Patent Application
20130168567
The present invention relates generally to collimators for collimating photons, and more particularly to collimators for collimating high-energy photons for use in medical imaging, such as in nuclear medicine.
In most forms of medical diagnostic imaging, images are created by one of two methods: transmission or emission. Transmission imaging occurs when radiation is directed through a patient and onto a radiation detector, such as when using X-rays in X-ray imaging (XR) or Computed Tomography imaging (CT). Emission imaging occurs when radiation in the form of a radiopharmaceutical is injected into a patient (or ingested or inhaled by the patient) and radioactive particles are emitted by the patient's body, such as when gamma photons are emitted and detected in Single Photon Emission Computed Tomography (SPECT) or Positron Emission Tomography (PET). In both transmission and emission imaging, the detectors used are selected based on their sensitivity to high energy photons of a certain energy which are used by that imaging modality, such as X-rays used in XR and CT (80-120 keV), or gamma rays used in SPECT (140 keV) or PET (511 keV).
These imaging modalities typically utilize collimators to help assure that the photons received by the detector are from within a known incidence angle with respect to the detector surface. Collimators are very well known within these imaging modalities, and come in various known configurations such as parallel hole, slant hole, converging, diverging, fan beam, cone beam, pinhole and multi-pinhole. The most common collimators (i.e., parallel hole, slant hole, converging, diverging, fan beam and cone beam) are essentially a collection of straight, narrow “tubes” that are typically much longer than the “hole” or channel of each tube. A given collimator of this type typically has all of its channels of a certain diameter (e.g., 2 mm) and a certain cross-sectional shape (e.g., square, round, hexagonal, triangular, etc.).
Collimator septa 14 are typically made of lead, tungsten or other material that is effective at stopping or absorbing high energy photons. Collimators are typically constructed by connecting foils which when connected form the desired shape and size of channels, or by other well-known additive or subtractive fabrication techniques such as casting, machining or extruding.
In the design of collimators, a compromise is struck between sensitivity and resolution. This is because stronger collimation results in more blocking and better selected photons, and weaker collimation results in less blocking and less selected photons. Thus, with all other things being kept equal, sensitivity and resolution are inversely proportional to each other; as one is increased, the other generally decreases. With the increased use of pixelated detectors, there is a trend toward use of smaller, more densely packed detector pixels. However, the resolution-sensitivity tradeoff remains a limitation on overall image quality and performance. Prior art work by Weinmann et al. at the Mayo Clinic (see “Design of optimal collimation for dedicated molecular breast imaging systems”, Med. Phys. 36 (3), March 2009, 845-56) has shown a sensitivity improvement of about 18% at constant resolution and septal penetration if the pixel size can be reduced from 2.46 mm to 1.6 mm. However, pixel size is a factory process standard which is difficult and costly to change, and changes to smaller pixels may result in reduced detector efficiency from increased pixel boundary-to-area ratio, It would be better to find alternative optimization approaches that do not change pixel size.
It would be desirable, therefore, to provide an improved collimator design which overcomes the disadvantages discussed above, and which provides advantages that are lacking in the prior art.
In a first embodiment of the present invention, there is provided a registered collimator having holes therein arranged in groups, wherein the holes within each group are separated by intra-group septa having a first thickness Tintra and the groups are separated from one another by inter-group septa having a second thickness Tinter. The collimator is adapted for use with a pixelated detector having multiple pixel elements, such that each of the groups of collimator holes may be registered with a respective one of the pixel elements.
In a second embodiment of the present invention, there is provided a collimator having holes therein arranged in groups, wherein the holes within each group are separated by intra-group septa having a first thickness Tintra and wherein the groups are separated from one another by inter-group septa having a second thickness Tinter, such that Tintra<Tinter.
In a third embodiment of the present invention, there is provided a collimator having a top surface, a bottom surface and an overall height Hc extending between the surfaces. The collimator has holes therein arranged in groups, wherein the holes within each group are separated by intra-group septa and the groups are separated from one another by inter-group septa. The intra-group septa have a height hs and are recessed from the top surface and/or the bottom surface of the collimator, such that hs<Hc. For example, hs may be about 0.8 Hc.
