Development and performance evaluation of the first model of 4-D CTscanner

Four-dimensional (4-D) computed tomography (CT) is a dynamic volume imaging system of moving organs with an image quality comparable to conventional CT and it is realized by continuous and high-speed cone-beam CT. We have developed a novel two-dimensional (2-D) detector for 4-D CT, which is based on the present CT technology, and mounted it on the gantry frame of the state-of-the-art CTscanner. In the present paper, we describe the design and the performance evaluation results of the first model of the 4-D CT-scanner. The X-ray detector for the 4-D CT-scanner is a discrete pixel detector in which pixel data are measured from independent detector elements. The numbers of elements are 912 (channels) * 256 (segments) and the element size is approximately 1 mm * 1 mm. Data sampling rate is 900 views (frames)/s and the dynamic range of the A/D converter is 16 bits. The rotation speed of the gantry is 1.0 s/rotation. The data transfer system between rotating and stationary parts in the gantry consists of laser diode and photodiode pairs and it achieves a net transfer speed of 5 Gbps. Volume data of 512*512*256 voxels are reconstructed with an Feldkamp-Davis-Kress (FDK) algorithm by parallel use of microprocessors. The image characteristics such as noise, uniformity, and spatial resolution were evaluated with stationary phantoms in a single rotation. The exposure dose to an object was measured with an extension of the standard measurement method of CT dose index (CTDI). Several volunteers were scanned to explore clinical potentials. For the 4-D CT-scanner, the image characteristics of stationary objects were almost the same as that of conventional CT, while CTDI of the 4-D CT was slightly higher than that of conventional CT. The isotropic resolving power of less than 0.5 mm was achieved for the stationary object.