Endoscopic endonasal surgical anatomy through the prechiasmatic sulcus: the key window to suprachiasmatic and infrachiasmatic corridors.

BACKGROUND: Classically, the transtuberculum and transplanum approaches have been utilized to reach the suprachiasmatic and infrachiasmatic corridors. The aim of this study was to provide a better understanding of the key endoscopic endonasal anatomy of the suprachiasmatic and infrachiasmatic corridors provided through selective removal of the prechiasmatic sulcus (SRPS). METHOD: A SRPS was performed in 16 sides of 8 alcohol-fixed head specimens. Twenty anatomical measurements were collected on the suprachiasmatic and infrachiasmatic corridors. The transplanum and transtuberculum approaches were also performed. RESULTS: In the suprachiasmatic corridor, the SRPS exposed the anterior communicating artery (AComm) and the post-communicating segment of the anterior cerebral arteries in all the cases, while the pre-communicating segment of the anterior cerebral arteries, recurrent arteries of Heubner, and fronto-orbital arteries were visualized in 75% (12/16), 31% (5/16), and 69% (11/16) of cases, respectively. In the infrachiasmatic corridor, the ophthalmic segment of the internal carotid artery and superior hypophyseal arteries were always visible through the SRPS. The mean width and height of the prechiasmatic sulcus were 13.2 mm and 9.6 mm, respectively. The mean distances from the midpoint of the AComm to the anterior margin of the optic chiasm (OCh) was 5.3 mm. The mean width of the infrachiasmatic corridor was 12.3 mm at the level of the proximal margin of the ophthalmic segment of the internal carotid artery. The mean distances from the posterior superior limit of the pituitary stalk to the basilar tip and oculomotor nerve were 9.7 mm and 12.3 mm, respectively. CONCLUSIONS: The SRPS provides access to the main neurovascular and cisternal surgical landmarks of the suprachiasmatic and infrachiasmatic corridors. This anatomical area constitutes the key part of the approach to the suprasellar area. To afford adequate surgical maneuverability, the transplanum or transtuberculum approaches are usually a necessary extension.

Variation in the Anatomy of the Normal Human Optic Chiasm: An MRI Study.

BACKGROUND: Compression of the optic chiasm typically leads to bitemporal hemianopia. This implies that decussating nasal fibers are selectively affected, but the precise mechanism is unclear. Stress on nasal fibers has been investigated using finite element modeling but requires accurate anatomical data to generate a meaningful output. The precise shape of the chiasm is unclear: A recent photomicrographic study suggested that nasal fibers decussate paracentrally and run parallel to each other in the central arm of an "H." This study aimed to determine the population variation in chiasmal shape to inform future models. METHODS: Sequential MRI scans of 68 healthy individuals were selected. 2D images of each chiasm were created and analyzed to determine the angle of elevation of the chiasm, the width of the chiasm, and the offset between the points of intersection of lines drawn down the centers of the optic nerves and contralateral optic tracts. RESULTS: The mean width of the chiasm was 12.0 ± 1.5 mm (SD), and the mean offset was 4.7 ± 1.4 mm generating a mean offset:width ratio of 0.38 ± 0.09. No chiasm had an offset of zero. The mean incident angle of optic nerves was 56 ± 7°, and for optic tracts, it was 51 ± 7°. CONCLUSIONS: The human optic chiasm is "H" shaped, not "X" shaped. The findings are consistent with nasal fibers decussating an average of 2.4 mm lateral to the midline before travelling in parallel across the midline. This information will inform future models of chiasmal compression.

The anatomic variations and surgical windows among optic chiasm/nerves and carotid arteries in the sellar region play a role in choosing the best surgical approaches: A Cadaveric study / Las variaciones anatómicas y las vías de acceso quirúrgico en el quiasma, los nervios ópticos y las arterias carótidas de la región selar influyen en la elección de los mejores abordajes quirúrgicos: un estudio en cadáveres

