Evaluation of spatial-encoding-related geometric distortion in magnetic resonance imaging

This study demonstrated the characteristics of spatial encoding-related geometric distortion in magnetic resonance imaging (MRI) by using a phantom with known physical dimensions to detect geometric distortion in MR scans. The amount of distortion was calculated as the difference between the physical coordinates of control points in the phantom and those of the corresponding points in the distorted MR image of the phantom. The phase and the frequency encoding directions were swapped to acquire a phantom image for movement of a patient table 120 mm to the right and left, allowing comprehensive distortion mapping over the isocentric plane and the entire field of view (FOV) along the spatial encoding. The geometric distortion of phase encoding directions over the entire FOV was small compared to the distortion of frequency encoding directions. The maximum absolute deviations were 28.00 mm and 20.00 mm along the frequency and the phase encoding directions over the entire FOV, respectively. The mean absolute deviations along the frequency and the phase encoding directions were 2.85 mm and 1.97 mm, respectively. Although geometric distortion along the phase encoding axis near the isocenter was small, the distortion increased slightly toward the peripheral regions. The distortion of the phase encoding direction in the peripheral region can be severely affected by the imaging gradient’s nonlinearity.