Magnetic 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.
A new PhD student in the team working on craniovascular anatomy! Stanislava Eisová was in our laboratory few years ago, publishing a paper on parietal bone and vessels in which she investigated correlations between craniovascular morphology, skull size, and bone thickness. She got a Master Degree in Anthropology of Past Populations at the University of West Bohemia, Czech Republic. Now she begins a PhD program on Anthropology and Human Genetics at the Faculty of Science of the Charles University, Prague. Her project will be on craniovascular traits, anthropology, paleoanthropology, and paleopathology. She will investigate craniovascular characters in normal samples, pathological conditions, and fossil specimens. The project is co-directed by Petr Velemínský.
The human brain is the most expensive and costly organ in terms of energetic resources and management. However, the current understanding of its sophisticated thermal control mechanisms remains insufficient. Wang et al., 2016, have reviewed the most recent studies on brain thermoregulation and examined the anatomical and physiological elements associated with selective brain cooling. Modern humans have a brain that is approximately three times larger than a primate with a similar body size, which uses 20%– 25% of the total body energy compared with a maximum of 10% in other primates and 5% in other mammals. The evolution of a large and expensive brain in modern humans effectively influences critical factors such as temperature, and functional limits can affect cerebral complexity and neural processes. Brain thermoregulation depends on many anatomical components and physiological processes, and it is sensitive to various behavioral and pathological factors, which have specific relevance for clinical applications and human evolution. The anatomical structures protecting the brain, such as the human calvaria, the scalp, and the endocranial vascular system, act as a thermal interface, which collectively maintains and shield the brain from heat challenges, and preserves a stable equilibrium between heat production and dissipation. Future advances in biomedical imaging techniques would allow a better understanding of the physiological and anatomical responses related to the cerebral heat management and brain temperature in modern humans.
Gizéh Rangel de Lázaro
This month we have published a review on craniovascular traits and anthropology, freely available to download from the Journal of Anthropological Sciences. The article describes many vascular traits that can be analyzed on skulls, through the traces they leave on the bone surface or within the bone itself. The traces of the middle meningeal vessels, the traces of the venous sinuses, the diploic channels, and the endocranial foramina, can provide information on the vascular networks and, indirectly, on the physiological processes associated with their growth and development. The functional information available from these imprints is partial and incomplete, but it is the only one we have on blood flow when dealing with fossils, archaeological remains, or forensic cases. Methods are an issue, because of the difficulties with small samples, scoring procedure, statistics of ordinal and nominal variables, and with an intrinsic limitation in current anatomy: we still ignore the variations and processes behind many macroanatomical features, even in our own species. Previous articles on this topic deal with middle meningeal artery, vessels and thermoregulation, diploic channels, and parietal bone vascularization. Most of these papers are part of a project funded by the Wenner-Gren Foundation through an International Collaborative Research Grant, entitled “Cranial anatomy, anthropology, and the vascular system”. This beautiful drawing of a sectioned skull is by Eduardo Saiz.
The Rhoton Collection is composed by an outstanding anatomical presentations of the brain created by the renowned surgeon and educator Dr. Albert Rhoton Jr throughout his life. These presentations were made using bright blue and red dyes in the blood vessels, so that surgeons could easily visualize and explore the brain and vascular structures for planning surgical interventions.
[Here a post in Spanish]
Gizéh Rangel de Lázaro
In general, members of the Primate Order possess larger brains for body size than other mammals, with modern humans (Homo sapiens) evolving the largest brains. The Internal Carotid Artery (ICA) provides blood supply to the brain but there are distinct anatomical differences between the primate groups. While vascular and other soft-tissues are very rarely fossilised, evidence of the ICA passage is retained where it was encapsulated in bony tubes or as distinct grooves of the endocranial surface. The ICA evolved in primates from two main pathways in the auditory complex: the promontory artery branched from the cochlear space to supply blood to the brain while the stapedial artery branched from the obturator foramen of the stapes to provide blood to the cerebral meninges and the orbito-facial complex. Primate groups are recognised by ICA anatomy: Within the Strepsirrhini, Lorisiformes and Cheirogaleids both lack the stapedial and promontory arteries with the External Carotid Artery (ECA) supplying blood to the brain instead of the ICA; whereas, non-Cheirogaleids possess the ICA and stapedial artery but often retain a much smaller promontory artery, while the anthropoids (apes and monkeys) lack the stapedial artery entirely retaining only the promontory artery.
The evolution of these distinct differences was examined by comparing living and fossil primates and reconstructing the hypothetical phylogenetic pathways. The size differences between the stapedial and promontory arteries were compared to endocranial volume (ECV) to investigate the influence on brain size. Only the size of the promontory artery had a consistent correlation with brain size. There was no reported correlation between brain size and size of the stapedial artery, with the stapedial only correlating with size of the promontory artery. This suggests that throughout primate evolution, the trend for body and brain size increases caused the stapedial artery to become restricted as head size increased but size of the obturator foramen did not scale equally with the head. Most early primates evolved a reduction in the size of the stapedial artery and quickly accommodated an increase in the promontory artery allowing even greater blood flow to the brain and driving the encephalisation process and the eventual loss of the stapedial artery in anthropoids.
