Zeitschrift für Medizinische Physik (Z MED PHYS)

Publications in this journal

• Article: Improvement of the penumbra for small radiosurgical fields using flattening filter free low megavoltage beams.

  Mehran Yarahmadi, Mahmoud Allahverdi, Hassan A Nedaie, Khadijeh Asnaashari, Sayed A Vaezzadeh, Otto A Sauer
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  ABSTRACT: BACKGROUND: In stereotactic radiosurgery, sharp beam edges have clear advantages to spare normal tissues. In general, the dose gradient is a limiting factor in minimizing dose to nearby critical structures for clinical cases. Therefore the penumbral width should be diminished. METHODS: A Varian Clinac 2100 linear accelerator equipped with in-house designed radiosurgical collimator was modeled using the EGSnrc/BEAMnrc Monte Carlo code and compared with the measurements. The 0.015 cm(3) PinPoint chamber was used to measure the 6 MV photon beam characteristics and to validate Monte Carlo calculations. Additional to the standard (STD) linac, a flattening filter free (FFF) linac was simulated. Percent depth doses, beam profiles and output factors were calculated for small field sizes with diameter of 5, 10, 20 and 30mm with DOSXYZnrc. The mean energy and photon fluence at the water surface were calculated with BEAMDP for both FFF linac and STD linacs. RESULTS: The penumbra width (80%-20%) was decreased by 0.5, 0.3, 0.2 and 0.2mm for field sizes of 5, 10, 20 and 30mm respectively when removing the FF. The fluence of photons at the surface increased up to 3.6 times and the mean energy decreased by a factor of 0.69 when removing the FF. The penumbra width (80%-20%) decreased by 17% when a 2 MeV monoenergetic electron pencil beam incident on the target is used instead of 6.2 MeV. CONCLUSIONS: It was found that the penumbra of small field sizes is decreased by removing the FF. Likewise using low megavoltage photons reduced the beam penumbra maintaining adequate penetration and skin sparing.
  Zeitschrift für Medizinische Physik 05/2013;
• Article: Supplementary values of the dosimetric parameters kNR and Em for various types of detectors in 6 and 15 MV photon fields.

  Ndimofor Chofor, Dietrich Harder, Björn Poppe
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  ABSTRACT: The present communication broadens the data base for determinations of the non-reference condition correction factor kNR needed in high-energy photon dosimetry to accomplish the use of various detectors under non-reference conditions. Following our previous strategy of calculating semiempirical values of kNR and correlating them with the mean photon energy Em at the point of measurement in a large water phantom, the values of Em are now stated for 6 and 15 MV photon radiations of accelerators with and without flattening filters, square field sizes from 1 to 30 cm side length and depths from 0 to 28 cm. The unambiguity of the kNR-Em correlation is again confirmed and is quantified by fitting formulae for air-filled ionization chambers, TLD detectors and Si diodes. This survey provides a practicable access to the kNR values, particularly for the non-water equivalent detectors to be used in small-field dosimetry.
  Zeitschrift für Medizinische Physik 05/2013;
• Article: Ion range estimation by using dual energy computed tomography.

  Nora Hünemohr, Bernhard Krauss, Julien Dinkel, Clarissa Gillmann, Benjamin Ackermann, Oliver Jäkel, Steffen Greilich
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  ABSTRACT: Inaccurate conversion of CT data to water-equivalent path length (WEPL) is one of the most important uncertainty sources in ion treatment planning. Dual energy CT (DECT) imaging might help to reduce CT number ambiguities with the additional information. In our study we scanned a series of materials (tissue substitutes, aluminum, PMMA, and other polymers) in the dual source scanner (Siemens Somatom Definition Flash). Based on the 80kVp/140SnkVp dual energy images, the electron densities ϱe and effective atomic numbers Zeff were calculated. We introduced a new lookup table that translates the ϱe to the WEPL. The WEPL residuals from the calibration were significantly reduced for the investigated tissue surrogates compared to the empirical Hounsfield-look-up table (single energy CT imaging) from (-1.0±1.8)% to (0.1±0.7)% and for non-tissue equivalent PMMA from -7.8% to -1.0%. To assess the benefit of the new DECT calibration, we conducted a treatment planning study for three different idealized cases based on tissue surrogates and PMMA. The DECT calibration yielded a significantly higher target coverage in tissue surrogates and phantom material (i.e. PMMA cylinder, mean target coverage improved from 62% to 98%). To verify the DECT calibration for real tissue, ion ranges through a frozen pig head were measured and compared to predictions calculated by the standard single energy CT calibration and the novel DECT calibration. By using this method, an improvement of ion range estimation from -2.1% water-equivalent thickness deviation (single energy CT) to 0.3% (DECT) was achieved. If one excludes raypaths located on the edge of the sample accompanied with high uncertainties, no significant difference could be observed.
  Zeitschrift für Medizinische Physik 04/2013;
• Article: Treatment planning in molecular radiotherapy.

