Simulation of shadowing effects in ultrasound imaging from computed tomography images

Simulation of ultrasound (US) images based on computed tomography (CT) data has previously been performed with different approaches. Shadowing effects are normally pronounced in US images, so they should be included in the simulation. In this study, a new method to introduce shadowing effects has been developed which makes the simulated US images appear more realistic. US images of a cod (Gadus morhua) were obtained with a BK Medical 2202 ProFocus US scanner with a dedicated research interface giving access to beamformed RF data. The center frequency of the transmit pulse was 10 MHz. In transmit mode, the focus point was at 45 mm. 384 US focused beams were emitted to create the image. CT images with a slice thickness of 0.5 mm, and a pixel size of 0.2 × 0.2 mm, were obtained with an Aquilion ONE Toshiba CT scanner. CT data were mapped from Hounsfield Units (HU) to backscatter (BST), attenuation (ATT) coefficients, and characteristic acoustic impedance (CAI) with a new mapping method. The new approach uses focused beam tracing to create maps of the transmission coefficient (TRC) and then the scattering strength map (SSM). There were 384 maps of SSM corresponding to 384 emissions. Finally an average SSM map was calculated. Field II was used to simulate an US image with dimensions of 38.9 mm × 55.3 mm × 4.5 mm, using 10⁶ point scatterers. Since no quantitative method to assess quality of a simulated US image compared to a measured one exists, visual inspection was used. The method gives diffuse shadows that are similar to the ones observed in measurements on real objects.