Category Archives: Magnetic Resonance

Brain regional scaling

Reardon and colleagues recently published a study on the variation of human brain organization and its relationship with brain size. Using neuroimaging data from more than 3000 individuals they calculated the local surface area and estimated the areal scaling in relation to the total cortical area in order to generate a reference map for areal scaling in cortical and subcortical structures. By using three separate cohorts, three different platforms of image-acquisition, and two distinct imaging processing pipelines, they obtained the same results. Regions with positive scaling, i.e. which area increases with increasing total cortical size, were found with the prefrontal, lateral temporoparietal, and medial parietal cortices, whereas the limbic, primary visual, and primary somatosensory regions showed negative scaling. These patterns of cortical area distribution relatively to normative brain size variation were also reproduced at the individual level in terms of proportion, as, for instance, areas of positive scaling regions were positively correlated with the total cortical area. These patterns of areal scaling distribution are also comparable with patterns of brain expansion during human development and primate evolution (humans vs. macaques). In terms of cytoarchitecture, the regions of positive scaling were concentrated within association cortices, such as the default mode, dorsal attention, and frontoparietal networks, while the negative scaling regions were found within the limbic network. The association of areal scaling patterns with known patterns of mitochondria-related gene expression suggests these regions that are expanded in larger brains might differ in their metabolic profile. The authors concluded that the similarity of the areal scaling maps across development and evolution, and at the individual level, suggests a shared scaling gradient of the primate cortex. Larger brains tend to preferentially expand association cortex, specialized for integration of information, which might point to a need for an increase of the neural subtracts, such as dendrites or synapses, in order to maintain or enhance brain function in an expanded brain. Further study designs are required to investigate the relationship between cortical areal patterns and brain function.

Sofia Pedro

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Intracranial vessels in traumatic brain injury

vesselsTraumatic brain injury (TBI) is a frequent cause of death and disability affecting millions of people every year worldwide. It is initiated by mechanical forces that cause sudden head motion. Such motion produces deformation of the brain and surrounding tissues and thus may result in axonal injury, contusion, or hematoma. The trauma launches a cascade of biochemical reactions often leading to ischemia, hypoxia, brain swelling, and edema. TBI may also induce damages of the cranial vasculature, with alterations of the blood vessel that put the neural tissue at risk. TBI can either cause vessel rupture and hemorrhage (bleeding), or a pathophysiological change of the vessel structure which is secondary associated with some kind of dysfunction. Hemorrhage is easy to recognize, commonly categorized according to its location as epidural (between the skull and the dura mater – associated with disruption of the middle meningeal vessels), subdural (between the dura mater and arachnoid membrane – usually concerning the bridging veins), or subarachnoidal (between arachnoid membrane and the pia mater). Intracerebral bleeding may also occur when the membranes surrounding the brain are impaired. In case of contusion, vascular damage and mechanical cortical damage can occurr at the same time. Even if bleeding is not present, function and microstructure of the injured vessels might be impaired. Vessel disruption and hemorrhage alter the cerebral blood flow, increase the intracranial pressure, affect the maintenance of the blood-brain barrier (exchange of nutrients and waste that occurs at the capillary level), and disrupt the CNS homeostasis by exposing the neural tissue to disregulated blood flow.

Understanding cerebrovascular injuries and the mechanisms behind them is crucial for diagnostics and treatment strategies. Monson et al. (in press) describe the current state of knowledge on the mechanics of cerebral vessels during head trauma and how they respond to the applied loads. They provide a summary of the experimental research focused on the loading conditions during the TBI. Experiments with physical models for instance show that there is significant relative motion between the brain and skull during the trauma. In the sagittal plane, this motion tends to be largest at the vertex and smallest at the brain base. Constraints at the base can lead to brain rotation which pushes the parietal cortex into the cranial bones, possibly causing of contusion and subdural hematoma. In general, it appears that rotation is more damaging than translation. Computer models represent another approach of the TBI research and provide accurate predictions of brain deformation for many loading scenarios. These models enable to estimate exterior loading conditions according to the internal deformation of tissues that are directly involved in injury. However, validation of these models is needed, as well as specific models focusing on blood vessels. The authors also provide a summary of what is known about cerebral vessel response to extreme deformations, passive physical properties, structural failure, and subfailure damage and dysfunction. In another article, Saad et al. (in press) describe various kinds of intracranial hemorrhage in biomedical imaging. Both macroscopic hematomas and microhemorrhages are described according to the distinctions based on intracranial compartments, and traumatic vs nontraumatic cases.

