Determination of Chemical Exchange Rate in Chemical Exchange Saturation Transfer (CEST) Phenomenon in Magnetic Resonance Imaging through Analytical Solution of Bloch-McConnell Equations

Magnetic Resonance Imaging (MRI), by enabling the non-invasive measurement of certain physiological markers, facilitates the study and tracking of molecular states, ultimately leading to the early diagnosis of diseases. Chemical Exchange Saturation Transfer (CEST) serves as a novel contrast mechanism in MRI for molecular studies. This contrast depends on multiple parameters, including relaxation times, the chemical exchange rate between water molecules and the contrast agent, the concentration of the contrast agent, and the properties of the applied radiofrequency (RF) pulse. The chemical exchange rate is a crucial parameter, as it correlates with various clinical indicators such as pH, temperature, and metabolite concentration. However, its direct measurement remains challenging. In this study, a method for determining this rate using RF pulse width is proposed. First, the CEST effect is expressed in an analytical relationship, demonstrating that this contrast reaches its maximum at a specific RF pulse width. This analytical expression distinguishes the CEST effect from the magnetic transfer effects caused by macromolecules in biological tissues, which act as confounding factors. Assuming a known contrast concentration, the chemical exchange rate can be determined through an analytical relationship based on the RF pulse width that maximizes the CEST effect. The validity of this analytical relationship is confirmed by comparison with widely accepted definitions of the CEST effect. Furthermore, by applying a Taylor series expansion to the analytical expression, relevant formulas from reputable publications are derived. The study employs validated data from a three-pool structure, which is consistent with biological tissue models referenced in authoritative sources. It is recommended that the findings of this study be practically implemented on MRI scanners.

کلیدواژه ها:

Bloch-McConnell equations ، Chemical Exchange Saturation Transfer (CEST) ، chemical exchange rate ، Magnetization Transfer ، Electromagnetic pulse ، Z-Spectrum