# Patent

## Title
Rib suppression in radiographic images

## Abstract

A method for rib suppression in a chest x-ray image detects and labels one or more ribs in a region of interest and detects rib edges of the one or more detected ribs. Cross rib profiles are generated along the detected ribs. The original x-ray image is conditioned according to at least one of the detected rib edge and cross rib profiles. The conditioned x-ray image can be stored, displayed, or transmitted.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to U.S. Provisional Patent Application Ser. No. 61/552,658 filed Oct. 28, 2011 in the names of Huo et al., entitled “RIB SUPPRESSION IN RADIOGRAPHIC IMAGES”, incorporated in its entirety herein by reference.


FIELD OF THE INVENTION
The present invention relates generally to the field of radiographic imaging and more particularly to methods for detecting and suppressing rib features from a radiographic image, such as the chest.
BACKGROUND
The chest x-ray is useful for detecting a number of patient conditions and for imaging a range of skeletal and organ structures. Radiographic images of the chest are useful for detection of lung nodules and other features that indicate lung cancer and other pathologic structures and other life-threatening conditions. In clinical applications such as in the Intensive Care Unit (ICU), chest x-rays can have particular value for indicating pneumothorax as well as for tube/line positioning, and other clinical conditions. To view the lung fields more clearly and allow more accurate analysis of a patient's condition, it is useful to suppress the rib cage and related features in the chest x-ray, without losing detail of the lung tissue or other features within the chest cavity.
Methods have been proposed for detecting and suppressing rib structures and allowing the radiologist to view the lung fields without perceptible obstruction by the ribs. Some methods have used template matching, rib edge detection, or curve fitting edge detection. Applicants have recognized that it can be challenging to remove rib features from the chest x-ray image without degrading the underlying image content that can include lung tissue.
US Patent Application Publication No. 2009/0290779 entitled “Feature-based neural network regression for feature suppression” (Knapp) describes the use of a trained system for predicting rib components and subsequently subtracting the predicted rib components.
US Patent Application Publication No. 2009/0060366 entitled “Object segmentation in images” (Worrell) describes alternative techniques using detected rib edges to identify rib structures.
The article entitled “Image-Processing Technique for Suppressing Ribs in Chest Radiographs by Means of Massive Training Artificial Neural Network (MTANN)” by Suzuki et al. in IEEE Transactions on Medical Imaging, Vol. 25 No. 4, April 2006 describes methods for detection of lung nodules and other features using learned results from a database to optimize rib suppression for individual patient images.
The article entitled “Detection and Compensation of Rib Structures in Chest Radiographs for Diagnose Assistance” in Proceedings of SPIE, 3338:774-785 (1998) by Vogelsang et al. describes methods for compensating for rib structures in a radiographic image. Among techniques described in the Vogelsang et al. article are template matching and generation and selection from candidate parabolas for tracing rib edges.
The article entitled “Model based analysis of chest radiographs”, in Proceedings of SPIE 3979, 1040 (2000), also by Vogelsang et al. describes Bezier curve matching to find rib edges in a chest radiograph for alignment of a model and subsequent rib shadow compensation.
While some of these methods may have achieved a level of success using rib edge detection to identify rib structures that can then be suppressed in the x-ray image, improvements are desired.
Thus, there is a need for a method of rib suppression that accurately detects ribs, including clavicles, in chest x-ray images and suppresses the rib area in a chest x-ray image, while preserving the image content of underlying lung tissue.
SUMMARY
At least one embodiment of the present invention is directed to rib suppression in chest x-ray images, while preserving other image content.
Any objects are given only by way of illustrative example, and such objects may be exemplary of one or more embodiments of the invention. Other desirable objectives and advantages inherently achieved by the disclosed invention may occur or become apparent to those skilled in the art. The invention is defined by the appended claims.
According to one aspect of the invention, there is provided a method for rib suppression in a chest x-ray image, the method executed at least in part by a computer system and comprising: detecting and labeling one or more ribs in a region of interest in the x-ray image; detecting rib edges of the one or more detected ribs; generating cross rib profiles along the detected ribs; conditioning the original x-ray image according to at least one of the detected rib edge and cross rib profiles; and displaying the conditioned x-ray image.



BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of the embodiments of the invention, as illustrated in the accompanying drawings. The elements of the drawings are not necessarily to scale relative to each other.
FIG. 1 is a logic flow diagram that shows steps of a procedure for rib suppression according to an embodiment of the present invention.
FIG. 2 is a logic flow diagram that shows processing that is performed in lung segmentation and rib detection.
FIG. 3 is a logic flow diagram that shows iterative processing that is performed for each detected or labeled rib as part of rib edge segmentation.
FIG. 4A shows a section of a rib with an identified portion for generating a rib profile in a chest x-ray image.
FIG. 4B is a schematic diagram that shows how a cross rib profile for a chest x-ray is generated.
FIG. 5 shows an original chest x-ray image prior to processing for rib suppression.
FIG. 6A shows results from rib detection.
FIG. 6B shows results from rib labeling.
FIG. 7 shows labeled ribs overlaid onto the original image of FIG. 5.
FIGS. 8A and 8B show examples of rib growing algorithms.
FIG. 9 shows a chest x-ray image with suppressed rib content, following a subtraction operation.



DETAILED DESCRIPTION OF THE EMBODIMENTS
The following is a detailed description of the preferred embodiments of the invention, reference being made to the drawings in which the same reference numerals identify the same elements of structure in each of the several figures.
Applicants have recognized that improvements are desired for methods using rib edge detection to identify rib structures that can then be suppressed in the x-ray image. For example, to adapt rib detection methods to individual patient images. Methods using template or function-fitting of the detected rib edge have limitations for successfully characterizing large variations in the shape of ribs as well as limitations related to image quality, especially when foreign objects, e.g., tubes/lines and other devices, are captured in ICU portable chest images. Methods such as the MTANN approach described above are not suited to conventional x-ray images, but require dual energy images as part of the training database. Further, the MTANN technique may not be able to accurately estimate the edge of the bone as well as it estimates bone density elsewhere. In addition, non-zero density estimation in non-rib areas can contribute to added noise in these areas, which affects the overall image quality of the rib suppressed images. The detection methods described have proved to be memory-intensive, requiring significant computational resources. Robustness is also desirable.
Applicants have recognized a need for a method of rib suppression that detects ribs, including clavicles, in chest x-ray images and suppresses the rib area in a chest x-ray image, meanwhile preserving the image content of underlying lung tissue.
Conventional rib detection techniques typically first locate rib/line edges, then use rib edge information to identify rib structures that lie between the rib edges. The inventors have found results from this conventional approach to be disappointing, often failing to provide accurate enough information on rib structures for acceptable levels of rib suppression. Embodiments of the present invention address the problem of rib suppression in a different manner, by detecting rib regions first, then, once features of individual rib structures have been identified, more accurately and robustly locating rib edges. This approach allows the complete rib structure to be identified and its affect on image content more accurately profiled than has been achieved using conventional methods.
The logic flow diagram of FIG. 1 shows a sequence for automated rib suppression consistent with an embodiment of the present invention for chest x-ray image processing. The chest x-ray image can originate from a digital radiography (DR) detector or from scanned image data. This image data may also be obtained from an image archive, such as a PACS (picture archiving and communication system). In a lung segmentation process 20, the lung and rib cage portions of the image are segmented, thus extracting the lung region of interest from the image. A number of approaches to lung segmentation have been proposed, including, for example, that described in U.S. Pat. No. 7,085,407 entitled “Detection of Ribcage Boundary from Digital Chest Image” to Ozaki that employs landmark detection and other utilities to detect the boundaries of the rib cage. Other methods for lung detection and segmentation include methods that detect the spine structure and use a bounding box for coarse detection, with subsequent processing for more closely identifying the boundaries of the lung or rib cage. Neural network-based logic can also be employed for generating a pixel-based lung segmentation. Boundary smoothing can also be employed, such as by using morphological filtering or other suitable processing technique, for example.
Continuing with FIG. 1 processing, with the lung region of interest or area including the lungs identified, a rib detection process 30 follows, in which structural information about the rib features is used in conjunction with image pixel intensities to separate likely rib content from non-rib image content. This step helps to eliminate from processing the image content that is not obstructed by rib features and has been found to provide improved results. Further processing of the candidate rib content is executed in a rib labeling step 40 that groups and organizes the detected rib contents. In rib labeling step 40, classification of the rib content groups likely rib pixels into corresponding categories for labeling as part of individual ribs, labels these pixels as part of the rib content of the image, and helps to remove false positives from rib detection process 30. Position, shape information, and gradient are used, for example, to help eliminate false positives. Processing in step 40 provides for classifying pixels into one or more of multiple ribs, by using some amount of prior knowledge of rib structures, such as shape, position, and general direction, and by applying morphological filtering. Among features that have been found to be particularly useful for rib classification are rib width and position, including percentage of pixels initially determined to be part of a rib feature. Other features could similarly be extracted and used for false-positive removal. Rib labeling in labeling step 40 alternately calculates a medial axis for one or more ribs to generate a skeletal image for validating rib detection and for subsequent processing including rib modeling for retrieving missing or missed-labeled ribs or portion of ribs. The skeletal image has medial axis information and, optionally, other anatomical data relevant to rib location.
