Hiroshi Aradate

Algorithm for image reconstruction in multi-slice helical CT

Efforts are being made to develop a new type of CT system that can scan volumes over a large range within a short time with thin slice images. One of the most promising approaches is the combination of helical scanning with multi-slice CT, which involves several detector arrays stacked in the z direction. However, the algorithm for image reconstruction remains one of the biggest problems in multi-slice CT. Two helical interpolation methods for single-slice CT, 360LI and 180LI, were used a starting points and extended to multi-slice CT. The extended methods, however, had a serious image quality problem due to the following three reasons: (1) excessively close slice positions of the complementary and direct data, resulting in a larger sampling interval; (2) the existence of several discontinuous changeovers in pairs of data samples for interpolation; and (3) the existence of cone angles. Therefore we have proposed a new algorithm to overcome the problem. It consists of the following three parts: (1) optimized sampling scan; (2) filter interpolation; and (3) fan-beam reconstruction. Optimized sampling scan refers to a special type of multi-slice helical scan developed to shift the slice position of complementary data and to acquire data with a much smaller sampling interval in the z direction. Filter interpolation refers to a filtering process performed in the z direction using several data. The normal fan-beam reconstruction technique is used. The section sensitivity profile (SSP) and image quality for four-array multi-slice CT were investigated by computer simulations. Combinations of three types of optimized sampling scan and various filter widths were used. The algorithm enables us to achieve acceptable image quality and spatial resolution at a scanning speed that is about three times faster than that for single-slice CT. The noise characteristics show that the proposed algorithm efficiently utilizes the data collected with optimized sampling scan. The new algorithm allows suitable combinations of scan and filter parameters to be selected to meet the purpose of each examination.

Physical performance evaluation of a 256-slice CT-scanner for four-dimensional imaging.

We have developed a prototype 256-slice CT-scanner for four-dimensional (4D) imaging that employs continuous rotations of a cone-beam. Since a cone-beam scan along a circular orbit does not collect a complete set of data to make an exact reconstruction of a volume [three-dimensional (3D) image], it might cause disadvantages or artifacts. To examine effects of the cone-beam data collection on image quality, we have evaluated physical performance of the prototype 256-slice CT-scanner with 0.5 mm slices and compared it to that of a 16-slice CT-scanner with 0.75 mm slices. As a result, we found that image noise, uniformity, and high contrast detectability were independent of z coordinate. A Feldkamp artifact was observed in distortion measurements. Full width at half maximum (FWHM) of slice sensitivity profiles (SSP) increased with z coordinate though it seemed to be caused by other reasons than incompleteness of data. With regard to low contrast detectability, smaller objects were detected more clearly at the midplane (z = 0 mm) than at z = 40 mm, though circular-band like artifacts affected detection. The comparison between the 16-slice and the 256-slice scanners showed better performance for the 16-slice scanner regarding the SSP, low contrast detectability, and distortion. The inferiorities of the 256-slice scanner in other than distortion measurement (Feldkamp artifact) seemed to be partly caused by the prototype nature of the scanner and should be improved in the future scanner. The image noise, uniformity, and high contrast detectability were almost identical for both CTs. The 256-slice scanner was superior to the 16-slice scanner regarding the PSF, though it was caused by the smaller transverse beam width of the 256-slice scanner. In order to compare both scanners comprehensively in terms of exposure dose, noise, slice thickness, and transverse spatial resolution, K=Dsigma2ha3 was calculated, where D was exposure dose (CT dose index), sigma was magnitude of noise, h was slice thickness (FWHM of SSP), and a was transverse spatial resolution (FWHM of PSF). The results showed that the K value was 25% larger for the 16-slice scanner, and that the 256-slice scanner was 1.25 times more effective than the 16-slice scanner at the midplane. The superiority in K value for the 256-slice scanner might be partly caused by decrease of wasted exposure with a wide-angle cone-beam scan. In spite of the several problems of the 256-slice scanner, it took a volume data approximately 1.0 mm (transverse) x 1.3 mm (longitudinal) resolution for a wide field of view (approximately 100 mm long) along the zeta axis in a 1 s scan if resolution was defined by the FWHM of the PSF or the SSP, which should be very useful to take dynamic 3D (4D) images of moving organs.

