Collimator for imaging systems and methods for making same

A method for fabricating a collimator includes mixing an x-ray absorbent material with at least one of a temporary binder and a temporary gel, and extruding the mixed x-ray absorbent material through a die to form a unitary collimator structure that is at least one of substantially honeycomb in shape and substantially rectangular in shape.

BACKGROUND OF THE INVENTION

This invention relates generally to methods for making a collimator used in an imaging system, and to the collimator made from these methods.

In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile of the object.

In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object.

One method for reconstructing an image from a set of projection data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.

Detector elements are configured to perform optimally when impinged by x-rays travelling a straight path from the x-ray source to the detector elements. Particularly, detector elements typically include scintillation crystals which generate light events when impinged by an x-ray beam. These light events are output from each detector element and directed to photoelectrically responsive materials in order to produce an electrical signal representative of the attenuated beam radiation received at the detector element. Typically, the light events are output to photomultipliers or photodiodes which produce individual analog outputs. Detector elements thus output a strong signal in response to impact by a straight path x-ray beam.

X-rays often scatter when passing through the object being imaged. Particularly, the object often causes some, but not all, x-rays to deviate from the straight path between the x-ray source and the detector. Therefore, detector elements are often impinged by x-ray beams at varying angles. System performance is degraded when detector elements are impinged by these scattered x-rays. When a detector element is subjected to multiple x-rays at varying angles, the scintillation crystal generates multiple light events. The light events corresponding to the scattered x-rays generate noise in the scintillation crystal output, and thus cause artifacts in the resulting image of the object.

To reduce the effects of scattered x-rays, scatter collimators are often disposed between the object of interest and the detector array. Such collimators are constructed of x-ray absorbent material and positioned so that scattered x-rays are substantially absorbed before impinging upon the detector array. For one known collimator, it is important that the scatter collimator be properly aligned with both the x-ray source and the detector elements so that only x-rays travelling on a substantially straight path impinge on the detector elements.

Known collimators are complicated and cumbersome to construct. In addition, it is difficult to satisfactorily align known collimators with the x-ray source and the detector elements to both absorb scattered x-rays and shield sensitive portions of the detector elements.

BRIEF SUMMARY OF THE INVENTION

In one embodiment, a method for fabricating a collimator is provided. The method includes mixing an x-ray absorbent material with at least one of a temporary binder and a temporary gel, and extruding the mixed x-ray absorbent material through a die to form a unitary collimator structure that is at least one of substantially honeycomb in shape and substantially rectangular in shape.

In another embodiment, a collimator for an imaging system is provided. The collimator includes an extruded x-ray absorbent material, and is unitary and at least one of substantially honeycomb in shape and substantially rectangular in shape.

In a further embodiment, a computed tomographic (CT) imaging is provided. The CT system includes a detector array, at least one radiation source, and a collimator including an extruded x-ray absorbent material wherein the collimator is unitary and at least one of substantially honeycomb in shape and substantially rectangular in shape.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIGS. 1 and 2, a computed tomograph (CT) imaging system10is shown as including a gantry12representative of a “third generation” CT scanner. Gantry12has an x-ray source14that projects a beam of x-rays16toward a radiation detector array18on the opposite side of gantry12. Detector array18is formed from a plurality of detector elements20including a plurality of scintillators (not shown), which together sense the projected x-rays that pass through an object22, for example a medical patient. Detector array18may be fabricated in a single slice or multi-slice configuration. Each detector element20produces light in response to x-ray radiation, which is converted to an electrical signal by a sensing region of a semiconductor array optically coupled thereto. Each detector element20produces an electrical signal that represents the intensity of an impinging x-ray beam on that detector element and hence the attenuation of the beam as it passes through patient22at a corresponding angle. During a scan to acquire x-ray projection data, gantry12and the components mounted thereon rotate about a center of rotation24.

Rotation of gantry12and the operation of x-ray source14are governed by a control mechanism26of CT system10. Control mechanism26includes an x-ray controller28that provides power and timing signals to x-ray source14and a gantry motor controller30that controls the rotational speed and position of gantry12. A data acquisition system (DAS)32in control mechanism26samples analog data from detector elements20and converts the data to digital signals for subsequent processing. An image reconstructor34receives sampled and digitized x-ray data from DAS32and performs high speed image reconstruction. The reconstructed image is applied as an input to a computer36which stores the image in a mass storage device38.

