Method and apparatus for performing multi-energy (including dual energy) computed tomography (CT) imaging

Abstract

An improved dual energy CT imaging system for providing improved imaging and improved material identification.

Claims

1. A multi-energy computed tomography (CT) imaging system for providing an image of an object, the multi-energy computed tomography (CT) imaging system comprising: a polychromatic X-ray source; a detector for detecting X-rays from the polychromatic X-ray source after the X-rays have passed through an object and for providing a first set of polychromatic energy measurements relating to a first polychromatic X-ray spectrum passed through the object and for providing a second set of polychromatic energy measurements relating to a second polychromatic X-ray spectrum passed through the object; and a processor configured to: (i) transform the first set of polychromatic energy measurements into a corresponding first monochromatic data set associated with X-rays at a selected first monochromatic energy level, and to transform the second set of polychromatic energy measurements into a corresponding second monochromatic data set associated with X-rays at a selected second monochromatic energy level; (ii) transform the first monochromatic data set into a first monochromatic image and transform the second monochromatic data set into a second monochromatic image; and (iii) use at least one of the first monochromatic image and the second monochromatic image to perform at least one of (a) provide an image free from beam hardening artifacts, and (b) provide identification of material properties within the object.

2. The multi-energy computed tomography (CT) imaging system according to claim 1, wherein the detector comprises a first section and a second section, wherein the multi-energy computed tomography (CT) imaging system further comprises a first filter and a second filter, and further wherein the first set of polychromatic energy measurements is provided by positioning the first filter between the polychromatic X-ray source and the first section of the detector, and the second set of polychromatic energy measurements is provided by positioning the second filter between the polychromatic X-ray source and the second section of the detector.

3. The multi-energy computed tomography (CT) imaging system according to claim 2, wherein at least one of the first filter and the second filter is disposed between the polychromatic X-ray source and the object.

4. The multi-energy computed tomography (CT) imaging system according to claim 2, wherein at least one of the first filter and the second filter is disposed between the object and the detector.

5. The multi-energy computed tomography (CT) imaging system according to claim 2, wherein the first filter is disposed between the object and the detector.

6. The multi-energy computed tomography (CT) imaging system according to claim 1, wherein the detector comprises a first section and a second section, wherein the multi-energy computed tomography (CT) imaging system further comprises a first filter, and further wherein the first set of polychromatic energy measurements is provided by positioning the first filter between the polychromatic X-ray source and the first section of the detector, and the second set of polychromatic energy measurements is provided by calculating a difference between an output of the first section of the detector and an output of the second section of the detector.

7. The multi-energy computed tomography (CT) imaging system according to claim 6, wherein the first filter is disposed between the polychromatic X-ray source and the object.

8. The multi-energy computed tomography (CT) imaging system according to claim 1, wherein the corresponding first monochromatic data set comprises a first sinogram, and wherein the corresponding second monochromatic data set comprises a second sinogram.

9. The multi-energy computed tomography (CT) imaging system according to claim 1, wherein the multi-energy computed tomography (CT) imaging system further comprises a lookup table, and further wherein the lookup table is used to transform at least one of the first set of polychromatic energy measurements into a corresponding first monochromatic data set and the second set of polychromatic energy measurements into a corresponding second monochromatic data set.

10. The multi-energy computed tomography (CT) imaging system according to claim 9, wherein the lookup table provides at least one multiplier for converting at least one of the first set of polychromatic energy measurements into the corresponding first monochromatic data set and the second set of polychromatic energy measurements into the corresponding second monochromatic data set.

11. The multi-energy computed tomography (CT) imaging system according to claim 9, wherein indices of the lookup table comprise: (i) at least one of the first set of polychromatic energy measurements and the second set of polychromatic energy measurements; and (ii) a function which is sensitive to differences between the first set of polychromatic energy measurements and the second set of polychromatic energy measurements.

12. The multi-energy computed tomography (CT) imaging system according to claim 9, wherein indices of the lookup table comprise: (i) at least one of the first set of polychromatic energy measurements and the second set of polychromatic energy measurements; and (ii) a ratio between the first set of polychromatic energy measurements and the second set of polychromatic energy measurements.

