1. I. Introduction Objectives  To solve geometric problem(misalignment) in cone-beam computed tomography (CBCT) system.  To adjust X-ray focal spot using metal-hole phantom and physical center of the detector.  Five pin-hole phantoms on detector for geometric alignment between source part and detector part.  To obtain the calibration parameters through the difference between reference point and rotation point. II. Materials and Methods Gantry geometric measurement method IV. Conclusions  Two-step process for misaligned geometry of CBCT system.  The center of the detector was forcibly aligned in order to adjust X- ray focal spot by using metal-hole phantom.  The geometric alignment of CBCT system was measured by using the difference between reference point (start point) and each rotation point based on the 2D projection.  The X-ray image results of before and after alignment of CBCT system will be analyzed and compared through the proposed method. III. Results and Discussions Cone-beam computed tomography (CBCT) system for medical diagnosis X-ray source in CBCTAcquired projection in detector Gantry geometric calibration Position of pin-hole phantom in flat panel detector Gantry distortion measurement(center pin-hole phantom)  X-ray tube (A-132/B-100, Varian inc.) Focal spot: 0.6 and 1.2 Focal spot: 0.6 and 1.2 Rhenium-tungsten molybdenum target Rhenium-tungsten molybdenum target  Flat panel detector (PaxScan 4030CB, Varian inc.) 397mm x 298mm active area 397mm x 298mm active area 2048 x 1536 pixels with 194μm pixel pitch 2048 x 1536 pixels with 194μm pixel pitch  Phantom research Correction focal spot using 14 x 14 metal-hole phantom Correction focal spot using 14 x 14 metal-hole phantom Acquiring 360 projection with five pin-hole phantom Acquiring 360 projection with five pin-hole phantom Focal Spot Correction  14 x 14 metal-hole phantom 1.5mm hole diameter with 196 holes 1.5mm hole diameter with 196 holes 2mm between hole centers with 0.5mm gap 2mm between hole centers with 0.5mm gap 5 layers with 10mm layer 5 layers with 10mm layer Analysis Method  Rotation of gantry system from 1 degree to 360 degree 360 projections in 5rpm rotation 360 projections in 5rpm rotation  Difference where r and o are reference point and rotation point of U where r and o are reference point and rotation point of U and V coordinates, respectively. and V coordinates, respectively.  2048 x 1536 pixels 1.5mm hole diameter with 196 holes 1.5mm hole diameter with 196 holes 2mm between hole centers with 0.5mm gap 2mm between hole centers with 0.5mm gap 5 layers with 10mm layer 5 layers with 10mm layer  U- and V-axis U-axis : front-rear direction U-axis : front-rear direction V-axis : left-right direction V-axis : left-right direction Gantry distortion measurement(V-axis)  Total pattern in each angle Pin-hole (3) : much distortion after 180 angle Pin-hole (3) : much distortion after 180 angle Pin-hole (5) : droop problem from starting and 50 angle Pin-hole (5) : droop problem from starting and 50 angle Gantry distortion measurement(U-axis)  Total pattern in each angle Pin-hole (2,3) : droop problem from 90 to 270 angle Pin-hole (2,3) : droop problem from 90 to 270 angle Pin-hole (4,5) : droop problem from starting and 50 angle Pin-hole (4,5) : droop problem from starting and 50 angle