Stáňa Eisová


Woolly mammoth brain

Endocranial casts are usually the only resource for studying brain gross anatomy of an extinct species. However, sometimes, a frozen mummy can add information not only on the cortical features but also on the internal structures of the brain. Some years ago, Anastasia Kharlamova and Paul Manger published a study on the mummified brain of a Pleistocene woolly mammoth. This Yuka woolly mammoth specimen, dated approximately to 38,000 years ago, was determined to be an adolescent female aged between 6 and 9 years. Its mummified brain is unique in the state of preservation, allowing access to its external and internal morphology through the use of CT imaging techniques. Furthermore, it gave the opportunity to compare mammoths with extant African elephants’ brain, in order to characterize Elephantidae brain evolution, by determining whether elephant-specific features were already present in this specimen. The authors first compared the brains of the two species in terms of overall organization, concluding the Yuka mammoth displays the typical structure of the Elephantidae family. Direct volume comparisons were possible only for the structures which borders were clearly visible on the CT scans. The brain volume of the Yuka mammoth shrank during mummification, due to dehydration, and occupies only about 55% of the endocranial volume. Therefore, the calculated structure masses had to be corrected for such shrinkage. Moreover, due to differences in tissue fat composition, this shrinkage was heterogeneous, differing between hemispheres and between these and the cerebellum, which showed the least shrinkage. Based on subdural volume and on regression equations using extant mammals, the brain mass was determined to range between 4,230 and 4,340g. Given the average brain mass of an adult female elephant is only 300 to 400 g heavier, and that this specimen is immature, the values appear to be close to what would be expected for an adolescent woolly mammoth female. The size of the corpus callosum was also similar to that of female African and Asian elephants suggesting that, like elephants, mammoths also displayed sexual dimorphism in this structure. The comparable size of the amygdala suggests a similar organization of the limbic system, and the similarity in size and organization of the cerebellum point to a similar role in control of the trunk. This further indicates the Elephantidae family holds the largest cerebellum of all mammals, and that the cerebellar sensorimotor integration and learning movements of the trunk is a feature of this family. As the Elephantidae brain structure seems to be evolutionarily conservative, it can be assumed that the woolly mammoth could have achieved the same cognitive capacities as the extant elephants. However, further predictions of behavior and specializations would need a more detailed histological examination, which was not possible in the Yuka specimen. Nonetheless, this study provided an exceptional glimpse into the brain of an extinct species, and helped extending the understanding of the Elephantidae family.

 

Sofia Pedro


Parietal lobes and tool use

The parietal lobe has a unique central location in the brain, and it is involved in higher cognitive functions. Investigating its functions and connectivity is essential to understand its role in uniquely human abilities. Two recent works have put emphasis on the importance of the parietal lobe for tool use.

Catani and colleagues investigated the intralobar parietal connectivity in human and monkey brains, using diffusion imaging tractography. In general, the patterns of white matter connectivity are similar in both species, although with some differences for areas that are distinct in humans. The larger tract connects the superior parietal lobule (SPL) to the angular and supramarginal gyri of the inferior parietal lobule, within the IPS. The authors suggest it might act to mediate the interaction between the two lobules during object manipulation, and to coordinate both dorsal and ventral visuospatial networks. The second and third larger tracts link the postcentral gyrus to the inferior parietal lobule and to the SPL, respectively. These might transmit tactile and proprioceptive information on the body orientation relatively to an object for guiding motor actions and grasp. The connection between the postcentral and the angular gyri was only observed in humans, leading the authors to highlight its role in specific cognitive functions. Particularly, its connections to SPL are key for tool use, mathematical thinking, and language and communication.

