P3D-1 An Analysis of Refraction Artifacts in Time-of-Flight Tomography Regarding their Impact on Image Definition and Contrast Resolution

Time-of-flight tomography within the first order Born approximation has an inherent limitation on spatial and contrast resolution. The deviation of the propagation path from a straight line mainly caused by refraction is dependent on the speed of sound distribution. Some examples in published literature simulate cylindrical phantoms lying at the center of rotation. This paper investigates the effect of location, size and speed of sound of the lesion on its reconstruction. Such an analysis necessitates an algorithm capable of tracing rays through media possessing well defined speeds of sound. The algorithm implemented here can trace rays through a medium defined by any arbitrary number of right cylindrical lesions which may even intersect each other or lie completely within each other. It is essentially a 2D shooting algorithm that yields all the points where the ray changes its direction staring from transmitter and reaching receiver. This information is used to calculate the time of flight for each ray. The inversion is carried out under the assumption of straight line propagation. The reconstructions show that two neighboring lesions are merged together to form an elongated dumbbell shaped image for lesion sizes below 5 mm and for speeds of sound above 1530 m/s. The asymmetrical location of the individual lesions results in twisted dumbbell shape. The two lesions become separately visible when the distance between them is larger than the mean of their diameters.