Method and System for Multi-Label Image Segmentation

Abstract

The invention is a computer-method that pertains to the segmentation of images. The invention extends the Sim Cut model for image segmentation that allows for the segmentation of images into two parts to allow for the segmentation of images into an indefinite number of parts.

Description

FIELD OF THE INVENTION

The disclosure relates to segmenting digital images, and more particularly to segmenting digital images in the case of more than two labels.

BACKGROUND OF THE INVENTION

A method and system has been invented that allow for image segmentation in the case of more than two labels using a novel graph cut model.

The method is based on graphs that have the form of undirected flow networks. The flow network is an undirected weighted graph, G is defined as a set of nodes or vertices V, a set of edges E and a positive weight function, w associated with the edges:

G=(V, E) and w:E→R₊  (1)

where each edge, {p, q} is defined as a pair of nodes p, q∈V. Additionally, a subset of the nodes, L are “label” nodes.

A graph cut, C is defined as a subset of the edges in the graph such that when those edges are removed from the graph the remaining graph consists of |L| connected components and each connected component contains one node from the set L

A minimum-weight graph cut or minimum cut is a graph cut that has a minimum weight of all the possible graph cuts:

$\begin{array}{cc}{C}_{\min }=\underset{C\in \chi }{\arg \min }\sum _{\left\{p,q\right\}\in C}w_{\mathrm{pq}}& \left(2\right)\end{array}$

where χ is the set of all possible graph cuts.

An exact solution for the graph cut can be obtained for the case with binary labels. Methods for obtaining this solution are based on establishing a flow through the graph. All flows are allowed that conserve flow at each node and, for each edge in the graph, are less than the weight of the edge. Another approach is to establish flow through electrical network models. In these methods, an electrical network is defined using resistors that represent the weights at each edge in the graph. A competitive algorithm for obtaining the graph cut has arisen based on the electrical model. A novel methodology is presented here that generalizes prior methods to allow for image segmentation in the case of more than two labels.

SUMMARY OF THE INVENTION

A computer-implemented method for image segmentation is disclosed, the method comprising acquiring a digital image; defining a set of image vertices from the digital image; defining a set of label vertices; constructing a graph from said image vertices and label vertices wherein said graph; calculating a non-negative cost function; constructing an electrical network based upon the constructed graph; simulating the electrical network one time for each label; and segmenting the digital image based upon the voltages obtained from the simulations of the electrical network.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 schematically shows a flowchart of appearance-based image segmentation, in accordance with an embodiment the present disclosure;

FIG. 2 is an exemplary medical image before applying appearance-based image segmentation disclosed herein, in accordance with an embodiment the present disclosure;

FIG. 3 is an exemplary medical image after applying appearance-based image segmentation disclosed herein, in accordance with an embodiment the present disclosure;

FIG. 4 schematically shows a flowchart of depth-based image segmentation, in accordance with an embodiment the present disclosure;

FIG. 5 is an exemplary gray-scale verseion of a photograph used in depth-based image segmentation before applying the image segmentation disclosed herein, in accordance with an embodiment the present disclosure;

FIG. 6 is an exemplary photograph used in depth-based image segmentation after applying the image segmentation disclosed herein, in accordance with an embodiment the present disclosure;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Various embodiments of the present invention will be described in detail with reference to the drawings. Reference to various embodiments does not limit the scope of the invention, which is limited only by the scope of the claims attached hereto. Additionally, any examples set forth in this specification are not intended to be limiting and merely set forth some of the many possible embodiments for the claimed invention.

Referring now to the drawings, wherein the depictions are for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same, FIG. 1 schematically shows a flowchart of a segmentation algorithm according to an exemplary embodiment. The algorithm begins at step 105 by acquiring an n-dimensional digital image from an imaging scanner. For the purposes of this application, an image is any scalar or vector function on n-dimensional coordinates. The image may have two or a higher number of dimensions. The digital image may be provided by any one of a number of known imaging devices including medical-based imaging device such as a magnetic resonance image, a computed tomography image, an optical coherence tomography image, or an ultrasound-originating image. It is to be appreciated that imaging, for use in embodiments of the present disclosure, can be achieved utilizing traditional scanners or any other image acquisition technique as would be appreciated by those having ordinary skill in the art.