13. The multi-energy computed tomography (CT) imaging system according to claim 1, wherein the processor generates an image free from beam hardening artifacts by presenting one of the first monochromatic image and the second monochromatic image.

14. The multi-energy computed tomography (CT) imaging system according to claim 1, wherein the processor generates an image free from beam hardening artifacts by presenting a composite image of the first monochromatic image and the second monochromatic image.

15. The multi-energy computed tomography (CT) imaging system according to claim 14, wherein the composite image is a weighted composite image.

16. The multi-energy computed tomography (CT) imaging system according to claim 14, wherein the composite image of the first monochromatic image and the second monochromatic image comprises a plurality of voxels, and further wherein each voxel of the composite image comprises a voxel selected from the first monochromatic image or the second monochromatic image.

17. The multi-energy computed tomography (CT) imaging system according to claim 1, wherein the processor uses at least one of the first monochromatic image and the second monochromatic image to provide identification of materials within the object by generating an image whose voxel values reflect material properties of the scanned object.

18. The multi-energy computed tomography (CT) imaging system according to claim 1, wherein the processor uses at least one of the first monochromatic image and the second monochromatic image to provide identification of materials within the object by generating an image whose voxel values reflect an electron density (rho value) of the scanned object.

19. The multi-energy computed tomography (CT) imaging system according to claim 1, wherein the processor uses at least one of the first monochromatic image and the second monochromatic image to provide identification of materials within the object by generating an image whose voxel values reflect Z.sub.effective of the scanned object.

20. The multi-energy computed tomography (CT) imaging system according to claim 1, wherein the detector provides a third set of polychromatic energy measurements relating to a third polychromatic X-ray spectrum passed through the object.

21. A multi-energy computed tomography (CT) imaging method for providing an image of an object free from beam hardening artifacts and/or providing identification of material properties of the object, the multi-energy computed tomography (CT) imaging method comprising: providing a first set of polychromatic energy measurements relating to a first polychromatic X-ray spectrum passed through the object and providing a second set of polychromatic energy measurements relating to a second polychromatic X-ray spectrum passed through the object; transforming the first set of polychromatic energy measurements into a corresponding first monochromatic data set associated with X-rays at a selected first monochromatic energy level, and transforming the second set of polychromatic energy measurements into a corresponding second monochromatic data set associated with X-rays at a selected second monochromatic energy level; transforming the first monochromatic data set into a first monochromatic image, and transforming the second monochromatic data set into a second monochromatic image; and using at least one of the first monochromatic image and the second monochromatic image to provide at least one of (a) an image free from beam hardening artifacts, and (b) identification of material properties within the object.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other objects and features of the present invention will be more fully disclosed or rendered obvious by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein:

(2) FIGS. 1 and 2 are schematic views showing the exterior of an exemplary prior art CT imaging system;

(3) FIG. 3 is a schematic view showing various components in the torus of the exemplary prior art CT imaging system shown in FIGS. 1 and 2;

(4) FIG. 4 is a schematic view showing components in the torus of a novel multi-energy (including dual energy) CT imaging system formed in accordance with the present invention;

(5) FIG. 5 is a schematic view of a dual-filter detector which may be utilized in the X-ray detector assembly of the novel multi-energy (including dual energy) CT imaging system shown in FIG. 4;

(6) FIG. 6 is a schematic view of a single-filter detector which may be utilized in the X-ray detector assembly of the novel multi-energy (including dual energy) CT imaging system shown in FIG. 4;

(7) FIG. 7 is a schematic representation showing how polyenergetic data may be transformed to monoenergetic data;

(8) FIG. 8 is a schematic representation showing how multiple monoenergetic data sets and images may be produced, and multiple monoenergetic images may be transformed into an image conveying material properties;

(9) FIG. 9 is a schematic view showing how polyenergetic data sets may be transformed into any number of monoenergy data sets, and how two or more monoenergetic images may be transformed into an image conveying material properties; and

(10) FIG. 10 is a table showing how raw polychromatic data may be transformed into equivalent monochromatic data.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(11) In accordance with the present invention, there is provided a novel multi-energy (including dual energy) CT imaging system providing improved imaging (e.g., images free of beam hardening artifacts or approximations) and improved material identification.

