Patent Application
20090086884
The present application hereby claims priority under 35 U.S.C. §119 on German patent application number DE 10 2007 046 514.0 filed Sep. 28, 2007, the entire contents of which is hereby incorporated herein by reference.
Embodiments of the invention generally relate to a method for recognizing and marking contrast agents in blood vessels of the lung with the aid of a CT examination using at least two different x-ray energy spectra, and/or to an evaluation unit of a CT system, with a patient being scanned at least in the region of the lungs with two different x-ray energy spectra, while the patient has contrast agents in the blood stream, and an at least two or three dimensional tomographic display being reconstructed which reproduces the local absorption properties of the scanned region.
By way of example, a method is described in the patent application US 2004/0101089 A1, the entire contents of which are hereby incorporated herein by reference.
When examining lung tissue, it is sometimes medically necessary to also evaluate the circulatory state of thin blood vessels. Due to partial volume effects, this evaluation is difficult using reconstructed CT images which display HU values, especially in the case of fine lung vessels. The HU values in the interior of the vessel calculated in the process depend inter alia on the vessel diameter, the reconstruction kernel and, if applicable, also on the slice thickness of the reconstructed slice images.
If the vessels are very thin, partial volume effects prevent a clear display of vessels supplied with contrast agent. In this case, these partial volume effects have effective diameters which can be significantly larger than the pixel diameter or voxel diameter of a CT image.
In at least one embodiment of the invention, a method permits automated determination of the circulatory state of blood vessels in the lung tissue and displays it.
In at least one embodiment, the inventor has recognized the following:
In the case of CT images, which are recorded using a single x-ray spectrum, it is a problem in fine lung vessels that, due to the finite image focus, the HU value expected for blood is not measured in the center of the vessel because the adjacent air lowers this HU value. For this reason it is not possible to read-out whether or not the vessel contains contrast agent in addition to blood, using the HU value in the center of it.
Although scanning an examination object using a plurality of x-ray energy spectra—different technical methods are generally known to this end—cannot resolve the problem of the finite image focus, it is however possible to determine the ratio of the quantity of the contrast agent to the quantity of tissue within the same volume and use this to characterize the circulatory state. For a fixed contrast agent concentration in the blood, this ratio is independent of the size of the considered volume and the actual size of the vessel contained therein, provided that the vessel walls are disregarded. Large ratios characterize normal circulation; values in the vicinity of zero characterize circulatory disorders.
To calculate this ratio with the aid of tomographic images of a “dual energy” CT system, a two or three dimensional neighborhood can first of all be determined for each voxel. In principle, this neighborhood can have an arbitrary shape, but a convex shape lends itself to this purpose. As an additional criterion, it is possible to regard only voxels in the neighborhood whose arbitrarily linearly weighted HU value from the HU value of a first low energy radiation spectrum xS1 and the HU value of a second, higher energy radiation spectrum xS2 exceeds and/or drops below a certain threshold, where x=axS1+bxS2+c. Coefficients a, b and c can be chosen arbitrarily. However, in the following text it is assumed for the purposes of simplification that, for the HU value x′,
b=1−a, and c=0.
A lower threshold for x′ is expedient, because for low values of x′, that is to say for voxels in the lung parenchyma, the contrast/noise ratio is significantly poorer than for voxels in the region of the vessel. Additionally, constant systematic errors in the subsequent processing have, relatively speaking, a more pronounced effect on low HU values than high HU values. Advantageously, this threshold can be set to be just above typical CT values for the parenchyma including contrast agents.
An upper threshold for x′ can be used to remove calcium or metallic foreign bodies, for example, from the analysis.
