Computed tomography scanner and method for controlling the...

X-ray or gamma ray systems or devices – Specific application – Computerized tomography

Reexamination Certificate

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C378S110000, C378S112000, C378S146000

Reexamination Certificate

active

06826252

ABSTRACT:

The present invention relates to a method for controlling a computed tomography scanner, and a computed tomography scanner that is suitable for performing the claimed method.
In many problems in medicine, examinations are performed with the aid of computed tomography scanners. In some fields of mechanical engineering as well, especially materials science and aviation safety, such examinations are also used for testing purposes.
In them, X-radiation is used, which is partly capable of penetrating solid bodies, especially nonmetal bodies. As the radiation passes through a body being examined or observed, the radiation is partly absorbed, and so the intensity of the radiation is less after passing through the body being examined than before passing through it. This effect is also known as attenuation. After penetrating the body being examined, the resulting radiation can be detected, for instance by means of a semiconductor detector, and the attenuation in the radiation can thus be determined. In this way, findings about a distribution of material within the body being examined can be obtained.
A disadvantage of using X-radiation is that beyond a certain dose, it can harm biological tissue. There is accordingly a need, especially in medicine, to keep the radiation dose required for a measurement low.
In medical technology, computed tomography systems are typically used to produce pictures of slices or volumes of the human body.
A decisive factor in the diagnostic value of images obtained from projections produced by the computed tomography scanner is their image quality. The term “projection” is used to mean raw data obtained from a computed tomography scanner, from which data the desired images are then calculated. Thus, a projection as a rule includes a distribution, measured by means of a receiving unit of a computed tomography scanner, of intensity values (attenuation values) of an X-ray beam (i.e. X-radiation) emitted by an X-ray source of the computed tomography scanner.
The quality of the projections and thus the image quality of the images generated from the projections is influenced by numerous factors:
First, artifacts can occur, which can be ascribed for instance to the motion of an X-ray source and of a receiving unit of the computed tomography scanner, or to motion of the measurement object being observed while a projection is being produced or during the productions of successive projections.
Second, pixel noise in the images produced from the projections obtained from the computed tomography scanner are of definitive significance for the image quality. One important manipulated variable for pixel noise is the intensity of the radiation received. This in turn depends on the primary intensity of an X-ray beam emitted by an X-ray source of the computed tomography scanner. If the intensity of the beam, received by means of a receiving unit of the computed tomography scanner, for producing projections is high, then the pixel noise in the images generated from the projections can as a rule also be kept low.
In known computed tomography scanners, for controlling the computed tomography scanner, before images are produced, a user can specify a value for a primary intensity of the x-ray source. This primary intensity of the X-ray source is then maintained in the production of images of the body being examined. The magnitude of the primary intensity is selected such that the projections generated by means of the computed tomography scanner are expected to have adequate quality. The intention is thus to assure that even the images generated from the projections will have no more than a maximum allowable pixel noise.
Hence in the choice of the primary intensity of the X-ray source of the computed tomography scanner, the level of experience of the physician using it is of substantial significance.
Users of the known computed tomography scanners tend as a rule, in controlling the computed tomography scanner, to set a relatively high primary intensity of the X-ray source, so that a measurement object under observation, such as a patient, is exposed to a higher does of X-radiation than would actually be necessary for an image of the desired quality.
The quality of the projections obtained by means of a computed tomography scanner can also fluctuate sharply as the system moves around a patient, because of the patient geometry, since the human body overall, in contrast to its individual organs, is a non-homogeneous entity. As a result, a directional noise pattern can occur in the projections obtained and in the images generated from them.
From German Patent Disclosure DE 199 19 423 A1, a method for controlling a computed tomography scanner is known in which an aid in use is made available to a user, if before producing projections, the user has set a combination of operating parameters that are not at least inside the technological limits for the individual operating parameters. Such operating parameters of a computed tomography scanner that can be specified by a user are, besides the primary intensity of the X-ray source, the scanning time, that is, the time during which the object being examined is exposed to X-radiation for performing an examination, and the scanning length, that is, the length of the object being examined in the direction of the system axis, along which the object being examined is scanned with X-radiation for performing an examination. An operating parameter can lead to an impermissible operating state, for instance if the X-ray source is thermally overloaded, or if because of the mechanical design of the computed tomography scanner the specified scanning length cannot be scanned completely. By the known method, for at least those operating parameters, a value deviating from the pre-selected combination of operating parameters is ascertained for which the intended examination can be expected to be performed with avoidance of the impermissible operating state, without significant loss in image quality compared to the pre-selected combination of operating parameters. The changes in the operating parameters can either be set automatically or made available to the user as a suggestion. The expected quality of the projections, and thus of the images produced from the projections, is calculated using a product of the X-ray tube current and the scanning time.
Also in the method described in DE 199 19 423 A1, the primary intensity of the X-ray source is defined by a user of the computed tomography scanner. Thus in this known method for controlling a computed tomography scanner, the problems discussed above also occur, so that here as well, it is quite likely that the X-radiation dose to a body being observed will be unnecessarily high, because too high a primary intensity of the X-ray source has been selected.
It is the object of the present invention to make available a possible way of controlling a computed tomography scanner for producing projections of a measurement object, in order to generate images, that makes it possible to keep the radiation dose on the measurement object under observation as low as possible, without sacrifices in quality for the projections to be produced by the computed tomography scanner and thus for the images generated from them.
This object is attained with the characteristics of the independent claims. Refinements of the invention are defined by its dependent claims.
The object is attained by a method for controlling a computed tomography scanner for producing projections of a measurement object in which the projections are used to create images of the measurement object, which has the following steps:
specifying a set-point value for a factor that is characteristic of the quality of the projections to be produced by the computed tomography scanner;
sequentially producing projections of the measurement object, by determining an actual value of the factor that is characteristic of the quality of the particular projection produced by the computed tomography scanner between the productions of successive projections and by regulating the p

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