Willi A. Kalender

Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Bavaria, Germany

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Publications (539)970.76 Total impact

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    ABSTRACT: Various applications require information on breast parameters, such as breast length and volume. An optical system was designed and tested for measuring these parameters with subjects in a prone position. The study results were used for optimizing patient positioning and handling for a future breast computed tomography (BCT) system. Measurements were conducted using an optical measurement system. To test the functionality and accuracy of the system, measurements were performed using reference phantoms. Additionally, 20 women and 5 men were examined to calculate breast parameters in alternative positions and breathing states. The results of the optical measurements were compared with magnetic resonance imaging (MRI) measurements. Volume and length of the reference phantoms were determined with errors below 2%. The patient study demonstrated a mean breast volume of 530.7 ml for women during normal breathing. During an exhalation state, breast volume increased significantly by 17.7 ml in comparison with normal breathing. Differences with MRI measurements were found to be 3% for breast length and 9% for breast volume on average. The proposed optical measurement system was found to be suitable for measuring the dimensional parameters of the breast in a prone position and provides a tool for evaluating breast coverage for BCT.
    07/2015; 2(3-3):034001. DOI:10.1117/1.JMI.2.3.034001
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    ABSTRACT: Purpose: The presented study aimed at optimizing a patient table design for breast CT (BCT) systems with respect to breast tissue coverage and patient comfort. Additionally, the benefits and acceptance of an immobilization device for BCT using underpressure were evaluated. Materials and Methods: Three different study parts were carried out. In a positioning study women were investigated on an MRI tabletop with exchangeable inserts (flat and cone-shaped with different opening diameters) to evaluate their influence on breast coverage and patient comfort in various positioning alternatives. Breast length and volume were calculated to compare positioning modalities including various opening diameters and forms. In the second study part, an underpressure system was tested for its functionality and comfort on a stereotactic biopsy table mimicking a future CT scanner table. In the last study part, this system was tested regarding breast tissue coverage. Results: Best results for breast tissue coverage were shown for cone-shaped table inserts with an opening of 180 mm. Flat inserts did not provide complete coverage of breast tissue. The underpressure system showed robust function and tended to pull more breast tissue into the field of view. Patient comfort was rated good for all table inserts, with highest ratings for cone-shaped inserts. Conclusion: Cone-shaped tabletops appeared to be adequate for BCT systems and to allow imaging of almost the complete breast. An underpressure system proved promising for the fixation of the breast during imaging and increased coverage. Patient comfort appears to be adequate. Key points: Tissue coverage in breast CT is highly dependent on patient table design. An underpressure fixation system shows potential to increase breast coverage. The proposed breast CT patient table design combines good coverage and patient comfort.
    Senologie - Zeitschrift für Mammadiagnostik und -therapie 06/2015; 12(02):96-103. DOI:10.1055/s-0035-1553177
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    ABSTRACT: To investigate the dose saving potential of direct-converting CdTe photon-counting detector technology for dedicated breast CT. We analyzed the modulation transfer function (MTF), the noise power spectrum (NPS) and the detective quantum efficiency (DQE) of two detector technologies, suitable for breast CT (BCT): a flat-panel energy-integrating detector with a 70 μm and a 208 μm thick gadolinium oxysulfide (GOS) and a 150 μm thick cesium iodide (CsI) scintillator and a photon-counting detector with a 1000 μm thick CdTe sensor. The measurements for GOS scintillator thicknesses of 70 μm and 208 μm delivered 10% pre-sampled MTF