W. Van Hecke - Academia.edu (original) (raw)

Papers by W. Van Hecke

Therapeutic Advances in Neurological Disorders, 2019

Background: Whole brain atrophy (WBA) estimates in multiple sclerosis (MS) correlate more robustl... more Background: Whole brain atrophy (WBA) estimates in multiple sclerosis (MS) correlate more robustly with clinical disability than traditional, lesion-based metrics. We compare Structural Image Evaluation using Normalisation of Atrophy (SIENA) with the icobrain longitudinal pipeline (icobrain long), for assessment of longitudinal WBA in MS patients. Methods: Magnetic resonance imaging (MRI) scan pairs [1.05 (±0.15) year separation] from 102 MS patients were acquired on the same 3T scanner. Three-dimensional (3D) T1-weighted and two-dimensional (2D)/3D fluid-attenuated inversion-recovery sequences were analysed. Percentage brain volume change (PBVC) measurements were calculated using SIENA and icobrain long. Statistical correlation, agreement and consistency between methods was evaluated; MRI brain volumetric and clinical data were compared. The proportion of the cohort with annualized brain volume loss (aBVL) rates ⩾ 0.4%, ⩾0.8% and ⩾0.94% were calculated. No evidence of disease activ...

Diffusion Tensor Imaging, 2016

Journal of Neuroscience, Psychology, and Economics, 2014

The neuroradiology journal, Jan 30, 2007

This study discusses the influence of user-defined parameters on fiber tracking results obtained ... more This study discusses the influence of user-defined parameters on fiber tracking results obtained from a standard deterministic streamline tractography algorithm. Diffusion tensor imaging with fiber tractography was performed in five healthy volunteers. A region of interest was highlighted in the ventral part of the pons at the level of the middle cerebellar peduncle. The parameters studied were angle threshold, fractional anisotropy threshold, step length and number of seed samples per voxel. Changes in fiber tracts were described for increasing values per parameter. Increasing the angle threshold resulted in more and longer fibers. A higher fractional anisotropy threshold resulted in decreased length and fiber tracts that were not representative. Increasing the step length decreased the fiber continuity and altered its position. A higher number of seed samples per voxel resulted in a higher fiber tract density. When interpreting diffusion tensor images, the reader should understand...

Introduction: Recently, an extension of the concept of track-density imaging (TDI) that also enco... more Introduction: Recently, an extension of the concept of track-density imaging (TDI) that also encodes the angular distribution was presented [1]. Furthermore, the resulting track orientation distribution (TOD) of a dense full-brain short-tracks tractogram was used as an alternative to the fiber orientation distribution (FOD) from constrained spherical deconvolution (CSD) for the purpose of guiding tractography itself. It was reasoned that the TOD in each voxel has inherent information on the support of directions by the surroundings and can therefore guide a track along directions that are more likely to correspond to continuous structure over a longer distance [1]. Backed up by this powerful concept, TOD-based tractography was shown to yield promising qualitative results on in vivo data of challenging quality. A quantitative validation was, however, lacking. In this work, we provide such validation by employing concepts of the recently proposed Tractometer approach [2]. Data: The data consists of an acquisition of the phantom that was also used in [2]. The general layout is shown in Fig.1. The voxel size equals 3×3×3mm³ (there are 3 slices). We chose to employ the acquisition that provided the overall best results in [2]: two repetitions of 64 uniformly distributed DWI-directions at b=1500s/mm². FOD, TOD-1, TOD-2: FODs were obtained in a "white matter" (WM) region by performing CSD up to a spherical harmonic (SH) order of 8. Outside the WM region, FODs were defined to have zero amplitude. The TODs were constructed as explained in [1]; 80 million short-tracks were generated by probabilistic FOD-based tracking, seeded from the WM region, and using the following parameters: stepsize = 0.2mm, min. radius of curvature = 1mm, min. track length = 24mm, max. track length = 48mm, min. FOD amplitude to initiate/continue tracks = 0.2/0.1. TOD-1 was then obtained by integrating an apodized point spread function of maximum SH order 16 along each track. TOD-2 was obtained in the same way from TOD-1; albeit by setting min. TOD amplitude to initiate/continue tracks = 200000/100000. For further explanation, we refer the reader to [1]. In Fig.2 (top), FOD, TOD-1 and TOD-2 are compared in a crossing region. Large qualitative improvements are shown. Tractography: Deterministic as well as probabilistic FOD-and TOD-based tractography was performed using the same (above) parameters; yet without a max. track length, and-specifically for deterministic tracking-without a curvature threshold. MRtrix [3] generated tracks until 1000000 were accepted for each experiment. The amount of tracks that had to be generated to achieve this (including rejected tracks) is given in Tab.1. "Out of the box", TOD-based tracking produces less tracks that are unable to meet the criteria. A random subset of the deterministic tracks is shown in Fig.2 (bottom); qualitative improvements are in line with those in Fig.2 (top). Tractometer: Using the ROIs shown in Fig.1, each set of 1000000 tracks was subdivided in valid connections (VC), invalid connections (IC) and no connections (NC); as defined in [2]. The percentage of tracks in each of these categories is given in Tab.2. For TOD-based tracking, consistent (large) increases of VC and reductions of IC and NC are seen. The additional measure of VC/(VC+IC) reflects the amount of VC one would obtain by enforcing anatomical constraints such as in [4] (i.e. tracks must connect (gray matter) ROIs, thus NC are absent). In this case, probabilistic TOD-based tracking even scores up to 80%, while its deterministic counterpart has the potential to reach over 90%! As was already reasoned in [2], however, probabilistic methods are surely disadvantaged by the limitations of this particular phantom. The actual VC are shown in Fig.3. The TOD-based results provide qualitatively better looking VC; e.g. nicer boundaries and less bulging of bundle 6 into 7, amongst others. Conclusion: In this work, we provided a much-needed quantitative validation of TOD-based tractography. As the Tractometer approach focusses on the connectivity aspect, our results in favour of TOD-based tracking promote its use in the domain of connectomics. Qualitative results, as obtained in [1], are furthermore confirmed.

