Spyridon Kollias

Reduced field‐of‐view MRI using outer volume suppression for spinal cord diffusion imaging

A spin‐echo single‐shot echo‐planar imaging (SS‐EPI) technique with a reduced field of view (FOV) in the phase‐encoding direction is presented that simultaneously reduces susceptibility effects and motion artifacts in diffusion‐weighted (DW) imaging (DWI) of the spinal cord at a high field strength (3T). To minimize aliasing, an outer volume suppression (OVS) sequence was implemented. Effective fat suppression was achieved with the use of a slice‐selection gradient‐reversal technique. The OVS was optimized by numerical simulations with respect to T1 relaxation times and B1 variations. The optimized sequence was evaluated in vitro and in vivo. In simulations the optimized OVS showed suppression to <0.25% and ∼3% in an optimal and worst‐case scenario, respectively. In vitro measurements showed a mean residual signal of <0.95% ± 0.42 for all suppressed areas. In vivo acquisition with 0.9 × 1.05 mm2 in‐plane resolution resulted in artifact‐free images. The short imaging time of this technique makes it promising for clinical studies. Magn Reson Med 57:625–630, 2007.

Resolving fiber crossing using advanced fast marching tractography based on diffusion tensor imaging.

Magnetic resonance diffusion tensor tractography is a powerful tool for the non-invasive depiction of the white matter architecture in the human brain. However, due to limitations in the underlying tensor model, the technique is often unable to reconstruct correct trajectories in heterogeneous fiber arrangements, such as axonal crossings. A novel tractography method based on fast marching (FM) is proposed which is capable of resolving fiber crossings and also permits trajectories to branch. It detects heterogeneous fiber arrangements by incorporating information from the entire diffusion tensor. The FM speed function is adapted to the local tensor characteristics, allowing in particular to maintain the front evolution direction in crossing situations. In addition, the FMs discretization error is reduced by increasing the number of considered possible front evolution directions. The performance of the technique is demonstrated in artificial data and in the healthy human brain. Comparisons with standard FM tractography and conventional line propagation algorithms show that, in the presence of interfering structures, the proposed method is more accurate in reconstructing trajectories. The in vivo results illustrate that the elucidated major white matter pathways are consistent with known anatomy and that multiple crossings and tract branching are handled correctly.

Combining fMRI and DTI: a framework for exploring the limits of fMRI-guided DTI fiber tracking and for verifying DTI-based fiber tractography results.

A powerful, non-invasive technique for estimating and visualizing white matter tracts in the human brain in vivo is white matter fiber tractography that uses magnetic resonance diffusion tensor imaging. The success of this method depends strongly on the capability of the applied tracking algorithm and the quality of the underlying data set. However, DTI-based fiber tractography still lacks standardized validation. In the present work, a combined fMRI/DTI study was performed, both to develop a setup for verifying fiber tracking results using fMRI-derived functional connections and to explore the limitations of fMRI based DTI fiber tracking. Therefore, a minor fiber bundle that features several fiber crossings and intersections was examined: The striatum and its connections to the primary motor cortex were examined by using two approaches to derive the somatotopic organization of the striatum. First, an fMRI-based somatotopic map of the striatum was reconstructed, based on fMRI activations that were provoked by unilateral motor tasks. Second, fMRI-guided DTI fiber tracking was performed to generate DTI-based somatotopic maps, using a standard line propagation and an advanced fast marching algorithm. The results show that the fiber connections reconstructed by the advanced fast marching algorithm are in good agreement with known anatomy, and that the DTI-revealed somatotopy is similar to the fMRI somatotopy. Furthermore, the study illustrates that the combination of fMRI with DTI can supply additional information in order to choose reasonable seed regions for generating functionally relevant networks and to validate reconstructed fibers.