The ratios which determine the circulatory state in the lung vessels considered can now be determined for the remaining voxels in various ways. In principle it is possible to use different methods for this, but these methods describe the same fundamental idea. The ratios are each designated Ri, with each index referring to a particular method of calculation:
with the aid of a determined contrast agent image xI and a virtual native image xvn.
or by directly calculating the ratio from the corresponding HU values by calculating
As an alternative to this, it is also possible to firstly calculate the ratio for every voxel in the selected neighborhood and subsequently carry out an averaging process. A further alternative is to generate images of the equivalent densities of the soft tissue and contrast agent with the aid of a raw-data based material decomposition, as is described, for example, in “Energy-selective Reconstructions in X-ray Computerized Tomography”, R. E. Alvarez and A. Macovski, Phys. Med. Biol., 1976, Vol. 21, No. 5, 733-744, the entire contents of which are hereby incorporated herein by reference.
Subsequently, it is possible in turn to calculate the ratio from the averages of the equivalent densities over a neighborhood or individually for each voxel in the neighborhood, and then average over all voxels.
A threshold can then be selected for the ratio, with all values above this threshold being evaluated as characteristic of a healthy circulation. All values below the threshold are evaluated as having reduced circulation. The vessels with proper circulation can subsequently be marked on a CT image.
Furthermore, the ratio can be superposed onto the normal CT image or a suitable mixed image and be color-coded.
Hence, the method according to at least one embodiment of the invention can be structured into the following individual steps:
According to at least one of abovementioned ideas of at least one embodiment of the invention, the inventor proposes in the most general form a method for recognizing and marking blood vessels of the lung with the aid of a CT examination using at least two different x-ray energy spectra, comprising at least the following method steps:
scanning a patient at least in the region of the lung with two different x-ray energy spectra, with the patient having contrast agents in the blood stream,
reconstructing an at least two or three dimensional tomographic display for each x-ray energy spectrum which reproduces the local spectrum-specific absorption properties of the scanned region,
a neighborhood is defined for which for a multiplicity of voxels an average ratio is calculated with the aid of the local absorption values of the at least two x-ray energy spectra, which neighborhood indicates a measure of the ratio of the proportion of contrast agent to the proportion of soft tissue in the neighborhood of the respectively considered voxel,
in the case of this ratio dropping below a predetermined threshold value this voxel is considered to have reduced circulation and is marked in a tomographic display.
Advantageously, the average ratio can be formed by the local absorption values of the at least two x-ray energy spectra. In this case, the ratio can be calculated as the quotient of the absorption coefficients determined from the two x-ray energy spectra
or as the quotient of the HU values determined from the two X-ray energy spectra
In another embodiment of the method according to the invention, it is proposed that the scanned region's material is decomposed into two different materials, and two tomographic data records with local equivalent densities of each of the two different materials are determined, as described for example in Kalender W, Bautz W. Felsenberg D, Süβ C, Klotz E, “Materialselektive Bildgebung und Dichtemessung mit der Zwei-Spektren-Methode I. Grundlagen und Methodik”[Material-selective imaging and density measurements using the two-spectra method I. Theory and methodology], Digitale Bilddiagn. 1987 June; 7(2):66-72 (the entire contents of which are hereby incorporated herein by reference), and that the average ratio is formed from the local equivalent densities of the two materials using R1=zI/zG.
As an alternative, it is also possible to decompose the material the scanned region into three different materials, as is described in an example manner in the patent application with the reference number DE 10 2006 009 222.8 (the entire contents of which are hereby incorporated herein by reference), by determining at least one tomographic virtual native image and a tomographic contrast agent image, with the average ratio in this case being formed from the tomographic virtual native image and the tomographic contrast agent image. In this case, the ratio R2=c/fG can be calculated as the quotient of the equivalent density values c of the contrast agent image and the equivalent density values fG of the virtual native image. It is likewise also possible to calculate the ratio using
with xI being the voxel values of the contrast agent image and xvn being the voxel values of the virtual native image.
In order to reduce the computational complexity and speed up the method, it is possible for the image region in which further calculations are carried out to be limited in a particularly advantageous variant of the method before calculating the ratios by calculating a linearly weighted absorption value x=a*xS1+b*xS2+c for each voxel of the scanned region using the CT image data xS1 und xS2 which was scanned with the two x-ray energy spectra S1 and S2 and reconstructed, and by checking whether a preset boundary value of the absorption value is exceeded. The rest of the method is carried out only on those voxels which exceed this bound. It should be mentioned in this regard that it is also within the scope of the invention to set the weighting factors a or b and the constant c to zero, that is to say that only the image data obtained by one of the two radiation spectra is checked for the bounds as described above.
