In dual source CT (DSCT) with two X-ray sources and two data measurement systems mounted on a CT gantry with a mechanical offset of 90 deg, cross scatter radiation, (essentially 90 deg Compton scatter) is added to the detector signals. In current DSCT scanners the cross scatter correction is model based: the idea is to describe the scattering surface in terms of its tangents. The positions of these tangent lines are used to characterize the shape of the scattering object. For future DSCT scanners with larger axial X-ray beams, the model based correction will not perfectly remove the scatter signal in certain clinical situations: for obese patients scatter artifacts in cardiac dual source scan modes might occur. These shortcomings can be circumvented by utilizing the
non-diagnostic time windows in cardiac scan modes to detect cross scatter online. The X-ray generators of both systems have to be switched on and off alternating. If one X-ray source is switched off, cross scatter deposited in the respective other detector can be recorded and processed, to be used for efficient cross scatter correction. The procedure will be demonstrated for cardiac step&shoot as well as for spiral acquisitions. Full rotation reconstructions are less sensitive to cross scatter radiation; hence in non-cardiac case the model-based approach is sufficient. Based on measurements of physical and anthropomorphic phantoms we present image data for DSCT systems with various collimator openings demonstrating the efficacy of the proposed method. In addition, a thorough analysis of contrast-to-noise ratio (CNR) shows, that even for a X-ray beam corresponding to a 64x0.6 mm collimation, the maximum loss of CNR due to cross scatter is only about 7% in case of obese patients.
Truncation of CT projection data is always coupled with incomplete angular sampling and can lead to severe image
artifacts in clinical CT. Extrapolation of projection data is needed to restore CT values inside and outside the scan field
of view (SFOV).
We present three types of extrapolation schemes. The first type (M1) is characterized by extrapolation of projection data
using a virtual object of constant attenuation. For multi-slice helical CT this extrapolation scheme is applied in a row-wise
manner. The second type (M2) utilizes consistency conditions of parallel projection data. The conservation of mth
order moments of non-truncated projections can be utilized for the extrapolation of truncated projection data by fitting
extrapolation functions of variable length. The third method (M3) extrapolates truncated data by sinogram
decomposition and completion. For each voxel in image space the corresponding trace in the 3D-sinogram is computed.
The minimum signal within each trace is extrapolated to the extended sinogram parts, which represent the extended
Based on the evaluation of both simulation data of an anthropomorphic thorax phantom and clinical data, we evaluate
the three reconstruction techniques. Sinogram decomposition proofs to be better than the other techniques, but is
computationally very demanding.
In cardiac CT temporal resolution is directly related to the gantry rotation time of 3rd generation CT scanners. This time
cannot be substantially reduced below current standards of 0.33 s - 0.35 s due to mechanical limitations. As an
alternative we present a dual source CT (DSCT) system. The system is equipped with two X-ray tubes and two
corresponding detectors that are mounted onto the rotating gantry with an angular offset of 90°. Due to the simultaneous
data acquisition and the angular offset, complementary quarter-scan data are measured at the same phase in the cardiac
cycle. Hence, the exposure time of any image slice is reduced by a factor of two and the temporal resolution is
improved by the same factor. In contrast to single source cardiac CT with multi-segment image reconstruction, the
temporal resolution does not depend on the heart rate.
Since multi-segment reconstruction techniques applied in single source cardiac CT, which limit the table speed, are no
longer needed, faster volume coverage in cardiac spiral imaging can be achieved. As a consequence of these concepts,
patient dose in cardiac CT can be significantly reduced.
ECG correlated image reconstruction is based on 3D backprojection of the Feldkamp type. Data truncation coming from
the fact that one detector (A) covers the entire scan field of view (50 cm in diameter), while the other detector (B) is
restricted to a smaller, central field of view (26 cm in diameter), has to be treated.
We evaluate temporal resolution and dose efficiency by means of phantom scans and computer simulations. We present
first patient scans to illustrate the performance of DSCT for ECG correlated cardiac imaging.
