Mitigating slice cross‐talk in multi‐slice multi‐echo spin echo T₂ mapping

Abstract

PurposeTo investigate whether a T2 inter‐slice variation could occur when a multi‐slice multi‐echo spin echo (MESE) sequence is used for image acquisition and to propose an enhanced method for reconstructing T2 maps that can effectively address and correct these variations.MethodsBloch simulations were performed accounting for the direct saturation effect to evaluate magnetization changes in multi‐slice 2D MESE sequence. Experimental phantom scans were performed to validate these simulations. An improved version of the dictionary‐based reconstruction approach was proposed, enabling the creation of a multi‐slice dictionary of echo modulation curves (EMC). The corresponding method has been assayed considering inter‐slice T2 variation with phantoms and in lower leg.ResultsExperimental and numerical study illustrate that direct saturation leads to a bias of EMCs. This bias during the T2 maps reconstructions using original single‐slice EMC‐dictionary method led to inter‐slice T2 variation of 2.03% in average coefficient of variation (CV) in agarose phantoms, and up to 2.8% in vivo (for TR = 2 s, slice gap = 0%). A reduction of CV was observed when increasing the gap up to 100% (0.36% in phantoms, and up to 1.5% in vivo) or increasing TR up to 4 s (0.76% in phantoms, and up to 1.9% in vivo). Matching the multi‐slice experimental data with multi‐slice dictionaries provided a reduced CV of 0.54% in phantoms and up to 2.3% in vivo.ConclusionT2 values quantified from multi‐slice MESE images using single‐slice dictionaries are biased. A dedicated multi‐slice EMC method providing the appropriate dictionaries can reduce the inter‐slice T2 variation.

Dictionary-based T2-mapping with multi-echo turbo-spin echo

“…In addition, we assessed the in uence of multi-slice mode on T2 variation among slices. Such a variation has been previously investigated for T2 maps reconstructed from MESE images using dictionary tting [13]. Accordingly, we showed that T2 inter-slice variation was small when 100% slice gaps were utilized (CV < 1%).…”

“…In order to double-check the T2 values, a "gold-standard" T2 single slice mapping was computed from MR images acquired using a spin echo pulse sequence with the following parameters: TR/TEs = 5.5 s / 10-30-50-100-150-200-300 ms, voxel size = 1.17x1.17x2 mm. In order to assess the effect of a multi-slice ME-TSE acquisition on the inter-slice T2 values homogeneity, single-slice and multi-slice images of the phantoms were obtained using an ME-TSE sequence with the following parameters: TR = 2 s, rst TE = 10.3 ms, inter-echo time = 10.3 ms, turbo factor = 3, number of contrasts = 3, slice thickness = 2 mm, number of slices = 5, inter-slice gap = 0 and 100% (100% gap was used for reference T2 maps, as this gap provides negligible direct saturation between slices [13]). For a ME-TSE acquisition, the k-space for each image is lled with data from different echo times and the number of such echoes is called turbo-factor.…”

“…Experimental EMCs were initially obtained from the MR images. Then, dictionaries of echo modulation curves, containing 9 echo signals, were generated using the ME-TSE pulse sequence diagram with the relevant parameters and taking into account the inter-slice interactions as previously described [13].…”

“…Similarly to what we proposed in our seminal paper [13,14], the matching criteria were based on the minimal L2-norm difference between the experimental curve and curve from dictionary. Each experimental ME-TSE EMC contained only 3 points.…”

“…In addition, in multi-slice scanning, direct saturation between neighboring slices can also bias T2 values [12]. These issues have been elegantly addressed using a dictionary-based method aiming at mapping T2 from multi-slice MESE images [13,14].…”