem) cell transplants.

Neurotransplantation of magnetically labeled oligodendrocyte progenitors: magnetic resonance tracking of cell migration and myelination

Demyelination is a common pathological finding in human neurological diseases and frequently persists as a result of failure of endogenous repair. Transplanted oligodendrocytes and their precursor cells can (re)myelinate axons, raising the possibility of therapeutic intervention. The migratory capacity of transplanted cells is of key importance in determining the extent of (re)myelination and can, at present, be evaluated only by using invasive and irreversible procedures. We have exploited the transferrin receptor as an efficient intracellular delivery device for magnetic nanoparticles, and transplanted tagged oligodendrocyte progenitor cells into the spinal cord of myelin-deficient rats. Cell migration could be easily detected by using three-dimensional magnetic resonance microscopy, with a close correlation between the areas of contrast enhancement and the achieved extent of myelination. The present results demonstrate that magnetic resonance tracking of transplanted oligodendrocyte progenitors is feasible; this technique has the potential to be easily extended to other neurotransplantation studies involving different precursor cell types.

Water diffusion and acute stroke

The occlusion of the middle cerebral artery was used as an experimental acute stroke model in 30 cats. The diffusion of water was followed by diffusion-sensitized MRI between 1 and 15 h after induction of stroke. It is demonstrated that images representing the trace of the diffusion tensor provide a much more accurate delineation of affected area than images representing the diffusion in one direction only. The reason is that the strong contrast caused by the anisotropy and orientation of myelin fibers is completely removed in the trace of the diffusion tensor. The trace images show a small contrast between white and gray matter. The diffusion coefficient of white matter is decreased in acute stroke to approximately the same extent as gray matter. It is further shown that the average lifetime of water in extra and intracellular space is shorter than 20 ms both for healthy and ischemic tissue indicating that myelin fibers are permeable to water. The anisotropy contrast did not change before or after induction of stroke, nor after sacrifice. Together, these observations are consistent with the view that the changes in water diffusion during acute stroke are directly related to cytotoxic oedema, i.e., to the change in relative volume of intra- and extracellular spaces. Changes in membrane permeability do not appear to contribute significantly to the changes in diffusion.

The relative metabolic demand of inhibition and excitation

By using the (14C)2-deoxyglucose method, inhibition has been shown to be a metabolically active process at the level of the synapse. This is supported by recent results from magnetic resonance spectroscopy that related the changes in neuroenergetics occurring with functional activation to neurotransmitter cycling. However, inhibitory synapses are less numerous and strategically better located than excitatory synapses, indicating that inhibition may be more efficient, and therefore less energy-consuming, than excitation. Here we test this hypothesis using event-related functional magnetic resonance imaging in volunteers whose motor cortex was inhibited during the no-go condition of a go/no-go task, as demonstrated by transcranial magnetic stimulation. Unlike excitation, inhibition evoked no measurable change in the blood-oxygenation-level-dependent signal in the motor cortex, indicating that inhibition is less metabolically demanding. Therefore, the 'activation' seen in functional imaging studies probably results from excitation rather than inhibition.

Functional magnetic resonance imaging brain mapping in psychiatry: methodological issues illustrated in a study of working memory in schizophrenia

Functional magnetic resonance imaging (fMRI) is a potential paradigm shift in psychiatric neuroimaging. The technique provides individual, rather than group-averaged, functional neuroimaging data, but subtle methodological confounds represent unique challenges for psychiatric research. As an exemplar of the unique potential and problems of fMRI, we present a study of 10 inpatients with schizophrenia and 10 controls performing a novel "n back" working memory (WM) task. We emphasize two key design steps: (1) the use of an internal activation standard (i.e., a physiological control region) to address activation validity, and (2) the assessment of signal stability to control for "activation" artifacts arising from unequal signal variance across groups. In the initial analysis, all but one of the patients failed to activate dorsolateral prefrontal cortex (DLPFC) during the working memory task. However, some patients (and one control) also tended to show sparse control region activation in spite of normal motor performance, a result that raises doubts about the validity of the initial analysis and concerns about unequal subject motion. Subjects were then matched for signal variance (voxel stability), producing a subset of six patients and six controls. In this comparison, the internal activation standard (i.e., motor activation) was similar in both groups, and five of six patients, including two whom were neuroleptic-naive, failed to activate DLPFC. In addition, a tendency for overactivation of parietal cortex was seen. These results illustrate some of the promise and pitfalls of fMRI. Although fMRI generates individual brain maps, a specialized survey of the data is necessary to avoid spurious or unreliable findings, related to artifacts such as motion, which are likely to be frequent in psychiatric patients.

