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Fluorine‐19 (19F) MRI of injected perfluorocarbon emulsions (PFCs) allows for the non‐invasive quantification of inflammation and cell tracking, but suffers from a low signal‐to‐noise ratio and extended scan time. To address this limitation, we tested the hypotheses that a 19F MRI pulse sequence that combines a specific undersampling regime with signal averaging has both increased sensitivity and robustness against motion artifacts compared with a non‐averaged fully sampled pulse sequence, when both datasets are reconstructed with compressed sensing. As a proof of principle, numerical simulations and phantom experiments were performed on selected variable ranges to characterize the point spread function of undersampling patterns, as well as the vulnerability to noise of undersampling and reconstruction parameters with paired numbers of x signal averages and acceleration factor x (NAx‐AFx). The numerical simulations demonstrated that a probability density function that uses 25% of the samples to fully sample the k‐space central area allowed for an optimal balance between limited blurring and artifact incoherence. At all investigated noise levels, the Dice similarity coefficient (DSC) strongly depended on the regularization parameters and acceleration factor. In phantoms, the motion robustness of an NA8‐AF8 undersampling pattern versus NA1‐AF1 was evaluated with simulated and real motion patterns. Differences were assessed with the DSC, which was consistently higher for the NA8‐AF8 compared with the NA1‐AF1 strategy, for both simulated and real cyclic motion patterns (P < 0.001). Both strategies were validated in vivo in mice (n = 2) injected with perfluoropolyether. Here, the images displayed a sharper delineation of the liver with the NA8‐AF8 strategy than with the NA1‐AF1 strategy. In conclusion, we validated the hypotheses that in 19F MRI the combination of undersampling and averaging improves both the sensitivity and the robustness against motion artifacts. [ABSTRACT FROM AUTHOR]

Introduction: Drug-resistant essential tremor (ET) can benefit from open standard stereotactic procedures, such as deep-brain stimulation or radiofrequency thalamotomy. Non-surgical candidates can be offered either high-focused ultrasound (HIFU) or radiosurgery (RS). All procedures aim to target the same thalamic site, the ventro-intermediate nucleus (e.g., Vim). The mechanisms by which tremor stops after Vim RS or HIFU remain unknown. We used voxel-based morphometry (VBM) on pretherapeutic neuroimaging data and assessed which anatomical site would best correlate with tremor arrest 1 year after Vim RS.Methods: Fifty-two patients (30 male, 22 female; mean age 71.6 years, range 49–82) with right-sided ET benefited from left unilateral Vim RS in Marseille, France. Targeting was performed in a uniform manner, using 130 Gy and a single 4-mm collimator. Neurological (pretherapeutic and 1 year after) and neuroimaging (baseline) assessments were completed. Tremor score on the treated hand (TSTH) at 1 year after Vim RS was included in a statistical parametric mapping analysis of variance (ANOVA) model as a continuous variable with pretherapeutic neuroimaging data. Pretherapeutic gray matter density (GMD) was further correlated with TSTH improvement. No a priori hypothesis was used in the statistical model.Results: The only statistically significant region was right Brodmann area (BA) 18 (visual association area V2, p = 0.05, cluster size Kc = 71). Higher baseline GMD correlated with better TSTH improvement at 1 year after Vim RS (Spearman’s rank correlation coefficient = 0.002).Conclusions: Routine baseline structural neuroimaging predicts TSTH improvement 1 year after Vim RS. The relevant anatomical area is the right visual association cortex (BA 18, V2). The question whether visual areas should be included in the targeting remains open. [ABSTRACT FROM AUTHOR]