We developed and evaluated a prototype flat-panel detector based Volume CT (fpVCT) scanner. The fpVCT scanner consists of a Varian 4030CB a-Si flat-panel detector mounted in a multi slice CT-gantry (Siemens Medical Solutions). It provides a 25 cm field of view with 18 cm z-coverage at the isocenter. In addition to the standard tomographic scanning, fpVCT allows two new scan modes: (1) fluoroscopic imaging from any arbitrary rotation angle, and (2) continuous, time-resolved tomographic scanning of a dynamically changing viewing volume. Fluoroscopic imaging is feasible by modifying the standard CT gantry so that the imaging chain can be oriented along any user-selected rotation angle. Scanning with a stationary gantry, after it has been oriented, is equivalent to a conventional fluoroscopic examination. This scan mode enables combined use of high-resolution tomography and real-time fluoroscopy with a clinically usable field of view in the z direction. The second scan mode allows continuous observation of a timeevolving process such as perfusion. The gantry can be continuously rotated for up to 80 sec, with the rotation time ranging from 3 to 20 sec, to gather projection images of a dynamic process. The projection data, that provides a temporal log of the viewing volume, is then converted into multiple image stacks that capture the temporal evolution of a dynamic process. Studies using phantoms, ex vivo specimens, and live animals have confirmed that these new scanning modes are clinically usable and offer a unique view of the anatomy and physiology that heretofore has not been feasible using static CT scanning. At the current level of image quality and temporal resolution, several clinical applications such a dynamic angiography, tumor enhancement pattern and vascularity studies, organ perfusion, and interventional applications are in reach.
We designed, assembled and evaluated a prototype volume CT scanner (VCT) for the purpose of investigating various calibration methods and cone beam reconstruction algorithms as well as the potential clinical benefits of a high-resolution volume CT scanner. The new VCT is based on SIEMENS Sensation4 CT scanner. To achieve larger volume coverage and higher spatial resolution we replaced the prior 4-slices detector with a flat-panel detector. We also modified the prior x-ray tube to achieve a very small focus size by a smaller emitter and wider axial coverage by a larger anode angle. In addition the high-voltage generator was enhanced to support pulsed operation. Special measurement methods were elaborated and applied to measure the focus size, shape and position as well as the uniformity of the flat field x-ray exposure. The accuracy and stability of gantry rotation speed has been evaluated to decide for the most appropriate exposure trigger. New methods are applied to measure and calibrate the resulted x-ray geometry. One prototype VCT scanner is installed at a pre-clinical site to evaluate the application potential of the new VCT technology. The new volume scanner achieves unprecedented spatial resolution, slice sensitivity and spatial coverage. In a complementary paper we present the image quality, contrast resolution and dose issues associated with this scanner.
We developed and evaluated a prototype flat-panel detector based Volume CT (VCT) scanner. We focused on improving the image quality using different detector settings and reducing x-ray scatter intensities. For the presented results we used a Varian 4030CB flat-panel detector mounted in a multislice CT-gantry (Siemens Medical Systems). The scatter intensities may severely impair image quality in flat-panel detector CT systems. To reduce the impact of scatter we tested bowtie shaped filters, anti-scatter grids and post-processing correction algorithms. We evaluated the improvement of image quality by each method and also by a combination of the several methods. To achieve an extended dynamic range in the projection data, we implemented a novel dynamic gain-switching mode. The read out charge amplifier feedback capacitance is changing dynamically in this mode, depending on the signal level. For this scan mode dedicated corrections in the offset and gain calibration are required. We compared image quality in terms of low contrast for both, the dynamic mode and the standard fixed gain mode. VCT scanners require different types of dose parameters. We measured the dose in a 16 cm CTDI phantom and free air in the scanners iso-center and defined a new metric for a VCT dose index (VCTDI). The dose for a high quality VCT scan of this prototype scanner varied between 15 and 40 mGy.