Analog non-linear transformation-based tone mapping for image enhancement in C-arm CT

Flat-Panel C-arm Computed Tomography (CT) suffers from pixel saturation due to the detector's limited dynamic range. We describe a novel approach of analog, non-linear tone mapping (TM) for preventing detector saturation. An analog TM operator (TMO) applies a non-linear transformation in a CMOS (complementary metal-oxide semiconductor) sensor and its inverse TMO based on 14-bit digital raw data. This is done in order to prevent overexposure and to enhance image quality to 32 bits. The method was applied to the cases of low-contrast head imaging and to that of imaging both knees. Cone-beam projection data with and without overexposure was simulated for a 200 degree short-scan of the knees and a 360 degree full-scan of a Forbild head phantom. The results show an increased correlation coefficient with respect to ground truth of 0.99 for TMO compared to 0.96 for overexposed knee data and a higher low-contrast visibility (CC=0.99) compared to linear quantization (CC=0.97).