DEFORMITY-WEIGHTED REGISTRATION OF MEDICAL IMAGES

Abstract

Disclosed is a computer-implemented method of determining a spatial relationship between planning image data and current surface data which leads to improved surface registration accuracy by considering the elasticity and deformability of the tissue. The knowledge about the tissue can be estimated based on type of tissue and atlas information. For the process of generating surface registration points on specific anatomical regions, e.g. the face or forehead, are acquired with a classical navigated pointer or laser pointer. It is also possible to acquire points with surface scanners. Confidence values defining a probability for certain parts of the surface registration points being deformed in comparison to a planning image are read from atlas data and used to compensate for the deformation in the registration between the surface registration points and the planning image in order to render the registration valid.

Claims

1. A computer-implemented medical method of determining a spatial relationship between planning image data and current surface data, the method comprising the following steps: acquiring planning image data which describes a three-dimensional medical image of an anatomical body part having an external surface; acquiring current surface data which describes a current surface scan describing the external surface; acquiring atlas data which describes a model of the external surface and confidence values for elements of the external surface which describe a probability for the geometry of the external surface to be subject to a change; determining confidence assignment data based on the atlas data and the current surface data, wherein the confidence assignment data describes an assignment of the confidence values for elements of the external surface described by the atlas data to the corresponding elements of the external surface described by the current surface scan; determining registration data based on the planning image data and the current surface data and the confidence assignment data, wherein the registration data describes a spatial relationship between the external surface described by the planning image data and the external surface described by the current surface data, and further wherein the spatial relationship is weighted for the elements of the external surface according to the confidence value assigned to the respective element.

2. The method according to claim 1, wherein the change is due to at least one of positioning a patient, gravity, swelling of tissue included in the external surface, or elastic deformation of tissue included in the external surface.

3. The method according to claim 1, wherein the probability depends on whether the geometry of the external surface is at least substantially defined by the hardness of tissue included in or lying closely below the external surface.

4. The method according to claim 3, wherein the probability is higher the lower the hardness of the tissue included in or lying closely below the external surface.

5. The method according to claim 1, wherein the confidence values depend on a time interval lying in between the point in in time at which the registration data has been determined and the point in time at which the anatomical body part will be re-positioned, or the confidence values depend on an imaging modality used to generate the current surface data.

6. The method according to claim 1, wherein the confidence values have been determined by at least one of the following: determining a typical deformation of the external surface determined from a plurality of comparisons of planning image data with current surface data acquired for different patients; using pre-defined confidence values derived from expert knowledge as the confidence values; using a physical property of tissue included in or lying closely below the external surface as a basis for computing the confidence values.

7. The method according to claim 1, wherein the confidence assignment data is determined by determining preliminary confidence assignment data based on the atlas data and the planning image data, wherein the preliminary confidence assignment data describes an assignment of the confidence values for elements of the external surface described by the atlas data to the corresponding elements of the external surface described by the planning surface scan, and wherein the registration data is determined by establishing the spatial relationship between the external surface described by the planning image data and the external surface described by the current surface data and applying the confidence values to the spatial relationship for the respective element of the external surface to which the confidence value is assigned and for which the spatial relationship has been or is being established, whereby the spatial relationship is weighted for that element by the confidence value assigned to that element.

8. The method according to claim 1, wherein the atlas data describes a direction and a distance in which the elements of the external surface move when the geometry of the external surface is subject to the change, and wherein the spatial relationship is determined by compensating for that movement.

9. The method according to claim 1, wherein the spatial relationship between the external surface described by the planning image data and the external surface described by the current surface data is determined by computing a norm between each point defining the current surface scan and the external surface described by the planning image data, or by applying an iterative closest point algorithm to the planning image data and the current surface data.

10. The method according to claim 1, wherein the planning image data has been generated by applying magnetic resonance tomography imaging or computed x-ray tomography imaging to the anatomical body part.

11. The method according to claim 1, wherein the current surface data has been generated by acquiring positional information using a tracked pointing instrument to describing the position of the external surface, or applying a surface scanning imaging modality including use of at least one of a range camera, thermographic camera, structured light camera or time-of-flight camera.

