Magnetic Resonance Relaxation Anisotropy: Physical Principles and Uses in Microstructure Imaging

Abstract

Magnetic resonance imaging (MRI) provides an excellent means of studying tissue microstructure noninvasively since the microscopic tissue environment is imprinted on the MRI signal even at macroscopic voxel level. Mesoscopic variations in magnetic field, created by microstructure, influence the transverse relaxation time (T) in an orientation-dependent fashion (T is anisotropic). However, predicting the effects of microstructure upon MRI observables is challenging and requires theoretical insight. We provide a formalism for calculating the effects upon T of tissue microstructure, using a model of cylindrical magnetic field perturbers. In a cohort of clinically healthy adults, we show that the angular information in spin-echo T is consistent with this model. We show that T in brain white matter of nondemented volunteers follows a U-shaped trajectory with age, passing its minimum at an age of ∼30 but that this depends on the particular white matter tract. The anisotropy of T also interacts with age and declines with increasing age. Late-myelinating white matter is more susceptible to age-related change than early-myelinating white matter, consistent with the retrogenesis hypothesis. T mapping may therefore be incorporated into microstructural imaging.

Cerebral White Matter Maturation Patterns in Preterm Infants: An MRI T2 Relaxation Anisotropy and Diffusion Tensor Imaging Study

“…Fitted T2 surfaces as a function of fiber-to-field angle and FA across all WM are shown ( Fig 5). As in previously published work on the adult brain, 31,33 T2 is longest when parallel to B0. 31,33 This effect is smaller at lower FA, such that T2 anisotropy occurs only where there is considerable order.…”

“…As in previously published work on the adult brain, 31,33 T2 is longest when parallel to B0. 31,33 This effect is smaller at lower FA, such that T2 anisotropy occurs only where there is considerable order. At higher FA values, the peak-to-trough T2 is up to ß20 milliseconds in the preterm group but only 10 milliseconds in the LP group.…”

“…At higher FA values, the peak-to-trough T2 is up to ß20 milliseconds in the preterm group but only 10 milliseconds in the LP group. Although peak-to-trough distances are similar to the adult brain, the amplitudes of anisotropy are about 10fold lower 31 (and FA two to fourfold). Surprisingly, the amplitude of anisotropy is largest in the VP group for any given FA.…”

“…We found that T2 is anisotropic in the infant brain at termequivalent age, with amplitude in WM that is approximately 1/10th of that reported in adults. 31 The lower anisotropy implies a more homogeneous magnetic field on a mesoscopic scale, from which we may infer a lower susceptibility difference between axon walls and their surroundings. This implies less, or less ordered, myelin within the developing myelin sheaths.…”

“…Anisotropy of T2 was analyzed in the DTI native space, supersampled to 1 mm isotropic resolution. 31 Two subgroups were defined: a VP and a LP group. Data in each subgroup were pooled.…”