In place of a lower bound, it is also possible to use a bounded range with an upper and lower bound by determining a linearly weighted absorption value for each voxel of the scanned region from the CT image data of the two x-ray energy spectra, and by checking whether a predetermined bounded range for the absorption value is adhered to, with the method being carried out only for those voxels which lie in this bounded range.
It is also advantageous if the defined neighborhood of a voxel comprises a preset radius r. The preset radius r can in this case be selected as a function of an observed vessel diameter and/or the image noise occurring and/or possible motion unsharpness, with the preset radius r increasing with increasing vessel diameter, increasing noise and/or increasing offset or increasing movement between the CT images recorded using the different spectra.
Furthermore, the marked voxels can be output in color.
An image evaluation unit for a CT system comprising a processor and a program memory, with the program memory comprising a program code which executes the method according to at least one embodiment of the invention when the evaluation unit is operating, is also included within the scope of at least one embodiment of the invention.
In the following text, embodiments of the invention are described in more detail with the aid of the figures, in which only features which are necessary for understanding the embodiments of invention are illustrated. In this case, the following reference symbols are used: 1: dual energy CT system; 2: first x-ray tube for the first spectrum; 3: first detector; 4: second x-ray tube for the second spectrum; 5: second detector; 6: gantry housing; 7: patient; 8: displaceable patient couch; 9: system axis; 10: image evaluation unit; 11: CT slice image created using the first energy spectrum; 12: CT slice image created using the second energy spectrum; 13: ideal HU value profile of a vessel; 14: actual diameter of a vessel; 15: measured HU value profile of a vessel; 15.1: HU value profile of a vessel measured using the first energy spectrum; 15.2: HU value profile of a vessel measured using the second energy spectrum; 16: apparent vessel diameter; 17: spatial offset; 18: position of the voxels of a lung without contrast agents in the HU value diagram; 19: boundary line; 20.1: lower bound; 20.2: upper bound; 21: region of voxels with reduced circulation; A, A′, B, B′: vectors in the HU value diagram; c: equivalent density value of the contrast agent; fG: equivalent density value of the virtual native image; Prg1-Prgn: computer programs, Ri: ratios of contrast agent quantity and tissue quantity; S1, S2: x-ray energy spectra; xS1, xS2: spectrum-specific absorption values in HU (=Hounsfield Units); xvm: absorption values of the native image in HU; xI: absorption values of the contrast agent image in HU; zG: equivalent density of the tissue; zI: equivalent density of the contrast agent: z1, z2: equivalent densities; μS1, μS2: absorption coefficients.
In more detail:
Various example embodiments will now be described more fully with reference to the accompanying drawings in which only some example embodiments are shown. Specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. The present invention, however, may be embodied in many alternate forms and should not be construed as limited to only the example embodiments set forth herein.
Accordingly, while example embodiments of the invention are capable of various modifications and alternative forms, embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments of the present invention to the particular forms disclosed. On the contrary, example embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of the invention. Like numbers refer to like elements throughout the description of the figures.
It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments of the present invention. As used herein, the term “and/or,” includes any and all combinations of one or more of the associated listed items.
It will be understood that when an element is referred to as being “connected,” or “coupled,” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected,” or “directly coupled,” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between,” versus “directly between,” “adjacent,” versus “directly adjacent,” etc.).
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments of the invention. As used herein, the singular forms “a,” “an,” and “the,” are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the terms “and/or” and “at least one of” include any and all combinations of one or more of the associated listed items. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper”, and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, term such as “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein are interpreted accordingly.
Although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, it should be understood that these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are used only to distinguish one element, component, region, layer, or section from another region, layer, or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of the present invention.