values of 6.6 mm(-1) and 3.2 mm(-1), and DQE(0) values of 23% and 61%. The 10% pre-sampled MTF value for the 150 μm thick CsI scintillator 6.9 mm(-1), and the DQE(0) value was 49%. The CdTe sensor reached a 10% pre-sampled MTF value of 8.5 mm(-1) and a DQE(0) value of 85%. The photon-counting CdTe detector technology allows for significant dose reduction compared to the energy-integrating scintillation detector technology used in BCT today. Our comparative evaluation indicates that a high potential dose saving may be possible for BCT by using CdTe detectors, without loss of spatial resolution. Copyright © 2015 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
    Physica Medica 04/2015; 31(4). DOI:10.1016/j.ejmp.2015.03.007 · 2.40 Impact Factor
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    ABSTRACT: The aim of this study was the registration of digitized thin 2D sections of mouse vertebrae and tibiae used for histomorphometry of trabecular bone structure into 3D micro computed tomography (μCT) datasets of the samples from which the sections were prepared. Intensity-based and segmentation-based registrations (SegRegs) of 2D sections and 3D μCT datasets were applied. As the 2D sections were deformed during their preparation, affine registration for the vertebrae was used instead of rigid registration. Tibiae sections were additionally cut on the distal end, which subsequently undergone more deformation so that elastic registration was necessary. The Jaccard distance was used as registration quality measure. The quality of intensity-based registrations and SegRegs was practically equal, although precision errors of the elastic registration of segmentation masks in tibiae were lower, while those in vertebrae were lower for the intensity-based registration. Results of SegReg significantly depended on the segmentation of the μCT datasets. Accuracy errors were reduced from approximately 64% to 42% when applying affine instead of rigid transformations for the vertebrae and from about 43% to 24% when using B-spline instead of rigid transformations for the tibiae. Accuracy errors can also be caused by the difference in spatial resolution between the thin sections (pixel size: 7.25 μm) and the μCT data (voxel size: 15 μm). In the vertebrae, average deformations amounted to a 6.7% shortening along the direction of sectioning and a 4% extension along the perpendicular direction corresponding to 0.13-0.17 mm. Maximum offsets in the mouse tibiae were 0.16 mm on average.
    Computer Methods in Biomechanics and Biomedical Engineering 12/2014; 18(15):1-16. DOI:10.1080/10255842.2014.941824 · 1.77 Impact Factor
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    ABSTRACT: Articular cartilage and subchondral bone are the key tissues in osteoarthritis (OA). The role of the cancellous bone increasingly attracts attention in OA research. Because of its fast adaptation to changes in the loading distribution across joints, its quantification is expected to improve the diagnosis and monitoring of OA. In this study, we simulated OA progression-related changes of trabecular structure in a series of digital bone models and then characterized the potential of texture parameters and bone mineral density (BMD) as surrogate measures to quantify trabecular bone structure. Five texture parameters were studied: entropy, global and local inhomogeneity, anisotropy and variogram slope. Their dependence on OA relevant structural changes was investigated for three spatial resolutions typically used in micro computed tomography (CT; 10 μm), high-resolution peripheral quantitative CT (HR-pQCT) (90 μm) and clinical whole-body CT equipment (250 μm). At all resolutions, OA-related changes in trabecular bone architecture can be quantified using a specific (resolution dependent) combination of three texture parameters. BMD alone is inadequate for this purpose but if available reduces the required texture parameter combination to anisotropy and global inhomogeneity. The results are summarized in a comprehensive analysis guide for the detection of structural changes in OA knees. In conclusion, texture parameters can be used to characterize trabecular bone architecture even at spatial resolutions below the dimensions of a single trabecula and are essential for a detailed classification of relevant OA changes that cannot be achieved with a measurement of BMD alone.
