Single-shot T2 mapping through overlapping-echo detachment planar imaging (OLED) sequence
Congbo Cai1, Yuchuan Zhuang2, Shuhui Cai3, Jianhui Zhong2, and Zhong Chen3

1Department of Communication Engineering, Xiamen University, Xiamen, China, People's Republic of, 2Department of Imaging Sciences, University of Rochester, ROCHESTER, NY, United States, 3Department of Electronics Science, Xiamen University, Xiamen, China, People's Republic of

### Synopsis

Magnetic Resonance parameters mapping can provide useful quantitative information for characterization of tissue properties. However, the long acquisition time usually hinder the real-time MR parameter mapping. In this abstract, a novel single-shot T2 mapping method was proposed based on spin-echo EPI method. Two overlapping echo signals with the different T2 weighting were obtained simultaneously. A detachment algorithm based on joint sparsity constraint was proposed to separate the two echo signals. The robustness and efficiency of the sequence were demonstrated through phantom experiments. The reliable T2 mapping can be obtained in the order of milliseconds.

### Target audience

The target audience is basic scientists and clinical scientists who are interested in ultrafast T2 mapping.

### Purpose

MR parameters mapping provides useful quantitative information for characterization of tissue properties [1]. However, the long acquisition time usually hinder its real-time applications. Problems will occur due to the motion; especially the unrepeatable physiological activities will make the real-time MR parameter mapping impossible [2]. Many kinds of imaging methods have been proposed to overcome this problem [3]. However, several seconds are still needed. In this work, a novel single-shot T2 mapping method was proposed based on conventional SE-EPI acquisition scheme, and termed overlapping-echo detachment planar imaging (OLED) method. Two overlapping echo signals with the different T2 weighting were obtained simultaneously through two small flip angle excitation pulses and corresponding echo-shifting gradients. The detachment algorithm based on joint sparsity constraint was proposed to separate the two echo signals. The robustness and efficiency of the OLED sequence were demonstrated through phantom experiments. The reliable T2 mapping can be obtained in the order of milliseconds.

### Methods

Two echo signals with different evolution time for OLED sequence will be acquired synchronously, expressed respectively as: $$\begin{cases} s_{1}(TE_{1})=\int_{}^{} \rho(r)|sina\times cosa|e^{-TE_{1}/T_{2}(r)}dr , first echo \\ s_{2}(TE_{2})=\int_{}^{} \rho(r)\frac{1}{2}|sina\times (1+cosa)|e^{-TE_{2}/T_{2}(r)}dr , second echo\end{cases}$$ , where α=45o is the flip angle of the excitation pulses, the echo times TE1 and TE2 are determined by the echo-shifting gradients G1 and G2. The T2 value is calculated in a pixel-wise fashion from Eq. (1). To separate the two echo signals, the following minimization problem is resolved: , $$\left\{x_{1},x_{2}\right\}=argmin\left[||x_{1}-x_{10}||_2^2+\lambda_{1}||\triangledown x_{1} ||_1+\lambda_{2}||\triangledown x_{2} ||_1+\lambda_{3}||\triangledown \left(x_{1}-\beta x_{2}\right) ||_1\right]$$ (2) where x10, x20, x1 and x2 are the preliminary and separated images from the first and second echo signals respectively, λ1, λ2, and λ3 are Lagrange multipliers adjusting constraint weights, and we have: $$x_{1}e^{i\phi_{1}\left(r\right)}+x_{2}e^{i\phi_{2}\left(r\right)}=x_{0}$$ , where φ1(r) and φ2(r) are the linear phase ramps of the first and second images .

### Results

The experiments were performed on a whole-body 3T scanner ( MAGNETOM Trio TIM, Siemens Healthcare, Erlangen, Germany) with a phantom consisting of multiple vials filled with water doped with MnCl2 of different concentrations. For single-shot EPI and OLED sequences, scan time was 153.5 ms and 141.6ms respectively with acquisition matrix 64×128 and FVO 20x20cm2. Conventional SE sequence was also used for comparison, with 8 different TEs and acquisition matrix = 128×128, and the total scan time was about 1 hour. The results are shown in Fig. 2.

### Discussion

In Fig.2, we can see that the T2 values from single-shot OLED sequence are coincident with those from conventional SE sequence (Pearson’s correlation coefficient = 0.9995 for the average T2 value in the different compartments). The maximum deviation is about 4.7% (△T2=-1.5ms) for compartment 1 which has the shortest T1 and T2 values. However, the deviation is mainly from the relative low SNR for compartment 1. The distortions are obvious for EPI and OLED sequences because of the inhomogeneous background field. All the amplitude images (b~e), denoted by the red arrows in Fig. 2, have the inhomogeneous signal intensities in the same compartment. The inhomogeneous signal intensities in SE image mainly come from the inhomogeneous coil sensitivity maps, while the peripheral hyperintense in EPI and OLED sequences may also come from the B0 inhomogeneity. From Fig. 2 (g), we can see that the reconstructed T2 map from OLED is still quite homogeneous comparing to the corresponding signal intensity.

### Conclusion

Robustness of the OLED method was demonstrated by phantom experiments. Our preliminary results indicate that the OLED can provide single-shot T2 mapping in the order of milliseconds, which is promising for dynamic imaging, and will propel single-shot EPI from a qualitative method to a quantitative method.

### Acknowledgements

This work was supported by the NNSF of China under Grants 81171331.

### References

[1] D. Ma, et al. Nature, 495 (2013) 187-193. [2] O. Speck, et al. Magn. Reson. Med., 40(1996) 243-248. [3] T. J. Sumpf, et al. IEEE Trans. Med. Imag., 33 (2014) 2213-2222.

### Figures

Fig.1 Single-shot OLED sequence

Fig.2 MR images from water phantom with different concentration of MnCl2. (a) OLED Image including two echo signals; (b) and (c) Separated images from the first and second echo signals of (a); (d) SE image; (e) SE-EPI image; (f) T2 map from SE images; (g) T2 map from (a); (h) Compartment order.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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