Herbert Bruder

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.

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.

Weighted FBP-a simple approximate 3D FBP algorithm for multislice spiral CT with good dose usage for arbitrary pitch

A new 3D reconstruction scheme, weighted filtered backprojection (WFBP) for multirow spiral CT based on an extension of the two-dimensional SMPR algorithm is described and results are presented. In contrast to other 3D algorithms available, the algorithm makes use of all available data for all pitch values. The algorithm is a FBP algorithm: linear convolution of the parallel data along the row direction followed by a 3D backprojection. Data usage for arbitrary pitch values is maintained through a weighting scheme which takes into account redundant data. If proper row weighting is applied, the image quality is superior to the image quality of the SMPR algorithm.

Segmented multiple plane reconstruction: a novel approximate reconstruction scheme for multi-slice spiral CT

A new reconstruction scheme for multi-row spiral CT is described and results are presented. The spiral path is decomposed into small, overlapping segments which are used for a separate convolution and backprojection yielding a stack of segment images which contain only projection data of a partial scan (typically in the range of 20). These segment image stacks are, in a second step, reformatted to the requested image planes. In a third step, the reformatted segment images are added to obtain full images. The main benefit of the proposed algorithm is superior images quality. A 64-row dataset with a cone angle of 6.4 and a table feed of 80 mm per spiral turn has been reconstructed with excellent image quality. A filter direction for three-dimensional (3D) backprojection algorithms is suggested by investigating the limit where the partial scan size goes to zero.

Novel approximate approach for high-quality image reconstruction in helical cone-beam CT at arbitrary pitch

We present a novel approximate image reconstruction technique for helical cone-beam CT, called the Advanced Multiple Plane Reconstruction (AMPR). The method is an extension of the ASSR algorithm presented in Medical Physics vol. 27, no. 4, 2000 by Kachelriess et al. In the ASSR, the pitch is fixed to a certain value and dose usage is not optimum. These limitations have been overcome in the AMPR algorithm by reconstructing several image planes from any given half scan range of projection angles. The image planes are tilted in two orientations so as to optimally use the data available on the detector. After reconstruction of several sets of tilted images, a subsequent interpolation step reformats the oblique image planes to a set of voxels sampled on a cartesian grid. Using our novel approach on a scanner with 16 slices, we can achieve image quality superior to what is currently a standard for four-slice scanners. Dose usage in the order of 95% for all pitch values can be achieved. We present simulations of semi-antropomorphic phantoms using a standard CT scanner geometry and a 16 slice design.

Performance evaluation of a multi-slice CT system with 16-slice detector and increased gantry rotation speed for isotropic submillimeter imaging of the heart.

