T₂ Relaxometry of Normal Pediatric Brain Development

INTRODUCTION

Quantitative MRI acquisition and analysis techniques allow investigations beyond conventional qualitative interpretation employed in routine clinical practice. Such approaches are aimed at quantifying specific tissue characteristics, providing reproducible indices mirroring the underlying biological system. Relaxometry, for instance, combines acquisition and analysis techniques to generate MR relaxation time constants that directly reflect the local environment of protons. In particular, given its sensitivity to alterations in tissue microstructure, T₂ relaxation provides a quantitative monitoring tool in both health and disease conditions.

Past studies have examined brain development by assessing grey and white matter contrast variations observed in early childhood (1-4). These qualitative contrast assessments examined conventional T₁- and T₂-weighted images and are often acquired in the context of clinical examinations, from which only the subjects deemed to have no neurological pathology were retained (1). Some of these studies presented more quantitative grey/white matter contrast ratios which clearly captured the well-known qualitative contrast change associated with the stages of myelination (2,4).

The finding that grey and white matter contrast in infancy is reversed from that of adults is striking. This reversed contrast is observed during the first 4-6 months postnatal and the change to the adult pattern occurs at around the age of 9-12 months (5). The actual timing of the contrast reversal is dependent on the field strength, the imaging sequence, and the brain region studied (3). These qualitative contrast changes stem from the governing relaxation parameters, T₁ and T₂, as well as from proton density (PD), which all reflect microstructural changes associated with maturation of the underlying tissue. Ideally, a quantitative relaxometry approach to assessing maturation avoids imaging sequence dependence, and permits the detection of changes earlier and more consistently than with qualitative methods (6).

A consistent observation throughout quantitative relaxometry studies is prolonged relaxation times for neonates, followed by a steep decline in both T₁ and T₂ values, especially during the first year of life. Subsequently, a slower decrease extends into the third year, at which age the relaxation parameters approach adult values (5-9). Because the rate of T₂ shortening is much faster than that observed for T₁, it is assumed that T₂ is more sensitive to tissue changes and is therefore often preferred as an index of early brain development (10).

In many previous studies, cohorts originated from clinical investigations, and data collection was limited to infants showing no apparent MR abnormalities. Thus, the study subjects were not necessarily samples of a strictly normal, healthy population. Recognizing this need for data encompassing populations of normal, healthy young children, the NIH sponsored a multi-centre study, entitled the MRI Study of Normal Brain Development (a.k.a. the NIH Pediatric MRI Data Repository, when referring to the database) (11), for which recruitment is demographically diverse and representative of the US population in terms of gender, race and ethnicity, and family income. This NIH study consists of examining approximately 500 children over a seven-year period, through two distinct objectives, partitioned by age groups: 4.5 to 18 years (Objective 1), and birth to 4-years, 5-months (Objective 2).

The data acquisition includes several domains, each aimed at monitoring different aspects of development such as gross morphological changes via anatomical MRI, biochemical characteristics using MR spectroscopy, tissue microstructure through diffusion tensor imaging, and relaxometry (Objective 2 only), as well as behavioral and cognitive development with the aid of a large battery of age-appropriate neurological and neuropsychological tests. The ultimate goal of this project is to provide a publicly available normative pediatric MRI brain and behavioral database, which can subsequently be used in studies assessing normal brain development and brain deviations associated with neurological, neuropsychiatric and developmental disorders (11). The objectives of the present work were to estimate T₂ values in several brain regions in 344 brain scans of a representative group of healthy children aged birth through 4-years, 5-months of age and to subsequently model the evolution of the T₂ relaxation time constant with age.

