# 31 P magnetic resonance fingerprinting for rapid quantification of creatine kinase reaction rate in vivo

## EarlyView Article

• Published: Sep 15, 2017
• Author: Charlie Y. Wang, Yuchi Liu, Shuying Huang, Mark A. Griswold, Nicole Seiberlich, Xin Yu
• Journal: NMR in Biomedicine

The purpose of this work was to develop a 31P spectroscopic magnetic resonance fingerprinting (MRF) method for fast quantification of the chemical exchange rate between phosphocreatine (PCr) and adenosine triphosphate (ATP) via creatine kinase (CK).

A 31P MRF sequence (CK‐MRF) was developed to quantify the forward rate constant of ATP synthesis via CK ( ${k}_{\mathrm{f}}^{\mathrm{CK}}$), the T1 relaxation time of PCr ( ${T}_{1}^{\mathrm{PCr}}$), and the PCr‐to‐ATP concentration ratio ( ${M}_{\mathrm{R}}^{\mathrm{PCr}}\right)$. The CK‐MRF sequence used a balanced steady‐state free precession (bSSFP)‐type excitation with ramped flip angles and a unique saturation scheme sensitive to the exchange between PCr and γATP. Parameter estimation was accomplished by matching the acquired signals to a dictionary generated using the Bloch‐McConnell equation. Simulation studies were performed to examine the susceptibility of the CK‐MRF method to several potential error sources. The accuracy of nonlocalized CK‐MRF measurements before and after an ischemia–reperfusion (IR) protocol was compared with the magnetization transfer (MT‐MRS) method in rat hindlimb at 9.4 T (n = 14). The reproducibility of CK‐MRF was also assessed by comparing CK‐MRF measurements with both MT‐MRS (n = 17) and four angle saturation transfer (FAST) (n = 7).

Simulation results showed that CK‐MRF quantification of ${k}_{\mathrm{f}}^{\mathrm{CK}}$ was robust, with less than 5% error in the presence of model inaccuracies including dictionary resolution, metabolite T2 values, inorganic phosphate metabolism, and B1 miscalibration. Estimation of ${k}_{\mathrm{f}}^{\mathrm{CK}}$ by CK‐MRF (0.38 ± 0.02 s−1 at baseline and 0.42 ± 0.03 s−1 post‐IR) showed strong agreement with MT‐MRS (0.39 ± 0.03 s−1 at baseline and 0.44 ± 0.04 s−1 post‐IR). ${k}_{\mathrm{f}}^{\mathrm{CK}}$ estimation was also similar between CK‐MRF and FAST (0.38 ± 0.02 s−1 for CK‐MRF and 0.38 ± 0.11 s−1 for FAST). The coefficient of variation from 20 s CK‐MRF quantification of ${k}_{\mathrm{f}}^{\mathrm{CK}}$ was 42% of that by 150 s MT‐MRS acquisition and was 12% of that by 20 s FAST acquisition.

This study demonstrates the potential of a 31P spectroscopic MRF framework for rapid, accurate and reproducible quantification of chemical exchange rate of CK in vivo.

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