T1ρ at Adjacent Level Discs after Surgery
Authors: Borthakur A. 
Published: June 2012

Introduction

Disk degenerative disease (DDD) is a strong etiologic risk factor of chronic low back pain (LBP) and is a common disabling condition for millions of individuals. Total disk arthroplasty is rapidly growing in popularity as an alternative to fusion to treat disc-related back pain. One motivation for arthroplasty is to preserve motion in order to prevent DDD at adjacent disk levels, a serious side effect of fusion surgery1. However, standard diagnostic tools such as x-ray measurements of disk height and T2 magnetic resonance imaging (MRI) signal intensity measurements are insensitive to adjacent level DDD.2 A better approach is to utilize noninvasive imaging methods that are responsive to subtle chemical biomarkers. To this end, T1ρ MRI, which has been demonstrated to correlate with disk degeneration, proteoglycan content, and disk pressure in cadaver tissue,3–6 was used as a quantitative biomarker to assess discs adjacent to surgically fused discs pre- and postsurgery. We evaluated whether a change in T1ρ value was related to presurgical discography opening pressure7 or T2-MRI based Pfirrmann degenerative grades8 of discs adjacent to fused discs. The hypothesis tested is that adjacent discs with low opening pressure (indicating low osmotic pressure and proteoglycan content) are more prone to degeneration and manifest decreased T1ρ values. The objective of this study is to determine changes in T1ρ relaxation times in adjacent level discs in patients following surgery to relate these to presurgical opening pressure as measured by provocative discography.

Materials and Methods

All MRI scans were performed on a 3 Tesla Siemens Tim Trio clinical scanner (using the vendor-supplied spine array coil with approval from the IRB and with subjects' consent). T1ρ and T2-weighted MRI were performed on patients pre- and postsurgery ( n = 4, 12 levels {2 points per level - nucleus & total disc}, mean age 42 ± 3 years, range 39–45) at time points ∼2 years apart. Discography opening pressure (OP), the pressure where an injected contrast dye first overcomes the internal osmotic pressure, was obtained following the placement of 22 gauge needles into the center of the L2/L3 through L5/S1 disks, using the IntelliSystem with digital pressure display. Lohexol (Omnipaque 300), a low osmolar, nonionic, iodinated-contrast agent, was injected into each disk under continuous fluoroscopic imaging. Following coregistration and segmentation procedures, average T1ρ (in milli seconds) was recorded from the discs a level above or below the surgically treated disk using algorithms written in Matlab. Pfirrmann grading was performed on T2 MRI of the same discs by a single-reader with several years of experience in disk MRI analysis. Statistical descriptives and regressions were performed in SPSS 19.0 to evaluate any relationships between changes in T1ρ pre- and postsurgery 2 years apart (δT1ρ) in the discs adjacent to fused discs and presurgery opening pressure and Pfirrmann grade of the adjacent discs.

Results

There was a significant (p1ρ and presurgery opening pressure of discs (Fig. 1) that were above surgically fused discs. Adjacent discs were with low OP (T1ρ 2 years postsurgery). Dashed lines indicate 95% confidence intervals. Graph of δT1ρ and presurgery Pfirrmann grade (Fig. 2) shows poor correlation (r = 0.25, p = 0.16) demonstrating the inability of Pfirrmann grade to predict T1ρ, and hence adjacent level DDD.

Conclusion

Presurgical opening pressure measurements of adjacent discs were significantly related to changes in T1ρ pre- and postsurgery. The data suggests that a low OP may be used to establish a threshold below which these disks are likely to degenerate following surgical intervention at an adjacent disc. At the same time, in this limited number of subjects, Pfirrmann grading could not be related to changes in T1ρ. We are continuing recruitment of more patients with an aim to develop T1ρ MRI to detect adjacent level DDD to evaluate patient outcomes from back surgery. I confirm having declared any potential conflict of interest for all authors listed on this abstract Yes Disclosure of Interest None declared Bertagnoli R, et al. European Spine Journal 2002;11(2):S131-S136 Chou R, et al. Annals of Internal Medicine 2007;147(7):478–491 Johannessen W, et al. Spine 2006;31(11):1253–1257 Auerbach J, et al. European Spine Journal 2006;15 Suppl 3:S338–344 Blumenkrantz G, et al. Magnatic Resonance in Medicine2010;63(5):1193–1200 Borthakur A, et al. Spine 2011;36(25):2190–6 Tehranzadeh J. Radiologic Clinics of North America 1998;36(3):463–495 8.Pfirrmann CW, et al. Spine 2001;26(17):1873–1878