2 of the inner arms 22 a 22 b the positions of the first ends 20 a 1 and 20 b 1 of the outer arms 20 a 20 b are moved accordingly so that no part of the outer arms 20 a 20 b blocks the projected u201c field of view u201d of the inner arms 22 a 22 b and also so that there is no gap or discontinuity in the detected signal at the junctions where the outer arms 20 a 20 b meet the inner arms 22 a 22 b as discussed in more detail hereinafter the value of the first angle u03c8 1a between the first arm 22 a and the radiation axis may be different from the value of the third angle u03c8 1b between the third arm 22 b and the radiation axis also the value of the second angle u03c8 2a between the second arm 20 a and the source axis may be different from the value of the fourth angle u03c8 2b between the fourth arm 20 b and the source axis thus the number of combinations of positions of the array arms 20 a 20 b and 22 a 22 b is literally infinite fig2 depicts a view in the x direction of the detector array 18 see the xyz coordinate axis indicator in fig1 this is an example of a view looking toward the array 18 from the position of the source 12 the array 18 depicted in fig2 has fewer cells shown than would be present in the preferred embodiment of the invention so as not to overcomplicate the representation of the array features this view most clearly indicates that the projected spacing of.