Measurement of micron level distortions in a macroscopic cylindrical object from x-ray CT images

A technique for the measurement or tracking of fine distortion in macroscopic objects of nominally circular section is described and assessed. Distortions of order several microns have been measured in a 14 cm diameter, thick-walled polymethyl methacrylate (PMMA or Perspex) cylinder by tracking the location of multiple edge points of the object in its x-ray CT images using the circle-PLUS   subpixel edge detection procedure. The cylinder was deformed by compressing it along a diameter over a total range of 340μm (0.7 pixel in its CT image) in 10 steps. The successive cylinder wall deflections in the direction of the compression, and the corresponding deflections in the perpendicular direction, were calculated from images obtained using a GE Lightspeed medical CT scanner. The calculated shifts in cylinder diameter were compared directly with mechanical measurements obtained with a micrometer. The minimum detectable shift in object diameter of 0.033 pixel or 1/30th of a pixel (16μm) was calculated from the image and verified by mechanical measurement. The circle-PLUS edge detection procedure should be applicable to both precision industrial measurements and clinical measurements such as vessel or bone dimensions or applications requiring precise organ size, shape or position determination. The procedure is not restricted to x-ray CT applications and could also be readily applied to precision optical measurements or MRI data.