방사선의학 웹진

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Abstract
Conventional intraoperative computed tomography (CT) has a long scan time, degrading the image quality. Its large size limits the position of a surgeon during surgery. Therefore, this study proposes a CT system comprising of eight carbon-nanotube (CNT)-based x-ray sources and 16 detector modules to solve these limitations. Gantry only requires 45° of rotation to acquire the whole projection, reducing the scan time to 1/8 compared to the full rotation. Moreover, the volume and scan time of the system can be significantly reduced using CNT sources with a small volume and short pulse width and placing a heavy and large high-voltage generator outside the gantry. We divided the proposed system into eight subsystems and sequentially devised a geometry calibration method for each subsystem. Accordingly, a calibration phantom consisting of four polytetrafluoroethylene beads, each with 15 mm diameter, was designed. The geometry calibration parameters were estimated by minimizing the difference between the measured bead projection and the forward projection of the formulated subsystem. By reflecting the estimated geometry calibration parameters, the projection data were obtained via rebinning to be used in the filtered-backprojection reconstruction. The proposed calibration and reconstruction methods were validated by computer simulations and real experiments. Additionally, the accuracy of the geometry calibration method was examined by computer simulation. Furthermore, we validated the improved quality of the reconstructed image through the mean-squared error (MSE), structure similarity (SSIM), and visual inspections for both the simulated and experimental data. The results show that the calibrated images, reconstructed by reflecting the calibration results, have smaller MSE and higher SSIM values than the uncalibrated images. The calibrated images were observed to have fewer artifacts than the uncalibrated images in visual inspection, demonstrating that the proposed calibration and reconstruction methods effectively reduce artifacts caused by geometry misalignments.

그림1. (a) 제안한 CT 시스템의 top view. (b) 한 서브시스템의 trimetric view.

그림 2. 보정 팬텀의 (a) 보정 전, (b) 보정 후의 복원 영상.

그림 3. 트레일믹스 팬텀의 (a) 보정 전, (b) 보정 후의 복원 영상.

시뮬레이션을 통해 제안된 보정 방법으로 추정된 매개변수의 값과 실제 값의 차이로 보정 오차를 계산했습니다. 보정 오차를 통해, 기하 보정의 목표 정밀도(복원 영상의 1 복셀 미만)를 만족함을 확인했습니다. 그리고 보정 전후 복원 영상의 mean square error (MSE), structural similarity (SSIM)을 비교하여 제안된 보정 방법이 효과적으로 인공음영을 제거함을 정량적으로 확인했습니다. 또한 제안된 시스템의 프로토타입을 제작하고, 실제 시스템에서의 보정 성능을 보정 팬텀과 트레일믹스 팬텀의 복원 영상을 통해 육안으로 확인했습니다. 보정 전 영상의 빨간색 화살표는 기하학적 오정렬로 인한 인공음영을 나타냅니다. 보정된 영상에서는 이러한 인공음영이 감소하여 영상 품질이 향상되었으며, 녹색 화살표로 표시된 것처럼 비드 부착에 사용된 테이프까지도 명확하게 관찰되었습니다.

affiliations

Seunghyuk Moon3,1, Seungwon Choi3,1, Hanjoo Jang1, Minsik Shin2, Youngjun Roh2 and Jongduk Baek1
1 School of Integrated Technology, Yonsei University, Incheon, Republic Of Korea

2 KohYoung Technology, Yongin, Republic Of Korea