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A Comparative Analysis of CEST Techniques for Mapping Muscle Creatine and Phosphocreatine at 3T
Licheng Ju1, Kexin Wang1, Michael Schär1, Su Xu2, Joshua Rogers2, Dan Zhu1, Qin Qin1, Robert G. Weiss1, and Jiadi Xu1
1Johns Hopkins University, Baltimore, MD, United States, 2University of Maryland, Baltimore, MD, United States

Synopsis

Keywords: CEST / APT / NOE, CEST & MT

Motivation: Creatine and phosphocreatine metabolites imaging at 3T are essential for related disease in muscle.

Goal(s): Estimate creatine proton exchange rate in muscle; Simultaneous mapping of PCr and Cr by PLOF CEST method at 3T.

Approach: Antemortem and postmortem animal study was to validate PCr/Cr CEST peak position and creatine exchange rate. Three types of CEST acquisition methods were compared on human leg muscle.

Results: Z-spectra in mouse hindlimb before and after euthanasia indicated CrCEST is a slow-exchanging process (<150 s-1). This allowed us to simultaneously extract PCr/CrCEST signals and mapping in muscle at 3T using the PLOF method on both human and animal.

Impact: Amide, Cr, and PCr CEST in the skeletal muscle can be mapped simultaneously at 3T by PLOF CEST within a clinically feasible acquisition duration, which has potential to assist in the diagnosis of related diseases.

INTRODUCTION

Phosphocreatine (PCr) is a compound that is synthesized from creatine (Cr) and adenosine triphosphate (ATP) in muscle cells (1,2) via the creatine kinase reaction, serving as a reservoir of high-energy phosphate bonds which is particularly important for short-term, high-intensity activities (3). For decades, researchers have been exploring non-invasive methods to detect PCr or Cr using MRI. While phosphorus (31P) magnetic resonance spectroscopy (MRS) has proven to be specific method for assessing high-energy phosphates and mitochondrial impairment (4-12), it is not commonly used in clinical practice due to several practical challenges. The chemical exchange saturation transfer (CEST) method (13-18) has presented a novel prospect for the detection of low concentration Cr (19-23) and PCr (19,24-28) in tissues. However, a major challenge in this area is the extraction and quantification of PCr/CrCEST from the crowded in vivo Z-spectrum (29). Recently, a Polynomial and Lorentzian line-shape Fitting (PLOF) approach was proposed to suppress both DS and MTC/NOE and to extract Cr and PCr CEST signals with high specificity in both brain and muscle at high MRI fields (25,26,30,31). Due to its rapid exchange rate, CrCEST coalesces with water at 3T as observed in phantoms (19,24), which poses a significant challenge for the extraction of CrCEST at 3T. The objective of this investigation is to estimate the CrCEST exchange rate in muscle and to evaluate the feasibility of the PLOF CEST method for simultaneous mapping of PCr and Cr in mouse and human skeletal muscle at 3T.

METHODS

Five adult wild-type mice were scanned on a 3T Bruker Biospec system equipped with a 40 mm quadrature volume resonator for both transmission and reception. A Philips Ingenia Elition 3.0 T and a dStream Flex M Coil were used for CEST human experiments, which include five healthy subjects. The FOV was set to 220 × 220 × 70 mm3, and a matrix size of 64 × 64 × 7 was used for image acquisition. Three fast 3D CEST techniques, pulsed-GRASE, ssEPI and pulsed-GRE, were compared.

RESULTS

A comparison of the Z-spectra in mouse hindlimb before and after euthanasia (Fig. 1) indicated that CrCEST is a slow-exchanging process with exchange rate between 45.5 s-1 to 150.7 s-1 in muscle (Fig. 2). This allowed us to simultaneously extract and assign PCr/CrCEST signals at 3T using the PLOF method. We determined optimal B1 values ranging from 0.3-0.6 µT for CrCEST and 0.3-1.2 µT for PCrCEST (Fig. 3). For the comparative study on human calf muscle, pulsed-GRE method showed larger variations across the five studies compared to those of pulsed-GRASE and ssEPI methods (Fig. 4). Mean SNR values for ssEPI, pulsed-GRASE, and pulsed-GRE were 959, 611 and 1460, respectively. While the CEST signal standard deviation σCEST value for pulsed-GRE is much higher compared to ssEPI and pulsed-GRASE methods (p<0.01). Pulsed-GRASE method yields much higher CEST signals than the ssEPI method. Across all seven slices, the mean PCrCEST values were 2±0.09% and 1±0.12% for pulsed-GRASE and ssEPI, respectively, while amideCEST values were 1.1±0.04% for pulsed-GRASE and 0.4±0.06% for ssEPI. CrCEST values were also higher in pulsed-GRASE (0.9±0.08%) than in ssEPI (0.7±0.07%) as shown in Fig. 5.

