Gavin C. Houston

Fiber density asymmetry of the arcuate fasciculus in relation to functional hemispheric language lateralization in both right- and left-handed healthy subjects: A combined fMRI and DTI study

Previously reported leftward asymmetry in language-related gray and white matter areas of the brain has been proposed as a structural correlate of left-sided functional hemispheric language lateralization. However, structural asymmetry in non-left-sided functional language lateralization has as yet not been studied. Furthermore, the neuroanatomical basis of the reported volumetric white matter asymmetry is not fully understood. In 20 healthy volunteers, including 13 left-handers, we performed functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI). We studied the relative fiber density (RFD) of the arcuate fasciculus (AF), using DT-tractography, in relation to functional hemispheric language lateralization. Hemispheric language lateralization was right-sided in five left-handed individuals. We demonstrated an overall significant leftward asymmetry in RFD of the AF, irrespective of handedness or functional language lateralization. Furthermore, in right-handers, the degree of structural asymmetry was found to be correlated with the degree of functional lateralization. We conclude that structural asymmetry in the AF does not seem to reflect functional hemispheric language lateralization, as has been proposed previously. Our findings suggest that the previously reported white matter asymmetry may be explained by a structural asymmetry in the arcuate fasciculus. These findings have important implications for the understanding of the functional and structural lateralization of brain regions as well as for the clinical evaluation of language function.

Postconcussion syndrome after minor head injury: Brain activation of working memory and attention

After minor head injury (MHI) postconcussive symptoms (PCS) such as memory and attention deficits frequently occur. It has been hypothesised that PCS are caused by microstructural damage to the brain due to shearing injury, which is not detectable with conventional imaging, and may be responsible for a functional deficit. The purpose of this study was to correlate functional magnetic resonance imaging brain activation of working memory and selective attention with PCS. 21 MHI patients and 12 healthy controls were scanned at 3T. Stimulation paradigms were the n‐back and Counting Stroop tasks to engage working memory and selective attention, respectively. Functional data analysis consisted of random effects group analyses, correlating brain activation patterns with the severity of PCS as evaluated with the Rivermead postconcussion symptoms questionnaire. At minimal working memory load, activation was seen in patients with greater severity of PCS in the working memory network. With an increase of working memory load, increase of activation was more pronounced in patients with greater severity of PCS. At high and increased working memory load, activation associated with the severity of PCS was seen in the posterior parietal area, parahippocampal gyrus, and posterior cingulate gyrus. Activation related to selective attention processing was increased with greater severity of PCS. The increased activity in relation to working memory and attention, and the recruitment of brain areas outside the working memory network at high working memory load, may be considered a reflection of the brains compensatory response to microstructural injury in patients with PCS. Hum Brain Mapp, 2009.

Incorporating functional MR imaging into diffusion tensor tractography in the preoperative assessment of the corticospinal tract in patients with brain tumors

BACKGROUND AND PURPOSE: Our goal was to improve the preoperative assessment of the corticospinal tract (CST) in patients with brain tumors. We investigated whether the integration of functional MR imaging (fMRI) data and diffusion tensor (DT) tractography can be used to evaluate the spatial relationship between the hand and foot fibers of the CST and tumor borders. MATERIALS AND METHODS: We imaged 10 subjects: 1 healthy volunteer and 9 patients. Imaging consisted of a 3D T1-weighted sequence, a gradient-echo echo-planar imaging (EPI) sequence for fMRI, and a diffusion-weighted EPI sequence for DT tractography. DT tractography was initiated from a seed region of interest in the white matter area subjacent to the maximal fMRI activity in the precentral cortex. The target region of interest was placed in the cerebral peduncle. RESULTS: In the healthy volunteer, we successfully tracked hand, foot, and lip fibers bilaterally by using fMRI-based DT tractography. In all patients, we could track the hand fibers of the CST bilaterally. In 4 patients who also performed foot tapping, we could clearly distinguish hand and foot fibers. We were able to depict the displacement of hand and foot fibers by tumor and the course of fibers through areas of altered signal intensity. CONCLUSION: Incorporating fMRI into DT tractography in the preoperative assessment of patients with brain tumors may provide additional information on the course of important white matter tracts and their relationship to the tumor. Only this approach allows a distinction between the CST components, while visualization of the CST is improved when fiber tracking is hampered by tumor (infiltration) or perifocal edema.

