UC San Diego

Purpose

To develop 3D ultrashort-TE (UTE) sequences with tight TE intervals (δTE), allowing for accurate T2*$$ {\mathrm{T}}_2^{\ast } $$ 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 T2*$$ {\mathrm{T}}_2^{\ast } $$ fit within an acceptable scan time. A validation study was conducted on a phantom with known short T2*$$ {\mathrm{T}}_2^{\ast } $$ 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. T2*$$ {\mathrm{T}}_2^{\ast } $$ 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 T2*$$ {\mathrm{T}}_2^{\ast } $$ compared with the regular six-echo acquisition. In the phantom study, the T2*$$ {\mathrm{T}}_2^{\ast } $$ was measured up to three times more accurately compared with standard six-echo UTE. In human lungs, T2*$$ {\mathrm{T}}_2^{\ast } $$ maps were successfully obtained from 10 echoes, yielding average values T2*$$ {\mathrm{T}}_2^{\ast } $$  = 1.62 ± 0.48 ms for mono-exponential and T2s*$$ {\mathrm{T}}_{2s}^{\ast } $$  = 1.00 ± 0.53 ms for bi-exponential models.

Conclusion

A UTE sequence using δTE was implemented and validated on short T2*$$ {\mathrm{T}}_2^{\ast } $$ 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.