X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2000-11-28
2003-01-07
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S015000, C378S094000
Reexamination Certificate
active
06504893
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to a method for examining a body region executing a periodic motion in an examination subject be means of computed tomography (CT) apparatus of the type having an X-ray source continuously rotating around a system axis and from which an X-ray beam penetrating the examination subject proceeds, and having a detector system for the X-rays emanating from the X-ray source that has at least a first detector line and a last detector line, wherein the examination subject, and the X-ray source and the detector system, are displaced relative to one another in the direction of the system axis during the rotation of the X-ray source, and wherein a tomogram of at least the body region executing the periodic motion is determined with an electronic calculating means from the output data of the detector system corresponding to the detected X-rays.
DESCRIPTION OF THE PRIOR ART
The technique of prospectively ECG-triggered exposure of individual slices with single-line CT apparatus of the 3
rd
generation (X-ray source and detector system rotate in common around a system axis) has been known since the early 1980's. A characteristic feature of the ECG signal, for example the R-wave, is used to implement an axial exposure in a defined heart phase for a fixed position (z-position) of the examination subject relative to the X-ray source and detector system in the direction of the system axis (z-direction). A full-revolution scan or sub-revolution scan is triggered after a selectable time delay (in % of the average RR interval of the ECG signal or absolutely in msec) relative to the respectively preceding R-wave. The data required for the image reconstruction at this z-position can be collected in a number of successive revolutions in order to improve the time resolution.
Given a single-line CT apparatus, i.e. a CT apparatus whose detector system is a single line of individual detectors, it is also known to implement a spiral scan with the ECG signal registered in parallel therewith (referred to as retrospective gating). Here, tomograms are subsequently calculated in the image reconstruction only from data in permitted data regions (in the respectively desired heart phase, for example in the diastole). In a single-line CT apparatus, this has the drawback that a gap-free coverage of the volume z-direction with tomograms is not achieved. In a spiral scan, a relative displacement of the X-ray source and detector system and the examination subject in the z-direction ensues simultaneously with a continuous rotation of X-ray-source and detector system around the system axis.
With the introduction of new CT apparatuses of the 3
rd
generation with sub-second rotation, i.e. X-ray source and detector system need less than one second for a complete revolution around the system axis, and multi-slice technology, i.e. a detector system with more than one line of individual detectors, heart diagnostics with CT apparatuses is experiencing a new boom. ECG-triggered axial exposures as well as spiral exposures with ECG signal registered in parallel (retrospective ECG gating) have been expanded to multi-slice CT apparatuses, i.e. CT apparatuses with multi-slice technology, which are also referred to as multi-line CT apparatuses. Due to the multi-cell quality, new possibilities also exist in retrospectively gated spiral examination with suitable reconstruction techniques such as, for example, the gap-free presentation of the heart volume in the z-direction in any desired phase of the heart cycle.
In ECG-triggered axial exposures, radiation is only triggered within the time span during which the data actually required for the image reconstruction are registered. The method is thus dose-sensitive; it uses only the X-ray dose actually needed for the image calculation. However, one tomogram of a slice (given single-line CT apparatus) or a number of tomograms of slices (given multi-line CT apparatus) are respectively registered at fixed table positions per exposure (scan). Between two scans, the examination subject and the X-ray source and the detector system—which assume a fixed z-position relative to one another during a scan—must be brought into the new, desired z-position. This takes time and is the reason that tomograms usually cannot be registered in every heart cycle (heart period) but only in every second or every third. The examination time is considerably lengthened as a result, and it is often not possible to acquire tomograms of the desired, thin slices of the entire heart volume in one breath-holding phase. Given multi-slice CT apparatus, moreover, the tomograms arise automatically at the spacing of a detector line in z-direction. For qualitatively high-grade 3D applications, for example volume renderings for presentation of the coronary arteries, however, tomograms with a smaller spacing in the z-direction are required. A reconstruction of corresponding tomograms is not possible given conventional single-slice exposures.
Given spiral scans with a multi-line CT apparatus having the ECG signal registered in parallel, data are registered during the entire duration of the spiral scan. The data registered in the desired heart phases of the heart cycle are identified later (retrospectively) from the ECG signal registered during the spiral scan and are utilized for the reconstruction. In a multi-slice CT apparatus, this method has the advantage that tomograms in every desired heart phase can be reconstructed overlapping in the z-direction at arbitrarily small spacings. Due to the continuous relative motion between the examination subject and the X-ray source and detector system, the coverage of the entire heart volume with thin slices is possible in one breath-holding phase in a multi-slice CT apparatus. Both features are preconditions for qualitatively high-grade 3D representations of the heart.
In such exposure techniques, the patient absorbs an X-ray dose that is not inconsiderable.
In the context of reducing the dose applied to a patient, U.S. Pat. No. 5,625,662 teaches modulating the tube current of an X-ray tube provided as the X-ray source in a CT apparatus, dependent on the rotational angle of the X-ray tube as well as on weighting factors to be applied to the data acquired at the respective rotational angle.
In the same context, U.S. Pat. No. 5,485,494 discloses a CT apparatus wherein the tube current of an X-ray tube provided as the X-radiation source is modulated dependent on the rotational angle, this modulation being undertaken according to a stored function that is preferably acquired on the basis of a test scan of the patient.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of the type initially described, wherein a body region of an examination subject executing a periodic motion can be registered with a reduced radiation dose.
The above object is achieved in accordance with the principles of the present invention in a method for examining a body region of an examination subject executing a periodic motion with a CT apparatus having a multi-line detector system in spiral mode, wherein the X-ray source is activated and deactivated for the emission of X-rays substantially synchronously with the periodic motion, so that the X-ray source is activated only during a phase of the periodic motion to be imaged with the CT apparatus.
In the invention, thus, the advantages of a prospective triggering (only the dose actually required is applied) are united with the advantages of the spiral scan with a multi-line CT apparatus (gap-free volume coverage, possibility of overlapping reconstruction of tomograms) for examinations of, for example, the heart in a previously defined (selected) heart phase. To that end, full revolution or sub-revolution scans are prospectively triggered during the individual heart cycles with a selectable time delay relative to the respectively preceding R-wave of the ECG signal (in % or as a fraction of the average duration of the R R interval of the ECG signal or absolutely in msec), with
Flohr Thomas
Ohnesorge Bernd
Barber Therese
Porta David P.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
LandOfFree
Method for examining a body region executing a periodic motion does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for examining a body region executing a periodic motion, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for examining a body region executing a periodic motion will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3070917