Super‐resolution methods in MRI: Can they improve the trade‐off between resolution, signal‐to‐noise ratio, and acquisition time?

Improving the resolution in magnetic resonance imaging comes at the cost of either lower signal‐to‐noise ratio, longer acquisition time or both. This study investigates whether so‐called super‐resolution reconstruction methods can increase the resolution in the slice selection direction and, as such, are a viable alternative to direct high‐resolution acquisition in terms of the signal‐to‐noise ratio and acquisition time trade‐offs. The performance of six super‐resolution reconstruction methods and direct high‐resolution acquisitions was compared with respect to these trade‐offs. The methods are based on iterative back‐projection, algebraic reconstruction, and regularized least squares. The algorithms were applied to low‐resolution data sets within which the images were rotated relative to each other. Quantitative experiments involved a computational phantom and a physical phantom containing structures of known dimensions. To visually validate the quantitative evaluations, qualitative experiments were performed, in which images of three different subjects (a phantom, an ex vivo rat knee, and a postmortem mouse) were acquired with different magnetic resonance imaging scanners. The results show that super‐resolution reconstruction can indeed improve the resolution, signal‐to‐noise ratio and acquisition time trade‐offs compared with direct high‐resolution acquisition. Magn Reson Med, 2012.

Diagnostic classification of arterial spin labeling and structural MRI in presenile early stage dementia

Because hypoperfusion of brain tissue precedes atrophy in dementia, the detection of dementia may be advanced by the use of perfusion information. Such information can be obtained noninvasively with arterial spin labeling (ASL), a relatively new MR technique quantifying cerebral blood flow (CBF). Using ASL and structural MRI, we evaluated diagnostic classification in 32 prospectively included presenile early stage dementia patients and 32 healthy controls. Patients were suspected of Alzheimers disease (AD) or frontotemporal dementia. Classification was based on CBF as perfusion marker, gray matter (GM) volume as atrophy marker, and their combination. These markers were each examined using six feature extraction methods: a voxel‐wise method and a region of interest (ROI)‐wise approach using five ROI‐sets in the GM. These ROI‐sets ranged in number from 72 brain regions to a single ROI for the entire supratentorial brain. Classification was performed with a linear support vector machine classifier. For validation of the classification method on the basis of GM features, a reference dataset from the AD Neuroimaging Initiative database was used consisting of AD patients and healthy controls. In our early stage dementia population, the voxelwise feature‐extraction approach achieved more accurate results (area under the curve (AUC) range = 86 − 91%) than all other approaches (AUC = 57 − 84%). Used in isolation, CBF quantified with ASL was a good diagnostic marker for dementia. However, our findings indicated only little added diagnostic value when combining ASL with the structural MRI data (AUC = 91%), which did not significantly improve over accuracy of structural MRI atrophy marker by itself. Hum Brain Mapp 35:4916–4931, 2014.

Variations in labeling protocol influence incorporation, distribution and retention of iron oxide nanoparticles into human umbilical vein endothelial cells