A graph is constructed at step 100 from the image or, in one embodiment, a series of images. Each pixel i.e., point, of said image is associated with a vertex of said graph and said graph includes an edge connecting each pair of vertices corresponding to adjacent points in said image. The graph may be defined as G=(V, E, w) with V representing a set of vertices of a graph, and E representing a set of edges of a graph such that every pixel u or v has a corresponding vertex, and w is a set of non-negative values for the weight of each edge in the graph. A set L is defined representing labels. Image segmentation will consist of the assignment of one of these labels to each point in the image.

In one embodiment, optional limited segmentation may be performed by a user-operator 110. The optional limited segmentation, or initial segmentation, may be outputted to the graph construction 120. In one embodiment, optional limited segmentation may be performed by assigning seeds, either interactively or automatically by acquiring landmarks that belong to each partition of the image.

An electrical network will be constructed based on the graph 130. The electrical network is a network of resistors with each resistor representing one of the edges in the graph with the same connectivity as the corresponding edges in the graph. The electrical behavior of the resistors is non-linear and is given by the relation:

$\begin{array}{cc}{i}_{\mathrm{pq}}=\frac{w_{\mathrm{pq}}}{1+x_p-x_q}\left(x_p-x_q\right)& \left(3\right)\end{array}$

where i and x are vectors with |L| components. The vector i is analogous to electrical current and the vector x is analogous to electrical voltage.

Each of the components of the current and voltage vectors are associated with a label in the graph. The relationship between labels and the voltage and current vectors is given by a map θ:

θ:L→{u₁, u₂, . . . u_|L|)}  (4)

where u₁, u₂ . . . are vectors R^|V|. For each label, the vector is 1 for the component whose index is that of the node in the graph that is the label. All other components of the vectors are zero.

Input to the electrical network is applied at the nodes in the electrical network corresponding to the nodes in the graph in the set L. A vector-valued voltage is applied to each of the input nodes with the vector direction given by θ:

x _l =v _inθ(l)   (5)

where v_in is a large number and l∈L

Constraints are used to obtain a system of equations for the voltage. At each node, p∈V\L the net electrical current is constrained to be zero.

$\begin{array}{cc}\sum _{q\in N_p}i_{\mathrm{pq}}=0& \left(6\right)\end{array}$

where N_p is the set of nodes in the electrical network that are directly connected to the node p

The systems of equations from (5) and (6) can be written in matrix form:

A(X)X=B   (7)

where A(X)X represents a factorization of the non-constant terms in the system of equations. X is a |V|×|L| matrix. Each row of X represents the unknown voltage vector at one of the nodes in electrical network, x_p. A(X) is a |V|×|V| matrix and B is a |V|×|L| matrix and represents the constant terms in the system of equations.

The system of equations is solved using the fixed point method:

A({tilde over (X)}_k){tilde over (X)}_k+1=B   (8)

where {tilde over (X)} is an approximate solution and k is the iteration. The fixed point solution is initialized with the zero matrix {tilde over (X)}₀=0

Partition of the graph and the image 140 is represented by a map, S, from the set of nodes, V, to the indices of the labels, L. The partition is obtained by selecting the label at each node that corresponds to largest component of the voltage at that node.

S={(j, s_j)|s_j=argmax({tilde over (x)}_i^j), j∈V}  (9)

where s_j is the index of the label assigned to the j^th node of the graph. {tilde over (x)}_i^j is the voltage vector at the j^th node after the i^th iteration.

