The measurement of quantitative, tissue-specific MR properties, e.g., water content, longitudinal relaxation time (T1) and effective transverse relaxation time (T2⁎), using quantitative MRI at a ...clinical field strength (1.5 T to 3T) is a well-explored topic. However, none of the commonly used standard brain atlases, such as MNI or JHU, provide quantitative information. Within the framework of quantitative MRI of the brain, this work reports on the development of the first quantitative brain atlas for tissue water content at 3T. A methodology to create this quantitative atlas of in vivo brain water content based on healthy volunteers is presented, and preliminary, practical examples of its potential applications are also shown.
Established methods for the fast and reliable measurement of the absolute water content were used to achieve high precision and accuracy. Water content and T2⁎ were mapped based on two different methods: an intermediate-TR, two-point method and a long-TR, single-scan method. Twenty healthy subjects (age 25.3 ± 2.5 years) were examined with these quantitative imaging protocols. The images were normalised to MNI stereotactic coordinates, and water content atlases of healthy volunteers were created for each method and compared. Regions-of-interest were generated with the help of a standard MNI template, and water content values averaged across the ROIs were compared to water content values from the literature.
Finally, in order to demonstrate the strength of quantitative MRI, water content maps from patients with pathological changes in the brain due to stroke, tumour (glioblastoma) and multiple sclerosis were voxel-wise compared to the healthy brain.
The water content atlases were largely independent of the method used to acquire the individual water maps. Global grey matter and white matter water content values between the methods agreed with each other to within 0.5 %. The feasibility of detecting abnormal water content in the brains of patients based on comparison to a healthy brain water content atlas was demonstrated.
In summary, the first quantitative water content brain atlas in vivo has been developed, and a voxel-wise assessment of pathology-related changes in the brain water content has been performed. These results suggest that qMRI, in combination with a water content atlas, allows for a quantitative interpretation of changes due to disease and could be used for disease monitoring.
The TAPIR sequence is an accurate and efficient method for T
mapping. It combines a slice-interleaving Look-Locker read-out with an acquisition of multiple k-space lines in 1 shot. Whereas the ...acquisition of multiple lines per excitation increases imaging speed, the corresponding increase in TR and TE is detrimental to the T
fitting performance. This is especially problematic for substances exhibiting rapid T
relaxation (e.g., myelin water).
The T
fitting performance of TAPIR is enhanced by using an interleaved spiral read-out with shorter TE and TR. Furthermore, an improvement to a method for fast gradient delay estimation is presented. Whereas previous methods assume the gradient delay to be stationary, the presented approach corrects the spiral k-space trajectory by using a polynomial fit of the measured gradient delays.
Gradient delay artifacts are largely eliminated, requiring very little additional scanning time. The sampling efficiency of the spiral read-out allows for a significant reduction of the acquisition time in comparison to Cartesian TAPIR. Spiral TAPIR enables the sampling of more slices and an accurate measurement of rapidly relaxing compartments. Over a wide T
range (448-3115 ms), spiral TAPIR reduces the mean fitting error from -2.5% to -0.1%. Combining 50% undersampling with the shorter TR of spiral TAPIR, an increase in imaging speed by a factor of up to 3.3 was achieved.
Using a spiral read-out trajectory, the established TAPIR sequence enables measurement of rapidly relaxing T
compartments, while improving T
mapping performance and imaging speed.
