Xiaozhi Cao^{1}, Qing Li^{1}, Huihui Ye^{1,2}, Hongjian He^{1}, and Jianhui Zhong^{1,3}

A spiral projection acquisition scheme was implemented
for 3D MR fingerprinting to achieve isotropic resolution of 1x1x1 mm^{3} in whole brain T1
and T2 mapping within 2.5 minutes by using efficient L1SPIRiT
reconstruction (ESPIRiT) and back-projection reconstruction.

Methods

The acquisition and reconstruction process were shown in Figure 1.

a. A spiral trajectory was rotated about x-axis along time points (TP) dimension and simultaneously about z-axis along acquisition group (AG) dimension.

b. By combining the spiral interleaves acquired from the same time point but different acquisition groups, spiral interleaves formed an undersampled disk-like k-space coverage in a same plane.

c. Undersample the “disk” with acceleration factor R=2, which means it could reduce the number of acquisition groups by half, resulting in a two-fold reduction of acquisition time.

d. ESPIRit ^{5} reconstruction method was applied on the undersampled disk-like k-space data.
Since the “disk” was rotated along time points dimension, the reconstructed
image of each time point was actually a parallel-beam projected image from a
specific angle.

e. By using a
sliding-window method ^{6} to select adjacent projected images and applying a
back-projection reconstruction method ^{7} on these images, a series of 3D
images with varying mixed contrast were obtained along the time point
dimension.

f. Reconstructed images
were template matched with a pre-calculated dictionary voxel-by-voxel to
generate T_{1} and T_{2} maps by using the extended phase graph (EPG) ^{8} method. The
dictionary entries were also implemented with sliding-window method in the same
way as we did it in step e.

Figure
2 shows a typical pulse sequence design of a 3D MRF fingerprinting with fast
imaging with steady state precession (FISP) readout ^{9} and SPI trajectory. A
slab-selective gradient was used to achieve a 240-mm slab thickness and a
dephase gradient was used to achieve a 4- dephase. Each acquisition group includes 500
time points in which the flip angles vary in a preset way as Figure 2b shows
and the TRs were set at 16 msec constantly. Between two adjacent acquisition
groups, a waiting time of 2sec was inserted for the recovery of longitudinal
magnetization. A total of 30 acquisition groups were acquired for fully-sampled
data as reference and 15 of them were used for the validation of the proposed
method. The spiral rotates about x-axis at tiny golden angle (23.63°) along time points dimension. The
isotropic spatial resolution of 1x1x1 mm^{3} was achieved for the
identified parametric maps in a field of view (FOV) of 240x240x240 mm^{3}. The
measurements of a phantom and in vivo brains were performed on a Siemens 3T
Prisma scanner with a 64-channel head coil.

Discussion and Conclusion

We proposed a reconstruction scheme by combining the ESPIRiT and back-projection reconstruction on 3D MRF with SPI trajectory. Compared to original 3D SPI MRF using 3D INUFFT method [4], the reconstruction was split into two part, 2D INUFFT which transforms k-space data to projected images and back-projection which transforms projected images to 3D images. With a simple 2D INUFFT replaced by ESPIRiT, acquisition acceleration could be achieved by undersampling the “disk” data with less spiral interleaves. Therefore, the T1. Ma D, Gulani V, Seiberlich N, Liu K, Sunshine JL, Duerk JL, Griswold MA. Magnetic resonance fingerprinting. Nature 2013;495(7440):187-192.

2. Ma D, Jiang Y, Chen Y, McGivney D, Mehta B, Gulani V, Griswold M. Fast 3D magnetic resonance fingerprinting for a whole-brain coverage. Magnetic resonance in medicine 2017; DOI:10.1002/mrm.26888.

3. Liaoc C, Bilgic B, Manhard M, Zhao B, Cao X, Zhong J, Wald L, Setsompop K. 3D MR fingerprinting with accelerated stack-of-spirals and hybrid sliding-window and GRAPPA reconstruction. NeuroImage 2017;162:13-22.

4. Cao X, Liao C, Li Q, Ye H, He H, Zhong J. Fast 3D MR fingerprinting with spiral projection acquisition for whole brain quantification imaging. In Proceeding of the 26th Annual Meeting of the ISMRM, Paris, France, 2018. p. 1017.

5. Uecker M, Lai P, Murphy M, Virtue P, Elad M, Pauly J, Vasanawala S, Lustig M. ESPIRiT-an eigenvalue approach to autocalibrating parallel MRI: Where SENSE meets GRAPPA. Magnetic resonance in medicine 2014;71(3):990-1001.

6. Cao X, Liao C, Wang Z, Chen Y, Ye H, He H, Zhong J. Robust sliding-window reconstruction for Accelerating the acquisition of MR fingerprinting. Magnetic resonance in medicine 2017;78(4):1579-1588.

7. Herman GT. Image reconstruction from projections. Real-time imaging 1995;1(1):3-18.

8. Weigel M. Extended phase graphs: dephasing, RF pulses, and echoes - pure and simple. Journal of magnetic resonance imaging : JMRI 2015;41(2):266-295.

9. Jiang Y, Ma D, Seiberlich N, Gulani V, Griswold MA. MR fingerprinting using fast imaging with steady state precession (FISP) with spiral readout. Magnetic resonance in medicine 2015;74(6):1621-1631.

Figure.1

Process of acquisition and reconstruction for the proposed method.

Figure.2

(a) Pulse sequence of 3D MRF with spiral projection acquisition.

(b) Flip angles pattern. TR was kept at 16 ms constantly for whole scan.

(c) Acquisition groups distribution.

Figure.3

The projected images reconstructed from full-sampled (a) and under-sampled using a simple 2D INUFFT (b) and ESPIRiT algorithm (d) as well as corresponding difference map compared with the full-sampled data (c,e).

Figure.4

T1 (a) and T2 (b) maps using different reconstruction schemes with full-sampled and under-sampled (acceleration factor R=2) data.