    12/2014; 3:615. DOI:10.1038/bonekey.2014.110
  • C Steiding · D Kolditz · W Kalender ·
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    ABSTRACT: Purpose: The aim of this work was to implement, validate, and compare two procedures for routine image quality (IQ) assurance in dental cone-beam computed tomography (CBCT): 1. the German standard DIN 6868 - 161 introduced in 2013 and 2. the established standard IEC 61 223 - 3-5 for clinical CT x-ray equipment referenced as DIN and IEC below. Materials and Methods: The approximated in-plane modulation transfer function (MTF), the contrast-to-noise indicator (CNI), and the uniformity indicator (UI) were determined in accordance with DIN. Image noise, the uniformity index (UI), the contrast-to-noise ratio (CNR), and the 3 D MTF were measured according to IEC 61 223 - 3-5 using a previously proposed quality assurance (QA) framework. For this, a modular phantom was used. All experiments were performed on a clinical dental CBCT unit. The severity of image artefacts was measured at different z-positions. A dedicated computer program was implemented to allow for automated QA procedure. Results: The position and orientation of the phantoms were detected automatically in all of the measurements providing a reproducible placement of the evaluation regions and volumes. 50 and 10 in-plane MTF values of the approximated and the exact MTF calculation procedure were in agreement to within 5. With increasing axial distance from the isocentre, UI∗ and CNI dropped by 30 and 19, respectively. Conventional IQ parameters showed higher sensitivity to image artefacts; i. e., UI and CNR were reduced by about 197 and 37. Conclusion: The implemented automated QA routines are compatible with both the DIN and the IEC approach and offer reliable and quantitative tracking of imaging performance in dental CBCT for clinical practice. However, there is no equivalence between the DIN and the IEC metrics. In addition, direct measurements of physical IQ parameters such as image contrast and noise, uniformity, and axial resolution are not supported by the new concept according to DIN.
    RöFo - Fortschritte auf dem Gebiet der R 11/2014; 187(04). DOI:10.1055/s-0034-1385333 · 1.40 Impact Factor
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    ABSTRACT: The estimation of patient dose using Monte Carlo (MC) simulations based on the available patient CT images is limited to the length of the scan. Software tools for dose estimation based on standard computational phantoms overcome this problem; however, they are limited with respect to taking individual patient anatomy into account. The purpose of this study was to generate whole-body patient models in order to take scattered radiation and over-scanning effects into account. Thorax examinations were performed on three physical anthropomorphic phantoms at tube voltages of 80 kV and 120 kV; absorbed dose was measured using thermoluminescence dosimeters (TLD). Whole-body voxel models were built as a combination of the acquired CT images appended by data taken from widely used anthropomorphic voxel phantoms. MC simulations were performed both for the CT image volumes alone and for the whole-body models. Measured and calculated dose distributions were compared for each TLD chip position; additionally, organ doses were determined. MC simulations based only on CT data underestimated dose by 8%-15% on average depending on patient size with highest underestimation values of 37% for the adult phantom at the caudal border of the image volume. The use of whole-body models substantially reduced these errors; measured and simulated results consistently agreed to better than 10%. This study demonstrates that combined whole-body models can provide three-dimensional dose distributions with improved accuracy. Using the presented concept should be of high interest for research studies which demand high accuracy, e.g. for dose optimization efforts.
    Physica Medica 10/2014; DOI:10.1016/j.ejmp.2014.09.005. · 2.40 Impact Factor

  • Cancer Research 10/2014; 74(19 Supplement):2071A-2071A. DOI:10.1158/1538-7445.AM2014-2071A · 9.33 Impact Factor
  • A C Rößler · E Wenkel · F Althoff · W Kalender ·
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    ABSTRACT: Purpose: The presented study aimed at optimizing a patient table design for breast CT (BCT) systems with respect to breast tissue coverage and patient comfort. Additionally, the benefits and acceptance of an immobilization device for BCT using underpressure were evaluated. Materials and methods: Three different study parts were carried out. In a positioning study women were investigated on an MRI tabletop with exchangeable inserts (flat and cone-shaped with different opening diameters) to evaluate their influence on breast coverage and patient comfort in various positioning alternatives. Breast length and volume were calculated to compare positioning modalities including various opening diameters and forms. In the second study part, an underpressure system was tested for its functionality and comfort on a stereotactic biopsy table mimicking a future CT scanner table. In the last study part, this system was tested regarding breast tissue coverage. Results: Best results for breast tissue coverage were shown for cone-shaped table inserts with an opening of 180 mm. Flat inserts did not provide complete coverage of breast tissue. The underpressure system showed robust function and tended to pull more breast tissue into the field of view. Patient comfort was rated good for all table inserts, with highest ratings for cone-shaped inserts. Conclusion: Cone-shaped tabletops appeared to be adequate for BCT systems and to allow imaging of almost the complete breast. An underpressure system proved promising for the fixation of the breast during imaging and increased coverage. Patient comfort appears to be adequate.
