Martin Petersilka

Technical principles of dual source CT

During the past years, multi-detector row CT (MDCT) has evolved into clinical practice with a rapid increase of the number of detector slices. Todays 64 slice CT systems allow whole-body examinations with sub-millimeter resolution in short scan times. As an alternative to adding even more detector slices, we describe the system concept and design of a CT scanner with two X-ray tubes and two detectors (mounted on a CT gantry with a mechanical offset of 90 degrees) that has the potential to overcome limitations of conventional MDCT systems, such as temporal resolution for cardiac imaging. A dual source CT (DSCT) scanner provides temporal resolution equivalent to a quarter of the gantry rotation time, independent of the patients heart rate (83 ms at 0.33 s rotation time). In addition to the benefits for cardiac scanning, it allows to go beyond conventional CT imaging by obtaining dual energy information if the two tubes are operated at different voltages. Furthermore, we discuss how both acquisition systems can be used to add the power reserve of two X-ray tubes for long scan ranges and obese patients. Finally, future advances of DSCT are highlighted.

Image reconstruction and image quality evaluation for a dual source CT scanner

The authors present and evaluate concepts for image reconstruction in dual source CT (DSCT). They describe both standard spiral (helical) DSCT image reconstruction and electrocardiogram (ECG)-synchronized image reconstruction. For a compact mechanical design of the DSCT, one detector (A) can cover the full scan field of view, while the other detector (B) has to be restricted to a smaller, central field of view. The authors develop an algorithm for scan data completion, extrapolating truncated data of detector (B) by using data of detector (A). They propose a unified framework for convolution and simultaneous 3D backprojection of both (A) and (B) data, with similar treatment of standard spiral, ECG-gated spiral, and sequential (axial) scan data. In ECG-synchronized image reconstruction, a flexible scan data range per measurement system can be used to trade off temporal resolution for reduced image noise. Both data extrapolation and image reconstruction are evaluated by means of computer simulated data of anthropomorphic phantoms, by phantom measurements and patient studies. The authors show that a consistent filter direction along the spiral tangent on both detectors is essential to reduce cone-beam artifacts, requiring truncation of the extrapolated (B) data after convolution in standard spiral scans. Reconstructions of an anthropomorphic thorax phantom demonstrate good image quality and dose accumulation as theoretically expected for simultaneous 3D backprojection of the filtered (A) data and the truncated filtered (B) data into the same 3D image volume. In ECG-gated spiral modes, spiral slice sensitivity profiles (SSPs) show only minor dependence on the patients heart rate if the spiral pitch is properly adapted. Measurements with a thin gold plate phantom result in effective slice widths (full width at half maximum of the SSP) of 0.63-0.69 mm for the nominal 0.6 mm slice and 0.82-0.87 mm for the nominal 0.75 mm slice. The visually determined through-plane (z axis) spatial resolution in a bar pattern phantom is 0.33-0.36 mm for the nominal 0.6 mm slice and 0.45 mm for the nominal 0.75 mm slice, again almost independent of the patients heart rate. The authors verify the theoretically expected temporal resolution of 83 ms at 330 ms gantry rotation time by blur free images of a moving coronary artery phantom with 90 ms rest phase and demonstrate image noise reduction as predicted for increased reconstruction data ranges per measurement system. Finally, they show that the smoothness of the transition between image stacks acquired in different cardiac cycles can be efficiently controlled with the proposed approach for ECG-synchronized image reconstruction.

Dual-source spiral CT with pitch up to 3.2 and 75 ms temporal resolution: image reconstruction and assessment of image quality.