DISCUSSION

The breakdown of creatine phosphate into creatine and inorganic phosphate, which is catalyzed by creatine phosphatase, occurs after death (1,26,32-35). Therefore, the PCr to Cr conversion during postmortem provides a suitable method for validating PCr and Cr CEST at 3T (28). Due to the slow-exchanging rate of CrCEST, the current study provides a three-peak PLOF method to simultaneously yield high-resolution amide, PCr and Cr maps at 3T. Our study demonstrates that the SNR of the pulsed-GRE method is actually higher than that of ssEPI and pulsed-GRASE on 3T clinical scanner. However, the major issue with pulsed-GRE is its high σCEST, which poses significant difficulty for reliably extracting CEST contrasts (Fig. 4). Therefore, we only performed amide, PCr and Cr CEST mapping with ssEPI and pulsed-GRASE in this study.

CONCLUSION

Our study revealed that CrCEST displayed a noticeable peak at 2.0 ppm in the mouse hindlimb post-euthanasia at 3T, indicating that it is a slow-exchanging process in muscle. Furthermore, we used the proposed PLOF method on human muscle at 3T to obtain high-resolution maps of amide, PCr, and Cr simultaneously. Comparison of several fast 3D CEST approaches indicated that the pulsed-GRASE method provides high PCr/Cr CEST values compared to the ssEPI method, while the noise background in the Z-spectra by ssEPI is much lower than pulsed-GRASE. These findings suggest that PCr/CrCEST has the potential to be a cost-effective and widely available method for measuring PCr/Cr in muscle at 3T.

Acknowledgements

This work was supported by P41EB031771, R01HL149742, R01AG080104, R01HL63030, R01AG063661 and R21AG074978. The authors thank Dr. Abubakr Eldirdiri, Mr. Joseph S. Gillen, Mrs. Terri Lee Brawner, Ms. Kathleen A. Kahl, and Ms. Ivana Kusevic for experimental assistance.

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Figures

Figure 1. Illustration of the experimental design. During the antemortem stage, we collected T1 map and CEST experiments at various B1 values (ranging from 0.3 to 1.6 μT) in live mice. The mice were subsequently euthanized in the scanner using an overdose of isoflurane (5%). To capture changes in T1 map and CEST during the euthanasia process and postmortem stage, interleaved scans were acquired using B1=0.6 μT. Finally, T1 map and multi-B1 CEST experiments (0.3 to 1.6 μT) were collected at the half-hour mark post-euthanasia.

Figure 2. Determination of CrCEST guanidino proton exchange rate in muscle. (A) Lineshape fitting of the average CrCEST (n = 5) signal extracted by the PLOF method at antemortem state. (B) Lineshape fitting of the average ΔZ-spectra (n = 5) at postmortem state. (C) Simulated CrCEST lineshape as a function of exchange rate. MTC parameters used (T2MTC=52.1/59.8 μs and fMTC=26.2%/28.1% for antemortem / postmortem, respectively) were those from the background fitting, while the Cr concentration was fixed at 10 mM and 39 mM for the antemortem and postmortem states, respectively.

Figure 3. Optimization of B1 for PCr/Cr CEST in mouse hindlimb muscle using cwRARE. (A-B) The averaged differential Z-spectrum (n=5), i.e., ΔZ spectrum, in mouse hindlimb for the whole slice before (A) and after (B) euthanasia for B1=0.4, 0.8, 1,2, 1.6 μT, respectively. (C) The averaged PCr CEST signals (n=5) before (green) and after (red) euthanasia in mouse hindlimb as a function of B1. (D) The averaged Cr CEST signals (n=5) before (green) and after (red) euthanasia as a function of B1. Error bars show the standard derivation of five studies. Cr, creatine; PCr, phosphocreatine.

Figure 4. SNR, Z-spectrum noise evaluation and image quality comparison of pulsed-GRASE, ssEPI and pulsed-GRE. (A) Exemplary M0 images demonstrate consistently high image quality. (B) The averaged SNR (n=5) of the center slice in the M0 images for ssEPI, pulsed-GRASE and pulsed-GRE. (C) CEST noise characterizations (σCEST) over the seven slices. CEST noise was evaluated by the standard deviation of the difference values between the Z values and the smoothed Z-spectrum (span=5), i.e., Z-Zsmooth. (D) The averaged σCEST (n=5) for the three methods over the seven slices.

Figure 5. Typical 3D amide, PCr and Cr CEST maps and averaged values on human skeletal muscle. (A, B) Typical 3D amide, PCr, Cr CEST maps by ssEPI (A) and pulsed-GRASE (B), respectively. (C, D) The corresponding averaged amide (blue circle), PCr (green square) and Cr (orange triangle) CEST values (n=5) for each slice by ssEPI (C) and pulsed-GRASE (D), respectively. Error bars represent the standard deviation over five subjects. GRASE, gradient- and spin-echo; EPI, echo planar imaging; Cr, creatine; PCr, phosphocreatine.

Proc. Intl. Soc. Mag. Reson. Med. 32 (2024)
4453
DOI: https://doi.org/10.58530/2024/4453