The in-plane modulation transfer function (MTF) for multi-slice computed tomography (CT) can be found by scanning a phantom with cylindrical contrast inserts and making use of the circular edges presented in reconstructed axial images. Pixel data across the edge are used to establish an edge spread function, which is then used to obtain the line spread function and finally the MTF. A crucial step in this approach is to accurately locate the centroid of the circular region. Since the ESF is usually established in subpixel scale, slight deviation of the centroid may result in large errors. It has been a common practice to apply a preset threshold and calculate the center of mass in the binary output on each individual slice. It has also been suggested to locate the centroid on each slice by maximizing the sum of pixel values lying under a predefined template. In this paper, we propose a new algorithm based on registering the entire cylindrical object in 3D space. In a test on a high-noise low-contrast edge, both the threshold and the maximization algorithm showed scattered distribution of centroids across consecutive slices, resulting in underestimation of the MTF up to 10% at intermediate frequencies. In comparison, the method based on 3D registration has been found more robust to noise and the centroid locations are more consistent in the longitudinal direction. It is therefore recommended to use the proposed algorithm for centroid determination in evaluating the MTF with a circular edge in CT images.
The aim of this work was to investigate the influence of backscatter radiation from the orbital bone and the intraorbital fat on the eye lens dose in the dental CBCT energy range. To this end we conducted three different yet interrelated studies; A preliminary simulation study was conducted to examine the impact of a bony layer situated underneath a soft tissue layer on the amount of backscatter radiation. We compared the Percentage Depth Dose (PDD) curves in soft tissue with and without the bone layer and we estimated the depth in tissue where the decrease in backscatter caused by the presence of the bone is noticeable. In a supplementary study, an eye voxel phantom was designed with the DOSxyznrc code. Simulations were performed exposing the phantom at different x-ray energies sequentially in air, in fat tissue and in realistic anatomy with the incident beam perpendicular to the phantom. Finally, a virtual head phantom was implemented into a validated hybrid Monte Carlo (MC) framework to simulate a large Field of View protocol of a real CBCT scanner and examine the influence of scattered dose to the eye lens during the whole rotation of the paired tube-detector system. The results indicated an increase in the dose to the lens due to the fatty tissue in the surrounding anatomy. There is a noticeable dose reduction close to the bone-tissue interface which weakens with increasing distance from the interface, such that the impact of the orbital bone in the eye lens dose becomes small.