Kastner and coworkers reviewed the organization and function of the dorsal pathway of the visual system of monkey and human brains, focusing on the areas of the posterior parietal cortex within and adjacent to the intraparietal sulcus (IPS). Monkeys and humans have diverged in the functional contributions of the IPS since their last common ancestor, as some functionally-defined areas that are located within the IPS in monkeys have been partially relocated outside this sulcus in humans. The authors suggest this relocation might be due to expansion for accommodation of human-specific abilities, such as tool use. They hypothesize that humans might have developed a derived and advanced tool network from the modification of the macaque circuit for object manipulation. First, the human dorsal vision pathway must provide object shape information regardless of size and viewpoint, facilitating object recognition and mental manipulation. Second, object information is integrated with cognitive information such as working memory, allowing maintaining the information over a period of time. Finally, humans have areas that respond specifically to tool use, some of which also integrate frontal and temporal networks involved in action recognition and semantic knowledge related to tools and actions, respectively.

Both studies point at the parietal lobes and visuospatial integration as key elements for human cognitive capacity, as suggested by evidence in paleoneurology, evolutionary neuroanatomy, and cognitive archaeology.

 

Sofia Pedro


Primates brain shape

We have published one more paper on the morphology of the precuneus, this time featuring a sample of non-human primates, in collaboration with James Rilling and Todd Preuss from the Emory University (Atlanta, USA). Modern humans have a much larger precuneus than chimpanzees both in absolute and relative size. Taking into account the large brain size in our species, we investigated the midsagittal morphology in non-human primates as to test whether precuneus proportions are influenced by allometric factors. We did a geometric morphometric analysis on a total of 42 MRIs from the National chimpanzee brain resource database, including 5 species of apes and 4 species of monkeys. A first analysis, conducted on the species averages, showed that the main pattern of midsagittal variation involves the general shape of the braincase, which might be due to cranial constraints rather than to changes in proportions of specific brain regions. This main shape pattern separates monkeys from apes, as the former display flatter, elongated brains (with capuchins being the flattest), while the latter exhibit rounder brains with frontal bulging (especially orangutans). This morphological variation correlates with brain size, except for gorillas (which brain is large but elongated), and gibbons (which have smaller but round brains). A second analysis was conducted only on chimpanzees and macaques, to compare two species with different brain size. In neither case the proportions of the precuneus displayed major differences between species or size-related changes. However, as in humans, precuneus size is very variable within each species, suggesting a remarkable plasticity. Overall, the results suggest that precuneus expansion in modern humans is a species-specific characteristic of our species, rather than a simple consequence of increase in brain size. Further studies should address the histological and functional processes involved in this morphological change.

Sofia Pedro


Vessels and neurosurgery

obrMagnetic Resonance Imaging (MRI) methods has a primary use in medicine, especially in diagnosis and image-guided surgery. In neuroscience, attention is mainly focused on the brain, and vessels are not always a target of the imaging procedures. The crucial aspect of using imaging during surgery concerns the correspondence with the real physical structures. This correspondence is affected by a displacement of the brain during surgery called brain shift, which can result in 5 – 10 mm difference from the MRI data. Several technical procedures are used in order to avoid this mismatch. Since intraoperative MRI devices are not always available, using local markers for orientation and navigation could be a plausible alternative. In a recent paper, Grabner and colleagues suggest to use the system of veins on the surface of the cerebral cortex as reference landmarks. These veins are well visible during the surgery, and can potentially improve navigation. The study is focused on developing a non-invasive MRI technique for the visualization of the superficial cortical veins and validation of that method by comparing MR images with high resolution photographs of human cadavers.