PURPOSE To present the theory for image reconstruction of a high-pitch, high-temporal-resolution spiral scan mode for dual-source CT (DSCT) and evaluate its image quality and dose. METHODS With the use of two x-ray sources and two data acquisition systems, spiral CT exams having a nominal temporal resolution per image of up to one-quarter of the gantry rotation time can be acquired using pitch values up to 3.2. The scan field of view (SFOV) for this mode, however, is limited to the SFOV of the second detector as a maximum, depending on the pitch. Spatial and low contrast resolution, image uniformity and noise, CT number accuracy and linearity, and radiation dose were assessed using the ACR CT accreditation phantom, a 30 cm diameter cylindrical water phantom or a 32 cm diameter cylindrical PMMA CTDI phantom. Slice sensitivity profiles (SSPs) were measured for different nominal slice thicknesses, and an anthropomorphic phantom was used to assess image artifacts. Results were compared between single-source scans atpitch=1.0 and dual-source scans at pitch=3.2. In addition, image quality and temporal resolution of an ECG-triggered version of the DSCT high-pitch spiral scan mode were evaluated with a moving coronary artery phantom, and radiation dose was assessed in comparison with other existing cardiac scan techniques. RESULTS No significant differences in quantitative measures of image quality were found between single-source scans atpitch=1.0 and dual-source scans at pitch=3.2 for spatial and low contrast resolution, CT number accuracy and linearity, SSPs, image uniformity, and noise. The pitch value (1.6≤pitch≤3.2) had only a minor impact on radiation dose and image noise when the effective tube current time product (mA s/pitch) was kept constant. However, while not severe, artifacts were found to be more prevalent for the dual-source pitch=3.2 scan mode when structures varied markedly along the z axis, particularly for head scans. Images of the moving coronary artery phantom acquired with the ECG-triggered high-pitch scan mode were visually free from motion artifacts at heart rates of 60 and 70 bpm. However, image quality started to deteriorate for higher heart rates. At equivalent image quality, the ECG-triggered high-pitch scan mode demonstrated lower radiation dose than other cardiac scan techniques on the same DSCT equipment (25% and 60% dose reduction compared to ECG-triggered sequential step-and-shoot and ECG-gated spiral with x-ray pulsing). CONCLUSIONS A high-pitch (up topitch=3.2), high-temporal-resolution (up to 75 ms) dual-source CT scan mode produced equivalent image quality relative to single-source scans using a more typical pitch value (pitch=1.0). The resultant reduction in the overall acquisition time may offer clinical advantage for cardiovascular, trauma, and pediatric CT applications. In addition, ECG-triggered high-pitch scanning may be useful as an alternative to ECG-triggered sequential scanning for patients with low to moderate heart rates up to 70 bpm, with the potential to scan the heart within one heart beat at reduced radiation dose.

Erratum: First performance evaluation of a dual-source CT (DSCT) system (European Radiology (2006) vol. 16 (2) (256-268) 10.1007/ s00330-005-2919-2)

Unfortunately, the acronym ECG was incorrectly defined throughout the text. The correct term should be electrocardiogram /electrocardiograph in the Abstract, the first word of the Introduction, and in the legends to Figs. 4, 11, 12. The original article can be found at http://dx. doi.org/10.1007/s00330-005-2919-2. T. G. Flohr (*) . H. Bruder . M. Petersilka . K. Gruber . C. Süß . M. Grasruck . K. Stierstorfer . B. Krauss . R. Raupach . B. M. Ohnesorge Siemens Medical Solutions, Computed Tomography CTE PA, Siemensstrasse 1, 91301 Forchheim, Germany e-mail: thomas.flohr@siemens.com Tel.: +49-9191-188195

Design considerations in cardiac CT

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.

Flash imaging in dual source CT (DSCT)

We present new acquisition modes of a recently introduced dual-source computed tomography (DSCT) system equipped with two X-ray tubes and two corresponding detectors, mounted onto the rotating gantry with an angular offset of typically 90°. Due to the simultaneous acquisition of complementary data, the minimum exposure time is reduced by a factor of two compared to a single-source CT system (SSCT). The correspondingly improved temporal resolution is beneficial for cardiac CT. Also, maximum table feed per rotation in a spiral mode can be increased by a factor of 2 compared to SSCT, which provides benefits both for cardiac CT and non-cardiac CT. In an ECG-triggered mode the entire cardiac volume can be scanned within a fraction of one cardiac RR-cycle. At a rotation time of 0.28s using a detector with 64×0.6 mm beam collimation, the scan time of the entire heart is less than 0.3s at a temporal resolution of 75 ms. It will be shown, that the extremely fast cardiac scan reduces the patient dose to a theoretical lowest limit: for a 120 kV scan the dose level for a typical cardiac CT scan is well below 2 mSv. Using further protocol optimization (scan range adaptation, 100kV), the radiation dose can be reduced below 1mSv.

