Quantitative assessment of the effect of anode surface roughness on diagnostic X-ray spectra: A Monte Carlo simulation study

An experimental measurement study involving surface profilometry and scanning electron microscopy (SEM) combined with MCNP4C-based Monte Carlo simulations was performed to evaluate anode surface roughness in aged X-ray tubes and to quantitatively predict its impact and relevance on generated diagnostic X-ray spectra. Surface profilometry determined that the center-line average roughness in the most aged x-ray tube evaluated in our study was around 50 ?m. SEM measurements also revealed valuable details about the morphology of cracks and irregularities on the anode's focal path. An image-based modeling method was followed for defining more realistic models from the surface-cracked anodes into MCNP's environment. In this approach, the matrices of focal spot images were numerically processed such that the mean depth of cracks along with their spatial pattern to be incorporated into pixel values in agreement with the measured average depths. By invoking MCNP4C's Gaussian energy broadening (GEB), the correctness of this code in simulating diagnostic x-ray spectra was well validated against experimentally-measured spectra. The simulated spectra in surface deteriorated anode models were compared with those simulated in perfectly plain anodes considered as reference. From these comparisons, an intensity loss of 4.5% and 16.8% was predicted for anodes aged by 5 and 50 ?m-deep cracks in conditions of 50 kVp, 6 degree target angle, 2.5 mm Al total filtration. By deploying several point detectors (F5 tallies) along the anode-cathode direction and averaging exposure over them, it was found that for a 12 degree anode roughened by 50 ?m-deep cracks, the reduction of overall exposure is 14.9% and 13.1% for 70 and 120 kVp tube voltages, respectively. Finally, change in patient entrance skin dose (ESD), as a result of anode roughness, was assessed under various conditions in a chest X-ray radiography examination. In conclusion, anode surface roughness can have a non negligible effect on output spectra in aged X-ray imaging tubes and as such, depending on X-ray tube's workload, its impact should be carefully considered in medical X-ray imaging systems.