X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Utility Patent
1999-03-22
2001-01-02
Porta, David P. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S004000
Utility Patent
active
06169778
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a computed tomography (CT) of the type having a stationary frame, a rotary unit mounted carrying an X-ray source which emits an X-ray beam, and a detector system for the X-rays wherein an examined subject is irradiated by the X-ray beam, and the detector system emits a dataset corresponding to the intensity of the received radiation at a number of projection angles in each rotation of the rotary unit; and wherein an electronic computing unit reconstructs an image of at least a part of the region of the subject which is irradiated with the X-ray beam from the datasets, in consideration of the respective projection angles.
2. Description of the Prior Art
In the above type of CT device, a slotted disk is provided for determining the projection angles, this disk being scanned by an optical sensor and having a number of slots equal at least to the number of projection angles. The technical realization of such a slotted disk is problematic, given the necessary accuracy or precision of at least {fraction (1/100)} mm. Furthermore, due to its large diameter, the slotted disk must be composed of a number of segments, whose positioning is very difficult, particularly at the joints. In addition, contamination in the region of the very narrow slots of the slotted disk can result in functional errors.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a CT device of the initially-described type wherein the determination of the projection angles appertaining to the individual projections can occur simply and cost-effectively.
The above object is achieved in accordance with the principles of the present invention in a computed tomography (CT) device having a stationary frame, and a rotary unit carrying an X-ray source and a radiation detector, which is rotatably mounted at the frame. The rotary unit has a number of marks thereon respectively disposed at known angular intervals, these marks being scanned by a sensor which is attached to the frame, and which emits an output signal as each mark passes the sensor. The number of marks is significantly lower than the number of projection angles which are employed for irradiating the examination subject during each rotation of the rotary unit. Datasets from the radiation detector for each of the projection angles, and signals indicating the passage of the respective marks passed the sensor, are supplied to a computing unit, from which the respective projection angles associated with the datasets are determined by interpolation from the output signals of the sensor.
In the case of the Invention, a significant simplification is achieved because a relatively low number of marks is required compared to the number of projection angles, thus the marks need not be produced with extreme precision, since it suffices for the subsequent detection of the projection angles by interpolation if the angular distances of the marks from each other are known. Simulations have shown that, in the case of an inventive CT device, it is possible to determine the projection angles with such a high degree of accuracy that there are no discernible adverse effects on the image quality compared to a CT device according to the prior art, which operates with a slotted disk. It is advantageous, though not imperative, to arrange the marks at substantially equal intervals. The precision of the angle calculation increases with the number of marks, up to an upper limit of twelve marks. The delay after which the angles can be calculated decreases with as the number of marks increases.
A particularly exact detection of the projection angles is possible according to an embodiment of the invention wherein the detector system is prompted to emit the datasets that respectively correspond to the projections by a first clock signal, and wherein the detection of the respective projection angles appertaining to the projections occurs on the basis of a second clock signal, which is synchronized with the first clock signal. In certain circumstances it is possible for the first and the second clock signals to be identical.
A particularly high degree of precision in identifying the angle of projection is possible if the detection of the projection angle occurs with a resolution of at least 15,000 angular increments per rotation of the rotary unit.
It enhances the precision of the detection of the projection angle if the interpolation occurs according to a second order polynomial.
According to an embodiment of the Invention, at least six marks, preferably twelve, are provided. It is clear that, as a result of their low number, the marks are constructed so as to be relatively large, and are consequently robust, and thus less susceptible to disturbances as well as simpler and more economic to produce. In addition, significantly larger sensors can be employed.
According to a preferred inventive embodiment, the sensor operates on a magnetic, inductive, optical or capacitive basis, and that the marks are composed of a corresponding material. The marks can be of a virtually arbitrarily thickness without compromising the operability, while, in the case of the known slotted disks, the thickness of the slotted disk may not exceed a few tenths of a millimeter, due to the necessity of making the slots with such a small width.
REFERENCES:
patent: 5175754 (1992-12-01), Casey et al.
patent: 5636255 (1997-06-01), Ellis
patent: 5841830 (1998-11-01), Barni et al.
Schmidt Martin
Sembritzki Otto
Porta David P.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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