title: Magnetic resonance imaging of the Amine-Proton EXchange (APEX) dependent contrast.
abstract: Chemical exchange between water and labile protons from amino-acids, proteins and other molecules can be exploited to provide tissue contrast with magnetic resonance imaging (MRI) techniques. Using an off-resonance Spin-Locking (SL) scheme for signal preparation is advantageous because the image contrast can be tuned to specific exchange rates by adjusting SL pulse parameters. While the amide-proton transfer (APT) contrast is obtained optimally with steady-state preparation, using a low power and long irradiation pulse, image contrast from the faster amine-water proton exchange (APEX) is optimized in the transient state with a higher power and a shorter SL pulse. Our phantom experiments show that the APEX contrast is sensitive to protein and amino acid concentration, as well as pH. In vivo 9.4-T SL MRI data of rat brains with irradiation parameters optimized to slow exchange rates have a sharp peak at 3.5 ppm and also broad peak at -2 to -5 ppm, inducing negative contrast in APT-weighted images, while the APEX image has large positive signal resulting from a weighted summation of many different amine-groups. Brain ischemia induced by cardiac arrest decreases pure APT signal from ~1.7% to ~0%, and increases the APEX signal from ~8% to ~16%. In the middle cerebral artery occlusion (MCAO) model, the APEX signal shows different spatial and temporal patterns with large inter-animal variations compared to APT and water diffusion maps. Because of the similarity between the chemical exchange saturation transfer (CEST) and SL techniques, APEX contrast can also be obtained by a CEST approach using similar irradiation parameters. APEX may provide useful information for many diseases involving a change in levels of proteins, peptides, amino-acids, or pH, and may serve as a sensitive neuroimaging biomarker.
authors.affiliation: Neuroimaging Laboratory, Department of Radiology, University of Pittsburgh, Pittsburgh, PA 15203, USA. taj6@pitt.edu
authors.forename: Tao
authors.initials: T
authors.lastname: Jin
authors.forename: Ping
authors.initials: P
authors.lastname: Wang
authors.forename: Xiaopeng
authors.initials: X
authors.lastname: Zong
authors.forename: Seong-Gi
authors.initials: SG
authors.lastname: Kim
mesh_terms: D000588:Amines / Q000378:metabolism*; D000818:Animals; D002545:Brain Ischemia / Q000378:metabolism* / Q000473:pathology; D006323:Heart Arrest / Q000378:metabolism* / Q000473:pathology; D007089:Image Enhancement / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D057054:Molecular Imaging / Q000379:methods*; D011522:Protons; D051381:Rats; D017207:Rats, Sprague-Dawley; D014018:Tissue Distribution
chemical_list: D000588:Amines; D011522:Protons