The most common modality of diffusion MRI used in the ageing and development studies is diffusion tensor imaging (DTI) providing two key measures, fractional anisotropy and mean diffusivity. Here, we ...investigated diffusional changes occurring between childhood (average age 10.3 years) and mitddle adult age (average age 54.3 years) with the help of diffusion kurtosis imaging (DKI), a recent novel extension of DTI that provides additional metrics quantifying non-Gaussianity of water diffusion in brain tissue. We performed voxelwise statistical between-group comparison of diffusion tensor and kurtosis tensor metrics using two methods, namely, the tract-based spatial statistics (TBSS) and the atlas-based regional data analysis. For the latter, fractional anisotropy, mean diffusivity, mean diffusion kurtosis, and other scalar diffusion tensor and kurtosis tensor parameters were evaluated for white matter fibres provided by the Johns-Hopkins-University Atlas in the FSL toolkit (http://fsl.fmrib.ox.ac.uk/fsl/fslwiki/Atlases). Within the same age group, all evaluated parameters varied depending on the anatomical region. TBSS analysis showed that changes in kurtosis tensor parameters beyond adolescence are more widespread along the skeleton in comparison to the changes of the diffusion tensor metrics. The regional data analysis demonstrated considerably larger between-group changes of the diffusion kurtosis metrics than of diffusion tensor metrics in all investigated regions. The effect size of the parametric changes between childhood and middle adulthood was quantified using Cohen's d. We used Cohen's d related to mean diffusion kurtosis to examine heterogeneous maturation of various fibres. The largest changes of this parameter (interpreted as reflecting the lowest level of maturation by the age of children group) were observed in the association fibres, cingulum (gyrus) and cingulum (hippocampus) followed by superior longitudinal fasciculus and inferior longitudinal fasciculus. The smallest changes were observed in the commissural fibres, forceps major and forceps minor. In conclusion, our data suggest that DKI is sensitive to developmental changes in local microstructure and environment, and is particularly powerful to unravel developmental differences in major association fibres, such as the cingulum and superior longitudinal fasciculus.
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To apply the MB (multiband) excitation and blipped-CAIPI (blipped-controlled aliasing in parallel imaging) techniques in a spin and gradient-echo (SAGE) EPI sequence to improve the slice coverage for ...vessel architecture imaging (VAI).
Both MB excitation and blipped-CAIPI with in-plane parallel imaging were incorporated into a gradient-echo (GE)/spin-echo (SE) EPI sequence for simultaneous tracking of the dynamic MR signal changes in both GE and SE contrasts after the injection of contrast agent. MB and singleband (SB) excitation were compared using a 20-channel head coil at 3 Tesla, and high-resolution MB VAI could be performed in 32 glioma patients.
Whole-brain covered high resolution VAI can be achieved after applying multiband excitation with a factor of 2 and in-plane parallel imaging with a factor of 3. The quality of the images resulting from MB acceleration was comparable to those from the SB method: images were reconstructed without any loss of spatial resolution or severe distortions. In addition, MB and SB signal-to-noise ratios (SNR) were similar. A relative low g-factor induced from the MB acceleration method was achieved after using a blipped-CAIPI technique (1.35 for GE and 1.33 for SE imaging). Performing quantitative VAI, we found that, among all VAI parametric maps, microvessel type indicator (MTI), distance map (I) and vascular-induced bolus peak-time shift (VIPS) were highly correlated. Likewise, VAI parametric maps of slope, slope length and short axis were highly correlated.
Multiband accelerated SAGE successfully doubles the number of readout slices in the same measurement time when compared to conventional readout sequences. The corresponding VAI parametric maps provide insights into the complexity and heterogeneity of vascular changes in glioma.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The aim of this work is to quantify the metabolic profile of the human putamen in vivo in a cohort of elderly subjects using single-voxel proton magnetic resonance spectroscopy. To obtain metabolite ...concentrations specific to the putamen, we investigated a correction method previously proposed to account for the tissue composition of the volume of interest. We compared the method with the conventional approach, which a priori assumes equal metabolite concentrations in GM and WM. Finally, we compared the concentrations acquired at 3 Tesla (T) and 7 T MRI scanners. Spectra were acquired from 15 subjects (age: 67.7 ± 8.3 years) at 3 T and 7 T, using an ultra-short echo time, stimulated echo acquisition mode sequence. To robustly estimate the WM-to-GM metabolite concentration ratio, five additional subjects were measured for whom the MRS voxel was deliberately shifted from the putamen in order to increase the covered amount of surrounding WM. The concentration and WM-to-GM concentration ratio for 16 metabolites were reliably estimated. These ratios ranged from ~0.3 for γ-aminobutyric acid to ~4 for N-acetylaspartylglutamate. The investigated correction method led to significant changes in concentrations compared to the conventional method, provided that the ratio significantly differed from unity. Finally, we demonstrated that differences in tissue voxel composition cannot fully account for the observed concentration difference between field strengths. We provide not only a fully comprehensive quantification of the neurochemical profile of the putamen in elderly subjects, but also a quantification of the WM-to-GM concentration ratio. This knowledge may serve as a basis for future studies with varying tissue voxel composition, either due to tissue atrophy, inconsistent voxel positioning or simply when pooling data from different voxel locations.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The simultaneous quantification of T
and T
* maps based on fast sequences for combined GE and SE acquisition has rekindled research and clinical interest by offering a wide range of attractive ...applications, e.g., dynamic tracking of oxygen extraction fraction (OEF). However, previously published methods based on EPI-readouts have been hindered by resolution and the number of acquired echoes.