    RöFo - Fortschritte auf dem Gebiet der R 09/2014; 36(2). DOI:10.1055/s-0034-1385208 · 1.40 Impact Factor
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    ABSTRACT: The purpose of this study was to validate the use of a single shaped filter (SF) for computed tomography (CT) using variable source-to-filter distance (SFD) for the examination of different object diameters.A SF was designed by performing simulations with the purpose of achieving noise homogeneity in the reconstructed volume and dose reduction for arbitrary phantom diameters. This was accomplished by using a filter design method thats target is to achieve a homogeneous detector noise, but also uses a correction factor for the filtered back projection process. According to simulation results, a single SF designed for one of the largest phantom diameters meets the requirements for all diameters when SFD can be adjusted. To validate these results, a SF made of aluminium alloy was manufactured. Measurements were performed on a CT scanner with polymethyl methacrylate (PMMA) phantoms of diameters from 40-100 mm. The filter was positioned at SFDs ranging from 97-168 mm depending on the phantom diameter. Image quality was evaluated for the reconstructed volume by assessing CT value accuracy, noise homogeneity, contrast-to-noise ratio weighted by dose (CNRD) and spatial resolution. Furthermore, scatter distribution was determined with the use of a beam-stop phantom. Dose was measured for a PMMA phantom with a diameter of 100 mm using a calibrated ionization chamber.The application of a single SF at variable SFD led to improved noise uniformity and dose reduction: noise homogeneity was improved from 15% down to about 0%, and dose was reduced by about 37%. Furthermore, scatter dropped by about 32%, which led to reduced cupping artifacts and improved CT value accuracy. Spatial resolution and CNRD was not affected by the SF.By means of a single SF with variable SFD designed for CT, significant dose reduction can be achieved and image quality can be improved by reducing noise inhomogeneity as well as scatter-induced artifacts.
    Physics in Medicine and Biology 09/2014; 59(19):5691-5706. DOI:10.1088/0031-9155/59/19/5691 · 2.76 Impact Factor
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    Natalia Saltybaeva · Mary Ellen Jafari · Martin Hupfer · Willi A Kalender ·
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    ABSTRACT: Purpose: To determine the dose-length product (DLP)-effective dose (ED) (DLP/ED) conversion coefficient (k) tables for the lower extremities that can be used for calculating ED. Materials and methods: Dose calculations were performed on standard phantoms using a validated Monte Carlo calculation tool. Calculations were performed to obtain ED values for tube voltages from 80 kV to 140 kV in steps of 20 kV for the following examinations: hip (femur), knee, ankle, and computed tomographic (CT) angiography of the lower extremities. Values of the DLP were calculated by multiplying measured CT dose index values by the scan length; k values resulted as the quotients of the ED and DLP values. DLP/ED coefficients averaged over the range of voltage values and their standard deviations were determined for the given lower-extremity CT examinations for all age groups and for both sexes. Results: Coefficients depend strongly on the phantom age and size, but little on the kilovolt value. In the case of the newborn, for example, k values were 0.0612, 0.0046, 0.0014, and 0.047 for hip, knee, ankle, and CT angiography, respectively, while in the case of the adult, these respective values were 0.0110, 0.0004, 0.0002, and 0.0062. A substantial difference up to 20% between coefficients in male and female phantoms was observed for CT angiographic examination. Conclusion: DLP/ED conversion coefficients are provided for lower extremities and allow estimation of ED for commonly used clinical musculoskeletal CT and CT angiographic protocols.