  Gerhard Glatting, Manuel Bardiès, Michael Lassmann
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  ABSTRACT: In molecular radiotherapy a radionuclide or a radioactively labelled pharmaceutical is administered to the patient. Treatment planning therefore comprises the determination of activity to administer. This administered activity should maximize tumour cell sterilization while minimizing normal tissue damage. In this work we present different approaches that are frequently used for determining the suitable activity. These approaches may be cohort- based as in chemotherapy, or patient-specific using dosimetry based on individual biokinetics. The approaches are different with respect to the input complexity, the corresponding costs and - in consequence - the quality of the therapy. In addition, a general scheme for data collection and analysis is proposed. To develop an effective and safe treatment, elaborate data need to be obtained. The main challenges, however, are collecting these complex data and analyse them properly.
  Zeitschrift für Medizinische Physik 04/2013;
• Article: Evaluation of the potential of phase-contrast computed tomography for improved visualization of cancerous human liver tissue.

  Peter B Noël, Julia Herzen, Alexander A Fingerle, Marian Willner, Marco K Stockmar, Dieter Hahn, Marcus Settles, Enken Drecoll, Irene Zanette, Timm Weitkamp, Ernst J Rummeny, Franz Pfeiffer
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  ABSTRACT: PURPOSE: Phase-contrast X-ray computed tomography (PCCT) is currently investigated and developed as a potentially very interesting extension of conventional CT, and can offer several advantages for specific indications in diagnostic imaging. Current absorption-based computed tomography (CT) without the application of contrast material is limited in the detection of minor density differences in soft-tissue. The purpose of this study is to test whether PCCT can improve soft tissue contrast in healthy and tumorous human liver specimens. MATERIALS AND METHODS: Two specimens of human liver (one healthy and one metastasized liver sample) were imaged with brilliant X-ray beam at the synchrotron radiation source ESRF in Grenoble, France. For correlation the same specimens were imaged with a magnetic resonance imaging system at 1.5 T. The histopathology confirmed our findings in the corresponding sections of the specimens. RESULTS: In the phase-contrast CT images we observed a significantly enhanced soft-tissue contrast when compared to simultaneously recorded standard absorption CT measurements. Further, we found that the pathological and morphological information in the PCCT reconstructions show significant improvement when compared to those performed on MRI. Based on matching of prominent features, a good correlation between PCCT and the histological section is demonstrated; especially the tumor capsule and the surrounding vascular structures are visible in PCCT. In addition, our study revealed the ability of PCCT to visualize the blood vessels structure in the tumorous liver without the need of any contrast agents. CONCLUSION: Grating-based PCCT significantly improves the soft-tissue contrast in ex-vivo liver specimens and holds the potential to overcome the need of contrast materials for visualization of the tumor vascularization.
  Zeitschrift für Medizinische Physik 04/2013;
• Article: Flat Detector Cone-Beam CT.

  Marc Kachelrieß
  Zeitschrift für Medizinische Physik 04/2013;
• Article: Dependence of image quality on acquisition time for the PET/CT Biograph mCT.

