X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Utility Patent
1998-10-20
2001-01-02
Bruce, David V. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S015000, C378S901000
Utility Patent
active
06169779
ABSTRACT:
BACKGROUND OF THE INVENTION
In a conventional third-generation computed tomography (CT) scanner, a plurality of detectors, more generally detector channels,
53
are positioned in a row
52
along a circular arc centered about an X-ray source
54
as shown in FIG.
1
. The detector array
52
and the X-ray source
54
together are mounted on a rotatable device called a gantry. During a scan, the gantry rotates about an object
55
to be imaged. Data are acquired as fan-beam projections
50
of the object
55
at successive increments of gantry rotation angle. The fan beam projections at successive rotation angles can be reconstructed to form an image of a slice through the object through which the X-rays have passed. Alternatively, the fan-beam projections are reordered into sets of parallel-beam projections, each at a corresponding common projection angle, prior to reconstruction of the image. Parallel-beam reconstruction is generally preferred to fan-beam reconstruction because it requires fewer computations and therefore provides faster results.
Each set of parallel-beam projections thus represents a collection of projection data at a corresponding projection angle &phgr; for all detectors. Suppose the angular spacing of the detectors is &dgr; (i.e., the angle subtended by the portion of the fan beam projected onto one detector segment of the detector array) and the rotation angle of the gantry is &thgr;. As can be seen from
FIG. 2
, the projection angle &phgr; of the jth channel of the detector array is related to gantry rotation angle &thgr; and detector angular spacing &dgr; by:
&phgr;=&thgr;−(
j−j
o
)*&dgr;, (1)
where j
o
is the central detector channel, which lies on the X-ray path projecting from the source
54
through the center of rotation
20
.
Fan-beam projection data are sampled at successive rotation angles of interval &Dgr;&thgr;. In the specific case where the rotation-angle interval &Dgr;&thgr; is equal to the detector angular spacing, i.e., &Dgr;&thgr;=&dgr;, the process of reordering from fan-beam projections to parallel-beam projections is relatively straightforward. A map of projection angle &phgr; versus detector channel number j for this case is plotted in FIG.
3
. Each vertical column
22
of data points on the map represents the projection angles of the collected projection data P
j
(&thgr;) at detector channel j at successive rotation angles &thgr;. Each diagonal row
24
of data points represents the projection angles of fan beam data collected by the detector array at a single rotation angle &thgr;
k
. Each horizontal row
26
of data points on the map constitutes the reordered parallel-beam data R
j
(&phgr;) used for image reconstruction at projection angle &phgr;. Therefore, in the simple case where &Dgr;&thgr;=&dgr;, the reordering from fan-beam projection data to parallel-beam projection data is simply a data sorting procedure.
CT scanners are often configured such that projection data is collected at a rotation-angle interval greater than the detector angular spacing, i.e., &Dgr;&thgr;>&dgr;, for reducing the data collection rate or for hastening the reconstruction process. Such a configuration affects the fan-beam to parallel-beam reordering procedure. If &Dgr;&thgr;=3&dgr;, for example, the parallel-beam projection data cannot simply be sorted out from the fan-beam projection data, as only one out of three parallel-beam projections coincides with a fan-beam projection. The remaining two-thirds of the parallel-beam projection data must be interpolated from fan-beam projections at successive rotation angles. This holds true for conventional third-generation scanners having a single detector row (FIG.
1
), and for cone-beam scanners having multiple detector rows (FIG.
4
). Interpolation has a filtering effect on the data, which degrades spatial resolution of the reconstructed image. It is therefore desirable to avoid such interpolation during reordering.
SUMMARY OF THE INVENTION
The present invention is directed to a method of and system for data acquisition which avoids the need for interpolation of data from adjacent rotation angles during reordering of data. The present invention takes advantage of the rotation of the gantry and the fact that during the transition between rotation angles at increments &Dgr;&thgr;, detectors are moving through positions which correspond accurately to a projection angle &phgr;. If the appropriate detectors are sampled at the proper rotation angles between the rotation angles of increments &Dgr;&thgr;, referred to herein as sub-rotation angles, then the resulting data is properly aligned for reordering, without the need for the interpolation.
In accordance with one embodiment of the method of the present invention, data is collected from a plurality of detector channels of a detector array during a CT scan of the type where at least a source of a diverging beam of radiation projected toward the detector array is rotatable about a rotation axis through a plurality of rotation angles &thgr;, and the angle &dgr; subtended by the part of the beam projected toward each of the detector channels is smaller than the incremental angle &Dgr;&thgr; between successive rotation angles. The method comprises collecting data from the detector channels in a predetermined sequence so that all of the data represents parallel projection data at the projection angles without the need to interpolate any of the parallel projection data.
In accordance with another embodiment of the system of the present invention, a CT scanning system comprises:
a detector array including at least one row of detector channels;
a source of energy defining a diverging beam of radiation directed toward the array, wherein at least the source is rotatable about a rotation axis through a plurality of rotation angles &thgr; and the angle &dgr; subtended by the part of the beam projected toward each of the detector channels is smaller than the incremental angle &Dgr;&thgr; between rotation angles; and
a subsystem for acquiring data from the detector array within a predetermined time interval as the rotation of the source passes through each of the rotation angles;
wherein data is collected from the detector channels in a predetermined sequence so that all of the data represents parallel projection data at the projection angles without the need to interpolate any of the parallel projection data.
Thus, rather than the conventional practice of sampling all channels in the array at each rotation angle, in the technique of the present invention, data are sampled in a preselected number of groups of detector channels. In this manner, parallel-beam projection data can be obtained from the fan-beam projection data without interpolation. This technique is applicable to both conventional fan-beam scanners and cone-beam scanners, and is applicable to both simultaneously sampled systems and successively sampled systems.
In one embodiment, the present invention comprises a method of acquiring projection data in a computed tomography system. The system includes an energy source and a detector array having a plurality of channels rotatable about an object to be imaged for interrogating the object at incremental rotation angles. At each incremental rotation angle, first fan-beam projection data is sampled from a first group of the detector elements. Each fan-beam projection is oriented at a projection angle different from other projections in the first group. During transition of the detector array and source to a subsequent incremental rotation angle, second fan-beam projection data are sampled from a second group of detector channels paired and interleaved with the first group of detector channels. Sampling of data at the second group preferably occurs when the detector channels of the second group are in such a rotational position that the projection angle of the first group of detector channels is substantially aligned with the projection angle of the second group of detector channels for each detector channel
Analogic Corporation
Bruce David V.
McDermott & Will & Emery
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