Investigating the effect of exchange and multicomponent T₁ relaxation on the short repetition time spoiled steady‐state signal and the DESPOT1 T₁ quantification method

Abstract

Purpose: To examine the spoiled steady-state (spoiled gradient-recalled echo sequence [SPGR]) signal arising from two-compartment systems and the role of experimental parameters, in particular TR for resolving signal from each compartment. Materials and Methods:Using Bloch-McConnell simulations, we examined the SPGR signal from two-component systems in which T 1 is much greater than the mean residence time ( m ) of proton spins in each component. Specifically, we examined the role of TR on the ability to resolve each components signal, as well as the influence of experimental parameters on derived DESPOT1 T 1 values. Results:Results revealed that when TR Յ 0.01 m , the measured SPGR signal may be modeled as a summation of signal from each species using a no-exchange approximation. Additionally, under this short TR condition, the driven equilibrium single pulse observation of T 1 (DESPOT1) mapping approach provides T 1 values preferentially biased toward the short or long T 1 species, depending on the choice of flip angles. Conclusion:The ability to model the SPGR signal using a no-exchange approximation may permit the quantification multicomponent T 1 relaxation in vivo. Additionally, the ability to preferentially weight the DESPOT1 T 1 value toward the short or long T 1 may provide a useful window into these components.

Improved determination of the myelin water fraction in human brain using magnetic resonance imaging through Bayesian analysis of mcDESPOT

“…Neglecting exchange between components, the two-component SPGR signal is conventionally given by (33) $$M_{SPGR}^k=M_{SPGR}^0\text{sin}\left(B_1{\alpha }_k\right)true(f_s\frac{1-E_{1,s}}{1-E_{1,s}\text{cos}\left(B_1{\alpha }_k\right)}+(1-f_s)\frac{1-E_{1,l}}{1-E_{1,l}\text{cos}\left(B_1{\alpha }_k\right)}true)$$ where the dimensionless quantity B 1 denotes an RF transmit scaling factor, s and l respectively denote the short- and long- T 2 components, f s is the fraction of the short T 2 component, $M_0^{SPGR}$ represents the signal amplitude at echo time TE = 0 and incorporates proton density and machine factors, and α k is the k th excitation FA out of a total of K FAs. We also define E 1, j = exp (−TR SPGR /T 1, j ), where T 1, j is the spin-lattice relaxation time of the j th component.…”

Articular Cartilage of the Human Knee Joint: In Vivo Multicomponent T2 Analysis at 3.0 T

“…However, authors of previous multicomponent T2 mapping studies (24)(25)(26)(27)(28) have been limited by their use of CarrPurcell-Meiboom-Gill techniques with long acquisition times, which allowed for cartilage assessment on only a single section of ex vivo specimens. Multicomponent driven equilibrium single-shot observation of T1 and T2 (mcDESPOT) is a rapid method for multicomponent T2 mapping (29)(30)(31)(32)(33)(34). The use of mcDESPOT allows acquisition of three-dimensional (3D) voxel-based measurements of the fractions and T2 values of the fast-relaxing and slow-relaxing water components of the articular cartilage of the human knee joint at 3.0 T with high spatial resolution, large volume coverage, and relatively Abbreviations: AUC = area under the curve BLOKS = Boston-Leeds osteoarthritis knee scoring CI = confidence interval F F = fraction of the fast-relaxing water component FSE = fast spin echo mcDESPOT = multicomponent driven equilibrium singleshot observation of T1 and T2 T2 F = T2 of the fast-relaxing water component T2 Single = single-component T2 T2 s = T2 of the slow-relaxing water component 3D = three-dimensional…”

“…Results of previous studies have shown high pixelby-pixel correlation between cartilage T2 Single measurements obtained by using mcDESPOT and those obtained by using conventional Carr-Purcell-Meiboom-Gill techniques (35). Multicomponent T2 maps for the fast-relaxing water component (T2 F ) and the slow-relaxing water component (T2 S ) and maps for the fraction of the fast-relaxing water component (F F ) were created by using the two-pool mcDESPOT reconstruction method (29)(30)(31). The mcDESPOT method fits the observed spoiled gradient-echo and balanced steady-state free-precession signal at various flip angles with the use of the mathematical model described in the Appendix E1 (Fig E1 [online] …”

Investigating exchange and multicomponent relaxation in fully‐balanced steady‐state free precession imaging

“…It remains unclear how this method will fare in vivo, where the ability to resolve multicomponent T 1 relaxation has been nontrivial. Recent work in this area with rapid SPGR imaging, however, has shown it may be possible to quantify multicomponent T 1 relaxation in fast relaxing species provided the repetition time satisfies the condition TR Ͻ m (20,21). We are nevertheless confident that a combination of rapid, multiangle (and perhaps multiple TR) SPGR and SSFP imaging will permit similar analysis in vivo.…”