Biexponential T1ρ relaxation mapping of human knee menisci

Abstract

Background: Measuring T_1ρ in the knee menisci can potentially be used as noninvasive biomarkers in detecting early-stage osteoarthritis (OA).

Purpose: To demonstrate the feasibility of biexponential T_1ρ relaxation mapping of human knee menisci.

Study type: Prospective.

Population: Eight healthy volunteers with no known inflammation, trauma, or pain in the knee and three symptomatic subjects with early knee OA.

Field strength/sequence: Customized Turbo-FLASH sequence to acquire 3D-T_1ρ -weighted images on a 3 T MRI scanner.

Assessment: T_1ρ relaxation values were assessed in 11 meniscal regions of interest (ROIs) using monoexponential and biexponential models.

Statistical tests: Nonparametric rank-sum tests, Kruskal-Wallis test, and coefficient of variation.

Results: The mean monoexponential T_1ρ relaxation in the lateral menisci were 28.05 ± 4.2 msec and 37.06 ± 10.64 msec for healthy subjects and early knee OA patients, respectively, while the short and long components were 8.07 ± 0.5 msec and 72.35 ± 3.2 msec for healthy subjects and 2.63 ± 2.99 msec and 55.27 ± 24.76 msec for early knee OA patients, respectively. The mean monoexponential T_1ρ relaxation in the medial menisci were 34.30 ± 3.8 msec and 37.26 ± 11.38 msec for healthy and OA patients, respectively, while the short and long components were 7.76 ± 0.7 msec and 72.19 ± 4.2 msec for healthy subjects and 3.06 ± 3.24 msec and 55.27 ± 24.59 msec for OA patients, respectively. Statistically significant (P ≤ 0.05) differences were observed in the monoexponential relaxation between some of the ROIs. The T_1ρ,short was significantly lower (P = 0.02) in the patients than controls. The rmsCV% ranges were 1.51-16.6%, 3.59-14.3%, and 4.91-15.6% for T_1ρ -mono, T_1ρ -short, and T_1ρ -long, respectively.

Data conclusion: Our results showed that in all ROIs, T_1ρ relaxation times of outer zones (red zones) were less than inner zones (white zones). Monoexponential T_1ρ was increased in medial, lateral, and body menisci of early OA while the biexponential numbers were decreased in early OA patients.

Level of evidence: 2 Technical Efficacy Stage: 1 J. Magn. Reson. Imaging 2019. J. Magn. Reson. Imaging 2019;50:824-835.

Keywords: bicomponent T1ρ relaxation; biexponential and monoexponential T1ρ; knee menisci.

FIGURE 1:

Representative sagittal T_1ρ-weighted (TSL = 2 msec) illustrating the ROIs of meniscus subregions in lateral, medial, and body menisci.

FIGURE 2:

Ex vivo study on the bovine menisci. (a) Mono, (b) short, and (c) long component spatial T_1ρ maps. (d) The estimated parameters from scans acquired with an AF = 3 show less than a 6% difference from the fully sampled scans. (e) The estimated relaxations using 10 TSLs showed less than 6% difference from the estimated relaxations using 15 TSLs points.

FIGURE 3:

Monte Carlo simulations. Effect of different parameters on biexponential model. (a) The estimation mean-absolute percentage error (MAPE) decreases by increasing the number of TSLs. (b) Higher SNR leads to less estimation error. (c) The estimation is more accurate for shorter short component (T_1ρs). (d) The estimation is more accurate for larger long component (T_1ρl). (e,f) The component with larger fraction (short component, A_s, in (e) and long component, A_l in (f)) has better estimation than the component with a shorter fraction.

FIGURE 4:

A representative example of T_1ρ relaxation maps of knee menisci in healthy subject in (a1–a6) lateral, (b1–b6) medial, (c1–c6) body menisci. (a1–c1) The binary maps show the pixels that were included in the biexponential map (yellow) by satisfying the Eq. 3 condition.

FIGURE 5:

A representative example of T_1ρ relaxation maps of knee menisci in early OA patient in (a1–a6) lateral, (b1–b6) medial, (c1–c6) body menisci. (a1–c1) The binary maps show the pixels that were included in the biexponential map (yellow) by satisfying the Eq. 3 condition

FIGURE 6:

The boxplot shows the T_1ρ relaxation times component between nine different ROIs. The line in the middle of each box is the median and the bars extending from the boxes are the minimum and maximum values. The dashed line with numbers show the pairwise rank sum test between two ROIs (PLWZ: Posterior-Lateral White Zone; PLRZ: Posterior-Lateral Red Zone; ALWZ: Anterior-Lateral White Zone; ALRZ: Anterior-Lateral Red Zone; TLM: Total Lateral Menisci; PMWZ: Posterior-Medial White Zone; PMRZ: Posterior-Medial Red Zone; AMWZ: Anterior-Medial White Zone; AMRZ: Anterior-Medial Red Zone; TMM: Total Medial Menisci; MB: Meniscus Body).

FIGURE 7:

The boxplot shows the T_1ρ relaxation times component (mono, short and long) between different genders.

FIGURE 8:

Monoexponential vs. biexponential fitting models. T_1ρ decay on a logarithmic scale and the fitted residuals of representative pixels in lateral (a), medial (b), and body (c) of knee meniscus. The deviation of the data from a straight line shows the existence of more than one exponential term.