OBJECTIVE: Understanding the relationship between the carotid artery, optic nerve and the anterior clinoid process is the basis of surgical approaches performed in the subchiasmal region. The location of the optic chiasm, the length of the optic nerves, and the distance and angle between the optic nerves determine the route of surgical approaches. We have determined the types of optic chiasm to study the relationship between vascular and neural structures in this region. MATERIALS AND METHODS: Thirty autopsy specimens were investigated at the Bursa Forensic Medicine Institute for optic chiasm types and the relationship between the neural and vascular anatomical structures of the sellar-parasellar and subchiasmal region was examined between June 2016 and November 2016. RESULTS: In this study, 4 prefix types (13%), 6 postfix types (20%), and 20 central types (67%) of chiasm were defined. Furthermore, we measured this angle between two optic nerves, which indirectly shows the location of chiasm according to the diaphragma sellae, and then detected the mean value of this angle as 87.1 ± 11.6°. The "limit" value to designate a chiasm as prefix was measured in the current study as ≥ 101.1°. The angle between optic nerves ranged from a mean value of 69.9 ± 3.7° in 6 cases with postfix chiasm, to a mean value of 104.0±2.1° in 4 cases with prefix chiasm and a mean value of 88.8 ± 6.7° in 20 cases with central chiasm. CONCLUSIÓN: In this study, we showed that the relationship among optic chiasma types, optic nerves and bony and vascular structures around the sellar area was effective at determining the surgical approach to this región

OBJETIVO: Comprender la relación entre la arteria carótida, el nervio óptico y la apófisis clinoides anterior es la base de los tratamientos quirúrgicos realizados en la región subquiasmática. La ubicación del quiasma óptico, la longitud de los nervios ópticos, y la distancia y el ángulo entre dichos nervios determinan la vía de acceso quirúrgico. Hemos determinado los tipos de quiasma óptico para estudiar la relación entre las estructuras vasculares y neurales en esta región. MATERIALES Y MÉTODOS: Entre junio y noviembre de 2016, se analizaron 30 muestras de autopsia en el Instituto de Medicina Legal de Bursa (Turquía) para determinar los tipos de quiasma óptico, y examinar la relación entre las estructuras anatómicas neurales y vasculares de las regiones selar-paraselar y subquiasmática. RESULTADOS: En este estudio, se definieron 4 casos de quiasma prefijado (13%), 6 de quiasma posfijado (20%) y 20 de quiasma central (67%). Además, medimos el ángulo entre 2 nervios ópticos que muestra indirectamente la ubicación del quiasma según el diafragma selar, y luego detectamos el valor medio de este ángulo (87,1 ± 11,6°). El valor «límite» para designar un quiasma como «prefijado» se midió en el estudio actual como ≥ 101,1°. El valor medio del ángulo entre los nervios ópticos osciló entre 69,9 ± 3,7° en los 6 casos de quiasma posfijado, 104,0 ± 2,1° en los 4 casos de quiasma prefijado y 88,8 ± 6,7° en los 20 casos de quiasma central. CONCLUSIÓN: En este estudio, pusimos de manifiesto que la relación entre los tipos de quiasma óptico, los nervios ópticos y las estructuras óseas y vasculares alrededor del área selar fue eficaz para determinar el acceso quirúrgico en esta región

The determination of the pituitary gland, optic chiasm, and intercavernous distance measurements in healthy subjects according to age and gender.

BACKGROUND: This paper was undertaken to determine the morphometry of pituitary gland diameter, pituitary gland height, intercavernous distance, optic chiasm diameter and optic chiasm height in skulls of Turkish population aged between 18 and 60 years. MATERIALS AND METHODS: It was a retrospective study in which 292 subjects were included 187 females and 105 males, ranging from 18 up to 60 years. Subjects underwent brain magnetic resonance imaging in the Radiology Department. Statistical analysis was performed with SPSS 21.00 programme. ANOVA test, χ2 test, and Pearson correlation analysis were used to determine the relation and significance between measurements and age group. The p < 0.05 value was considered as significant. RESULTS: The groups were divided into five groups according to age. The overall means and standard deviations of the measurements were: pituitary gland width, 13.09 ± 1.99 mm; pituitary gland height, 4.91 ± 1.10 mm; intercavernous distance, 15.93 ± 3.05 mm; optic chiasm width, 12.82 ± 1.27 mm; and optic chiasm height, 2.80 ± 0.49 mm in females, respectively whereas, the same measurements were 12.96 ± 1.74 mm; 4.79 ± 0.95 mm; 16.08 ± 3.11 mm; 13.13 ± 1.37 mm; 2.86 ± 0.70 mm in males, respectively. Height of the pituitary gland reached a maximum in the age group of 18 to 20 years in both females and males and there was a decrease in the pituitary gland height in the subsequent age groups. CONCLUSIONS: Knowledge of the variation in the size of pituitary gland, intercavernous distance and optic chiasm is important to evaluate the dimensions of these structures for clinical and pathological processes.