Of course, the type of scanning described here does not constitute a limitation of the method according to an embodiment of the invention. Thus, it is known to a person skilled in the art that it is also possible to obtain energy-specific tomographic images of an examination object using different methods and differently constructed CT systems. Hence, it is also possible to operate a CT system with a single x-ray tube alternately with different radiation spectra, or an energy-resolving detector can be used when scanning with a single spectrum. It is essential to the invention only that eventually CT images of the same object with two energy specific absorption data records are available for processing.
Due to the peculiarities of the measurement method in computed tomography and of the reconstruction method, it is known that partial volume effects occur when displaying small structures, that is to say also when displaying thin vessels in the lung, which additionally complicate recognizing the circulatory state of small vessels. This is substantially due to the limited resolution of the CT and the noise due to applying the lowest dosage possible in the scan.
When evaluating two CT images recorded at different times, there is an additional problem due to the fact that it is impossible to avoid movements between the records and thus the respectively congruent voxels do not reproduce the identical location in a displayed organ when comparing two CT images of the same object.
The definition of the ratio, which is a measure of the ratio of the proportion of contrast agent to the proportion of soft tissue in the neighborhood of the respectively considered voxel, will be explained now on the basis of
In a perfused lung, the blood contains dissolved iodine with a constant concentration. The magnitude of the vector B, which represents the absorption properties of iodine, is directly proportional to the magnitude of the vector A, which corresponds to the absorption of blood, and thus:
|B|=k|A|
Independent of how much air is now contained in a considered neighborhood, the proportion of iodine B′ and proportion of blood A′ of the total absorption also satisfy:
|B′|=k|A′|
Hence all voxels in the lung lie on the dashed line 18.
On this graph, the partial volume effect is indicated by the shortening of the vector A to A′, and the shortening of vector B to B′. The circles show the position of the lung tissue without a contrast agent.
If the iodine concentration is lower in one region of the lung, all voxels there lie on a line through the point of intersection of ordinate and abscissa at (−1000,−1000), the gradient of which line is less than the gradient of the line 18.
This means that a limiting concentration is represented by a boundary line through the point (−1000,−1000). All voxels above this line have normal circulation. All voxels below the line have reduced circulation.
When considering the images, all voxels located outside the area of the boundary lines 20.1 and 20.1 are rejected first. Subsequently, the voxels whose ratio lies above the boundary line 19 are rejected, and the remaining voxels belonging to the shaded region 21 are considered to be voxels characterizing a vessel with reduced circulation.
According to an embodiment of the invention, it is now possible to use different variables to determine whether a voxel lies above or below the boundary line:
If the method above, of an embodiment of the invention, is applied to the two CT images in
Further, elements and/or features of different example embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims.
Still further, any one of the above-described and other example features of the present invention may be embodied in the form of an apparatus, method, system, computer program and computer program product. For example, of the aforementioned methods may be embodied in the form of a system or device, including, but not limited to, any of the structure for performing the methodology illustrated in the drawings.
Even further, any of the aforementioned methods may be embodied in the form of a program. The program may be stored on a computer readable media and is adapted to perform any one of the aforementioned methods when run on a computer device (a device including a processor). Thus, the storage medium or computer readable medium, is adapted to store information and is adapted to interact with a data processing facility or computer device to perform the method of any of the above mentioned embodiments.
The storage medium may be a built-in medium installed inside a computer device main body or a removable medium arranged so that it can be separated from the computer device main body. Examples of the built-in medium include, but are not limited to, rewriteable non-volatile memories, such as ROMs and flash memories, and hard disks. Examples of the removable medium include, but are not limited to, optical storage media such as CD-ROMs and DVDs; magneto-optical storage media, such as MOs; magnetism storage media, including but not limited to floppy disks (trademark), cassette tapes, and removable hard disks; media with a built-in rewriteable non-volatile memory, including but not limited to memory cards; and media with a built-in ROM, including but not limited to ROM cassettes; etc. Furthermore, various information regarding stored images, for example, property information, may be stored in any other form, or it may be provided in other ways.
Example embodiments being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 046 514.0 | Sep 2007 | DE | national |