A 71-year-old male has been diagnosed with drug-resistant essential tremor (ET), in a familial context, with symptoms that started in childhood, at the age of 11. Tremor was predominant in the right upper limb, in a right-handed patient, provoking dramatic functional impairment. Due to a relative contra-indication to DBS (anticoagulation treatment for pulmonary embolism), an initial Gamma Knife (GK) thalamotomy of the left ventro-intermediate nucleus (Vim) was performed. After progressive and dramatic clinical alleviation, which persisted up to two years, the tremor relapsed. Magnetic resonance imaging (MRI) showed the presence of a small contrastenhancement (CE) surrounding a hypodense T1 necrotic core which, after co-registration with the dosimetry palnning in the Leksell GammaPlan, corresponded to the GK targeting. Additionally, the DWI data made possible the semi-automated segmentation of 7 clusters of thalamic nuclei, with the ventro-lateral ventral (VLV, nomenclature as from Morel et al.) cluster containing the contrast-enhancement area, as corresponding to the GKS target and in a ventral position, having anatomical relevance. A deep-brain stimulation (DBS) procedure was decided after multidisciplinary discussion. Due to the fact that the patient had bilateral tremor, and DBS could offer bilateral implantation without additional risks, it has been decided to treat both sides. For the left previously treated by GK, we aimed at stimulating the vicinity of the preexisting lesion. On this side, intraoperative microrecording showed that the center of the CE visualized lesion on MRI was silent (with no cells) and was surrounded by an area of normally active neurons; however, there was a clear difference in terms of potentials of action between the left (with previous GK) and the right (with no previous GK), raising the question whether new or surviving cells were present on the left. Additional data while comparing left and right tracks showed that neuronal noise, single unit amplitude and frequency were significantly higher on the non-GK treated side, suggesting that the cells on the previously treated left GK side were surviving and not new ones, different than those on the right. The patient had immediate and complete clinical alleviation after DBS. However, three days after the DBS procedure, he presented with dysarthria and left hemisyndrome. A CT scan and angio-CT showed right sylvian hypoperfusion, with right carotid artery thrombus. An MRI performed at 1.5T revealed an ischemia on the territory of the right anterior choroidal artery and progression of the carotid thrombus. The patient has been putted under anticoagulation treatment, with further disappearance of symptoms. Additionally, tremor on the left side remained suppressed after the stroke, with no need for stimulation on that specific side. Eighteen months latter, this important alleviation persisted bilaterally, by unilateral stimulation. Electrophysiological findings suggest functional reorganization at the periphery of the necrotic core visualized on the followup MRI after GK. Consequently, this peripheral area containing neuronal activity is most probably the one responding to DBS, although the former is not identhical on both sides, but with a final identhical clinical efect. Deep-brain stimulation after GK thalamotomy, aiming at a target closed to the previous lesion is possible and can lead to tremor suppression with durable effect. The analysis of the electrophysiological findings in this unique case helps to better understand the functional neuronal reorganization after GK thalamotomy. [ABSTRACT FROM AUTHOR]

The lack of possibility to directly visualize the Ventro-intermediate thalamic nucleus (Vim) on current routine MRI lead to application of indirect targeting methods in stereotactic neurosurgical procedures, such as Gamma Knife Surgery (GKS) and Deep Brain Stimulation (DBS), for treatment of drug-resistant tremor. Recently, our group proposed an automated robust and reproducible method across healthy subjects and tremor patients for parceling the thalamus in seven main groups of nuclei while exploring the local diffusion information from the spherical harmonics representation of the orientation distribution functions (ODFs) in k-means clustering framework. One among the resulting parcel is the Ventral-Lateral-Ventral group (VLV) enclosing all motor-related nuclei including the Vim. We aim at further automatic subdivision the VLV cluster. To this end, within the VLV, we first build a k-nn graph with edges corresponding to the respective ODFs distances and then, we perform a partition in 3 sub-clusters using the Normalized Cut algorithm. The outcome of the proposed subdivision was compared to the radiological response in the follow-up images of 17 patients treated for tremor with GKS unilaterally. The MRI protocol included pre-operative diffusion-weighted images with 64 (or 72) gradient direction, b=1000s/mm2 and voxel size of 2.2x2.2x2.2mm3, pre-operative and post-operative MPRAGEs with isotropic resolution of ~1mm3, all of them acquired at field of 3T. In general, the sub-partition followed a spatial-distribution pattern and for 12 out of 17 cases we observed that one specific sub-cluster encloses entirely or the major part of the contrast enhancement corresponding to the GKS target that appeared on the follow-up images.The evaluation of the sub-partition outcome was further extended in one additional tremor patient treated with DBS, bilaterally. Similar pre-operative MRI data as for the GKS-treated patients was acquired. We observed that in each hemisphere both initial targeting point and the final position of the electrode are inside the anticipated sub-cluster. This study shows that the diffusion-MRI-based sub-clustering of the VLV thalamic nuclei could potentially allow, in an automated manner, to narrow the area of Vim's localization. Our preliminary results will be further investigated in a larger cohort of patients (treated by either GKS or DBS) and on diffusion images with higher spatial resolution. [ABSTRACT FROM AUTHOR]