Considering Magnetic Resonance Angiography (MRA), the main concern of using this method is the use of a contrast agent, and the possibility of overlooking the superficial cortical veins because of the slow blood flow. Alternatively, the authors suggest to use Susceptibility-Weighted Imaging (SWI), which is a blood-oxygen-level dependent technique of MRI with an ability to image vessels smaller than a voxel. Gradient-echo based T2-weighted imaging was performed in this study using a 7 Tesla MRI scanner. Image processing relied on automated vessel segmentation and overlaid on anatomical MRI. The results showed high correlation between segmented veins in MRI and actual venous anatomy of the sample, and therefore surface venograms could serve as alternative navigation system for neurosurgery.

Reliable methods of imaging and segmentation of vessels are valuable also in theoretical fields, where those methods could contribute to the investigation of the function of the endocranial venous system. The importance of the veins is usually estimated according to their size, although functional information on these vessels is still scanty, and methodological research to improve craniovascular studies can be beneficial in both anthropology and medicine.

Stáňa Eisová


Microgravity and sensorimotor function

Space missions can have adverse effects on astronauts, such as the already-mentioned vision deterioration and cognitive impairment. Spending a long time on space can also impact sensorimotor function. Koppelmans et al. have recently investigated the influence of microgravity environment on sensorimotor performance and brain structure. They conducted a longitudinal study with a group of male subjects remaining in a 6-degrees head down tilt bed rest (HDBR) position, an analog environment to study the effects of spaceflight microgravity, during 70 days. MR images were collected before, during, and after HDBR, to explore changes in gray matter (GM) volume, and functional mobility and postural equilibrium tests were conducted pre- and post-HDBR, to check sensorimotor performance. For control, they used data from other subjects who had completed the same measurements at four different times over 90 days for another study, not being exposed to HDBR. Relative to controls, the HDBR subjects showed widespread changes in GM volume, as the percent of brain volume, from pre- to the last assessment during HDBR. More specifically, GM volume increased in the posterior parietal region and decreased in the fronto-temporal regions, and these changes are strongly correlated. The sensorimotor performance was decreased in HDBR subjects from pre- to post-HDBR, as they needed more time to complete the test, while controls showed no difference in performance. Following the HDBR period, both GM volume and sensorimotor changes started to recover, though not totally 12 days later. Regarding the association between brain and behavior, researchers found that larger increases in GM volume in precuneus and pre- and postcentral gyri correlated with better balance performance, though not significantly after Bonferroni correction. They propose these changes in GM volume might reflect cortical plasticity as an adaptive response to alterations in somatosensory input caused by HDBR position. The observed patterns of GM change could also be explained by alterations in intracranial fluids distribution and pressure due to posture, though this hypothesis would need further examination. The authors conclude their findings match the sensorimotor deterioration observed in astronauts, but are also of interest for individuals who are temporarily or permanently confined to a bed and will probably experience the same GM and sensorimotor alterations.

Sofia Pedro


Cortical and scalp development

In a recent study, Tsuzuki and colleagues analysed the co-development of the brain and head surfaces during the first two years of life using a sample of 16 infant MRIs, aged from 3 to 22 months. First, they digitized a set of cortical landmarks defined by the major sulci. Then they determined the position of cranial landmarks according to the 10-10 system, a standard method to place electrodes for electroencephalography, using  nasion, inion, and the pre-auriculars as a reference. Besides analysing the spatial variability of the cortical and scalp landmarks with age, they compared the variability of the cortical landmarks to the 10-10 positions, in order to evaluate the validity of the scalp system as a reference for brain development. For that, they transferred a given cortical landmark to the head surface by expressing its position as a composition of vectors in reference to the midpoint between the two pre-auriculars and to the three neighbor 10-10 points. The scalp-transferred landmarks were then transformed to the scalp template of a 12-month-old infant and depicted in reference to the 10-10 system.