In one embodiment, the invention is applied to the segmentation of an image of the heart using computed tomography. The computed tomography is shown in FIG. 2. A graph was constructed from the image:

G _CT=(V_CT, E_CT) and w_CT:E→R₊  (10)

where V_CT=P_CT∪L_CT where P_CT is the set of pixels in the image and L_CT is a set of 10 labels. Each node in the image has a value, f assigned to it. For pixel nodes, the value of f is the image intensity of the corresponding pixel. For label nodes, the value of f is the class intensity of the label. The class intensity is a value in the set {−100, −50, 0, . . . 350}. For each edge, e_pq, the difference in the intensity function f is given by a function δ_pq:

δ_pq=|f(p)−f(q)|  (11)

The set of edges in the graph includes edges that produce a regularization effect, E_CT^N and edges that relate the pixel nodes to the labels E_CT^L:

E _CT =E _CT ^N ∪E _CT ^L   (12)

E_CTN includes edges between all pairs of adjacent pixels in the image while E_CTL includes edges between all pixel nodes in the image and all label nodes.

The weight of the edges is given by the function:

$\begin{array}{cc}{w}_{\mathrm{CT}}\left(e_{\mathrm{pq}}\right)=\{\begin{array}{cc}\frac{2.0}{1.0+{\delta }_{\mathrm{pq}}^2},& \mathrm{if}e_{\mathrm{pq}}\in E_{\mathrm{CT}}^N\\ \frac{1.0}{1.0+{\delta }_{\mathrm{pq}}^2},& \mathrm{if}e_{\mathrm{pq}}\in E_{\mathrm{CT}}^L\end{array}& \left(13\right)\end{array}$

For construction of the graph, edges were included based on the 8-neighbor adjacency. The input voltage magnitude was v_in=10⁶. A solution was obtained for 10 fixed-point iterations. A direct solver was used at each fixed-point iteration. The algorithm was implemented in Python with the Numpy, Scipy, and Networkx modules. The segmentation result is shown in FIG. 3.

In another embodiment, the invention is applied to the segmentation of a photograph for the purpose of depth estimation, FIG. 4. The segmentation is performed using one additional view 200. The images are then rectified 210 and a graph 220 is constructed from the rectified images. Simulation 230 of the corresponding electrical network is performed and assignment of labels 240 is performed to obtain the segmentation.

In this embodiment, each label represents the shift in position of where an object appears in photographs taken at two different horizontal positions. That shift is geometrically related to the distance of the object from the cameras. A graph, G_stereo is constructed from a pair of photographs:

G _stereo=(V_stereo, E_stereo) and w_stereo:E_stereo→R₊  (14)

The graph, G_stereo has the same structure as the graph used for computed tomography segmentation. The set of nodes in the graph V_stereo are the pixels in the photograph from the left camera and the set of labels L_stereo. Edges are included between nodes of adjacent pixels and between each pixel and all of the label nodes:

E _stereo =E _stereo ^N ∪E _stereo ^L   (15)

For the edges in E_stereo, the weights are given by:

$\begin{array}{cc}{w}_{\mathrm{stereo}}\left(e_{\mathrm{pq}}\right)=\{\begin{array}{cc}S\left(f_{\mathrm{left}}\left(p\right),f_{\mathrm{left}}\left(q\right)\right),& \mathrm{if}e_{\mathrm{pq}}\in E_{\mathrm{stereo}}^N\\ S\left(f_{\mathrm{left}}\left(p\right),f_{\mathrm{right}}\left(t\left(e_{\mathrm{pq}}\right)\right)\right),& \mathrm{if}e_{\mathrm{pq}}\in E_{\mathrm{stereo}}^L\end{array}& \left(16\right)\end{array}$

where the function f_left and f_right are vectors representing the RGB channels of the color in the left and right photographs, respectively. t(e_pq) is a map from an edge to a pixel in the right photograph. The location of the pixel in the right photograph is the position of the pixel p shifted to the left by the shift associated with the label node q.