    Radiology 06/2014; 273(1):132903. DOI:10.1148/radiol.14132903 · 6.87 Impact Factor

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    ABSTRACT: Dynamic contrast-enhanced (DCE) micro-computed tomography (micro-CT) has emerged as a valuable imaging tool to noninvasively obtain quantitative physiological biomarkers of drug effect in preclinical studies of antiangiogenic compounds. In this study, we explored the ability of DCE micro-CT to assess the antiangiogenic treatment response in breast cancer xenografts and correlated the results to the structural vessel response obtained from 3-dimensional (3D) fluorescence ultramicroscopy (UM). Two groups of tumor-bearing mice (KPL-4) underwent DCE micro-CT imaging using a fast preclinical dual-source micro-CT system (TomoScope Synergy Twin, CT Imaging GmbH, Erlangen, Germany). Mice were treated with either a monoclonal antibody against the vascular endothelial growth factor or an unspecific control antibody. Changes in vascular physiology were assessed measuring the mean value of the relative blood volume (rBV) and the permeability-surface area product (PS) in different tumor regions of interest (tumor center, tumor periphery, and total tumor tissue). Parametric maps of rBV were calculated of the tumor volume to assess the intratumoral vascular heterogeneity. Isotropic 3D UM vessel scans were performed from excised tumor tissue, and automated 3D segmentation algorithms were used to determine the microvessel density (MVD), relative vessel volume, and vessel diameters. In addition, the accumulation of coinjected fluorescence-labeled trastuzumab was quantified in the UM tissue scans to obtain an indirect measure of vessel permeability. Results of the DCE micro-CT were compared with corresponding results obtained by ex vivo UM. For validation, DCE micro-CT and UM parameters were compared with conventional histology and tumor volume. Examination of the parametric rBV maps revealed significantly different patterns of intratumoral blood supply between treated and control tumors. Whereas control tumors showed a characteristic vascular rim pattern with considerably elevated rBV values in the tumor periphery, treated tumors showed a widely homogeneous blood supply. Compared with UM, the physiological rBV maps showed excellent agreement with the spatial morphology of the intratumoral vascular architecture. Regional assessment of mean physiological values exhibited a significant decrease in rBV (P < 0.01) and PS (P < 0.05) in the tumor periphery after anti-vascular endothelial growth factor treatment. Structural validation with UM showed a significant reduction in reduction of relative vessel volume (rVV) (P < 0.01) and MVD (P < 0.01) in the corresponding tumor region. The reduction in rBV correlated well with the rVV (R = 0.73 for single values and R = 0.95 for mean values). Spatial maps of antibody penetration showed a significantly reduced antibody accumulation (P < 0.01) in the tumor tissue after treatment and agreed well with the physiological change of PS. Examination of vessel diameters revealed a size-dependent antiangiogenic treatment effect, which showed a significant reduction in MVD (P < 0.001) for vessels with diameters smaller than 25 μm. No treatment effect was observed by tumor volume. Noninvasive DCE micro-CT provides valuable physiological information of antiangiogenic drug effect in the intact animal and correlates with ex vivo structural analysis of 3D UM. The combined use of DCE micro-CT with UM constitutes a complementary imaging toolset that can help to enhance our understanding of antiangiogenic drug mechanisms of action in preclinical drug research.
    Investigative radiology 03/2014; 49(7). DOI:10.1097/RLI.0000000000000038 · 4.44 Impact Factor
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    Christian Steiding · Daniel Kolditz · Willi A Kalender ·
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    ABSTRACT: Purpose: Thousands of cone-beam computed tomography (CBCT) scanners for vascular, maxillofacial, neurological, and body imaging are in clinical use today, but there is no consensus on uniform acceptance and constancy testing for image quality (IQ) and dose yet. The authors developed a quality assurance (QA) framework for fully automated and time-efficient performance evaluation of these systems. In addition, the dependence of objective Fourier-based IQ metrics on direction and position in 3D volumes was investigated for CBCT.