The anatomic variations and surgical windows among optic chiasm/nerves and carotid arteries in the sellar region play a role in choosing the best surgical approaches: A Cadaveric study.

OBJECTIVE: Understanding the relationship between the carotid artery, optic nerve and the anterior clinoid process is the basis of surgical approaches performed in the subchiasmal region. The location of the optic chiasm, the length of the optic nerves, and the distance and angle between the optic nerves determine the route of surgical approaches. We have determined the types of optic chiasm to study the relationship between vascular and neural structures in this region. MATERIALS AND METHODS: Thirty autopsy specimens were investigated at the Bursa Forensic Medicine Institute for optic chiasm types and the relationship between the neural and vascular anatomical structures of the sellar-parasellar and subchiasmal region was examined between June 2016 and November 2016. RESULTS: In this study, 4 prefix types (13%), 6 postfix types (20%), and 20 central types (67%) of chiasm were defined. Furthermore, we measured this angle between two optic nerves, which indirectly shows the location of chiasm according to the diaphragma sellae, and then detected the mean value of this angle as 87.1±11.6°. The "limit" value to designate a chiasm as prefix was measured in the current study as ≥101.1°. The angle between optic nerves ranged from a mean value of 69.9±3.7° in 6 cases with postfix chiasm, to a mean value of 104.0±2.1° in 4 cases with prefix chiasm and a mean value of 88.8±6.7° in 20 cases with central chiasm. CONCLUSION: In this study, we showed that the relationship among optic chiasma types, optic nerves and bony and vascular structures around the sellar area was effective at determining the surgical approach to this region.

The Organization of the Second Optic Chiasm of the Drosophila Optic Lobe.

Visual pathways from the compound eye of an insect relay to four neuropils, successively the lamina, medulla, lobula, and lobula plate in the underlying optic lobe. Among these neuropils, the medulla, lobula, and lobula plate are interconnected by the complex second optic chiasm, through which the anteroposterior axis undergoes an inversion between the medulla and lobula. Given their complex structure, the projection patterns through the second optic chiasm have so far lacked critical analysis. By densely reconstructing axon trajectories using a volumetric scanning electron microscopy (SEM) technique, we reveal the three-dimensional structure of the second optic chiasm of Drosophila melanogaster, which comprises interleaving bundles and sheets of axons insulated from each other by glial sheaths. These axon bundles invert their horizontal sequence in passing between the medulla and lobula. Axons connecting the medulla and lobula plate are also bundled together with them but do not decussate the sequence of their horizontal positions. They interleave with sheets of projection neuron axons between the lobula and lobula plate, which also lack decussations. We estimate that approximately 19,500 cells per hemisphere, about two thirds of the optic lobe neurons, contribute to the second chiasm, most being Tm cells, with an estimated additional 2,780 T4 and T5 cells each. The chiasm mostly comprises axons and cell body fibers, but also a few synaptic elements. Based on our anatomical findings, we propose that a chiasmal structure between the neuropils is potentially advantageous for processing complex visual information in parallel. The EM reconstruction shows not only the structure of the chiasm in the adult brain, the previously unreported main topic of our study, but also suggest that the projection patterns of the neurons comprising the chiasm may be determined by the proliferation centers from which the neurons develop. Such a complex wiring pattern could, we suggest, only have arisen in several evolutionary steps.

The neural pathway midline crossing theory: a historical analysis of Santiago Rámon y Cajal's contribution on cerebral localization and on contralateral forebrain organization.

Throughout history, many scientists have wondered about the reason for neural pathway decussation in the CNS resulting in contralateral forebrain organization. Hitherto, one of the most accepted theories is the one described by the renowned Spanish physician, Santiago Rámon y Cajal at the end of the 19th century. This Nobel Prize winner, among his many contributions to science, gave us the answer to this question: the key lies in the optic chiasm. Based on the fact that the ocular lenses invert the image formed in the retina, Cajal explained how the decussation of the fibers in the optic chiasm is necessary to obtain a continuous image of the outside in the brain. The crossing of the tactile and motor pathways occurred posteriorly as a compensatory mechanism to allow the cortical integration of the sensory, motor, and visual functions. This theory had a great influence on the scientific community of his time, and maintains its importance today, in which none of the theories formulated to date has managed to entirely refute Cajal's. In addition, the decussation of neural pathways plays a significant role in different diseases, especially in the recovery process after a hemispheric lesion and in several congenital pathologies. The advantages of cerebral lateralization have also recently been published, although the evolutionary connection between fiber decussation and cortical function lateralization remains a mystery to be solved. A better understanding of the molecular and genetic substrates of the midline crossing processes might result in significant clinical advances in brain plasticity and repair.

