Edward D. Wirth

Visualization of neural tissue water compartments using biexponential diffusion tensor MRI

The apparent diffusion tensor (ADT) imaging method was extended to account for multiple diffusion components. A biexponential ADT imaging experiment was used to obtain separate images of rapidly and slowly diffusing water fractions in excised rat spinal cord. The fast and slow component tensors were compared and found to exhibit similar gross features, such as fractional anisotropy, in both white and gray matter. However, there were also some important differences, which are consistent with the different structures occupying intracellular and extracellular spaces. Evidence supporting the assignment of the two tensor components to extracellular and intracellular water fractions is provided by an NMR spectroscopic investigation of homogeneous samples of brain tissue. Magn Reson Med 45:580–587, 2001.

Feasibility and Safety of Neural Tissue Transplantation in Patients with Syringomyelia

Transplantation of fetal spinal cord (FSC) tissue has demonstrated significant potential in animal models for achieving partial anatomical and functional restoration following spinal cord injury (SCI). To determine whether this strategy can eventually be translated to humans with SCI, a pilot safety and feasibility study was initiated in patients with progressive posttraumatic syringomyelia (PPTS). A total of eight patients with PPTS have been enrolled to date, and this report presents findings for the first two patients through 18 months postoperative. The study design included detailed assessments of each subject at multiple pre- and postoperative time points. Outcome data were then compared with each subjects own baseline. The surgical protocol included detethering, cyst drainage, and implantation of 6-9-week postconception human FSC tissue. Immunosuppression with cyclosporine was initiated a few days prior to surgery and continued for 6 months postoperatively. Key outcome measures included: serial magnetic resonance imaging (MRI) exams, standardized measures of neurological impairment and functional disability, detailed pain assessment, and extensive neurophysiological testing. Through 18 months, the first two patients have been stable neurologically and the MRIs have shown evidence of solid tissue at the graft sites, without evidence of donor tissue overgrowth. Although it is still too soon to draw any firm conclusions, the findings from the initial two patients in this study suggest that intraspinal grafting of human FSC tissue is both feasible and safe.

Diffusion anisotropy in excised normal rat spinal cord measured by NMR microscopy

A conventional spin-echo NMR imaging pulse sequence was used to obtain high-resolution images of excised normal rat spinal cord at 7 and 14 T. It was observed that the large pulsed-field gradients necessary for high-resolution imaging caused a diffusion weighting that dominated the image contrast and that could be used to infer microscopic structural organization beyond that defined by the resolution of the image matrix (i.e., fiber orientation could be assigned based on diffusion anisotropy). Anisotropic diffusion coefficients were therefore measured using apparent diffusion tensor (ADT) imaging to assess more accurately fiber orientations in the spinal cord; structural anisotropy information is portrayed in the six unique images of the complete ADT. To reduce the dimensionality of the data, a trace image was generated using a separate color scale for each of the three diagonal element images of the ADT. This new image retains much of the invariance of the trace to the relative orientations of laboratory and sample axes (inherent to a greyscale trace image) but provides, by the use of color, contrast reflecting diffusion anisotropy. The colored trace image yields a pseudo-three-dimensional view of the rat spinal cord, from which it is possible to deduce fiber orientations.

Neurophysiological assessment of the feasibility and safety of neural tissue transplantation in patients with syringomyelia.

The feasibility and safety of a procedure involving fetal spinal cord tissue transplantation in patients with syringomyelia was assessed using a neurophysiological protocol designed to quantitate peripheral nerve function, spinal cord reflex excitability, and spinal cord conduction pathways essential for somatosensory evoked potentials. We report here data obtained before and for 18 months following the transplantation procedure performed on the first two patients in this study. The neurophysiological assessment protocols included measures of cortical and spinal cord evoked potentials, H-reflex excitability, and peripheral nerve conduction. Prior to the procedure, both patients had significant deficits on some of the neurophysiological measures, for example, lower extremity cortical evoked potentials. However, robust measures of intact pathways, such as upper extremity cortical evoked potentials, were also observed preoperatively in both patients. Thus, it was anticipated that conduction in these intact pathways could be at risk either from complications from the transplantation procedure and/or from continued expansion of the syrinx. Following the transplantation procedure, no negative changes were observed in any of the neurophysiological measures in either patient. In addition, patient 1 showed a decrease in the rate potentiation of tibial H-reflexes on the right side and an increase in the response probability of left tibial H-reflexes. The results of this postoperative longitudinal assessment provide a first-level demonstration of the safety of the intraspinal neural tissue transplantation procedure. However, the consideration of safety is currently limited to the grafting procedure itself, since the long-term fates of the donor tissue in these two patients remain to be shown more definitively.