  Flavia Molina-Duran, Dietmar Dinter, Frederic Schoenahl, Stefan O Schoenberg, Gerhard Glatting
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  ABSTRACT: The impact of acquisition time on reconstructed PET image quality is analyzed for different acquisition times (1, 2, 3 and 4min). Image quality was tested according to the National Electrical Manufacturers Association (NEMA) NU 2-2007, the evaluation for the signal to noise ratio (SNR) and the reconstructed activity ratio (RAR) for three algorithms, i.e. OSEM, TrueX and TOF applying different effective iteration numbers. The present work shows that the image quality of 3 and 4min acquisition time for spherical lesions of 10mm diameter are not significantly different between TrueX, TOF and OSEM. The 2min acquisition time should be used carefully for the TrueX and OSEM algorithms in small lesions, because the levels of background noise are high compared to 3 or 4min measurements. Also, the reconstructed activity ratio is underestimated to be approximately half of the expected value. For large lesions the three algorithms perform similarly for all acquisition durations, however, OSEM has the advantage of a more accurately reconstructed activity ratio compared to TrueX and TOF, which are more strongly influenced by noise.
  Zeitschrift für Medizinische Physik 04/2013;
• Article: Evaluation of phase-contrast CT of breast tissue at conventional X-ray sources - presentation of selected findings.

  Susanne Grandl, Marian Willner, Julia Herzen, Doris Mayr, Sigrid D Auweter, Alexander Hipp, Franz Pfeiffer, Maximilian Reiser, Karin Hellerhoff
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  ABSTRACT: BACKGROUND: Grating-based phase contrast computed tomography (PC-CT) at synchrotron radiation sources has been shown to provide improved visualization of breast tumors. However, broad clinical application of phase-contrast imaging will likely depend on transferring the technology to standard polychromatic X-ray sources. On the basis of selected findings, we demonstrate the potential of grating-based PC-CT using a conventional X-ray source. MATERIALS AND METHODS: Grating-based PC-CT of two ex-vivo formalin fixed breast specimens containing lobular carcinoma was conducted using a Talbot Lau interferometer run at a polychromatic X-ray source of 40kVp. Phase-contrast and absorption-based 3D-datasets of both specimens were simultaneously recorded. Radiological images were manually matched with corresponding histological sections. The visualization of selected histological findings in phase contrast was compared to absorption contrast. RESULTS: Grating-based PC-CT was able to depict the 3-dimensional structure of dilated ducts and high phase contrast was found as a correlate to thickened fibrous ductal walls. Differences in contrast between fibrous and less fibrous breast tissue were observed in phase- but not in absorption-contrast images. Furthermore, regions of low phase contrast correlated with the extension of compact tumor components. CONCLUSIONS: On the basis of selected findings, we show that grating-based PC-CT at a polychromatic X-ray source provides complementary information to conventional absorption contrast; albeit at lower spatial resolution than synchrotron-based imaging.
  Zeitschrift für Medizinische Physik 03/2013;
• Article: A three-dimensional model of tracheobronchial particle distribution during mucociliary clearance in the human respiratory tract.

  Robert Sturm
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  ABSTRACT: Although theoretical approaches to tracheobronchial (TB) clearance have been continuously refined during the past decades, questions concerning the exact course of particle removal from the TB tree have been largely remained unsolved. In order to clarify this problem, three-dimensional patterns of mucociliary particle clearance have to be generated at pre-defined time points after particle exposure. Here, we present a mathematical method for the generation of respective clearance patterns. Three-dimensional transport paths of inhaled particles as well as spatial deposition patterns were generated by determining spatial information of all airway tubes passed by the particles and the particle deposition sites. Three-dimensional data were converted to a coordinate system, within which the trachea represented the z-axis. Visualization of stored data was realized with the help of a freely available program code that is specialized in processing huge data sets. Mucociliary clearance of deposited particular mass was computed by assuming (1) an interrelationship between mucus velocity and airway caliber and (2) an average tracheal mucus velocity of 5.5mm min-1. Position of cleared particles within the spatial TB tree was determined at t=0 h (immediately after exposure), t=12 h and t=24 h. Spatial patterns of mucociliary clearance were computed for particles with a uniform geometric diameter of 5μm and a density of 1g cm-3. Inhalation of the aerosol loaded with those particles took place under sitting breathing conditions (breathing frequency: 15min-1, tidal volume: 750 ml). As demonstrated by the generated clearance patterns, mucociliary transport of 5μm particles is completed after 30 h. Within the first 12 h following aerosol exposure, about 75% of the initially deposited particular mass is removed from the TB tree. After 24 h, 95% of the particles have been cleared. Clearance patterns are characterized by a successive transition of maximal particle concentrations towards more proximal airway generations. For 0.1μm particles and 1μm particles clearance times are significantly prolonged, whilst 10μm particles are even faster removed from the TB tree than the 5μm particles. Based on the results of this study the time span between initial deposition of particular matter and complete evacuation of deposited particles ranges from several hours to some days and depends on (1) the preferential deposition site of the inhaled material and (2) the mean mucus velocities in the bronchial airway generations.
  Zeitschrift für Medizinische Physik 03/2013;
• Article: Biomechanical eye model and measurement setup for investigating accommodating intraocular lenses.

