Investigation of the contribution of total creatine to the CEST Z‐spectrum of brain using a knockout mouse model

Abstract

The current study aims to assign and estimate the total creatine (tCr) signal contribution to the Z-spectrum in mouse brain at 11.7 Tesla. Creatine (Cr), phosphocreatine (PCr) and protein phantoms were used to confirm presence of a guanidinium resonance at this field strength. Wild type (WT) and knockout mice with Guanidinoacetate N-Methyltransferase deficiency (GAMT−/−) that have low Cr and PCr concentrations in the brain were used to assign the tCr contribution to the Z-spectrum. To estimate the total guanidinium concentrations, two pools for the Z-spectrum around 2 ppm were assumed: (i) a Lorentzian function representing the guanidinium CEST at 1.95 ppm in the 11.7 T Z-spectrum; (ii) a background signal that can be fitted by a polynomial function. Comparison between the WT and GAMT−/− mice provided strong evidence for three types of contributions to the peak in the Z-spectrum at 1.95 ppm, namely proteins, Cr and PCr, the latter fitted as tCr. A ratio of 20±7% (Protein) and 80±7% tCr was found in brain with 2 μT and 2 s saturation. Based on phantom experiments, the tCr peak was estimated to consist of about 83±5% Cr and 17±5% PCr. Maps for tCr of mouse brain were generated based on the peak at 1.95 ppm after concentration calibration with in vivo MRS.

High‐resolution creatine mapping of mouse brain at 11.7 T using non‐steady‐state chemical exchange saturation transfer

“…The tCr concentration in the thalamus was found to be 10.0 ± 0.2mM ( n = 3) by in vivo MRS, which leads to a Cr concentration of 5.0 ± 0.1mM, while was determined to be 0.11 ± 0.01 s −1 with a saturation power of 2 μT and a saturation length of 1 s. Hence, r Cr = 0.018 s −1 mM −1 was derived according to Equation by assuming λ = 0.8, in which λ is a ratio that defines Cr and protein guanidium CEST signal at 1.95 ppm and was determined in our previous study . This was then used for scaling the Cr concentration maps for the brain.…”

“…The T 1 map is needed for Cr quantification with the PLOF method. The Cr concentration in mouse brain was determined from in vivo proton MRS experiments, which were performed on a voxel of 2 × 2 × 2 mm 3 using a stimulated echo acquisition mode (STEAM) sequence ( T E = 3 ms, T M = 10 ms, T R = 2.5 s, number of average (NA) = 256) …”

“…Δ Z 1 is the typical CEST buildup curve when a single CEST pool is considered, which has been well studied previously. As seen from Equation , Δ Z 1 is scaled down by a factor of Z ss2 , as demonstrated in the previous studies . Δ Z 2 is usually neglected in the CEST studies since it is zero under steady‐state saturation.…”

“…Recently, a combined polynomial and Lorentzian line-shape fitting (PLOF) that removes broad CEST background signals was proposed to extract and quantify Cr and phosphocreatine (PCr). 33,34 A similar strategy was also implemented by a different group for PCr mapping with CEST. 35 The successful application of PLOF requires sufficient signal-to-noise ratio (SNR) and image stability as do other CEST quantification methods.…”

Chemical exchange saturation transfer imaging of phosphocreatine in the muscle

“…Thus, a number of groups have held interest in imaging creatine (Cr) using CEST methods, as creatine has labile protons in its guanidyl group. However, a portion of this CEST signal will come from the guanidyl groups of mobile proteins . Conversely, targeting PCr in this phosphorylation process may be crucial for obtaining more specific information about this metabolic process.…”