Lung T2 * mapping using 3D ultrashort TE with tight intervals δTE

Abstract

Purpose: To develop 3D ultrashort-TE (UTE) sequences with tight TE intervals (δTE), allowing for accurate $T_2^{*}$ mapping of lungs under free breathing.

Methods: We have implemented a four-echo UTE sequence with δTE (< 0.5 ms). A Monte-Carlo simulation was performed to identify an optimal number of echoes that would result in a significant improvement in the accuracy of the $T_2^{*}$ fit within an acceptable scan time. A validation study was conducted on a phantom with known short $T_2^{*}$ values (< 5 ms). The scanning protocol included a combination of a standard multi-echo UTE with six echoes (2.2-ms intervals) and a new four-echo UTE (TE < 2 ms) with tight TE intervals δTE. The human imaging was performed at 3 T on 6 adult volunteers. $T_2^{*}$ mapping was performed with mono-exponential and bi-exponential models.

Results: The simulation for the proposed 10-echo acquisition predicted over 2-fold improvement in the accuracy of estimating the short $T_2^{*}$ compared with the regular six-echo acquisition. In the phantom study, the $T_2^{*}$ was measured up to three times more accurately compared with standard six-echo UTE. In human lungs, $T_2^{*}$ maps were successfully obtained from 10 echoes, yielding average values $T_2^{*}$ = 1.62 ± 0.48 ms for mono-exponential and $T_{2s}^{*}$ = 1.00 ± 0.53 ms for bi-exponential models.

Conclusion: A UTE sequence using δTE was implemented and validated on short $T_2^{*}$ phantoms. The sequence was successfully applied for lung imaging; the bi-exponential signal model fit for human lung imaging may provide valuable insights into the diseased human lungs.

Keywords: bi-exponential model; lung T 2 * $$ {\mathrm{T}}_2^{\ast } $$ ; short and long T2 components; ultrashort TE (UTE).