Zeitschrift für Medizinische Physik (Z MED PHYS)

Publications in this journal

• Article: Perspectives in medical optics.

  Achim Langenbucher, Edgar Janunts
  Zeitschrift für Medizinische Physik 10/2010; 20(4):248.
• Article: Modeling and experimental verification for a broad beam light transport in optical tomography.

  Edgar Janunts, Thomas Pöschinger, Fabian Eisa, Achim Langenbucher
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  ABSTRACT: This paper describes a general theoretical model for computing a broad beam excitation light transport in a 3D diffusion medium. The model is based on the diffusion approximation of the radiative transport equation. An analytical approach for the light propagation is presented by deriving a corresponding Green's function. A finite cylindrical domain with a rectangular cross section was considered as a 3D homogeneous phantom model. The results of the model are compared with corresponding experimental data. The measurements are done on solid and liquid phantoms replicating tissue-like optical properties.
  Zeitschrift für Medizinische Physik 10/2010; 20(4):277-86.
• Article: Simultaneous examination of tear film break-up and the lipid layer of the human eye: a novel model eye for time course simulation of physiologic tear film behavior (Part 2).

  Stefan Arnold, Holger Bruenner, Achim Langenbucher
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  ABSTRACT: This article describes a model eye created to mimic the three-layered human tear-film. The model replicates the properties of the mucin, liquid and lipid layer as well as the time-course behavior including blinking process and tear film break-up for measurements with the tear film sensor described in Part 1. The setup basically consists of a sphere that is moistened by a dilution of tear film substitute. Therefore, the time point of the first break-up strongly depends on the consistency of the dilution. By means of an integrated heater, the evaporation rate of the artificial tear-film can be increased. To illustrate the applicability of the model eye, exemplary images during tear-film simulation, taken with and without using the heater, are shown. All images were captured by the sensor. Results and other potential applications are briefly discussed.
  Zeitschrift für Medizinische Physik 09/2010; 20(4):316-9.
• Article: Simultaneous examination of tear film break-up and the lipid layer of the human eye: a novel sensor design (Part 1).

  Stefan Arnold, Annette Walter, Timo Eppig, Holger Bruenner, Achim Langenbucher
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  ABSTRACT: This article presents a non-invasive sensor setup for objective analysis of the pre-corneal human tear-film involving a time-resolved videokeratoscopy and simultaneous imaging of the lipid layer of the tear-film by a second CMOS-camera. This paper describes in detail the mechanical and optical design of the sensor setup, the realization of the entire illumination component with an integrated Placido grid projection, and the calculation of the image formation using the Placido grid. This concept is demonstrated here with a test subject in the full inter-blink period. All images were taken under physiological conditions. The sensor can assist the ophthalmologist in diagnosing Dry-Eye Syndrome. Results and other potential applications are discussed.
  Zeitschrift für Medizinische Physik 09/2010; 20(4):309-15.
• Article: [A comparison of Monte-Carlo simulated and measured dose distributions in oblique electron beams].

  Frederic Schweizer, Gunter Christ
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  ABSTRACT: Oblique incidence of electron beams is a rare clinical application which can be used e.g. at the irradiation of the internal mammary lymph nodes in the case of mamma carcinoma. 3-D treatment planning systems are accepted standard for photon beams but not for electron beams. The investment for measuring, implementing and testing basic data seems many customers not to be worthwhile when considering the well known inaccuracies of conventional algorithms. From Monte-Carlo (MC) based algorithms however higher accuracies can be expected. It was the aim of this paper to test the MC algorithm of the treatment planning system Oncentra Treatment Planning (OTP, Theranostic) in the application for oblique incidence of electron beams. Measured and simulated values are compared for electron energies from 4 MeV up to 18 MeV. For standard electron applicators, even at extended source-surface distances, satisfying results are achieved. However, the agreement gets lost with increasing oblique incidence, especially for small individual and asymmetrical cut outs. We therefore still use measured values instead of simulated dose distributions.
  Zeitschrift für Medizinische Physik 03/2010; 20(1):17-24.
• Article: [Analysis of flow in artificial stenosis models of mid-sized arteries using 3D PC-MRI].

