I. Introduction

1. I. Introduction Cone-beam computed tomography (CBCT) system for medical diagnosis • 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. Misaligned 2D projection geometry based calibration method for cone-beam computed tomography 1Chang-Woo Seo, 1Bo Kyung Cha,, 1Sungchae Jeon , 1Young Huh , 2Kisung Lee 1Advanced Medical Device Research,, Cent er, Korea Electrotechnology Research Institute, Gyeonggi-do 426-170, Korea 2Department of Radiologic Science , Korea University, S eoul, 136-703, Korea II. Materials and Methods • Gantry geometric measurement method • Focal Spot Correction • 14 x 14 metal-hole phantom • 1.5mm hole diameter with 196 holes • 2mm between hole centers with 0.5mm gap • 5 layers with 10mm layer X-ray source in CBCT Acquired projection in detector • X-ray tube (A-132/B-100, Varian inc.) • Focal spot: 0.6 and 1.2 • Rhenium-tungsten molybdenum target • Flat panel detector (PaxScan 4030CB, Varian inc.) • 397mm x 298mm active area • 2048 x 1536 pixels with 194μm pixel pitch • Phantom research • Correction focal spot using 14 x 14 metal-hole phantom • Acquiring 360 projection with five pin-hole phantom • Analysis Method • Rotation of gantry system from 1 degree to 360 degree • 360 projections in 5rpm rotation • Difference • where r and o are reference point and rotation point of U • and V coordinates, respectively. III. Results and Discussions Gantry distortion measurement(U-axis) • Gantry geometric calibration Position of pin-hole phantom in flat panel detector • 2048 x 1536 pixels • 1.5mm hole diameter with 196 holes • 2mm between hole centers with 0.5mm gap • 5 layers with 10mm layer • Total pattern in each angle • Pin-hole (2,3) : droop problem from 90 to 270 angle • Pin-hole (4,5) : droop problem from starting and 50 angle Gantry distortion measurement(center pin-hole phantom) 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. • U- and V-axis • U-axis : front-rear 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 (5) : droop problem from starting and 50 angle