In each of the various embodiments, the collimator has multiple groups of collimator holes registered with at least some of the detector pixels on a one-group-to-one-pixel (i.e., multiple-holes-per-pixel) basis, rather than simply one hole per pixel as is the case in prior art registered collimators. Each of the embodiments may further comprise an imaging equipment arrangement including a pixelated detector having multiple pixel elements, in which the detector is operatively coupled to the collimator such that each group of collimator holes is registered with a respective one of the pixel elements. Further, the collimator of each embodiment may have a reduced height (e.g., about one-half or less) as compared to comparable conventional registered collimators that do not have the multiple-holes-per-pixel aspect described herein
The foregoing summary, as well as the following detailed description of certain embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. It should be understood that the various embodiments are not limited to the arrangements shown in the drawings.
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 of said elements or steps, unless such exclusion is explicitly stated. Furthermore, any references to a particular embodiment of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property.
Various embodiments of the invention provide a collimator for collimating high-energy photons for use in performing molecular imaging of an anatomy or physiology of interest in a patient. A technical effect of the various embodiments is to provide a collimator that is configured to improve the sensitivity and/or resolution of a pixelated detector when used therewith. The collimator is also configured to help identify tumors or lesions during or after an imaging examination and optionally to facilitate performing a biopsy of the identified tumors or lesions in the anatomy of interest, such as in molecular breast imaging using one or more gamma cameras. An advantage of the various embodiments of the present invention is an improvement over the prior art approaches to the sensitivity-resolution tradeoff in designing collimators.
To assist the reader in understanding the embodiments of the present invention that are disclosed, all reference numbers used herein are summarized below, along with the elements they represent:
Referring now to the drawings,
It may be easier to make and use a collimator fashioned according to an embodiment of the present invention wherein the holes are uniform within a group, and also uniform from group to group, in terms of shape, size and centeredness. However, there may be some applications where having different hole shapes, sizes and/or centeredness/orientations may be beneficial, such as when the collimator is designed for special applications where it may be desired to accept photons from a particular organ or region of interest differently from surrounding or nearby tissue. Also, there may be some detector arrangements (e.g., non-uniform distributions, shapes or sizes of pixels), some imaging procedures, and/or some reconstruction/calibration schemes or algorithms (e.g., utilizing differential photon acceptances) that are particularly well suited to using non-uniform hole shapes, sizes and/or centeredness/orientations within groups and/or from group to group as described above.
In the first embodiment illustrated by
Because of the thinner intra-group septa 46 in the second embodiment 40, the collective area of all the holes 42 in each group 44 is larger than the collective area of all the holes 32 in each group 34 in the first embodiment 30, thereby providing more photons to each detector pixel than would be the case for the first embodiment 30 at constant collimator resolution.