This work presents a novel 10-echo GE-SE EPIK (EPI with keyhole) sequence for the rapid quantification of T
'. T
/T
* maps from the GE-SE EPIK sequence were validated using three phantoms and 15 volunteers at 3T. The incorporation of a sliding window approach, combined with the full sampling of the k-space center inherent to EPIK, enables a high effective temporal resolution. That is, for an eight-slice breath-hold experiment, a temporal sampling rate of eight reconstructed slices per 1.1 s.
In comparison with repeated single-echo SE, multi-echo GE, and spectroscopy methods, the GE-SE EPIK sequence shows good agreement in quantifying T
/T
* values, while the gray matter/white matter separation yielded the expected contrast differentiation. The OEF was calculated with a view to an initial application with clinical relevance, producing results comparable to those in the literature and with good sensitivity in breath-hold experiments.
GE-SE EPIK provides increased resolution and more echoes, including two SEs, than comparable sequences. Moreover, GE-SE EPIK achieves this within an acquisition time of 57 s for 20 slices (matrix size = 128×128; FOV = 24 cm) and with a reasonably short TE for the final echo (114 ms). The sequence can dynamically track OEF changes in a breath-hold experiment.
Alterations in the substantia nigra are strongly associated with Parkinson's disease. However, due to low contrast and partial volume effects present in typical MRI images, the substantia nigra is ...not of sufficient size to obtain a reliable segmentation for region-of-interest based analysis. To combat this problem, the approach proposed here offers a method to investigate and reveal changes in quantitative MRI parameters in the vicinity of substantia nigra without any a priori delineation. This approach uses an alternative method of statistical, voxel-based analysis of quantitative maps and was tested on 18 patients and 15 healthy controls using a well-established, quantitative free water mapping protocol. It was possible to reveal the topology and the location of pathological changes in the substantia nigra and its vicinity. Moreover, a decrease in free water content, T1 and T2* in the vicinity of substantia nigra was indicated in the Parkinson's disease patients compared to the healthy controls. These findings reflect a disruption of grey matter and iron accumulation, which is known to lead to neurodegeneration. Consequently, the proposed method demonstrates an increased sensitivity for the detection of pathological changes-even in small regions-and can facilitate disease monitoring via quantitative MR parameters.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Human cognition relies on attentional capacities which, among others, are influenced by factors like tiredness or mood. Based on their inherent preferences in sleep and wakefulness, individuals can ...be classified as specific “chronotypes”. The present study investigated how early, intermediate and late chronotypes (EC, IC, LC) differ neurally on an attention-to-motion task.
Twelve EC, 18 IC and 17 LC were included into the study. While undergoing functional magnetic resonance imaging (fMRI) scans, subjects looked at vertical bars in an attention-to-motion task. In the STATIONARY condition, subjects focused on a central fixation cross. During Fix-MOVING and Attend-MOVING, bars were moving horizontally. Only during the Attend-MOVING, subjects were required to attend to changes in the velocity of bars and indicate those by button presses. A two-way repeated measures ANOVA probed group by attentional load effects.
The dorsolateral prefrontal cortex (DLPFC), insula and anterior cingulate cortex showed group by attention specific activations. Specifically, EC and LC presented attenuated DLPFC activation under high attentional load (Attend-MOVING), while EC showed less anterior insula activation than IC. LC compared to IC exhibited attenuation of superior parietal cortex.
Our study reveals that individual sleep preferences are associated with characteristic brain activation in areas crucial for attention and bodily awareness. These results imply that considering sleep preferences in neuroimaging studies is crucial when administering cognitive tasks. Our study also has socio-economic implications. Task performance in non-optimal times of the day (e.g. shift workers), may result in cognitive impairments leading to e.g. increased error rates and slower reaction times.
•Individual sleep preferences are linked to characteristic brain activation patterns.•Subjects that wake up late show attenuated DLPFC and superior parietal activation.•People that tend to wake up very early show less anterior insula recruitment.•Sleep preferences should be considered in imaging studies applying cognitive tasks.
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