The present disclosure relates to the use of X-ray detector cassettes that may be abutted or overlapped to form a detector assembly suitable for imaging objects that are too large to image using a single X-ray detector cassette. Such a detector assembly may be customized in terms of the size and/or shape of the field-of-view (FOV). In certain embodiments the radiation-sensitive electronics (e.g., readout electronics) are positioned to the side of the X-ray detecting components (e.g., scintillator, TFT array, and so forth), allowing the cassette to be thin relative to other detector devices and allowing the electronics to remain outside the X-ray beam path.