  Stefan Scheuer, Frank G Zöllner, Erik Tumat, Lothar R Schad
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  ABSTRACT: Phase contrast MRI allows access to tri-directional encoded velocity information and therefore, measurement of flow in the human hemodynamic system. The aim of this work was to investigate whether this technology could be applied to support the grading of stenosis in mid-size arteries. Using a specially constructed flow phantom and a stenosis model with tube diameter of 5mm and 8mm and a stenosis of 50%, experiments at different flow rates (180-640 ml/min), slice thickness (1-4 mm), field strength (1.5 and 3.0 T), and multi-slice as well as 3D volume acquisition were performed. The observations were assessed visually and evaluated by signal-to-noise (SNR) ratios in regions before and after the stenosis. The obtained results show that examinations should be performed at high field (3.0 T) and at flow rates up to 500 ml/min without hampering the measurements by areas of signal loss. In comparison, no detectable differences in the flow patterns of the two acquisition schemes could be observed. However, the SNR was higher using the 3D volume acquisition and thick slices. In summary, 3D PC-MRI of mid-size vessels with stenosis is feasible for certain flow rates. The presented results could be seen as guidance for in vivo situations to assess if an examination of a patient is reasonable in terms of outcome.
  Zeitschrift für Medizinische Physik 03/2010; 20(1):34-45.
• Article: [Quality control in PET/CT systems: experiences and requirements].

  Lilli Geworski, Cornelia Karwarth, Eduard Fitz, Michail Plotkin, Bernd Knoop
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  ABSTRACT: Today, in most cases PET examinations are performed using PET/CT hybrid systems. While acceptance testing and routine control of the basic modalities PET and CT, respectively, are described by appropriate regulations, corresponding instructions with regard to the interface connecting both are still missing. This interface includes the adjustment of gantries and patient bed to each other as well as the energy scaling of attenuation coefficients from CT energy to 511 keV. Measurements checking the mechanical adjustment (determination of off-set parameters) are performed following manufacturer's recommendation, with a typical frequency twice a year. On a Biograph 16 (Siemens, Inc.), these measurements were systematically extended to a weekly frequency over an observation period of 10 months, supplemented by measurements with additional load to the patient bed (up to 135 kg), and different vertical bed positions. The results show, that for the construction tested additional off-set measurements for routine control extending well beyond manufacturer's recommendation are not necessary. The energy scaling of attenuation coefficients is depending on methodological aspects and software implementation, and therefore is not part of routine control. On the contrary, the development of appropriate methods for acceptance testing to assess and to determine the mechanical adjustment in all its degrees of freedom as well as the accuracy of attenuation corrected emission data is urgently needed.
  Zeitschrift für Medizinische Physik 03/2010; 20(1):46-50.
• Article: Verification of a commercial implementation of the Macro-Monte-Carlo electron dose calculation algorithm using the virtual accelerator approach.

  Jakob Tertel, Jörg Wulff, Heiko Karle, Klemens Zink
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  ABSTRACT: In this work, the accuracy of the implementation of the Macro Monte Carlo electron dose calculation algorithm into the radiation therapy treatment planning system Eclipse is evaluated. This implementation - called eMC - uses a particle source based on the Rotterdam Initial Phase-Space model. A three-dimensional comparison of eMC calculated dose to dose distributions resulting from full treatment head simulations with the Monte Carlo code package EGSnrc is performed using the 'virtual accelerator' approach. Calculated dose distributions are compared for a homogeneous tissue equivalent phantom and a water phantom with air and bone inhomogeneities. The performance of the eMC algorithm in both phantoms can be considered acceptable within the 2%/2 mm Gamma index criterion. A systematic underestimation of dose by the eMC algorithm within the air inhomogeneity is found.
  Zeitschrift für Medizinische Physik 03/2010; 20(1):51-60.
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  Article: Investigation of correction factors for non-reference conditions in ion chamber photon dosimetry with Monte-Carlo simulations.