Development of a contouring guide for three different types of optic chiasm: A practical approach.

INTRODUCTION: Sparing of the organs at risk (OARs) is a crucial task in daily radiotherapy practice. Irradiation of the optic chiasm (OC) results in radiation-induced optic neuropathy (RION). The structure of the OC is complex, and OC morphology can vary in axial images. Therefore, a standard atlas can result in inaccurate descriptions of OC morphology in different patients. The aim of our study was to provide a guide based on computed tomography (CT) for the delineation of different types of OC. METHODS: Thirty-six patients were selected to participate in our study. These patients underwent CT analysis of the brain, head and neck regions in a supine position. Axial images 3 mm in thickness were obtained at 3-mm intervals. A magnetic resonance imaging (MRI) study was also performed using the same set-up. The OC was then delineated. The contours were revised by three neuroradiologists and nine radiation oncologists with > 5 years of expertise. RESULTS: Three types of OC were distinguished by magnetic resonance (MR). The location and boundaries of normal, prefixed and postfixed chiasms were developed with a CT-based atlas. Discrepancies were observed in the delineation of the prefixed and postfixed OC. CONCLUSIONS: Our guide allows improved definitions of the anatomical boundaries for different types of OC. Our experience could provide useful information for radiation oncologists in daily practice.

Measurements and Clinical Application of Anatomical Space for Transfrontal Pituitary Surgery Through Magnetic Resonance Imaging Reconstruction.

OBJECTIVE: This study aims to clarify the relative position of the normal important structures and anatomical spaces formed by the structures passed through during the transfrontal pituitary surgery, and discuss how to avoid some eloquent structures. METHODS: A total of 120 cases of magnetic resonance imaging images from normal adult brains were selected as the object of study and divided into male and female groups. The important adjacent structures of the pituitary passed through during the transfrontal pituitary surgery were marked on the reconstructed images. In all planes of the spaces passing through successively during the pituitary surgery, the morphological parameters such as the size, boundary, structure, and spatial extent of the spaces were measured. RESULTS: The size, boundary, structure, and spatial extent of the space between the 2 optic nerves, the space between the optic nerves and the pituitary stalk, and the space between the tuber cinereum and the interal carotid artery in the plane of the pituitary stalk were measured, the anterior part and the posterior part in male were shorter than those in female (P = 0.021; P = 0.029); no statistically significant difference was found in the measurements of the lengths and angles of these spaces. CONCLUSIONS: The authors' findings provide the surgeons with the detailed anatomical data and help to provide a morphological basis for intraoperative protection of the pituitary and vital adjacent structures and surgical approach.

The Superior Hypophyseal Arteries: Anatomical Study with an Endoscopic Endonasal Perspective.

BACKGROUND: The use of high-definition endoscopes in extended transsphenoidal approaches to the suprasellar area has significantly improved visualization of its vascularization. OBJECTIVE: To systematically examine the superior hypophyseal arteries (SHAs) anatomy from an endonasal endoscopic perspective. METHODS: The endoscopic endonasal transsphenoidal trans-tuberculum approach was performed in 19 adult, fresh and latex injected specimens. Dissections recordings were reviewed to analyze SHAs type, number, and branches, as well as internal carotid arteries (ICA) branches that vascularized optic nerves and chiasm. RESULTS: Identification of SHAs was possible in all specimens (37/38 sides). The number of SHAs varied from 1 to 3 per side (mean: 1.7). The anterior superior hypophyseal artery was visible in almost all cases (35/37 sides) and originated at the level of the carotid cave in 18/35 specimens; number of branches ranged from 1 to 6 (mean: 3.5), directed to the optic nerve (86%), chiasm (57%), infundibulum (86%), and/or parallel to the pituitary stalk (74%). The 4 main branches and patterns, originally described by McConnell in 1953, were confirmed. The posterior superior hypophyseal artery was evident in 28/37 sides with number of branches ranging from 0 to 4 (mean: 2.1), directed to the optic chiasm (50%), optic tract (32%), infundibulum (79%), and/or pituitary stalk (36%). The surgical implications of this study, together with anatomical and clinical videos, are also briefly discussed. CONCLUSION: SHAs constitute a complex of anterior and posterior branches that stem from the medial ICA with different patterns, vascularizing the optic apparatus and pituitary stalk.