Characteristics such as gradient orientation and shape for the labeled rib content can then be used for subsequent processing in a rib edge segmentation step 50. In rib edge segmentation step 50, edge portions of the ribs are identified, and this identification is refined using iterative processing. Guided growth processing may alternately be used to enhance rib edge detection. A cross rib profiling step 56 generates a cross rib profile that provides values for rib compensation along the detected ribs. Finally, a rib subtraction step 80 is executed, subtracting rib edges and values from the rib profile from the chest x-ray image, to condition the image and provide a rib-suppressed x-ray image as the conditioned image for display. Other types of conditioning can be used for combining the detected rib information with the original x-ray image to generate a rib-suppressed image for display or for further analysis.
The logic flow diagram of FIG. 2 shows processing that is performed in lung segmentation process 20, rib detection process 30, and labeling process 40, and shows how the results of this processing are used. In an optional scaling step 22, the image can be scaled to a lower resolution in order to speed subsequent processing. An extract ROI step 24 helps to generate position features information for more accurate definition of the region of interest (ROI). An image normalization step 26 then provides normalized information on image features, consistent with multiple images.
Rib detection process 30 determines, for pixels in the region of interest, whether or not each pixel corresponds to a rib feature. Rib detection process 30 has a features computation step 28 that computes features for each pixel, such as providing Gaussian derivative features information and position information, for example. Next, as part of rib detection step 30, a pixel classification step 32 determines whether each pixel within the lung region is a rib or non-rib pixel. Classifier techniques such as artificial neural network, supporting vector machine or random forests that are well known in the art can be used to perform the pixel classification.
In this sequence, labeling process 40 is also shown in more detail. A false positive removal step 42 executes for identifying individual ribs. False-positive pixels are first removed as part of this processing. A subsequent grouping step 36 then determines whether or not one or more groups of detected pixels can themselves be grouped together as one individual rib, based on factors such as positional relationship, connectedness and adjacence, gradient features, and the position relative to the central axis of individual groups. These ribs can be labeled according to rib pattern. Global rib modeling, based on ribs that have already been labeled and known anatomical relationships, can be used to detect a missing rib from the previous steps.
The logic flow diagram of FIG. 3 shows iterative processing that is performed for each detected or labeled rib, after the processing described with respect to FIG. 2, as part of rib edge segmentation 50 (FIG. 1). The input to this processing is the set of labeled ribs. A medial axis extraction step 52 obtains the medial axis of each rib. An initial smoothing step 58 performs any necessary fitting to smooth rib edges, according to the extracted medial axis. As part of smoothing step 58, the smoothed boundaries provide a starting point for more closely approximating rib edges. Using the smoothed rib contour, one or more line segments for the upper or lower rib boundaries are generated as initial rib edge candidates. Next, one or more additional line segment candidates for each segment are generated based on calculated gradients or other features. A set of the best-fit edge candidates for the upper and lower rib edge is selected, using optimization of a model based on factors such as edge gradients, rib width, line segment smoothness, and rib shape constraints.
Continuing with the sequence of FIG. 3, a rib growing step 64 continues the line segment optimization process of modeling step 60 to extend existing line segments and merging disconnected line segments as they are detected or extrapolated from existing segments. A growing algorithm is useful where segments of the ribs are foreshortened or missing. As part of the growing algorithm, existing segments are aligned according to an anatomy model. Segments are iteratively extended and tested to determine whether or not growth is completed. Segment growth can also use edge extension techniques such as those employed for tubing detection and described in commonly assigned, copending U.S. Patent Application No. 2009/0190818 entitled “Computer-Aided Tubing Detection” by Huo.
Repeated iteration of the sequence of steps 58, 60, and 64, as many times as needed, helps to improve the collected rib profiles that are generated and provided in a cross-rib profile generation step 86, so that rib data that is combined with the image data in image conditioning step 92 more accurately characterizes the rib content.
FIGS. 4A and 4B show how a cross rib profile is generated and its relationship to the chest x-ray image. In FIG. 4A, a line 74 shows the basic direction over which the profile is obtained, across the rib in a cross-sectional manner. In FIG. 4B, a rib 70 is shown schematically in cross section, representing a bony shell and a soft interior portion. A profile 72 shows how rib 70 affects image data, with peak values along the edges. X-rays are generally incident in the direction indicated V in this figure.
