Comprehensive quantitative evaluations of reconstruction method using oil-gel-based phantom in ultrasound computed tomography

The performance of a reconstruction method in ultrasound computed tomography (CT) ideally should be evaluated using various kinds of phantoms at a wide range of speeds of sound when inclusions are made. However, generating real phantoms is more time consuming than generating simulated ones. In our previous study, we developed an oil-gel-based phantom by including water or salt water. In this study, we designed an evaluation method including various contrast conditions using the oil-gel-based phantom by changing the liquid and temperature. The phantom including water or salt water in 10-, 7-, 5-, or 3-mm holes was measured using our prototype ultrasound CT at temperatures of 15, 17.5, 20, 22.5, 25, 27.5, and 30°C, making the number of measurements 14. For these conditions, the difference (= contrast) in the speed of sound between the inclusions and the oil gel was −37 to 92 [m/s]. The filtered back projection (FBP) and full waveform inversion (FWI) were evaluated. The mean error of the speeds of sound in inclusions with the FBP and FWI were 17.1 ± 14.9 and 8.8 ± 10.1 [m/s], respectively. The mean percentage error of the sizes of the phantom (51 mm) and inclusions with the FBP and FWI were 22.5 ± 22.5% and 3.9 ± 4.3%, respectively. A single oil-gel-based phantom provided various contrast conditions after the temperature and liquid were changed. This kind of phantom can be used for comprehensive quantitative evaluations of the reconstruction method.