Indirect Susceptibility Mapping of Thin-Layer Samples Using Nuclear Magnetic Resonance Imaging

We measured and imaged magnetic field distributions of thin layers (2-D objects with negligible thickness) of biological and physical samples, by using nuclear magnetic resonance (NMR). The image represents the magnetic susceptibility distribution in the sample. We used a standard gradient echo imaging method, susceptible to magnetic field homogeneity, for detection. Since the physical and biological samples we investigated do not generate any NMR signal, we used a homogeneous phantom reference - a container filled with water - as a medium. The image acquired by this method is actually a projection of the sample properties onto the homogeneous phantom. The method can be applied in nanotechnology, microelectronics, and especially in the biological and medical sciences.     

Asymmetric spin echo sequence and requirements on static magnetic field of NMR scanner

The article describes studies with two NMR sequences frequently used for static magnetic field distribution measurement. The gradient echo sequence is suitable for such measurements, it has good sensitivity and fast measurements are possible. The asymmetric spin echo sequence is also suitable for inhomogeneity measurements, but the time taken for measurement is longer. The purpose of the studies was the comparison of the features and the quality of results of both sequences under conditions of an imperfect static magnetic field. The imperfection deliberately was inserted into the static magnetic field of a low-field NMR scanner. Two common measurements were used for comparison, nevertheless due to the low-field NMR scanner the processing of the measured data was not entirely standard and processing using optimisation was required. The experiments revealed that the asymmetric spin echo sequence can provide images of a good quality and also under interference from a static magnetic field where measurement using the gradient echo sequence is not able to provide useable images.     

A simple method for mapping the B1 field distribution of linear RF coils

Inhomogeneity of the radio frequency (RF) field B₁ leads to intensity variations in MR images and to spatial dependence of spectral line amplitudes. In this paper, a simple method of measuring the B₁ field components of an unsegmented linear coil is described. The method is designed for the coils operating up to 20 MHz. The B₁ field distribution is replaced by the static magnetic field caused by DC current flowing through the coil. The technique involves rotating the coil 90° so that measured B₁ component is aligned with B₀ and measuring the shift of resonance frequency using a spectroscopic imaging sequence. Experimental results were in good agreement with the theoretical computations.