Ingo Hermann^{1}, Benedikt Rieger^{1}, Jascha Zapp^{1}, Sebastian WeingĂ¤rtner^{1,2,3}, and Lothar R. Schad^{1}

In this work, an optimized fast group matching reconstruction for magnetic resonance fingerprinting based on echo-planar imaging was evaluted to enhance clinical usability. This scanner based 'on the fly' reconstruction reduced the reconstruction time by an acceleration factor of 10 shortening the reconstruction to 10 seconds. The fast group matching algorithm was tested in-vivo and compared with full dictionary matching and resulted in virtually no deviation in T_{1} and T_{2}* maps facilitating the use of MRF in clinical routine.

A comparison of pattern matching and group matching reconstruction showed complete agreement with virtually no deviations in T_{1} and T_{2}* for T_{1} < 300 ms and T_{2}* < 20 ms with an acceleration factor of 10. Figure 2 shows the MRF maps of 5 different slices with group matching reconstruction. In Fig. 3 the absolute deviation of full reconstruction versus group matching is depicted in percentage for 1 and 100 matched groups (k). For 1 matched group the deviation is RSME = 2.3 % and for 100 matched groups RSME = 1.3 % respectively. With 1,000 matched groups there is no deviation (RSME = 0 %) when matching the dictionary with both algorithms. An acceleration factor of more than 10 was achieved for k = 1,000 leading to an entire reconstruction time for the generation of T_{1} and T_{2} maps for 60 slices on Prisma around 10 seconds and on Skyra around 30 seconds.

Dictionary matching as compared with conventional optimization schemes, bears the advantage of high robustness against local minima. The group matching algortihm provides the means, to exploit local similarities within the dictionary, to speed up the reconstruction while still sampling across the entire dictionary, therefore, maintaining the advantage of local minima robustness.

The group matching algorithm is a reliable and fast method for MRF acquisitions, and is compatible with MRF-EPI scans. Best results are obtained for a high number (10%) of small groups (~10 per group). Slower group sizes result in longer reconstruction times. For group sizes larger than 10, the acceleration rate is comparable to the number of entries per group. This provides means to achieve increased reconstruction fidelity by using larger dictionaries while maintaining clinically acceptable reconstruction times.

[1] Cauley, S. F. et al. Fast group matching for MR fingerprinting
reconstruction. Magnetic Resonance in Medicine **74**, 523-528 (2015).

[2]
Rieger, B., Zimmer, F., Zapp, J., Weingartner, S. & Schad, L. R.
Magnetic resonance fingerprinting using echo-planar imaging: Joint
quantication of T_{1} and relaxation times. Magnetic Resonance in Medicine **78**, 1724-1733 (2016).