Development of a Cest MRI 3T Sequence for pH-Sensitive Imaging
Abstract
Purpose
To develop a novel CEST-based MRI technique at 3T for robust Z-spectrum imaging enabling quantitative pH sensitivity in a brain tissue-like phantom.
Methods
A 3D Spoiled Gradient Recalled Echo (SPGR) sequence was extended with a Magnetization Transfer (MT) preparation module. The module consisted of an off-resonance Gaussian-shaped RF saturation pulse, followed by a crusher gradient, applied once prior to each SPGR excitation. Z-spectrum data was acquired using logarithmically spaced frequency offsets spanning –7 to +7 ppm (-894 Hz to 894 Hz), referenced to the water resonance at 0 ppm. Each 3D volume corresponded to a single offset frequency. Imaging was performed on a 3T scanner using a 48-channel head coil. Acquisition parameters included TR = 15.9 ms and TE = 1.3 ms. A pH-sensitive phantom with three compartments composed of egg whites, water, and vinegar, spanning physiologically relevant pH values, was constructed to evaluate sequence performance. Z-spectra were generated by extracting normalized signal intensities (Ssat/S0) from regions of interest and plotted as a function of saturation offset to assess pH-dependent contrast behaviour.
Results
The acquired Z-spectra demonstrated clear and reproducible modulation of the amide proton transfer effect at approximately 3.5 ppm as a function of pH for pH values of 7.2, 6.8, and 6.4. Increased chemical exchange was observed in more basic pH compartments, with reduced chemical exchange at more acidic pH values. The spectra exhibited a well-defined amide dip with minimal off-resonance contamination beyond ±5 ppm, indicating effective frequency selectivity and stable saturation performance across the sampled offset range.
Conclusion
The proposed SPGR CEST Z-spectrum pulse sequence enables reliable detection of pH-dependent contrast and demonstrates robust frequency-selective saturation across k-space. This sequence provides a flexible framework for quantitative pH-sensitive MRI and establishes a foundation for future optimization of acquisition efficiency in studies of tissue acidosis.