Time‐division multiplexing (TDM) sequence removes bias in T2 estimation and relaxation‐diffusion measurements

Abstract

AbstractPurposeTo compare the performance of multi‐echo (ME) and time‐division multiplexing (TDM) sequences for accelerated relaxation‐diffusion MRI (rdMRI) acquisition and to examine their reliability in estimating accurate rdMRI microstructure measures.MethodThe ME, TDM, and the reference single‐echo (SE) sequences with six TEs were implemented using Pulseq with single‐band (SB) and multi‐band 2 (MB2) acceleration factors. On a diffusion phantom, the image intensities of the three sequences were compared, and the differences were quantified using the normalized RMS error (NRMSE). Shinnar–Le Roux (SLR) pulses were implemented for the SB‐ME and SB‐SE sequences to investigate the impact of slice profiles on ME sequences. For the in‐vivo brain scan, besides the image intensity comparison and T2‐estimates, different methods were used to assess sequence‐related effects on microstructure estimation, including the relaxation diffusion imaging moment (REDIM) and the maximum‐entropy relaxation diffusion distribution (MaxEnt‐RDD).ResultsTDM performance was similar to the gold standard SE acquisition, whereas ME showed greater biases (3–4× larger NRMSEs for phantom, 2× for in‐vivo). T2 values obtained from TDM closely matched SE, whereas ME sequences underestimated the T2 relaxation time. TDM provided similar diffusion and relaxation parameters as SE using REDIM, whereas SB‐ME exhibited a 60% larger bias in the <R2> map and on average 3.5× larger bias in the covariance between relaxation‐diffusion coefficients.ConclusionOur analysis demonstrates that TDM provides a more accurate estimation of relaxation‐diffusion measurements while accelerating the acquisitions by a factor of 2 to 3.