Accelerated Magnetic Resonance Spectroscopic Imaging (MRSI) using Magnetic Resonance Fingerprinting (MRF)

Rashmi Reddy^{1}, Imam Shaik^{1}, Arush Honnedevasthana^{1}, Pavan Poojar^{1}, and Sairam Geethanath^{1}

A retrospective analysis was performed on the Ethanol (CH_{3}CH_{2}OH)
phantom data sets. These datasets were acquired on Siemens 1.5T scanner with
dimensions of 16 X 16 X 1024 with 1024 being the time points.
A spectra was obtained with TE/TR combination of 30ms/1590ms respectively and Averages=1.
4 bottles having pure distilled water (for reference) in 1 of the bottles and 3
other bottles with different concentrations of ethanol (25%, 50% and 100%) were
scanned with total acquisition time of 4 minutes and 30 seconds. The phantom set
up is as shown in figure 1. Spectra of ethanol obtained from scanner was noisy
due to low TR/TE and less averages.

MRF-Dictionary
generation: Free Induction Decay (FID) closely approximating the response of metabolites for the molecular groups of ethanol i.e., CH_{3}, CH_{2} and OH was generated
based on equation as in^{3} given below,

$$S(k)=\sum_{q=1}^NA_{q}exp(-t_{k}/T_{2}^{q})exp(j(2\pi f_{q}t_{k} + \phi_{q}))$$

Where, S is the simulated FID having ‘q’ number of
metabolite peaks each with amplitude A_{q} having an apparent
relaxation time T_{2}^{q}, a frequency f_{q} and a
phase φ_{q} and t_{k} is the k^{th} time point in the
FID. FIDs for above mentioned metabolites were
generated for given TE/TR and for different T_{2} values between the
range 50-720ms, 50-750ms and 20-90ms for CH_{3}, CH_{2} and OH
respectively. Spectra obtained by taking Inverse Fast Fourier Transform (IFFT) were
added up and stored in dictionary.

Pattern matching: Pattern
matching between the original spectra and dictionary was performed using vector
dot product. To find the ppm shift of the spectra, cross correlation and
circular shift operations were performed on the matched spectra from
dictionary. The original and matched spectra were subjected
to the following processing steps in jMRUI^{4}: (a) Apodization (b) Phase Correction (c)
Frequency Shift.

[1] Posse S, Otazo R, Dager SR, Alger J . MR spectroscopic imaging: principles and recent advances. J Magn Reson Imaging. 2013, 1301-25

[2] Dan Ma, Vikas Gulani, Nicole Seiberlich, Kecheng Liu, Jeffrey L. Sunshine, Jeffrey L.Duerk and Mark A. Griswold. Magnetic Resonance Fingerprinting. Nature. 2013 March 14; 495(7440): 187–192

[3] M. Joliot, B. M. Mazoyer, and R. H. Huesman. Invivo Nmr Spectral Parameter-Estimation - a Comparison between Time and Frequency-Domain Methods. Magnetic Resonance in Medicine, 18(2), 358-370 (1991)

[4] A. Naressi, C. Couturier, I. Castang et al. Java-based graphical user interface for MRUI, a software package for quantitation of in vivo/medical magnetic resonance spectroscopy signals. Computers in Biology and Medicine, 31(4), 269-286 (2001)

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

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