In both the first and second embodiments 30/40, the individual holes or channels 32/42 have dimensions (e.g., lh and wh for square/rectangular holes, diameter for round holes, area, etc.) which are one-half of or even smaller than the dimensions of holes 12 found in typical prior art registered collimators 10. Because these individual holes 32/42 are smaller, the overall height/thickness Hc of the collimator 30/40 can also be smaller than comparable conventional prior art registered collimators 10 (e.g., on the order of about one-half or less), while still maintaining generally the same aspect ratio (i.e., the ratio of collimator height Hc to hole dimension). (A comparable conventional registered collimator used with a pixelated detector, as compared to a collimator constructed according to the present invention used with that same pixelated detector, would be a similar type of collimator (e.g., parallel hole) having a comparable aspect ratio (i.e., ±10%), but having a one-to-one registration between its holes and the detector's pixels, as opposed to the multiple-smaller-holes-per-pixel arrangement of the present invention.) This is illustrated in
A third embodiment of the present invention is shown in
For the sake of comparison,
Simulations were conducted using various configurations of the three embodiments compared against known prior art collimators. The results of these comparisons are shown below in TABLES 1 and 2. Rows 1 and 2 describe two actual collimators known in the art: one used by Gamma-Medica (designated as “GM actual”) and another by GE Healthcare (designated as “GE actual”). As shown in Row 1, the GM collimator is used with a pixelated detector having a pixel pitch of 1.6 mm, and is constructed out of tungsten (W). This collimator has a height Hc of 9.4 mm and uses septa that are 0.38 mm in thickness, providing a sensitivity of 1972 cpm/μCi, a FWHM resolution at 3.0 cm of 5.061 mm, and a septal penetration of 1.44%. As shown in Row 2, the GE collimator is used with a pixelated detector having a pixel pitch of 2.46 mm, and is constructed out of lead (Pb). This collimator has a height Hc of 21.0 mm and uses septa that are 0.40 mm in thickness, providing a sensitivity of 1267 cpm/μCi, a FWHM resolution at 3.0 cm of 4.800 mm, and a septal penetration of 0.87%. Using the sensitivity of the GM collimator as a standard (i.e., Relative Sensitivity (RS)=1.00), it can be seen that the GE collimator has a RS of 0.64 as compared to the GM collimator.
Rows 3-6 show known ways in which the GE collimator of Row 2 could be modified to match the resolution and septal penetration of the GM collimator of Row 1. Choosing 5.000 mm and 5.100 mm as two target values for the resolution (i.e., just a little above and below the 5.061 mm GM resolution), it can be seen in Rows 3 and 4 that simply lowering the collimator height Hc to 19.8 mm and 18.8 mm, respectively, would meet the two target values. Alternatively, rows 5 and 6 show that the same performance can be achieved with tungsten, namely switching from lead to tungsten, and making the septum thickness (Tinter) smaller, would also meet the two resolution targets. However, none of these modifications in rows 3-6 raises the sensitivity up to the chosen standard of the GM collimator.
By contrast, rows 7 and 8 describe a collimator designed according to the first embodiment of the present invention, wherein not one but four collimator holes per detector pixel are provided. Keeping the same 2.46 mm pixel pitch as used with the GE collimator of row 2, and using tungsten as the detector material, rows 7 and 8 indicate that the collimator height Hc can be dramatically reduced (down to the range of 6.1 to 6.4 mm), while maintaining the collimator's original septum thickness of 0.40 mm and still meeting the 5.000 or 5.100 mm resolution criteria. Moreover, the sensitivities achieved by these modifications are generally better than those achieved by the known approaches of rows 3-6, while still maintaining excellent septal penetration and the required resolution.
Rows 9 and 10 show modifications made according to the second embodiment, in which the collimator also uses four holes per pixel, but also includes reducing the intra-group septum thickness Tintra to 0.025 mm. This allows the inter-group septum thickness Tinter to be reduced, while still maintaining a thin collimator (i.e., very low Hc) with high sensitivity and low septal penetration. Rows 11 and 12 show the intra-group septum thickness Tintra being further reduced to 0.020 mm, which enables the inter-group septum thickness Tinter and collimator height Hc to remain about the same as rows 9 and 10, but with improved sensitivity compared to rows 9 and 10.
