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Positive Contrast Chemical Exchange Imaging using RACETE1Comprehensive Heart Failure Center, University Hospital Wuerzburg, Wuerzburg, Germany, 2Experimental Physics 5, University Wuerzburg, Wuerzburg, Germany
Synopsis
The RACETE sequence is a novel method for imaging chemical exchange. As it refocuses the magnetization transferred while stored in the longitudinal state, it yields a positive contrast. By creating a refocused sequence other properties of standard MR sequences, such as excitation pulse phase variation, can be exploited.
Introduction
Chemical exchange is a rich source for new contrasts in MRI. As chemical exchange is a stochastic process usually excitation cannot be transferred. Chemical exchange is therefor measured using Chemical Exchange dependent Saturation Transfer1. However with the RACETE2 sequence a new method that can generate echos from exchanged magnetization was introduced recently. By generating a positive refocused signal, phase information can also be exploited.Methods
The RACETE method is based on the stimulated echo sequence. It exploits the fact that magnetization is stored in the longitudinal direction by the first two pulses of a simulated echo sequence. If the exchange occurs in this state, the phase information will be retained even over the exchange process. Experiments using different phantoms with exchangeable pools (iopamidol and salicylic acid) have been conducted to demonstrate the ability to transfer and utilize excitation phase transfer. Furthermore the methods sensitivity to magnetization transfer contrast (MTC) is examined using a phantom with cross-linked proteins.Results
Figure 1 (top) shows a drawing of a phantom containing two different substances with exchangeable protons at different chemical shifts and the corresponding spin echo and RACETE magnitude image. In the bottom row three RACETE phase maps are shown for measurements with different excitation pulse phases. Both substances show the phase imprinted by the excitation pulses.
For examination of MTC effects on the RACETE sequence a phantom containing cross linked proteins was used. The RACETE sequence yields no signal in a pure protein sample. A tube containing both the protein as well as an exchangeable agent shows reduced signal in comparison to a tube containing the same concentration of the chemical exchange agent without proteins.
Discussion
RACETE is able to transfer the excitation phase. This can be exploited to reduce artifacts or discern different substances in a single measurement. Further investigation into the sensitivity of the phase contrast is underway. The RACETE-sequence inherently filters out any positive contribution from pure MTC samples, as the MTC exchange is too fast in comparison to the frequency selective pulses used to store the magnetization in the longitudinal direction. In samples with chemical exchange and MTC the signal is reduced, as the pool storing the transferred excitation is decreased by MTC effects.Conclusion
It could be shown that the RACETE sequence generates a positive chemical exchange contrast, while not showing any sensitivity to the sample exhibiting only MTC. The phase is retained over the exchange process.Acknowledgements
This work was supported by grants from the Deutsche Forschungsgemeinschaft (SFB 688 Z2) and the Bundesministerium für Bildung und Forschung (BMBF Projekt 01EO1504)References
Ward, K. M., Aletras, A. ., & Balaban, R. A New Class of Contrast Agents for MRI Based on Proton Chemical Exchange Dependent Saturation Transfer (CEST). JMR, 143(1), 79–87. 2000.
Gutjahr, F. T., Munz, E., Jakob, P. M. Positive chemical exchange contrast in MRI using Refocused Acquisition of Chemical Exchange Transferred Excitations (RACETE). ZMedPhys. 2018, Early Access.
Figures
From left to right, top row: Schematic drawing of the phantom (concentration in mMol/l), spin echo image, RACETE magnitude image. Bottom row: RACETE phase maps, showing the different substances following the phase imprinted to the different substances by the excitation pulses.