Bias field inhomogeneity measurements

Magnetic resonance imaging (MRI) is affected by intensity inhomogeneity where the illuminated areas alternate with shadow areas. The phenomenon of inhomogeneity is barely noticeable by the human observer, but in the field of automated image segmentation or registration, the unwanted intensity variations can cause significant errors. An important step in image processing is the evaluation of the inhomogeneity and the adequate correction of this artifact. In this paper we have proposed to measure the inhomogeneity with the most well-known quantitative formulas. We shall also put forward two new measurement methods: one for direct measurement and one for indirect measurement. The first method measures the smoothness of the bias field ratio, regardless the used correction algorithm. The second defines a procedure and an image function for inhomogeneity evaluation. The values obtained point the inhomogeneity level out and suppress the white noise better than the usual algorithms. We used simulated MR images obtained from the Brain Web site in the quantitative and comparative evaluations. The results obtained, when compared to existing methods, show a significant confidence level for the proposed measurements.