Increased iron in the dentate nucleus of patients with Friedrich's ataxia

Friedreich's ataxia (FA) is the most frequently inherited ataxia. To test the hypothesis that iron is increased in the cerebellum of patients with FA, we developed a multigradient echo magnetic resonance sequence for the three-dimensional imaging of brain iron-induced contrast. Relaxation rate (R2*) values in the unaffected globus pallidus were equal in FA patients and controls, although R2* values in the dentate nucleus of patients were significantly higher, which is most likely due to increased iron.

Real-time shimming to compensate for respiration-induced B0 fluctuations

In MRI of human brain, the respiratory cycle can induce B0-field fluctuations through motion of the chest and fluctuations in local oxygen concentration. The associated NMR frequency changes can affect the MRI data in various ways and lead to temporal signal fluctuations, and image artifacts such as ghosting and blurring. Since the size of the effect scales with magnetic field strength, artifacts become particularly problematic at fields above 3.0T. Furthermore, the spatial dependence of the B0-field fluctuations complicates their correction. In this work, a new method is presented that allows compensation of field fluctuations by modulating the B0 shims in real time. In this method, a reference scan is acquired to measure the spatial distribution of the B0 effect related to chest motion. During the actual scan, this information is then used, together with chest motion data, to apply compensating B0 shims in real time. The method can be combined with any type of scan without modifications to the pulse sequence. Real-time B0 shimming is demonstrated to substantially improve the phase stability of EPI data and the image quality of multishot gradient-echo (GRE) MRI at 7T.

Pittfalls of MRI measurement of white matter perfusion based on arterial spin labeling

Although arterial spin labeling (ASL) MRI has been successfully applied to measure gray matter (GM) perfusion in vivo, accurate detection of white matter (WM) perfusion has proven difficult. Reported literature values are not consistent with each other or with perfusion measured with other modalities. In this work, the cause of these inconsistencies is investigated. The results suggest that WM perfusion values are substantially affected by the limited image resolution and by signal losses caused by the long transit times in WM, which significantly affect the label. From gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) bolus-tracking experiments (N=6), it is estimated that the transit time can be several seconds long in deep WM. Furthermore, simulations show that even at a spatial resolution of 7 microl voxel size, contamination by the GM signals can exceed 40% of the actual WM signal. From 10-min long flow-sensitive alternating inversion recovery ASL (FAIR-ASL) measurements at 3T in normal subjects (N=7), using highly sensitive detectors, it is shown that single-voxel (7 mul) deep WM perfusion values have an signal-to-noise ratio (SNR) less than 1. The poor sensitivity and heterogeneous transit time limit the applicability of ASL for measurement of perfusion in WM.

Evaluation of restricted diffusion in cylinders. Phosphocreatine in rabbit leg muscle

Interpretation of NMR diffusion data is complicated in the presence of diffusion barriers. In this study, restricted diffusion in cylinders is evaluated. A method is presented to determine the diameter of the cylinder and the unrestricted diffusion coefficient from the dependence of the trace of the diffusion tensor with diffusion time. This method is valid even if the orientation of the cylinders is unknown and varies within the sample. An example is given for the diffusion of phosphocreatine in the cylindrically shaped fibers of rabbit skeletal muscle.

Restricted and anisotropic displacement of water in healthy cat brain and in stroke studied by NMR diffusion imaging

The occlusion of the middle cerebral artery in cat brain was used as an experimental stroke model to investigate the physical basis of the recently reported lowered diffusion constant of water in acute infarcted brain tissue (Moseley et al., Magn. Reson. Med. 14, 330, 1990). The original findings were confirmed in this study of 12 animals investigated with the diffusion-sensitized stimulated echo sequence. The following additional results were obtained: First, the onset of significant lowering of the diffusion constant in the stroke area varied significantly (up to 2.5 h depending on the animal). Second, the affected area is much more clearly outlined in diffusion-weighted images than in T2-weighted images, even in the period between 3 to 12 h following occlusion. Third, for diffusion times between 50 and 2000 ms. the diffusion constant of water is independent of diffusion time in healthy tissue, as well as in the stroke area. Fourth, the diffusion anisotropy is similar in healthy and in stroke area and remains similar regardless of the diffusion time used.

