RF coils

The most difficult materials to study by NMR microscopy are those with short T2 or T2 relaxation times and/or with low concentrations of the nudear spins, which normally result in poor NMR signal intensities. One possibility for improving the image quality is to adapt the shape and size of the rf coils to the size of the objects in order to achieve the best possible filling factor and therefore the best sensitivity [1]. In addition, methods with short echo or detection times have been developed, such... 

The whole NMR imaging sensor system usually consists of a magnet, a shim system mounted inside the room-temperature bore of the magnet, a gradient system mounted inside the shim system and the rf coil mounted inside the gradient system. In the case of a saddle coil or a birdcage resonator, open access can be realized from the bottom to the top of the entire system with the coil diameter. 

Micro-coils can be used to reach higher spatial resolutions, as a result of the increase in sensitivity of very small rf coils. The possible resolution in NMR... 

Small signal unit Magnet Gradient coil RF coil... 

The compact MRI system has an advantage for rf coil design because solenoid coils can be used in most applications. The solenoid coil has about three times better SNR than that of the saddle-shaped coil (14). Even if the saddle-shaped or birdcage coil is used in the quadrature mode, the solenoid coil will still have better SNR because an SNR gain of only about 1.4 times is obtained in that mode. 

Figure 2.2.8 shows a typical tuning and matching circuit for an rf coil. If the selfinductance of the rf coil is too large for the tuning and matching, the rf coil wire... 

Figure 2.2.13 shows an overview of the MRI system developed for salmon selection. A 0.2-T C-shaped yoked permanent magnet with a 25-cm gap [50-ppm homogeneity for 15-cm DSV (diameter spherical volume), weight 1.4 tons] is used for the magnet. For the rf coils, two solenoid coils with a 14-cm circular aperture and 14 cm x 18 cm oval aperture were developed. 

The profiling method requires the sensitive slice to be shifted through the object. Figure 2.4.2 shows the mechanical lift used to move the sensor with respect to the sample. The object under study, for instance the lower surface of the arm in the picture, is positioned on top of a flat holder (A) and the NMR sensor is placed under it on a movable plate (B). The mechanical construction allows one to move the sensor up and down with a precision of 10 pm. The distance between the rf coil and the sensitive slice defines the maximum penetration depth into the sample (maximum field of view of the ID image). Depending on the application, the position of the rf coil with respect to the sensitive slice can be changed to maximize the sensitivity. 

In order to use Eqs. (2.6.1) and (2.6.2) to estimate the sensitivity gain obtained by remote detection, knowledge of the relative sensitivity of the detector and the encoding circuit, A, is required. Here we discuss the sensitivity of an rf coil detector as an example, because all the experiments presented in this text use inductive detection at high field. The signal-to-noise ratio of inductive NMR detection can be approximated by the following simplified equation [12] ... 

Fig. 2.6.5 Hardware for high field NMR remote probe in (c) contains a relatively large saddle-detection. Photographs (a) and (b) show la- coil and is used for (flow) imaging. The detec-boratory-built remote detection probes with tor probe in (d) contains a microsolenoid coil both rf coils built into the same body (c), (d) for optimized mass sensitivity, which is parti-and (e) are detector-only remote probes that cularly useful for microfluidic NMR applica-can be inserted from the top or bottom into the tions. The same probe is shown in (e) with a NMR imaging assembly, so that the well mounted holder for a microfluidic chip that is...

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