| BackgroundVision is the main source of organism to outside, accounting for more than 70%of all afferent sensory. Relying on visual perception system, humans and animals canjudge and distinguish the relationship with objects, including size, shape, color,brightness, dynamic or static condition, distance, etc., can also activate their thinking,feeling and emotion, and develop action plans. Because of the special anatomy ofvisual pathway, the lesions could not be inspected by routine ophthalmic examination,apart from papillopathy, must be detected by assistance of modern equipment.However, ultrasound and CT have lesser diagnostic value for lesions of visual whitematter due to low sensitive for distinguishing gray matter from white matter. MRI hasproven to be beneficial for the assessing normal and abnormal brains with its highsoft tissue and spatial resolution, avoiding from bone artifact and refraining fromradiation damage, but it has less sensitive for the white matter fibers. So, theseconventional imaging modalities currently available in the clinic cannot reveal theanatomic structure of white matter fibers.Recently, however, depiction of complex white matter architecture in vivo hasbeen made possible with magnetic resonance diffusion tensor imaging (MR-DTI).MR-DTI is the only one noninvasive technique in vivo to reflect the microstructural architecture and orientation of white matter fibers and, through the technique oftractography, reveals the trajectories of cerebral white matter tracts. In addition,quantification of diffusion can be especially helpful as it may allow early to detect thephysiological and pathological changes of white matter fibers. DTI can probestructural properties of tissue not accessible to other techniques and may represent aunique tool for noninvasively mapping fiber tracts in the living human brain. DTIallows the quantification of rotationally invariant measures of water diffusionincluding mean diffusivity and fractional anisotropy, which provide informationrelated to the integrity and orientation of tracts in the brain in vivo. It is generallyagreed that DTI is benefit for studies including brain development and maturation,leukodystrophy, tumors, head trauma. However, there had not yet paid close attentionto studying normal or abnormal visual pathway, and had short of relevant literaturesabout utilizing DTI. Not only can they reveal axonal disruption and demyelination ofwhite matter fibers, and demonstrate the anatomical and pathological changes ofvisual fibers with lesions, but also have important value for treatment of preoperativeplans, intraoperative localization, and prognosis evaluating. Therefore, the DTItechnique was selected to systemically study brain white matter fibers of visualpathway.Objective1. To search for the optimal scanning parameters for MR diffusion tensor imaging inthe study of the visual pathway white matter fibers.2. To investigate the normal reference value of the fractional anisotropy(FA) andmean diffusivity(MD) of white matter fibers including the optic nerve, optic tract andoptic radiation in healthy adult. To study the trajectory and three dimensionalconformation of visual pathway white matter fibers in healthy adult using magneticresonance diffusion tensor tractography. 3. To investigate the fractional anisotropy and mean diffusivity feature and evaluateclinical applications value of diffusion tensor imaging in lesions involved the opticnerve and optic radiation.Material and methods1. Subjects:30 cases of healthy volunteers and 145 patients with lesions involved the opticpathway were studied. Within the patients, there had acute optic neuritis(n=16), opticatrophy(n=21), orbital tumor(n=3), occipital and/or temporal ischemic infarct(n=71),congenital septo-optic dysplasia(n=1), acquired blindness(n=20), and intracerebraltumor(n=34).2. Equipment and examination technique:All studies were performed on a 1.5T whole-body MR system(Signa Twin,General Electric Medical System). The imager is equipped with magnetic fieldgradients that permit up to 40 mT/m/s amplitude and magnetic field gradientsswitching rate that up to 120mT/m/s. A 8-channel head phased array coil was used forall subjects. Patients with fixed head, ears plugging appropriate size cotton ball inorder to diminish the influence of noise, were asked to close their eyes and avoiddeliberate eye movements during scanning. DTI scan parameters: single-shot spinecho planar imaging (SE-EPI) sequence, TR/TE 6000/60.1ms, b value with1000s/mm2. diffusion-sensitive gradient direction with 13, matrix 128×128. FOV240 mm×240 mm, 