Том 47, № 1 (2016)

Compressed sensing (CS) is an effective method to reduce k-space sampling for accelerated MRI data acquisition and reconstruction. Iterative-shrinkage algorithms provide an efficient numerical technique to minimize mixed l_l − l₂ norm minimization problems. These algorithms utilize a regularization parameter to introduce sparsity in the solution for CS recovery problem. This paper introduces a new method based on geometry factor (g-Factor) as an adaptive regularization parameter. For this purpose, Projection onto Convex Sets (POCS) algorithm is modified to include regularization term in the form of g-Factor and a priori constraint (data consistency) for image reconstruction from the highly under-sampled data. The performance of the proposed algorithm is verified using simulated and actual MRI data. The results show that g-Factor as a regularization parameter provides better image reconstruction from the highly under-sampled data as compared to a fixed regularization parameter in POCS.

A novel nuclear magnetic resonance (NMR) experimental scheme is presented, which has promising applications in pulsed magnetic fields. The experimental scheme, broadband continuous wave NMR (BB-CW-NMR), is validated with numerical solutions of Bloch equations in pulsed fields under broadband continuous radio frequency (RF) irradiation. Furthermore, the influence of experimental parameters such as relaxation times and RF power on the waveform and amplitude of the broadband continuous NMR signal is analyzed briefly. To verify the reliability of the numerical calculation program, numerical solutions of the Bloch equations under the irradiation of RF pulse sequence are given. There is good agreement between simulation and expected experimental results, indicating the validity of the program. Finally, we estimate the amplitude of the NMR signal, the level of noise, and RF interference in a BB-CW-NMR experiment. The results of simulation and analysis demonstrate that the BB-CW-NMR experiment scheme is feasible under the conditions of appropriate relaxation times and RF power if the leakage of the RF field into the receiver coil is reduced by at least a factor of 10⁶.

Imaging methods have an important role in the management of patient’s health care. Some of the advantages made magnetic resonance imaging (MRI) as an exclusive modality. Gd-DTPA is one of the most common contrast agents in clinical applications. In this study, the concentrations of three gadolinium-based MRI contrast agents were measured and quantification accuracy of these contrast agents by MRI method was investigated. Different concentrations from the Gd-DTPA, Gd₂O₃–DEG and paramagnetoliposome nanoparticles (encapsulated Gd₂O₃–DEG nanoparticles in liposome) samples were prepared. Physical characteristics of the contrast agents were investigated by DLS and TEM methods. The T₁-weighted images of the prepared samples were recorded using MRI scanner. For each sample, gadolinium concentrations were determined using the relaxation rates and relaxivities. Determined concentrations by the experimental and ICP/OES methods were compared and the standard errors of the results were determined. Morphology, dimension and hydrodynamic diameter of the contrast agents were investigated. The hydrodynamic diameter of Gd₂O₃–DEG and PML nanoparticles were 90 ± 7.2 nm (with PdI = 0.328) and 96.8 ± 6.5 nm (with PdI = 0.299), respectively. In assessment of gadolinium concentrations, standard deviations of the experimental and ICP/OES data were ranged from 0.007 to 0.04. P values of all data points were higher than 0.05 that confirm there is no significant difference between the experimental and ICP/OES measurements. From the results, it could be concluded that MR systems could be used as an accurate and available method to estimate gadolinium concentrations.

The results of the research of a non-cyclic geometrical phase of the ³⁵Cl nuclear quadrupole resonance (NQR) signals are presented, caused by the character of nuclear magnetization trajectory under radio-frequency excitation by means of PEANUT sequence for powdered samples. The analytical math expressions for a geometrical phase in NQR for a spin I = 3/2 while rotating nuclear magnetization by means of the PEANUT pulse sequence with frequency detuning and variable duration of the pulse were obtained. It is shown that the measured phase for this sequence is a geometrical phase and it may accumulate up to Δω ≠ 0 with a change of duration of the radio-frequency pulse \(t_{\text{w}}^{\prime }\). The experiment with the phase inversion and with nutation detection by means of echo amplitude was first applied in NQR jointly with measuring a geometrical phase. Since nutations detection and excitation in the PEANUT method are completely separate in time, experiments with high nutation frequencies become possible. The experimental examples supporting theoretically predicted distinctions of the geometrical phase for the PEANUT method in ³⁵Cl NQR are presented. An alternative nutation experiment is proposed to determine asymmetry parameter η for the nuclei with the spin I = 3/2 in powders proceeding from the nature of a non-cyclic geometrical phase accumulation.

The objective of this study is to investigate the neural activities in the human brain after stellate ganglion block (SGB) treatment using functional magnetic resonance imaging (fMRI). Thirteen healthy female volunteers from the local community participated in the study. An echo planar imaging sequence for fMRI has used to examine the effect on the cerebral cortex before and after SGB stimulation at 1.5T MRI. We used 6 mL of 1 % mepivacaine hydrochloride as a local anesthetic. Two control groups with and without a saline injection were also included. FMRI data were processed using statistical parametric mapping. The blood-oxygen-level-dependent fMRI signal intensities increased in contralateral anterior cingulate cortex, hypothalamus, ventral putamen, and parahippocampal gyrus after SGB stimulation, while the signal intensities had decreased in contralateral thalamus and dorsal putamen, and ipsilateral caudate nucleus. In the saline control group, the signal intensities in ipsilateral posterior putamen and superior temporal gyrus and both insular cortices have increased significantly. The present study could show the changes by SGB in the several important brain areas associated with sympathetic nervous system. The non-invasive and repeatable imaging technique of fMRI could provide some useful information for better understanding of the neural mechanisms involved in SGB treatments.