  Timo Eppig, Melanie Gillner, Katja Zoric, Janine Jäger, André Löffler, Achim Langenbucher
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  ABSTRACT: We present a biomechanical eye model to induce pseudophakic accommodative movement for evaluation of the focal shift of accommodative intraocular lenses. Therefore, an accommodative intraocular lens (IOL) was implanted into freshly enucleated porcine eyes. The eyes were glued into a mechanical apparatus to expand the ciliar body effectuating mechanical accommodation. An optical coherence tomographer was used to measure positional and geometrical changes of the IOL for different levels of expansion. The expansion unit allowed stretching of the globe of several millimeters. With the biomechanical eye model we were able to simulate the mechanical functionality of accommodation as well as to measure the lens vault and change in geometry. Accommodative vault could only be measured with an intact vitreous, indicating that the vitreous plays an important role for the functionality of accommodative IOLs.
  Zeitschrift für Medizinische Physik 03/2013;
• Article: Theoretical model for design and analysis of protectional eyewear.

  B Zelzer, A Speck, A Langenbucher, T Eppig
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  ABSTRACT: INTRODUCTION: Protectional eyewear has to fulfill both mechanical and optical stress tests. To pass those optical tests the surfaces of safety spectacles have to be optimized to minimize optical aberrations. MATERIAL AND METHODS: Starting with the surface data of three measured safety spectacles, a theoretical spectacle model (four spherical surfaces) is recalculated first and then optimized while keeping the front surface unchanged. Next to spherical power, astigmatic power and prism imbalance we used the wavefront error (five different viewing directions) to simulate the optical performance and to optimize the safety spectacle geometries. RESULTS: All surfaces were spherical (maximum global deviation 'peak-to-valley' between the measured surface and the best-fit sphere: 0.132mm). Except the spherical power of the model Axcont (-0.07m-1) all simulated optical performance before optimization was better than the limits defined by standards. The optimization reduced the wavefront error by 1% to 0.150 λ (Windor/Infield), by 63% to 0.194 λ (Axcont/Bolle) and by 55% to 0.199 λ (2720/3M) without dropping below the measured thickness. CONCLUSION: The simulated optical performance of spectacle designs could be improved when using a smart optimization. A good optical design counteracts degradation by parameter variation throughout the manufacturing process.
  Zeitschrift für Medizinische Physik 03/2013;
• Article: Grating-Based X-ray Phase Contrast for Biomedical Imaging Applications.

  Franz Pfeiffer, Julia Herzen, Marian Willner, Michael Chabior, Sigrid Auweter, Maximilian Reiser, Fabian Bamberg
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  ABSTRACT: In this review article we describe the development of grating-based x-ray phase-contrast imaging, with particular emphasis on potential biomedical applications of the technology. We review the basics of image formation in grating-based phase-contrast and dark-field radiography and present some exemplary multimodal radiography results obtained with laboratory x-ray sources. Furthermore, we discuss the theoretical concepts to extend grating-based multimodal radiography to quantitative transmission, phase-contrast, and dark-field scattering computed tomography.
  Zeitschrift für Medizinische Physik 02/2013;
• Article: Characterization of a chip-based bioreactor for three-dimensional cell cultivation via magnetic resonance imaging.