The fossil record offers several possible approaches to study the evolution of the human brain. Besides cerebral size and shape, we can make inferences about cognitive functions and metabolic processes. Analyses of the craniovascular system are required to better understand both aspects. A recent article in the Royal Society Open Science journal adds new evidence into this issue comparing cerebral blood flow rate and endocranial volume in fossil hominids. The metabolic rate of the human brain is tightly related to the cerebral blood flow, which is mainly supplied by internal carotid arteries (ICAs). The authors measured the dimensions of the carotid foramen, the external opening of the carotid canal, in 35 fossil skulls, and calculate the size of the internal carotid arteries lumen. Then, they calculated the blood flow based on the shear stress, arterial lumen radius and blood viscosity (using supporting data from human and rats models). Their results show that the ICAs blood flow rate increases disproportionately in hominids, when scaled against brain volume. The authors then speculate about metabolic rate and its association with greater synaptic activity, cognitive functions, and life-history evolution. The paleoneurological information considered in the article is not much updated, and the sample includes many casts, which reliability is not comparable with original specimens. Also, inferences on cognition or life-history sound probably too much speculative when dealing with a simple carotid canal. Nonetheless, this paper supplies a good perspective in vascular biology, with a clear application in paleoanthropology. The possibility of calculating the cerebral blood flow in fossil specimens is interesting and opens new research opportunities.
Studying anatomical variability in paleontological and archaeological context is a challenge to look behind (and beyond) the bones. In the case of cranial remains we are able to make inferences not only on bone morphology but also on part of the vascular system. With computed tomography we can observe the diploic channels inside the bone matrix, and the imprints of the middle meningeal vessels on the endocranial surface of the vault. In the parietal bone both networks are particularly developed, most of all in modern humans. This month we have published a new study focusing the size and morphology of these vascular imprints in adult humans, and on their relationship with bone size and thickness. Our aim was to reveal possible influences between vascular and bone morphology. Vessels and bones share morphogenetic processes, and there can be shared functional and structural relationships between angiogenesis and osteogenesis. Shared growth factors can generate a positive correlation between bones and vessels dimensions or, conversely, biomechanical constraints between bone matrix and its embedded soft tissues can generate an inverse relationship between their volumes. We used CT data of human adult crania to measure cranial size, parietal bone thickness, and lumen size of these vascular traces. We provide a metric description of the size variation and size distribution of the diploic channels and meningeal imprints, for different orders of branches. The diploe largely influences the overall thickness of the bone. The upper part of the parietal bone shows the thickest values. The lumen size of the diploic channels and meningeal imprints is very similar, with no patent sexual or hemispheric differences. The correlation analysis did not revealed any clear relationship between vessels size, cranial size, and cranial thickness. Therefore, these results do not support the hypothesis of a reciprocal influence between bone and vascular morphology, which are likely to respond to different factors. Actually, although some vascular changes may be described in extreme cases of cranial deformation, also according to a previous survey on the endocranial vascular pattern in normal variation there is no apparent correspondence between gross cranial form and craniovascular traces.
There are plenty of reports about anatomical and morphological variation of cranial foramina; however, their developmental mechanisms fundamental for interpretation of such a variation and understanding of vital medical conditions related to their aberrant formation are poorly known. Cranial foramina transmitting the vessels and nerves emerge within the cranial bones which themselves show complex origin and development. Recent embryological study in chicks by Akbareian et al. (2015) presents development of cranial foramina in mesoderm derived occipital bone arising through endochondral ossification. Unexpectedly, the formation mechanism did not show any extensive apoptotic cell activity and target proliferation. Instead as a “clearing” mechanism forming the cavity of foramina was proposed localized restriction of ossification caused by the presence of vessel and nerve elements with minimal mesenchymal cell death. Further importance for morphological studies of foramina can bear a discovery that the shape of vessel dictates the overall shape of the foramen.
The diploic channels are placed between the vault cortical layers (external and internal). The calvarial diploe contains large and valveless diploic veins interconnected through a complex network of microscopic channels. We have now published a procedure for segmentation of diploic channels and localization of the main vessel pathways by reducing the noise of the cancellous bone. We also provide quantitative description of the diploic vessel variation in modern humans and three Neanderthals. One modern human was reformatted at three different resolutions namely high, medium and low, to estimate the effect of the pixel resolution on the final anatomical rendering. The use of computed tomography at high resolution can hamper semi-automatic segmentation of the diploic channels. Optimal resolution should be sufficient to reveal the channels without increasing noise associated with the trabecular structure. We have found that modern humans present a remarkable variation of diploic channels in their morphological patterns, being the parietal area the most vascularized. There is a correlation in the degree of vascularization of the frontal, parietal, and occipital bone, and no asymmetries can be apparently detected. The three Neanderthals analyzed in the study also display a parietal vascular network, but less developed than modern human, suggesting these vessels may be involved in evolutionary changes. The diploic network is commonly connected with the meningeal artery at the temporal fossa, with the emissary veins at the occipital bone, and with the venous sinuses at the parieto-occipital areas. The brain and braincase of our specie are characterized by larger parietal areas, and changes in the vascular organization can be associated with thermoregulation and heat management. In this sense future research may help us to understand the possible involvement of the diploic veins in brain thermoregulation. The study of diploic channels may be relevant in anthropology, medicine, paleontology, and forensic sciences.
Gizéh Rangel de Lázaro