Apparatus for Performing Dual Energy

Computed Tomography (CT) Imaging

(12) In accordance with the present invention, and looking now at FIG. 4, there is provided a novel multi-energy (including dual energy) CT imaging system 105. Multi-energy (including dual energy) CT imaging system 105 is substantially the same as the exemplary prior art CT imaging system 5 previously discussed, except as will hereinafter be discussed.

(13) More particularly, in the preferred form of the present invention, multi-energy (including dual energy) CT imaging system 105 generally comprises a torus 110 which is supported by a base 115. A center opening 120 is formed in torus 110. Center opening 120 receives the object (e.g., the body or the container) which is to be scanned by multi-energy (including dual energy) CT imaging system 105.

(14) Still looking now at FIG. 4, torus 110 generally comprises a fixed gantry 125, a rotating disc 130, an X-ray tube assembly 135 and an X-ray detector assembly 140. More particularly, fixed gantry 125 is disposed concentrically about center opening 120. Rotating disc 130 is rotatably mounted to fixed gantry 125. X-ray tube assembly 135 and X-ray detector assembly 140 are mounted to rotating disc 130 in diametrically-opposing relation, such that an X-ray beam 145 (generated by X-ray tube assembly 135 and detected by X-ray detector assembly 140) is passed through the object (e.g., the body or the container) disposed in center opening 120.

(15) In one preferred form of the invention, X-ray tube assembly 135 comprises a polychromatic X-ray tube assembly, i.e., X-ray tube assembly 135 emits X-rays with a range of different energies. In one preferred form of the invention, X-ray tube assembly 135 comprises a single X-ray tube which is driven by a single voltage.

(16) Inasmuch as X-ray tube assembly 135 comprises a single polychromatic X-ray tube driven by a single voltage, in order to allow the imaging system to be used for multi-energy (including dual energy) CT imaging, it is necessary for X-ray detector assembly 140 to provide, for each of its detectors, measurements at two or more different X-ray energy ranges, e.g., a high energy measurement which maximizes the detection of higher energy photons and a low energy measurement which maximizes the detection of lower energy photons, with or without additional energy measurements.

(17) For clarity of description, multi-energy (including dual energy) CT imaging system 105 will generally hereinafter be discussed in the context of a dual energy CT imaging system, however, it should be appreciated that the CT imaging system may utilize three or more energy measurements without departing from the scope of the present invention.

(18) If desired, this may be accomplished in the manner of the prior art, i.e., by providing a dual-filter detector such as the dual-filter detector shown in FIG. 5. More particularly, in this form of the invention, each detector 150 (FIG. 5) of X-ray detector assembly 140 is provided with two separate detection regions 155, 160, with detection region 155 being provided with a first filter 165 which is configured to maximize the detection of higher energy photons (whereby to provide the high energy measurement used for dual energy CT scanning), and with detection region 160 being provided with a second, different filter 170 which is configured to maximize the detection of lower energy photons (whereby to provide the low energy measurement used for dual energy CT scanning). However, this prior art approach has the drawback of increased cost and lower photon yield, particularly with respect to the low energy measurement since the low energy measurement is restricted to low energy photons and such low energy photons are more heavily attenuated as they encounter the object being scanned, thereby yielding lower photon yields.

(19) For this reason, the present invention provides an improved approach for providing, for each of its detectors, measurements at two different X-ray energy ranges (i.e., a high energy measurement and a low energy measurement) in order to enable dual energy CT scanning.

(20) More particularly, and looking now at FIG. 6, with the improved approach of the present invention, each detector 150 of X-ray detector assembly 140 is provided with two separate detection regions 155, 160. Detection region 155 is provided with a filter 165 which is configured to maximize the detection of higher energy photons (whereby to provide the high energy measurement used for dual energy CT scanning). Detection region 160 is not provided with a filter (hence, the detector shown in FIG. 6 may be considered to be a single-filter detector, rather than a dual-filter detector such as is shown in FIG. 5).

