Attenuation correction is an essential requirement for quantification of positron emission tomography (PET) data. In PET/CT acquisition systems, attenuation maps are derived from computed tomography (CT) images. However, in hybrid PET/MR scanners, magnetic resonance imaging (MRI) images do not directly provide a patient-specific attenuation map. The aim of the proposed work is to improve attenuation correction for PET/MR scanners by generating synthetic CTs and attenuation maps. The synthetic images are generated through a multi-atlas information propagation scheme, locally matching the MRI-derived patient's morphology to a database of MRI/CT pairs, using a local image similarity measure. Results show significant improvements in CT synthesis and PET reconstruction accuracy when compared to a segmentation method using an ultrashort-echo-time MRI sequence and to a simplified atlas-based method.

A growing body of preclinical evidence indicates that addiction to cocaine is associated with neuroadaptive changes in frontostriatal brain systems. Human studies in cocaine-dependent individuals have shown alterations in brain structure, but it is less clear how these changes may be related to the clinical phenotype of cocaine dependence characterized by impulsive behaviours and compulsive drug-taking. Here we compared self-report, behavioural and structural magnetic resonance imaging data on a relatively large sample of cocaine-dependent individuals (n = 60) with data on healthy volunteers (n = 60); and we investigated the relationships between grey matter volume variation, duration of cocaine use, and measures of impulsivity and compulsivity in the cocaine-dependent group. Cocaine dependence was associated with an extensive system of abnormally decreased grey matter volume in orbitofrontal, cingulate, insular, temporoparietal and cerebellar cortex, and with a more localized increase in grey matter volume in the basal ganglia. Greater duration of cocaine dependence was correlated with greater grey matter volume reduction in orbitofrontal, cingulate and insular cortex. Greater impairment of attentional control was associated with reduced volume in insular cortex and increased volume of caudate nucleus. Greater compulsivity of drug use was associated with reduced volume in orbitofrontal cortex. Cocaine-dependent individuals had abnormal structure of corticostriatal systems, and variability in the extent of anatomical changes in orbitofrontal, insular and striatal structures was related to individual differences in duration of dependence, inattention and compulsivity of cocaine consumption.

Research from JAMA Psychiatry — Brain Anatomy and Its Relationship to Behavior in Adults With Autism Spectrum Disorder — A Multicenter Magnetic Resonance Imaging Study

Previous studies suggest that horizontal reaching movements are planned vectorially with independent specification of direction and extent. The transformation from visual to hand-centered coordinates requires the learning of a task-specific reference frame and scaling factor. We studied learning of a novel reference frame by imposing a screen-cursor rotation and learning of a scaling factor by imposing a novel gain. Previous work demonstrates that rotation and gain learning have different time courses and patterns of generalization. Here we used PET to identify and compare brain areas activated during rotation and gain learning, with a baseline motor-execution task as the subtracted control. Previous work has shown that the time courses of rotation and gain adaptation have a short rapid phase followed by a longer slow phase. We therefore also sought to compare activations associated with the rapid and slower phases of adaptation. We isolated the rapid phase by alternating opposite values of the rotation or gain every 16 movements. The rapid phase of rotation adaptation activated the preSMA. More complete adaptation to the rotation activated right ventral premotor cortex, right posterior parietal cortex, and the left lateral cerebellum. The rapid phase of gain learning only activated subcortical structures: bilateral putamen and left cerebellum. More complete gain learning failed to show any significant activation. We conclude that the time course of rotation adaptation is paralleled by a frontoparietal shift in activated cortical regions. In contrast, early gain adaptation involves only subcortical structures, which we suggest reflects a more automatic process of contextual recalibration of a scaling factor.

In functional magnetic resonance imaging, the brain's response to experimental manipulation is almost always assumed to be independent of endogenous oscillations. To test this, we addressed the possible interaction between cognitive task performance and endogenous fMRI oscillations in an experiment designed to answer two questions: 1) Does performance of a cognitively effortful task significantly change fractal scaling properties of fMRI time series compared to their values before task performance? 2) If so, can we relate the extent of task-related perturbation to the difficulty of the task? Using a novel continuous acquisition “rest-task-rest” design, we found that endogenous dynamics tended to recover their pre-task parameter values relatively slowly, over the course of several minutes, following completion of one of two versions of the n-back working memory task and that the rate of recovery was slower following completion of the more demanding (n = 2) version of the task. This result supports the model that endogenous low frequency oscillatory dynamics are relevant to the brain's response to exogenous stimulation. Moreover, it suggests that large-scale neurocognitive systems measured using fMRI, like the heart and other physiological systems subjected to external demands for enhanced performance, can take a considerable period of time to return to a stable baseline state.