Background: 4-slice CT scanners have shown limitations in clinical application for noninvasive coronary CT angiography (CTA). We evaluate advances in ECG-gated scanning of the heart and the coronary arteries with recently introduced 16-slice CT equipment (SOMATOM Sensation 16, Siemens, Forchheim, Germany). Materials and Methods: The technical principles of ECG-gated cardiac scanning, scan parameters, and detector design of the new scanner are presented. ECG-gated scan and image reconstruction techniques and ECG-controlled dose modulation (“ECG pulsing”) for a reduction of the patient dose are described, key parameters for image quality and simulation results presented, and phantom studies and initial patient experience discussed. The impact of reduced gantry rotation time (0.42 s) on temporal resolution and initial estimations of the patient dose are presented. Results: Extensions of ECG-gated reconstruction algorithms used for 4-slice CT provide adequate image quality for up to 16 slices. For each detector collimation different slice widths are available for retrospective reconstruction with well-defined slice sensitivity profiles (SSPs). For coronary CTA the heart can be covered with 0.75 mm collimation within a 20-s breathhold. The best possible spatial resolution is 0.5 × 0.5 × 0.6 mm. For 0.42 s gantry rotation time, temporal resolution reaches its optimum (105 ms) at a heart rate of 81 bpm. Effective patient dose for coronary CTA is 4–5 mSv using ECG-pulsed acquisition. Conclusion: The clinical performance of coronary CTA by means of spatial resolution, temporal resolution and scan time is substantially improved with the evaluated 16-slice CT scanner. Also, display of smaller coronary segments and instent visualization are substantially improved.Hintergrund: Die Einführung der Mehrschicht-CT im Jahr 1998 stellte einen Durchbruch der mechanischen CT in der nichtinvasiven Bildgebung des Herzens dar. Klinische Studien zeigten jedoch Limitationen gängiger 4-Schicht-CT-Geräte, welche die breite klinische Anwendung der Methode einschränken. Material und Methoden: Die neue Generation von Mehrschicht-CT-Geräten bietet gleichzeitige Aufnahme von bis zu 16 Submillimeter-Schichten und verbesserte Zeitauflösung für Herzuntersuchungen durch Verringerung der Gantry-Rotationszeit auf 0,42 s. Diese Arbeit stellt die technischen Grundlagen und mögliche Applikationen dieser neuen Technologie in der Untersuchung des Herzens und der Koronargefäße am Beispiel eines neuartigen 16-Schicht-CT-Gerätes dar (SOMATOM Sensation 16, Siemens AG, Forchheim). Das neue Systemkonzept, Detektordesign und Dosiseffizienz sowie Datenaufnahme- und Bildrekonstruktionstechniken für die EKG-synchronisierte Untersuchung des Herzens werden diskutiert. Ergebnisse: Die Erweiterung gängiger Rekonstruktionstechniken von 4- auf 16-Schicht-Detektoren liefert eine diagnostisch adäquate Bildqualität. Die neuen Verfahren sind in der Lage, für jede kollimierte Schichtdicke Schichten verschiedener Dicke mit gut definierten Schichtempfindlichkeitsprofilen zu rekonstruieren. Basierend auf einer kollimierten Schichtdicke von 0,75 mm kann eine hochaufgelöste CT-Angiographie (CTA) des Herzens und der Koronargefäße in einer 20-sekündigen Atempause durchgeführt werden. Die bestmögliche räumliche Auflösung beträgt 0,5 × 0,5 × 0,6 mm. Mit einer Rotationszeit des Messsystems von 0,42 s wird die bestmögliche zeitliche Auflösung von 105 ms bei einer Herzfrequenz von 81 Schlägen/min erreicht. Unter Verwendung einer EKG-synchronisierten Röhrenstrommodulation ist für eine CTA des Herzens und der Herzkranzgefäße mit einer effektiven Patientendosis von 4–5 mSv zu rechnen. Schlussfolgerung: Mit den neuen 16-Schicht-CT-Systemen lassen sich bei reduzierter Untersuchungszeit sowohl die räumliche als auch zeitliche Auflösung für die CTA des Herzens und der Koronargefäße deutlich verbessern. Auch die Darstellung kleinerer Koronarsegmente und die In-Stent-Visualisierung werden ermöglicht.

Single-slice rebinning reconstruction in spiral cone-beam computed tomography

At the advent of multislice computed tomography (CT) a variety of approximate cone-beam algorithms have been proposed suited for reconstruction of small cone-angle CT data in a spiral mode of operation. The goal of this study is to identify a practical and efficient approximate cone-beam method, extend its potential for medical use, and demonstrate its performance at medium cone-angles required for area detector CT. The authors investigate two different approximate single-slice rebinning algorithms for cone-beam CT: the multirow Fourier reconstruction (MFR) and an extension of the advanced single-slice rebinning method (ASSR), which combines the idea of ASSR with a z-filtering approach. Thus, both algorithms, MFR and ASSR, are formulated in the framework of z-filtering using optimized spiral interpolation algorithms. In each view, X-ray samples to be used for reconstruction are identified, which describe an approximation to a virtual reconstruction plane. The performance of approximate reconstruction should improve as the virtual reconstruction plane better fits the spiral focus path. The image quality of the respective reconstruction is assessed with respect to image artifacts, spatial resolution, contrast resolution, and image noise. It turns out that the ASSR method using tilted reconstruction planes is a practical and efficient algorithm, providing image quality comparable to that of a single-row scanning system even with a 46-row detector at a table feed of 64 mm. Both algorithms tolerate any table feed below the maximum value associated to the detector height. Due to the z-filter approach, all detector data sampled can be used for image reconstruction.

Improving best-phase image quality in cardiac CT by motion correction with MAM optimization.