    Medical Physics 03/2014; 41(3):031901. DOI:10.1118/1.4863507 · 2.64 Impact Factor
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    ABSTRACT: Background While the association of BMLs with joint space narrowing and pain in knee OA is verified, the correlation between BMLs and local BMD is a topic of major interest with a potential to strongly improve the understanding of the pathogenesis of the disease. Objectives To investigate whether trabecular BMD in BMLs is increased compared to the surrounding tissue. Methods 18 BMLs (WORMS grade ≥1) detected on coronal turbo spin echo MR images of 14 patients with knee OA (KL grade 2 or 3) have been automatically 3D segmented (left figure, BML). To compare these volumes of interest (VOI) with the circumjacent lesion-free trabecular bone tissue, a neighboring VOI was defined by a dilation of the BML VOI (left figure, Neighborhood). Via a 3D multimodality registration between MR and CT datasets of the same patient both VOIs were transferred to the CT datasets (right figure). Mean BMD values were calculated in both VOIs and their difference was determined for each BML. To account for the preferred occurrence of BMLs in proximity of the bone surface and the presence of a BMD gradient in the epiphysis, reference CT datasets (reference group) of patients with knee OA without BMLs were used. In these datasets, a BMD analysis was performed in VOIs corresponding to the BML and Neighborhood VOIs in the patients with BMLs (BML group). Further, the effect of a change from fatty to water equivalent tissue within the BML VOI was estimated. This effect has to be considered due to dependence of single energy QCT accuracy on fat concentration. Results Despite different spatial resolutions between MR and CT, the registrations showed excellent overlap. %BMD differences between BML- and neighborhood-VOIs were significant in both groups, but differences were significantly higher (p=0.01) in the BML (37.6%±32.6%) compared to the reference group (11.9%±22.2%). Under the assumption of a maximum 29% contribution of water equivalent material in the BML group, the %BMD difference would decrease to 30.1±30.7%, which would still be significantly different from the reference group. (Assumptions: fat value of -100 HU and 68% fat in the BML VOI (80% fat in 85% yellow marrow) of the reference group). Conclusions Highly accurate segmentation and registration methods ensure the precise investigation of BMD at BML locations and circumjacent bone regions. Locally increased BMD at the BML locations seems to be present in patients with knee OA. However, the effect of compositional changes within the BML needs to be further discussed as a high content of water equivalent material may impair the BMD results. The outcome may reflect increased loading conditions within the knee joint. In the presented study, the benefits of both MR and CT acquisitions regarding OA changes in soft tissue and bone, respectively, were utilized to gain information about the pathogenesis of knee OA and therefore help to improve its diagnosis. Disclosure of Interest None Declared
    Annals of the Rheumatic Diseases 01/2014; 71(Suppl 3):134-135. DOI:10.1136/annrheumdis-2012-eular.1916 · 10.38 Impact Factor
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    T. Lowitz · O. Museyko · V. Bousson · W.A. Kalender · J.-D. Laredo · K. Engelke ·
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    ABSTRACT: The quantification of changes in the trabecular bone structure induced by musculoskeletal diseases like osteoarthritis, osteoporosis, rheumatoid arthritis, and others by means of a texture analysis is a valuable tool which is expected to improve the diagnosis and monitoring of a disease. The reaction of texture parameters on different alterations in the architecture of the fine trabecular network and inherent imaging factors such as spatial resolution or image noise has to be understood in detail to ensure an accurate and reliable determination of the current bone state. Therefore, a digital model for the quantitative analysis of cancellous bone structures was developed. Five parameters were used for texture analysis: entropy, global and local inhomogeneity, local anisotropy, and variogram slope. Various generic structural changes of cancellous bone were simulated for different spatial resolutions. Additionally, the dependence of the texture parameters on tissue mineralization and noise was investigated. The present work explains changes in texture parameter outcomes based on structural changes originating from structure modifications and reveals that a texture analysis could provide useful information for a trabecular bone analysis even at resolutions below the dimensions of single trabeculae.
