ABSTRACT
Cost-effective and portable MRI systems operating at Earth-field would be helpful in poorly accessible areas or in developing nations. Furthermore Earth-field MRI can provide new contrasts opening the way to the observation of pathologies at the biochemical level. However low-field MRI suffers from a dramatic lack in detection sensitivity even worsened for molecular imaging purposes where biochemical specificity requires detection of dilute compounds. In a preliminary spectroscopic approach, it is proposed here to detect protease-driven hydrolysis of a nitroxide probe thanks to electron-nucleus Overhauser enhancement in a home-made double resonance system in Earth-field. The combination of the Overhauser effect and the specific enzymatic modification of the probe provides a smart contrast reporting the enzymatic activity. The nitroxide probe is a six-line nitroxide which lines are shifted according to its substrate/product state, which requires quantum mechanical calculations to predict EPR line frequencies and Overhauser enhancements at Earth field. The NMR system is equipped with a 13-mT prepolarization coil, a 153-MHz EPR coil and a 2-kHz NMR coil. Either prepolarized NMR or DNP-NMR without prepolarization provide NMR spectra within 3 min. The frequency dependence of Overhauser enhancement was in agreement with theoretical calculations. Protease-mediated catalysis of the nitroxide probe could only be measured through the Overhauser effect with 5 min time resolution. Future developments shall open the way for the design of new low-field DNP-MRI systems.
ABSTRACT
An inversion recovery (IR) artifact was used to delineate the blood/wall boundary in left ventricles. The artifact consisted of a hypointensity signal in pixels located at the boundary of two contiguous tissues with different T(1) relaxation times. The feasibility of measuring the ejection fraction using the artifact was tested in ten healthy volunteers, with two IR snapshot-FLASH sequences possessing different times of repetition (TR = 11msec and TR = 3.5msec) and appropriate times of inversion. The comparison with a cine-MRI sequence showed that ejection fraction measurements are feasible when performed with a snapshot-FLASH sequence that has a sufficiently short TR (3.5msec).
