Karl Stierstorfer

Publications of Karl Stierstorfer

• Exact Efficient Handling of Interrupted Illumination in Helical Cone-Beam Computed Tomography with Arbitrary Pitch.

  Authors: Harald Schöndube, Karl Stierstorfer, Frédéric Noo

  Tsinghua science and technology. 02/2010; 15(1):36-43.

  We present a theoretically-exact and stable computed tomography (CT) reconstruction algorithm that is capable of handling interrupted illumination and therefore of using all measured data atWe present a theoretically-exact and stable computed tomography (CT) reconstruction algorithm that is capable of handling interrupted illumination and therefore of using all measured data at arbitrary pitch. This algorithm is based on a differentiated backprojection (DBP) on M-lines. First, we discuss the problem of interrupted illumination and how it affects the DBP. Then we show that it is possible to take advantage of some properties of the DBP to compensate for the effects of interrupted illumination in a mathematically exact way. From there, we have developed an efficient algorithm which we have successfully implemented. We show encouraging preliminary results using both computer-simulated data and real data. Our results show that our method is capable of achieving a substantial reduction of image noise when decreasing the helix pitch compared with the maximum pitch case. We conclude that the proposed algorithm defines for the first time a theoretically-exact and stable reconstruction method that is capable of beneficially using all measured data at arbitrary pitch.

• Evaluation of z-axis resolution and image noise for nonconstant velocity spiral CT data reconstructed using a weighted 3D filtered backprojection (WFBP) reconstruction algorithm.

  Authors: Jodie A Christner, Karl Stierstorfer, Andrew N Primak, Christian D Eusemann, Thomas G Flohr, Cynthia H McCollough

  Medical physics. 02/2010; 37(2):897-906.

  To determine the constancy of z-axis spatial resolution, CT number, image noise, and the potential for image artifacts for nonconstant velocity spiral CT data reconstructed using a flexibly weightedTo determine the constancy of z-axis spatial resolution, CT number, image noise, and the potential for image artifacts for nonconstant velocity spiral CT data reconstructed using a flexibly weighted 3D filtered backprojection (WFBP) reconstruction algorithm. A WFBP reconstruction algorithm was used to reconstruct stationary (axial, pitch=0), constant velocity spiral (pitch = 0.35-1.5) and nonconstant velocity spiral CT data acquired using a 128 x 0.6 mm acquisition mode (38.4 mm total detector length, z-flying focal spot technique), and a gantry rotation time of 0.30 s. Nonconstant velocity scans used the system's periodic spiral mode, where the table moved in and out of the gantry in a cyclical manner. For all scan types, the volume CTDI was 10 mGy. Measurements of CT number, image noise, and the slice sensitivity profile were made for all scan types as a function of the nominal slice width, table velocity, and position within the scan field of view. A thorax phantom was scanned using all modes and reconstructed transverse and coronal plane images were compared. Negligible differences in slice thickness, CT number, noise, or artifacts were found between scan modes for data taken at two positions within the scan field of view. For nominal slices of 1.0-3.0 mm, FWHM values of the slice sensitivity profiles were essentially independent of the scan type. For periodic spiral scans, FWHM values measured at the center of the scan range were indistinguishable from those taken 5 mm from one end of the scan range. All CT numbers were within +/- 5 HU, and CT number and noise values were similar for all scan modes assessed. A slight increase in noise and artifact level was observed 5 mm from the start of the scan on the first pass of the periodic spiral. On subsequent passes, noise and artifact level in the transverse and coronal plane images were the same for all scan modes. Nonconstant velocity periodic spiral scans can achieve z-axis spatial resolution, CT number accuracy, image noise and artifact level equivalent to those for stationary (axial), and constant velocity spiral scans. Thus, periodic spiral scans are expected to allow assessment of four-dimensional CT data for scan lengths greater than the detector width without sacrificing image quality.