    01/2014; Vol 2014(Article ID 946574). DOI:10.1155/2014/946574
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    ABSTRACT: PURPOSE Intrascan Motion-artifact-correction in C-arm-based flat-detector CT (FD-CT) is an important issue in interventional imaging because of longer scan times as compared to Multi-Slice CT. Our aim was the development and evaluation of an online image-content-based motion-correction technique without using any kind of markers or external motion knowledge. METHOD AND MATERIALS The correction method is based on a gradient descent method, minimizing a gray-value entropy criterion optimizing the underlying acquistion trajectory parameters. It is formed as a multistep approach, including a global, local and projection wise optimization. We are using a locally rigid variation of the systems trajectory parameters like detector- or source-translation or a detector rotation to compensate patient motion. The retrospective evaluation of 30 arbitrary (with weak and strong motion, without motion artifacts) patient head scans included 5s 3D angiography and 20s soft-tissue protocols. All scans were performed on an Artis Q System (Siemens AG). For each dataset three volumes were computed: 1) original reconstruction using the system’s geometry calibration (OR), 2) motion corrected reconstruction without any system information (MCR) and 3) motion corrected reconstruction using the system’s geometry calibration as initialization (MCR+). Two neuroradiology experts performed a visual evaluation according to a 5-point grading scale with respect to general image quality, motion-artifact-content and spatial resolution of the structures of interest, e.g. 3D vessels. RESULTS The average scores for OR, MCR and MCR+ were 2.75, 3.0 and 3.15, respectively. The combined compensation of unknown trajectories and unknown patient motion (MCR) can lead to comparable results to OR. Both experts confirmed a distinct reduction of artifacts by the motion correction algorithm (MCR+) , e.g. blurring and streaks. Especially for 3D angiography even small distal vessels were depicted clearly. MCR+ application on soft-tissue protocols illustrated a constantly better delineation of bone und soft-tissue in the border zones. CONCLUSION Image-based motion correction is possible without a-priori knowledge of the motion pattern and can improve interventional FD-CT imaging. CLINICAL RELEVANCE/APPLICATION Using the proposed algorithm enables good image quality even for unsteady patients and can be helpful for longer FD-CT acquisitions in cases where anaesthesia is contraindicated.
    Radiological Society of North America 2013 Scientific Assembly and Annual Meeting; 12/2013
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    ABSTRACT: Purpose Objective evaluation of image quality based on known phantom structures fixed in known positions is hard to do. We designed phantoms which allow arranging tumor-like masses, microcalcifications (µCa) and fibers freely with and without superpositioned structures to enable observer studies with the observer blinded to the ground truth. Material and Methods Phantoms were made of breast-equivalent material as a 40 mm slab and as a 14 cm diameter cylinder simulating the compressed breast for digital mammography (DM) and breast tomosynthesis (BT) and the pendant breast for breast CT (bCT), respectively. Both phantoms provide cavities (45x40² mm³) to insert structures such as cubes (10³ mm³) and plates with different thicknesses simulating normal breast tissue or containing spherical soft-tissue masses (2-8 mm), µCa (0.1-0.4 mm) or connective tissue (fibers, 1 mm) which can be placed in different locations and superimpose each other. Measurements were done on clinical DM and BT systems and on a prototype high-resolution bCT system with and without superimposed structures. Exposure parameters for all systems were kept consistent with the standard clinical settings; i.e., spatial resolution was better than 100 µm and dose was kept below 5 mGy. Images were acquired using random structure arrangements; ROC curves were generated based on 5 observers. Results For DM and BT, low-contrast masses down to 4 mm were recognized without and down to 6 mm with added superimposing structures. All µCa objects were detected down to 200 µm with and without superpositioned structures. Fibers were not seen in either arrangement. For bCT, low-contrast masses down to a size of 2 mm diameter, all µCa objects and all fibers were recognized by the observers. Standard ROC analysis revealed higher sensitivity and specificity for bCT in low-contrast detectability than for DM and BT. Conclusion Objective evaluation of low-contrast detectability with the reader blinded to structure type and position is feasible. In this test, high-resolution bCT showed performance superior to DM and DT, especially when confounding structures were superimposed. Clinical Relevance/Application The proposed phantom setup allows conducting receiver operating characteristic (ROC) studies for the objective evaluation of scanners and scan protocols with ground truth clearly defined.