Characteristics of Human Fetal Spinal Cord Grafts in the Adult Rat Spinal Cord: Influences of Lesion and Grafting Conditions ☆

The present study evaluated the growth potential and differentiation of human fetal spinal cord (FSC) tissue in the injured adult rat spinal cord under different lesion and grafting conditions. Donor tissue at 6-9 weeks of gestational age was obtained through elective abortions and transplanted either immediately into acute resection (solid grafts) or into chronic contusion (suspension and solid grafts) lesions (i.e., 14-40 days after injury) in the thoracic spinal cord. The xenografts were then examined either histologically in plastic sections or immunocytochemically 1-3 months postgrafting. Intraspinal grafts in acute lesions demonstrated an 83% survival rate and developed as well-circumscribed nodules that were predominantly composed of immature astrocytes. Solid-piece grafts in chronic contusion lesions exhibited a 92% survival rate and also developed as nodular masses. These grafts, however, contained many immature neurons 2 months postgrafting. Suspension grafts in chronic contusion lesions had an 85% survival rate and expanded in a nonrestrictive, diffuse pattern. These transplants demonstrated large neuronally rich areas of neural parenchyma. Extensive neuritic outgrowth could also be seen extending from these grafts into the surrounding host spinal cord. These findings show that human FSC tissue reliably survives and differentiates in both acute and chronic lesions. However, both the lesion environment and the grafting techniques can greatly influence the pattern of differentiation and degree of host-graft integration achieved.

Water diffusion measurements in perfused human hippocampal slices undergoing tonicity changes.

Diffusion MRI has the potential to probe the compartmental origins of MR signals acquired from human nervous tissue. However, current experiments in human subjects require long diffusion times, which may confound data interpretation due to the effects of compartmental exchange. To investigate human nervous tissue at shorter diffusion times, and to determine the relevance of previous diffusion studies in rat hippocampal slices, water diffusion in 20 perfused human hippocampal slices was measured using a wide‐bore 17.6‐T magnet equipped with 1000‐mT/m gradients. These slices were procured from five patients undergoing temporal lobectomy for epilepsy. Tissue viability was confirmed with electrophysiological measurements. Diffusion‐weighted water signal attenuation in the slices was well‐described by a biexponential function (R2 > 0.99). The mean diffusion parameters for slices before osmotic perturbation were 0.686 ± 0.082 for the fraction of fast diffusing water (Ffast), 1.22 ± 0.22 × 10−3 mm2/s for the fast apparent diffusion coefficient (ADC), and 0.06 ± 0.02 × 10−3 mm2/s for the slow ADC. Slice perturbations with 20% hypotonic and 20% hypertonic artificial cerebrospinal fluid led to changes in Ffast of −8.2% and +10.1%, respectively (ANOVA, P < 0.001). These data agree with previous diffusion studies of rat brain slices and human brain in vivo, and should aid the development of working models of water diffusion in nervous tissue, and thus increase the clinical utility of diffusion MRI. Magn Reson Med 49:856–863, 2003.