Large-area two-dimensional detector for real-time three-dimensional CT (4D CT)

Real-time 3D CT is a high-speed cone-beam CT (4D-CT) with good low-contrast detectability. In a single rotation, a voxel data set with higher spatial resolution over a wide z- axis range can be obtained. Using continuous rotation, temporarily continuous voxel data sets and 3D dynamic images can be acquired. We have developed a large area 2D detector for 4D-CT, and we have got the first 4D-CT test images.

Development and evaluation of a real-time three-dimensional CT (4D-CT) scanner

4D-CT scanner is a high-speed cone-beam CT with good low- contrast detectability. In a single rotation, a voxel data set with higher spatial resolution over a wide z-axis can be obtained. Using continuous rotation, temporarily continuous voxel data sets and 3D dynamic images can be acquired. We have developed a 4D-CT prototype system using a large area 2D detector with high speed rotating gantry. The key technologies for 4D-CT scanner are a large area 2D detector, high-speed and continuous rotating gantry with high-speed data transfer. The full size 2D detector for prototype system has 912 channels, 256 detector rows for 0.5 mm slice thickness, and the sampling speed is 900 view/s. The rotation speed of the gantry is 1.0 s/rotation, and the speed of the data transfer system must be more than 5Gbps. The data transfer system consists of laser diode and photo diode. We have got real-time volume images of crayfish, moving jaw and breathing lung. We will start clinical research at next stage, and continue to improve the image quality.

4-dimensional computed tomography (4D CT) - its concepts and preliminary development

4D CT is a dynamic volume imaging system of moving organs with an image quality comparable to conventional CT. Dynamic cone-beam CT can realize it with several breakthroughs. They are: 1) large-area 2-dimensional (2D) detector, 2) high-speed data transfer system, 3) reconstruction algorithm, 4) ultra-high-speed reconstruction computer and 5) high-speed and continuous rotating gantry. Among them the development of the 2D detector is one of the biggest tasks because it should have as wide dynamic range and high data acquisition speed (view rate) as present CT detectors. We are now developing a 4D CT-scanner together with the key-components. It will take one volume in 0.5 sec with a 3D matrix of 512 /spl times/ 512 /spl times/ 512. This paper describes its concepts and design, as well as preliminary development of the 2D detector.

Performance evaluation of the first model of 4D CT-scanner

Abstract Four-dimensional computed tomography (4D CT) is a dynamic volume imaging system of moving organs with an image quality comparable to conventional CT. With 4D CT, one could carry out not only new diagnoses but also provide new interventional therapy by real-time observation of its procedure. In order to realize 4D CT, we have developed a novel 2D detector on the basis of the present CT technology, and mounted it on the gantry frame of the state of the art CT-scanner. We have evaluated its performances with standard stationary phantoms and scanned normal volunteers. In the present report, we describe the results of such performance evaluations.

Basic performance evaluation of the first model of four-dimensional CT scanner

We have developed a prototype of 4-dimensional (4D) CT-scanner that employs continuous rotation of cone-beam. Because a cone-beam scan along a circle orbit did not collect a complete set of data to make rigid reconstruction of volume (3D image), it might bring disadvantages or artifacts. To examine effects of the cone-beam data collection on image quality, we have evaluated basic performances of the prototype and compared them to those of a state-of-the-art multi-detector (MD) CT-scanner. As the results image characteristics such as noise, uniformity, high contrast and low contrast detectability of 4D CT were independent of z-coordinate, and comparable to those of MD CT. The transverse spatial resolution of 4D CT was independent of z-coordinate, and showed slightly better performance than that of MD CT, while the longitudinal spatial resolution of 4D CT was the same as the transverse one, and much better than that of MD CT in the present scan conditions. Isotropic resolving power of 0.5mm was achieved for 4D CT. A Feldkamp artifact was observed in distortion measurement though its clinical meaning has not been clarified. Exposure dose measured with CT dose index (CTDI) for 4D CT was comparable to that for MD CT. As a whole our first model of 4D CT-scanner was successful to take a volume data of 10cm long along longitudinal direction in a single rotation scan with comparable image quality and exposure dose to the state-of-the-art MD CT-scanner.