S is defined as:

$\begin{array}{cc}S\left(x,y\right)=0.1+\exp \left(\frac{-{x-y}^2}{100}\right)& \left(17\right)\end{array}$

It is to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Claims

1. A computer-implemented method for image segmentation, the method comprising: (a) acquiring a digital image;(b) defining a set of image vertices from the digital image wherein each vertex is associated with a point of said image or one of a set of labels;(c) defining a set of label vertices from a set of labels wherein each vertex is associated with one of a set of labels;(d) constructing a graph from said image vertices and label vertices wherein said graph includes an edge connecting each vertex to all adjacent image vertices corresponding to adjacent points in said image and wherein an edge connects each label vertex to one or more image vertices and wherein all edges are undirected.(e) calculating a non-negative cost function, wherein each calculated cost is associated with each edge in said graph and is derived from the image intensities;(f) constructing an electrical network based upon the constructed graph wherein the current-voltage relationship for the devices in the electrical network are related to the weight of the corresponding edges in the graph.(g) simulating the electrical network one time for each label wherein for each simulation the voltage of the node corresponding to the given label is set to a input high voltage and the voltage of the nodes corresponding to the remaining labels are set to a low input voltage;(h) segmenting the digital image based upon the voltages obtained from the simulations of the electrical network.

2. The method of claim 1 wherein the cost function is associated with a similarity metric associated with an image intensity at the pair of vertices joined by the directed edge.

3. The method of claim 1 wherein electrical devices corresponding to edges in the graph have a current-voltage characteristic given by:

4. The method of claim 1 wherein simulation of the electrical network utilizes fixed-point linearization. A({tilde over (X)}k){tilde over (X)}k+1=B   (19)where {tilde over (X)} is an approximate solution and k is the iteration. The fixed point solution is initialized with the zero matrix {tilde over (X)}0=0. A({tilde over (X)}k) is a |V|×|V| matrix whose elements are:

5. The method of claim 4 wherein vin is 1,000,000.

6. The method of claim 1 wherein simulation of the electrical network is obtained using serial processing methods for solving the systems of linear equations.

7. The method of claim 6 wherein the serial processing method for solving the systems of linear equations is algebraic multigrid.

8. The method of claim 1 wherein simulation of the electrical network is obtained using parallel processing methods for solving the systems of linear equations.

9. The method of claim 8 wherein the parallel processing method for solving the systems of linear equations is algebraic multigrid.

10. The method of claim 1 wherein segmentation of the image is obtained by assigning a label to each node based on the component of the voltage vector that has the maximum magnitude.

11. The method of claim 1 wherein the digital image comprises a medical image and segmentation is obtained based on appearance.

12. The method of claim 1 wherein image segmentation is applied to a photograph and the segmentation is based on appearance.

13. The method of claim 1 wherein image segmentation is applied to a photograph and the segmentation is based on depth.

14. The method of claim 1 wherein partial manual segmentation of the image is used to select points in the image that will be node in the graph that are connected to each label node.

15. A computer-readable medium on which is stored computer instructions for performing the steps of claim 1.

16. A system for image segmentation, the system comprising: (a) acquiring a digital image;(b) defining a set of image vertices from the digital image wherein each vertex is associated with a point of said image or one of a set of labels;(c) defining a set of label vertices from a set of labels wherein each vertex is associated with one of a set of labels;(d) constructing a graph from said image vertices and label vertices wherein said graph includes an edge connecting each vertex to all adjacent image vertices corresponding to adjacent points in said image and wherein an edge connects each label vertex to one or more image vertices and wherein all edges are undirected.(e) calculating a non-negative cost function, wherein each calculated cost is associated with each edge in said graph and is derived from the image intensities;(f) constructing an electrical network based upon the constructed graph wherein the current-voltage relationship for the devices in the electrical network are related to the weight of the corresponding edges in the graph.(g) simulating the electrical network one time for each label wherein for each simulation the voltage of the node corresponding to the given label is set to a input high voltage and the voltage of the nodes corresponding to the remaining labels are set to a low input voltage;(h) segmenting the digital image based upon the voltages obtained from the simulations of the electrical network.