2 NEX, 2/0mm thickness / interval paralleling optic nervescanning for anterior visual pathway and 3/0mm with axial scanning for posteriorvisual pathway3. Image processing, image quality evaluation, regions of interest(ROIs) definitionand measurement:Primary data of diffusion tensor imaging were processed off-line in the personal computer, with the post-processing software of the Volume-one 1.72 and thediffusion TENSOR Visualizer (dTV) 1.72 which were provided by GE Corp. Foreach section, a T2-weighted map, a directionally encoded color(DEC) map, afractional anisotropy (FA) map, an apparent diffusion coefficient (ADC) map, and adiffusion weighted (DW) map were calculated. Color hue indicates direction in DECmap as follows: red represented the x element (left to right); green the y element(anterior-posterior); blue the z element (superior-inferior). This convention applied toall the directional maps. The brightness was proportional to fractional anisotropy.According to the brink of fiber bundle and the background noise in DT images,the map quality was divided into excellent, good and poor in three different gradesand evaluated by three neuro-radiology doctors.To dissect the visual pathway, three different ROIs were defined, according tothe classic description of neuroanatomy and related literatures. The ROIs were drawnrespectively on the optic nerve (shown as green) and optic tract (shown as green orred) in coronal reconstructed DEC maps, and on the optic radiation (shown as green)in the section of trigone of lateral cerebral ventricle. The measurement wereundertook three times in front, middle and behind of each part of anterior visualpathway, and in three consecutive layers of optic radiation. Each parameter wasobtained with average value of three times measurement. The data were expressed asa mean±standard deviation (X±SD).The methods of tractography with anterior visual pathway as follows: the seedpoint was set with manual selection bilateral optic nerve, chiasm, and optic tract inthe chiasm section of DEC map, expressing X shape, the termination conditions wereset with FA＜0.20, step＜160, So＜160. The methods of tractography with posteriorvisual pathway as follows: the seed point was set within lateral geniculate body(LGB)in the DEC map or DWI map, occipital lobe was the region of termination. The termination conditions were FA＜0.18, step＜160, So＜160.4. statistical analysis.SPSS 11.0 software package was applied to statistical analysis in the study. Thedata were expressed as a mean±standard deviation ((?)±SD). A paired samples t-testor an independent samples t-test was used to compare the differences of two groupsamples means. One-way ANOVA was applied to analysis each parameter of eachregion of interest as homogeneity of variances in multiple group samples means. Thestatistical significance was defined as a P value less than 0.05.Results1. Results of partâ… Different scan parameters had different impact on the quality of the FA maps andthe DEC maps. The imaging quality was the best when b value was 1000s/mm2, butnoise became greater when b value was 3000s/mm2. The imaging quality was bestwhen 13 or 21 directions were used which did not any significant differences, and theimaging quality of 6 directions was worst. Slice thickness had the greater impact onthe signal-to-noise ratio(SNR) of the images. The optimal slice thickness were 2mmfor anterior visual pathway and 3mm for posterior visual pathway, because there hadhigher good imaging and avoiding from partial volume effect simultaneously.2. Results of partâ…¡The bilateral visual conducting fibers were high signal in FA maps and showedgreen color in DEC maps. The fibers and their rims were distinct and smooth in DECmaps. The FA value of optic nerve was 0.595±0.067 in left and 0.589±0.066 in right,the MD value was (0.948±0.112)×10-3mm2/s in left and (0.932±0.088)×10-3mm2/s inright. The FA value of optic tract was 0.531±0.062 in left and 0.526±0.052 in right, ndthe MD value was (0.944±0.131)×10-3mm2/s in left and (0.935±0.113)×10-3mm2/s inright. The FA value of optic radiation was 0.509±0.029 in left and 0.502±0.026 in opposite side, and the MD value was (0.763±0.050)×10-3mm2/s in left and(0.748±0.052)×10-3mm2/s in opposite side respectively. There were not anysignificant differences with the FA value or the MD value of bilateral normal opticfibers(P＞0.05).Diffusion tensor tractography (DTT) could visualize the trajectory and threedimensional conformation of the optic nerve, optic tract and optic radiation in vivo. Inthe study, DTT could demonstrate that optic radiation fibers