Biomedical subjects

J Granwehr

Publications and source records attributed to J Granwehr.

6 recordsLinked to original sources

PubMed2006

Sensitivity quantification of remote detection NMR and MRI.

A sensitivity analysis is presented of the remote detection NMR technique, which facilitates the spatial separation of encoding and detection of spin magnetization. Three different cases are considered: remote detection of a transient signal that must be encoded point-by-point like a free induction decay, remote detection of an experiment where the transient dimension is reduced to one data point like phase encoding in an imaging experiment, and time-of-flight (TOF) flow visualization. For all cases, the sensitivity enhancement is proportional to the relative sensitivity between the remote detector and the circuit that is used for encoding. It is shown for the case of an encoded transient signal that the sensitivity does not scale unfavorably with the number of encoded points compared to direct detection. Remote enhancement scales as the square root of the ratio of corresponding relaxation times in the two detection environments. Thus, remote detection especially increases the sensitivity of imaging experiments of porous materials with large susceptibility gradients, which cause a rapid dephasing of transverse spin magnetization. Finally, TOF remote detection, in which the detection volume is smaller than the encoded fluid volume, allows partial images corresponding to different time intervals between encoding and detection to be recorded. These partial images, which contain information about the fluid displacement, can be recorded, in an ideal case, with the same sensitivity as the full image detected in a single step with a larger coil.

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PubMed2005

Time-of-flight flow imaging using NMR remote detection.

A time-of-flight imaging technique is introduced to visualize fluid flow and dispersion through porous media using NMR. As the fluid flows through a sample, the nuclear spin magnetization is modulated by rf pulses and magnetic field gradients to encode the spatial coordinates of the fluid. When the fluid leaves the sample, its magnetization is recorded by a second rf coil. This scheme not only facilitates a time-dependent imaging of fluid flow, it also allows a separate optimization of encoding and detection subsystems to enhance overall sensitivity. The technique is demonstrated by imaging gas flow through a porous rock.

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PubMed2005

NMR detection using laser-polarized xenon as a dipolar sensor.

Hyperpolarized (129)Xe can be used as a sensor to indirectly detect NMR spectra of heteronuclei that are neither covalently bound nor necessarily in direct contact with the Xe atoms, but coupled through long-range intermolecular dipole-dipole interactions. To reintroduce long-range dipolar couplings the sample symmetry has to be broken. This can be done either by using an asymmetric sample arrangement, or by breaking the symmetry of the spin magnetization with field gradient pulses. Experiments are performed where only a small fraction of the available (129)Xe magnetization is used for each point, so that a single batch of xenon suffices for the point-by-point acquisition of a heteronuclear NMR spectrum. Examples with (1)H as the analyte nucleus show that these methods have the potential to obtain spectra with a resolution that is high enough to determine homonuclear J couplings. The applicability of this technique with remote detection is discussed.

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PubMed2004

Hyperpolarized xenon nuclear spins detected by optical atomic magnetometry.

We report the use of an atomic magnetometer based on nonlinear magneto-optical rotation with frequency-modulated light to detect nuclear magnetization of xenon gas. The magnetization of a spin-exchange-polarized xenon sample (1.7 c m(3) at a pressure of 5 bars, natural isotopic abundance, polarization 1% ), prepared remotely to the detection apparatus, is measured with an atomic sensor. An average magnetic field of approximately 10 nG induced by the xenon sample on the 10 cm diameter atomic sensor is detected with signal-to-noise ratio approximately 10 , limited by residual noise in the magnetic environment. The possibility of using modern atomic magnetometers as detectors of nuclear magnetic resonance and in magnetic resonance imaging is discussed. Atomic magnetometers appear to be ideally suited for emerging low-field and remote-detection magnetic resonance applications.

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PubMed2001

Longitudinally detected EPR: improved instrumentation and new pulse schemes.

A new setup for longitudinal detection (LOD) of EPR signals based on a commercial pulse EPR spectrometer equipped with an ENDOR probehead is presented. The design is suited for pulse LOD EPR and amplitude-modulated cw LOD EPR experiments. The sensitivity is substantially increased compared with earlier designs. Two new pulse schemes that take full advantage of the special properties of the setup are invented. In transient-nutation longitudinally detected EPR (TN-LOD EPR), the nutation of magnetization during a microwave pulse is used to measure the EPR signal. In pulse-train excited longitudinally detected EPR (PT-LOD EPR), a train of microwave pulses that periodically inverts the magnetization is applied. First experimental results on radicals and metal complexes at room temperature are presented.

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PubMed1998

Two-dimensional nuclear-Zeeman-resolved electron spin echo envelope modulation (NZ-ESEEM) spectroscopy.

A two-dimensional electron spin echo envelope modulation (ESEEM) experiment, called nuclear-Zeeman-resolved ESEEM (NZ-ESEEM), that correlates nuclear transition frequencies with nuclear Zeeman frequencies is introduced. NZ-ESEEM is basically a three-pulse ESEEM experiment complemented by a magnetic-field pulse applied during part of the free evolution period between the second and third microwave pulse. The inner working of the new approach is explained and the instrumentation is discussed. The capacity of the method is illustrated by two examples of applications.

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