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

  Authors: Thomas G Flohr, Shuai Leng, Lifeng Yu, Thomas Aiimendinger, Herbert Bruder, Martin Petersilka, Christian D Eusemann, Karl Stierstorfer, Bernhard Schmidt, Cynthia H McCollough

  Medical physics. 12/2009; 36(12):5641-53.

  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. With the use of two x-rayTo 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. 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 at pitch = 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. No significant differences in quantitative measures of image quality were found between single-source scans at pitch = 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). A high-pitch (up to pitch = 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.

• Accurate helical cone-beam CT reconstruction with redundant data.

  Authors: Harald Schöndube, Karl Stierstorfer, Frédéric Noo

  Physics in medicine and biology. 09/2009; 54(15):4625-44.

  We present a new image reconstruction algorithm for helical cone-beam computed tomography (CT). This algorithm is designed for data collected at or near maximum pitch, and provides a theoreticallyWe present a new image reconstruction algorithm for helical cone-beam computed tomography (CT). This algorithm is designed for data collected at or near maximum pitch, and provides a theoretically exact and stable reconstruction while beneficially using all measured data. The main operations involved are a differentiated backprojection and a finite-support Hilbert transform inversion. These operations are applied onto M-lines, and the beneficial use of all measured data is gained from averaging three volumes reconstructed each with a different choice of M-lines. The technique is overall similar to that presented by one of the authors in a previous publication, but operates volume-wise, instead of voxel-wise, which yields a significantly more efficient reconstruction procedure. The algorithm is presented in detail. Also, preliminary results from computer-simulated data are provided to demonstrate the numerical stability of the algorithm, the beneficial use of redundant data and the ability to process data collected with an angular flying focal spot.

• Technical principles of dual source CT.

  Authors: Martin Petersilka, Herbert Bruder, Bernhard Krauss, Karl Stierstorfer, Thomas G Flohr

  European journal of radiology. 11/2008;

  During the past years, multi-detector row CT (MDCT) has evolved into clinical practice with a rapid increase of the number of detector slices. Today's 64 slice CT systems allow whole-bodyDuring the past years, multi-detector row CT (MDCT) has evolved into clinical practice with a rapid increase of the number of detector slices. Today's 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 patient's heart rate (83ms at 0.33s 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.

• Dose performance of a 64-channel dual-source CT scanner.

  Authors: Cynthia H McCollough, Andrew N Primak, Osama Saba, Herbert Bruder, Karl Stierstorfer, Rainer Raupach, Christoph Suess, Bernhard Schmidt, Bernd M Ohnesorge, Thomas G Flohr

  Radiology. 07/2007; 243(3):775-84.

  PURPOSE: To prospectively compare the dose performance of a 64-channel multi-detector row computed tomographic (CT) scanner and a 64-channel dual-source CT scanner from the same manufacturer.PURPOSE: To prospectively compare the dose performance of a 64-channel multi-detector row computed tomographic (CT) scanner and a 64-channel dual-source CT scanner from the same manufacturer. MATERIALS AND METHODS: To minimize dose in the cardiac (dual-source) mode, the evaluated dual-source CT system uses a cardiac beam-shaping filter, three-dimensional adaptive noise reduction, heart rate-dependent pitch, and electrocardiographically based modulation of the tube current. Weighted CT dose index per 100 mAs was measured for the head, body, and cardiac beam-shaping filters. Kerma-length product was measured in the spiral cardiac mode at four pitch values and three electrocardiographic modulation temporal windows. Noise was measured in an anthropomorphic phantom. Data were compared with data from a 64-channel multi-detector row CT scanner. RESULTS: For the multi-detector row and dual-source CT systems, respectively, weighted CT dose index per 100 mAs was 14.2 and 12.2 mGy (head CT), 6.8 and 6.4 mGy (body CT), and 6.8 and 5.3 mGy (cardiac CT). In the spiral cardiac mode (no electrocardiographically based tube current modulation, 0.2 pitch), equivalent noise occurred at volume CT dose index values of 23.7 and 35.0 mGy (coronary artery calcium CT) and 58.9 and 61.2 mGy (coronary CT angiography) for multi-detector row CT and dual-source CT, respectively. The use of heart rate-dependent pitch values reduced volume CT dose index to 46.2 mGy (0.265 pitch), 34.0 mGy (0.36 pitch), and 26.6 mGy (0.46 pitch) compared with 61.2 mGy for 0.2 pitch. The use of electrocardiographically based tube current-modulation and temporal windows of 110, 210, and 310 msec further reduced volume CT dose index to 9.1-25.1 mGy, dependent on the heart rate. CONCLUSION: For electrocardiographically gated coronary CT angiography, image noise equivalent to that of multi-detector row CT can be achieved with dual-source CT at doses comparable to or up to a factor of two lower than the doses at multi-detector row CT, depending on heart rate of the patient.

