1. A computer-implemented method of compressing data in a materials-detection system, the method comprising: generating a first multi-dimensional digital image of a scan region, the scan region being included in a materials-detection apparatus and being configured to receive and move containers through the materials-detection apparatus; accessing a pre-defined background range of values, the background range of values representing a range of values associated with non-target materials and the background range of values being distinct from values associated with the target materials; comparing a value of a voxel included in the multi-dimensional digital image to the background range of values to determine whether the value of the voxel is within the background range of values; if the value of the voxel is within the background range of values, identifying the voxel as a voxel representing a low-density material; and generating a second multi-dimensional digital image that disregards the identified voxel to compress the first multi-dimensional digital image.