  Jörg Wulff, Johannes T Heverhagen, Heiko Karle, Klemens Zink
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  ABSTRACT: Current dosimetry protocols require geometrical reference conditions for the determination of absorbed dose in external radiotherapy. Whenever these geometrical conditions cannot be maintained the application of additional corrections becomes necessary, in principle. The current DIN6800-2 protocol includes a corresponding factor k(NR), but numerical values are lacking and no definite information about the magnitude of this correction is available yet. This study presents Monte-Carlo based calculations within the 6 MV-X photon field of a linear accelerator for a common used ion chamber (PTW31010) employing the EGSnrc code system. The linear accelerator model was matched to measurements, showing good agreement and is used as a realistic source. The individual perturbation correction factors as well as the resulting correction factor k(NR) were calculated as a function of depth for three field sizes, as a function of central axis distance for the largest field and within the build-up region. The behaviour of the ion chamber was further investigated for an idealized hypothetical field boundary. Within the field of the linear accelerator where charged particle equilibrium is achieved the factor k(NR) was generally below approximately 0.5%. In the build-up region a depth dependent correction of up to 2% was calculated when positioning the chamber according to DIN6800-2. Minimizing the depth dependence of the corrections in the build-up region lead to a slightly different positioning of the ion chamber as currently recommended. In regions of the hypothetical field boundary with missing charged particle equilibrium and high dose gradients, the ion chamber response changed by up to approximately 40%, caused by the comparatively large volume (0.125 cm(3)) of the investigated chamber.
  Zeitschrift für Medizinische Physik 03/2010; 20(1):25-33.
• Article: Optimizing an analytical dose calculation algorithm for fast 2D calculations.

  Friedlieb Lorenz, Henning Richter, Piotr Zygmanski
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  ABSTRACT: Previously, an analytical dose calculation algorithm for MLC-based radiotherapy was developed and commissioned, which includes a detailed model of various MLC effects as a unique feature [1]. The algorithm was originally developed as an independent verification of the treatment planning system's dose calculation and it explicitly modeled spatial and depth dependent MLC effects such as interleaf transmission, the tongue-and-groove effect, rounded leaf ends, MLC scatter, beam hardening, and divergence of the beam, which in turn resulted in a gradual MLC transmission fall-off with increasing off-axis distance. Originally the algorithm was implemented in Mathematica (Wolfram). To speed up the calculation time and to be able to calculate high resolution 2D dose distributions within a reasonable time frame (<2 s) the algorithm needs to be optimized and to be embedded in a user friendly environment. To achieve this goal, the dose calculation model is implemented in VisualBasic 6.0, which decreases the calculation time moderately. More importantly, the numerical algorithm for dose calculation is changed at two levels: the dose contributions are split into their x- and y-contributions and the calculation is aperture- rather than as originally point-based. Implementing these three major changes, the calculation time is reduced considerably without loosing accuracy. The time for a typical IMRT field with about 2500 calculation points decreased from 2387 seconds to 0.624 seconds (a factor of about 3800). The mean agreement of the optimized and the not optimized calculation algorithm at the isocenter for a fairly complex IMRT plan with 23 fields is better than 1% relative to the local dose at the measuring point.
  Zeitschrift für Medizinische Physik 03/2010; 20(1):61-7.
• Article: Advanced contrast modalities for X-ray radiology: Phase-contrast and dark-field imaging using a grating interferometer.

  Martin Bech, Torben H Jensen, Oliver Bunk, Tilman Donath, Christian David, Timm Weitkamp, Geraldine Le Duc, Alberto Bravin, Peter Cloetens, Franz Pfeiffer
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  ABSTRACT: Here we review our recent progress in the field of X-ray dark-field and phase-contrast imaging using a grating interferometer. We describe the basic imaging principles of grating-based phase-contrast and dark-field radiography and present some exemplary results obtained for simple test objects and biological specimens. Furthermore, we discuss how phase-contrast and dark-field radiography can be combined with the concept of computed tomography, and yield highly detailed three-dimensional insights into biomedical sample. Exemplary results obtained with standard X-ray tube sources and highly brilliant synchrotron sources are presented.
  Zeitschrift für Medizinische Physik 03/2010; 20(1):7-16.
• Article: Improving the performance of direct Monte Carlo optimization for large tumor volumes.