(21) As a result of this construction, detection region 155 will provide a measurement X which is representative of the higher energy portion of the X-ray spectrum passing through the object which is being scanned, and detection region 160 will provide a measurement Y which is representative of the total X-ray spectrum passing through the object which is being scanned. The present invention recognizes that if the value of the aforementioned measurement X is subtracted from the value of the aforementioned measurement Y, the resulting value Z will be representative of the lower energy portion of the X-ray spectrum passing through the object which is being scanned. Thus, with this approach, by measuring the high energy photons passing through filter 165 and striking detection region 155 (i.e., the aforementioned measurement X), the high energy measurement used for dual energy CT scanning can be obtained. And by measuring the total X-ray spectrum striking detection region 160 (i.e., the aforementioned measurement Y), and then subtracting the value of the higher energy portion of the X-ray spectrum striking detection region 155 (i.e., the aforementioned measurement X), the low energy measurement (i.e., the aforementioned measurement Z) can be obtained. In this way, the single-filter detector shown in FIG. 6 can be used to acquire the high energy measurement and the low energy measurement used for dual energy CT scanning.

(22) Alternatively, a proxy for the high energy measurement can be taken as the signal striking region 160, as its average spectral response represents a higher energy than that of region 155, and the low energy measurement can be taken as the signal striking region 155, since filter 165 will ensure that those photons striking region 155 will be at a lower energy level than the photons striking region 160.

Method for Performing Multi-Energy (Including Dual Energy) Computed Tomography (CT) Imaging

(23) In accordance with the present invention, there is also provided a new process for using the measurements taken at two or more different polychromatic X-ray energies to provide multi-energy (including dual energy) CT imaging.

(24) For clarity of description, multi-energy (including dual energy) CT imaging system 105 will generally hereinafter be discussed in the context of a dual energy CT imaging system, however, it should be appreciated that the CT imaging system may utilize three or more energy measurements without departing from the scope of the present invention.

(25) The process described below shows how polychromatic dual energy data can be processed into synthetic monochromatic images which are both free of beam hardening artifacts and which contain information on material composition.

(26) More particularly, when dual energy CT imaging is to be performed on an object (e.g., a body or a container), the object is placed in center opening 120 of novel dual energy CT imaging system 105, rotating disc 130 is rotated about fixed gantry 125, X-ray tube assembly 135 is energized so as to emit polychromic X-ray beam 145, and X-ray detector assembly 140 is operated so as to collect two energy measurements for each detector of X-ray detector assembly 140, i.e., a polychromatic high energy measurement and a polychromatic low energy measurement.

(27) As discussed above, and as shown in FIG. 5, the two energy measurements may be obtained for each detector by using a dual-filter detector where two different filters are positioned over each detector 150, i.e., by positioning filter 165 over detection region 155 and by positioning filter 170 over detection region 160. In this way, the high energy measurement is obtained from the output of detection region 155 and the low energy measurement is obtained from the output of detection region 160.

(28) Alternatively, and more preferably, and as also discussed above, the two energy measurements may be obtained for each detector of X-ray detector assembly 140 by positioning a filter over half of the detector and leaving the other half of the detector exposed (i.e., by positioning filter 165 over detection region 155 and leaving detection region 160 unfiltered, in the manner shown in FIG. 6). In this way, the polychromatic high energy measurement may be obtained from the output of detection region 155 and a polychromatic low energy measurement may be obtained from the the output of detection region 160.

(29) These two polychromatic energy measurements are made for each detector of X-ray detector assembly 140 for each rotational position of rotating disc 130 about fixed gantry 125, i.e., for each line of response of the X-ray beam passing through the object which is being scanned), thereby yielding two polyenergetic sinograms, g.sub.HIGH (high energy) and g.sub.LOW (low energy).

(30) Then, for each line of response, the ratio g.sub.HIGH/g.sub.LOW (or R) is computed, e.g., using a processor 175 (FIGS. 7-9).

