METHOD AND SYSTEM FOR CALIBRINGING AN X-RAY IMAGE SYSTEM

Description

[0001] The invention relates to a method and a system for determining, during operation, the geometric characteristics of a three-dimensional (3D) image reconstruction system from images acquired with an X-ray imaging system. The invention thus allows for online calibration of an X-ray imaging system.

[0002] Geometric features are estimated during rotational acquisition of a C-arm type device.

[0003] It is common to use a mobile radiology system for surgical or interventional procedures. These systems, also called mobile C-arms (or block amplifiers), allow the surgeon to acquire X-ray images during the procedure and to monitor the positioning of the instruments used (catheters, needles, prostheses, etc.) in real time in a minimally invasive manner. Most of these systems provide two-dimensional images with a video stream of thirty frames per second. The practitioner then uses these images to perform a mental reconstruction of the patient's anatomy in order to precisely position the instrument in the area to be operated on, in real time. More recently, more sophisticated systems have emerged that allow for the acquisition of a 3D image of the instrument used by a surgeon during a procedure.The radiology system rotates around the patient to acquire a set of two-dimensional (2D) images. These 2D images are then processed by a reconstruction algorithm to generate a 3D volumetric image. To reconstruct the image, the algorithm needs to know, for each 2D image, the exact geometry of the C-arm, namely the position of the detector and the X-ray source relative to the patient. Current systems offer offline calibration of the C-arm during preventive maintenance, typically every six months to one year.

[0004] US patent 6510241 describes a method for calibrating a radiology device, in which a virtual volume surrounding the object to be imaged is generated and decomposed into voxels (3D pixels). The method includes a step of acquiring the set of numbered projected two-dimensional images, and a reconstruction of the three-dimensional image from the projected image is performed.

[0005] US patent 6320928 describes an image reconstruction method in which multiple two-dimensional digital images of an object are acquired from different camera positions as the camera rotates around the object. The projected images are calibrated within a volume containing the object and divided into voxels whose spatial coordinates are identified within a chosen calibration reference frame.

[0006] US patent 6049582 relates to a C-arm device calibration method for 3D reconstruction in an imaging system comprising an imaging source and an imaging plane that uses a planar transformation to link voxels in a voxel space and pixels in the imaging plane.

[0007] Patent application EP3141187 relates to a calibration target for geometrically calibrating an X-ray imaging device designed to generate three-dimensional images of an object by reconstructing two-dimensional projections of said object. The calibration target comprises a volumetric support equipped with markers exhibiting radiological absorption in contrast to the volumetric support, the markers being arranged in a three-dimensional pattern. The markers are distributed into subsets along substantially parallel lines, such that cross-ratio sequences can be constructed from the respective marker subsets.Each cross-ratio sequence includes a unique cross-ratio for each set of four markers in which the markers are ordered according to an order dependent on the order numbers of the respective markers along the line along which they are aligned, according to a first predefined direction, said order being common to all cross-ratios, and where a subset of markers includes at least five markers, the order of cross-ratios in the respective cross-ratio sequences follows a predefined rule common to all cross-ratio sequences.

[0008] These systems assume that rotational acquisition is sufficiently reproducible so that the C-arm geometry determined "offline" is applicable to images acquired during a surgical procedure.

[0009] The mechanics of the systems have been improved to make the C-arm stable during the rotational acquisition of 2D images. However, these improvements (reduced mechanical play, use of more rigid parts, etc.) lead to more expensive devices. Furthermore, it is not always straightforward to make these modifications to existing devices.

[0010] Other methods from the prior art offer online calibration.

[0011] One method is based on the use of markers. A calibration target is positioned on or next to a patient during the procedure. This allows for precise, online estimation of the device's geometry without concern for the reproducibility of measurement conditions. US patent application 201000284601 describes such a method.

[0012] The document by Madan Hennadii et al., entitled "Device and methods for gold standard registration of clinical 3D and 2D cerebral angiograms," published in Progress in Biomedical Optics and Imaging, SPIE, XP 060051334, discloses a calibration method in the field of angiography for constructing a "gold standard." The calibration step uses a group of markers arranged in a band positioned at the level of the patient's head.

[0013] However, known prior art methods that use a target are not optimal for surgical use or other applications with equivalent usage constraints, for the reasons set out below: The target must be manufactured with precision so that the 3D position of the points is known accurately, which represents a cost. The target itself can be bulky and difficult to use when the patient is present. The target must have been sterilized, as it is used in a sterile environment, and undergo chemical / thermal treatment before and after its use.

[0014] A second type of image reconstruction method is based on the use of an image. These methods exploit the anatomical content of an image to perform both 3D image reconstruction and geometric calibration.

[0015] In patent EP2868277, the method uses markers; however, it is necessary that the 3D positions of the markers be known precisely to determine the geometric parameters. Summary of the invention

[0016] The invention is based on a new approach using self-adhesive radio-opaque markers without needing to know the 3D position of the markers.

[0017] The invention relates to a method for calculating during operation the geometric parameters of an X-ray imaging system according to claim 1 and a corresponding device according to claim 11.

