Chaohui Ye

Radiation damping effects on spin—lattice relaxation time measurements

Abstract Radiation damping effects in NMR spin—lattice relaxation experiments have been studied. After an inversion, a damped spin system recovers to the equilibrium state under the influence of radiation damping rather than through a relaxation process. Therefore, the spin—lattice relaxation time cannot be measured by inversion-recovery techniques. Precise measurements of the relaxation times in the presence of strong radiation damping can be performed by a saturation-recovery technique, but the data must be fitted by integral areas instead of by intensities.

Line shapes of strongly radiation‐damped nuclear magnetic resonance signals

Line shapes of strongly radiation‐damped nuclear magnetic resonance (NMR) signals are theoretically analyzed. Analytical expressions for the radiation‐damped signals in the frequency domain are reported for the first time. It is shown that the line shapes are generally dependent on the flip angle θ0 of the excitation pulse. In the range of 0≤θ0<π, the peak height increases linearly, but the linewidth decreases monotonically with the flip angle. An oscillating feature is predicted for the line shape when θ0≳π/2. The theoretical predictions are confirmed by the experimental results.

Measurement of 13C chemical shift tensor principal values with a magic-angle turning experiment

The magic-angle turning (MAT) experiment introduced by Gan is developed into a powerful and routine method for measuring the principal values of 13C chemical shift tensors in powdered solids. A large-volume MAT probe with stable rotation frequencies down to 22 Hz is described. A triple-echo MAT pulse sequence is introduced to improve the quality of the two-dimensional baseplane. It is shown that measurements of the principal values of chemical shift tensors in complex compounds can be enhanced by using either short contact times or dipolar dephasing pulse sequences to isolate the powder patterns from protonated or non-protonated carbons, respectively. A model compound, 1,2,3-trimethoxybenzene, is used to demonstrate these techniques, and the 13C principal values in 2,3-dimethylnaphthalene and Pocahontas coal are reported as typical examples.

Competition between radiation damping and transverse relaxation effects on NMR signal intensities

Abstract It has been found in experiments that the NMR signal intensity of the photon in water increases linearly with the pulse flip angle θ 0 (θ 0 2 O is as low as 4%. This results from the competition between radiation damping and transverse relaxation effects, which can be interpreted by the solution of the Bloch equations in the presence of the radiation damping terms but with the longitudinal relaxation term neglected. A saw-tooth-like peak height as a function of the pulse flip angle is predicted as long as T 1 ⪢ T * 2 ⩾ T r (where T r is the radiation damping time). Thus for a fairly strong magnetization, the maximum peak height cannot be obtained with a π/2 pulse, but with a pulse very close to π instead.

Radiation damping effects on NMR signal intensities

Abstract Preliminary data of radiation damping effects on NMR signal intensities are reported. In a sample of 90% H 2 O/10% D 2 O, where radiation damping is the dominant mechanism of the decay of the time domain proton signal, the intensity in the frequency domain versus the flip angle θ 0 of the pulse shows a saw-toothed profile with positive and negative maxima at θ 0 = 180° ±δ°, where δ is small, respectively, rather than at 90° and 270° in the usual sinusoidal case.

Small Infrared Target Detection Based on Weighted Local Difference Measure

Against an intricate infrared cloudy-sky background, jamming objects such as the edges of clouds in the scene have a similar thermal intensity measure with respect to the background as small targets. This may cause high false alarm rates and low probabilities of detection according to conventional small target detection methods. In this paper, we propose a weighted local difference measure (WLDM)-based scheme for the detection of small targets against various complex cloudy-sky backgrounds. Initially, a WLDM map is achieved to simultaneously enhance targets and suppress background clutters and noise. In this way, the true targets can be easily separated from jamming objects. After that, a simple adaptive threshold is used to segment the targets. More than 460 infrared small target images against diverse intricate cloudy-sky backgrounds were utilized to validate the detection capability of the WLDM-based method. Experimental results demonstrate that the proposed algorithm not only works more robustly for different cloudy-sky backgrounds, target movements, and signal-to-clutter ratio (SCR) values but also has a better performance with regard to the detection accuracy, in comparison to traditional baseline methods. In particular, the proposed method is able to significantly improve SCR values of the images.

