X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-03-31
2001-07-03
Porta, David P. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S009000
Reexamination Certificate
active
06256369
ABSTRACT:
BACKGROUND OF THE INVENTION
In a third-generation computerized tomography (CT) scanner, an X-ray source and a detector array are mounted to a disk assembly, which is rotatable about an object to be scanned. The rotating disk assembly is supported by a rigid, stationary gantry. The detector array comprises a plurality of channels extending laterally from a central channel. During a scan, the source and detectors scan the object at incremental rotation angles. A process referred to as reconstruction generates a series of two-dimensional images, or slices, of the object from the captured data.
FIG. 1
is an axial or front view of a third-generation X-ray CT scanner, including an X-ray source
20
and a detector array
22
mounted on a disk assembly rotatable about an object at an angular velocity of &ohgr;. The detector array
22
is typically disposed as a row in the shape of a circular arc, and centered about a point
26
, referred to as the “focal spot”, where X-ray radiation emanates from the X-ray source
20
. The X-ray beam emanating from the focal spot and incident on the detector array therefore define a fan beam. A rotating coordinate system x′y′z′, fixed relative to the disk assembly, is used in FIG.
1
and subsequent figures to illustrate the position of the focal spot of the X-ray source
20
with respect to the detector array
22
. The z′ axis coincides with the z axis of a stationary coordinate system (x,y,z). The x′y′ plane is coplanar with the xy plane, and rotates about the z′ axis at angular velocity &ohgr;. The source
20
and the detector array
22
lie on the x′y′ plane, and the y′ axis intersects the rotation center or isocenter
24
and the center channel
22
A of the detector array. In a conventional fixed focal spot system, the focal spot position remains fixed on the y′ axis throughout a scan.
For the purpose of discussion, the direction on the x′y′ plane, i.e. within the rotation plane of the fan beam, parallel to, or substantially parallel to, the x′ axis, is referred to herein as the “lateral” direction, and the direction parallel to, or substantially parallel to, the z′ axis is referred to as the “longitudinal” direction.
It is well understood that oscillatory movement of the focal spot in the x′y′ plane during a scan can improve the image-quality of a CT scanner, as described, for example, in U.S. Pat. No. 5,841,829, incorporated herein by reference. This is referred to as a “flying focal spot”. In this configuration, the focal spot
26
is made to oscillate in a lateral direction, between positions a and b, as shown in FIG.
1
. The displacement of the focal spot in the lateral direction, parallel to the x′ axis is plotted with respect to time in
FIG. 2
for a standard case of simple harmonic oscillation of the lateral movement.
During a scan, X-ray intensity is sensed at each detector channel over a range of rotation or view angles, as the disk assembly rotates at an angular velocity &ohgr;, and while the X-ray source continuously irradiates the object being scanned. All detector signals are typically sampled at the same time at each rotation angle. The signal of the X-ray intensity incident on a detector is integrated or filtered over a short time duration. During this time duration, focal spot displacement varies slightly near the peak of the sinusoidal oscillation. The lateral positions a and b indicated in
FIGS. 1 and 2
represent the mean displacements following the signal integration or filtration.
Oscillating focal spot displacement in the lateral direction, parallel to the x′ axis, is equivalent to a tangential offset of the detector array. Usually, the flying focal spot is operated to have displacements a and b located at the equivalent of positive and negative quarter-detector offsets. Under these conditions, the data collected from the focal spot at position a are shifted by a half detector width with respect to the data collected from the focal spot at position b.
By alternating the focal spot between displacement positions a and b at successive rotation angles, the effective number of detectors is doubled. At alternate even and odd rotating angle increments, the data are sampled at the even and odd detection positions respectively, resulting in data that are interleaved in the sampling space. As a result, the spatial resolution of the reconstructed image is doubled. The flying focal spot is applicable to both step-and-shoot scanning and helical scanning.
SUMMARY OF THE INVENTION
In the present invention, the focal spot is made to oscillate in the longitudinal direction, in other words, along a direction parallel or substantially parallel to the z′ or the scanner rotation axis. In this manner, an improvement in scan throughput rate is achieved.
In a first aspect, the present invention is directed to an apparatus for and method of computed tomography (CT) scanning. The apparatus of the present invention comprises a CT scanner including an energy source having a focal spot and a detector array for imaging of an object at successive incremental rotation angles about a longitudinal axis. The focal spot has a variable position along a longitudinally directed path during a scan of an object.
In a preferred embodiment, the focal spot moves along a predefined path including a set of multiple positions displaced from one another in a longitudinal direction at each successive rotation angle. The longitudinal movement of the focal spot is preferably in accordance with a oscillatory waveform function selected from the group consisting of square, trapezoidal, and sinusoidal waveform functions.
The focal spot may oscillate between first and second peak amplitude positions in accordance with a sinusoidal waveform function. In this case, the detected signals may be acquired over a short time duration when the focal spot is substantially at each of the first and second peak amplitude positions.
The energy source preferably comprises an X-ray source, and the detector array may comprise a single-row (or more generally a single set of arrayed detectors), or a dual-row detector array (or more generally two sets of arrayed detectors). A collimator may be included for collimating the radiation emitted by the energy source focal spot.
In an embodiment where the focal spot is made to oscillate between first and second positions along a path in the longitudinal direction, the collimator may further comprise: first and second apertures corresponding with the first and second positions, and a baffle, made for example as an extrusion, configured to prevent radiation emitted from a focal spot at the first position from entering the second aperture, and to prevent radiation emitted from a focal spot at the second position from entering the first aperture.
The focal spot may be configured to vary in position along a lateral direction, within one or more of the planes of rotation of the focal spot.
The focal spot may oscillate between first and second positions in the longitudinal direction generating first and second fan beams respectively centered about first and second fan beam planes. In this configuration, the first and second fan beam planes are preferably substantially parallel and are incident at the detector array at first and second longitudinal positions. The detector array may comprise a dual-row detector array having first and second detector subarrays wherein the first fan beam plane is incident at the first detector subarray and wherein the second fan beam plane is incident at the second detector subarray.
Alternatively, the first and second fan beam planes may be tilted so as to (a) converge and intersect before reaching the detector array so that they are diverging when they intersect with the detector array, (b) converge and intersect beyond the detector array, or (c) converge and intersect directly or nearly directly at the detector array.
As mentioned, the detector array may comprise a detector array having first and second de
Analogic Corporation
McDermott & Will & Emery
Porta David P.
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