MATERIALS AND METHODS

RESULTS

DISCUSSION

Based on the results for both ACR and Living phantom studies, the relaxation parameter estimates show good reproducibility over time (standard deviation<8%) and are within 12% (ROI₁) and 5% (ROI₂) of the values obtained by gold-standard methods (applied on the ACR phantom). The fact that the estimates for the shorter T₂ (74±2.7 ms) in ROI₂ are in better agreement with the gold-standard data (~70 ms) is probably due to the choice of echo times in the protocol. In addition to the expected loss in estimate accuracy from a 4-echo versus a 32-echo sequence, the shorter maximum echo time of the TSE sequence (TE_max(TSE) = 165 ms vs TE_max(CPMG) = 259 ms) might account for the increased discrepancy in estimating the longer T₂ values in ROI₁ (T_2(TSE) = 153.4±5.3 ms, T_2(CPMG)~135 ms). There is also a consistent T₂ estimate bias between the two sites, by which values from SITE 2 are higher than those from SITE 1. This is probably a result of systematic differences between the GE and Siemens scanners. More specifically, the multiple slice selective refocusing pulses, characteristic of 2D-FT multi-slice imaging techniques, have transition zones in which the flip angles vary from nominally 180 to 0 degrees. With these non-ideal pulses, resulting from either B₁-field nonuniformity or slice-profile imperfections, stimulated echoes with T₁ rather than T₂ decay constants will contribute to the measured signal intensities at the different echo times in a manner which depends on the precise slice profiles. These effects will undoubtedly vary from manufacturer to manufacturer. Nonetheless, the expected range of relaxation time constants during infancy far exceeds the variability in the phantom studies over time, so that the overall observations should be quite robust. In addition, despite the difference in T₂ estimates, the progression with age is in general consistent in all ROIs and it is thus reasonable to combine the data from the two sites for this particular analysis. Although this provides support for system independence, it does not guarantee it and careful consideration is required when combining multi-site data.

There is inevitably a trade-off between accuracy and speed in terms of relaxometry acquisition sequences. For example, to achieve a similar resolution, a single slice 32-echo acquisition would require close to 6 minutes of scan time whereas a full-brain 4-echo acquisition requires at most 10 minutes. In the context of this study, precision and brain coverage were deemed more important than accuracy, such that results should be reproducible and robustly capture changes with time but need not necessarily capture the exact time constant value. Moreover, scanning a population of unsedated children in the birth to less than 5-year age range necessitates very short scan times. The time limitations also preclude the use of multi-component T₂ analysis, which would require at least 32 echo times and sufficient signal to noise ratio to reliably differentiate relaxation components.

The average T₂ relaxation parameters at birth in the current study for the minor (S1: T_2(mf) = 372±66 ms; S2: T_2(mf) = 476±64 ms) and major (S1: T_2(MF) = 352±44 ms; S2: T_2(MF) = 464±33 ms) forceps are comparable with the values reported by Ding et al. (8) at 1.5 T for white matter (~400 ms). Similarly for grey matter, the current results in the caudate nucleus (S1: T_2(cn) =197±9.5 ms; S2: T_2(cn) = 233±10 ms) and the thalamus (S1: T_2(thal) = 183±9.0 ms; S2: T2(thal) = 214±8.5 ms) are consistent with those reported by Ding et al. (8) (~200 ms). Because an unexpected decrease in the T₂ relaxation parameter has been observed with increasing field strength (22,23), our results are also qualitatively comparable to those obtained at a higher field strength. For example, T₂ values reported by Ferrie et al. (6) at 2.35 T for healthy preterm newborns are shorter, but in the same relative order as those for the current study (T_2(mf) = 266±35 ms; T_2(MF) = 213±28 ms; T_2(cn) = 172±10 ms; T_2(thal) = 120±6 ms). The relatively lower standard deviations reported in that study can be explained by the very restricted cohort investigated (7 subjects at a post-conceptional age of 37 weeks). In the present study, the gestational age was not provided and thus a greater variability between subjects is expected. A similar argument applies to the results shown by Thornton et al. (10) at 2.4 T (T_2(mf) = 228±32ms; T_{2(occipital WM)} = 217±33 ms; T_2(thal) = 136±13 ms), where the subject cohort was limited to an age range of 37-42 weeks, post-conception. A consistent result in these studies and the present study is that the T₂ value of white matter at birth significantly exceeds that of grey matter. While earlier relaxometry studies failed to detect this difference (5,9,24), it was argued by Ding et al. (8) that the results were affected by relatively short repetition times on the order of 2000-2500 ms as opposed to the 3000-4500 ms repetition times used in later studies. This refers to the potential left-over T₁ signal component, which is relatively long-lived in neonates (T_1(max)~2.5 s). However, we expect the effect of this residual magnetization to be minimal in our study. A more likely explanation is that the short echo times in these early reports (TE_max = 56 ms (5); 80 ms(9); 90 ms (24)), used to measure the relatively long T₂'s expected for this age range, might be inadequate. Using longer echo times (2-echo: TE_max = 121 ms; 4-echo: TE_max = 165 ms for the present protocol) effectively allows sufficient magnetization decay and thus more accurately captures infant T₂ relaxation times, which are on the order of 400 ms.

