Biomedical subjects

M P Ledbetter

Publications and source records attributed to M P Ledbetter.

5 recordsLinked to original sources

PubMed2005

Nonlinear amplification of small spin precession using long-range dipolar interactions.

In measurements of small signals using spin precession the precession angle usually grows linearly in time. We show that a dynamic instability caused by spin interactions can lead to an exponentially growing spin-precession angle, amplifying small signals and raising them above the noise level of a detection system. We demonstrate amplification by a factor of greater than 8 of a spin-precession signal due to a small magnetic field gradient in a spherical cell filled with hyperpolarized liquid 129Xe. This technique can improve the sensitivity in many measurements that are limited by the noise of the detection system, rather than the fundamental spin-projection noise.

Journal Article↗

PubMed2004

Numerical and experimental studies of long-range magnetic dipolar interactions.

We describe several numerical methods developed to analyze the behavior of spin polarized liquids in the presence of long-range magnetic dipolar interactions and external field gradients. Two of the methods use a discrete lattice of spins. In the first we calculate the magnetic field from the lattice of spins directly, either in the rotating frame, or in the lab frame. In the second method we include the dipolar fields from linear magnetization gradients analytically and calculate the dipolar fields from higher order gradients in Fourier space, where they are a local function of the magnetization. In the third method the magnetization is expanded in a Taylor series and the dipolar fields are calculated analytically for each term. The results of these calculations are compared to experimental data, in which we use two superconducting quantum interference device magnetometers adjacent to a spherical sample of hyperpolarized liquid 129Xe to detect the evolution of magnetization gradients. In particular, we observe an increase by a factor of 100 of the spin dephasing time in a longitudinal magnetic field gradient due to dipolar interactions of the spins. While each of the numerical techniques has certain limitations, they are generally in agreement with each other and with experimental data.

Anisotropy↗

PubMed2003

Enhancement of SQUID-detected NMR signals with hyperpolarized liquid 129Xe in a 1 microT magnetic field.

We report an enhancement of proton NMR signals by a factor of 10(6) by cross polarization with hyperpolarized liquid 129Xe in an ultralow magnetic field of 1 microT. The NMR signals from cyclopentane, acetone, and methanol are detected using a commercial high-T(c) SQUID magnetometer with a signal-to-noise ratio of up to 1000 from a single 90 degrees tipping pulse. This technique allows a wide range of low-field NMR measurements and is promising for the detection of intermolecular scalar spin-spin couplings. Scalar intermolecular couplings can produce a shift of the average NMR frequency in a hyperpolarized sample even in the presence of rapid chemical exchange.

Journal Article↗

PubMed2002

Nonlinear effects from dipolar interactions in hyperpolarized liquid 129Xe.

We investigate nonlinear effects of long-range dipolar interactions in a spherical sample of hyperpolarized liquid 129Xe. Using two high-T(c) SQUID detectors we directly measure the evolution of the magnetization gradients. For small initial rf tip angles we observe an increase in the transverse relaxation time T2* by a factor of 5 and coherent oscillations of magnetization gradients. For large tip angles we observe an exponential growth of the magnetization gradients and demonstrate a gain in sensitivity to magnetic field gradients by a factor of 10. Our results are in quantitative agreement with simple analytical predictions. We discuss applications of these nonlinear effects for precision measurements.

Journal Article↗

PubMed2001

Transverse spin relaxation in liquid 129Xe in the presence of large dipolar fields.

Using spin-echo NMR techniques we study the transverse spin relaxation of hyperpolarized liquid 129Xe in a spherical cell. We observe an instability of the transverse magnetization due to dipolar fields produced by liquid 129Xe, and find that imperfections in the pi pulses of the spin-echo sequence suppress this instability. A simple perturbative model of this effect is in good agreement with the data. We obtain a transverse spin relaxation time of 1300 sec in liquid 129Xe, and discuss applications of hyperpolarized liquid 129Xe as a sensitive magnetic gradiometer and for a permanent electric dipole moment search.

Journal Article↗