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

Abstract

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.

Cardiovascular Circulation and Hepatic Perfusion of Pigs in 4-Dimensional Films Evaluated by 256-Slice Cone-Beam Computed Tomography

“…Compared with a conventional 16-slice CT scanner with 0.75 mm slices, with this new 256-slice cone-beam CT, image noise, uniformity, and high contrast detectability are independent of the zaxis. 8 Furthermore, the scanning mechanism can accommo- date a rotation speed of up to 0.5 s/rotation and ECG-gated acquisition will be possible in the next generation of scanner. 8 In the present MSCT, using retrospective ECG-gating acquisition or respiratory motion-gating acquisition, 24,25 which use helical scanning with simultaneous recording of an ECG or respiratory motion signal, following acquisition, it is possible to obtain any volume data of the heart or lung at any cardiac or respiratory phase desired.…”

“…8 Furthermore, the scanning mechanism can accommo- date a rotation speed of up to 0.5 s/rotation and ECG-gated acquisition will be possible in the next generation of scanner. 8 In the present MSCT, using retrospective ECG-gating acquisition or respiratory motion-gating acquisition, 24,25 which use helical scanning with simultaneous recording of an ECG or respiratory motion signal, following acquisition, it is possible to obtain any volume data of the heart or lung at any cardiac or respiratory phase desired. Furthermore, using this system, it is possible to obtain maximum 100 volumetric data points divided into maximum 100 serial segments of one cardiac or respiratory phase such as 0, 1, 2....97, 98 and 99% of the R-to-R interval of the ECG or respiratory motion.…”

“…[8][9][10] Furthermore, maximum continuous 25 s scanning enables 4-D analysis. Although this CT technique does not involve electorcardiogram (ECG)-gated acquisition, the combination of its synchrony with volumetric data and reconstruction technique in which 1 scanning period is divided into a maximum of 100 phases, facilitates selection of the most static images and allows entire heart data acquisition without cardiac motion artifacts.…”

Radiation exposure in patients with subarachnoid hemorrhage: a quality improvement target

“…The number of slices, or data channels, acquired per axial rotation has increased, with 16-and 64-slice systems now available (as well as models having 2, 6, 8, 10, 32, and 40 slices). Soon even larger detector arrays and axial coverage per rotation (>4 cm) will be commercially available, with results from a 256-slice scanner having already been published 4 . These tremendous strides in technology have resulted in many changes in the clinical use of CT.…”

Respiratory Signal Extraction for 4D CT Imaging of the Thorax from Cone-Beam CT Projections

“…The 4D CT imaging of a free-breathing thorax must face one major technical problem: the time required ( 0.5 second per scan) for acquiring several complete 3D CT images along one respiratory cycle ( 4 second). Mori et al [2] have proposed a prototype 256-slice CT-scanner dedicated to real-time 4D imaging. Other methods currently in use address the slowness of actual CT acquisitions relatively to the respiratory cycle on both scanner geometries: spiral/helical and cone-beam (CB).…”