[0018] The process may include an offline calibration step to initially calculate the calibration matrices used in the final step of determining the geometry parameters.

[0019] In another variant, the initial projection matrices are calculated by exploiting the system's orientation sensors or positioning sensors.

[0020] The markers can be inserted into or contained within patches positioned on or near the patient or object, and the patches used are, for example, adhesive patches defined as follows: An adhesive strip that will be applied to an object or patient, A set of radio-opaque markers distributed over the surface of the patch, An outer surface resistant to a fluid.

[0021] It is possible to distribute the markers within a patch in order to cover the entire surface of the patch.

[0022] Another possibility is to use small markers distributed across the entire compression garment before covering the area to be reconstructed.

[0023] Another variant involves using one or more anatomical markers or using radio-opaque markers implanted in the patient's anatomy, for example in the bone.

[0024] The process may include a step exploiting the geometric characteristics of the system to reconstruct a 3D image.

[0025] Other features and advantages of the present invention will become clearer upon reading the description of illustrative and non-limiting examples of embodiments, accompanied by figures which represent: Figure 1 a diagram representing a patient in position, Figure 2 , an example of a patch placed on the patient, and Figure 3 , a succession of steps implemented by the process according to the invention.

[0026] There figure 1 This illustrates an example of a device that allows a practitioner to monitor the position of a tool used during a surgical procedure in real time. The device includes an imaging system comprising an X-ray source, 10, and an X-ray detector, 11. The device consists of a C-arm, 12, supporting the X-ray source at one end, 121, and the X-ray detector at the other end, 122. The C-arm, 12, is held on a frame, 13, by a retaining piece, 14.

[0027] A horizontal guide 15, fixed to the frame 13 via a vertical piece 16 and to the support piece 14 of the hoop, allows for a horizontal translational movement of the hoop, arrow H .

[0028] The retaining piece 14 allows the arched arm 12 to perform an "orbital rotation" type movement, according to the arrow R .

[0029] The rotation of part 14 and the hoop results in an angular rotation along the arrow A .

[0030] The vertical movement is ensured by the horizontal translation of the guide and the vertical part.

[0031] The joints used between the various components of the system enabling the aforementioned rotational movements are known to those skilled in the art and will not be described in detail. Similarly, the aforementioned movements of the radiology system are known to those skilled in the art.

[0032] The device also includes a processing unit 17 comprising a processor 18 adapted to perform the steps of the process according to the invention, in order to determine the geometric characteristics of the device during the surgeon's intervention. The device may include a screen 19 on which the surgeon can view the position of the tool in real time.

[0033] The device is also equipped with orientation sensors 22 or positioning sensors 23.

[0034] Patient 20 is positioned on an operating table 21. The figure 2 diagram an example of implementation of the process where the markers are incorporated or integrated into a patch.

[0035] Patch 30 is positioned in the upper part of the body and includes at least one marker q. The coordinates of the patch or the position of the patches are not initially known. It is possible to use one or more patches to implement the method according to the invention.

[0036] There figure 3 illustrates an example of the sequence of steps implemented by the process according to the invention, in the case where the markers are integrated into a patch. Etape 1

[0037] One or more patches 30 incorporating radio-opaque markers are applied to the skin of the patient or object 20, in the vicinity, for example, of an area to be operated on, 31. This will allow the practitioner to obtain with precision a reconstruction of the organ that he has to operate on. Etape 2

[0038] Several 2D images are acquired from different viewpoints to perform a 3D reconstruction. The X-ray source and detector are moved around the body to be imaged to create multiple projections of the body from different angles. These projections are then used to reconstruct a three-dimensional image of the body. Etape 3

[0039] The radio-opaque markers contained in patch 30 are detected in each 2D image acquired at the X-ray detector and paired, from one image to another, 33. Geometric or radiometric similarity criteria are used to perform the matching of the markers. Etape 4

[0040] A 3D reconstruction of the markers is performed using an initial estimation of the projection matrices, 34. These projection matrices Mi can be determined during prior offline calibration or predicted from the system's position sensors. These 4*3 projection matrices allow each point of the object or patient in 3D space, for example relative to the Earth's reference frame, to be mapped onto a 2D plane detector linked to the detector.

[0041] At the end of this matching step, a first estimate of the 3D position of the markers is obtained. Etape 5

[0042] The 3D position of the markers and the knowledge of the projection matrices are then refined iteratively, 35. The geometric parameters, namely the projection matrices and the 3D position of the markers, are jointly estimated by minimizing the criterion presented below.

[0043] Consider a set of N projections, and therefore N matrices, to be determined. Consider a set of L points to be reconstructed; the criterion is given by: X , M = argmin ∑ i = 1 N ∑ j = 1 L M i X j − q ij Or q ij denotes the 2D coordinates of a marker of number j detected in an image i obtained by the system, X is the set of L 3D points to be reconstructed, X j the point of number j, M is the set of N projection matrices, M i the projection matrix of image i.

[0044] At the end of this step 35, it is possible to accurately reconstruct the corresponding 3D image.

[0045] The 3D images thus obtained can be used to allow a practitioner to precisely position their tools during the operation, 36.