Computer36also receives commands and scanning parameters from an operator via console40that has a keyboard. An associated cathode ray tube display42allows the operator to observe the reconstructed image and other data from computer36. The operator supplied commands and parameters are used by computer36to provide control signals and information to DAS32, x-ray controller28and gantry motor controller30. In addition, computer36operates a table motor controller44which controls a motorized table46to position patient22in gantry12. Particularly, table46moves portions of patient22through gantry opening48.

In one embodiment, computer36includes a device50, for example, a floppy disk drive or CD-ROM drive, for reading instructions and/or data from a computer-readable medium52, such as a floppy disk or CD-ROM. In another embodiment, computer36executes instructions stored in firmware (not shown). Computer36is programmed to perform functions described herein, accordingly, as used herein, the term computer is not limited to just those integrated circuits referred to in the art as computers, but broadly refers to computers, processors, microcontrollers, microcomputers, programmable logic controllers, application specific integrated circuits, and other programmable circuits.

FIG. 3is a side view of detector array18including a plurality of scintillator elements58, and a collimator60positioned proximate to scintillator elements58such that x-rays16pass through object22, and collimator60before impinging on detector array18.FIG. 4is a perspective view of an exemplary collimator60positioned proximate scintillator elements58.FIG. 5is a perspective view of another exemplary collimator shown inFIG. 3. In the exemplary embodiment, collimator60includes a length62and a width64that defines a collimator area, and a thickness68. Collimator60also includes a plurality of sidewalls70surrounding a plurality of openings72. In one embodiment, sidewalls70are formed such that openings72define a substantially honeycomb shape (i.e. a plurality of interconnected hexagons). In another embodiment, sidewalls70are formed such that openings72form a shape that is substantially rectangular in shape. Each scintillator element58includes a length82and a width84that defines a scintillator area, and a thickness88. In the exemplary embodiment, the scintillator area is substantially greater than a single collimator opening72such that a random distribution of collimator openings72are positioned proximate scintillator elements58and such that at least two partial openings72are positioned proximate a single scintillator element58. For example, a single scintillator element58may have a single opening72and several partial openings or a single scintillator element58may have a plurality of openings72and a plurality of partial openings72proximate each scintillator element58.

FIG. 6is a method90for fabricating collimator60. Method90includes mixing92an x-ray absorbent material with at least one of a temporary binder or a temporary gel. In one embodiment, the x-ray absorbent material includes a metallic material having a high atomic weight (Z number). In one embodiment, the metallic material has a Z number greater than seventy-two such as, but not limited to, tungsten, tantalum, and lead. Suitable organic binders include organic binders or gels used in ceramic molding, such as polyethylene glycol, methylcellulose, ethylhydroxy ethylcellulose, hydroxybutyl methylcellulose, hydroxymethylcellulose, hydroxypropyl methylcellulose, hydroxyethyl methylcellulose, hydroxybutylcellulose, hydroxyethyl-cellulose, hydroxypropylcellulose, sodium carboxy methylcellulose, and mixtures thereof. The collimator is partially solidified or dried to make a flexible “cake”. Method90also includes extruding94the mixed x-ray absorbent material through a die to form a unitary collimator structure that is substantially honeycomb in shape or substantially rectangular in shape, and sintering96the unitary structure at an appropriate temperature to form a substantially solid honeycomb structure. The substantially solid honeycomb structure is then sliced to a length dependent upon the embodiment to form collimator60.

In use, collimator60is positioned proximate a scintillator x-ray incidence side100(shown inFIG. 4) such that x-rays16pass through object22and collimator60before impinging on scintillator elements58. Scattered x-rays passing through object22are substantially reduced using collimator60prior to being sensed by detector18. In the exemplary embodiment, collimator60is mechanically attached to detector18using at least one mechanical fastener (not shown).

In the exemplary embodiment, collimator60is positioned to form a series of high aspect ratio channels proximate each scintillator element58to facilitate attenuating scattered x-rays. Additionally, the accuracy of openings72can be reduced since a specific collimator opening72is not aligned with a specific scintillator element58, but rather openings72form a random distribution of collimating openings proximate the scintillator array. Collimator60also facilitates reducing an aspect ratio since collimation is accomplished in two directions.

The above-described collimator provides for alignment with both the focal spot and the detector elements without the collimator being precisely aligned between the detector and the radiation source. Also, the collimator is not complex, and is more simple to fabricate than some known collimators. In addition, the scatter collimator sufficiently shields the detector elements from undesirable scattered x-rays and other radiation, thereby facilitating a reduction in x-rays, that are not travelling on a substantially straight path, from impinging on the detector elements and thus reducing artifacts in the resulting image of the object. Accordingly, the herein described collimator is believed to provide improved system performance as compared to known collimators.