Search Results (4)

A mathematical discussion is introduced to describe the gradient coils characterizing the NMRI system. A comparison between the physical laws characterizing the magnetic field of winding coils and a magnetohydrodynamic analysis is built up. The requirements for gradient coil performance are described and analyzed. Design methods are characterized by mathematical manipulation to reduce the nonlinearities that make the magnetic field not suitable for image extraction. A mathematical study has been shown to represent the Golay’s pairs as generators of linear magnetic fields. The entire study shows how the choice of the magnetic field pattern inside the NMRI can occur by choosing the proper geometrical parameters that are a function of the nonlinearities involved in the gradient coils design. Full article

The evaluation of surveillance imaging of OSCC patients is a difficult task physicians have to face daily. Multiple patients experience a recurrence of this disease, which underlines the importance of regular patient monitoring programs. Our study analysed the value of surveillance imaging, such as computed tomography (CT) and nuclear magnetic resonance imaging (NMRI), as a patient monitoring programme and its effectiveness in achieving improvement in early recurrence detection. The study comprised 125 patients, out of which 56 (n = 56) showed radiological and 69 (n = 69) showed clinical and radiological conspicuous patterns in domestic follow-ups, respectively. The use of CT and NMRI showed a significant dependence on the histological result (p = 0.03). However, the different groups showed no significant dependence on the histological result (p = 0.96). The distribution of the histological biopsies, which were taken due to radiological changes, were prone to wrong positive diagnoses (false positives) in 71 percent. To conclude, imaging modalities should be chosen for each patient individually to reduce false positives, improve the early detection of recurrence, and increase the cure rate. Full article

Artificial ground freezing method is widely applied in the construction of metro tunnel and significantly impact the microstructure of soils in artificial frozen-walls. To delve into the pore distribution characteristics of thawed residual soils, Nuclear Magnetic Resonance Imaging (NMRI) measurements were performed to investigate the relaxation time (T₂) spectrums and T₂-weighted images of saturated samples after freezing at different temperatures. The pore volume distributions were determined from T₂ spectrums based on the surface relaxation coefficient ( ${\rho }_2$ ) and the pore structures were visualized by T₂-weighted images. Subsequently, the pore size distribution curves from NMRI were compared and validated by mercury intrusion porosimetry (MIP) tests. According to the results, the peak areas of T₂ spectrums were linearly related to freezing temperatures in a positive manner. Pore volume distribution curves of thawed soils have two peaks, which are the major peaks with diameters of 0.5–20 μm and the secondary peaks with diameters of 20–500 μm. As the freezing temperature drops, the volumes of pores with different diameters all increased. The damage degree of microstructure in thawed soils increases as the temperature drops, according to the visualized pore structure. Besides, NMRI measurements of saturated soils are more accurate to reflect the full diameter range of pores, compared to MIP method. Full article

In this paper, the nuclear magnetic resonance (NMR) technique is applied to exploring the spontaneous imbibition mechanism in tight sandstones under all face open (AFO) boundary conditions, which will benefit a better understanding of spontaneous imbibition during the development of oil & gas in tight formations. The advantages of nuclear magnetic resonance imaging (NMRI) and NMR T₂ are used to define the distribution of remaining oil, evaluate the effect of micro structures on imbibition and predict imbibition recovery. NMR T₂ results show that pore size distributions around two peaks are not only the main oil distributions under saturated condition but also fall within the main imbibition distributions range. Spontaneous imbibition mainly occurs in the first 6 h and then slows down and even ceases. The oil signals in tiny pores stabilize during the early stage of imbibition while the oil signal in large pores keeps fluctuating during the late stage of imbibition. NMRI results demonstrate that spontaneous imbibition is a replacement process starting slowly from the boundaries to the center under AFO and ending with oil-water mixing. Furthermore, the wetting phase can invade the whole core in the first 6 h, which is identical with the main period of imbibition occurring according to NMR T₂ results. Factors influencing the history of oil distribution and saturation differ at different periods, while it is dominated by capillary imbibition at the early stage and allocated by diffusion at later time. Two imbibition recovery curves calculated by NMRI and NMR T₂ are basically consistent, while there still exists some deviations between them as a result of the resolutions of NMRI and NMR T₂. In addition, the heterogeneity of pore size distributions in the two samples aggravates this discrepancy. The work in this paper should prove of great help to better understand the process of the spontaneous imbibition, not only at the macroscopic level but also at the microscopic level, which is significant for oil/gas recovery in tight formations. Full article