Quantifying the effect of posterior spinal instrumentation on the MRI signal of adjacent intervertebral discs

Study design: Ex vivo porcine imaging study. Objectives: Quantitatively evaluate change in MRI signal at the discs caudal to spinal fusion instrumentation. Summary of background data: Individuals who receive posterior spinal instrumentation are at risk of developing accelerated disc degeneration at adjacent levels. Degeneration is associated with a loss of biochemical composition and mechanical integrity of the disc, which can be noninvasively assessed through quantitative T2* (qT2*) MRI techniques. However, qT2* is sensitive to magnetic susceptibility introduced by metal. Methods: Nine ex vivo porcine lumbar specimens were imaged with 3 T MRI. Fast spin-echo T2-weighted (T2w) images and gradient-echo qT2* maps were acquired, both without and with posterior spinal fusion instrumentation. Average T2* relaxation times of the nuclei pulposi (NP) were measured at the adjacent and sub-adjacent discs and measurements were compared using t tests before and after instrumentation. The size of the signal void and metal artifact were determined (modified ASTM F2119-07) within the vertebral body and spinal cord for both MRI sequences. The relationship between T2* signal loss and distance from the instrumentation was evaluated using Pearson’s correlation. Results: There was no significant difference between adjacent and sub-adjacent NP T2* relaxation time prior to instrumentation (p = 0.86). Following instrumentation, there was a significant decrease in the T2* relaxation time at the adjacent NP (average = 20%, p = 0.02), and no significant difference at the sub-adjacent NP (average = − 3%, p = 0.30). Furthermore, there was a significant negative correlation between signal loss and distance to disc (r = − 0.61, p ' 0.01). Conclusions: Spinal fusion instrumentation interferes with T2* relaxation time measurements at the adjacent disc but not at the sub-adjacent discs. However, there is sufficient signal at the adjacent disc to quantify changes in the T2* relaxation time following spinal fusion. Hence, baseline MRI scan following spinal fusion surgery are required to interpret and track changes in disc health at the caudal discs. Level of evidence: N/A.