Joint myocardial T₁ and T₂ mapping

Background

Recent studies suggest that quantitative myocardial T₁ mapping allows assessment of focal and diffuse fibrosis in the myocardium [1]. Quantitative T₂ mapping has also been proposed to overcome challenges associated with T₂ weighted imaging [2]. These maps are traditionally acquired with different sequences, necessitating image registration to evaluate them jointly. A sequence that can jointly estimate T₁ and T₂ maps has been proposed [3], but it requires multiple relaxation cycles, which necessitates a lengthy free-breathing acquisition. In [4], an alternative joint estimation sequence was proposed based on the inversion-recovery SSFP curve. In this study, we sought to develop a saturation-recovery based heart-rate independent sequence that can be acquired in a breath-hold and that allows for simultaneous estimation of quantitative T₁ and T₂ maps.

Methods

The sequence diagram is depicted in Figure 1. At every heartbeat, a saturation pulse is applied to eliminate the magnetization history. The longitudinal magnetization then recovers for T_sat based on the T₁ value. Subsequently a T₂-prep pulse [5] with echo length TE_prep is applied to generate the additional T₂ weighting, after which a single shot SSFP image is acquired. The process is repeated for 13 heartbeats with various (T_sat^k, TE_prep^k) corresponding to heartbeat k, to sample different T₁-T₂ weighted images. The first heartbeat is acquired with no magnetization preparation.

Figure 1

a) The sequence diagram for the proposed technique. A saturation pulse is applied in every R-R interval to eliminate the magnetization history. The longitudinal magnetization then recovers for T_sat. Subsequently a T₂-prep with echo length TE_prep is applied to generate the additional T₂ weighting, after which a single shot SSFP image is acquired. b) The mapping sequence acquires the first image with no magnetization preparation (corresponding to T_sat = ∞ and TE_prep = 0), followed by 12 images (3 are shown) acquired with different T_sat and TE_prep values. The major characteristics of the longitudinal magnetization signal curve are depicted under the pulse sequence diagram.

The T₁ and T₂ maps were estimated jointly by voxel-wise least squares fitting to a 4-parameter signal model, A (1- exp(-T_sat^k/T₁)) exp(-TE_prep^k/T₂) + B. Phantom imaging of 14 vials with different T₁/T₂ values were performed and compared to inversion-recovery and CPMG spin-echo references, respectively. Breath-held in-vivo imaging was performed on 5 healthy adult subjects, and the maps were compared to SASHA T₁ maps [6] and to T₂ maps [7].

Results

Phantom imaging resulted in T₁ and T₂ values not significantly different than the references (P = 0.481 and 0.479 respectively). Example in-vivo T₁ and T₂ maps are depicted in Figure 2, comparing various techniques. The T₁ and T₂ values were in good agreement (1211 ± 82 ms vs. 1210 ± 92 ms for T₁; 49.0 ± 5.8 ms and 47.3 ± 6.5 ms for T₂).

Figure 2

T₁ and T maps from a healthy subject, acquired using the proposed technique, as well as SASHA T₁ mapping, and conventional T₂ mapping using 4 T₂prep echo times. Both the T₁ and T₂ maps generated jointly with the proposed method are similar to the individual maps with similar magnetization preparations. The myocardial T₁ and T₂ values in the septum were 1211 ± 82 ms (SASHA T₁), 1210 ± 92 ms (Joint T₁-T₂), 49.0 ± 5.8 ms (conventional T₂) and 47.3 ± 6.5 ms (Joint T₁-T₂) for each technique. The methods generated with the proposed method were acquired in the same time as each individual map, and are jointly registered by design.

Conclusions

The proposed sequence allows for the simultaneous estimation of accurate and jointly registered quantitative T₁ and T₂ maps with similar accuracy and precision to saturation-based T₁ mapping and to T₂ mapping of same duration.

Funding

NIH:K99HL111410-01; R01EB008743-01A2.