Does Time of Day, Unloaded Recovery, or Longitudinal Time Between Disc MRI Scans Affect T2 Time or Disc Height?

Introduction: The intervertebral disc supports load transfer and allows for motion in the spine. Disc degeneration, often implicated in low back pain, includes structural breakdown and reduction of disc function. Magnetic resonance imaging (MRI) is used to quantify disc degeneration using measures of disc T2 relaxation time (correlated with water content) and grading schemes that are based on disc geometry and signal intensity [1].

While MRI is a critical tool to assess disc degeneration, it is unknown whether the time of day at which the MR scan is acquired affects the outcome measures. For example, it is known that spine loads cause fluid and height loss throughout the day [1,2], but the time course for these changes is unknown. Moreover, it is unknown whether a period of unloaded rest can mediate these effects. Therefore, the objective of this study was to evaluate the T2 time and disc height from MRI scans at four timepoints across the day, as well as to compare T2 time before and after a 45-minute unloaded rest. Understanding how time of day and unloaded rest affect disc outcome measures will provide guidance for study design and interpretation of MRI studies.

With age, the disc progressively degenerates, including loss of water content [1,3]. However, little is known about the timeframe spanning over years for the associated T2 changes. Therefore, this study also investigated longitudinal changes in T2 relaxation time in a young population up to 6 years.

Materials and

Methods: Subjects with no history of low back pain (25-36 years old, n=5, 3F/2M) underwent repeated MRI 1) at multiple timepoints throughout a day, 2) before and after a 45-minute unloaded rest, and 3) longitudinally across 0.5- 6 years. The five lumbar discs from L1-S1 were evaluated.

Subjects were scanned first thing in the morning, with no unloaded rest, to obtain T2 relaxation time. Next, a T1-weighted scan was acquired to obtain disc geometry. Subjects then remained supine for a 45-minute rest and the T2 scan was repeated. This protocol was repeated at 4 timepoints throughout the day, with 2 hours of regular activity in between scanning sessions. All subjects were also scanned in the morning after 45-minute unloaded rest in a previous study 0.5 to 6 years prior [1].

T2 relaxation time was analyzed by the intensity decay of a circular ROI in the nucleus, fit to a noise corrected exponential decay [4]. Disc height was calculated as the disc mid sagittal area divided by the anterior-posterior width. To compare T2 relaxation time and disc height across timepoints throughout the day, a linear mixed model with Tukey’s t-test for multiple comparisons was used. To compare T2 relaxation time pre/post rest and longitudinally paired t-tests were done.

Results, Conclusions, and Discussions: There were some notable changes throughout the day, as T2 relaxation time decreased, between timepoints 2 and 3 (ΔT2 = -11 ± 17 ms, p ≤ 0.05) and between timepoints 2 and 4 (ΔT2 = -3 ± 13 ms, p≤0.05) (Figure 1A). The disc height was significantly decreased from timepoint 1 to 4 (Δheight = -0.24 ± 0.56 mm, p ≤ 0.05), and timepoint 2 to 4 (Δheight = -0.25 ± 0.43 mm, p ≤ 0.05) (Figure 1B). Before and after supine unloaded rest, there was no significant difference in the T2 relaxation time (Pre T2 = 121 ± 31 ms, Post T2 = 120 ± 29 ms, p>0.05) (Figure 1A).

Because there was no difference between the morning timepoints 1 and 2, the time of day for MRI scanning can be anytime within 4 hours of the start of day. After 4 hours, small alterations occur in both T2 time and disc height that cannot be eliminated with 45-minute unloaded rest. Supine unloaded rest for 45-minutes was insufficient to recover disc water content and alter T2 time measures, which is consistent with previous results [5].

These diurnal changes were evaluated in young subjects; however, it is also known that T2 relaxation time decreases with age (longitudinally) and degeneration [3]. Longitudinally, the T2 relaxation time was not significantly different over 0.5 to 6 years (ΔT2 = 4 ± 27 ms, p>0.05) (Figure 2). The range of time examined in this study with young subjects was too short to see age related changes (0.5-6 years).

The study investigated the effects of time of day, unloaded rest, and longitudinal time on MRI based measures. Ultimately it was found that scanning in the morning with or without rest did not significantly impact T2 time and disc height. Applying this new information will improve future scanning consistency and enable better data interpretation.

[1] Meadows+ JOR Spine, 2023
[2] Martin+ Eur Spine J, 2022
[3] Menezes-Reis+ Quant Imaging Med Surg, 2016
[4] Meadows+ JOR Spine, 2021
[5] Stelzeneder+ TSJ, 2012

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