X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-09-10
2001-08-14
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S004000, C250S363030
Reexamination Certificate
active
06275561
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a computer tomography method which involves helical scanning of an examination zone by means of a scanning unit which includes a radiation source and a detector unit, an object present in the examination zone and the scanning unit simultaneously rotating about an axis of rotation relative to one another and performing a motion parallel to the direction of the axis of rotation, resulting in a relative motion in the form of a helix, and also involves a reconstruction of the spatial distribution of the absorption within the examination zone from the measuring data acquired by the detector unit. The invention also relates to a computer tomography apparatus for performing a method of this kind.
2. Description of the Related Art
A method and a computer tomography apparatus of the kind set forth are known from DE-OS 195 45 778 (Tam). This known method enables the scanning of an extended examination zone in the direction of the axis of rotation by means of a cone beam and the reconstruction of the absorption distribution in the examination zone also in case the object present therein, for example a patient, is longer than the part of the examination zone for which data has been acquired.
However, for this method it is necessary to define in advance the so-called region of interest (ROI) and at the beginning and at the end of the ROI (in relation to the direction of rotation) an additional scan of the examination zone must be performed along a circular path which extends perpendicularly to the axis of rotation. The reconstruction of the absorption distribution may commence only after the scanning of the examination zone has been completed. The change over from a circular scan to a helical scan and back to a circular scan of the examination zone necessitates an abrupt acceleration or deceleration of either the scanning unit or the object in the examination zone; this could cause unsharpness. It is a further drawback that the scanning region must be defined in advance.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide a method of the kind set forth which does not require additional circular scanning motions and enables the reconstruction already during the acquisition of the measuring data. This object is achieved by a method of the kind set forth in that it includes the following steps:
a) using exclusively the measuring data for the reconstruction which are obtained from rays which pass exactly through the area between two adjacent turns of the helix, the points within the examination zone thereby being irradiated over an angular range of exactly 180 degrees—as seen from the point itself,
b) rebinning the measuring data and the associated rays so as to form a number of groups, each group comprising a plurality of planes which extend parallel to the axis of rotation and each of which contains a respective fan beam,
c) filtering the data of each group formed by the rebinning
d) reconstructing of the spatial distribution of the absorption from the filtered data of different groups.
The invention utilizes exclusively measuring data acquired while the radiation source irradiates the points within the examination zone from an angular range of exactly 180° as seen from the relevant point itself; the rays associated with the measuring data then pass exactly through the area between two adjacent turns of the helix. On the one hand this angular range suffices to enable exact reconstruction while on the other hand it avoids the use of redundant measuring data. The type of rebinning performed in relation to these measuring data is essential (rebinning is to be understood to mean the resorting of the measuring data from the sequence yielded by the acquisition as well as the re-interpolation of the measuring data on a different grid). Rebinning is performed with data having been acquired along rays which emanate from the same source point and form a fan beam extending in a plane parallel to the rotation axis. The subsequent processing steps, i.e. the preferably one-dimensional filtering and the reconstruction, are thus significantly facilitated.
In principle there are various possibilities for grouping the data emanating from the above-mentioned fan beams. For most of these groups, however, it will then be necessary to weight the measuring data with suitably chosen weighting factors (which may be dependent on the type of detector and on the type of rebinning). This necessity is eliminated, however, in the preferred version which is disclosed in claim
2
and in which each group contains only mutually parallel planes so that the further processing is significantly facilitated. Notably an outstanding image quality is thus obtained. In the further version according to claim
5
, for each group the rebinning is performed on a virtual detector which extends perpendicularly to the planes associated with the relevant group and has a rectangular surface. The re-interpolation on an equidistant measuring point grid as required for the subsequent processing steps is thus significantly facilitated.
The absorption distribution could in principle be reconstructed from the filtered data of different groups by means of so-called generalized projections as described in the document published by Schaller et al. in SPIE, Vol. 3032, 32, pp. 213 to 224. A preferred type of reconstruction, however, is realized by back projection of the filtered data in conformity with claim
3
.
The filtering operation could, also be performed, for example, by subjecting the data produced by the rebinning operation to a convolution with an appropriate filter kernel. The filtering operation defined in claim
4
, however, requires less calculation time.
Claim
6
discloses a computer tomography apparatus for performing the method according to the invention and claim
7
defines an advantageous embodiment thereof. The shape of the collimator arrangement and/or the shape of the detector unit in this embodiment ensure that each point within the examination zone “sees” the radiation source, upon emerging from the radiation beam generated thereby, at an angle which has been shifted through exactly 180°(&Xgr;) with respect to the angle upon entering of the radiation beam. The advantage of this step resides in the fact that all measuring data required for an exact reconstruction is measured (and no other data). Thus, removal or co-weighting of redundant measuring data is not necessary.
REFERENCES:
patent: 5323007 (1994-06-01), Wernick et al.
patent: 5744802 (1998-04-01), Muehllehner et al.
patent: 5828718 (1998-10-01), Ruth et al.
patent: 5848117 (1998-12-01), Urchuk et al.
Hobden Pamela R.
Kim Robert H.
U.S. Philips Corporation
Vodopia John F.
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