³¹P Magnetization Transfer spectroscopy (MT-MRS) has been used to measure high energy phosphate metabolism via creatine kinase (CK) in vivo. However, current ³¹P MT-MRS methods require prohibitively long experiment time, which limits their clinical applications. In this study, we propose a novel Magnetic Resonance Fingerprinting¹ (MRF) framework based method, the Creatine Kinase encoding MRF method (CK-MRF), to increase measurement efficiency of ³¹P metabolism through CK. Measurement accuracy and reproducibility of the creatine kinase forward rate constant (k_f,CK) using CK-MRF was compared with conventional methods in rat hind limb. Changes in k_f,CK following Ischemia/Reperfusion (IR) were also measured.

CK-MRF acquisition consisted of an inversion preparation pulse followed by 320 acquisitions arranged in blocks, alternating between phosphocreatine (PCr) and γATP excitation (Fig. 1). The acquisition was organized into two sections, composed of blocks of 10 ramped excitations with 12.8 ms TR. Excitation flip angle (FA) in both sections was modulated by a sinusoidal envelope. Following each excitation block, frequency selective saturation was achieved with a 470 ms BISTRO scheme. Saturation frequency at either 2.4 ppm upfield of PCr (γATP saturation, section 2) or 2.4 ppm downfield of PCr (control saturation, section 1) was used. Total acquisition time for 1 fingerprint average was 20 s. RF excitations used Gaussian shaped spectrally selective pulses (4 ms duration) with alternated phase. FID signals were acquired in a 7.8 ms window with 30 ms dwell time. Signal evolutions for both PCr and γATP were extracted by Fourier transform.

A dictionary was constructed using Matlab-based Bloch-McConnell simulator that included four matching parameters: intrinsic PCr longitudinal relaxation time (T_1,PCr), pseudo-first order CK exchange rate (k_f,CK), PCr to ATP ratio (M_0,PCr/M_0,γATP), and PCr chemical shift (δ_PCr). Dictionary resolution for T_1,PCr, M_0,PCr/M_0,γATP, and δ_PCr was 0.25 s, 0.3, and 6.25 Hz respectively. k_f,CK used an adaptive resolution of 0.006 s^-1 and 0.03 s^-1 for values below and above 0.415 s^-1 respectively. The dictionary totaled 132,660 entries.

Animal studies (n=3) were performed on rat hind-limb at 9.4T using a custom-built ³¹P saddle coil. An inflatable cuff was placed at the thigh of the rat to induce ischemia. During baseline, 3 methods were compared for measurement precision: CK-MRF, conventional MT by ³¹P saturation transfer², and Four Angle Saturation Transfer³ (FAST). A total of 1800 s of data was acquired for each method. Acquired data was retrospectively averaged without view-sharing and the results of parameter estimation with different number of averages were compared. Following baseline data acquisition, 20 min ischemia was induced by inflating the cuff to 300 mmHg followed by 10 minutes of reperfusion for stabilization. Afterwards, CK-MRF and conventional ³¹P saturation transfer were acquired in interleaved fashion for a total of 20 min each.

Figs. 2a&b show representative fingerprints acquired in vivo. Fig. 3 shows the measurement accuracy and reproducibility of baseline k_f,CK for all 3 methods using different number of signal averages, corresponding to an acquisition time of 20 s (CK-MRF and FAST methods only), 160 s, 320 s, and 1800 s, respectively. Even with 20 s acquisition (single average), CK-MRF demonstrated similar or better precision compared to 160 s signal averages of either conventional MT or FAST acquisition. Data from 20 s FAST acquisition yielded nonsensical data. Both CK-MRF and conventional MT detected an increase in post-ischemia k_f,CK. CK-MRF observed an increase from 0.315 s^-1 at baseline to 0.349 s^-1 after reperfusion, while conventional ³¹P saturation transfer observed an increase from 0.338 to 0.374 s^-1. In this study, we present the use of a spectroscopic MRF method to increase the sensitivity to creatine kinase activity. The current iteration shows good agreement with conventional ³¹P MT measurement methods with far superior measurement efficiency leading to the possibility of performing spatial mapping measurements.