Study of natural diamonds by dynamic nuclear polarization-enhanced 13C nuclear magnetic resonance spectroscopy

The results of a study of two types of natural-diamond crystals by dynamic nuclear polarization (DNP)-enhanced high-resolution solid-state 13C nuclear magnetic resonance (NMR) are reported. The home-built DNP magic-angle spinning (MAS) 13C NMR spectrometer operates at 54 GHz for electrons and 20.2 MHz for carbons. The power of the microwave source was about 30 W and the highest DNP enhancement factor came near to 10(3). It was shown that in the MAS spectra the 13C NMR linewidths of the Ib-type diamond were broader than those of IaB3-type diamond. From the hyperfine structure of the DNP enhancement as a function of frequency, four kinds of nitrogen-centred and one kind of carbon-centred free radicals could be identified in the Ib-type diamond. The hyperfine structures of the DNP enhancement curve that originated from the anisotropic hyperfine interaction between electron and nuclei could be partially averaged out by MAS. The 13C polarization time of DNP was rather long, i.e. 1500 s, and the spin-lattice relaxation time (without microwave irradiation) was about 300 s, which was somewhat shorter than anticipated. Discussions on these experimental results have been made in this report.

Rotation operator approach to spin dynamics and the Euler geometric equations

The rotation operator approach proposed previously is applied to spin dynamics in a time‐varying magnetic field. The evolution of the wave function is described, and that of the density operator is also treated in terms of a spherical tensor operator base. It is shown that this formulation provides a straightforward calculation of accumulated phases and probabilities of spin transitions and coherence evolutions. The technique focuses, not on the rotation matrix, but on the three Euler angles and its characteristic equations are equivalent to the Euler geometric equations long known to describe the motion of a rigid body. The method usually depends on numerical calculations, but analytical solutions exist in some situations. In this paper, as examples, a hyperbolic secant pulse is solved analytically, and a Gaussian‐shaped pulse is calculated numerically.

MRI-visible liposome nanovehicles for potential tumor-targeted delivery of multimodal therapies

Real-time diagnosis and monitoring of disease development, and therapeutic responses to treatment, are possible by theranostic magnetic resonance imaging (MRI). Here we report the synthesis of a multifunctional liposome, which contains Gd-DOTA (an MRI probe), paclitaxel and c(RGDyk) (a targeted peptide). This nanoparticle overcame the insolubility of paclitaxel, reduced the side effects of FDA-approved formulation of PTX-Cre (Taxol®) and improved drug delivery efficiency to the tumor. c(RGDyk) modification greatly enhanced the cytotoxicity of the drug in tumor cells A549. The T1 relaxivity in tumor cells treated with the targeted liposome formulation was increased 16-fold when compared with the non-targeted group. In vivo, the tumors in mice were visualized using T1-weighted imaging after administration of the liposome. Also the tumor growth could be inhibited well after the treatment. Fluorescence images in vitro and ex vivo also showed the targeting effect of this liposome in tumor cells, indicating that this nanovehicle could limit the off-target side effects of anticancer drugs and contrast agents. These findings lay the foundation for further tumor inhibition study and application of this delivery vehicle in cancer therapy settings.

Tumor-selective macromolecular MRI contrast agents

Tumor-selective macromolecular ligands containing 5-(p-Carbonyloxyphenyl)-10,15,20-triphenylporphyrin (HPTP) moiety (PHEA-DTPAHPTP and PAEA-DTPA-HPTP) were prepared by the incorporation of diethylenetriaminepentaacetic acid (DTPA) and 5-(p-hydroxylphenyl)-10,15, 20-triphenylporphyrin (HPTP) as the tumor-selective group in poly [α,β-(N-(2-hydroxyethyl)-L-aspartamide)] (PHEA) and poly-[α-β-(N-(2-aminoethy1)-L-aspartamide)] (PAEA). These ligands were further complexed with gadolinium chloride to form two tumor-selective macromolecular MRI contrast agents PHEA-Gd-DTPA-HPTP and PAEA-Gd-DTPA-HPTP. Relaxivity studies showed that both the polyaspartamide-gadolinium complexes possess higher relaxation effectiveness than that of the clinically used Gd-DTPA. Magnetic resonance imaging of tumors in mice indicated that these two polyaspartamide MRI contrast agents containing 5-(p-Carbonlyoxyphenyl)-10,15,20-triphenylporphyrin moiety can significantly enhance the contrast MRIs of Hepatoma (H22) and Ehrlich ascites carcinoma after injection and are taken up selectively by these cancers in mice.