Changes in T2 relaxation times associated with maturation of the human intervertebral disk

Abstract

Background and purpose: By calculating T2 relaxation times for intervertebral disks, we tested the hypothesis that disk water concentration increases between the first and second decades of life.

Materials and methods: In subjects younger than 10 years old (group 1) and subjects between 19 and 20 years old (group 2), a sagittal MR image of the lumbar spine was obtained with a modified 3D fast spin-echo (FSE) multi-echo sequence. T2 relaxation times for each voxel were calculated by fitting a logarithmic regression to the signal intensity in images at 16 different echo times. T2 times were averaged for each spinal disk in each group and differences tested for statistical significance by analysis of variance (ANOVA). T2 times along the vertical axis of the disk at the midline were plotted and inspected for evidence of a central lower signal intensity region (CLSIR) in the 2 groups. We tested the differences between groups for significance with the Student t test.

Results: Maps of T2 relaxation times showed different patterns in groups 1 and 2. The mean T2 relaxation times in each disk level in group 1 ranged from 74-95 ms and in group 2, from 91-119 ms. Differences between the 2 groups were significant (P<.001, ANOVA, P=.0002, Student t test of means); differences between levels were not. In group 2, development of a CLSIR was significantly more common than in group 1 (P=.0001, Student t test).

Conclusions: T2 increases in the intervertebral disk between the first and second decades of life.

Fig 1.

Four selected images at TE of 19, 38, 56, and 75 ms from the modified FSE sequence illustrate the loss of signal intensity from the spinal tissues and L4-L5 disk with increasing TE. Disk tissue loses signal intensity less rapidly than vertebrae. The T2 relaxation time, the length of time required for the signal intensity to diminish by 50%, is calculated for each voxel from 16 such images.

Fig 2.

Sagittal MR image showing the placement of a region of interest (dotted line) and the line superior to inferior in the intervertebral disk along which T2 values were sampled and plotted as a function of distance to calculate the effect of the central region of low signal intensity on the T2 relaxation times.

Fig 3.

Sagittal T2-weighted MR images (A), color map of the T2 relaxation times in a sagittal section (B) and surface contour plot of T2 relaxation times in a sagittal section (C) for an L2-L3 disk in a 6-year-old subject (group 1). The plot shows the anterior edge of the disk to the reader's left and the posterior edge to the reader's right. The upper margin of the disk is toward the top of the page and the lower margin, toward the bottom. The plot shows highest values of T2 near the central portion of the disk. In the central region, the T2 values are relatively uniform.

Fig 4.

Sagittal T2-weighted MR images (A), color map of the T2 relaxation times in a sagittal section (B), and surface contour plot of T2 relaxation times in a sagittal section (C) for an L2-L3 disk in a 19-year-old subject, displayed in the same manner as Fig 3. This plot shows a band of diminished T2 values in the central portion of the disk, where fibrous tissue content increases during maturation of the disk. This band is found commonly in the 19- and 20-year-olds in this study and rarely in the younger subjects.

Fig 5.

T2 relaxation times on a line superior to inferior through the center of an L4-L5 disk in a 6-year-old subject and of an L2-L3 disk in a 19-year-old. This plot shows the CLSIR as a dip in the T2 relaxation times along the superior-inferior axis of the disk in the older subject. It also shows the relatively greater relaxation times of T2 in the older group. T2 relaxation times are shown on the vertical axis, and the distance from the superior endplate is shown along the horizontal axis.