Using Three-Dimensional Printing to Create Individualized Cranial Nerve Models for Skull Base Tumor Surgery.

OBJECTIVE: Using three-dimensional (3D) printing to create individualized patient models of the skull base, the optic chiasm and facial nerve can be previsualized to help identify and protect these structures during tumor removal surgery. METHODS: Preoperative imaging data for 2 cases of sellar tumor and 1 case of acoustic neuroma were obtained. Based on these data, the cranial nerves were visualized using 3D T1-weighted turbo field echo sequence and diffusion tensor imaging-based fiber tracking. Mimics software was used to create 3D reconstructions of the skull base regions surrounding the tumors, and 3D solid models were printed for use in simulation of the basic surgical steps. RESULTS: The 3D printed personalized skull base tumor solid models contained information regarding the skull, brain tissue, blood vessels, cranial nerves, tumors, and other associated structures. The sphenoid sinus anatomy, saddle area, and cerebellopontine angle region could be visually displayed, and the spatial relationship between the tumor and the cranial nerves and important blood vessels was clearly defined. The models allowed for simulation of the operation, prediction of operative details, and verification of accuracy of cranial nerve reconstruction during the operation. Questionnaire assessment showed that neurosurgeons highly valued the accuracy and usefulness of these skull base tumor models. CONCLUSIONS: 3D printed models of skull base tumors and nearby cranial nerves, by allowing for the surgical procedure to be simulated beforehand, facilitate preoperative planning and help prevent cranial nerve injury.

MRI measurements of the normal pediatric optic nerve pathway.

The purpose of this work is to establish a reference scale of optic nerve pathway measurements in pediatric patients according to age using MRI. Optic nerve pathway measurements were retrospectively analyzed using an orbits equivalent sequence on brain MRI scans of 137 pediatric patients (72 male, 65 female, average age = 7.7 years, standard deviation  = 5.3). The examinations were performed on a 1.5-T or 3-T Siemens MR system using routine imaging protocols. Measurements include diameters of the orbital optic nerves (OON), prechiasmatic optic nerves (PON), optic tracts (OT), and optic chiasm (OC). Measurements were performed manually by 2 neuroradiologists, using post-processing software. Patients were stratified into five age groups for measurement analyses: (I) 0-1.49 years, (II) 1.5-2.99 years, (III) 3-5.99 years, (IV) 6-11.99 years, and (V) 12-18 years. The observed value range of OON mean diameter was 2.7 mm (Interquartile range (IQR) = 2.4-2.9), PON was 3.2 mm (IQR  =  3.05-3.5), OT 2.6 mm (IQR = 2-2.9). A strong positive correlation was established between age and mean diameter of OON (r = 0.73, p < .001), PON (r = 0.59, p < .001), and OT (r = 0.72, p < .001). A significant difference in mean OON diameters was found between age groups I-II (d = 0.3, p = .01), II-III (d = 0.5, p < .001), III-IV (d = 0.5, p < .001) followed by a plateau between IV-V (d = 0.l0, p = .19). OON/OT ratio maintained a steady mean value 1 (IQR = 0.93-1.1) regardless of age (p = .7). The diameter of optic pathways was found to increase as a function of age with consistent positive correlation between nerve and tract for all ages.

Prechiasmatic sulcus and optic strut: an anatomic study in dry skulls.

