I. Introduction

• ObjectivesTo solve geometric problem(misalignment) in cone-beam computed tomography To solve geometric problem(misalignment) in cone-beam computed tomography (CBCT) system.(CBCT) system.

To adjust X-ray focal spot using metal-hole phantom and physical center of the To adjust X-ray focal spot using metal-hole phantom and physical center of the detector.detector.

Five pin-hole phantoms on detector for geometric alignment between source part Five pin-hole phantoms on detector for geometric alignment between source part and detector part. and detector part.

To obtain the calibration parameters through the difference between reference To obtain the calibration parameters through the difference between reference point and rotation point.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

Cone-beam computed tomography (CBCT) system for medical diagnosis

X-ray source in CBCTX-ray source in CBCT Acquired projection in detectorAcquired projection in detector

• Gantry geometric calibration

Position of pin-hole phantom in flat panel detectorPosition of pin-hole phantom in flat panel detector

Gantry distortion measurement(center pin-hole phantom)Gantry distortion measurement(center pin-hole phantom)

X-ray tube (A-132/B-100, Varian inc.) X-ray tube (A-132/B-100, Varian inc.) Focal spot: 0.6 and 1.2Focal spot: 0.6 and 1.2 Rhenium-tungsten molybdenum targetRhenium-tungsten molybdenum target

Flat panel detector (PaxScan 4030CB, Varian inc.) Flat panel detector (PaxScan 4030CB, Varian inc.) 397mm x 298mm active area397mm x 298mm active area2048 x 1536 pixels with 194μm pixel pitch2048 x 1536 pixels with 194μm pixel pitch

Phantom researchPhantom researchCorrection focal spot using 14 x 14 metal-hole phantomCorrection focal spot using 14 x 14 metal-hole phantomAcquiring 360 projection with five pin-hole phantomAcquiring 360 projection with five pin-hole phantom

• Focal Spot Correction

14 x 14 metal-hole phantom 14 x 14 metal-hole phantom 1.5mm hole diameter with 196 holes1.5mm hole diameter with 196 holes2mm between hole centers with 0.5mm gap2mm between hole centers with 0.5mm gap5 layers with 10mm layer5 layers with 10mm layer

14 x 14 metal-hole phantom 14 x 14 metal-hole phantom 1.5mm hole diameter with 196 holes1.5mm hole diameter with 196 holes2mm between hole centers with 0.5mm gap2mm between hole centers with 0.5mm gap5 layers with 10mm layer5 layers with 10mm layer

• Analysis Method

Rotation of gantry system from 1 degree to 360 degreeRotation of gantry system from 1 degree to 360 degree360 projections in 5rpm rotation360 projections in 5rpm rotation

DifferenceDifference

where where rr and and oo are reference point and rotation point of U are reference point and rotation point of U and V coordinates, respectively.and V coordinates, respectively.

Rotation of gantry system from 1 degree to 360 degreeRotation of gantry system from 1 degree to 360 degree360 projections in 5rpm rotation360 projections in 5rpm rotation

DifferenceDifference

where where rr and and oo are reference point and rotation point of U are reference point and rotation point of U and V coordinates, respectively.and V coordinates, respectively.

VVVVVUUUUU orordorord ),(,),(

2048 x 1536 pixels 2048 x 1536 pixels 1.5mm hole diameter with 196 holes1.5mm hole diameter with 196 holes 2mm between hole centers with 0.5mm gap2mm between hole centers with 0.5mm gap5 layers with 10mm layer5 layers with 10mm layer

2048 x 1536 pixels 2048 x 1536 pixels 1.5mm hole diameter with 196 holes1.5mm hole diameter with 196 holes 2mm between hole centers with 0.5mm gap2mm between hole centers with 0.5mm gap5 layers with 10mm layer5 layers with 10mm layer

U- and V-axis U- and V-axis U-axis : front-rear directionU-axis : front-rear direction V-axis : left-right directionV-axis : left-right direction

U- and V-axis U- and V-axis U-axis : front-rear directionU-axis : front-rear direction V-axis : left-right directionV-axis : left-right direction

Gantry distortion measurement(V-axis)Gantry distortion measurement(V-axis)

Total pattern in each angleTotal pattern in each anglePin-hole (3) : much distortion after 180 angle Pin-hole (3) : much distortion after 180 angle Pin-hole (5) : droop problem from starting and 50 anglePin-hole (5) : droop problem from starting and 50 angle

Total pattern in each angleTotal pattern in each anglePin-hole (3) : much distortion after 180 angle Pin-hole (3) : much distortion after 180 angle Pin-hole (5) : droop problem from starting and 50 anglePin-hole (5) : droop problem from starting and 50 angle

Gantry distortion measurement(U-axis)Gantry distortion measurement(U-axis)

Total pattern in each angleTotal pattern in each anglePin-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 Pin-hole (4,5) : droop problem from starting and 50 angleangle

Total pattern in each angleTotal pattern in each anglePin-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 Pin-hole (4,5) : droop problem from starting and 50 angleangle