Jonathan I Tamir^{1}, Valentina Taviani^{2}, Shreyas S Vasanawala^{3}, and Michael Lustig^{1}

Volumetric fast spin-echo (3DFSE) imaging is clinically desirable because of its robustness to off-resonance and its utility for obtaining many types of image contrasts at isotropic resolution. However, its routine clinical use is inhibited by blurring due to long echo trains needed to maintain scan efficiency. Her we present T1-T2 Shuffling, a 3DFSE-based acquisition and reconstruction scheme that mitigates image blur and retrospectively synthesize T1-weighted and T2-weighted image contrasts. The acquisition, an extension of T2 Shuffling, employs a randomizing echo train view ordering with variable repetition times (TRs). The use of short TRs increases scan efficiency while providing T1 sensitivity.

The CUBE 3DFSE pulse sequence (GE Healthcare) was previously modified to re-sample phase encodes at different echo times$$$^{3}$$$. Here we add the capability to specify the TR independently for each echo train to provide T1 sensitivity. Variable-density Poisson-disc sampling patterns are prescribed for each unique TE and TR combination. The signal evolution with driven-equilibrium at the $$$i^\mathrm{th}$$$ TE and the $$$j^\mathrm{th}$$$ TR is depicted in Figure 1 for different combinations of (T1,T2), and is given by$$$^{7}$$$$$S\left(\mathrm{TE}_i,\mathrm{TR}_j\right)=M_0f\left(T_1,T_2,\mathrm{TE}_i\right)\frac{1-E_1(\mathrm{TR}_j)}{1-E_1(\mathrm{TR}_j)f\left(T_1,T_2,\mathrm{TE}_T\right)}, $$where $$$f\left(T_1,T_2,\mathrm{TE}_i\right)$$$ is the relaxation signal at the $$$i^\mathrm{th}$$$ TE, $$$E_1(\mathrm{TR})=\exp\left(-\frac{\mathrm{TR}-T{\times}T_s}{T_1}\right)$$$, $$$T_s$$$ is the echo spacing, and $$$T$$$ is the echo train length. The relaxation signal $$$f\left(T_1,T_2,\mathrm{TE}\right)$$$ depends on the variable refocusing flip angle scheme and can be calculated with the extended phase graph algorithm$$$^{2}$$$.

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(a) Simulated steady-state transverse and longitudinal magnetization signal evolution curves for FSE with driven equilibrium across a TR of 1500 ms using variable-refocusing
flip angles shown in (b). Each curve represents a
different (T1, T2) pair.

Scan and imaging parameters used for the in vivo experiments. All scans were performed at 3T with IRB approval and informed consent.

(a) Fully sampled scans of 2D multi spin-echo and inversion recovery fast spin-echo were used to estimate T1 and T2 maps of the brain at 3T. These maps were used to derive a T1-T2 distribution and simulate an ensemble of signal evolutions with variable refocusing flip angles and variable TRs. (b) The signal evolutions at each TR were concatenated and a subspace was formed through SVD. (c) Using the subspace and the known T1 and T2 values, a linear combination was found to synthesize arbitrary image contrasts.

Reconstructed spin-echo image animations at different contrasts and orientations. Left: sagittal reformat at a TR of 7000 ms and TE increasing from 5 ms to 200 ms shows the T2 relaxation curve. Middle: coronal reformat at TE = 10 ms and TR increasing from 200 ms to 7000 ms shows the T1 saturation recovery curve. Right: Axial reformat at TE = 10 ms, TR = 7000 ms, and TI increasing from 50 ms to 7000 ms shows the T1 inversion recovery curve.

Comparison of conventional T1-weighted and T2-weighted images obtained from two separate CUBE acquisitions with synthesized image contrasts using T1-T2 Shuffling (single acquisition).