For the age range of the present study, a significant decrease in the T₂ relaxation parameter is characterized by a steep decline within the first year, followed by a less pronounced decline thereafter. Thus, as per a previous study (7), mono-exponential regressions with age were applied, providing high quality fits. In general, the mono-exponential fit was well suited to T₂ (mean ${{\mathrm{R}}_{\mathrm{a}}}^2$~88%) and fits were slightly better in white (${{\mathrm{R}}_{\mathrm{a}}}^2$=86-91%) than grey matter (${{\mathrm{R}}_{\mathrm{a}}}^2$= 86%). A bi-exponential model, as proposed by Ding et al. (8), was also tested, but failed to be justified for our dataset which had a more restricted age range (<5-years) than the older study (<40-years) (8). Thus, insufficient data from older subjects may impede the detection of a second exponential term, which is thought to reflect the subtle changes extending into adult age (25,26).

The expected result was obtained for relaxation parameters for subjects approaching 4-years, 5-months (the upper age limit in the cohort) in that they begin to approach the adult range and the T₂ value of grey matter exceeds that of white matter (as expressed through the T₂₍₀₎ parameter). The relaxation parameters are shown to exhibit a rapid decline until approximately 1 year of age, at which point the values reach the range expected for adults. This result is qualitatively comparable with the 10-month rapid decrease reported in the Ding et al. (8) study, but shorter than the 2-3 year period observed by Ono et al. (24), which were both carried out at 1.5 T. The discrepancy with the latter results is probably due to the relatively short echo times (TE = 40, 80 ms) used in the Ono et al. study (24) and the difference in the definition of the point at which values “approach” the adult range. Other reports have shown that the significant lengthening in relaxation times as compared to adults extends to of 3-4 years at 0.15 T (9) and 0.35 T (5). The difference in sequence, field strength and echo times could be the source of the discrepancies.

Our data also shows a first cross-over between relaxation parameters of white and grey matter (splenium of corpus callosum and caudate nucleus) at approximately 6 months, followed by one at approximately 13 months (between the major forceps and the caudate nucleus). These results approach the cross-over at ~7 months found by Ding et al. (8), between similar structures (frontal lobe white matter and caudate nucleus). The difference could be due to the fact that these estimates were derived from a mono-exponential model in our case and a bi-exponential model in the case of Ding et al. (8). Also, as is the case in all the comparisons made thus far, the choice of the regions of interest is an additional source of discrepancies among studies. Since there is no available standard model for pediatric brains, regions are selected manually and are therefore subject to inter-rater variability and partial volume effects.