Chronic Cervical Spinal Cord Injury: DTI Correlates with Clinical and Electrophysiological Measures

Diffusion tensor imaging (DTI) is rarely applied in spinal cord injury (SCI). The aim of this study was to correlate diffusion properties after SCI with electrophysiological and neurological measures. Nineteen traumatic cervical SCI subjects and 28 age-matched healthy subjects participated in this study. DTI data of the spinal cord were acquired with a Philips Achieva 3 T MR scanner using an outer volume suppressed, reduced field of view (FOV) acquisition with oblique slice excitation and a single-shot EPI readout. Neurological and electrophysiological measures, American Spinal Injury Association (ASIA) impairment scale scores, and motor (MEP) and somatosensory evoked potentials (SSEP) were assessed in SCI subjects. Fractional anisotropy (FA) values were decreased in the SCI subjects compared to the healthy subjects. In upper cervical segments, the decrease in FA was significant for the evaluation of the entire cross-sectional area of the spinal cord, and for corticospinal and sensory tracts. A decreasing trend was also found at the thoracic level for the corticospinal tracts. The decrease of DTI values correlated with the clinical completeness of SCI, and with SSEP amplitudes. The reduced DTI values seen in the SCI subjects are likely due to demyelination and axonal degeneration of spinal tracts, which are related to clinical and electrophysiological measures. A reduction in DTI values in regions remote from the injury site suggests their involvement with wallerian axonal degeneration. DTI can be used for the quantitative evaluation of the extent of spinal cord damage, and eventually to monitor the effects of future regeneration-inducing treatments.

Diffusion-weighted imaging of the entire spinal cord

In spite of their diagnostic potential, the poor quality of available diffusion‐weighted spinal cord images often restricts clinical application to cervical regions, and improved spatial resolution is highly desirable. To address these needs, a novel technique based on the combination of two recently presented reduced field‐of‐view approaches is proposed, enabling high‐resolution acquisition over the entire spinal cord. Field‐of‐view reduction is achieved by the application of non‐coplanar excitation and refocusing pulses combined with outer volume suppression for removal of unwanted transition zones. The non‐coplanar excitation is performed such that a gap‐less volume is acquired in a dedicated interleaved slice order within two repetition times. The resulting inner volume selectivity was evaluated in vitro. In vivo diffusion tensor imaging data on the cervical, thoracic and lumbar spinal cord were acquired in transverse orientation in each of four healthy subjects. An in‐plane resolution of 0.7 × 0.7 mm2 was achieved without notable aliasing, motion or susceptibility artifacts. The measured mean ± SD fractional anisotropy was 0.69 ± 0.11 in the thoracic spinal cord and 0.75 ± 0.07 and 0.63 ± 0.08 in cervical and lumbar white matter, respectively. Copyright

Influence of SENSE on image properties in high-resolution single-shot echo-planar DTI.

Limited spatial resolution is a key obstacle to the study of brain white matter structure with diffusion tensor imaging (DTI). In its frequent implementation with single‐excitation spin‐echo echo‐planar sequences, DTIs ability to resolve small structures is strongly restricted by T2 and T  2* decay, B0 inhomogeneity, and limited signal‐to‐noise ratio (SNR). In this work the influence of sensitivity encoding (SENSE) on diffusion‐weighted (DW) image properties is investigated. Computer simulations showed that the PSF becomes narrower with increasing SENSE reduction factors, R, enhancing the intrinsic resolution. After a brief theoretical discussion, we describe the estimation of SNR on a pixel‐by‐pixel basis as a function of R. The mean image SNR behavior is manifold: SENSE is capable of increasing SNR efficiency by reducing the echo time (TE). Each SNR(R) curve reveals a maximum that depends on the amount of partial Fourier encoding used. The overall best SNR efficiency for an eight‐element head coil array and a b‐factor of 1000 s/mm2 is achieved at R = 2.1 and partial Fourier encoding of 60%. In vivo tensor maps of volunteers and a patient, with an in‐plane resolution of 0.78 × 0.78 mm2, are also presented to demonstrate the practical implementation of the parallel approach. Magn Reson Med, 2006.

Grey matter changes associated with medication-overuse headache: correlations with disease related disability and anxiety.

