Visit DIX: German-Spanish-German dictionary | diccionario Alemán-Castellano-Alemán | Spanisch-Deutsch-Spanisch Wörterbuch

Medical Background

The registration of different subjects to compare the anatomical variability and statistically analyze this variability is an established process in different medical applications [30].

The basic approach to generating an atlas is to obtain images from a large number of subjects in a mathematical framework that produces a database that is probabilistic. Such an atlas allows the user to obtain relative information that takes into account the variance in structure and function in human populations. Once established such an atlas can interact with new data sets derived from individual subjects and patients [31].

When comparing a new subject with the atlas, abnormalities in this subject are easier to detect. Also the structures in the new subject can be labeled according to the atlas, to which it was registered. Building a temporal atlas allows observation of the mean development in a population.

A quantitative analysis of the brain development in newborns has been done [6]. In this study the total brain volume and total volumes of the cerebral gray matter, unmyelinated white matter, myelinated white matter, and cerebrospinal fluid in premature and mature newborns of postconceptional ages of 29 to 41 weeks have been quantified. The results show that the total brain tissue volume increases linearly at a rate of 22cm$^3$ per week. Total gray matter shows a linear increase in relative intracranial volume of approximately 1.4% or 15cm$^3$ in absolute volume per week. The increase in total gray matter is mainly due to a four-fold increase in cortical gray matter.

Unmyelinated white matter is the most prominent brain tissue class in the preterm infant below 36 weeks. Although minimal myelinated white matter is present in the preterm infant at 29 weeks, between 35 and 41 weeks an abrupt five-fold increase in absolute volume of myelinated white matter is documented. The extracerebral and intraventricular cerebrospinal fluid volume changes minimally during this observation period.

Subsequent neurological disability in infants born prematurely and in term infants who experience perinatal hypoxic-ischemic injury is common and serious [7]. In part the tendency for such injuries may relate to a particular vulnerability of actively developing cerebral gray matter and white matter in the last trimester of human gestation. The occurrence of these processes at this maturational time period may render the brain subjected to ischemia or related insults more vulnerable not only to injury but also to subsequent impairment of gray matter and white matter development. The description of the anatomical and temporal characteristics of these developmental processes in the living infant is of great importance in understanding such maturation-dependent vulnerabilities. Quantitative volumetric measurements of cerebral gray and white matter development, including particular myelination, in the living premature and term infant are necessary to describe these anatomical and temporal characteristics.

Another study [2] shows how the water diffusion in certain regions changes over the same period of time (30 to 40 weeks postconceptional age). The results show that the mean apparent diffusion coefficient at 28 weeks is high $(1.8 \mu$m$^2$/ms) and decreases towards term $(1.2 \mu$m$^2$/ms). Relative anisotropy is higher the closer birth is to term with greater absolute values in the internal capsule than in the central white matter. Preterm infants at term show higher mean diffusion coefficient in the central white matter $(1.4 \pm 0.24$ versus $1.15 \pm 0.09 \mu$m$^2$/ms) and lower relative anisotropy in both areas compared to fullterm infants (white matter: $10.9 \pm 0.6$ versus $22.9 \pm 3.0$ %; internal capsule $24.0 \pm 4.44\%$ versus $33.1 \pm 0.6\%$). Nonmyelinated fibers in the corpus callosum are visible by diffusion tensor MRI as early as 28 weeks. Fullterm and preterm infants at term show marked differences in white matter fiber organization. The data indicate that quantitative assessment of water diffusion by diffusion tensor Magnetic Resonance Imaging (MRI) provides insight into microstructural development in cerebral white matter in living infants.

The study shows that there are architectural differences between the preterm infants studied at term and the infants born at term. Fiber development and orientation, particularly in the white matter but also in the internal capsule are more evident in the infants born at term. The central white matter in preterm infants at term exhibits less directionality of the diffusion, thinner fiber bundles, and less organized fibers compared to the infants born at term.

A third study [5] shows that it is possible to identify areas of abnormalities with Diffusion Weighted Imaging (DWI) at a time where neither cranial ultrasonography nor conventional MRI detected any definite abnormality. Here the early detection of the diffuse component of the Periventricular Leukomalacia (PVL) with Diffusion Weighted Image is shown.

The findings of the studies presented demonstrate the fields of interest when observing the first possible observation periods of brain development.

Given a diffusion tensor image of a premature born baby (and therefore the stage of myelination of the brain white matter), possible questions when comparing a new subject to an atlas can be:

• What postconceptional age does this subject have?
• Is the development in every structure comparable to the mean development?
• Are there any lesions in the developed structures?
• Are all the expected structures present?