A rapid method for phosphocreatine‐weighted imaging in muscle using double saturation power‐chemical exchange saturation transfer

Abstract

Monitoring the variation in phosphocreatine (PCr) levels following exercise provides valuable insights into muscle function. Chemical exchange saturation transfer (CEST) has emerged as a sensitive method with which to measure PCr levels in muscle, surpassing conventional MR spectroscopy. However, existing approaches for quantifying PCr CEST signals rely on time‐consuming fitting methods that require the acquisition of the entire or a section of the CEST Z‐spectrum. Additionally, traditional fitting methods often necessitate clear CEST peaks, which may be challenging to obtain at low magnetic fields. This paper evaluated the application of a new model‐free method using double saturation power (DSP), termed DSP‐CEST, to estimate the PCr CEST signal in muscle. The DSP‐CEST method requires the acquisition of only two or a few CEST signals at the PCr frequency offset with two different saturation powers, enabling rapid dynamic imaging. Additionally, the DSP‐CEST approach inherently eliminates confounding signals, offering enhanced robustness compared with fitting methods. Furthermore, DSP‐CEST does not demand clear CEST peaks, making it suitable for low‐field applications. We evaluated the capability of DSP‐CEST to enhance the specificity of PCr CEST imaging through simulations and experiments on muscle tissue phantoms at 4.7 T. Furthermore, we applied DSP‐CEST to animal leg muscle both before and after euthanasia and observed successful reduction of confounding signals. The DSP‐CEST signal still has contaminations from a residual magnetization transfer (MT) effect and an aromatic nuclear Overhauser enhancement effect, and thus only provides a PCr‐weighted imaging. The residual MT effect can be reduced by a subtraction of DSP‐CEST signals at 2.6 and 5 ppm. Results show that the residual MT‐corrected DSP‐CEST signal at 2.6 ppm has significant variation in postmortem tissues. By contrast, both the CEST signal at 2.6 ppm and a conventional Lorentzian difference analysis of CEST signal at 2.6 ppm demonstrate no significant variation in postmortem tissues.