Abstract: Method for detecting variations in gas density within a volume surrounded by a closed metal wall opaque to optical light includes a source of x-rays positioned at a selected location outside the closed metal wall. Positioning a detector outside the closed metal wall at a location suitable to detect x-rays from the source passing entirely through a portion of the volume surrounded by the closed metal wall. Providing the detector with a plurality of sensors arranged in at least one row to capture a dimensionally distributed view of detected x-rays. Coupling a processor to an output of the detector to analyze the data which can be displayed in a suitable graphical or pictorial presentation, including processing the data to correct for any beam hardening of the x-rays as they pass through the closed metal wall, to apply the Maximum Likelihood Estimation method to generate on the display a reconstructed image of the gas density, and to use Inverse Radon Transforms for deconvolution.