## 4 Discussion  
### General Statements  
The developed mathematical model and the code for its implementation make it possible to simulate metal artifacts and reduce their magnitude by varying the maximum X-ray energy, the thickness, the material of the pre-filter, and the ADC bit depth.  
It should be noted that an increase in the ADC bit depth and pre-filter thickness is effective in the case of a sufficient number of \(N_{0}\) photons recorded by the detector behind the test object. The specific level \(N_{0}\) depends not only on the general parameters of the TO and the CT system but also on the number of projections.  
The title of the article contains a question about the nature of the metal artifact. This question is not idle for two reasons. The first refers to the expansion of the range of problems solved using various implementations of X-ray CT, with a simultaneous increase in the range of test objects in size range (from micro to macro) and the complication of their structure with the inclusion of high-density fragments, including those from heavy metals. Researchers interested in using X-ray CT for solving their specific problems have an easily justified temptation to use existing standard X-ray CT systems; as a rule, these are medical tomographs. The discrepancy between the low levels of radioscopic transparency of research objects and the parameters of the selected systems leads to pronounced metal artifacts. The second reason is no less important and is associated with the rapid development of algorithmic approaches to the compensation of metal artifacts [2; 18; 25; 26; 27; 49; 50; 51]. Algorithmic approaches implicitly and explicitly assume the presence of information about the X-ray opaque areas of the test objects.  
Figure 6: Reconstructed slices of an aluminum cylinder 200 mm in diameter with cylindrical inserts (Table 1, group 4).  
It follows from the law of attenuation of gamma radiation that for an object of any size with an arbitrary density and an arbitrary value of the effective atomic number of the material, the probability of a photon passing without attenuation is finite. From this follows the fundamental physical feasibility of the task of monitoring objects by the methods of DR and X-ray CT. The technical feasibility of the inspection task is understood as the availability of technical means that allow inspecting at a given time with the required quality with a set of restrictions specified by the consumer.  
The following proposition is theoretically substantiated and illustrated by the results of the computational experiment in Sections 1 and 2. The radioscopic transparency of objects in combination with the radiometric detector loading, the maximum energy of X-ray radiation, the ADC bit depth for the integral photon detection mode, and the thickness of the pre-filter determine the presence or absence of metal artifacts in the reconstructed TO slices. From the foregoing, it follows that the presence of metal artifacts is associated with the technical feasibility of the inspection task by the X-ray CT method. The answer to the question posed in the article title is formulated as follows: metal artifacts are of a technical nature due to the incorrect choice of the parameters of the X-ray CT systems, which determines the actual and physical laws of attenuation and registration of X-rays for specific objects.  
Such an answer should be an incentive for designers and developers when choosing the parameters of X-ray CT systems based on the inadmissibility of metal artifacts for specific test objects for which specialized X-ray CT systems are designed.