Age-related changes in the cortical landmarks were most obvious in the prefrontal and parietal regions. As the brain elongates, the frontal lobe shifts anteriorly and the precentral gyri widen. In addition, the intraparietal sulci and the posterior part of the left Sylvian fissure move forward, suggesting relative enlargement of the parietal region in the anterior direction. The same result was obtained by our team by analyzing cranial and brain landmarks in adults: larger brain size is associated with a relative forward position of the parietal lobe. The scalp showed relative anteroposterior elongation and lateral narrowing with growth. Regarding the contrast between the cortical landmarks and the 10-10 system, the authors observed that the variability in the position of the former was much smaller than the area defined by 10-10 landmarks, indicating this system can be useful to predict the underlying cortical structures. Hence, they conclude that the changes in brain shape during development are well described by cortical landmarks and that the relative scalp positioning based on the 10-10 system can adjust to preserve the correspondence between the scalp and the cortical surfaces.

 

Sofia Pedro


What the brain’s wiring looks like

The world’s most detailed scan of the brain’s internal wiring has been produced by scientists at Cardiff University. The MRI machine reveals the fibres which carry all the brain’s thought processes. It’s been done in Cardiff, Nottingham, Cambridge and Stockport, as well as London England and London Ontario. Doctors hope it will help increase understanding of a range of neurological disorders and could be used instead of invasive biopsies …

[keep on reading this article by Fergus Walsh on BBC News]


Structural MRI artifacts

Magnetic resonance imaging (MRI) is a valuable and increasingly used method for studying brain anatomy as it allows large-scale, high-quality in vivo analyses. However, some artifacts might influence the digital results, and thus require cautious interpretation. In a recent review, these issues are addressed along with possible solutions. First, we need to keep in mind that the images acquired are not mere photographs of the brain, but reflect some biophysical properties of the tissues, by measuring the radio-frequency signals emitted by hydrogen atoms (present in water and fat) after being excited by magnetic waves. Thus, MRI is an indirect analysis of the brain anatomy and depends greatly on specific tissue properties. Second, researchers can choose from a variety of methods, depending on the aim of the survey. Macrostructure, i.e. the size and shape across voxels, can be studied through manual volumetry or automatic segmentation, voxel- or deformation-based morphometry, surface- based algorithms, or diffusion tractography. Microstructure, i.e. within-voxel contents, is usually analyzed through diffusion MRI, but also magnetization transfer imaging, or quantitative susceptibility mapping.

When making inferences on the biological significance of the outputs, the researcher must account for the possible digital artifacts. These can occur both during image acquisition and processing and can be subject-related and methodological-related. A common problem is subject motion, which might contaminate or influence the results, as the amount of motion varies with other factors influencing brain changes (age, sex, and disease status), or can even correlate with a specific effect being studied. For instance, motion induces gray matter reduction, which might be perceived as brain atrophy. Subject motion is unavoidable, but its influence can be reduced by using a motion detector during acquisition, or by estimating the amount of motion allowing statistical adjustments, also useful to  detect outliers. The difficulty in manipulating the magnetic and radio-frequency fields might also introduce deformation. The main magnetic field should be spatially uniform, but it is dispersed by brain tissue while concentrated by air. This can be partially compensated by applying additional fields. The radiofrequency field is not homogeneous, which affects MRI contrast and intensity. Combining multiple transmit coils might help reduce this caveat, although the contribution and sensitivity of each coil must be taken into account when processing the image.

A particular case that can affect estimates of cortical volume and thickness is the difficulty in discriminating the dura and gray matter due to the similar intensity and anatomical proximity. In this case,  MRI parameters can be manipulated in order to increase the contrast between these tissues, without reducing the contrast between gray and white matter. Individual variability in folding patterns is a further major issue in voxel-based morphometry studies because it complicates the mapping of correspondences between subjects. Registration might be enhanced by analyzing regions with larger variation to find possible anatomical alterations, aligning cortical folding patterns to locate corresponding areas, and mapping sulcal changes to improve sulci identification. Finally, researchers should continuously keep track of the constant advances and innovations in the field. The authors conclude acknowledging the importance of structural MRI when coupled with other biological information, like genetic expression (Allen Brain Atlas), cytoarchitecture (JuBrain), and cognitive associations (Neurosynth).

Sofia Pedro