    RSNA 2013, Chicago; 12/2013
  • Conference Paper: The European Perspective
    Willi A. Kalender ·
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    ABSTRACT: LEARNING OBJECTIVES 1) Understand that CTDI is merely a technical concept for scanner acceptance and constancy testing, but not a measure for patient dose. 2) Learn about concepts for patient- and scanner-specific patient dose estimates. 3) Learn about the concept of diagnostic reference levels and its strengths and weaknesses. ABSTRACT There is no major debate regarding the validity of the computed tomography dose index (CTDI) in Europe because it is considered as a tool for scanner acceptance and constancy testing. Its use for that purpose is undisputed. Measures for patient dose have been a major topic for decades. There are no common regulations valid for all of Europe, but there are a number of initiatives and concepts in place already which originated here. Among these are primarily the generation of conversion coefficients k for estimating values of the effective dose E from the dose length product (DLP) by E = k×DLP and the concept of dose reference levels (DRL). DRLs for radiological examinations in the European Union were demanded by law already in 2000. Patient dose assessment relies predominantly on pre-tabulated values generated for anthropomorphic and voxel phantoms. Efforts are underway to provide more patient-specific dose estimates (PSDE) independent of CTDI phantom measurements. The lecture will review the above concepts and will point to both strengths and weaknesses.
    Radiological Society of North America 2013 Scientific Assembly and Annual Meeting; 12/2013
  • Natalia Saltybaeva · Daniel Kolditz · Willi A. Kalender ·
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    ABSTRACT: PURPOSE Monte Carlo (MC) simulations can be performed on patient CT image data. However, this data is limited to the scanned volume and does not allow estimating dose for organs outside the directly exposed range. Standard mathematical phantoms do not have this limitation, but they do not reflect individual patient anatomy. The aim of this work was to generate whole-body patient-specific voxel models for accurate organ and effective dose estimations. METHOD AND MATERIALS Three anthropomorphic phantoms representing an adult (Alderson Research Laboratories, New York, USA) and 5 and 1 y.o children (CIRS, Norfolk, VA, USA) were considered as patients and scanned with thorax routine protocols (SOMATOM Definition Flash, Siemens, Forchheim, Germany) with tube voltages of 80, 100 and 120 kV. The absorbed dose was measured using 90 calibrated TLD chips. Whole-body voxel models were generated by amending the patient CT volume by size-adapted versions of the ICRP Reference Male phantom in case of the adult and ORNL voxel phantoms in the case of the pediatric patients. Organ definitions provided with these phantoms were transferred to the combined models and adapted interactively. MC simulations were performed using the validated tool ImpactMC (CT Imaging GmbH, Erlangen, Germany) for a) the unamended patient volumes, b) the combined whole-body models. Simulated 3D dose distributions were compared with TLD measurements chip by chip. RESULTS The mean difference between measurements and simulations based on the unamended CT volumes was 19%; using the whole-body model reduced the difference to 6%. In contrast to unamended CT volumes, whole-body models intrinsically also provided the dose values for organs outside the scanned volume. These organs contributed 17%, 34% and 36% to effective dose for the adult, 5y.o. and 1 y.o. phantom, respectively. CONCLUSION Patient-specific whole-body models allow to increase accuracy of dose estimation and to calculate dose for all relevant organs. CLINICAL RELEVANCE/APPLICATION The approach is useful for individual patient dose estimation, especially for clinical studies.
    Radiological Society of North America 2013 Scientific Assembly and Annual Meeting; 12/2013

Publication Stats

13k Citations
970.76 Total Impact Points


  • 1996-2015
    • Friedrich-Alexander Universität Erlangen-Nürnberg
      • Institute of Physics
      Erlangen, Bavaria, Germany
  • 1995-2015
    • Universitätsklinikum Erlangen
      • Department of Obstetrics and Gynaecology
      Erlangen, Bavaria, Germany
  • 2001
    • Phoenix Zoo
      Phoenix, Arizona, United States
  • 1997-1999
    • IST Austria
      Klosterneuberg, Lower Austria, Austria
    • San Diego Zoo
      San Diego, California, United States
  • 1998
    • Humboldt-Universität zu Berlin
      Berlín, Berlin, Germany
  • 1994
    • Washington University in St. Louis
      San Luis, Missouri, United States
  • 1989
    • Vrije Universiteit Brussel
      • Department of Radiology
      Bruxelles, Brussels Capital, Belgium
  • 1988
    • Universität Stuttgart
      Stuttgart, Baden-Württemberg, Germany