PURPOSE Research in image reconstruction for cardiac CT aims at using motion correction algorithms to improve the image quality of the coronary arteries. The key to those algorithms is motion estimation, which is currently based on 3-D/3-D registration to align the structures of interest in images acquired in multiple heart phases. The need for an extended scan data range covering several heart phases is critical in terms of radiation dose to the patient and limits the clinical potential of the method. Furthermore, literature reports only slight quality improvements of the motion corrected images when compared to the most quiet phase (best-phase) that was actually used for motion estimation. In this paper a motion estimation algorithm is proposed which does not require an extended scan range but works with a short scan data interval, and which markedly improves the best-phase image quality. METHODS Motion estimation is based on the definition of motion artifact metrics (MAM) to quantify motion artifacts in a 3-D reconstructed image volume. The authors use two different MAMs, entropy, and positivity. By adjusting the motion field parameters, the MAM of the resulting motion-compensated reconstruction is optimized using a gradient descent procedure. In this way motion artifacts are minimized. For a fast and practical implementation, only analytical methods are used for motion estimation and compensation. Both the MAM-optimization and a 3-D/3-D registration-based motion estimation algorithm were investigated by means of a computer-simulated vessel with a cardiac motion profile. Image quality was evaluated using normalized cross-correlation (NCC) with the ground truth template and root-mean-square deviation (RMSD). Four coronary CT angiography patient cases were reconstructed to evaluate the clinical performance of the proposed method. RESULTS For the MAM-approach, the best-phase image quality could be improved for all investigated heart phases, with a maximum improvement of the NCC value by 100% and of the RMSD value by 81%. The corresponding maximum improvements for the registration-based approach were 20% and 40%. In phases with very rapid motion the registration-based algorithm obtained better image quality, while the image quality of the MAM algorithm was superior in phases with less motion. The image quality improvement of the MAM optimization was visually confirmed for the different clinical cases. CONCLUSIONS The proposed method allows a software-based best-phase image quality improvement in coronary CT angiography. A short scan data interval at the target heart phase is sufficient, no additional scan data in other cardiac phases are required. The algorithm is therefore directly applicable to any standard cardiac CT acquisition protocol.

Dual-source computed tomography for assessing cardiac function: a phantom study.

Purpose:To investigate the influence of heart rate and temporal resolution on the assessment of global ventricular function with dual-source computed tomography (DSCT). Materials and Methods:A dynamic cardiac phantom was repeatedly scanned with a DSCT scanner applying a standardized scan protocol at different heart rates, ranging from 40 to 140 bpm. Images were reconstructed with monosegmental and bisegmental algorithms using data from a single source and from both sources. Ventricular volumes and ejection fraction (EF) were computed by semiautomated analysis. Results were compared with the phantoms real volumes. Interscan, intraobserver, and interobserver variability were calculated. Results:For single-source data reconstruction temporal resolution was fixed to 165 milliseconds, whereas dual-source image reconstructions resulted in a temporal resolution of 83 milliseconds (monosegmental) and 67.7 ± 14.2 milliseconds (bisegmental), respectively. In general, deviation from the phantoms real volumes was less with dual-source data reconstruction when compared with single-source data reconstruction. Comparing dual-source data reconstruction with single-source data reconstruction, the percent deviation from the phantoms real volumes for EF was 0.7% (monosegmental), 0.7% (bisegmental), and 4.3% (single source), respectively. There was no correlation between heart rate and EF for dual-source data reconstruction (r = −0.168; r = −0.157), whereas a relevant correlation was observed for single-source data reconstruction (r = −0.844). Interscan, intraobserver, and interobserver variability for EF were 1.4%, 0.9%, and 0.3%, respectively. Conclusions:DSCT allows reliable quantification of global ventricular function independent of the heart rate. Multisegmental image reconstruction is not needed for DSCT assessment of global ventricular function.

Influence of heart rate and temporal resolution on left-ventricular volumes in cardiac multislice spiral computed tomography: a phantom study.

Purpose:We sought to investigate the influence of heart rate and temporal resolution on the assessment of left-ventricular (LV) function with multislice spiral computed tomography (CT). Material and Methods:A dynamic cardiac phantom was repeatedly scanned with a 64-slice CT scanner using a standardized scan protocol (64 × 0.6 mm, 120kV, 770mAseff, 330 milliseconds rotation time) at different simulated heart rates, ranging from 40 to 140 beats per minute. Images were reconstructed with an algorithm utilizing data from 1 to 4 cardiac cycles (RR intervals). Ejection fraction (EF), end-systolic, end-diastolic, and stroke volume as well as cardiac output were calculated. Results of the measurements were compared with the real volumes of the phantom. Interscan and intraobserver variability were calculated. Results:Using a monosegmental reconstruction algorithm, the temporal resolution was fixed to 165 milliseconds. With bi-, tri-, and quad-segmental image reconstruction, mean temporal resolution was 128.3 ± 33.2 milliseconds, 103.3 ± 49.2 milliseconds, and 87.8 ± 81.5 milliseconds, respectively. Multisegmental image reconstruction resulted in a lower deviation when comparing measured and real volumes. Using mono-, bi-, tri-, and quad-segmental image reconstruction, the percent deviation between measured and real values for EF was 8.2%, 4.5%, 3.3%, and 3.4%, respectively. Applying multisegmental image reconstruction with improved temporal resolution the deviation decreased with increasing heart rate when compared with mono-segmental image reconstruction. Interscan and intraobserver variability for EF were 1.1% and 1.9%, respectively. Conclusion:Enhanced temporal resolution improves the quantification of LV volumes in cardiac multislice spiral CT, enabling reliable assessment of LV volumes even at increased heart rates.