Relaxation anisotropy of quantitative MRI parameters in biological tissues

Abstract

Quantitative MR relaxation parameters vary in the sensitivity to the orientation of the tissue in the magnetic field. In this study, the orientation dependence of multiple relaxation parameters was assessed in various tissues. Ex vivo samples of each tissue type were prepared either from bovine knee (tendon, cartilage) or mouse (brain, spinal cord, heart, kidney), and imaged at 9.4 T MRI with T1, T2, continuous wave (CW-) T1ρ, adiabatic T1ρ and T2ρ, and Relaxation along fictitious field (RAFF2-4) sequences at five different orientations with respect to the main magnetic field. Relaxation anisotropy of the measured parameters was quantified and compared. The highly ordered collagenous tissues, i.e. cartilage and tendon, presented the highest relaxation anisotropy for T2, CW-T1ρ with spin-lock power < 1 kHz, Ad-T2ρ and RAFF2-4. Maximally anisotropy was 75% in cartilage and 30% in tendon. T1 and adiabatic T1ρ did not exhibit observable anisotropy. In the other measured tissue types, anisotropy was overall less than 10% for all the parameters. The results confirm that highly ordered collagenous tissues have properties that induce very clearly observable relaxation anisotropy, whereas in other tissues the effect is not as prominent. Quantitative comparison of anisotropy of different relaxation parameters highlights the importance of sequence choice and design in MR imaging.

Myelin biomarkers in the healthy adult brain: Correlation, reproducibility, and the effect of fiber orientation

“…23 The MWF has been found to vary with white matter fiber orientation, perhaps due to the orientation dependence of the underlying T 2 relaxation. [24][25][26] Our aim was to investigate the utility of these three myelin-sensitive techniques. To this end, we compared in vivo brain ihMTR, MTR and MWF, assessing the consistency of myelination rankings between white matter and grey matter regions, correlation between metrics, scan-rescan variability, and how each metric is affected by fiber orientation.…”

“…Current in vivo implementations of MWI achieve whole cerebrum coverage in reasonable scan times by accelerating the acquisition using both gradient and spin echoes (GRASE) 23 . The MWF has been found to vary with white matter fiber orientation, perhaps due to the orientation dependence of the underlying T 2 relaxation 24–26 …”

SPICY: a method for single scan rotating frame relaxometry

“…It might be possible to use adiabatic spin locking in the SPICY sequence. [42][43][44] Because the SPICY allows signal readout in a similar manner as the CPMG, 8 it could be added to multidimensional NMR or Laplace NMR experiments 64 to correlate 𝑇 1𝜌 with the other relaxation or diffusion parameters. Furthermore, the 1D imaging version of SPICY could replace the CPMG readout part in multidimensional ultrafast NMR 65 or Laplace NMR (LNMR) experiments.…”

“…[39][40][41] Alternatively, the T 1r experiments can be performed using adiabatic spin locking. [42][43][44] The conventional experiments must be repeated with incremented spin lock times, such that the total experiment time ranges from minutes to days depending on the number of repetitions. Furthermore, T 1r dispersion experiments are more time consuming since the experiments must be repeated with several SL frequencies.…”

Hydrated Collagen: Where Physical Chemistry, Medical Imaging, and Bioengineering Meet

“…The effect has been known in partially hydrated collagen since the early 1960s and was first reported in native biological tissues (mammalian tendon samples) by Fullerton and co-workers in 1985 . The MA effect is now well-known in clinical radiology: it is commonly observed in a range of ordered collagenous tissues, most notably tendon, − articular cartilage, − the intervertebral disc, and as reported recently, the heart muscle . A similar effect, albeit possibly with a different molecular mechanism, has also been reported in brain white matter. − …”