Three-dimensional functional magnetic resonance imaging of human brain on a clinical 1.5-T scanner

Functional magnetic resonance imaging (fMRI) is a tool for mapping brain function that utilizes neuronal activity-induced changes in blood oxygenation. An efficient three-dimensional fMRI method is presented for imaging brain activity on conventional, widely available, 1.5-T scanners, without additional hardware. This approach uses large magnetic susceptibility weighting based on the echo-shifting principle combined with multiple gradient echoes per excitation. Motor stimulation, induced by self-paced finger tapping, reliably produced significant signal increase in the hand region of the contralateral primary motor cortex in every subject tested.

A fast gradient-recalled MRI technique with increased sensitivity to dynamic susceptibility effects

A fast imaging method that is based on gradient-recalled echoes of spins whose excitation and echo formation are separated by more than one TR period is presented. This method does not incorporate chemical-shift refocusing and thus results in drastically increased sensitivity to dynamic susceptibility effects, while maintaining a short total imaging time. The efficiency of the new technique is demonstrated in dynamic contrast-enhanced experiments (bolus tracking) in the cat brain using a duration of 600 ms for each image. Blood volume maps are derived with expected contrast between white and gray matter.

In vivo measurement of cerebral oxygen consumption and blood flow using 17O magnetic resonance imaging

We used 17O NMR imaging techniques to measure the H2(17)O concentration in a 0.8-ml voxel in the cat brain following injection of an arterial bolus of enriched H2(17)O and during inhalation of enriched 17O2. We also measured the H2(17)O concentration in arterial blood during 17O2 inhalation. The data from the first measurement were used to calculate the blood flow in the voxel. The data from all three measurements were combined to calculate the oxygen consumption in the voxel. The values of cerebral blood flow and oxygen consumption calculated with 17O NMR techniques agree reasonably well with values calculated for a similar region of the cat brain using autoradiographic techniques.

In vivo proton spectroscopy and spectroscopic imaging of [1-13C]-glucose and its metabolic products

Metabolism of [1-13C]-glucose was studied in situ in cat brain using gradient-enhanced proton-detected heteronuclear spectroscopy. Proton detection of [1-13C]-glucose, [3-13C]-lactate, 4-[13C]-glutamine, 4-[13C]-glutamate and the combined signals 2-[13C]-glutamate/glutamine and 3-[13C]-glutamate/glutamine was achieved, despite the fact that some of the associated proton resonances are close to the water signal. Two-dimensional [1H-13C]-spectra demonstrate the possibility of in situ spectral assignment with 1H sensitivity and 13C resolution. Spectroscopic images of glucose and its metabolic products were also acquired, showing the possibility to study spatial dependence of metabolism.

The role of the medial wall and its anatomical variations for bimanual antiphase and in-phase movements

The medial wall of the frontal cortex is thought to play an important role for bimanual coordination. However, there is uncertainty regarding the exact neuroanatomical regions involved. We compared the activation patterns related to bimanual movements using functional magnetic resonance imaging in 12 healthy right-handed subjects, paying special attention to the anatomical variability of the frontal medial wall. The subjects performed unimanual right and left and bimanual antiphase and in-phase flexion and extension movements of the index finger. Activation of the right supplementary motor area (SMA) proper, right and left caudal cingulate motor area (CMA), and right and left premotor cortices was significantly stronger during bimanual antiphase than bimanual in-phase movements, indicating an important function of these areas with bimanual coordination. A frequent anatomical variation is the presence of the paracingulate sulcus (PCS), which might be an anatomical landmark to determine the location of activated areas. Seven subjects had a bilateral, three a unilateral right, and two a unilateral left PCS. Because the area around the PCS is functionally closer coupled to the CMA than to the SMA, activation found in the area around the PCS should be attributed to the CMA. With anatomical variations such as the presence of a PCS or a vertical branch of the cingulate sulcus, normalization and determination of the activation with the help of stereotaxic coordinates can cause an incorrect shift of CMA activation to the SMA. This might explain some of the discrepancies found in previous studies.

