X-ray or gamma ray systems or devices – Specific application – Tomography
Patent
1997-11-18
1999-08-24
Bruce, David Vernon
X-ray or gamma ray systems or devices
Specific application
Tomography
378 25, 378 53, G01N 2300
Patent
active
059433876
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of evaluating changes in the intensity of polychromatic radiation having a known frequency spectrum after it has passed through a certain thickness of a body consisting of one or more phases of a known composition, the purpose being to correct reconstruction or quantification artefacts.
The method of the invention allows various factors to be taken into account, for example: the chemical composition of the body, the energy distribution of the polychromatic beam, the elements making up the said body or changes in the absorption coefficient of the radiation, etc.
The method of the invention is particularly well suited to modelling and/or correcting the hardening of a beam of polychromatic radiation passing through an absorbent body.
Throughout the description, the terms "beam hardening" are used to denote the increase in the mean energy of a beam as it passes through an absorptive body.
The present invention is especially well suited to applications which simulate the hardening of a X-ray beam and for correcting the artefacts introduced by this hardening.
The method of the invention is applied in particular to measuring the "effective" thickness of a substance through which a polychromatic X-ray beam has passed in an absorptive body of a known chemical composition.
It can be applied, for example, as a means of determining the distribution and percentage of phases in a compound where the nature of the phases of which it is made up is known.
The method of the invention is especially suitable for applications involving rock characterisation, for example reservoir rocks in the petroleum industry.
When a beam of polychromatic radiation passes through an absorptive body, it undergoes a change which is specifically due to its interaction with the element or elements constituting this body. This being the case, the energy spectrum of the beam is different after it has passed through the body compared with what it was before. This change is due in particular to the energies contained in the spectrum and may vary depending on the position of the zone through which the beam has passed and its thickness.
Since the lowest energy values are associated with a high absorption coefficient, they produce the highest hardening artefacts in the majority of cases.
In the case of applications which are quantitative by nature, it is useful to have a method that will take account of any uncertainties which come into play due to the polychromatic nature of the radiation.
A certain number of methods have been described in the prior art as a means of minimising or even eliminating the uncertainties inherent in the polychromatic nature of the radiation.
One of the methods is to use a filter, for example a plate made from a material which absorbs at least some of the radiation and more specifically the weakest energies. However, this method has a disadvantage in that some of the beam is absorbed, causing a loss in intensity.
In another method, a calibration is undertaken using a model or reference sample. However, such a sample must be homogeneous in composition and must exhibit an attenuation close to that of the object to be analysed and it is not always possible to meet these two selection criteria.
Consequently, these methods do not provide an easy solution to the phenomena introduced by the polychromatic nature of the beam.
The objective of the method of the invention is to evaluate the change in the intensity of polychromatic radiation having a known frequency spectrum after it has passed through a certain thickness of a body or object consisting of one or more phases of a known composition, the purpose being to correct the errors or artefacts linked to the polychromatic nature of the radiation.
It is characterised in that this change in intensity is evaluated by building a model to represent the influence of a certain number of defined parameters linked to the nature and the physical-chemical properties of each of the phases which affect the absorption coefficient of the body in one or mor
REFERENCES:
patent: 5485492 (1996-01-01), Pelc
patent: 5533080 (1996-07-01), Pelc
de Paiva Rogerio Ferreira
Lynch John
Bruce David Vernon
Institut Francais du Pe'trole
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