A computer-implemented method for marking a target in a 3-Dimensional image, comprising the steps of: acquiring Computed Tomography image data of a scene; rendering a 3-Dimensional image of the scene using ray casting based on the Computed Tomography image data; removing a transparent region from the 3-Dimensional image based on a fixed 2-Dimensional transfer function in which a transparency mapping relation is defined; and marking the target in the 3-Dimensional image with the transparent region being removed; wherein said step of removing a transparent region comprises: building an octree for describing a 3-Dimensional space data structure of the Computed Tomography image data; traversing the octree to determine whether a node of the octree is transparent based on a minimal gray value, a maximal gray value, a minimal material value, and a maximal material value of the current node; and removing the determined transparent node; the method being characterized in that: said transparency mapping relation defines a relation between a maximum transparency as well as the minimal gray value, the maximal gray value, the minimal material value and the maximal material value for each node of the octree, and said transparency mapping relation is defined by dividing a 2-Dimensional colour table into a first granularity in a gray direction and into a second granularity in a material direction, the first granularity being coarser than the second granularity, and determining the maximum transparency for all rectangular regions of the divided 2-Dimensional colour table. acquiring Computed Tomography image data of a scene; rendering a 3-Dimensional image of the scene using ray casting based on the Computed Tomography image data; removing a transparent region from the 3-Dimensional image based on a fixed 2-Dimensional transfer function in which a transparency mapping relation is defined; and marking the target in the 3-Dimensional image with the transparent region being removed; wherein said step of removing a transparent region comprises: building an octree for describing a 3-Dimensional space data structure of the Computed Tomography image data; traversing the octree to determine whether a node of the octree is transparent based on a minimal gray value, a maximal gray value, a minimal material value, and a maximal material value of the current node; and removing the determined transparent node; building an octree for describing a 3-Dimensional space data structure of the Computed Tomography image data; traversing the octree to determine whether a node of the octree is transparent based on a minimal gray value, a maximal gray value, a minimal material value, and a maximal material value of the current node; and removing the determined transparent node; the method being characterized in that: said transparency mapping relation defines a relation between a maximum transparency as well as the minimal gray value, the maximal gray value, the minimal material value and the maximal material value for each node of the octree, and said transparency mapping relation is defined by dividing a 2-Dimensional colour table into a first granularity in a gray direction and into a second granularity in a material direction, the first granularity being coarser than the second granularity, and said transparency mapping relation defines a relation between a maximum transparency as well as the minimal gray value, the maximal gray value, the minimal material value and the maximal material value for each node of the octree, and said transparency mapping relation is defined by dividing a 2-Dimensional colour table into a first granularity in a gray direction and into a second granularity in a material direction, the first granularity being coarser than the second granularity, and determining the maximum transparency for all rectangular regions of the divided 2-Dimensional colour table.