Diffusion and susceptibility effects in NMR microscopy
A microscopy probe was developed to image small samples at 7 Tesla (300 MHz). The gradient coil set comprises two Golay coils and a Maxwell pair, generating gradients up to 2 T/m. A 3 turn solenoidal radiofrequency coil was used for excitation and signal reception. This probe was capable of imaging small samples up to 3 mm diameter. In-plane pixel sizes down to 25 m have been employed, and the images obtained have adequate quality for biological studies. This research was focused on a particular biological system: the excised mouse eye. The susceptibility-related field inhomogeneity was evaluated. Interfaces with air are the main source of field inhomogeneity, which depends strongly on the shape of the interface. Excised mouse eye images were particularly affected by susceptibility-related field inhomogeneity due to small quantities of blood and the optic nerve, which protrudes from the almost completely spherical eyeball. The signal loss and image distortion were analysed by imaging simple geometrical phantoms and comparing simulated image with the experimental results. Guidelines for imaging in the presence of magnetic field susceptibility were discussed. The measurement of diffusion coefficients by imaging techniques proved to be very sensitive to the presence of intrinsic gradients and imaging gradients. Diffusion sensitisation gradients were applied in opposite directions and the resulting images were combined to achieve sufficient accuracy. A new technique to manipulate these images was developed; this allowed the identification of the sources of error in the measurement, and the detection, of anisotropic diffusion, aiding the interpretation of diffusion weighted images and diffusion coefficient maps.