P. Van Hecke

Changes in brain activation during the acquisition of a new bimanual coordination task

Motor skill acquisition is associated with the development of automaticity and induces neuroplastic changes in the brain. Using functional magnetic resonance imaging (fMRI), the present study traced learning-related activation changes during the acquisition of a new complex bimanual skill, requiring a difficult spatio-temporal relationship between the limbs, i.e., cyclical flexion-extension movements of both hands with a phase offset of 90 degrees. Subjects were scanned during initial learning and after the coordination pattern was established. Kinematics of the movements were accurately registered and showed that the new skill was acquired well. Learning-related decreases in activation were found in right dorsolateral prefrontal cortex (DLPFC), right premotor, bilateral superior parietal cortex, and left cerebellar lobule VI. Conversely, learning-related increases in activation were observed in bilateral primary motor cortex, bilateral superior temporal gyrus, bilateral cingulate motor cortex (CMC), left premotor cortex, cerebellar dentate nuclei/lobule III/IV/Crus I, putamen/globus pallidus and thalamus. Accordingly, bimanual skill learning was associated with a shift in activation among cortico-subcortical regions, providing further evidence for the existence of differential cortico-subcortical circuits preferentially involved during the early and advanced stages of learning. The observed activation changes account for the transition from highly attention-demanding task performance, involving processing of sensory information and corrective action planning, to automatic performance based on memory representations and forward control.

Cerebellar and premotor function in bimanual coordination: parametric neural responses to spatiotemporal complexity and cycling frequency.

In the present functional magnetic resonance imaging (fMRI) study, we assessed the neural network governing bimanual coordination during manipulations of spatiotemporal complexity and cycling frequency. A parametric analysis was applied to determine the effects of each of both factors as well as their interaction. Subjects performed four different cyclical movement tasks of increasing spatiotemporal complexity (i.e., unimanual left-right hand movements, bimanual in-phase movements, bimanual anti-phase movements, and bimanual 90 degrees out-of-phase movements) across four frequency levels (0.9, 1.2, 1.5, and 1.8 Hz). Results showed that, within the network involved in bimanual coordination, functional subcircuits could be distinguished: Activation in the supplementary motor area, superior parietal cortex (SPS), and thalamic VPL Nc was mainly correlated with increasing spatiotemporal complexity of the limb movements, suggesting that these areas are involved in higher-order movement control. By contrast, activation within the primary motor cortex, cingulate motor cortex (CMC), globus pallidus, and thalamic VLo Nc correlated mainly with movement frequency, indicating that these areas play an important role during movement execution. Interestingly, the cerebellum and the dorsal premotor cortex were identified as the principal regions responding to manipulation of both parameters and exhibiting clear interaction effects. Therefore, it is concluded that both areas represent critical sites for the control of bimanual coordination.

Localized 1H NMR spectroscopy in fifty cases of newly diagnosed intracranial tumors

Fifty patients with newly diagnosed, untreated intracranial tumors were examined with 1H nuclear magnetic resonance single-volume spectroscopy (MRS) using a 1.5 T whole-body MR system. Prior to the MRS, contrast enhanced MR and/or CT imaging studies were carried out. Histological verification was obtained in all patients except one. All tumor spectra revealed distinct abnormalities as compared with the normal brain spectra. Although most meningiomas showed a rather characteristic spectral pattern, generally features specific for the various tumor types were not observed. For instance, though a strong lactic acid signal was seen in most malignant tumors, this signal was also evident in five benign neoplasms.

NMR imaging study of the pharmacodynamics of polylysine-gadolinium-DTPA in the rabbit and the rat.

The pharmacodynamics of polylysine-(Gd-DTPA) (Schering, Berlin, Germany), a new blood pooling contrast agent for MRI, were studied in the rabbit and the rat. Polylysine-(Gd-DTPA) is a compound with high LD50. Due to its high molecular weight (50.000) and physico-chemical properties, it remains in the vascular system; during the first hour, the plasma level is three times higher than for Gd-DTPA. MRI was performed at 1.5 T using a SE sequence with TR/TE = 300/15 or 20 msec. Signal intensities of muscle, liver and kidney were measured before and after intravenous injection of the contrast agent (0.1 mmol/kg) during 8 hours in the rat (n = 3) and up to 2 wk in the rabbit (n = 3). A dose response study in three additional rabbits confirmed that the 0.1 mmol/kg dose was optimal. The pharmacodynamics results show that the effects of polylysine-(Gd-DTPA) are similar in both the rabbit and the rat. The liver signal is enhanced by about 60% immediately after injection in both species. This enhanced signal decays to half its maximal value in about one hour, which makes the contrast agent useful for clinical applications at a dose of 0.1 mmol/kg. In the kidney medulla and cortex the signals are enhanced by much larger factors (about 3 to 4); it takes at least one day for the kidney to clear the contrast agent in both species.

Functional MRI of the brain: localisation of eloquent cortex in focal brain lesion therapy.