[0046] The general principle of beam fitting methods is described in the document entitled "Bundle adjustment - a modern synthesis" by B.trigs, PF Mc Lauchlan, RI Hartley, International Workshop on Vision Algorithms, Corfu, Greece, September 21-22, 1999 Proceedings.

[0047] Any other algorithm, taking as input the coordinates of a marker, determined by the execution of the process to deduce the geometric parameters of the device, may be used.

[0048] According to one embodiment, the process includes a preliminary offline calibration step leading to an imprecise geometry of the C-arm. The calibration matrices resulting from the "offline" calibration are used in the fourth step to perform the first reconstruction.

[0049] The markers used at the patch level are, for example, spherical markers to facilitate detection. They can also have shapes exhibiting rotational symmetry around axes of rotational symmetry.

[0050] The patch(s) containing the markers can be stuck directly onto the patient or positioned near the area to be imaged.

[0051] In the case of adhesive patches, it is possible to use a patch defined as follows: An adhesive strip that will be applied to the skin of a patient or an object before the acquisition of 2D images used for 3D image reconstruction; a set of radiopaque markers distributed across the surface of the patch; and an outer surface resistant to water, blood, and friction, protecting the patch from its environment. The outer surface is, for example, made of plastic.

[0052] Self-adhesive patches can be for single use.

[0053] For example, the markers are distributed in such a way as to cover the entire surface of the patch.

[0054] The patch and marker set, for example, has a thickness of approximately 1mm and an approximate size of 4×14 cm.

[0055] The markers can be integrated into a stretchable fabric, or "medical stretch suit." Small markers, such as opaque beads, are distributed across the entire compression garment before covering the area to be reconstructed, for example, a part of the patient. It is then possible to simultaneously calibrate the device using the information obtained during the fourth step and reconstruct the 3D surface of the object to be reconstructed. This surface will serve as a baseline for the 3D reconstruction.

[0056] According to one embodiment, the method uses one or more anatomical markers (characteristic radiopaque parts of the human body) that correspond to points of interest present in an image. The markers are extracted from the images using image processing techniques known to those skilled in the art. In this embodiment, the method omits step 31, patch placement. The first step consists of acquiring X-ray images.

[0057] In some cases, it will be possible to combine markers contained in patches and anatomical markers, the latter being able to be implanted in the patient's body, for example in a bone.

[0058] The method according to the invention also makes it possible to calibrate a C-arm device using one or more markers, the position of these markers not being initially known.

Claims

1. Method for calculating during use the geometric parameters of an x-ray imaging system, an object or a patient (20) to be observed being placed between the x-ray source and a detector of x-rays having passed through the object or patient, the object comprising a plurality of radiopaque markers, the method comprising at least the following steps, for each marker Xj of the object or patient: • Using the marker Xj of initially unknown 3D position, • Acquiring a plurality of 2D images for a plurality of viewpoints of the imaging system (32), • Detecting the position qij of said marker Xj in each of the acquired 2D images i (33), criteria of geometric or radiometric similarities being used to match each marker Xj of one image to another, and • Estimating the projection matrices Mi of the images i corresponding to the projections of the object at various viewing angles and reconstructing in 3D the position of said marker on the basis of the estimation of the projection matrices (34), the method further comprising a step (35) of jointly iteratively estimating from X, all of the 3D positions of the markers and from M, all of the N projection matrices, by minimising a criterion, the criterion being: X , M = argmin ∑ i = 1 N ∑ i = 1 L M i X j − q ij the method comprising a reconstruction in 3D of the image of the object or patient, in response to the end of said iterative estimation step (35).

2. Method according to claim 1, characterised in that it comprises an off-line calibration step, in order to calculate the initial projection matrices.

3. Method according to claim 1, characterised in that the initial projection matrices are calculating by utilising the system orientation or positioning sensors.

4. Method according to any one of claims 1 to 3, characterised in that the markers are contained in an adhesive patch positioned on or in the proximity of the patient or object, defined as follows: • an adhesive strip, • a set of radiopaque markers distributed over the surface of the patch, • an outer surface resistant to a fluid.

5. Method according to any one of claims 1 to 3, characterised in that the markers are distributed at a patch, in order to cover the entire surface of the patch.

6. Method according to any one of claims 1 to 4, characterised in that markers integrated in a stretch fabric are used.

7. Method according to any one of claims 1 to 5, characterised in that small markers are used, distributed over the entirety of a compressive piece of clothing before covering a part of a patient to be reconstructed.

8. Method according to any one of claims 1 to 3, characterised in that at least one anatomic marker is used.

9. Method according to any one of claims 1 to 3, characterised in that at least one radiopaque marker is used, implanted in the anatomy of the patient, such as in the bone.

10. Method according to any one of claims 1 to 9, characterised in that it comprises a step utilising the geometric features of the system to reconstruct an image in 3D.

11. Device for calculating during use the geometric parameters of an x-ray imaging system, an object or a patient (20) to be observed being placed between the x-ray source and a detector of x-rays having passed through the object or patient, the device further comprising at least one processing device (17) comprising a processor (18) adapted to executing the steps of the method according to any one of claims 1 to 10.