On the feasibility of MRI-guided focused ultrasound for local induction of gene expression

Gene therapy is a promising approach to the treatment of many forms of disease, including cancer. Of critical concern in its implementation is the ability to control the location, duration, and level of expression of the therapeutic gene. Here, we propose the use of local heat in combination with a heat-sensitive promoter to help accomplish this. Certain members of the family of heat shock protein (hsp) promoters display a regulation that depends strongly on temperature. We present a study of natural hsp70 induction in rat leg by MRI-guided focused ultrasound to investigate the hsp70 promoter as a possible candidate for use in control of gene expression with local heat. A temperature increase of 5-8 degrees C in the focal region for 45 minutes led to a differential expression of the hsp70 mRNA between the focal region and the surrounding tissue ranging from a factor of 3 to 67.

Fast 3D functional magnetic resonance imaging at 1.5 T with spiral acquisition

A new method to perform rapid 3D fMRI in human brain is introduced and evaluated in normal subjects, on a standard clinical scanner at 1.5 Tesla. The method combines a highly stable gradient echo technique with a spiral scan method, to detect brain activation related changes in blood oxygenation with high sensitivity. A motor activation paradigm with a duration of less than 5 min, performed on 10 subjects, consistently showed significant changes in signal intensity in the area of the motor cortex. In all subjects, these changes survived high statistical thresholds.

fMRI applications in schizophrenia research

fMRI has unique potential in the study of psychiatric patients, particularly in characterizing individual variations and changes over time. We have performed four studies of patients with schizophrenia, using three different fMRI acquisition protocols: (1) 3-D echo-shifted FLASH, a multishot volumetric approach; (2) 3-D PRESTO, a hybid of multishot and echo-planar imaging (EPI) methods that also acquires true volumetric data; and (3) a whole-brain isotropic, multislice EPI technique. Patients were studied during sensorimotor activation and during a novel "N back" working memory paradigm. In general, patients show normal sensorimotor activation responses, although motor cortical activation tends to be less completely lateralized. Prefrontal activation during working memory tends to be reduced in patients with schizophrenia even when performance is normal. A major potential confound in studying this patient population with fMRI is the effect of motion. We propose several methodological standards to address this problem, including comparisons of motion corrections parameters, voxel variances, and the use of an "internal activation standard."

A comparison of fast MR scan techniques for cerebral activation studies at 1.5 tesla

To evaluate the sensitivity of fast, gradient-echo MR scan techniques in their ability to detect blood oxygenation level dependent (BOLD) signal changes in task activation studies, three dedicated fast scan techniques, each with whole-brain coverage, were compared during a 3-min finger tapping paradigm on nine normal volunteers on a clinical 1.5 T scanner. Multislice (2D) single-shot spiral, 3D spiral, and multislice (2D) single-shot EPI scan techniques were done with similar temporal and spatial resolutions on each of the volunteers in random order. After image registration and statistical analysis, the sensitivity to detect activation was evaluated for the techniques by calculating t scores and number of activated voxels in predetermined regions of interest, including the contralateral primary sensorimotor cortex, the premotor region, the parietal region, the supplementary motor area, and the ipsilateral cerebellum. Baseline images acquired with the three techniques were qualitatively comparable and had a similar effective spatial resolution of around 5 x 5 x 5 mm3, as determined from autocorrelation analysis. The anatomical coverage was somewhat reduced (4 less slices per volume) with EPI at the identical temporal resolution of 1.76 s for all techniques. The use of multislice 2D spiral scan for motor cortex fMRI experiments provided for a superior overall temporal stability, and an increased sensitivity compared with multislice 2D EPI, and 3D spiral scan. The difference in sensitivity between multislice 2D spiral and EPI scans was small, in particular in the case of a ramp-sampled version of EPI. The difference in performance is attributed mainly to the difference in scan-to-scan stability.

fMRI study of effort and information processing in a working memory task

It is unclear how effort translates into brain function. In this study we endeavored to identify the activity in a working memory task that is related to the allocation of mental resources. Such activity, if present, would be a likely candidate to explain how effort works in terms of brain function. Eleven healthy participants performed a Sternberg task with a memory-set of one, three, or five consonants in an fMRI study. Probe stimuli were either one consonant or one digit. We expected digits to be processed automatically and consonants to require working memory. Because the probe type was unpredictable and subjects had to respond as fast as possible, we expected subjects to allocate mental resources on the basis of the memory-set size, not the probe type. Accordingly, we anticipated that activity in regions involved in effort would be a function of the size of the memory-set, but independent of the type of probe. We found that the reaction-time for digits increased in line with our expectation of automatic processing and the reaction time for letters increased in line with our expectation of controlled processing. fMRI revealed that activity in the right ventral-prefrontal cortex changed as a function of effort. The ventral anterior cingulate cortex and hypothalamus showed reduced activity as a function of effort. Activity in regions regarded as pivotal for working memory (among others, the left dorsolateral prefrontal cortex, anterior cingulate cortex) appeared to be predominantly related to information processing and not involved in effort.