BACKGROUND: Although safe surgical access to the cavernous sinus is related to understanding the anatomical and ethnic variants of the prechiasmatic sulcus and the optic strut, there remains a paucity of studies of the morphology and the bony relationships in the region. The present study provides a systematic morphological and morphometric analysis of the sulcal region and the optic strut anatomy and their relations in a Greek population. METHODS: The interoptic distance, length of planum sphenoidale, sulcal length and sulcal angle was determined in 96 Greek adult dry skulls. The prechiasmatic sulci and optic struts were morphologically classified and association of sulcal region measures according to type of prechiasmatic sulcus and optic strut were examined. RESULTS: Mean interoptic distance was 1.69 ± 0.25 cm; sulcal length, 0.72 ± 0.18 cm; length of planum sphenoidale, 1.86 ± 0.32 cm; sulcal angle, 24.05 ± 17.17°. The sulcal angle was significantly smaller in female skulls compared to males (14.82 ± 12.43 vs 28.29 ± 15.24; p < 0.05). Type I (narrow, steep) prechiasmatic sulci were the most commonly observed (35.8%), followed by Type IV (wide, flat) (32.1%), Type II (narrow, flat) (18.5%) and, finally, Type III (wide, steep) sulci (13.6%). The optic strut was presulcal in 8.3% of specimens, sulcal in 31%, postsulcal in 41.7% and asymmetric in 19%. CONCLUSIONS: The present study augments the current knowledge of the morphology of key anatomical landmarks, prechiasmatic sulcus and the optic strut, for cavernous sinus surgery and indicates population and gender differences. We report significant anatomical variations in the prechiasmatic sulcus, optic strut and surrounding structures. In addition to providing a better understanding of the anatomical landmarks, necessary for the safe navigation in transcranial and endoscopic procedures, the present results also suggest that surgeons must consider population differences in determining the anatomical landmarks and navigation points in the sellar region.

Can the angle between optic nerves indicate whether optic chiasm is prefixed, normofixed or postfixed? An anatomical study with radiologic and neurosurgical implications.

PURPOSE: The objective of this study was to measure the angle (Interneural angle, INA) between intracranial segments of optic nerves (ISON), and to look for any relation between it and the relative anteroposterior location (RAPL) of the optic chiasm (OC)-viz. prefixed, normofixed and postfixed. METHODS: The sample comprised of 100 autopsy specimens from South Indian population. INA was measured using software-aided processing of digital photographs. Length of the ISON was measured on each side using Vernier calipers. RAPL of the OC was noted during dissection. These were analysed with statistical methods. RESULTS: RAPL of OC was found to be prefixed in 24 %, normofixed in 65 % and postfixed in 11 %. The INA had an overall mean of 69.9° (SD 9.29°). ANOVA confirmed statistically significant difference in INA among different groups; the corresponding mean value for the group was as follows: 79.61° (prefixed), 68.10° (normofixed) and 59.48° (postfixed). ROC curve was plotted for the use of various 'cut off' values of INA to 'diagnose' prefixed OC; an INA ≥71.4° was seen to diagnostically correlate with prefixed OC with 83.3 % sensitivity and 75 % specificity. CONCLUSIONS: The INA is wider when OC is prefixed, intermediate when normofixed and narrowed when postfixed. This observation throws light on the possibility of using INA as a marker of RAPL of OC. As INA can be measured in axial MRI sections, it can be used in differentiation of the cases with prefixed OC from others during pre-operative work up for pituitary surgeries and to identify individuals 'at risk' during subfrontal approach for pituitary lesions.

Visualization of Nerve Fiber Orientations in the Human Optic Chiasm Using Photomicrographic Image Analysis.

PURPOSE: Hemidecussation of fibers entering the optic chiasm from the optic nerves is well recognized. The reason why bitemporal hemianopia results from chiasmal compression has not been fully explained. There is still a paucity of data relating to the precise details of the routes that the nerve fibers take through the chiasm and, in particular, where and how nerve fibers cross each other. This information is important to understanding why crossing fibers are selectively damaged as a result of chiasmal compression. METHODS: An optic chiasm obtained at postmortem was fixed, stained, and sectioned to allow high-resolution photomicrographs to be taken. The photomicrographs were integrated to allow regions of interest across entire sections to be analyzed for fiber direction and crossing. RESULTS: The results confirmed that fibers from the temporal retina pass directly backward in the lateral chiasm to the optic tract, whereas fibers from the nasal retina cross to the contralateral optic tract. Crossings take place in the paracentral regions of the chiasm rather than in the center of the chiasm (where the nerve fibers are traveling mostly in parallel). The paracentral crossing regions are distributed in a largely postero-superior to antero-inferior arrangement. CONCLUSIONS: These findings clarify the precise locations and crossing angles of crossing nerve fibers in the chiasm. This information may help explain the clinical observation of junctional scotoma and will provide a much better basis for structural modeling of chiasmal compression which, in turn, will improve our understanding of how and why bitemporal hemianopia occurs.

