Summary: Diffusion-tensor MR imaging of the brain is an objective method that can measure diffusion of water in tissue noninvasively. Five adult volunteers participated in this study that was performed to evaluate the potential of gradient-and spin-echo readout for diffusion-tensor imaging by comparing it with single-shot spin-echo echo-planar imaging. Gradient-and spin-echo readout provides comparable measures of water diffusion to single-shot spinecho echo-planar readout with significantly less geometrical distortion at the expense of a longer imaging time.Diffusion-tensor MR imaging of the brain has been proposed as a noninvasive technique that provides microstructural and physiological information regarding brain tissue in vivo by measuring the diffusion of water (1, 2). Most current applications of diffusion-tensor MR imaging implement ultrafast single-shot echo-planar readouts, which effectively freeze physiological effects (3-5) despite severe geometrical distortions caused by local magnetic field inhomogeneity. Gradient-and spinecho (GRASE) MR imaging has less distortion compared with single-shot echo-planar imaging because the multiple refocusing RF pulses implemented in GRASE MR imaging are responsible for reducing errors originating from static field inhomogeneity (6). For this technical note, we compared the signal-to-noise ratio (SNR), isotropic apparent diffusion coefficient (ADCi), fractional anisotropy (FA), and geometrical distortions from diffusion-tensor MR imaging by use of GRASE and echo-planar readout techniques. Description of the Technique MR Imaging and Data ProcessingMR imaging studies were performed on a 1.5-T MR system (ACS-NT; Philips Medical Systems, Best, The Netherlands) using a quadrature head coil operating in receive mode. The ᭧ American Society of Neuroradiology whole-brain MR imaging protocol included axial T1-weighted spin-echo 517/14/2 (TR/TE/excitations) imaging and axial diffusion-tensor imaging using peripheral gating. The diffusion tensor MR imaging was performed using single-shot spin-echo echo-planar and GRASE readouts. Diffusion sensitization was applied sequentially in six different non-colinear directions (G xx , G yy , G zz , G xy , G xz , G yz ). The parameters for single-shot spin-echo echo-planar diffusion-tensor imaging were 3429-5538/96/4 (TR/effective TE/excitations), with a b value of 600 s·mm Ϫ2 . The parameters for GRASE diffusion-tensor imaging with four spin echoes each comprising five gradient echoes were 4000-4615/119 (TR/TE eff ), with a b value of 600 s·mm Ϫ2 . To eliminate phase errors that originated during the diffusion preparation period, a dephasing gradient and a 90-degree RF pulse before the GRASE readout were used (7). The T1-weighted spin-echo sequence and both the diffusiontensor sequences were matched for total brain coverage (number of sections, 18; section thickness, 5 mm; intersection gap, 1 mm; field of view, 23 cm). The GRASE experiment was performed twice, once with four excitations (4-GRASE) and once with 10 excitations (10-GRASE)....

A method is described for the correction of geometric distortions occurring in echo planar images. The geometric distortions are caused in large part by static magnetic field inhomogeneities, leading to pixel shifts, particularly in the phase encode direction. By characterizing the field inhomogeneities from a field map, the image can be unwarped so that accurate alignment to conventionally collected images can be made. The algorithm to perform the unwarping is described, and results from echo planar images collected at 1.5 and 4 Tesla are shown.

Behavioral and neurophysiological studies suggest that skill learning can be mediated by discrete, experience-driven changes within specific neural representations subserving the performance of the trained task. We have shown that a few minutes of daily practice on a sequential finger opposition task induced large, incremental performance gains over a few weeks of training. These gains did not generalize to the contralateral hand nor to a matched sequence of identical component movements, suggesting that a lateralized representation of the learned sequence of movements evolved through practice. This interpretation was supported by functional MRI data showing that a more extensive representation of the trained sequence emerged in primary motor cortex after 3 weeks of training. The imaging data, however, also indicated important changes occurring in primary motor cortex during the initial scanning sessions, which we proposed may ref lect the setting up of a task-specific motor processing routine. Here we provide behavioral and functional MRI data on experience-dependent changes induced by a limited amount of repetitions within the first imaging session. We show that this limited training experience can be sufficient to trigger performance gains that require time to become evident. We propose that skilled motor performance is acquired in several stages: "fast" learning, an initial, withinsession improvement phase, followed by a period of consolidation of several hours duration, and then "slow" learning, consisting of delayed, incremental gains in performance emerging after continued practice. This time course may ref lect basic mechanisms of neuronal plasticity in the adult brain that subserve the acquisition and retention of many different skills.The performance of many tasks improves, throughout life, with repetition and practice. Even in adulthood simple tasks such as reaching to a target or rapidly and accurately tapping a short sequence of finger movements, which appear, when mastered, to be effortlessly performed, often require extensive training before skilled performance develops. What changes occur in the adult brain when a new skill is acquired through practice? When, and after how much practice, do these changes occur? Functional reorganization of adult mammalian sensory and motor cortical representations has been found to occur in many different animal models of brain plasticity in the last two decades, advancing the idea that throughout life the functional properties of central nervous system neurons, as well as the neural circuitry within different brain areas, are malleable and retain a functionally significant degree of plasticity (e.g., refs. 1-4). These representational changes have been shown to be induced not only in response to lesions of peripheral or central sensory input or motor output pathways but also, in normal individuals, as a result of practice and experience. The advent of new brain imaging techniques, especially functional MRI (fMRI) (5), which allows repeated mapping of cor...

