Disclosed herein is a method, comprising: for i=1, . . . , M, sending a pencil radiation beam (i) toward an image sensor, wherein the pencil radiation beam (i) is incident on an incident region (i) on the image sensor, wherein the pencil radiation beam (i) is aimed at a target region (i) on the image sensor, wherein M is a positive integer, wherein the image sensor comprises active areas spatially discontinuous from each other, and wherein the incident regions (i), i=1, . . . , M and the target regions (i), i=1, . . . , M are on the active areas; and for i=1, . . . , M, determining an offset (i) between the incident region (i) and the target region (i).