Medical image computing and computer-assisted intervention : MICCAI ... International Conference on Medical Image Computing and Computer-Assisted Intervention, 2011

In the field of diffusion weighted imaging (DWI), it is common to fit one of many available model... more In the field of diffusion weighted imaging (DWI), it is common to fit one of many available models to the acquired data. A hybrid diffusion imaging (HYDI) approach even allows to reconstruct different models and measures from a single dataset. Methods for DWI atlas construction (and registration) are as plenty as the number of available models. Therefore, it would be nice if we were able to perform atlas building before model reconstruction. In this work, we present a method for atlas construction of DWI data in q-space: we developed a new multi-subject multi-channel diffeomorphic matching algorithm, which is combined with a recently proposed DWI retransformation method in q-space. We applied our method to HYDI data of 10 healthy subjects. From the resulting atlas, we also reconstructed some advanced models. We hereby demonstrate the feasibility of q-space atlas building, as well as the quality, advantages and possibilities of such an atlas.

2010 IEEE International Symposium on Biomedical Imaging: From Nano to Macro, 2010

... [7] A. Barmpoutis, BC Vemuri, and JR Forder, Registration of high angular resolution diffusi... more ... [7] A. Barmpoutis, BC Vemuri, and JR Forder, Registration of high angular resolution diffusion MRI images using 4th order tensors, LNCS 4791, pp. 908915, 29 Oct - 2 Nov 2007. ... 5, no. 10, pp. 1435 1447, 1996b. [13] DC Alexander, C. Pierpaoli, PJ Basser, and JC Gee, Spa ...

Medical Imaging 2008: Image Processing, 2008

ABSTRACT Purpose: Voxel based morphometry (VBM) is increasingly being used to detect diffusion te... more ABSTRACT Purpose: Voxel based morphometry (VBM) is increasingly being used to detect diffusion tensor (DT) image abnormalities in patients for different pathologies. An important requisite for these VBM studies is the use of a high-dimensional, non-rigid coregistration technique, which is able to align both the spatial and the orientational information. Recent studies furthermore indicate that high-dimensional DT information should be included during coregistration for an optimal alignment. In this context, a population based DTI atlas is created that preserves the orientational DT information robustly and contains a minimal bias towards any specific individual data set. Methods: A ground truth evaluation method is developed using a single subject DT image that is deformed with 20 deformation fields. Thereafter, an atlas is constructed based on these 20 resulting images. Thereby, the non-rigid coregistration algorithm is based on a viscous fluid model and on mutual information. The fractional anisotropy (FA) maps as well as the DT elements are used as DT image information during the coregistration algorithm, in order to minimize the orientational alignment inaccuracies. Results: The population based DT atlas is compared with the ground truth image using accuracy and precision measures of spatial and orientational dependent metrics. Results indicate that the population based atlas preserves the orientational information in a robust way. Conclusion: A subject independent population based DT atlas is constructed and evaluated with a ground truth method. This atlas contains all available orientational information and can be used in future VBM studies as a reference system.