(31) The signal for any given line of response, whether associated with a filtered detector or an unfiltered detector (i.e., g.sub.HIGH or g.sub.LOW, respectively), can be converted to the predicted signal for a purely monochromatic system of ANY monoenergy. This is achieved using a lookup table where R and g (either g.sub.HIGH or g.sub.LOW) are used as the lookup indices to obtain the appropriate multiplier to transform the CT-acquired polyenergetic sinogram g (either g.sub.HIGH or g.sub.LOW) into a synthetic monoenergetic sinogram G.sub.ENERGY, where .sub.ENERGY may be any monoenergetic level (e.g., G.sub.160 which represents the synthetic monoenergetic sinogram at a monoenergy of 160 kev, G.sub.40 which represents the synthetic monoenergetic sinogram at a monoenergy of 40 kev, etc.). In practice, because the response of the filtered and unfiltered channels are different, separate tables and separate lookup values are required for each of the filtered and unfiltered channels (i.e., the multipliers A.sub.ENERGY and B.sub.ENERGY respectively to produce a corresponding monochromatic sinogram of G.sub.ENERGY). In this way, synthetic monoenergetic signal levels are predicted for every line of response, generating a full resolution monoenergetic synthetic sinogram. This may be done for any monoenergy using the appropriate multipliers.

(32) Next, CT reconstruction techniques are applied to monoenergetic sinograms to produce monoenergetic images, i.e., I.sub.ENERGY. The monoenergetic images may be produced for any monoenergy, e.g., I.sub.160 which is the synthetic monoenergetic image at a monoenergy of 160 kev, I.sub.40 which is the synthetic monoenergetic image at a monoenergy of 40 kev, etc. Any number of monoenergetic images can be generated from the data acquired using the methods described. These monoenergetic images are free of beam hardening artifacts and are therefore potentially useful for that reason alone. By way of example but not limitation, a single monoenergetic image could be potentially very useful for radiation planning. In radiation therapy it is useful to know how the radiation used to target and kill tumors attenuates as the radiation beam travels through the body. A monoenergetic image can be generated at the same energy as the radiation used for therapy purposes. Each location within the image (i.e., each voxel) contains the predicted attenuation properties of the material for the therapy beam.

(33) By way of example but not limitation, and looking now at FIG. 7, the top of the figure shows a representation of polychromatic dual energy data. In the field of tomographic imaging, raw data commonly takes on the form of a sinogram or fanogram. The data on the left side of FIG. 7 are in sinogram format. Each detector channel consists of several filtered detectors. For example, with a dual energy system, there are two polyenergetic signals which are measured, g.sub.HIGH and g.sub.LOW (for high and low polyenergetic ranges, respectively). The insert shows example numerical data for several detector pairs. This raw data could be turned into an uncorrected polyenergetic image using any number of industry-standard reconstruction techniques including filtered back projection. The polyenergetic image would suffer from beam-hardening image artifacts. The middle row of FIG. 7 shows an example multiplier map which transforms the raw polyenergetic data into monoenergetic data. The multiplier for each detector channel has two components, A and B, to convert the polyenergetic signals g.sub.HIGH and g.sub.LOW respectively. The bottom row of FIG. 7 shows the resulting monoenergetic data. The two polyenergetic signals g.sub.HIGH and g.sub.LOW are converted to a monoenergetic signal g.sub.ENERGY, where .sub.ENERGY may be any monoenergetic level (e.g., G.sub.160 which represents the synthetic monoenergetic sinogram at a monoenergy of 160 kev, G.sub.40 which represents the synthetic monoenergetic sinogram at a monoenergy of 40 kev, etc.). Monoenergetic data does not suffer from beam hardening artifacts when turned into an image.

(34) Synthetic monoenergetic images also contain information on the material composition along each line of response. For example, a mathematical function rho(I.sub.HIGH, I.sub.LOW) could be used to directly determine the electron density of the material at that location (i.e., voxel).

(35) From a computational efficiency point of view, the preferred embodiment uses a lookup table to transform the monoenergetic values at each location within the image into a material value (in this example, rho).

(36) The underlying physical interactions which determine the attenuation of X-rays are generally characterized by Compton scattering (interacting with loosely-bound electrons) and the photoelectric effect (resonant interactions with electrons in atomic shells). Any physical property strongly dependent on the measured strength of these two kinds of interactions can be evaluated. For example, the mass-density of a material, largely dependent on the kind of atoms the material is composed of (a function of the photoelectric effect) and the number of atoms per volume (a function of the electron density), can be determined using the multiple synthetic monoenergetic images I.sub.HIGH, I.sub.LOW (which could be, for example, I.sub.160, I.sub.40).