Diffusion Magnetic Resonance Imaging Study of a Rat Hippocampal Slice Model for Acute Brain Injury

Diffusion magnetic resonance imaging (MRI) provides a surrogate marker of acute brain pathology, yet few studies have resolved the evolution of water diffusion changes during the first 8 hours after acute injury, a critical period for therapeutic intervention. To characterize this early period, this study used a 17.6-T wide-bore magnet to measure multicomponent water diffusion at high b-values (7 to 8,080 s/mm2) for rat hippocampal slices at baseline and serially for 8 hours after treatment with the calcium ionophore A23187. The mean fast diffusing water fraction (Ffast) progressively decreased for slices treated with 10-μmol/L A23187 (—20.9 ± 6.3% at 8 hours). Slices treated with 50-μmol/L A23187 had significantly reduced Ffast 80 minutes earlier than slices treated with 10-μmol/L A23187 (P < 0.05), but otherwise, the two doses had equivalent effects on the diffusion properties of tissue water. Correlative histologic analysis showed dose-related selective vulnerability of hippocampal pyramidal neurons (CA1 > CA3) to pathologic swelling induced by A23187, confirming that particular intravoxel cell populations may contribute disproportionately to water diffusion changes observed by MRI after acute brain injury. These data suggest diffusion-weighted images at high b-values and the diffusion parameter Ffast may be highly sensitive correlates of cell swelling in nervous issue after acute injury.

Simultaneous diffusion MRI measurements from multiple perfused rat hippocampal slices

Rat brain slices provide a controllable tissue model in which to investigate the biophysical basis of diffusion‐weighted magnetic resonance (MR) signal changes observed clinically in nervous tissue after ischemic injury. This study describes a new multislice perfusion chamber that allows for the simultaneous acquisition of diffusion‐weighted MR images from multiple perfused rat hippocampal slices (eight slices in the present study). These images had a signal‐to‐noise ratio (SNR) of 48 ± 3 at b = 8080 s/mm2, which was sufficient to analyze the multicomponent diffusion properties of water in rat hippocampal slices. The tissue water diffusion parameters (ffast = 0.527 ± 0.041, Dfast = 1.268 ± 0.087 × 10–3 mm2/s, and Dslow = 0.060 ± 0.003 × 10–3 mm2/s) were stable for at least 8 hr after slice procurement (ANOVA, P > 0.05), suggesting that it may be possible to study the acute temporal evolution of diffusion changes in multiple brain slices following experimental perturbation. Magn Reson Med 48:565–569, 2002.

Dynamic Assessment of Intraspinal Neural Graft Survival Using Magnetic Resonance Imaging

Although previous work has demonstrated the usefulness of magnetic resonance imaging (MRI) for visualizing intraspinal transplants in vivo, the degree to which MRI can differentiate viable fetal neural tissue from evolving spinal cord pathology has not been investigated. Thus, the present study assessed whether MRI performed at earlier postgrafting intervals (0-20 weeks) could document the survival of fetal neural transplants in the injured cat spinal cord. Twelve adult female cats received a hemisection injury at the L1 level, followed immediately by implantation of either embryonic cat spinal cord or neocortex into the cavity. The spinal cords of three control animals were hemisected but received no transplant. Each animal was subsequently imaged at 4 and 8 weeks postoperative. Selected animals from each group were also studied at additional time points ranging from immediately postoperative to 20 weeks. Multislice T2-weighted and intermediate T1-weighted spin-echo images of the lesion or graft site were obtained. Correlative postmortem histological analyses revealed viable donor tissue in 6 of 12 transplant recipients. Spinal cords from the remaining hosts and the control animals all contained cysts at the surgical site that were devoid of donor neural tissue. The graft sites with viable tissue tended to exhibit a slightly hyperintense signal on both intermediate T1-weighted (T1WI) and T2-weighted images (T2WI) throughout the entire experiment. Control cats and cats with failed transplants also were slightly bright on T1WI, but were very hyperintense on T2WI.(ABSTRACT TRUNCATED AT 250 WORDS)

Fetal Grafting in Animal Models of Spinal Cord Injury

The pathological sequelae of spinal cord injury (SCI) are complex and generally result in chronic neuronal loss, axonal damage, and demyelination. Although there have been several experimental therapeutic approaches studied to repair the chronically injured spinal cord, one strategy that has shown considerable promise is fetal tissue grafting. Building upon early work with fetal rat allografts, more recent work reviewed herein showed that this approach could promote recovery of function in a phylogenetically higher species, such as adult cats. Additional preclinical studies have demonstrated successful xenografting of human fetal tissue into the adult rat spinal cord and have revealed the influence of lesion conditions and tissue preparation on graft development. These results suggest that fetal tissue grafting may play a role in the repair of chronic human SCI.