The effect of cartilage dehydration and rehydration on quantitative ultrashort echo time biomarkers

Abstract

Background: The effect of dehydration of ex vivo cartilage samples and rehydration with native synovial fluid or normal saline on quantitative ultrashort echo time (UTE) biomarkers are unknown. We aimed to investigate the effect of cartilage dehydration-rehydration on UTE biomarkers and to compare the rehydration capabilities of native synovial fluid and normal saline.

Methods: A total of 37 cartilage samples were harvested from patients (n=5) who underwent total knee replacement. Fresh cartilage samples were exposed to air to dehydrate for 2 hours after baseline magnetic resonance (MR) scanning, then randomly divided into two groups: one soaking in native synovial fluid (n=17) and the other in normal saline (n=20) to rehydrate for 4 hours. UTE-based biomarkers [T₁, adiabatic T_1r (AdiabT_1r), macromolecular fraction (MMF), magnetization transfer ratio (MTR), and T₂*] and sample weights were evaluated for fresh, dehydrated, and rehydrated cartilage samples. Differences and agreements between groups were assessed using the values of fresh cartilage samples as reference standard.

Results: Dehydrating in air for 2 hours resulted in significant weight loss (P=0.000). T₁, AdiabT_1r, and T₂* decreased significantly while MMF and MTR increased significantly (all P<0.02). Non-significant differences were observed in cartilage weights after rehydrating in both synovial fluid and normal saline, with P values being 0.204 and 0.769, respectively. There were no significant differences in T₁, AdiabT_1r, MMF, and MTR after rehydrating in synovial fluid (P>0.0167, with Bonferroni correction) while T₂* (P=0.001) still had significant differences compared with fresh samples. However, no significant differences were detected for any of the evaluated UTE biomarkers after rehydrating in normal saline (all P>0.05). No differences were detected in the agreement of UTE biomarker measurements between fresh samples and samples rehydrated with synovial fluid and normal saline.

Conclusions: Cartilage dehydration resulted in significant changes in UTE biomarkers. Rehydrating with synovial fluid or normal saline had non-significant effect on all the evaluated UTE biomarkers except T₂* values, which still had significant differences compared with fresh samples after rehydrating with synovial fluid. No significant difference was observed in the rehydration capabilities of native synovial fluid and normal saline.

Keywords: Cartilage; dehydration; fresh; rehydration; ultrashort echo time magnetic resonance imaging (UTE-MRI).

Figure 1

UTE pulse sequence diagram. (A) T₁ is measured by the VFA method using the 3D UTE-Cones sequence with a single TR. B₁ mapping is obtained by the UTE-AFI method with two different TRs. Accurate T₁ value can be obtained by the combination of 3D UTE-VFA and UTE-VFI methods. (B) A train of AFP pulses are used in the 3D UTE Cones AdiabT_1ρ sequence to obtain T_1ρ values. (C) A Fermi pulse is used in the 3D UTE-MT sequence followed by multi-spoke (Nsp) excitation. The usage of Nsp reduces the scan time effectively. (D) 3D UTE sequence is used for T₂* acquisition with a short rectangular hard pulse excitation followed by an initial short TE of 32 µs. TR, repetition time; AFP, adiabatic full passage; MT, magnetization transfer; TE, echo time; RF, radio frequency; FID, free induction decay; DAW, data acquisition window; TSL, spin-locking time; UTE, ultrashort echo time; VFA, variable flip angle; AFI, actual flip angle imaging; AdiabT_1ρ, adiabatic T_1ρ.

Figure 2

Representative fitting curves for UTE-based biomarkers for an articular cartilage sample, with the dots being the experimental data points. Data are presented as mean ± standard deviation. (A) The representative fitting curve for T₁, with uncertainties of 2%. (B) The representative fitting curve for T_1ρ, with uncertainties of 7%. (C) The representative fitting curve for MMF, with uncertainties of 6%. (D) The representative fitting curve for T₂*, with uncertainties of 2%. FA, flip angle; TSL, spin-locking time; MMF, macromolecular fraction; TE, echo time; UTE, ultrashort echo time.

Figure 3

The representative images of ROI and pixel maps for UTE biomarkers. (A) The three consecutive layers in the center of each cartilage sample (the nicked notch was shown as signal void) used for ROI analysis. (B) The representative T₁ map of fresh, dehydrated, and rehydrated cartilage samples. (C) The representative T_1ρ map of fresh, dehydrated, and rehydrated cartilage samples. (D) The representative MMF map of fresh, dehydrated, and rehydrated cartilage samples. MMF, macromolecular fraction; ROI, region of interest; UTE, ultrashort echo time.

Figure 4

The bar chart of cartilage weights and UTE biomarkers of fresh, dehydrated, and rehydrated cartilage samples in SF and NS groups and the corresponding P values. (A) The weights of cartilage decreased significantly after dehydration in air for 2 hours. No significant differences were detected between the fresh and rehydrated cartilage samples in both SF and NS groups after 4 hours of submersion. (B) T₁ values decreased significantly after dehydration across samples. There were no significant differences observed in T₁ values after rehydrating in SF or NS. (C) T_1ρ values decreased significantly after dehydration in both SF and NS groups. Non-significant differences were observed after rehydrating in SF or NS compared to the fresh samples. (D) MMF increased significantly after dehydration. MMF values after rehydrating with SF and NS had non-significant differences with the values of fresh samples. (E) Significant increases were observed in MTR after dehydration. MTR values had non-significant differences after rehydrating with SF and NS compared with the values of fresh samples. (F) T₂* values decreased significantly in both SF and NS groups after dehydration. Rehydrated with SF, T₂* values still exhibited significant post-rehydration differences. However, T₂* values after rehydrating with NS had non-significant differences with the measurements taken when samples were still fresh. Bonferroni correction was performed to calibrate type I errors, so P values less than 0.0167 were considered statistically significant. SF, synovial fluid; NS, normal saline; MMF, macromolecular fraction; MTR, magnetization transfer ratio; UTE, ultrashort echo time.

Figure 5

The Bland-Altman plots of UTE biomarkers for synovial fluid-rehydrated cartilage samples, comparing the rehydration states with fresh states. The differences between baseline and the rehydration state are depicted on the vertical axis, with their averages depicted on the horizontal axis. (A) 1/17 data points were outside the 95% LoA for T₁. (B) 1/17 data points were outside the 95% LoA for T_1ρ. (C) All data points were within the 95% LoA for MMF. (D) 1/17 data points were outside the 95% LoA for MTR. (E) All data points were within the 95% LoA for T₂*. MMF, macromolecular fraction; MTR, magnetization transfer ratio; UTE, ultrashort echo time; LoA, limit of agreement.

Figure 6

The Bland-Altman plots of UTE biomarkers for normal saline-rehydrated cartilage samples, comparing the rehydration states with fresh states. The differences between baseline and the rehydration state are depicted on the vertical axis, with their averages depicted on the horizontal axis. (a) All data points were within the 95% LoA for T₁. (B) 1/20 data points were outside the 95% LoA for T_1ρ. (C) All data points were within the 95% LoA for MMF. (D) 1/20 data points were outside the 95% LoA for MTR. (E) 1/20 points were outside the 95% LoA for T₂*. MMF, macromolecular fraction; MTR, magnetization transfer ratio; UTE, ultrashort echo time; LoA, limit of agreement.