Showing papers by "Steffen J. Glaser published in 2013"

Time-optimal control of SU(2) quantum operations

Abstract: We propose an analysis of the time-optimal control of SU(2) quantum operations. By using the Pontryagin maximum principle, we show how to determine the optimal trajectory for reaching a given target state. Explicit analytical solutions are given for two specific examples. We discuss the role of the detuning in the construction of the optimal synthesis.

Saturation-recovery metabolic-exchange rate imaging with hyperpolarized [1-13C] pyruvate using spectral-spatial excitation.

Abstract: Within the last decade hyperpolarized [1-13C] pyruvate chemical-shift imaging has demonstrated impressive potential for metabolic MR imaging for a wide range of applications in oncology, cardiology, and neurology. In this work, a highly efficient pulse sequence is described for time-resolved, multislice chemical shift imaging of the injected substrate and obtained downstream metabolites. Using spectral-spatial excitation in combination with single-shot spiral data acquisition, the overall encoding is evenly distributed between excitation and signal reception, allowing the encoding of one full two-dimensional metabolite image per excitation. The signal-to-noise ratio can be flexibly adjusted and optimized using lower flip angles for the pyruvate substrate and larger ones for the downstream metabolites. Selectively adjusting the excitation of the down-stream metabolites to 90° leads to a so-called "saturation-recovery" scheme with the detected signal content being determined by forward conversion of the available pyruvate. In case of repetitive excitations, the polarization is preserved using smaller flip angles for pyruvate. Metabolic exchange rates are determined spatially resolved from the metabolite images using a simplified two-site exchange model. This novel contrast is an important step toward more quantitative metabolic imaging. Goal of this work was to derive, analyze, and implement this "saturation-recovery metabolic exchange rate imaging" and demonstrate its capabilities in four rats bearing subcutaneous tumors.

Diffusion of hyperpolarized 13C-metabolites in tumor cell spheroids using real-time NMR spectroscopy

Abstract: The detection of tumors noninvasively, the characterization of their progression by defined markers and the monitoring of response to treatment are goals of medical imaging techniques. In this article, a method which measures the apparent diffusion coefficients (ADCs) of metabolites using hyperpolarized 13C diffusion-weighted spectroscopy is presented. A pulse sequence based on the pulsed gradient spin echo (PGSE) was developed that encodes both kinetics and diffusion information. In experiments with MCF-7 human breast cancer cells, we detected an ADC of intracellularly produced lactate of 1.06 ± 0.15 µm2/ms, which is about one-half of the value measured with pyruvate in extracellular culture medium. When monitoring tumor cell spheroids during progressive membrane permeabilization with Triton X-100, the ratio of lactate ADC to pyruvate ADC increases as the fraction of dead cells increases. Therefore, 13C ADC detection can yield sensitive information on changes in membrane permeability and subsequent cell death. Our results suggest that both metabolic label exchange and 13C ADCs can be acquired simultaneously, and may potentially serve as noninvasive biomarkers for pathological changes in tumor cells. Copyright © 2012 John Wiley & Sons, Ltd.

Chemical Shift Correlations from Hyperpolarized NMR Using a Single SHOT

Abstract: A significant challenge in realizing the promise of the dissolution dynamic nuclear polarization technique for signal enhancement in high-resolution NMR lies in the nonrenewability of the hyperpolarized spin state. This property prevents the application of traditional two-dimensional correlation spectroscopy, which relies on regeneration of spin polarization before each successive increment of the indirect dimension. Since correlation spectroscopy is one of the most important approaches for the identification and structural characterization of molecules by NMR, it is important to find easily applicable methods that circumvent this problem. Here, we introduce the application of scaling of heteronuclear couplings by optimal tracking (SHOT) to achieve this goal. SHOT decoupling pulses have been numerically optimized on the basis of optimal control algorithms to obtain chemical shift correlations in C–H groups, either by acquiring a single one-dimensional 13C spectrum with 1H off-resonance decoupling or vice ...

