Optical coherence tomography grading correlates with MRI T2 mapping and extracellular matrix content

Abstract

Optical coherence tomography (OCT) and T2 mapping are emerging clinical imaging technologies with potential to detect subsurface changes in cartilage retaining a macroscopically intact articular surface. This study tests the hypothesis that OCT correlates with magnetic resonance imaging (MRI) T2 values, and that OCT signal is sensitive to cartilage matrix degeneration. Forty-five osteochondral cores were harvested from five human tibial plateau explants after MRI T2 mapping. Cores underwent OCT imaging and were graded as follows: A, obvious birefringence; B, no birefringence; C, subsurface voids and/or irregular surface. Extracellular matrix content was determined and cores underwent histologic and polarized light microscopy (PLM) evaluation. Grade B and C cores had 25% higher superficial T2 values (p = 0.047) and 50% higher deep T2 values (p = 0.012) than grade A cores. Grade B and C cores had 36% higher glycosaminoglycan (GAG) content compared to grade A cores (p = 0.009). Histology and PLM demonstrated increased surface irregularity and structural disorganization with increasing OCT grade. OCT grade and T2 value increased with increasing collagen disorganization, suggesting that MRI T2 mapping and OCT are sensitive to changes in collagen structure. Our results demonstrate the ability of OCT and T2 mapping to detect early cartilage degeneration in clinically normal appearing cartilage.

Figure 1

(A) Tibial plateau with selected core region of interest highlighted in red. (B) The plateau has been mounted on a registration plate with MRI fiducial markers located at the sites designated by the open black circles. (C) MRI T2 map. image of a coronal slice corresponding to the red line shown in (B). The selected region of interest is highlighted in red. The MRI fiducial markers are seen below the tibial plateau.

Figure 2

(A) Superficial T2 relaxation time increased with increasing OCT grade (p = 0.12). Cores with OCT grades B, C had a 25% higher mean superficial T2 value compared to grade A cores (p = 0.047). (B) Deep T2 values increased with increasing OCT grade (p = 0.01). Within the groups, deep T2 values between grade A and C cores were significantly different (*p <0.05). Cores without birefringence (grades B and C) had an overall 50% higher mean deep T2 value compared to birefringent cores (p = 0.012). Error bars represent SEM.

Figure 3

GAG content increased with increasing OCT grade (p = 0.004). Grade A osteochondral cores had significantly lower average GAG values than grade C cores (*p = 0.003), while differences between grade A and B GAG values approached significance (p = 0.08). Overall, cores without birefringence (grades B, C) had a 36% higher GAG content compared to cores with birefringence (p = 0.009). Error bars represent SEM

Figure 4

Comparison of OCT grade to polarized light microscopy (PLM). Representative OCT images of osteochondral cores with grades A through C (a–c). Corresponding PLM images of sections stained with picrosirius red demonstrating increased collagen fiber disorganization with increasing OCT grade (d–f). Corresponding T2 MRI maps demonstrating a correlation with OCT and increasing collagen disorganization (g–i).

Figure 5

(A) Superficial T2 values were 31% higher in cores with evidence of collagen matrix disorganization as assessed by PLM, compared to cores with normal appearing collagen matrix (*p = 0.04). (B) Deep T2 values were 69% higher in cores with collagen disorganization compared to normal appearing cores (** p <0.001). Error bars represent standard error of the mean.