Measurement of RF non-homogeneity in MRI

Introduction: Non-uniformity is one of the most important parameters affecting MRI images which can lead to harmful effects in the diagnosis and analysis of qualitative and quantitative methods. A method is introduced for measuring RF non-homogeneity in MRI systems. Methods and materials: To verify the uniformity of B0 and B1 fields, a cylindrical phantom, with a diameter of 24 cm and a height of 25.5 cm, was made of plexiglass and filled with vegetable oil.in this method a single slice of a uniform phantom is imaged with different flip angles, using a standard spin echo protocol. The signal intensity in these images are then fitted to a mathematical47model which describes the relationship between signal intensity and flip angle in spin echo images. To assess the uniformity of B0 and B1 magnetic fields, multi-angled echo spin protocol and three-dimensional echo gradient protocol were used, respectively, with two different echo times using an MRI Siemens Prisma 3 Tesla in the National Brain Laboratory. Results: RF waves were sent uniformly but, as expected, no uniformity was observed in the received waves by the surface coil and the waves received by the head coil were uniform. The uniformity of B0 field was equal to 0.3125 ppm over a 24 cm DSV (diameter of spherical volume). Discussion and conclusion: By using this phantom, the accuracy of coils can be evaluated and the plan of B0 and B1 magnetic fields can be prepared, which can be later used to eliminate the artifacts caused by the lack of uniformity of these two fields on functional images such as diffusion and magnetic spectroscopy. In addition, this phantom can be used for assessing the uniformity of magnetic fields in devices of various powers.

Keywords:

MRI , RF non-homogeneity , Image non-uniformity , SE , Phantom