Mr. Johannes Köhrer Profile

Johannes Köhrer

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Proceedings Article | 1 April 2024 Poster + Paper

Determination of a detectability index for image quality evaluation using a PVA-microcapsules phantom

Chatsuda Songsaeng, Elisabeth Salomon, Markus Ortner, Lukas Zalka, Johannes Köhrer, Peter Homolka, Julia Pospischek, Christian Kollmann, Jack Miloslavsky, Uli Palli, Michael Figl, Johann Hummel

Proceedings Volume 12932, 1293214 (2024) https://doi.org/10.1117/12.3006046

KEYWORDS: Transducers, Polyvinyl alcohol, Image quality, Modulation transfer functions, Ultrasonography, Microspheres, X-ray imaging, Tunable filters, Silicon, Signal attenuation

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The goal of this study was the development of a phantom for the determination of the image quality of ultrasound (US) based on the Linear System Theory. Modular transfer Function (MTF) and noise power spectrum (NPS) were determined on two US phantoms. One contained a cylinder filled with water, which appears as a circle in the US images, the second was completely homogeneous. The base material of the phantom was Poly(vinyl alcohol) which was mixed with water in a 1:9 ratio. Additionally, micro-plastic spheres and starch, respectively, were included to increase echogenicity. An algorithm was developed that calculates a radial MTF from the circular structure representing spatial resolution averaged across all directions. Noise power spectrum was determined as described by Fredenberg et al., image quality was evaluated by means of a detectability index for different diameters. Two transducers with different bandwidths (4 to 13MHz and 3 to 8MHz) were used to show the dependence of the index on the main frequency of the US wave. In addition, three penetration depths, which also require different frequencies, were used. Detectability was higher with the transducer of higher frequency for all measurements i.e. for all depths and all diameters (paired t-tests, all p < 0.01). There was also a decrease of detectability with increasing depth for both transducers. The dependence of the index on the axial distance of the ROIs was highly significant (two-sided, paired Wilcoxon test, p < 0.00001). With respect to the comparison for the different phantom materials (PVA with starch and PVA with micro-spheres), the null hypothesis (equality of variances; unpaired, two-sided Wilcoxon test) could not be rejected (p=0.14). The results suggest that the concept of the detectability index can also be applied to US images with some reservations.

Proceedings Article | 1 April 2024 Poster + Paper

Adaptive design of ultrasound phantoms using additive manufacturing

Lukas Zalka, Johannes Köhrer, Chatsuda Songsaeng, Peter Homolka, Christian Kollmann, Johann Hummel, Michael Figl

Proceedings Volume 12932, 1293219 (2024) https://doi.org/10.1117/12.3006375

KEYWORDS: 3D printing, Ultrasonography, Materials properties

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Ultrasound imaging can be used to visualize the boundaries between two media with different properties, but less information is available about the medium penetrated. Therefore, conventional ultrasound devices used in routine medical imaging usually assume a constant value for the speed of sound of the structures. Thus, current research aims to develop new methods to obtain more information about the medium scanned. To support the research ultrasound phantoms with precisely known properties like speed of sound, attenuation or hardness can be used. We developed polyvinyl alcohol-based phantoms and investigated the correlation between the properties and the freeze-thaw cycles (FTC), as well as the effects of glycerol on the speed of sound. Additionally, we designed 3D printed molds for shaping the phantom and improving the FTC process. Furthermore, a construction was designed to prevent the measurement setup from being contaminated by PVA and glycerol defusing out of the phantom. The physical properties of the phantoms measured were the speed of sound, attenuation, and Shore hardness. The measurements conducted were carried out using the standards of the American Institute for Ultrasound in Medicine (AIUM) and the American Society for Testing and Materials (ASTM). There was an increase in Shore hardness and speed of sound as the number of FTCs increased. However, the differences in the speed of sound were only minimal, whereby higher differences could be achieved by the addition of glycerol. At present, no statements can be made about the attenuation. The measurements of the materials are in an early state and further improvement is needed. The manufacturing process and measurements were improved by 3D printed molds.

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