  Judith Alvarez-Moret, Thomas Dirscherl, Mark Rickhey, Ludwig Bogner
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  ABSTRACT: Direct Monte Carlo Optimization (DMCO) is a powerful method for dose optimization with Monte Carlo accuracy and direct aperture optimization with simulated annealing. Recently, we presented quasi intensity modulated arc therapy (qIMAT), a step-and-shoot technique that simulates a rotational technique by using a high number of beams and reducing the number of segments. In the present work, we applied a combination of both techniques to optimize an anal cancer case. Because of the limited memory of standard computers, two techniques for reducing the size of the inverse kernel (IK) were investigated. The standard deviation degradation technique (SDDT) and the reduced resolution technique (RRT) were applied to a 7-field IMRT plan on the CarPet phantom. Several IKs with an estimated standard deviation (SD) of the MC-calculation of 5%, 10% and 15% and another three IKs with voxel size of 4, 8 and 16mm were calculated. All IKs were optimized with DMCO; after optimization, a final dose calculation with 5% SD and 4mm resolution was carried out. SDDT was a better compromise between plan quality and IK-size reduction than RRT. PTV homogeneity and dose sparing to the OAR was almost identical for SDDT, while for RRT the quality was degraded by low resolution. Therefore, SDDT was applied to the anal cancer case. The IK-file of a quasi-IMAT plan with 30 beams was calculated with XVMC with 15% SD and a voxel size of 4mm. After optimization with DMCO using one segment per beam, a final dose calculation with 2% variance was performed. By comparing the DVHs of qIMAT with a 7-field IMRT (commercial therapy planning system) and with a 7-field IMRT (DMCO), qIMAT showed considerably advantages over IMRT in OARs dose sparing. In this way, the DMCO optimization with qIMAT of complex cases with large treatment volumes, such as anal cancer, are possible. Furthermore, for anal cancer, the comparison of qIMAT with IMRT showed that qIMAT can improve the plan quality.
  Zeitschrift für Medizinische Physik 01/2010; 20(3):197-205.
• Article: Artifact free T2*-weighted imaging at high spatial resolution using segmented EPI sequences.

  Patrick Michael Heiler, Sebastian Schmitter, Lothar Rudi Schad
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  ABSTRACT: The aim of this work was the development of novel measurement techniques that acquire high resolution T2*-weighted datasets in measurement times as short as possible without suffering from noticeable blurring and ghosting artifacts. Therefore, two new measurement techniques were developed that acquire a smoother k-space than generic multi shot echo planar imaging sequences. One is based on the principle of echo train shifting, the other on the reversed gradient method. Simulations and phantom measurements demonstrate that echo train shifting works properly and reduces artifacts in multi shot echo planar imaging. For maximum SNR-efficiency this technique was further improved by adding a second contrast. Both contrasts can be acquired within a prolongation in measurement time by a factor of 1.5, leading to an SNR increase by approximately 2. Furthermore it is demonstrated that the reversed gradient method remarkably reduces artifacts caused by a discontinuous k-space weighting. Assuming sequence parameters as feasible for fMRI experiments, artifact free T2*-weighted images with a matrix size of 256 × 256 leading to an in-plane resolution in the submillimeter range can be obtained in about 2s per slice.
  Zeitschrift für Medizinische Physik 01/2010; 20(3):166-74.
• Article: CCD-based projectional imaging of fluorescent probes in tissue-like media: experimental setup and characterization.

  Thomas Pöschinger, Edgar Janunts, Holger Brünner, Achim Langenbucher
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  ABSTRACT: In this article, a non-contact imaging setup for the acquisition of multiple 2D projections of fluorescent probes in tissue-like phantoms is described. The setup basically consists of a high-sensitivity CCD camera for the detection of fluorescence and a rotating broad-beam light source for the continuous illumination of a rotatable phantom located in the rotation center. This allows for imaging of various projections in a full angular projection range of 360°. Beside the detailed description of the system layout, important key characteristics of the setup are outlined. The setup is demonstrated with projectional measurements of a tissue-like phantom and the results are verified by comparison of the projection-dependent fluorescence intensity distributions with corresponding 2D simulations. It is shown that the instrument is suitable for the sensitive detection of fluorescence emanating from fluorescent objects in tissue-like phantoms. Such setup could facilitate the collection of large projection data sets as they are used in optical fluorescence tomography of small animals.
  Zeitschrift für Medizinische Physik 01/2010; 20(4):299-308.
• Article: ["Dose tolerance" in radiotherapy--development and foundations].

  Christian Streffer
  Zeitschrift für Medizinische Physik 01/2010; 20(1):1-2.
• Article: Theoretical models for dynamic shape factors and lung deposition of small particle aggregates originating from combustion processes.

