Final answer:
Recalibration of the detector element is the correct action to eliminate a ring artifact in CT imaging, as it directly addresses the potential miscalibration or defects in the scanner's detectors.
Step-by-step explanation:
The question pertains to eliminating a ring artifact in medical imaging, particularly in computed tomography (CT). Among the given options, recalibration of the detector element is the most likely to eliminate a ring artifact. Ring artifacts can be caused by miscalibrated or defective detector elements in a CT scanner. By recalibrating the detector, uniformity is restored, and thus the artifact can be corrected. The removal of metal pertains to patient preparation and can reduce artifacts, but it does not directly correct existing detector issues. Decreasing the matrix size or increasing mAs might affect the image quality or dose, but they do not directly target the issue of detector calibration.