Profile 72 is generated using known characteristics of the rib in the chest x-ray. One method for providing rib profile 72 is to apply a low-pass filter (LPF) to the chest image and use the results of this processing to provide a cross rib profile, which is known to those skilled in image processing and analysis. An alternate method employs a model to provide an initial approximation or starting point for developing the rib profile. Using information from the model also enables rib profile information to be identified and extracted from the image itself. Whatever method is used, the usefulness of the rib profile depends, in large part, upon accurate detection of rib edges.
The two Vogelsang et al. references cited earlier describe how the cross rib profile can be generated and used. In the article “Model based analysis of chest radiographs”, Vogelsang et al. particularly describe how the cross rib profile is used as a model, and show how six regions for vertical compensation values are identified and interpolation applied using this model.
By way of example, FIG. 5 and following show results of some of the steps of the processing sequence for rib removal according to an embodiment of the present invention. FIG. 5 shows an original chest x-ray image 38 that requires identification and removal of ribs in order to make underlying tissue more visible. FIG. 6A shows an image 44 that shows rib detection. FIG. 6B shows an image 46 following rib labeling that helps to more precisely identify the rib regions. In FIG. 7, an image 62 shows labeled ribs overlaid onto the original image 38.
FIGS. 8A and 8B show an example of rib growing using the overlaid results of FIG. 7. Rib growing algorithms are of particular value for extending the rib curvature along the ends of the rib, where features may be unclear, and help to provide improved edge detection. In an embodiment of the present invention, rib growing algorithms follow the general curvature of a medial axis 94. FIG. 8B shows an example medial axis 94 in dashed line form.
Subtraction or other ways of combining rib edge information with the final image provide a rib suppressed image, as shown in FIG. 9.
Embodiments of the present invention help to provide more accurate detection of rib edges than available using conventional methods, such as shape modeling. In an alternate embodiment of the present invention, only the rib edge profiles are subtracted from the original image to provide rib suppression.
Consistent with one embodiment, the present invention utilizes a computer program with stored instructions that perform on image data that is accessed from an electronic memory. As can be appreciated by those skilled in the image processing arts, a computer program of an embodiment of the present invention can be utilized by a suitable, general-purpose computer system, such as a personal computer or workstation. However, many other types of computer systems can be used to execute the computer program of the present invention, including an arrangement of networked processors, for example. The computer program for performing the method of the present invention may be stored in a computer readable storage medium. This medium may comprise, for example; magnetic storage media such as a magnetic disk such as a hard drive or removable device or magnetic tape; optical storage media such as an optical disc, optical tape, or machine readable optical encoding; solid state electronic storage devices such as random access memory (RAM), or read only memory (ROM); or any other physical device or medium employed to store a computer program. The computer program for performing the method of the present invention may also be stored on computer readable storage medium that is connected to the image processor by way of the internet or other network or communication medium. Those skilled in the art will further readily recognize that the equivalent of such a computer program product may also be constructed in hardware.
It is noted that the term “memory”, equivalent to “computer-accessible memory” in the context of the present disclosure, can refer to any type of temporary or more enduring data storage workspace used for storing and operating upon image data and accessible to a computer system, including a database. The memory could be non-volatile, using, for example, a long-term storage medium such as magnetic or optical storage. Alternately, the memory could be of a more volatile nature, using an electronic circuit, such as random-access memory (RAM) that is used as a temporary buffer or workspace by a microprocessor or other control logic processor device. Display data, for example, is typically stored in a temporary storage buffer that is directly associated with a display device and is periodically refreshed as needed in order to provide displayed data. This temporary storage buffer can also be considered to be a memory, as the term is used in the present disclosure. Memory is also used as the data workspace for executing and storing intermediate and final results of calculations and other processing. Computer-accessible memory can be volatile, non-volatile, or a hybrid combination of volatile and non-volatile types.
It is understood that the computer program product of the present invention may make use of various image manipulation algorithms and processes that are well known. It will be further understood that the computer program product embodiment of the present invention may embody algorithms and processes not specifically shown or described herein that are useful for implementation. Such algorithms and processes may include conventional utilities that are within the ordinary skill of the image processing arts. Additional aspects of such algorithms and systems, and hardware and/or software for producing and otherwise processing the images or co-operating with the computer program product of the present invention, are not specifically shown or described herein and may be selected from such algorithms, systems, hardware, components and elements known in the art.
It is noted that there can be any of a number of methods used for functions such as segmentation of ribs from other tissue in the chest x-ray image or for filtering portions of the image content.
The invention has been described in detail with particular reference to a presently preferred embodiment, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.