Abstract Objectives. Medication-overuse headache (MOH) is associated with psychiatric comorbidities. Neurobiological similarities to substance dependence have been suggested. This study investigated grey matter changes, focussing on pain and reward systems. Methods. Using voxel-based morphometry, structural MRIs were compared between 29 patients with both, MOH and migraine, according to International Headache Society criteria, and healthy controls. The Migraine Disability Assessment (MIDAS) score was used. Anxiety and depression were screened for with the Hospital Anxiety and Depression Scale (HADS) and confirmed by a psychiatrist, using the Mini International Neuropsychiatric Interview. Results. Nineteen patients (66%) had a present or past psychiatric disorder, mainly affective (N = 11) and anxiety disorders (N = 8). In all patients a significant increase of grey matter volume (GMV) was found in the periaqueductal grey matter of the midbrain, which correlated positively with the MIDAS and the HADS-anxiety subscale. A GMV increase was found bilaterally in the thalamus, and the ventral striatum. A significant GMV decrease was detected in frontal regions including orbitofrontal cortex, anterior cingulate cortex, the left and right insula, and the precuneus. Conclusion. These findings are consistent with dysfunction of antinociceptive systems in MOH, which is influenced by anxiety. Dysfunction of the reward system may be a neurobiological basis for dependence in a subgroup of MOH patients.

Reconstruction of the human visual system based on DTI fiber tracking.

To apply and to evaluate the newly developed advanced fast marching algorithm (aFM) in vivo by reconstructing the human visual pathway, which is characterized by areas of extensive fiber crossing and branching, i.e., the optic chiasm and the lateral geniculate nucleus (LGN).

Decrease of Gray Matter Volume in the Midbrain is Associated with Treatment Response in Medication-Overuse Headache: Possible Influence of Orbitofrontal Cortex

Patients with chronic daily headache and overuse of analgesics, triptans, or other acute headache compounds, are considered to suffer from medication-overuse headache (MOH). This implies that medication overuse is the cause of headache chronification. It remains a key question why only two-thirds of patients with chronic migraine-like headache and overuse of pain medication improve after detoxification, whereas the remainder continue to have chronic headache. In the present longitudinal MRI study, we used voxel-based morphometry to investigate gray matter changes related to medication withdrawal in a group of humans with MOH. As a main result, we found that only patients with significant clinical improvement showed a significant decrease of previously increased gray matter in the midbrain including periaqueductal gray matter and nucleus cuneiformis, whereas patients without improvement did not. Patients without treatment response had less gray matter in the orbitofrontal cortex. Another striking result is the correlation of treatment response with the amount of orbitofrontal gray matter. Thus, we demonstrate adaptive gray matter changes within the pain modulatory system in patients with MOH who responded to detoxification, probably reflecting neuronal plasticity. Decreased gray matter in the orbitofrontal cortex at baseline may be predictive of poor response to treatment.

Preliminary experience with visualization of intracortical fibers by focused high-resolution diffusion tensor imaging

BACKGROUND AND PURPOSE: The inherent low anisotropy of gray matter and the lack of adequate imaging sensitivity and resolution has, so far, impeded depiction of axonal fibers to their intracortical origin or termination. We tested the hypothesis that an experimental approach with high-resolution diffusion tensor imaging (DTI) provides anisotropic data for fiber tractography with sufficient sensitivity to visualize in vivo the fine distribution of white matter bundles at the intracortical level. MATERIALS AND METHODS: We conducted phantom measurements of signal-to-noise ratio (SNR) and obtained diffusion tensor maps of the occipital lobe in 6 healthy volunteers using a dedicated miniature phased array detector at 3T. We reconstructed virtual fibers using a standard tracking algorithm. RESULTS: The coil array provided a SNR of 8.0 times higher at the head surface compared with a standard quadrature whole head coil. Diffusion tensor maps could be obtained with an in-plane resolution of 0.58 × 0.58 mm2. The axonal trajectories reconstructed from the diffusion data penetrate into the cortical ribbon perpendicular to the pial surface. This is the expected pattern for the terminations of thalamocortical afferent fibers to the middle layers of the occipital cortex and is consistent with the known microstructural organization of the mammalian cerebral cortex. CONCLUSION: High-resolution DTI reveals intracortical anisotropy with a distinct parallel geometrical order, perpendicular to the pial surface, consistent with structures that may be identified as the terminal afferents in cortical gray matter.