A method for detecting significant and regionally specific correlations between sensory input and the brain's physiological response, as measured with functional magnetic resonance imaging (MRI), is presented in this paper. The method involves testing for correlations between sensory input and the hemodynamic response after convolving the sensory input with an estimate of the hernodynamic response function. This estimate is obtained without reference to any assumed input. To lend the approach statistical validity, it is brought into the framework of statistical parametric mapping by using a measure of cross-correlations between sensory input and hemodynamic response that is valid in the presence of intrinsic autocorrelations. These autocorrelations are necessarily present, due to the hemodynamic response function or temporal point spread function.

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Background The impact of COVID-19 on physical and mental health and employment after hospitalisation with acute disease is not well understood. The aim of this study was to determine the effects of COVID-19-related hospitalisation on health and employment, to identify factors associated with recovery, and to describe recovery phenotypes. MethodsThe Post-hospitalisation COVID-19 study (PHOSP-COVID) is a multicentre, long-term follow-up study of adults (aged ≥18 years) discharged from hospital in the UK with a clinical diagnosis of COVID-19, involving an assessment between 2 and 7 months after discharge, including detailed recording of symptoms, and physiological and biochemical testing. Multivariable logistic regression was done for the primary outcome of patient-perceived recovery, with age, sex, ethnicity, body-mass index, comorbidities, and severity of acute illness as covariates. A posthoc cluster analysis of outcomes for breathlessness, fatigue, mental health, cognitive impairment, and physical performance was done using the clustering large applications k-medoids approach. The study is registered on the ISRCTN Registry (ISRCTN10980107). Findings We report findings for 1077 patients discharged from hospital between March 5 and Nov 30, 2020, who underwent assessment at a median of 5•9 months (IQR 4•9-6•5) after discharge. Participants had a mean age of 58 years (SD 13); 384 (36%) were female, 710 (69%) were of white ethnicity, 288 (27%) had received mechanical ventilation, and 540 (50%) had at least two comorbidities. At follow-up, only 239 (29%) of 830 participants felt fully recovered, 158 (20%) of 806 had a new disability (assessed by the Washington Group Short Set on Functioning), and 124 (19%) of 641 experienced a health-related change in occupation. Factors associated with not recovering were female sex, middle age (40-59 years), two or more comorbidities, and more severe acute illness. The magnitude of the persistent health burden was substantial but only weakly associated with the severity of acute illness. Four clusters were identified with different severities of mental and physical health impairment (n=767): very severe (131 patients, 17%), severe (159, 21%), moderate along with cognitive impairment (127, 17%), and mild (350, 46%). Of the outcomes used in the cluster analysis, all were closely related except for cognitive impairment. Three (3%) of 113 patients in the very severe cluster, nine (7%) of 129 in the severe cluster, 36 (36%) of 99 in the moderate cluster, and 114 (43%) of 267 in the mild cluster reported feeling fully recovered. Persistently elevated serum C-reactive protein was positively associated with cluster severity.Interpretation We identified factors related to not recovering after hospital admission with COVID-19 at 6 months after discharge (eg, female sex, middle age, two or more comorbidities, and more acute severe illness), and four different recovery phenotypes. The severity of physical and mental health impairments were closely related, whereas cognitive health impairments w...

Performance of complex motor tasks, such as rapid sequences of finger movements, can be improved in terms of speed and accuracy over several weeks by daily practice sessions. This improvement does not generalize to a matched sequence of identical component movements, nor to the contralateral hand. Here we report a study of the neural changes underlying this learning using functional magnetic resonance imaging (MRI) of local blood oxygenation level-dependent (BOLD) signals evoked in primary motor cortex (M1). Before training, a comparable extent of M1 was activated by both sequences. However, two ordering effects were observed: repeating a sequence within a brief time window initially resulted in a smaller area of activation (habituation), but later in larger area of activation (enhancement), suggesting a switch in M1 processing mode within the first session (fast learning). By week 4 of training, concurrent with asymptotic performance, the extent of cortex activated by the practised sequence enlarged compared with the unpractised sequence, irrespective of order (slow learning). These changes persisted for several months. The results suggest a slowly evolving, long-term, experience-dependent reorganization of the adult M1, which may underlie the acquisition and retention of the motor skill.

Multiple sclerosis is still regarded primarily as a disease of the white matter. However, recent evidence suggests that there may be significant involvement of gray matter. Here, we have used magnetic resonance imaging and magnetic resonance spectroscopy in vivo and histopathology postmortem to estimate thalamic neuronal loss in patients with multiple sclerosis. Our results show that neuronal loss in multiple sclerosis can be substantial (30-35% reduction). We conclude that a neurodegenerative pathology may make a major contribution to the genesis of symptoms in multiple sclerosis.