Patent Application
20080101542
This invention relates generally to diagnostic imaging methods and apparatus, and more particularly, to methods and apparatus that provide for collimator stability during rotational velocity changes in a CT gantry.
The x-ray post-patient collimator used in CT detectors typically is a device made of a highly x-ray absorbing material such as Tungsten or Molybdenum which selects x-rays along a particular direction and rejects scattered radiation from other directions. In computed tomography (CT), collimators are placed in front of a detector bank to eliminate scattered radiation from the patient. The known collimators usually are made of Tungsten plates placed in front of interfaces of the detector cells, usually requiring a high precision manufacturing process and alignment or precision features within the individual parts for proper alignment. In the known designs, the collimator plates are aligned at pixel boundaries and have a plate in every channel or pixel. The dimensions of these plates are driven by the scatter/primary ratio and their ability to withstand higher g-forces without deflection.
In addition to the scatter rejection, the plates if misaligned to the cast septa (reflector) between the pixels, can cause spectral non-linearity and gain changes and introduce ring artifacts in imaging. In order to avoid or reduce these artifacts, the plates should be chosen so that they do not exhibit bow (flexing) in the beginning of a rotational change and should be locked with some mechanism so that they do not deflect more than a sufficiently small amount.
In one aspect, a method includes aligning a plurality of collimator plates to a plurality of cast reflector septa, and locking the collimator plates such that a gain change from changing rotational speeds is reduced or eliminated.
In another aspect, a method includes positioning a plurality of collimator plates each substantially in a pixel such that during rotational changes in velocity the collimator plates do not make contact or shadow with a reflector material surrounding each collimator plate.
In yet another aspect, a system includes an energy source, and an energy detector positioned to receive energy emitted from the source, wherein the energy detector includes a reflector and a plurality of collimator plates keyed to the reflector such the collimator plates do not move under rotational velocity changes of the energy detector.
In still another aspect, a system includes an energy source, and an energy detector positioned to receive energy emitted from the source, wherein the energy detector includes a reflector septa and a plurality of collimator plates mounted in the reflector septa, the collimator plates having a thickness of between about 250 μm and about 350 μm.
In still yet another aspect, a system includes an energy source, and an energy detector positioned to receive energy emitted from the source, wherein the energy detector includes a reflector and a plurality of collimator plates positioned on every other of a plurality of channels.
In yet still another aspect, a system includes an energy source, and an energy detector positioned to receive energy emitted from the source, wherein the energy detector includes a reflector septa and a plurality of collimator plates mounted in the reflector septa, the collimator plates having a thickness of between about 50 μm and about 150 μm.
There are herein described methods and apparatus useful for imaging systems such as, for example, but not limited to an x-ray system. The apparatus and methods are illustrated with reference to the figures wherein similar numbers indicate the same elements in all figures. Such figures are intended to be illustrative rather than limiting and are included herewith to facilitate explanation of an exemplary embodiment of the apparatus and methods of the invention. Although, described in the setting of an x-ray system, it is contemplated that the benefits of the invention accrue to all diagnostic imaging systems and modalities such as PET, SPECT, fused systems such as a CT/PET system, and/or any modality yet to be developed in which collimator plates are used.
The herein described CT detector collimators 10 may be made of the same material as the known collimators, which can be, for example, Tungsten or Molybdenum. Generally, a plurality of plates 12 are aligned to the cast reflector septa 14 as shown in
One way to avoid the gain change from one rotation speed to another rotation speed is to position the plates in the center of the pixels, so that, if the plates deflect, they do not reach the cast reflector, leading to no change in the gain.
In essence, it becomes desirable and required to lock the plates through a mechanism which allows the plates to be fixed and stable under different rotation speeds. As explained earlier, one solution is to create grooves over the top reflector, deep and wide enough to insert the plates in. In this case, the plates become locked to the pack (scintillator array) and will not move relative to the pixels under rotational load.
The “every other channel” plate collimator, advantages include using 50% less plates (which reduces cost,) any flatness requirement maybe can be relaxed, any spectral edge effect maybe will be reduced, longer height (>15 mm) will enable >40 mm aperture with the same scatter, and that the scatter/primary ratio (high order) may be better than the prior art.
The herein described methods and apparatus address the desire to improve the overall detector performance versus high rotation speeds of gantry. One embodiment uses thinner collimator plates (50% of the reflector width) and wider cast reflector septa. In this case, the alignment of the thin plates, will lead to a better tolerance range for manufacturability, which is much better than the prior arts' capability. In addition to the alignment relaxation, there is no trade-off on geometric efficiency because the scintillator aperture would not change. There is an additional advantage in this configuration in the reduction of the crosstalk. Because of wider cast reflector septa, the amount of crosstalk (optical and x-ray) will be significantly reduced.
Included in this disclosure, is the possibility to insert the plates in the reflector surface as shown in
These grooves can be used also as guides for the plates. The insertion of these plates not only helps locking the plates and renders the alignment easier but avoids their deflection during rotation as well.
Of course, the methods described herein are not limited to practice in system 40 and can be utilized in connection with many other types and variations of imaging systems. Although the herein described methods are described in a human patient setting, it is contemplated that the benefits of the invention accrue to non-human imaging systems such as those systems typically employed in small animal research. Although the herein described methods are described in a medical setting, it is contemplated that the benefits of the invention accrue to non-medical imaging systems such as those systems typically employed in an industrial setting or a transportation setting, such as, for example, but not limited to, a baggage scanning CT system for an airport or other transportation center.
In some known CT imaging system configurations, a radiation 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 an “imaging plane”. The radiation beam passes through an 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 radiation beam received at the detector array is dependent upon the attenuation of a radiation 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.
In third generation CT systems, the radiation source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged such that an angle at which the radiation beam intersects the object constantly changes. A group of radiation 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 includes a set of views made at different gantry angles, or view angles, during one revolution of the radiation source and detector.
To reduce the total scan time, a “helical” scan may be performed. To perform a “helical” scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a cone beam helical scan. The helix mapped out by the cone beam yields projection data from which images in each prescribed slice may be reconstructed.
Either in a helical or non-helical scan, the gantry rotation velocity can be changed. The herein described methods and apparatus decrease or eliminate any gain change from the detector during these velocity changes.
As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural said elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.
Technical effects include that the herein described methods and apparatus will allow for a high relaxation in plates bow and deflection tolerance of plates, no gain change from one speed to another, reduction in calibration time by only having a single speed calibration, Cost improvement by making the manufacturability easier, and Image Quality improvement by reducing the gain change from one speed to another. Use of thicker (x-direction) and taller (Z direction) plates in thin cast reflector septa will enable the improvement of detector performance and manufacturability process. This can be done through the relaxation of the alignment tolerance of both diode-to-scintillator alignment and the pack-to-collimator plates spacing. In addition to this, any deflection of plates under higher gantry rotations is significantly reduced, enabling one-speed calibration. In addition to performance, the cost of the collimator will be reduced by 50% of plates used, with less controllability and precision.
Exemplary embodiments are described above in detail. The assemblies and methods are not limited to the specific embodiments described herein, but rather, components of each assembly and/or method may be utilized independently and separately from other components described herein.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