Abstract. The aim of this study was to assess the feasibility of functional MRI (fMRI) in a clinical environment on a large patient group, and to evaluate the pretherapeutic value of localisation of eloquent cortex. Forty patients with focal brain lesions of different origin were studied using fMRI. Functional information was obtained using motor, somatosensory, auditory and phonological stimuli depending on the localisation of the lesions. To obtain information about the spatial accuracy of fMRI, the results were compared with postoperative electrocortical stimulation. Two patients with secondary trigeminal neuralgia were scanned using a motor protocol and were implanted with an extradural plate electrode. Imaging was successful in 40 of 42 patients (including the 2 with trigeminal neuralgia). These patients were analysed for strength of activation, the relation of the lesion to activation sites and the presence of mass effect. The correlation between these data and surgical findings provided significant additional clinical information. Functional MRI can be accurately performed in patients with focal brain lesions using a dedicated approach. Functional MRI offers important clinical information as a contribution to a decrease in posttherapeutic morbidity. The accuracy of the technique can be confirmed by other modalities, including invasive cortical electrostimulation.

Cerebral venous angiomas

SummarySixteen cases of cerebral venous angiomas, seven cerebellar and nine supratentorial were imaged by magnetic resonance, using a T1 sequence and a motion compensated T2 sequence. The transcerebral draining vein of the lesion could be identified in thirteen out of sixteen patients. On T1-weighted images all the lesions were hypointense. On T2-weighted images, five lesions were hypointense, and eleven were hyperintense consistent with slow flow. In three cases the lesion was hemorrhagic. Three lesions with hypointense aspect on T1 and hyperintense aspect on T2-weighted images were only seen in retrospect, due to partial volume averaging in two and concomitant chronic hemorrhage in one. Before contrast, the peripheral dilated medullary veins could only be identified in five cases. On T2-weighted images, in three of these lesions the signal was higher than in the draining veins, pointing to a slower flow. After intravenous administration of Gadolinium, performed in twelve patients, the main transcerebral draining vein as well as the peripheral medullary veins could be well identified in all patients.

A Superresolution Framework for fMRI Sequences and Its Impact on Resulting Activation Maps

This paper investigates the benefits of using a superresolution approach for fMRI sequences in order to obtain high-quality activation maps based on low-resolution acquisitions. We propose a protocol to acquire low-resolution images, shifted in the slice direction, so that they can be used to generate superresolution images. Adopting a variational framework, the superresolution images are defiend as the minimizers of objective functions. We focus on edge preserving regularized objective functions because of their ability to preserve details and edges. We show that applying regularization only in the slice direction leads more pertinent solutions than 3-dimensional regularization. Moreover, it leads to a considerably easier optimization problem. The latter point is crucial since we have to process long fMRI sequences. The solutions—the sought high resoltion images—are calculated based on a half-quadratic reformulation of the objective function which allows fast minimization schemes to be implemented. Our acquisition protocol and processing technique are tested both on simulated and real functional MRI datasets.

Parameter estimation for moisture transport in apples with the aid of NMR imaging

The moisture diffusivity of Jonagold apple flesh and skin was estimated based on NMR images obtained from drying experiments. It was assumed that the diffusivities were not dependent on the moisture concentration. A multi‐echo sequence was required to correct for the change of T2 during the experiment. The diffusivities were estimated by minimization of the squared error between measured and predicted moisture concentrations. The latter were calculated based on the analytical solution or a Galerkin finite element discretization of the diffusion equation. The average flesh diffusivity was 10.3×10‐11 m2 s‐1 and did not change significantly during ultra‐low oxygen storage. The corresponding skin diffusivity was 1.32×10‐13 m2 s‐1. If the flesh and skin diffusivity were estimated simultaneously, the values were 2.8×10‐12 and 6.3×10‐15 m2 s‐1, respectively. ©1998 John Wiley & Sons, Ltd.

Improved quantitative time-domain analysis of NMR data by total least squares

Abstract . Experiments show that the use of Total Least Squares (TLS) instead of Least Squares (LS) in linear prediction and state-space methods, two time-domain fitting methods for Nuclear Magnetic Resonance (NMR) data, improves the accuracy of all NMR parameter estimates for any number of data points considered. In particular, the damping factors benefit from the use of TLS especially when the peak Signal-to-Noise Ratio (SNR) is low thereby also improving the accuracy of the amplitudes and phases.

Evaluation of signal processing methods for the quantification of a multi-exponential signal: the glycogen 13C-1 NMR signal

The 13C‐1 NMR peak in proton‐decoupled spectra of liver glycogen solution was quantitatively analyzed by three types of model‐function fitting algorithms: iterative line‐fitting in the frequency domain (MDCON); iterative least‐squares fitting (VARPRO) in the time‐domain; and noniterative singular value decomposition‐based analysis (HTLS), also in the time domain. Quantification results were compared with manual integration values. Performance of the algorithms was tested at various signal‐to‐noise ratios (S/N) of the glycogen C‐1 peak. This was achieved by varying the number of scans summed prior to analysis. Since T2 relaxation in glycogen has been shown to be multiexponential [Overloop, K. et al. Magn. Reson. Med. 36, 45‐51 (1996)], the exact quantification of the C‐1 glycogen signal requires a model function comprising a sum of Lorentzian components, each with a different broadening at the glycogen frequency. This paper focuses on the performance of the above methods to fit such a multicomponent resonance line. In the frequency domain, VARPRO performs better than HTLS because fixed values can be imposed to the linewidth of the components at the common C‐1 frequency, thereby reducing convergence problems at low S/N.