Correction of cross-scatter in next generation dual source CT (DSCT) scanners

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.

3D image-based scatter estimation and correction for multi-detector CT imaging

The aim of this work is to implement and evaluate a 3D image-based approach for the estimation of scattered radiation in multi-detector CT. Based on a reconstructed CT image volume, the scattered radiation contribution is calculated in 3D fan-beam geometry in the framework of an extended point-scatter kernel (PSK) model of scattered radiation. The PSK model is based on the calculation of elemental scatter contributions propagating the rays from the focal spot to the detector across the object for defined interaction points on a 3D fan beam grid. Each interaction point in 3D leads to an individual elemental 2D scatter distribution on the detector. The sum of all elemental contributions represents the total scatter intensity distribution on the detector. Our proposed extended PSK depends on the scattering angle (defined by the interaction point and the considered detector channel) and the line integral between the interaction point on a 3D fan beam ray and the intersection of the same ray with the detector. The PSK comprises single- and multiple scattering as well as the angular selectivity characteristics of the anti-scatter grid on detector. Our point-scatter kernels were obtained from a low-noise Monte-Carlo simulation of water-equivalent spheres with different radii for a particular CT scanner geometry. The model allows obtaining noise-free scatter intensity distribution estimates with a lower computational load compared to Monte-Carlo methods. In this work, we give a description of the algorithm and the proposed PSK. Furthermore, we compare resulting scatter intensity distributions (obtained for numerical phantoms) to Monte-Carlo results.

The impact of dual energy CT on pseudo enhancement of kidney lesions

Renal lesion detection and characterization using Computed Tomography is an important application in genitourinary radiology. Although in general the detection of renal lesions has been shown to be exceedingly accuratce, the detection of benign renal cysts is still problematic. Under certain circumstances, the attenuation values inside a cyst increase incorrectly with an increase in the iodine concentration in the surrounding soft tissue. This so called pseudoenhancement complicates the classification of cysts and creates severe difficulties to distinguish a benign nonenhancing lesion from an enhancing mass. In the present study, the standard procedure based on a single energy 120 kV mode is compared to three dual energy modes available on the Siemens Somatom Definition Flash scanner. In order to simulate the kidney and the lesions, several plastic rods were placed inside a small container filled with different iodine concentrations. This phantom is then positioned inside water tanks of different sizes. The rods simulating the lesions are made out of a special plastic with constant HU value throughout the relevant X-ray energy range. During the project, three important aspects have been discovered: 1) for normal situations, a 100/140 Sn kV mode on the Siemens Flash scanner is similar to the traditional single energy 120 kV mode. 2) For small patient sizes, all dual energy modes show a reduction of pseudoenhancement. 3) For larger patients, only the 100/140 Sn kV mode results in a reduction of pseudoenhancement. Both the 80/140 kV and the 80/140 Sn kV mode show a worse performance than the 120 kV single energy mode in a very large phantom size.

Scatter correction for multi-slice CT system

Scatter radiation in multi-slice CT system nowadays is playing a more and more important role, as the slice number is getting larger and larger, e.g. from 16 to 64 and even more. Scatter radiation may downgrade image quality e.g. create inhomogeneity and decrease the image contrast. Although in current multi-slice CT, anti-scatter collimation is widely used to reduce the scatter radiation received, as the detector is getting wider, its efficiency is getting weaker. Although beam hardening correction can somehow guarantee image homogeneity, as beam hardening effect and scatter radiation have different physical origin, a delegated scatter correction algorithm is desired. In this paper, we would like to propose a scatter correction algorithm working during data pre-processing before image reconstruction. After we implemented this algorithm in Siemens latest released Somatom Go. CT system, we obtain good image quality especially under certain clinical cases.