(37) Looking now at FIG. 8, although any number of monoenergetic datasets and images can be made, in the preferred embodiment, three monochromatic images are made. The mid-energy image optimizes the signal/noise ratio and is free of beam-hardening artifacts. The low and high-energy monochromatic images are transformed to produce an image of the material atomic composition.

Generation of Lookup Tables for Monoenergetic Sinograms

(38) The conversion between polyenergetic sinogram data into monoenergetic sinogram data has been described above as a mathematical function, with the preferred embodiment generalized to a lookup table. The mathematical function and/or lookup table contents could be determined entirely from theoretical computations or from empirical data. For both approaches, the goal is to transform the polyenergetic signals measured at the detectors to the expected monoenergetic signals. To verify that the transform operation produces the expected monoenergetic signals, it is usually convenient to select test objects of simple shape and composition. In most embodiments, the expected values are the best scientifically derived values. In our present embodiment, these target values are obtained from data published by the National Institute of Standards and Technology (NIST), but could be obtained from any source.

(39) Using a completely theoretical approach, one would model the spectrum of the X-ray source, and compute the expected signal level for the different kinds of filtered detectors. The signal levels for the various filtered detectors would be computed a second time assuming a monoenergetic X-ray source. Taking the ratio of the computed detector signals for the monoenergetic X-ray source to the computed detector signals for the polyenergetic source would determine the multipliers necessary to transform a measured polyenergetic signal to the target monoenergy signal.

(40) Using a completely empirical approach would involve making measurements of many objects (with varying composition and thickness) using a standard broad-spectrum (i.e., polychromatic) X-ray source. The equivalent measurements would be acquired using a monoenergetic source. By comparing values, one could determine the multiplicative values (e.g., A.sub.ENERGY and B.sub.ENERGY) to translate one set of measurements (i.e., the polychromatic measurements) to another set of measurements (i.e., the monochromatic measurements at the monoenergetic energy level ENERGY, e.g., 160 kev, 40 kev, etc.). However, it will be appreciated that a purely empirical approach is challenging because generating monochromatic X-rays is generally difficult, and because the number measurements would be large.

(41) A practical approach, and the preferred embodiment for the present invention, involves a predominantly theoretical approach which is fine-tuned to match empirical measurements. Known material samples with varying thicknesses can be scanned using a polyenergetic X-ray source to generate polyenergetic data, and then the polyenergetic data can be transformed to corresponding monoenergetic data using the derived lookup tables. The monoenergetic detector response to these known samples are easily computed, and can be compared with the results using the lookup process. The table values can then be adjusted to produce the correct known response.

(42) FIGS. 9 and 10 are schematic views showing how lookup tables may be used to generate multipliers for transforming polyenergetic data sets into any number of monoenergetic data sets, each one free of beam hardening. Monoenergetic images can then be transformed into a mapping of material properties such as effective atomic number or electron density.

Generation of Lookup Tables for Determining Zeffective or Other Material Properties

(43) The conversion between monoenergetic sinogram data (i.e., g.sub.ENERGY, which could be, for example, g.sub.160, g.sub.40, etc.) and monoenergetic image data (i.e., I.sub.ENERGY, which could be, for example, I.sub.160, I.sub.40, etc.), and material properties, is a mathematical function. In the preferred embodiment, this function is reduced to a tabular form. The values within the table can be derived from the known physical properties of materials or from empirical analysis of data. In the preferred embodiment, the table is empirically derived. For example, samples of varying mean atomic number (Z.sub.effective) are scanned. The ratio of two monoenergetic images (e.g., I.sub.160, I.sub.40) is used as an index into a table which returns the Z.sub.effective value. Thus, to generate the table values, the image ratio and Z.sub.effective value are recorded for a sample material. Data from six sample materials is collected and a best fit function Z.sub.eff(Image Ratio) is determined.

Modifications

(44) It will be appreciated that still further embodiments of the present invention will be apparent to those skilled in the art in view of the present disclosure. It is to be understood that the present invention is by no means limited to the particular constructions herein disclosed and/or shown in the drawings, but also comprises any modifications or equivalents within the scope of the invention.