Throughout the literature, the generalized rapid decrease in relaxation parameters for white and grey matter is thought to be primarily indicative of water content and distribution changes. Several studies have confirmed that the steep decline in water content during early childhood is paralleled by pronounced decreases in T₂ values within the first year postnatal with more subtle but continual decreases extending into adulthood (5,27,28). Concurrent with water content decline, white matter myelination occurring during early development affects the relaxation parameters through alterations of brain water distribution. The increase in concentration of myelin precursors such as myelin basic proteins, cholesterol and glycolipids (29), as well as the proliferation and differentiation of glial cells and the development of axons and dendrites (24,30), increase the binding potential for protons of free water molecules to the surrounding macromolecules and effectively reduce the observed relaxation times (31,32). From postmortem studies, and paralleled by qualitative and quantitative MRI studies, it has been shown that myelination progresses most rapidly until 2 years of age, followed by a slower and less dramatic change extending well into the second decade of life (28,33). This temporal progression is reflected in the results of this study, where T₂ relaxation times exhibit a rapid decline until about 1 year of age in all regions of interest, at which point the values are within approximately 10% of the estimate at the age of 4-years, 5-months. The average decay rate in each type of tissue as a whole is 0.19±0.01 months^-1 in the corpus callosum, 0.26±0.01 months^-1 in white matter and 0.32±0.02 months^-1 in grey matter. In terms of the general spatial progression, the rate of decline for T₂ values with age is faster in the major forceps as compared to the minor forceps (C_MF = 0.29±0.010 months^-1 > C_mf = 0.22±0.009 months^-1) and similarly more rapid in the splenium than in the genu of the corpus callosum (C_CCs = 0.22±0.009 months^-1 > C_CCg = 0.16±0.010 months^-1). These relative rates are consistent with the results by Ding et al. (8) and with the expected posterior-to-anterior pattern of myelination. This is evidenced through perinatal histochemical studies (33) and paralleled through qualitative analysis (2-4), and measures of quantitative relaxometry (7,8), magnetization transfer experiments (34), and estimates of full brain cholesterol (28). In fact, during early brain development, myelination is initiated caudally, in the spinal cord, and spreads rostrally through the brain. Another interesting observation is the apparent slower rate of decline in the corpus callosum as compared to more peripheral white matter structures (C_CC~0.19 months^-1 < C_peripheralWM~0.26 months^-1). This may reflect a more advanced degree of myelination in the deeper white matter structures (35), such as a relatively high concentration of early myelinating fibers. Thus, at birth the process is already nearing maturation and exhibits a slower evolution with time (34).

The average rate of decline in deep grey matter is considerably faster than that in peripheral and deep white matter structures (C_cn~0.31 months^-1, C_thal~0.33 months^-1 versus C_pWM~0.26 months^-1 and C_CC~0.19 months^-1). This contradicts the rate constants of the first exponential term in the model proposed by Ding et al. (8) for the same structures (C1_cn~0.40 months^-1, C1_thal~0.31 months^-1 versus C_pWM~0.48 months^-1). It appears that this difference is related to the choice of model and weighting. For example, for the unweighted data that did converge to the bi-exponential model (major/minor forceps and caudate nucleus), the decay constants from the first exponential term for white matter (C_pWM~0.53 months^-1) is greater than that for grey matter (C_cn~0.45 months^-1). However, the data in general did not converge to the bi-exponential model and no significant increase in the adjusted coefficient of determination was observed.

The main cause of the decay in grey matter is attributed to decreases in water content, concurrent with the rapid proliferation and formation of oligodendroglial cells, synapses and dendrites, which further reduces the free water in grey matter (36,37). Another possible factor contributing to T₂ reductions in deep grey matter structures that has been suggested is the accumulation of paramagnetic metals such as iron (38,39), as well as the presence of myelinated white matter projections in regions such as the thalamus.

In conclusion, as per the objective of the NIH MRI Study of Normal Brain Development, normal age-related changes in T₂ relaxometry were investigated and shown to provide a sensitive index for the assessment of normal brain maturation, through relaxation time constants that reflect the alterations in water content and distribution. Of special interest is the progression of myelination in white matter, which is the dominant developmental process occurring in synchrony with the observed relaxation parameter decline. During the neonatal period, the changes are especially dramatic, as demonstrated through the rapid shortening of this parameter and modeled through a rapid mono-exponential decay with age. The culmination of the results to date represents a subset of a normative database, a portion of which is currently available through www.NIH-PediatricMRI.org and the remainder of which will become publicly available in the future. This will provide a comparison standard for other studies investigating normal brain development, as well as studies of relaxation parameter deviations associated with disease. Neural-Behavioral data, collected concurrently with the MR data, will also allow for the investigation of potential relationship between brain T₂ and behavioral functions.