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

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.