An order parameter without magic angle effect (OPTIMA) derived from dispersion in ordered tissue

Abstract

MR R 2 imaging of ordered tissue exhibits the magic angle effect, potentially masking subtle pathological changes in cartilage. This work aimed to develop an orientation-independent order parameter (S) exclusively sensitive to collagen degeneration. Methods: A theory was developed based on R 1 dispersion coupled with a simplified molecular motion model in which anisotropic R a 2 ( ) became directly proportional to correlation time b ( ) and S could be derived. This new parameter was validated with ex vivo R 1 dispersion reported on orientated (n = 4), enzymatically depleted bovine cartilage (n = 6), and osteoarthritic human knee specimens (n = 14) at 9.4 Tesla, which was further demonstrated on 1 healthy human knee in vivo at 3 Tesla. Results: b ( ) from orientation-dependent R 1 dispersion revealed a significantly high average correlation (r = 0.89 ± 0.05, P < 0.05) with R a 2 (θ) on cartilage samples and a moderate correlation (r = 0.48, P < 0.001) for the human knee in vivo. The derived S (10 −3 ) significantly decreased in advanced osteoarthritis (1.64 ± 0.03 vs.2.30 ± 0.11, P < 0.001) and collagen-depleted samples (1.30 ± 0.11 vs. 2.12 ± 0.12, P < 0.001) when compared with early osteoarthritis and the control, respectively. Conclusion:The proposed order parameter could be a potentially useful orientation-independent MR biomarker for collagen alterations in cartilage and other highly structured tissues. K E Y W O R D Sanisotropic R 2 , collagen, correlation time, magic angle effect, order parameter, R 1 dispersion How to cite this article: Pang Y. An order parameter without magic angle effect (OPTIMA) derived from R 1 dispersion in ordered tissue.

An efficient R1ρ dispersion imaging method for human knee cartilage using constant magnetization prepared turbo‐FLASH

“…8,15,18 The existing preclinical and clinical evidence suggests that R 1ρ itself is not specifically sensitive to PG alterations, but rather R 1ρ dispersion is predominantly susceptible to collagen changes, which are characterized by the residual dipolar interaction of ordered water molecules buried inside of collagen triple-helical microstructures. 7,8,19,20 These observations are in accordance with two previous studies from the early 2000s 21,22 as well as with some recent investigations relevant to the underlying R 1ρ relaxation mechanisms in cartilage. 19,20,23 Recently, a theoretical framework of R 1ρ dispersion in cartilage has been outlined, 7 suggesting that an orientation-independent MR metric, named the order parameter, S, 24 can be derived for detecting early collagen degeneration in joint osteoarthritis.…”

“…7,8,19,20 These observations are in accordance with two previous studies from the early 2000s 21,22 as well as with some recent investigations relevant to the underlying R 1ρ relaxation mechanisms in cartilage. 19,20,23 Recently, a theoretical framework of R 1ρ dispersion in cartilage has been outlined, 7 suggesting that an orientation-independent MR metric, named the order parameter, S, 24 can be derived for detecting early collagen degeneration in joint osteoarthritis. Traditionally, the R 1ρ dispersion profile was obtained by collecting a series of R 1ρ mappings by varying ω 1 , with each R 1ρ mapping in turn being created from another series of R 1ρweighted images with varying SL durations (T SL ).…”

“…R 1ρ could be viewed as a specific transverse relaxation rate (R 2 ) under the influence of an applied SL RF pulse, and it is particularly sensitive to low-frequency water molecular interactions. 5,7,14 R 1ρ mapping of articular cartilage has been motivated by the diagnostic utility of a noninvasive and sensitive imaging method that can detect early cartilage degeneration in the absence of structural changes apparent on standard MRI. 11,[15][16][17] R 1ρ was first proposed as a promising MR biomarker for characterizing changes in proteoglycan (PG) content, but the specificity of its changes with PG alterations is not yet well understood.…”

“…Traditionally, the R 1ρ dispersion profile was obtained by collecting a series of R 1ρ mappings by varying ω 1 , with each R 1ρ mapping in turn being created from another series of R 1ρweighted images with varying SL durations (T SL ). 7,8,25 As demonstrated schematically in Figure 1C, the standard R 1ρ dispersion imaging (white dots) takes an unrealistically long acquisition time; thus, it is deemed to be impractical for clinical studies.…”

“…7,8 Accordingly, R ex 2 was set to zero in this work unless stated otherwise. R a 2 θ ð Þ is normally written as R a 2 3 cos 2 θ − 1 2 =4, with θ the angle between the collagen fiber primary direction in the deep femoral cartilage and 7,34 ; consequently, R 1ρ will become R i 2 when θ = 54.7 , the so-called magic angle. On the other hand, the same result can also be obtained when ω 1 = ∞.…”