A multi-layer X-ray detector comprises a first X-ray converter, a first sensor, a second X-ray converter, a second sensor, and an internal anti-scatter device. The first sensor is located at a first sensor layer and is configured to detect radiation emitted from the first X-ray converter. The second sensor is located at a second sensor layer and is configured to detect radiation emitted from the second X-ray converter. The first X-ray converter and the first sensor form a first detector pair, and the second X-ray converter and the second sensor form a second detector pair. The internal anti-scatter device comprises a plurality of X-ray absorbing septa walls and is located between the first detector pair and the second detector pair. No structure of the internal anti-scatter device is located within either layer of the first detector pair, and no structure of the anti-scatter device is located within either layer of the second detector pair. The plurality of septa walls comprises a plurality of first septa walls substantially parallel to each other, and wherein a spacing between the first septa walls in a first direction is equal to an integer multiple n of detector pixel pitch of the first sensor and/or of the second sensor in the first direction, wherein n=2, 3, 4, . . . N.