Understanding fiber mixture by simulation in 3D Polarized Light Imaging.

3D Polarized Light Imaging (3D-PLI) is a neuroimaging technique that has opened up new avenues to study the complex architecture of nerve fibers in postmortem brains. The spatial orientations of the fibers are derived from birefringence measurements of unstained histological brain sections that are interpreted by a voxel-based analysis. This, however, implies that a single fiber orientation vector is obtained for each voxel and reflects the net effect of all comprised fibers. The mixture of various fiber orientations within an individual voxel is a priori not accessible by a standard 3D-PLI measurement. In order to better understand the effects of fiber mixture on the measured 3D-PLI signal and to improve the interpretation of real data, we have developed a simulation method referred to as SimPLI. By means of SimPLI, it is possible to reproduce the entire 3D-PLI analysis starting from synthetic fiber models in user-defined arrangements and ending with measurement-like tissue images. For the simulation, each synthetic fiber is considered as an optical retarder, i.e., multiple fibers within one voxel are described by multiple retarder elements. The investigation of different synthetic crossing fiber arrangements generated with SimPLI demonstrated that the derived fiber orientations are strongly influenced by the relative mixture of crossing fibers. In case of perpendicularly crossing fibers, for example, the derived fiber direction corresponds to the predominant fiber direction. The derived fiber inclination turned out to be not only influenced by myelin density but also systematically overestimated due to signal attenuation. Similar observations were made for synthetic models of optic chiasms of a human and a hooded seal which were opposed to experimental 3D-PLI data sets obtained from the chiasms of both species. Our study showed that SimPLI is a powerful method able to test hypotheses on the underlying fiber structure of brain tissue and, therefore, to improve the reliability of the extraction of nerve fiber orientations with 3D-PLI.

Organs at risk in the brain and their dose-constraints in adults and in children: a radiation oncologist's guide for delineation in everyday practice.

PURPOSE: Accurate organs at risk definition is essential for radiation treatment of brain tumors. The aim of this study is to provide a stepwise and simplified contouring guide to delineate the OARs in the brain as it would be done in the everyday practice of planning radiotherapy for brain cancer treatment. METHODS: Anatomical descriptions and neuroimaging atlases of the brain were studied. The dosimetric constraints used in literature were reviewed. RESULTS: A Computed Tomography and Magnetic Resonance Imaging based detailed atlas was developed jointly by radiation oncologists, a neuroradiologist and a neurosurgeon. For each organ brief anatomical notion, main radiological reference points and useful considerations are provided. Recommended dose-constraints both for adult and pediatric patients were also provided. CONCLUSIONS: This report provides guidelines for OARs delineation and their dose-constraints for the treatment planning of patients with brain tumors.

Photoreceptor projections and receptive fields in the dorsal rim area and main retina of the locust eye.

In many insect species, photoreceptors of a small dorsal rim area of the eye are specialized for sensitivity to the oscillation plane of polarized skylight and, thus, serve a role in sky compass orientation. To further understand peripheral mechanisms of polarized-light processing in the optic lobe, we have studied the projections of photoreceptors and their receptive fields in the main eye and dorsal rim area of the desert locust, a model system for polarization vision analysis. In both eye regions, one photoreceptor per ommatidium, R7, has a long visual fiber projecting through the lamina to the medulla. Axonal fibers from R7 receptors of the dorsal rim area have short side branches throughout the depth of the dorsal lamina and maintain retinotopic projections to the dorsal medulla following the first optic chiasma. Receptive fields of dorsal rim photoreceptors are considerably larger (average acceptance angle 33°) than those of the main eye (average acceptance angle 2.04°) and, taken together, cover almost the entire sky. The data challenge previous reports of two long visual fibers per ommatidium in the main eye of the locust and provide data for future analysis of peripheral networks underlying polarization opponency in the locust brain.

Decussation as an axial twist: A comment on Kinsbourne (2013).

One of the great mysteries of the brain, which has puzzled all-time students of brain form and function, is the contralateral organization of the forebrain and the crossings of its major afferent and efferent connections. As a novel explanation, two recent studies have proposed that the rostral part of the head, including the forebrain, is rotated by 180° with respect to the rest of the body (de Lussanet and Osse, 2012; Kinsbourne, 2013). Kinsbourne proposes one 180° turn while we consider the 180° being the result of two 90° turns in opposite directions. We discuss the similarities and differences between the two hypotheses.