Image noise in X-ray imaging caused by radiation scattering and source leakage, a qualitative and quantitative analysis.

All irradiated objects in the vicinity of an imaging set-up increase the amount of scattered radiation. It is a well-known fact that if this radiation is allowed to impinge on the image collector it naturally degrades the image quality. It also causes problems with equipment characterisation, such as X-ray energy spectra measurements. However, it may not be a well-known fact that when a small radiation safety enclosure is used - common for industrial applications - this type of scattered radiation is astonishing large. In this work a quantitative and qualitative analysis has been carried out in an attempt to explain the origin of the problem, why it occurs and how it may be treated.The scattered radiation increases rapidly when the fractions of X-ray photons in the primary X-ray spectrum above the K-edges of the enclosure wall material increase. High-energy photons are scattered and/or will generate characteristic radiation in the enclosure wall material instead of being absorbed. For lead, which is commonly used as shielding and wall material, this noise becomes significant when tube potentials over 90-100 kV are used, since the K-edges for lead are 72-74 keV (Kα) and 85-87 keV (Kβ). Below tube potentials of 100 kV, the noise from scattered radiation is insignificant, even if the primary beam is wide enough to hit the enclosure walls. Above 100 kV it increases rapidly and for this application the number of scattered photons was 50% of the primary photons at 170 kV.In this particular case this problem was accentuated above tube potentials of 100 kV since the steel housing of the micro focal X-ray source used was penetrated by high-energy photons in unwanted directions. Collimation of the X-ray beam just in front of the X-ray source output window to narrow the solid angle of the primary X-ray beam to avoid direct radiation of the enclosure walls was not enough to suppress this scattered noise. A wider X-ray shield, covering the entire front of the X-ray source had to be applied. Other solutions to decrease scattered radiation would be the following: Increased internal shielding of the X-ray source is the first choice if the same possibilities for geometrical magnification of the imaged object are to be maintained. Increased distances to the enclosure walls and other objects inside, that is, to use larger enclosures or even separate X-ray chambers would also decrease scattered radiation. To line the enclosure with a series of X-ray shielding materials with K-edges at lower and lower energies is another alternative. The lowest K-edge material should be placed closest to the detector followed by the second lowest K-edge material etc., to effectively absorb backscatter and characteristic radiation from the enclosure walls. This would, however, be a rather expensive and complicated solution.