Original Research The Evolution of the Apparent Diffusion Coefficient in the Pediatric Brain at Low and High Diffusion Weightings Richard A. Jones, PhD, 1,2 * Susan Palasis, MD, 1,2 and J. Damien Grattan-Smith MD 1,2 Purpose: To evaluate the evolution of the apparent diffu- sion coefficient (ADC) with age for different degrees of dif- fusion weighting using a clinically feasible approach. Materials and Methods: Data was acquired using separate scans with b values in the range typically used for clinical studies (100 –900 seconds/mm 2 ) and higher b values (1800 –3000 seconds/mm 2 ). The ADC was calculated for each of the data sets by fitting the data to a monoexponen- tial function. Results: The results from 50 children aged three years and less showed some deviations from literature values derived using a full biexponential fit, with these differences reflect- ing the approximations inherent in this approach. The val- ues obtained with this technique appear to be reproducible but the resulting “institutional values” are comparable to those from other centers only if identical measurement criteria are used. Conclusion: A significant decline in both components of the ADC during the first few months of life was observed; in addition, the attenuated slow ADC values seen in adult white matter were only present at birth in early myelinating regions. The subsequent development of the slow ADC in white matter suggests that it is associated with myelination or processes associated with axonal development. Key Words: MRI; diffusion; pediatric; brain development; myelination J. Magn. Reson. Imaging 2003;18:665– 674. © 2003 Wiley-Liss, Inc. MAGNETIC RESONANCE (MR) DIFFUSION imaging has become a standard technique for the detection of acute stroke in adults, and an increasing number of applications for quantitative diffusion imaging are be- ing described in the literature (1–3). Recent research suggests that the use of very strong diffusion weight- ings may further enhance the utility of diffusion- weighted imaging (4,5). The apparent diffusion coeffi- cient (ADC) of cerebral tissue changes significantly during the initial year of life and exhibits significant regional variations in contrast to the homogeneous ADC of the normal adult brain (6 – 8). This complicates the interpretation of both diffusion-weighted and ADC images obtained from neonates and infants and may account for some of the difficulty in interpreting the changes in diffusion occurring subsequent to hypoxic- ischemic injury in the pediatric brain (9,10). Studies performed to characterize the dependence of the ADC on the diffusion weighting (b value) have shown that when the decay of the in vivo water signal is sampled over an extended range of b values, the result- ing decay curve is well characterized by a two-compo- nent model with the two components having fast and slow diffusion coefficients, respectively (4,5,11–13). The fast and slow apparent diffusion coefficients (ADC fast and ADC slow ) and the size of the components can be estimated using biexponential fitting; however, robust biexponential fitting requires both a large number of b values and high signal-to-noise ratio (SNR), making it unsuitable for routine clinical use. The studies per- formed to date have either fitted the data from regions of interest (4,11,12), have used low spatial resolution and limited spatial coverage to allow the collection of para- metric maps in a reasonable amount of time (13), or have used exams with a very long duration (5). An alternative approach to examining the form of the ADC decay curve is to use q-space imaging in which the diffusion related decay is subject to a Fourier analysis (14,15); however, this technique also requires a rela- tively large number of b values and is hence also time consuming. In addition, the q-space theory was devel- oped for very short gradient pulses and the validity of this approach to clinical systems where the duration of the gradient pulses has to be relatively long remains to be proven. Recently, a more clinically feasible method for studying the fast and slow diffusion coefficients has been suggested in which diffusion-weighted images are acquired using separate sets of low and high b values. ADC maps are then calculated by fitting each data set to 1 Department of Radiology, Emory University School of Medicine, At- lanta, Georgia. 2 Department of Radiology, Children’s Healthcare of Atlanta, Atlanta, Georgia. Presented in part at the 10th Annual Meeting of ISMRM, Honolulu, 2002. *Address reprint requests to: R.A.J., Department of Radiology, Chil- dren’s Healthcare of Atlanta, 1001 Johnson Ferry Road, Atlanta, GA 30342. E-mail: Richard.Jones@choa.org Received February 24, 2003; Accepted August 1, 2003. DOI 10.1002/jmri.10413 Published online in Wiley InterScience (www.interscience.wiley.com). JOURNAL OF MAGNETIC RESONANCE IMAGING 18:665– 674 (2003) © 2003 Wiley-Liss, Inc. 665