Fast echo-shifted gradient-recalled MRI: combining a short repetition time with variable T2* weighting

The principles of a fast T2*-sensitized MR imaging method (Moonen et al., Magn. Reson. Med. 26, 184 (1992)) are extended to further increase T2* sensitivity. It is shown that the period of T2*-weighting can be lengthened by n TR-periods by appropriate gradient schemes without RF refocusing resulting in progressively delayed gradient-recalled echoes. This extension of the echo-shifting concept thus introduces large flexibility in the choice of T2*-weighting without changing total imaging time. The coherence pathway formalism is used to evaluate and describe the selection of the desired echo and the attenuation of unwanted coherences. The new techniques are demonstrated for tracking a bolus of susceptibility contrast agent in cat brain. Relative blood-volume maps are derived with expected contrast between white and gray matter.

The effect of movement amplitude on activation in functional magnetic resonance imaging studies

To evaluate the effect of movement amplitude on the "blood oxygen level-dependent effect," the authors studied six normal subjects while they extended their index finger with two different amplitudes. Images were analyzed using SPM96. In five subjects, the signal intensity increase in the primary sensorimotor area was significantly greater with the larger amplitude movement. In other areas of interest (supplementary motor area, premotor cortex, insula, postcentral area, cerebellum), the large-amplitude movement often showed significant activation when the small-amplitude movement did not. The authors conclude that, in studies of the motor system, movement amplitude needs to be controlled.

High-sensitivity single-shot perfusion-weighted fMRI

A method is presented for measurement of perfusion changes during brain activation using a single-shot pulsed spin labeling technique. By employing a double-inversion labeling strategy, stationary tissue (background) signal was suppressed while minimally affecting perfusion sensitivity. This allowed omission of the otherwise required reference scan, resulting in twofold-improved temporal resolution. The method was applied to visual and motor cortex activation studies in humans, and compared to standard FAIR-type perfusion labeling techniques. Experiments performed at 1.5T and 3.0T indicate a close to 90% suppression of background signal, at a cost of an 11% and 9%, respectively, reduction in perfusion signal. Combined with the twofold increase in signal averaging, and a reduction in background signal fluctuations, this resulted in a 64% (1.5T, N = 3) and a 128% (3T, N = 4) overall improvement in sensitivity for the detection of activation-related perfusion changes. Magn Reson Med 46:88-94, 2001. Published 2001 Wiley-Liss, Inc.

Reproducibility of human 3D fMRI brain maps acquired during a motor task

This study is an investigation into the reproducibility of brain activation in the sensorimotor cortex obtained with 3D "PRESTO" fMRI on eleven normal subjects. During one session, two series of functional scans were acquired while the subjects performed a finger opposition task (2 Hz). Nine subjects were tested once more on a different day. Each individual motor trial was analyzed separately, with a conservative zt-based method. Using these results, the agreement between repeated series was examined in a number of ways, comparing the two series within one session, and the two series across sessions. In 28 of the 31 series (90%) significant signal change was found in the contralateral primary sensorimotor cortex (PSM). Overall, 0.20% of all voxels (total about 11,000) in the scanned volume reached significance, and approximately 60% of the significant positive signal changes were located in the PSM (P<5x10(-7) for a chance occurrence). Comparisons within and across sessions yielded similar results: there was a 20-30% overlap of the clusters of activated voxels in the PSM (chance overlap within the PSM: P<0.01). The mean distance between zt-weighted centers of mass was 4.0-4.4 mm (chance distance within the PSM: P=0.033 and 0.058, respectively). No significant difference was found between series in the magnitude of significant signal change. Whereas the number of activated voxels in the PSM was not consistently correlated between series, the ratio of this number over the total number of activated voxels in the scanned volume was significantly correlated (rho=0.75-0.79, P<0.05). These results indicate that activation in sensorimotor cortex associated with oppositional finger movement is reliably mapped with 3D PRESTO fMRI.