Physics in Medicine and Biology, 2012

Accurate simulations on detailed realistic head models are necessary to gain a better understandi... more Accurate simulations on detailed realistic head models are necessary to gain a better understanding of the response to transcranial magnetic stimulation (TMS). Hitherto, head models with simplified geometries and constant isotropic material properties are often used, whereas some biological tissues have anisotropic characteristics which vary naturally with frequency. Moreover, most computational methods do not take the tissue permittivity into account. Therefore, we calculate the electromagnetic behaviour due to TMS in a head model with realistic geometry and where realistic dispersive anisotropic tissue properties are incorporated, based on T1-weighted and diffusion-weighted magnetic resonance images. This paper studies the impact of tissue anisotropy, permittivity and frequency dependence, using the anisotropic independent impedance method. The results show that anisotropy yields differences up to 32% and 19% of the maximum induced currents and electric field, respectively. Neglecting the permittivity values leads to a decrease of about 72% and 24% of the maximum currents and field, respectively. Implementing the dispersive effects of biological tissues results in a difference of 6% of the maximum currents. The cerebral voxels show limited sensitivity of the induced electric field to changes in conductivity and permittivity, whereas the field varies approximately linearly with frequency. These findings illustrate the importance of including each of the above parameters in the model and confirm the need

PAIN, 2008

Central post-stroke pain (CPSP) is one of the most common types of intractable pain. We reported ... more Central post-stroke pain (CPSP) is one of the most common types of intractable pain. We reported that repetitive transcranial magnetic stimulation (rTMS) of primary motor cortex relieves pain for patients who were refractory to medical treatment. But the mechanism is unclear. In the present study, we investigated relations between the characteristics of CPSP and the results of fiber tracking, which is the only noninvasive method of evaluating the anatomical connectivity of white matter pathways. Fiber tracking of the corticospinal tract (CST) and thalamocortical tract (TCT) was investigated in 17 patients with CPSP. The stroke lesion was located in a supratentorial region in all cases (corona radiata, one case; thalamus, seven cases; putamen, nine cases). Relations between the delineation ratio (defined as the ratio of the cross section of the affected side to that of the unaffected side) of the CST and of the TCT, manual muscle test score, pain score, region of pain, and efficacy of rTMS were evaluated. Fiber tracking was successful in 13 patients with the stroke lesion involving the TCT. The rTMS-effective group had higher delineation ratio of the CST (p=0.02) and the TCT (p=0.005) than the rTMS-ineffective group. Previous studies suggested that an intact CST allows pain control but did not discuss the TCT. Our results suggest that the TCT also plays a role in pain reduction by rTMS of the primary motor cortex and that the efficacy of rTMS for patients with CPSP is predictable by fiber tracking.

NeuroImage, 2011

Rats are widely used in experimental neurobiological research, and rat brain atlases are importan... more Rats are widely used in experimental neurobiological research, and rat brain atlases are important resources for identifying brain regions in context of experimental microsurgery, tissue sampling, and neuroimaging, as well as comparison of findings across experiments. Currently, most available rat brain atlases are constructed from histological material derived from single specimens, and provide two-dimensional or three-dimensional (3-D) outlines of diverse brain regions and fiber tracts. Important limitations of such atlases are that they represent individual specimens, and that finer details of tissue architecture are lacking. Access to more detailed 3-D brain atlases representative for a population of animals is needed. Diffusion tensor imaging (DTI) is a unique neuroimaging modality that provides sensitive information about orientation structure in tissues, and is widely applied in basic and clinical neuroscience investigations. To facilitate analysis and assignment of location in rat brain neuroimaging investigations, we have developed a population-averaged three-dimensional DTI atlas of the normal adult Sprague Dawley rat brain. The atlas is constructed from high resolution ex vivo DTI images, which were nonlinearly warped into a population-averaged in vivo brain template. The atlas currently comprises a selection of manually delineated brain regions, the caudate putamen complex, globus pallidus, entopeduncular nucleus, substantia nigra, external capsule, corpus callosum, internal capsule, cerebral peduncle, fimbria of the hippocampus, fornix, anterior commisure, optic tract, and stria terminalis. The atlas is freely distributed and potentially useful for several purposes, including automated and manual delineation of rat brain structural and functional imaging data.

NeuroImage, 2009

SA-AM A framework for a hierarchical fiber bundle model inference using high angular resolution d... more SA-AM A framework for a hierarchical fiber bundle model inference using high angular resolution diffusion imaging,

Magnetic Resonance in Medicine, 2013

Diffusion weighted magnetic resonance images are often acquired with single shot multislice imagi... more Diffusion weighted magnetic resonance images are often acquired with single shot multislice imaging sequences, because of their short scanning times and robustness to motion. To minimize noise and acquisition time, images are generally acquired with either anisotropic or isotropic low resolution voxels, which impedes subsequent posterior image processing and visualization. In this article, we propose a superresolution method for diffusion weighted imaging that combines anisotropic multislice images to enhance the spatial resolution of diffusion tensor data. Each diffusion weighted image is reconstructed from a set of arbitrarily oriented images with a low through-plane resolution. The quality of the reconstructed diffusion weighted images was evaluated by diffusion tensor metrics and tractography. Experiments with simulated data, a hardware DTI phantom, as well as in vivo human brain data were conducted. Our results show a significant increase in spatial resolution of the diffusion tensor data while preserving high signal to noise ratio. Magn Reson Med 000:000-000, 2012.