Comparison of flat-panel detector and image-intensifier detector for cone-beam CT
Introduction
Cone-beam computed tomography (CT) uses a two-dimensional X-ray detector, and a cone- or pyramid-shaped X-ray beam. In addition to fluoroscopy and radiography functions, it can acquire a full set of projection images during a single rotation scan, and reconstruct an isotropic high-spatial resolution three-dimensional image.
A cone-beam CT system using several X-ray tubes and detectors (DSR) was developed as an ultrafast CT, especially for heart, lungs, and circulation [1]. On the other hand, the system with an X-ray tube and a detector has been developed especially for angiography [2], [3], [4], [5]. We have developed a new cone-beam CT system using an X-ray image intensifier and charge coupled device (CCD) television camera as the detector. We have evaluated the system for two years; it has been shown to be very effective for planning operations and confirming their results, for orthopedic imaging [6], [7], [8], [9], [10]. However, it is necessary to enlarge its field of view and improve its spatial resolution by future development.
A flat-panel detector (FPD) can form a higher spatial resolution image than an image-intensifier detector (IID) [11], [12], [13], [14], [15]. In addition, it can greatly enlarge the field of view. Accordingly, we evaluated a FFD and compared it with an IID for use in a cone-beam CT detector.
Section snippets
Detectors
The FPD (provided by Varian Medical Systems, Inc.) consists of a scintillator screen and a-Si photo-sensor array (Fig. 1). An X-ray beam is converted to an optical signal by the scintillator, and detected by a-Si sensor array. The FPD does not generate veiling glare or image distortion and has a smaller imaging pixel than that of the IID.
The IID consists of a 16-inch X-ray image intensifier, optics, and a CCD camera. An X-ray beam is converted to an optical signal by the input phosphor screen,
Spatial resolution
A 165-mm diameter cylindrical water phantom containing a high-contrast bar pattern was used to evaluate the contrast resolution on the plane perpendicular to the rotation-axis (X–Y plane) and parallel to it (Y–Z plane). During a scan, 576 projection images were obtained and several reconstructed images were added in order to evaluate spatial resolution without influence of the streak artifact and noise.
A 0.35-mm bar pattern can be resolved on both the X–Y and Y–Z planes of the reconstructed
Conclusions
The FPD cone-beam CT system has a higher spatial resolution than the IID system. At equal pixel sizes, the standard deviation of noise intensity of the FPD system is equal to that of the IID system. However, the circuit noise of the FPD must be reduced so that the standard deviation of its noise will become equal to that of the IID using the iris control, especially at low doses. Moreover, the X-ray-dose control method is effective to improve the S/N of three-dimensional images.
Our evaluations
Rika Baba received her Master's degree of Agriculture from Kyoto University in 1991. Since 1991, she has been working for Central Research Laboratory, Hitachi Ltd. Her current research interests include three-dimensional medical imaging and processing.
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Cited by (0)
Rika Baba received her Master's degree of Agriculture from Kyoto University in 1991. Since 1991, she has been working for Central Research Laboratory, Hitachi Ltd. Her current research interests include three-dimensional medical imaging and processing.
Yasutaka Konno received his Master's degree of Science from Tokyo University in 1998. Since 1998, he has been working for Central Research Laboratory, Hitachi Ltd. His current research interests include designing X-ray detector for medical imaging.
Ken Ueda received his Master's degree of Engineering from Tokyo University in 1972. He worked for Central Research Laboratory, Hitachi Ltd from 1972 to 1999. Since 1999, he has been working for Research and Development Center, Hitachi Medical Corporation. His current research interests include three-dimensional medical imaging and processing.
Shigeyuki Ikeda received his Bachelor's degree of Science from The University of Electro-Communications in 1982. Since 1982, he has been working for Research and Development Center, Hitachi Medical Corporation. He has developed digital radiography system including flat panel detector system.