12. A computer implemented method which, when running on at least one processor on at least one computer, causes the at least one processor to: acquire planning image data which describes a three-dimensional medical image of an anatomical body part having an external surface; acquire current surface data which describes a current surface scan describing the external surface; acquire atlas data which describes a model of the external surface and confidence values for elements of the external surface which describe a probability for the geometry of the external surface to be subject to a change; determine confidence assignment data based on the atlas data and the current surface data, wherein the confidence assignment data describes an assignment of the confidence values for elements of the external surface described by the atlas data to the corresponding elements of the external surface described by the current surface scan; determine registration data based on the planning image data and the current surface data and the confidence assignment data, wherein the registration data describes a spatial relationship between the external surface described by the planning image data and the external surface described by the current surface data, and further wherein the spatial relationship is weighted for the elements of the external surface according to the confidence value assigned to the respective element.

13. A medical system, comprising: at least one processor on at least one computer which when executing instructions, causes the at least one processor to: acquire planning image data which describes a three-dimensional medical image of an anatomical body part having an external surface; acquire current surface data which describes a current surface scan describing the external surface; acquire atlas data which describes a model of the external surface and confidence values for elements of the external surface which describe a probability for the geometry of the external surface to be subject to a change; determine confidence assignment data based on the atlas data and the current surface data, wherein the confidence assignment data describes an assignment of the confidence values for elements of the external surface described by the atlas data to the corresponding elements of the external surface described by the current surface scan; determine registration data based on the planning image data and the current surface data and the confidence assignment data, wherein the registration data describes a spatial relationship between the external surface described by the planning image data and the external surface described by the current surface data, and further wherein the spatial relationship is weighted for the elements of the external surface according to the confidence value assigned to the respective element. at least one electronic data storage device storing at least planning surface data and atlas data; and a surface acquiring device for acquiring the current surface data, wherein the at least one computer is operably coupled to the at least one electronic data storage device for acquiring, from the at least one electronic data storage device, at least the planning image data and the atlas data, and the surface acquiring device for acquiring, from the surface acquiring device, the current surface data.

14. (canceled)

15. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In the following, the invention is described with reference to the appended figures which give background explanations and represent specific embodiments of the invention. The scope of the invention is however not limited to the specific features disclosed in the context of the figures, wherein

[0053] FIG. 1 illustrates the basic flow of the method according to the first aspect; and

[0054] FIG. 2 is a schematic illustration of the system according to the fifth aspect.

DESCRIPTION OF EMBODIMENTS

[0055] FIG. 1 illustrates the basic steps of the method according to the first aspect, in which step S11 encompasses acquisition of the planning image data, step S12 encompasses acquisition of the current surface data and subsequent step S13 encompasses acquisition of the atlas data. These data sets serve as input to the subsequent steps S14 of determining the confidence assignment data and S15 of determining the registration data.

[0056] The disclosed method uses anatomical knowledge for point accuracy estimation weighting. Points acquired on harder regions get higher confidence than points on softer regions. During a surface registration of a face, the swelling of the soft tissue areas (e.g. the cheeks) can lead to a tilted registration. This can be reduced or even avoided with the weighting of points with a confidence value for a specific region. Additionally the number of points versus confidence value need to be considered: A single point acquired on the front teeth can be more accurate than 20 points on a potentially swollen cheek.

[0057] This disclosure therefore proposes a multi-step registration:

[0058] 1. Rough pre-registration in the following two steps: [0059] a. The surface of the anatomical structure is scanned [0060] b. The surface is registered to a preoperative three-dimensional scan

[0061] 2. Using atlas information for anatomical area confidence value point weighting in the following three steps: [0062] a. The surface is registered to atlas information. [0063] b. In the atlas, the body regions are labelled with confidence values which define how elastic the tissue is, how sensitive the region is to swelling, how sensitive the region is to tissue shifting etc. [0064] c. The atlas confidence values are transferred to the 3D surface point cloud.

[0065] 3. The registration is adjusted by the following two steps: [0066] a. According to the added confidence values, every point on the surface is weighted. [0067] b. The registration is adapted based on the weighted point cloud.

[0068] FIG. 6 is a schematic illustration of the medical system 1 according to the fifth aspect. The system is in its entirety identified by reference sign 1 and comprises a computer 2, an electronic data storage device (such as a hard disc) 3 for storing at least the patient data and a surface detecting device 4. The components of the medical system 1 have the functionalities and properties explained above with regard to the fifth aspect of this disclosure.