• Ultra-high resolution flat-panel volume CT: fundamental principles, design architecture, and system characterization.

  Authors: Rajiv Gupta, Michael Grasruck, Christoph Suess, Soenke H Bartling, Bernhard Schmidt, Karl Stierstorfer, Stefan Popescu, Tom Brady, Thomas Flohr

  European radiology. 07/2006; 16(6):1191-205.

  Digital flat-panel-based volume CT (VCT) represents a unique design capable of ultra-high spatial resolution, direct volumetric imaging, and dynamic CT scanning. This innovation, when fullyDigital flat-panel-based volume CT (VCT) represents a unique design capable of ultra-high spatial resolution, direct volumetric imaging, and dynamic CT scanning. This innovation, when fully developed, has the promise of opening a unique window on human anatomy and physiology. For example, the volumetric coverage offered by this technology enables us to observe the perfusion of an entire organ, such as the brain, liver, or kidney, tomographically (e.g., after a transplant or ischemic event). By virtue of its higher resolution, one can directly visualize the trabecular structure of bone. This paper describes the basic design architecture of VCT. Three key technical challenges, viz., scatter correction, dynamic range extension, and temporal resolution improvement, must be addressed for successful implementation of a VCT scanner. How these issues are solved in a VCT prototype and the modifications necessary to enable ultra-high resolution volumetric scanning are described. The fundamental principles of scatter correction and dose reduction are illustrated with the help of an actual prototype. The image quality metrics of this prototype are characterized and compared with a multi-detector CT (MDCT).

• First performance evaluation of a dual-source CT (DSCT) system.

  Authors: Thomas G Flohr, Cynthia H McCollough, Herbert Bruder, Martin Petersilka, Klaus Gruber, Christoph Süss, Michael Grasruck, Karl Stierstorfer, Bernhard Krauss, Rainer Raupach, Andrew N Primak, Axel Küttner, Stefan Achenbach, Christoph Becker, Andreas Kopp, Bernd M Ohnesorge

  European radiology. 03/2006; 16(2):256-68.

  We present a performance evaluation of a recently introduced dual-source computed tomography (DSCT) system equipped with two X-ray tubes and two corresponding detectors, mounted onto the rotatingWe present a performance evaluation 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 90 degrees . We introduce the system concept and derive its consequences and potential benefits for electrocardiograph [corrected] (ECG)-controlled cardiac CT and for general radiology applications. We evaluate both temporal and spatial resolution by means of phantom scans. We present first patient scans to illustrate the performance of DSCT for ECG-gated cardiac imaging, and we demonstrate first results using a dual-energy acquisition mode. Using ECG-gated single-segment reconstruction, the DSCT system provides 83 ms temporal resolution independent of the patient's heart rate for coronary CT angiography (CTA) and evaluation of basic functional parameters. With dual-segment reconstruction, the mean temporal resolution is 60 ms (minimum temporal resolution 42 ms) for advanced functional evaluation. The z-flying focal spot technique implemented in the evaluated DSCT system allows 0.4 mm cylinders to be resolved at all heart rates. First clinical experience shows a considerably increased robustness for the imaging of patients with high heart rates. As a potential application of the dual-energy acquisition mode, the automatic separation of bones and iodine-filled vessels is demonstrated.

