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TH‐D‐201C‐01: Advancing an Integrated Overhauser‐Enhanced MRI (OMRI) ‐ Prepolarized MM (PMRI) System Toward Quantitative Longitudinal Studies of Tumor Hypoxia and Redox Status
Author(s) -
Ahn K,
Scott G,
Stang P,
Conolly S,
Hristov D
Publication year - 2010
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3469549
Subject(s) - electron paramagnetic resonance , nuclear magnetic resonance , imaging phantom , magnetic resonance imaging , chemistry , materials science , analytical chemistry (journal) , nuclear medicine , physics , medicine , chromatography , radiology
Purpose : To advance the imaging performance of an integrated Overhauser‐enhanced MRI (OMRI) ‐ prepolarized MRI (PMRI) system to enable quantitative longitudinal imaging studies of multi‐faceted tumor environment by using both electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) Method and Materials : A field‐cycled OMRI‐PMRI system was further developed to achieve the sensitivity that identifies radiobiological hypoxia and redox status. A dedicated 5‐cm saddle coil delivered 154‐MHz EPR radiofrequency (RF) pulses to induce the Overhauser effect with a high EPR B 1 efficiency. A 3‐cm 5.5‐MHz NMR Litz‐wire saddle coil concentric to the EPR coil achieved high signal‐to‐noise ratio with an efficient filling factor. B 0 was at 5 mT 0.13 T 0.5 T for EPR irradiation NMR readout NMR prepolarization respectively. Gradient echo and multi‐spin echo pulse sequences were implemented using a custom MRI console to acquire images with minimal phase distortion. Trityl phantoms were prepared under normoxic and anoxic environment for pO 2 calibration. Various amounts of ascorbic acid (AsA) were injected to the mixtures of trityl and nitroxide (3‐carbamoyl PROXYL) phantoms to characterize the redox sensitivity. Results : Oxygen resolution of 4.1 torr and 3.5 torr were obtained from 4‐min double power (0.3 32 W) spin‐echo OMRI (TR/TE 1600/30 ms) for pure deoxygenated 1‐mM and 2‐mM trityl phantoms. Trityl radicals were not reduced by AsA and did not alter the reduction decay rate of the nitroxides (−0.07/min −0.13/min for 5 10‐mM AsA). Saturation factor measurements at various EPR RF power levels indicated a feasibility of accurate pO 2 calibration for the mixtures of trityl and nitroxide radicals. Conclusion : Our OMRI‐PMRI system is capable of multi‐parametric imaging sensitive to pO 2 redox status proton T1 and T2. The imager is ready to acquire physiological information in small animals accurately co‐registered with diagnostic quality anatomic NMR images.