  Eric Gottwald, Tanja Kleintschek, Stefan Giselbrecht, Roman Truckenmüller, Brigitte Altmann, Matthias Worgull, Jörg Döpfert, Lothar Schad, Melanie Heilmann
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  ABSTRACT: We describe the characterization of a chip-based platform (3(D)-KITChip) for the three-dimensional cultivation of cells under perfusion conditions via magnetic resonance imaging (MRI). Besides the chip, the microfluidic system is comprised of a bioreactor housing, a medium supply, a pump for generating active flow conditions as well as a gas mixing station. The closed circulation loop is ideally suited for a characterization via MRI since the small bioreactor setup with active perfusion, driven by the pump from outside the coils, not only is completely MRI-compatible but also can be transferred into the magnetic coil of an experimental animal scanner. We have found that the two halves of the chip inside the bioreactor are homogeneously perfused with cell culture medium both with and without cells inside the 3(D)-KITChip. In addition, the homogeneity of perfusion is nearly independent from the flow rates investigated in this study, and furthermore, the setup shows excellent washout characteristics after spiking with Gadolinium-DOTA which makes it an ideal candidate for drug screening purposes. We, therefore, conclude that the 3(D)-KITChip is well suited as a platform for high-density three-dimensional cell cultures, especially those requiring a defined medium flow and/or gas supply in a precisely controllable three dimensional environment, like stem cells.
  Zeitschrift für Medizinische Physik 02/2013;
• Article: Grating-based darkfield imaging of human breast tissue.

  Gisela Anton, Florian Bayer, Matthias W Beckmann, Jürgen Durst, Peter A Fasching, Wilhelm Haas, Arndt Hartmann, Thilo Michel, Georg Pelzer, Marcus Radicke, Claudia Rauh, Jens Rieger, André Ritter, Rüdiger Schulz-Wendtland, Michael Uder, David L Wachter, Thomas Weber, Evelyn Wenkel, Lukas Wucherer
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  ABSTRACT: Mastectomy specimens were investigated using a Talbot-Lau X-ray imaging set-up. Significant structures in the darkfield were observed, which revealed considerably higher contrast than those observed in digital mammography. Comparison with the histomorphometric image proofs that the darkfield signal correlates with a tumor region containing small calcification grains of 3 to 30μm size.
  Zeitschrift für Medizinische Physik 02/2013;
• Article: Effect of instrument settings on liquid-containing lesion images characterized by radiofrequency ultrasound local estimators.

  Jian Wang, Chunsong Kang, Tinghua Feng, Jiping Xue, Kailing Shi, Tingting Li, Xiaofang Liu, Yu Wang
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  ABSTRACT: The purpose of this study was to investigate the effects of ultrasonic instrument gain, transducer frequency, and depth on the color variety and color filling of radiofrequency ultrasonic local estimators (RULES) images which indicated specific physical representation of liquid-containing lesions in order to find the optimal settings for the clinical application of RULES in liquid-containing lesions. Changing the ultrasonic instrument gain, transducer frequency, and depth affected the color filling and color variety of 21 pathologically-confirmed liquid-containing lesion images analyzed by RULES. Blue colored fill dominated the RULES images to represent the liquid-containing lesions. A frequency of 12.5MHz led to red and green colors along the inner edges of the liquid-containing lesions. Changing the gain resulted in significantly different blue colored filling that was highest when the gain was 90 to 100. Changing the frequency also significantly changed the blue color filling, with the highest filling occurring at 12.5MHz. Changing the depth did not affect the blue color filling. The liquid components of the lesions may be identified by their characteristic manifestations in RULES, where color variety is affected by transducer frequency and blue color filling which represent liquid-containing lesions in RULES images is affected by frequency and gain.
  Zeitschrift für Medizinische Physik 01/2013;
• Article: Acquisition-weighted chemical shift imaging improves SLOOP quantification of human cardiac phosphorus metabolites.

  Oliver Geier, Andreas Max Weng, Andreas Toepell, Dietbert Hahn, Matthias Spindler, Meinrad Beer, Herbert Köstler
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  ABSTRACT: PURPOSE: Phosphorous metabolite ratios in human myocardium were determined by a combination of acquisition weighted CSI and a SLOOP evaluation and the results were compared to corresponding SLOOP experiments using standard CSI. MATERIALS AND METHODS: 10 healthy subjects were examined at 1.5 T using both standard CSI and acquisition weighted CSI. Both experiments were performed with a similar acquisition time and the same spatial resolution. The PCr/ATP ratio was determined and the localization properties of both experiments were compared. RESULTS: The PCr/ATP ratio of 2.2±0.4 found for the experiment using acquisition weighted CSI was almost identical to the value of 2.0±0.4 for standard CSI. The sensitivity and the localization properties improved in all subjects using SLOOP evaluation of the acquisition weighted sampling in comparison to the standard CSI acquisition with an average of 3% and 18%, respectively. CONCLUSION: The employment of acquisition weighting allows for a further improvement of the (31)P SLOOP spectroscopy of the human heart.
  Zeitschrift für Medizinische Physik 01/2013;
• Article: The dose response functions of ionization chambers in photon dosimetry - Gaussian or non-Gaussian?