were located in thelateral sagittal stratum and passed from the lateral geniculate body of the thalamus tothe primary visual cortex in the occipital lobe. The dorsal and laterial bundles passedposteriorly to the superior bank of the calcarine cortex, while the ventral bundlepassed anteriorly before making a sharp turn, known as the Meyer loop, and coursingposteriorly, terminating in the inferior bank of the calcarine cortex in accord withclassic anatomical studies.3. Results of partⅢ①Acute optic neuritis: The affected optic nerves were hypo-intensity in the FAmaps and DEC maps, the FA value declined markedly and MD value,λ∥value andλ⊥value increased obviously. There had significant difference comparing withunaffected and control optic nerves(P＜0.001).â‘¡Optic nerve atrophy: Atrophic optic nerves were hypo-intensity in the FAmaps and DEC maps, the FA value declined markedly and the MD value,λ⊥valueincreased obviously, but theλ∥value decreased obviously. There had significantdifference comparing with control optic nerves(P＜0.001).â‘¢Orbital tumor: Three of intra-orbital and extra-nerve benign tumors werestudied in which one was meningioma and two were cavernous hemangioma. Thethree optic nerves of tumor side were all showed pressure and displacement in the FAmaps and DEC maps. In the two patients with normal visual function, the signal intensity of the injured side optic nerve was normal in the FA maps and DEC maps.The FA value and MD value were normal also with comparing to the uninjured sideoptic nerves. The signal intensity of injured optic nerve was lower obviously in onepatient with degraded visual function in the FA maps and DEC maps, the FA valuedeclined and MD value increased markedly.â‘£The occipital lobe and/or temporal lobe infarct: The FA value of opticradiation in the area of infarct decreased obviously and gradually, and the MD valuedecreased markedly in acute period, then increased gradually following time-lapse,and with according to cerebrospinal fluid in chronic period. The optic radiation ofinjured side un-infarct area could occur retrograde Wallerian degeneration, its FAvalue declined and MD value increased gradually to follow the course of disease.There had significant difference comparing with contra-lateral optic radiation (P＜0.001).⑤Congenital septo-optic dysplasia(SOD): The FA value decreased and MDvalue increased in the anterior segment of bilateral optic radiation, the posteriorsegment arranged confusely and disorderly in the intra-occipital lobe, and the normalarc form of optic radiation disappeared.â‘¥Acquired blind human: The course and signal intensity of bilateral opticradiation were normal. There did not any significant difference in the FA value andthe MD value comparing with control groups respectively (P＞0.05).⑦Tumors with involved the optic radiation: The abnormality of the opticradiation was classified into four types: disruption+infiltration occurred in 9 cases ofbrain parenchyma high-grade malignant tumors, disruption+displacement occurredin one case of ependymoma of right trigone of lateral cerebral ventricle and 6 cases ofbrain parenchyma metastatic tumors, infiltration+displacement occurred in 2 case ofbrain parenchyma astrocytoma with gradeâ…¡, and displacement occurred in 6 cases of meningiomas and 10 cases of metastatic tumors. DTI and DTT could demonstratethe pathological changes of optic radiation fibers and showed the relationship withtumors.Conclusion1. The optimal parameters of visual pathway white matter fibers with MR diffusiontensor imaging(MR-DTI) were established preliminarily in the study. The DTI mapswere distinct and could satisfy the measurement and quantitate analysis.2. MR diffusion tensor imaging was very sensitive for visual pathway white matterfibers. The parameters of diffusion tensor, such as FA value, MD value,λ∥value andλ⊥value, were feasible to analysis quantitately the white matter fibers of visual pathway.MR diffusion tensor tractography was useful for showing the main visual fibers, andopen a new field for researching white matter fibers in vivo.3.â‘ Magnetic resonance diffusion tensor imaging could show the abnormality ofwhite matter fibers of visual pathway. The FA value, the MD value,λ∥value andλ⊥value could used to analyze quantitatively the pathological changes and diffusionfeatures of the visual pathway white matter fibers.â‘¡Diffusion tensor tractographycould allow for visualization white matter fibers of visual pathway threedimensionally and was found to be beneficial in the treatment planning and visualfunction prediction. |
PDF Full Text Request