title: Simulation and optimization of pulsed radio frequency irradiation scheme for chemical exchange saturation transfer (CEST) MRI-demonstration of pH-weighted pulsed-amide proton CEST MRI in an animal model of acute cerebral ischemia.
abstract: Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) is capable of measuring dilute labile protons and microenvironmental properties. However, the CEST contrast is dependent upon experimental conditions-particularly, the radiofrequency (RF) irradiation scheme. Although continuous-wave RF irradiation has been used conventionally, the limited RF pulse duration or duty cycle of most clinical systems requires the use of pulsed RF irradiation. Here, the conventional numerical simulation is extended to describe pulsed-CEST MRI contrast as a function of RF pulse parameters (i.e., RF pulse duration and flip angle) and labile proton properties (i.e., exchange rate and chemical shift). For diamagnetic CEST agents undergoing slow or intermediate chemical exchange, simulation shows a linear regression relationship between the optimal mean RF power of pulsed-CEST MRI and continuous-wave-CEST MRI. The optimized pulsed-CEST contrast is approximately equal to that of continuous-wave-CEST MRI for exchange rates less than 50 s(-1), as confirmed experimentally using a multicompartment pH phantom. In the acute stroke animals, we showed that pulsed- and continuous-wave-amide proton CEST MRI demonstrated similar contrast. In summary, our study elucidated the RF irradiation dependence of pulsed-CEST MRI contrast, providing useful insights to guide its experimental optimization and quantification.
authors: Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts 02129, USA. pzhesun@nmr.mgh.harvard.edu
authors: Phillip Zhe
authors: PZ
authors: Sun
authors: Enfeng
authors: E
authors: Wang
authors: Jerry S
authors: JS
authors: Cheung
authors: Xiaoan
authors: X
authors: Zhang
authors: Thomas
authors: T
authors: Benner
authors: A Gregory
authors: AG
authors: Sorensen
mesh_terms: D000465:Algorithms; D000577:Amides / Q000032:analysis*; D000818:Animals; D001157:Arterial Occlusive Diseases / Q000175:diagnosis*; D015415:Biomarkers / Q000032:analysis; D002539:Cerebral Arterial Diseases / Q000175:diagnosis*; D003198:Computer Simulation; D004195:Disease Models, Animal; D006863:Hydrogen-Ion Concentration; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted / Q000379:methods; D016018:Least-Squares Analysis; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D019047:Phantoms, Imaging; D011522:Protons; D051381:Rats; D017208:Rats, Wistar
chemical_list: D000577:Amides; D015415:Biomarkers; D011522:Protons