Enhanced and robust contrast in CEST MRI: Saturation pulse shape design via optimal control

• Published in: Magnetic resonance in medicine Vol. 92; no. 5; pp. 1867 - 1880
• Main Authors: Stilianu, Clemens, Graf, Christina, Huemer, Markus, Diwoky, Clemens, Soellradl, Martin, Rund, Armin, Zaiss, Moritz, Stollberger, Rudolf
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.11.2024
• Subjects: Adiabatic; Algorithms; Artifact identification; B0 robustness; Computer Simulation; Continuous radiation; contrast mechanism; Contrast Media - chemistry; Humans; Image Processing, Computer-Assisted - methods; Inhomogeneity; Magnetic Resonance Imaging - methods; Normal Distribution; Optimal control; Phantoms, Imaging; pulse design; Pulse shape; pulsed CEST; Radio frequency; Radio Waves; Robust control; Shape optimization; Stability; optimal control; pulsed CEST; pulse design; contrast mechanism; B0 robustness
• ISSN: 0740-3194; 1522-2594; 1522-2594
• DOI: 10.1002/mrm.30164

Purpose To employ optimal control for the numerical design of Chemical Exchange Saturation Transfer (CEST) saturation pulses to maximize contrast and stability against B0$$ {\mathrm{B}}_0 $$ inhomogeneities. Theory and Methods We applied an optimal control framework for the design pulse shapes for CEST saturation pulse trains. The cost functional minimized both the pulse energy and the discrepancy between the corresponding CEST spectrum and the target spectrum based on a continuous radiofrequency (RF) pulse. The optimization is subject to hardware limitations. In measurements on a 7 T preclinical scanner, the optimal control pulses were compared to continuous‐wave and Gaussian saturation methods. We conducted a comparison of the optimal control pulses with Gaussian, block pulse trains, and adiabatic spin‐lock pulses. Results The optimal control pulse train demonstrated saturation levels comparable to continuous‐wave saturation and surpassed Gaussian saturation by up to 50 % in phantom measurements. In phantom measurements at 3 T the optimized pulses not only showcased the highest CEST contrast, but also the highest stability against field inhomogeneities. In contrast, block pulse saturation resulted in severe artifacts. Dynamic Bloch‐McConnell simulations were employed to identify the source of these artifacts, and underscore the B0$$ {\mathrm{B}}_0 $$ robustness of the optimized pulses. Conclusion In this work, it was shown that a substantial improvement in pulsed saturation CEST imaging can be achieved by using Optimal Control design principles. It is possible to overcome the sensitivity of saturation to B0 inhomogeneities while achieving CEST contrast close to continuous wave saturation.