  Robert Sturm
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  ABSTRACT: A theoretical model was developed which allows the generation of irregularly shaped aggregate particles due to the stepwise joining of spherical components with variable diameters. The mathematical approach is mainly thought to act as a supporting tool for the simulation of the transport and deposition behaviour of combustion aerosols in the atmosphere and the human respiratory tract. In combination with aggregate construction essential particle parameters (dynamic shape factor χ, aerodynamic diameter d(ae)) are computed using the model. As a main result of aggregate generation, an increasing particle size, expressed by an increasing number of spherical components, leads to an enhancement of χ and d(ae), whereby values of the first parameter range from 2 to 70. Deposition of small aggregates (sizes between 2 and 200nm) in the human respiratory tract is commonly marked by high rates of bronchial particle accumulation (40-60%) and declined rates of extrathoracic (20-30%) and alveolar accumulation (2-15%). Concerning aggregate deposition by airway generation, increased cluster size causes a significant decrease of particle accumulation in the proximal airways, whilst accumulation in the intermediate to distal airways is dramatically enhanced. The model was validated using experimental deposition data of tobacco smoke. An excellent correspondence between experimental and theoretical results was found.
  Zeitschrift für Medizinische Physik 01/2010; 20(3):226-34.
• Article: Radiobiological effect based treatment plan optimization with the linear quadratic model.

  Stefan Schell, Jan J Wilkens, Uwe Oelfke
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  ABSTRACT: As an approach towards more biology-oriented treatment planning for external beam radiation therapy, we present the incorporation of local radiation damage models into three dimensional treatment planning. This allows effect based instead of dose based plan optimization which could potentially better match the biologically relevant tradeoff between target and normal tissues. In particular, our approach facilitates an effective comparison of different fractionation schemes. It is based on the linear quadratic model to describe the biological radiation effect. Effect based optimization was integrated into our inverse treatment planning software KonRad, and we demonstrate the resulting differences between conventional and biological treatment planning. Radiation damage can be analyzed both qualitatively and quantitatively in dependence of the fractionation scheme and tissue specific parameters in a three dimensional voxel based system. As an example the potential advantages as well as the associated risks of hypofractionation for prostate cancer are analyzed and visualized with the help of effective dose volume histograms. Our results suggest a very conservative view regarding alternative fractionation schemes since uncertainties in biological parameters are still too big to make reliable clinical predictions.
  Zeitschrift für Medizinische Physik 01/2010; 20(3):188-96.
• Article: Iteratively re-weighted bi-cubic spline representation of corneal topography and its comparison to the standard methods.

  Zhongxia Zhu, Edgar Janunts, Timo Eppig, Tomas Sauer, Achim Langenbucher
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  ABSTRACT: The aim of this study is to represent the corneal anterior surface by utilizing radius and height data extracted from a TMS-2N topographic system with three different mathematical approaches and to simulate the visual performance. An iteratively re-weighted bi-cubic spline method is introduced for the local representation of the corneal surface. For comparison, two standard mathematical global representation approaches are used: the general quadratic function and the higher order Taylor polynomial approach. First, these methods were applied in simulations using three corneal models. Then, two real eye examples were investigated: one eye with regular astigmatism, and one eye which had undergone refractive surgery. A ray-tracing program was developed to evaluate the imaging performance of these examples with each surface representation strategy at the best focus plane. A 6 mm pupil size was chosen for the simulation. The fitting error (deviation) of the presented methods was compared. It was found that the accuracy of the topography representation was worst using the quadratic function and best with bicubic spline. The quadratic function cannot precisely describe the irregular corneal shape. In order to achieve a sub-micron fitting precision, the Taylor polynomial's order selection behaves adaptive to the corneal shape. The bi-cubic spline shows more stable performance. Considering the visual performance, the more precise the cornea representation is, the worse the visual performance is. The re-weighted bi-cubic spline method is a reasonable and stable method for representing the anterior corneal surface in measurements using a Placido-ring-pattern-based corneal topographer.
  Zeitschrift für Medizinische Physik 01/2010; 20(4):287-98.
• Article: Optical coherence tomography - development, principles, applications.

  Adolf Friedrich Fercher
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  ABSTRACT: This paper presents a review of the development of optical coherence tomography (OCT), its principles and important applications. Basic OCT systems are described and the physical foundations of OCT signal properties and signal recording systems are reviewed. Recent examples of OCT applications in ophthalmology, cardiology, gastroenterology and dermatology outline the relevance of this advanced imaging modality in the medical field.
  Zeitschrift für Medizinische Physik 01/2010; 20(4):251-76.