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.
KEYWORDS: Breast, 3D modeling, Polymethylmethacrylate, Digital breast tomosynthesis, Signal attenuation, Tissues, Target detection, Liquids, Photography, Quality measurement
Purpose: In this work we present equivalent breast thickness and dose sensitivity of a next iteration 3D structured breast phantom with lesion models to demonstrate its potential use for quality assurance measurements in breast imaging. Methods: PMMA equivalent thickness was determined employing the automatic exposure control (AEC) of Siemens Mammomat Inspiration and Siemens Mammomat Revelation. A 2D projection image of the phantom was acquired and the corresponding AEC settings recorded as reference. Equivalent PMMA thickness was found by interpolating between three PMMA thicknesses with mAs values close to the reference settings selected by AEC. Dose sensitivity of the reconstructed digital breast tomosynthesis (DBT) images was assessed by two experienced readers using a four alternative forced choice (4-AFC) study. Three different dose levels for lesion models and microcalcifications were evaluated. Results: PMMA equivalent thickness of the phantom was 46.8 mm and 47.0 mm for measurements on Siemens Mammomat Inspiration and Siemens Mammomat Revelation which equals to a breast equivalent thickness of 55.5 mm and 55.8 mm, respectively, compared to a physical phantom thickness of 53.5 mm. For lesion models dose sensitiviy of the detectability was not obvious. For microcalcification the diameter threshold was found to increase for decreasing dose from high dose to AEC to low dose. Conclusions: We found the measured equivalent breast thickness of our phantom to be close to its physical thickness. It can be concluded that changes in dose can be detected by the presented phantom for the tested dose levels.
In this work we tested different materials for 3D printing of spiculated mass models for their incorporation into an existing 3D structured phantom for performance testing of FFDM and DBT. Counting the number of spicules as a function of dose was then evaluated as a possible extra test feature expressing conspicuity next to detectability. Seven printable materials were exposed together with a PMMA step wedge and material samples with known linear attenuation coefficient to determine PMMA equivalent thickness and linear attenuation coefficient, respectively. Next, two models of spiculated masses were created each with a different complexity in terms of number of spicules. The visibility of the number of spicules of a 3D printed spiculated mass model loosely placed in the phantom or embedded into two different printing materials was assessed for FFDM and DBT. Vero White pure was chosen as the most appropriate material for the printing of masses whereas Vero Clear and Tango+ were chosen as background materials. The visibility of spicules was best in the loose mass models and better in the background material Tango+ compared to Vero Clear. While the discrimination of the different spicules could be assessed in FFDM and DBT, as expected only a limited dose sensitivity was found for the visibility of spicules evaluated for the different background materials and at different beam energies.
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