Magnetic resonance imaging
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To investigate the relationship between the Diffusion Kurtosis Imaging (DKI) parameters and conventional metrics provided by Diffusion-weighted imaging (DWI) in patients affected by Brain or Head and Neck (HN) cancer. ⋯ A significant association between the apparent diffusional kurtosis Kapp and the tissue diffusion coefficient Dmono emerged for both brain and HN tumors at 3 T, suggesting that both variables may consistently reflect deeper insight into the microstructural characteristics of tumors.
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Alterations in iron concentration in certain deep gray matter regions are known to occur in aging and several clinical conditions. In vivo measurements of R2∗ transverse relaxation rates and quantitative susceptibility mapping (QSM) have been shown to be strongly correlated with iron concentration in tissue, but their calculation requires the acquisition of a multi-echo gradient recalled echo sequence (MGRE). In the current study, we examined the feasibility of approximating R2∗ rates using metrics derived from fMRI-EPI and T2-weighted FLAIR images, which are widely available. ⋯ These relationships were preserved after referencing vEPI and vFLAIR with respect to the values in the control region. Effect sizes (above 0.5 for some of the regions, particularly the largest ones) were calculated and put in relation to those of the correlation between QSM and R2∗ rates. We propose that the metrics described here may be applied, possibly in a retrospective fashion, to analyze datasets with available EPI or T2-weighted FLAIR scans (and lacking a MGRE sequence), to devise new hypotheses regarding links between iron concentration in brain tissue and other variables of interest.
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Functional MRI (fMRI) has evolved from simple observations of regional changes in MRI signals caused by cortical activity induced by a task or stimulus, to task-free acquisitions of images in a resting state. Such resting state signals contain low frequency fluctuations which may be correlated between voxels, and strongly correlated regions are deemed to reflect functional connectivity within synchronized circuits. Resting state functional connectivity (rsFC) measures have been widely adopted by the neuroscience community, and are being used and interpreted as indicators of intrinsic neural circuits and their functional states in a broad range of applications, both basic and clinical. ⋯ In this mini-review, we summarize recent studies of rsFC within mesoscopic scale cortical networks (100μm-10mm) within a well defined functional region of primary somatosensory cortex (S1), as well as spinal cord and brain white matter in non-human primates, in which we have measured spatial patterns of resting state correlations and validated their interpretation with electrophysiological signals and anatomic connections. Moreover, we emphasize that low frequency correlations are a general feature of neural systems, as evidenced by their presence in the spinal cord as well as white matter. These studies demonstrate the valuable role of high field MRI and invasive measurements in an animal model to inform the interpretation of human imaging studies.
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Comparative Study
Comparison of BOLD, diffusion-weighted fMRI and ADC-fMRI for stimulation of the primary visual system with a block paradigm.
The blood oxygen level-dependent (BOLD) effect is extensively used for functional MRI (fMRI) but presents some limitations. Diffusion-weighted fMRI (DfMRI) has been proposed as a method more tightly linked to neuronal activity. This work proposes a protocol of DfMRI acquired for several b-values and diffusion directions that is compared to gradient-echo BOLD (GE-BOLD) and to repeated spin-echo BOLD (SE-BOLD, acquisitions performed with b=0s/mm2), which was also used to ensure the reproducibility of the response. ⋯ DfMRI and ADC-fMRI activation volumes were significantly smaller than those obtained with SE-BOLD. ADC-fMRI activations were more precisely localized in V1 than those of SE-BOLD-fMRI. This validated the increased capability of ADC-fMRI compared to BOLD to enhance the precision of localizing an fMRI activation in the cyto-architectural zone V1, thereby justifying the use of ADC-fMRI for neuro-scientific studies.
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The objective of this study is to develop rapid whole brain mapping of myelin water content (MWC) at 1.5T. The Fast Acquisition with Spiral Trajectory and T2prep (FAST-T2) pulse sequence originally developed for myelin water fraction (MWF) mapping was modified to obtain fast mapping of T1 and receiver coil sensitivity needed for MWC computation. The accuracy of the proposed T1 mapping was evaluated by comparing with the standard IR-FSE method. ⋯ We also compared MWC values obtained with either cerebrospinal fluid (CSF) or an external water tube attached to the subject's head as the water reference. Our results from healthy volunteers show that whole brain MWC mapping is feasible in 7min and provides accurate brain T1 values. Regional brain WC and MWC measurements obtained with the internal CSF-based water standard showed excellent correlation (R>0.99) and negligible bias within narrow limits of agreement compared to those obtained with an external water standard.