TABLE 2 shows a further set of simulations involving the second embodiment. In this set, the 0.246 pixel pitch, tungsten collimator material, and resolution targets of 5.000 and 5.100 mm were maintained, while the intra-group septum thickness Tintra was iterated from 0.015 mm (rows A and B), to 0.020 mm (rows C and D), and then to 0.025 mm (rows E and F). It may be noted that rows C and D have the same measurements as rows 11 and 12 in TABLE 1 (both having Tintra=0.020 mm), and that rows E and F have the same measurements as rows 9 and 10 (both having Tintra=0.025 mm); however, TABLE 2 also includes results for the Intra-group Septum Penetration, or IGSP, which is an estimation of how often along the intra-group septa 46 gamma photons penetrate before being absorbed. In these iterations, it was desired to find the design that had the highest sensitivity, while keeping the IGSP below 15%. This value is chosen to get 90% of the entitlement, and may be further optimized; it also shows that the GM collimator performance standard may be exceeded. TABLE 2 shows that while rows A and B had the best set of sensitivities, the IGSP values were unacceptably high. The design in rows E and F showed acceptable IGSP values, but their sensitivities were lower than those of rows C and D, which also had acceptable IGSPs. Thus, either of the designs in rows C and D (utilizing an intra-group septum thickness of Tintra=0.020 mm) would be a good choice, given the requirements presented.
An analysis for the third embodiment 50, utilizing a similar approach as that described for the second embodiment 40, indicates that a reduced intra-septum height hs of about 80% of the overall collimator height is a good choice. (That is, hs is about 0.8 Hc.)
As those skilled in the art will appreciate, the sensitivity, resolution and septal penetration/IGSP (SP) figures used in TABLES 1 and 2 may be calculated as follows:
Sensitivity=(0.28h2/Pp/He)2 (1)
Resolution=Pp(He+b)/He+Tintra (2)
SP=e
−μW (3)
where
h=(Pp−Tinter−Tintra)/2 (4)
and
H
e
=H
c−2/μ, (5)
and
W≈tH
c/(2h+t) (6)
In these equations, h represents the hole size, b is the source-to-collimator distance, μ is the linear attenuation coefficient for the collimator material (μ=34/cm for tungsten), W is the shortest path length for gamma rays to travel through a septum from one hole to the next, t is the thickness of the septum which the gamma rays pass through (i.e., Tinter or Tintra, as the case may be), and He is the effective height of the collimator (which has been reduced from the full length Hc due to septum penetration at both ends of the holes). The coefficient 0.28 in Eqn. (1) is a geometrical factor used for square holes.
If desired, two or more of the embodiments may be combined together. For example, the second embodiment 40 (which has intra-group septa 46 that are thinner than the inter-group septa 48) may be combined with the third embodiment 50 (which has intra-group septa 56 that are recessed from the top and/or bottom surface of the collimator thereby making them “shorter” than the inter-group septa 58). In such a combination, the intra-group septa designated in the drawings as elements 46 and 56 would be the same structure having the characteristics of both embodiments (i.e., the intra-group septa 46/56 would be both thinner and “shorter” than the inter-group septa 48/58). Other combinations of the embodiments 30/40/50 are also possible and within the scope of the present invention.
For those seeking further explanation of the collimator concepts used in this disclosure, the following references are suggested, all of which are incorporated herein by reference as if fully set forth herein: (1) http://www.nuclearfields.com/collimators-designs.htm; (2) http://www.nuclearfields.com/collimators-nuclear-medicine.htm; (3) Physics in Nuclear Medicine, Third Edition, by Simon R. Cherry, James A. Sorenson and Michael E. Phelps (W.B. Saunders Co.); and (4) Design of optimal collimation for dedicated molecular breast imaging systems, by Amanda L. Weinmann, Carrie B. Hruska and Michael K. O'Connor, Med. Phys. 36, pp. 845-856 (2009).
The above description is intended to be illustrative, and not restrictive. 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. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, a collimator according to the present invention may include the multiple-collimator-holes-per-detector pixel arrangement applied to only a portion of the overall collimator structure (e.g., corresponding to a particular organ or region of interest), with adjacent or other collimator structure conforming to the conventional one-hole-per-pixel arrangement. Moreover, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. While the dimensions and types of materials described herein are intended to illustrate the invention, they are by no means limiting and are exemplary embodiments. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. §112, sixth paragraph, unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure.
This written description uses examples to disclose the invention, including the best mode, and also to enable those skilled in the art to practice the invention, including making and using any devices or systems thereof and performing any methods thereof. It is the following claims, including all equivalents, which define the scope of the present invention.
