Bragg reflection from a logarithmic spiral dispersive element (LSDE) has been considered. LSDE is known to have a wide angle of reflection for an x-ray source placed in a spiral focus. Although this is true for a 2-dimensional spiral, it is not the case in 3 dimensions. For a 3- dimensional LSDE only narrow strip on the spiral surface contributes in reflection of radiation from a point source placed in the spiral focus. This imposes certain limitations on a spiral design and on detector dimensions.