Various studies have shown that various cell types can be labeled with iron oxide particles and visualized by magnetic resonance imaging (MRI). However, reported protocols for cell labeling show a large variation in terms of labeling dose and incubation time. It is therefore not clear how different labeling protocols may influence labeling efficiency. Systematic assessment of the effects of various labeling protocols on labeling efficiency of human umbilical vein endothelial cells (HUVEC) using two different types of iron oxide nanoparticles, i.e. super paramagnetic iron oxide particles (SPIOs) and microparticles of iron oxide (MPIOs), demonstrated that probe concentration, incubation time and particle characteristics all influence the efficiency of label incorporation, label distribution, label retention and cell behavior. For SPIO the optimal labeling protocol consisted of a dose of 12.5 µg iron/2 ml/9.5 cm(2) and an incubation time of 24 h, resulting in an average iron load of 12.0 pg iron/per cell (uptake efficiency of 9.6%). At 4 h many SPIOs are seen sticking to the outside of the cell instead of being taken up by the cell. For MPIO optimal labeling was obtained with a dose of 50 µg iron/2 ml/9.5 cm(2). Incubation time was of less importance since most of the particles were already incorporated within 4 h with a 100% labeling efficiency, resulting in an intracellular iron load of 626 pg/cell. MPIO were taken up more efficiently than SPIO and were also better tolerated. HUVEC could be exposed to and contain higher amounts of iron without causing significant cell death, even though MPIO had a much more pronounced effect on cell appearance. Using optimal labeling conditions as found for HUVEC on other cell lines, we observed that different cell types react differently to identical labeling conditions. Consequently, for each cell type separately an optimal protocol has to be established.

Validation of MR thermometry: Method for temperature probe sensor registration accuracy in head and neck phantoms

Abstract Purpose: Magnetic resonance thermometry (MRT) is an attractive means to non-invasively monitor in vivo temperature during head and neck hyperthermia treatments because it can provide multi-dimensional temperature information with high spatial resolution over large regions of interest. However, validation of MRT measurements in a head and neck clinical set-up is crucial to ensure the temperature maps are accurate. Here we demonstrate a unique approach for temperature probe sensor localisation in head and neck hyperthermia test phantoms. Methods: We characterise the proton resonance frequency shift temperature coefficient and validate MRT measurements in an oil–gel phantom by applying a combination of MR imaging and 3D spline fitting for accurate probe localisation. We also investigate how uncertainties in both the probe localisation and the proton resonance frequency shift (PRFS) thermal coefficient affect the registration of fibre-optic reference temperature probe and MRT readings. Results: The method provides a two-fold advantage of sensor localisation and PRFS thermal coefficient calibration. We provide experimental data for two distinct head and neck phantoms showing the significance of this method as it mitigates temperature probe localisation errors and thereby increases accuracy of MRT validation results. Conclusions: The techniques presented here may be used to simplify calibration experiments that use an interstitial heating device, or any heating method that provides rapid and spatially localised heat distributions. Overall, the experimental verification of the data registration and PRFS thermal coefficient calibration technique provides a useful benchmarking method to maximise MRT accuracy in any similar context.

Super-resolution reconstruction in MRI: better images faster?

Improving the resolution in magnetic resonance imaging (MRI) is always done at the expense of either the signal-to-noise ratio (SNR) or the acquisition time. This study investigates whether so-called super-resolution reconstruction (SRR) is an advantageous alternative to direct high-resolution (HR) acquisition in terms of the SNR and acquisition time trade-offs. An experimental framework was designed to accommodate the comparison of SRR images with direct high-resolution acquisitions with respect to these trade-offs. The framework consisted, on one side, of an image acquisition scheme, based on theoretical relations between resolution, SNR, and acquisition time, and, on the other side, of a protocol for reconstructing SRR images from a varying number of acquired low-resolution (LR) images. The quantitative experiments involved a physical phantom containing structures of known dimensions. Images reconstructed by three SRR methods, one based on iterative back-projection and two on regularized least squares, were quantitatively and qualitatively compared with direct HR acquisitions. To visually validate the quantitative evaluations, qualitative experiments were performed, in which images of three different subjects (a phantom, an ex-vivo rat knee, and a post-mortem mouse) were acquired with different MRI scanners. The quantitative results indicate that for long acquisition times, when multiple acquisitions are averaged to improve SNR, SRR can achieve better resolution at better SNR than direct HR acquisitions.