Myelin Imaging Using Dual-echo 3D Ultra-short Echo Time MRI with Rosette k-Space Pattern

Abstract

Purpose: This study aimed to develop a new 3D dual-echo rosette k-space trajectory, specifically for applications of ultra-short echo time (UTE) magnetic resonance imaging (MRI). The direct imaging of the myelin bilayer, which has ultra-short transverse relaxation time (uT2), was acquired to test the performance of the proposed UTE sequence. Theory and Methods: The rosette trajectory was developed based on rotations of a "petal-like" pattern in the kx-ky plane, with oscillated extensions in kz-direction for 3D coverage. Five healthy volunteers were recruited and underwent ten dual-echo rosette UTE scans with varied echo times (TEs). Dual-exponential model fitting was performed to separate uT2 signals, with the output of uT2 fraction, uT2 value and long T2 value. Results: The reconstructed images' signal contrast between white matter (WM) and grey matter (GM) increased with longer TEs. The WM regions had higher uT2 fraction values than GM (10.9% ± 1.9% vs. 5.7% ± 2.4%). The uT2 value was about 0.12 milliseconds in WM. Conclusion: The higher uT2 fraction value in WM compared to GM demonstrated the ability of the proposed sequence to capture rapidly decaying signals.

Proton Free Induction Decay MRSI at 7T in the Human Brain Using an Egg-Shaped Modified Rosette K-Space Trajectory

“…However, this comes at the cost of an increased acquisition time. To overcome this compressed sensing (CS) techniques have been implemented together with a 3D modified rosette trajectory, benefitting from its incoherent sampling pattern [22][23][24] . Compared to density-weighted-CRT 18 the rosette trajectory has a reduced SNR efficiency due to unfavorable k-space weighting.…”