  Hui Khee Looe, Tenzin Sonam Stelljes, Simon Foschepoth, Dietrich Harder, Kay Willborn, Björn Poppe
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  ABSTRACT: This study is concerned with the spatial resolution of air-filled ionization chambers in photon-beam dosimetry, i.e. with their dose response functions. These act as convolution kernels K(x,y), transforming true dose profiles D(x,y) into the measured signal profiles M(x,y). One-dimensional dose response functions have been experimentally determined for nine types of cylindrical ionization chambers both in their lateral and longitudinal directions, as well as across two plane-parallel chambers and for the single chambers of two 2D arrays. All these 1D dose response functions are closely described by Gaussian functions. The associated energy-dependent values of the standard deviations σ have been measured for 6 and 15 MV photons with an uncertainty of 0.02mm. At depths beyond secondary electron fluence build-up, there was no detectable depth dependence of the σ values. The general occurrence of Gaussian dose response functions, their extension beyond the geometrical boundaries of the chambers, and the energy dependence of their standard deviations can be understood by considering the underlying system of convolutions, which is the origin of the influences of secondary electron transport. Monte-Carlo simulations of the convolution kernels for a cylindrical, a square, and a flat ionization chamber and their Fourier analysis have been employed to show that the Gaussian convolution kernels are approximations to the true dose response functions, valid in the clinically relevant domain of the spatial frequency. This paper is conceived as the starting point for the deconvolution methods to be described in a further publication.
  Zeitschrift für Medizinische Physik 01/2013;
• Article: From molecular imaging markers to personalized image-guided therapy.

  Stefan O Schönberg, Björn Wängler
  Zeitschrift für Medizinische Physik 01/2013;
• Article: Grating-based X-ray phase-contrast tomography of atherosclerotic plaque at high photon energies.

  Holger Hetterich, Sandra Fill, Julia Herzen, Marian Willner, Irene Zanette, Timm Weitkamp, Alexander Rack, Ulrich Schüller, Mojtaba Sadeghi, Richard Brandl, Silvia Adam-Neumair, Maximilian Reiser, Franz Pfeiffer, Fabian Bamberg, Tobias Saam
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  ABSTRACT: BACKGROUND: Tissue characterization of atherosclerosis by absorption-based imaging methods is limited due to low soft-tissue contrast. Grating-based phase-contrast computed tomography (PC-CT) may become an alternative for plaque assessment if the phase signal can be retrieved at clinically applicable photon energies. The aims of this feasibility study were (i) to characterize arterial vessels at low and high photon energies, (ii) to extract qualitative features and (iii) quantitative phase-contrast Hounsfield units (HU-phase) of plaque components at 53 keV using histopathology as gold standard. MATERIALS AND METHODS: Five human carotid artery specimens underwent grating-based PC-CT using synchrotron radiation of either 23 keV or 53 keV and histological work-up. Specimens without advanced atherosclerosis were used to extract signal criteria of vessel layers. Diseased specimens were screened for important plaque components including fibrous tissue (FT), lipid (LIP), necrotic core (NEC), intraplaque hemorrhage (IPH), inflammatory cell infiltration (INF) and calcifications (CA). Qualitative features as well as quantitative HU-phase were analyzed. RESULTS: Thirty-three regions in 6 corresponding PC-CT scans and histology sections were identified. Healthy samples had the same signal characteristics at 23 keV and 53 keV with bright tunica intima and adventitia and dark media. Plaque components showed differences in signal intensity and texture at 53 keV. Quantitative analysis demonstrated the highest HU-phase of soft plaque in dense FT. Less organized LIP, NEC and INF were associated with lower HU-phase values. The highest HU-phase were measured in CA. CONCLUSION: PC-CT of atherosclerosis is feasible at high, clinically relevant photon energies and provides detailed information about plaque structure including features of high risk vulnerable plaques.
  Zeitschrift für Medizinische Physik 01/2013;