• Multi-detector row CT systems and image-reconstruction techniques.

  Authors: Thomas G Flohr, Stefan Schaller, Karl Stierstorfer, Herbert Bruder, Bernd M Ohnesorge, U Joseph Schoepf

  Radiology. 07/2005; 235(3):756-73.

  The introduction in 1998 of multi-detector row computed tomography (CT) by the major CT vendors was a milestone with regard to increased scan speed, improved z-axis spatial resolution, and betterThe introduction in 1998 of multi-detector row computed tomography (CT) by the major CT vendors was a milestone with regard to increased scan speed, improved z-axis spatial resolution, and better utilization of the available x-ray power. In this review, the general technical principles of multi-detector row CT are reviewed as they apply to the established four- and eight-section systems, the most recent 16-section scanners, and future generations of multi-detector row CT systems. Clinical examples are used to demonstrate both the potential and the limitations of the different scanner types. When necessary, standard single-section CT is referred to as a common basis and starting point for further developments. Another focus is the increasingly important topic of patient radiation exposure, successful dose management, and strategies for dose reduction. Finally, the evolutionary steps from traditional single-section spiral image-reconstruction algorithms to the most recent approaches toward multisection spiral reconstruction are traced.

• Flat panel computed tomography of human ex vivo heart and bone specimens: initial experience.

  Authors: Konstantin Nikolaou, Thomas Flohr, Karl Stierstorfer, Christoph R Becker, Maximilian F Reiser

  European radiology. 03/2005; 15(2):329-33.

  The aim of this technical investigation was the detailed description of a prototype flat panel detector computed tomography system (FPCT) and its initial evaluation in an ex vivo setting. TheThe aim of this technical investigation was the detailed description of a prototype flat panel detector computed tomography system (FPCT) and its initial evaluation in an ex vivo setting. The prototype FPCT scanner consists of a conventional radiographic flat panel detector, mounted on a multi-slice CT scanner gantry. Explanted human ex vivo heart and foot specimens were examined. Images were reformatted with various reconstruction algorithms and were evaluated for high-resolution anatomic information. For comparison purposes, the ex vivo specimens were also scanned with a conventional 16-detector-row CT scanner (Sensation 16, Siemens Medical Solutions, Forchheim, Germany). With the FPCT prototype used, a 1,024x768 resolution matrix can be obtained, resulting in an isotropic voxel size of 0.25x0.25x0.25 mm at the iso-center. Due to the high spatial resolution, very small structures such as trabecular bone or third-degree, distal branches of coronary arteries could be visualized. This first evaluation showed that flat panel detector systems can be used in a cone-beam computed tomography scanner and that very high spatial resolutions can be achieved. However, there are limitations for in vivo use due to constraints in low contrast resolution and slow scan speed.

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

  Authors: Karl Stierstorfer, Annabella Rauscher, Jan Boese, Herbert Bruder, Stefan Schaller, Thomas Flohr

  Physics in medicine and biology. 07/2004; 49(11):2209-18.

  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. InA 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.

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

  Authors: Thomas Flohr, Axel Küttner, Herbert Bruder, Karl Stierstorfer, Sandra S Halliburton, Stefan Schaller, Bernd M Ohnesorge

  Herz. 03/2003; 28(1):7-19.

  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 coronaryBACKGROUND: 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 x 0.5 x 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.

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

  Authors: Karl Stierstorfer, Thomas Flohr, Herbert Bruder

  Physics in medicine and biology. 09/2002; 47(15):2571-81.

  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 convolutionA 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.