# Analysis of the Refraction Effect in Ultrasound Breast Tomography
## 4 Discussion
However, breasts have different structures, thicker or thinner skin, and skin layers, so the same improvement in image quality may not always be achieved in actual measurements. In this paper, it is impossible to analyze all combinations of ultrasound velocity values present in the breast structure. However, to preliminarily check the changes in intensity of the refraction phenomenon related to the possible changes in ultrasound velocities in the skin, fat, and glandular tissue of the breast, the parameter (\(\Delta_{opt}\)) was calculated for selected minimum and maximum values (Table 1) using the developed model (Chapter 3.1). Figure 18: Illustrative representation of the spatial path of the refracted ultrasound wave ray through water and breast tissue from the selected sending transducer to the receiving transducer of the ring array and the selected horizontal measurement plane, and orthogonal projections of this path in the horizontal and vertical planes. It was also verified with the elaborated model that as the ultrasound velocity changes for a given tissue, the \(\Delta_{opt}\) values in the ranges determined in Table 1 change almost linearly. These results confirm the choice of an optimal water temperature of about 32 \({}^{\circ}\)C with respect to skin and fat. However, a low water temperature is beneficial for measurements of breasts with low ultrasound velocities in glandular tissue, and a high one for high velocities. The velocity of ultrasound in the glandular tissue of the breast has the greatest influence on the intensity of the refraction phenomenon, because in this case, the greatest changes in \(\Delta_{opt}\) values are observed. Results of calculations and their conclusions were verified by ultrasound transmission tomography imaging of the entire breast volume in vivo, which was performed using a prototype of the hybrid UBT scanner developed by DRAMINSKI S.A. Distributions of ultrasound tomography projection velocity values in the examined breast (Figure 16) show the formation of characteristic distortion areas around the breast/water interface determined earlier using calculations for each of these three water temperature values. The \"distortion ring\" is the widest at 20 \({}^{\circ}\)C. Furthermore, velocity projection values in this case decreased by about 10 m/s in the entire breast region compared to measurements for other values of water temperature. It can be assumed that this is related to an increase in the measured values of the transit time due to a decrease in the amplitude of transmitted pulses along deflected beam rays as a result of the refraction. Reconstructed images of breast cross-sections measured at different water temperatures show significant similarity in the visualized inner breast structure (Figure 17). Some differences and changes of cross-section size may be due to the slightly different position of the breast because the need to change the water temperature prolonged the scanning time and there was no possibility to ensure a stable breast position for the patient in vivo for such a long time. It should be noted that the images of the local ultrasound velocity distribution in the breast cross-section are the most important because they are quantitative images and it is possible to obtain mapping with the accuracy better than 5 m/s, which helps distinguish cancerous lesions in the early stages. Images of the local ultrasound attenuation distribution are reconstructed with much lower accuracy because of weakening of the amplitude of ultrasonic pulses as a result of many additional phenomena (scattering, diffraction, losses due to beam divergence and transmission across inhomogeneity boundaries, etc.). The reconstructed breast images for different values of water temperature do not differ significantly in terms of quality, but for the case of a water temperature of 20 \({}^{\circ}\)C, the distortions in the form of diagonal lines are most visible. Such lines appear in the case of jumps of projection ultrasound velocity values in the sinogram.