Analisys of Measurement Methods for Static Magnetic Field Uniformity Assessment in Clinical Magnetic Resonance Imaging

In this paper measurement methods of static Magnetic Field Uniformity (MFU) in clinical magnetic resonance imaging systems and relative indexes are analyzed. MFU concerns how much the actual static magnetic field intensity within the gantry differs from its nominal value and it is commonly evaluated for a spherical volume. Despite the importance of the assessment of this parameter a lack of standardization regarding measurement methods applied to obtain indexes must be noticed. MFU indexes are expressed in parts per million and can be obtained as the difference between the maximum and minimum field intensity value expressed as a ratio of nominal value (Peak- to-Peak, PP) over the entire imaging volume, or as a volumetric Root Mean Square of all magnetic field intensity values. This work describes the most common methods for MFU assessment; advantages and disadvantages of each technique are analyzed through simulation and experimental analysis performed on two 1.5 T Systems and one IT System. With the aim to find a relationship between PP and RMS uniformity indexes a novel index, called 'Gaussian Peak to Peak', is introduced. As expected, the same scanner shows different values for MFU indexes depending on the measurement method adopted; as far as it concerns the novel index, experimental data partially confirm the results obtained by simulation, suggesting that an index with reference to the expected distribution could be an interesting way to proceed on the way of standardization.