Purpose: The purpose of this study was to assess the accuracy of algorithms in registering pre- and post-contrast images with and without motion using a subtraction CT software.
Methods: A pre-contrast image of a cylindrical CT phantom was acquired. Five 28-mm wide water rods were replaced with iodine rods and the phantom was displaced vertically by 30 mm, in 5-mm increments, and post contrast images were acquired. Subtraction images were evaluated using a Matlab code by generating line profiles for each rod.The line profile data was used to estimate the diameter of each rod under stationary and all shifted positions by performing a linear interpolation to find the location in the image where the CT number exceeded 50 HU, and the location where it dropped below 50 HU. The difference in these locations determined the diameter of rods in each subtracted image. The rod diameters in the shifted images were compared with the rod diameters in the stationary image.
Results: The motion correction was successful and maintained similar rods shape, location, and CT number for all displacements up to 25 mm. For example, the measured vertical diameter of the 9 o’clock rod decreased by 4.1 mm(-8.5%- -10.6%). However, moving the phantom by 30 mm, more than the rod’s diameter size (28 mm), resulted in misregistration and inaccurate subtraction.
Conclusion: The subtraction algorithm corrected for motion between the pre- and post-contrast scans for displacements less than the size of the iodine rod.