Multicomponent T2* mapping of knee cartilage: technical feasibility ex vivo

Abstract

Disorganization of collagen fibers is a sign of early-stage cartilage degeneration in osteoarthritic knees. Water molecules trapped within well-organized collagen fibrils would be sensitive to collagen alterations. Multicomponent effective transverse relaxation (T2*) mapping with ultrashort echo time acquisitions is here proposed to probe short T(2) relaxations in those trapped water molecules. Six human tibial plateau explants were scanned on a 3T MRI scanner using a home-developed ultrashort echo time sequence with echo times optimized via Monte Carlo simulations. Time constants and component intensities of T2* decays were calculated at individual pixels, using the nonnegative least squares algorithm. Four T2*-decay types were found: 99% of cartilage pixels having mono-, bi-, or nonexponential decay, and 1% showing triexponential decay. Short T2* was mainly in 1-6 ms, while long T2* was ∼ 22 ms. A map of decay types presented spatial distribution of these T2* decays. These results showed the technical feasibility of multicomponent T2* mapping on human knee cartilage explants.

Fig. 1

Monte Carlo simulations for bi-component T2* fitting: long T₂*=(30ms, 75%), short T₂* =(1/2/3ms, 25%); SNR (at TE=0ms) =286 (noise_1), 143 (noise_2) or 95 (noise_3); noise trials=500; Type-1 (9-TE) (ms) = (0.5, 1.5, 3.0, 4.5, 7, 10, 20, 30, 40), Type-2 (11-TE) (ms) = (0.5, 1, 2, 3, 4, 5, 7, 10, 20, 30, 40). a-c) For short T₂* at 1, 2, and 3ms, Type-2 (11-TE) produced smaller SD values than Type-1 (9-TE) when T₂* became shorter from 3 to 1ms. d-e) For a long T₂* at 30ms, the SD values are less than 7% at all the three SNR levels for both Type-1 and Type-2. Note: in c) the mean and SD at noise_3 were excluded due to large SD value.

Fig. 2

Four typical decay curves found in our cartilage images with UTE acquisitions: mono-, bi-, tri-, and non-exponential decays. These decay curves were differentiated primarily in the first 10ms and became mono-exp decay thereafter.

Fig. 3

Multi-component T₂* mapping of a human tibial cartilage (asymptomatic, 18 yrs old, male): a) Cartilage image at TE=0.5ms (fat saturated, awsos sequence, resolution=0.39mm), b) Conventional T₂* map of the cartilage (mono-exp fitting, 11-TE), c) Multi-component T₂* decay map of the cartilage (multi-component exp fitting, 11-TE), and d) Distribution of pixel number over the four recognized types of T₂* decays. The multi-component T₂* decay map in c) unveiled more detailed spatial variations of T₂* relaxation in the cartilage than the single-component T₂* map in b). Almost all the cartilage pixels (99%) had decays of mono-, bi-, or non-exp type.

Fig. 4

A 2D plot of global averages of T₂* relaxation time and intensity of the investigated cartilages with three asymptomatic/healthy (green, shadow at HTP-5) and three diseased (red): a) Short-T₂* component from multi-component exponential fitting, b) Long-T₂* component, and c) Single-component from conventional mono-exponential fitting. The short-T₂* component differentiated the diseased cartilages from the asymptomatic ones, while the long-T₂* component or single-component did not. The percent intensities are not added up to 100 because they are averages instead of values at individual pixels.