X-ray or gamma ray systems or devices – Beam control – Collimator
Reexamination Certificate
1998-12-16
2001-07-10
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Beam control
Collimator
C378S019000, C378S098800
Reexamination Certificate
active
06259766
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a computer tomography device which includes an X-ray ray source for emitting an X-ray beam and a detector system for picking up density profiles of cross-sections of an object to be examined.
2. Description of Related Art
A computer tomography device of this kind is known from the article “Computed tomography scanning with simultaneous patient translation” by Carl R. Crawford and Kevin F. King in Medical Physics 17 (1990), 967-982.
Computer tomography involves the formation of images of cross-sections of the object to be examined, for example a patient to be radiologically examined. To this end, the patient is irradiated by means of X-rays from different directions and, due to local differences in the X-ray absorption within the patient, density profiles are formed for the various directions, said profiles being measured by the detector system. To this end, the X-ray source and the detector system are rotated about the patient. An image representing the density of the patient in a cross-section is derived from the various density profiles. It is usually necessary to form a plurality of images of cross-sections along different, usually parallel planes. Such an operation is also referred to as volume scanning. In the known computer tomography device the patient is then displaced in the longitudinal direction with respect to the X-ray source and the detection system while being exposed to X-rays from different directions. The patient is preferably displaced at a uniform speed so as to ensure that the patient does not become “car sick” during the measurement of the density profiles.
The density profiles measured have not been picked up in a fixed longitudinal position; because the patient is displaced while the direction wherefrom the density profiles are measured changes, the longitudinal position varies as a function of the direction within the density profiles. The density profiles are measured along an approximately helical path, the pitch of the helix being constant when the patient is displaced in the longitudinal direction at a uniform speed. The axis of the helical path extends in the longitudinal direction. The known computer tomography device includes a data processing unit for deriving computed density profiles from the measured density profiles, the computed density profiles always relating to a slice through the patient in a given longitudinal position. The reconstruction unit utilizes the computed density profiles for a given longitudinal position so as to derive an image of the cross-section therefrom for the relevant longitudinal position.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a computer tomography device which enables volume reconstruction to be performed faster and more accurately than by means of the known computer tomography device. It is another object of the invention to provide a computer tomography device for fast and accurate volume reconstruction while using only a limited number of individual detector elements.
This object is achieved by means of a computer tomography device according to the invention which includes an X-ray source for emitting an X-ray beam and a detector system for picking up density profiles of cross-sections of an object to be examined, which detector system includes a plurality of X-ray sensitive detector elements which are arranged in a two-dimensional pattern, in which detector elements in different positions along a transverse direction, parallel to the cross-sections, have substantially the same effective cross-section whereas at least some of the detector elements in different positions along a longitudinal direction, transversely of the cross-sections, have different effective cross-sections, and an X-ray collimator for spatially limiting the X-ray beam in the longitudinal direction, and substantially equal surface areas of detector groups, consisting of detector elements and/or parts of detector elements situated along the transverse direction, can be reached by the limited X-ray beam.
The computer tomography device according to the invention essentially enables simultaneous measurement of density profiles of individual cross-sections by means of the individual detector groups. Consequently, only a small amount of time is required to measure the density profiles of a substantial volume of the patient to be examined. Each of the density profiles represents density values in a cross-section of the patient to be examined. The individual cross-sections always relate to a part of the patient to be examined for the relevant, essentially the same longitudinal positions. Mathematically speaking this means that the cross-sections are planes at fixed longitudinal positions. The transverse direction extends perpendicularly to the longitudinal direction, so parallel to the cross-sections. It is not necessary to compute density profiles by interpolation: for individual cross-sections density values are measured for the longitudinal position of the relevant cross-section. Because density profiles are measured for the relevant longitudinal position, it is not necessary to compute these density profiles so that more accurate values can be obtained for the density profiles. Moreover, no time is wasted on carrying out the computations.
The respective detector groups of detector elements which have the same longitudinal positions and are reached by the X-rays over substantially the same surface areas pick up density values of essentially parallel cross-sections of the patient to be examined. From the incident X-rays, the individual detector elements derive a detector signal whose signal level represents the intensity of the incident X-rays. In order to ensure that substantially the same surfaces areas of detector elements of one and the same detector group are reached by X-rays, parts of detector elements are shielded by the X-ray collimator if the surface area of a relevant detector element per se is larger than the surface area to be reached, or detector signals of individual detector elements are combined when the surface area of the relevant detector elements that is reached by the X-rays is smaller than the surface area to be reached. Because reached surface areas of substantially the same size are formed by partly shielding detector elements, it is achieved that the density profiles are measured with a high spatial resolution despite the use of comparatively large detector elements. The resolution is determined by the effective surface area of the smallest effective surface area of the detector elements. Using a computer tomography device according to the invention, simultaneous measurement of density profiles of a larger part of the patient to be examined requires a number of detector elements which increases less than linearly as a function of the ratio of the smallest effective surface area to the linear dimension of the part of the patient for which density profiles are simultaneously measured. It is thus achieved that as a volume scan is made of a larger part of the patient in one operation, comparatively fewer additional, expensive detector elements will be required.
The detector system may be provided, for example with a sensor matrix with semiconductor photodiodes which are sensitive to X-rays and are connected to read-out lines via thin-film transistors per column. The gate contacts of the thin film transistors are connected to the addressing lines per row. Addressing signals, supplied via the addressing lines, open thin film transistors per row so as to read-out electric charges generated by the X-rays in the photodiodes in the relevant row. Said electric charges are thus read out via the read-out lines. An electronic multiplexer converts the electric charges read into an electric signal which represents the density profiles.
An alternative detector system comprises a plurality of detector elements. The detector elements include respective scintillators and photosensors. The scintillators convert X-rays in a low-energy
Dunn Drew A.
Kim Robert H.
U.S. Philips Corporation
Vodopia John F.
LandOfFree
Computer tomography device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Computer tomography device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Computer tomography device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2476050