## Claims

1. A method for rib suppression in a chest x-ray image, comprising:
detecting one or more ribs in a region of interest in the chest x-ray image;
applying a pixel classification to the one or more detected ribs in the region of interest;
using the pixel classification, labeling the one or more detected ribs in the region of interest;
detecting rib edges of the one or more detected ribs;
generating cross rib profiles along the detected rib edges;
conditioning the chest x-ray image according to at least one of the detected rib edges and cross rib profiles; and
storing, displaying, or transmitting the conditioned chest x-ray image.

2. The method of claim 1 wherein labeling the one or more ribs comprises segmenting a lung region of interest from the image.

3. The method of claim 1
wherein labeling the one or more ribs comprises extracting a medial axis from at least one of the one or more ribs.

4. The method of claim 1 further comprising adjusting segments of one or more of the detected rib edges to improve edge fitting.

5. The method of claim 1 further comprising scaling the image to a reduced resolution.

6. The method of claim 2 wherein labeling the one or more ribs in the image comprises classifying pixels within the lung region of interest as rib pixels or non-rib pixels.

7. The method of claim 1 wherein detecting the rib edges further comprises applying a model.

8. The method of claim 4 wherein adjusting segments comprises applying a growing or extending algorithm for rib edges.

9. The method of claim 3 further comprising obtaining a skeletal image comprising one or more of the medial axes.

10. The method of claim 1 further comprising smoothing the detected rib edges before generating cross rib profiles along the detected rib edges.

11. A method for rib suppression in a chest x-ray image, comprising:
segmenting one or more lung regions in the chest image;
determining one or more rib features for each pixel within the one or more segmented lung regions;
extracting a medial axis from at least one of the one or more determined rib features and iteratively segmenting one or more rib edges from the at least one or more rib features according to the extracted medial axis and according to an anatomical model;
forming a conditioned image by combining the one or more segmented rib edges with the original x-ray image; and
storing, displaying, or transmitting the conditioned image.

12. The method of claim 11 further comprising scaling the image to a reduced resolution.

13. The method of claim 11 wherein determining one or more rib features in the image comprises classification of pixels within the lung region of interest as rib pixels or non-rib pixels.

14. The method of claim 11 further comprising extending the one or more segmented rib edges by applying a growing algorithm.

15. The method of claim 11 wherein forming the conditioned image further comprises applying a filter to the image data.

16. The method of claim 11 wherein determining the one or more rib features further comprises grouping adjacent pixels associated with particular rib features.

17. A method for rib detection in a chest x-ray image, executed at least in part by a computer system, comprising:
detecting pixels for one or more ribs in a region of interest;
labeling one or more of the detected ribs as individual ribs by grouping the pixels to generate detected ribs and removing one or more false positive pixels from the detected pixels;
detecting one or more rib edges of the detected ribs by adjusting one or more edge segments; and
storing, displaying, or transmitting the detected results.

18. The method of claim 17 wherein detecting one or more rib edges further comprises detecting at least one medial axis for each rib and further comprises curve fitting.

19. The method of claim 17 wherein detecting one or more rib edges further comprises optimizing one or more line segments.

20. The method of claim 17 further comprising enhancing the one or more rib edges by applying a growth algorithm.