Browsing by Subject "Magnetic resonance"

In magnetic resonance microscopy, insufficient signal-to-noise ratio currently limits imaging performance. Superconducting probes can potentially increase the sensitivity of the magnetic resonance experiment. However, many superconducting probes failed to entirely deliver the expected increase in signal-to-noise ratio.

We present a method based on finite-element radiofrequency simulations. The radiofrequency model computes several figures of merit of a probe, namely: i) the resonant frequency, ii) the impedance, iii) the magnetic field homogeneity, iv) the filling factor, and v) the sensitivity. The probe is constituted by several components. The method calculates the electromagnetic losses induced by every component within the probe, and identifies the component limiting the sensitivity of the probe. Subsequently, the probe design can be improved iteratively.

We show that the sensitivity of an existing superconducting Helmholtz pair can be improved by increasing the filling factor of the probe and cooling the radiofrequency shield, which was implemented in the design of a new superconducting probe. The second probe exhibits a sensitivity three times as high, leading to improved imaging performance.

Magnetic resonance techniques are among the most powerful methods for characterization. However, they inherently suffer from an intrinsically low signal-to-noise due to the weak interaction of the nuclear spin with external magnetic fields. Hyperpolarization methods circumvent this limitation by deriving non-equilibrium spin polarization from an external source of spin order, dramatically increasing the magnetic resonance signals. Signal Amplification By Reversible Exchange, or SABRE, is a relatively new and promising method that derives spin hyperpolarization from parahydrogen, the singlet spin isomer of dihydrogen, allowing it to operate at a fraction of the cost of other hyperpolarization methods. A target molecule and parahydrogen transiently bind an organometallic complex, during which time polarization is transferred from the parahydrogen to target nuclei. The reversible nature of this interaction makes the hyperpolarization method readily scalable, giving SABRE the potential to supplant older, more expensive techniques and bring hyperpolarization technology to a broader audience. However, the current demonstrations of SABRE generate polarizations that are about an order of magnitude away from the upper theoretical limits of the technique, and variants of this experiment have been limited in target scope by the underlying physics. To address these limitations, this dissertation returns to examine the theoretical underpinnings of SABRE, and in doing so re-interrogates the unification of chemical exchange and quantum dynamics. We derive exact formulations of the chemical exchange interaction in both the magnetic resonance limits, which is used to construct a physically exhaustive computational model for SABRE. This model then facilitates in silico exploration of the system and permits us to address experimental limitations of the method. In particular, this dissertation utilizes simulations to develop and expand the capabilities of SABRE performed at arbitrarily high magnetic fields. We show that the limitations in the scope of SABRE imposed by the spin physics under these conditions may be removed, culminating in the first demonstration of simultaneous hyperpolarization of multiple components. This expansion is a key step towards translating SABRE into areas of conventional magnetic resonance such as biomolecular NMR and metabonomics. The theoretical framework presented here provides access to new routes for optimizing SABRE hyperpolarization, and we demonstrate that sequences may be developed to generate up to a five-fold increase in performance. Finally, we extend the theoretical treatment of chemical exchange within the Lindblad formalism to obtain exact master equations that are valid in any physical limit.