Phase cycling with a 240 GHz, free electron laser-powered electron paramagnetic resonance spectrometer

Abstract: Electron paramagnetic resonance (EPR) powered by a free electron laser (FEL) has been shown to dramatically expand the capabilities of EPR at frequencies above ∼100 GHz, where other high-power sources are unavailable. High-power pulses are necessary to achieve fast (<10 ns) spin rotations in order to alleviate the limited excitation bandwidth and time resolution that typically hamper pulsed EPR at these high frequencies. While at these frequencies, an FEL is the only source that provides ∼1 kW of power and can be tuned continuously up to frequencies above 1 THz, it has only recently been implemented for one- and two-pulse EPR, and the capabilities of the FEL as an EPR source are still being expanded. This manuscript presents phase cycling of two pulses in an FEL-EPR spectrometer operating at 240 GHz. Given that the FEL, unlike amplifiers, cannot be easily phase-locked to a reference source, we instead apply retrospective data processing to measure the relative phase of each FEL pulse in order to correct the signal phase accordingly. This allows the measured signal to be averaged coherently, and the randomly changing phase of the FEL pulse results in a stochastic phase cycle, which, in the limit of many pulses, efficiently cancels artifacts and improves sensitivity. Further, the relative phase between the first and second pulse, which originates from the difference in path length traversed by each pulse, can be experimentally measured without phase-sensitive detection. We show that the relative phase of the two pulses can be precisely tuned, as well as distinctly switched by a fixed amount, with the insertion of a dielectric material into the quasi-optical path of one of the pulses. Taken together, these techniques offer many of the advantages of arbitrary phase control, and allow application of phase cycling to dramatically enhance signal quality in pulsed EPR experiments utilizing high-power sources that cannot be phase-locked.

Time-optimal control of spin-1/2 particles with dissipative and generalized radiation-damping effects

Abstract: We analyze the time-optimal control of spin-1/2 particles with bounded field amplitudes in the presence of dissipative and radiation damping effects. Using tools of geometric optimal control theory, we determine different optimal syntheses for specific values of the system parameters. We show the nontrivial role of the effective radiation damping effect on the optimal control law.

On the application of geometric optimal control theory to Nuclear Magnetic Resonance

Abstract: We present some applications of geometric optimal control theory to control problems in Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI). Using the Pontryagin Maximum Principle (PMP), the optimal trajectories are found as solutions of a pseudo-Hamiltonian system. This computation can be completed by second-order optimality conditions based on the concept of conjugate points. After a brief physical introduction to NMR, this approach is applied to analyze two relevant optimal control issues in NMR and MRI: the control of a spin 1/2 particle in presence of radiation damping effect and the maximization of the contrast in MRI. The theoretical analysis is completed by numerical computations. This work has been made possible by the central and essential role of B. Bonnard, who has been at the heart of this project since 2009.

pH-biosensors based on compounds with pH-sensitive enolic groups for magnetic resonance imaging and spectroscopy and their uses

Abstract: The present invention relates to the use of compounds with at least one pH-sensitive chemical shift for determining pH and/or measuring pH changes in magnetic resonance. More specifically, the present invention is related to compounds with at least one pH-sensitive chemical shift, such compound being selected from pyruvic acid and its metabolites, compounds produced from pyruvic acid after interaction with acid, and compounds comprising at least one enolic group whose pK a value is lowered through effects of at least one neighboring group into a physiological and/or pathological pH-range, and wherein the compound exhibits at least one pH-sensitive chemical shift in an NMR spectrum. The present invention further relates to biosensors comprising at least one of the compounds. The present invention is furthermore related to in vitro and in vivo methods for determining pH and/or measuring pH changes using the compounds or biosensors. The present invention also relates to methods of diagnosing and/or monitoring treatment of a disease causing changes in pH wherein the compounds or biosensors are applied. The present invention also relates to use of the compounds or biosensors in quality control of food or in the examination of plants and organisms.