Prolonged T*2 values in newborn versus adult brain: Implications for fMRI studies of newborns.

Functional magnetic resonance imaging (fMRI) studies in adults are generally performed with echo planar imaging (EPI) sequences which utilize echo times (TE's) on the order of brain T2* values, a condition which maximizes signal differences due to small changes T2* accompanying activation. In newborns, we have measured T2* values which are longer by approximately a factor of two compared with adult brain T2* values. Our results suggest that improved reliability for fMRl studies in newborns may be achieved using longer echo times which reflect the longer T2* values. Introduction: Several groups have now reported results of functional magnetic resonance imaging (fMRI) studies of the visual and auditory responses in human infants (1-4). Such studies hold great interest because of the insight they may provide into functional cerebral development during infancy. Notably, the studies performed to date in infants have provided inconsistent fMRI data for the functional response of visual cortices to stimulation. For some infants, the positive signal change associated with "activation" in adults is reversed, while in others, results consistent with the adult pattern of positive signal activation have been obtained. It has been suggested that the reversal of the adult activation pattern in some infants is due to the inability of the infant's cerebral vasculature to meet the increased oxygen demand, resulting in a reduction of the oxy- to deoxyhemoglobin ratio (3). In balance, however, results of fMRI studies in infants require greater clarification due to the obfuscating effects of sedation, scanner noise, heterogeneous subject selection, and pulse sequence specifics on the observed signal response. In particular, infant fMRI studies to date have largely employed the same pulse sequence techniques and parameters used for adult studies. We have carried out brain T2* measurements at 1.5 T in a group of newborns and compared them with values measured in adults. Profoundly longer T2* values are found for the newborns, suggesting that optimal echo times (TE's) for fMRI studies of newborns should be increased to maximize signal intensity changes and improve the overall reliability of fMRI results in newborns. Methods: Ten newborns at approximately 42 weeks post menstrual age (PMA) were studied as part of a larger study on preterm and full term infant brain development. Magnetic resonance imaging was performed with a 1.5 T General Electric scanner operating at the LX 8.3 hardwarehoftware configuration. The T2* measurements were made using a spoiled gradient echo (SPGR) sequence to generate five 7 mm thick axial slices through the mid brain. The TR was fixed at 100 ms and image data sets were generated at TE values of 7, 24, 48, 64, and 91 ms with the total scan time for the T2* measurements requiring approximately 6 minutes. Infants were unsedated and sleeping. They were physiologically monitored and observed by a skilled attendant during each scan. Regions of interest (ROI's) in the frontal white matter (FWM), thalamus (TH), medial occipital lobe (MOL) and the lateral occipital lobe (LOL) were identified from which signal intensities as a function of TE were extracted and fit with monoexponential functions to calculate T2*. Four male adult volunteers with an age range of 24 to 47 years were studied in a comparable manner. Results: Table 1 shows the means and standard deviations (SD's) of the T2* values obtained for each ROI for the 42 week PMA newborns and the adults.