nips nips2011 nips2011-275 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Xinghua Lou, Fred A. Hamprecht
Abstract: We study the problem of learning to track a large quantity of homogeneous objects such as cell tracking in cell culture study and developmental biology. Reliable cell tracking in time-lapse microscopic image sequences is important for modern biomedical research. Existing cell tracking methods are usually kept simple and use only a small number of features to allow for manual parameter tweaking or grid search. We propose a structured learning approach that allows to learn optimum parameters automatically from a training set. This allows for the use of a richer set of features which in turn affords improved tracking compared to recently reported methods on two public benchmark sequences. 1
Reference: text
sentIndex sentText sentNum sentScore
1 de Abstract We study the problem of learning to track a large quantity of homogeneous objects such as cell tracking in cell culture study and developmental biology. [sent-6, score-0.915]
2 Reliable cell tracking in time-lapse microscopic image sequences is important for modern biomedical research. [sent-7, score-0.651]
3 Existing cell tracking methods are usually kept simple and use only a small number of features to allow for manual parameter tweaking or grid search. [sent-8, score-0.834]
4 We propose a structured learning approach that allows to learn optimum parameters automatically from a training set. [sent-9, score-0.209]
5 This allows for the use of a richer set of features which in turn affords improved tracking compared to recently reported methods on two public benchmark sequences. [sent-10, score-0.4]
6 1 Introduction One distinguishing property of life is its temporal dynamics, and it is hence only natural that time lapse experiments play a crucial role in current research on signaling pathways, drug discovery and developmental biology [17]. [sent-11, score-0.102]
7 Such experiments yield a very large number of images, and reliable automated cell tracking emerges naturally as a prerequisite for further quantitative analysis. [sent-12, score-0.602]
8 Even today, cell tracking remains a challenging problem in dense populations, in the presence of complex behavior or when image quality is poor. [sent-13, score-0.651]
9 Existing cell tracking methods can broadly be categorized as deformable models, stochastic filtering and object association. [sent-14, score-0.706]
10 Deformable models combine detection, segmentation and tracking by initializing a set of models (e. [sent-15, score-0.386]
11 active contours) in the first frame and updating them in subsequent frames (e. [sent-17, score-0.179]
12 Object association methods approximate and simplify the problem by separating the detection and association steps: once object candidates have been detected and characterized, a second step suggests associations between object candidates at different frames. [sent-27, score-1.058]
13 This class of methods scales well [21, 16, 13] and allows the tracking of thousands of cells in 3D [19]. [sent-28, score-0.342]
14 This was first accomplished by casting tracking as a local affinity prediction problem such as binary classification with either offline [1] or online learning [11, 5, 15], weakly supervised learning with imperfect oracles [27], manifold appearance model learning [25], or ranking [10, 18]. [sent-31, score-0.382]
15 However, these local methods fail to capture the very important dependency among associations, hence the resulting local affinities do not necessarily guarantee a better global association [26]. [sent-32, score-0.129]
16 To address this limitation, [26] extended the RankBoost method from [18] to rank global associations represented as a Conditional Random Field (CRF). [sent-33, score-0.31]
17 Firstly, it depends on a set of artificially generated false association samples that can make the training data particularly imbalanced and the training procedure too expensive 1 for large-scale tracking problems. [sent-35, score-0.621]
18 We first present an extended formulation of the object association models proposed in the literature. [sent-42, score-0.233]
19 We hence, secondly, propose to use structured learning to automatically learn optimum parameters from a training set, and hence profit fully from this richer description. [sent-44, score-0.209]
20 In section 2, we present the extended object association models and a structured learning approach for global affinity learning. [sent-47, score-0.329]
21 In section 3, an evaluation shows that our framework inherits the runtime advantage of object association while addressing many of its limitations. [sent-48, score-0.233]
22 1 Association Hypotheses and Scoring We assume that a previous detection and segmentation step has identified object candidates in all frames, see Fig. [sent-51, score-0.313]
23 We set out to find that set of object associations that best explains these observations. [sent-53, score-0.414]
24 To this end, we admit the following set E of standard events [21, 13]: a cell can move or divide and it can appear or disappear. [sent-54, score-0.514]
25 In addition, we allow two cells to (seemingly) merge, to account for occlusion or undersegmentation; and a cell can (seemingly) split, to allow for the lifting of occlusion or oversegmentation. [sent-55, score-0.26]
26 These additional hypotheses are useful to account for the errors that typically occur in the detection and segmentation step in crowded or noisy data. [sent-56, score-0.207]
27 The distinction between division and split is reasonable given that typical fluorescence stains endow the anaphase with a distinctive appearance. [sent-57, score-0.153]
28 Given a pair of object candidate lists x = {C, C } in two neighboring frames, there is a multitude of possible association hypotheses, see Fig. [sent-60, score-0.311]
29 We have two tasks: firstly, to allow only consistent associations (e. [sent-62, score-0.31]
30 making sure that each cell in the second frame is accounted for only once); and secondly to identify, among the multitude of consistent hypotheses, the one that is most compatible with the observations, and with what we have learned from the training data. [sent-64, score-0.543]
31 We express this compatibility of the association between c ∈ P(C) and c ∈ P(C ) by event e ∈ E e e as an inner product fc,c we . [sent-65, score-0.169]
32 Here, fc,c is a feature vector that characterizes the discrepancy (if any) between object candidates c and c ; and we is a parameter vector that encodes everything we 2 have learned from the training data. [sent-66, score-0.296]
33 Summing over all object candidates in either of the frames and over all types of events gives the following compatibility function: e e fc,c , we zc,c L(x, z; w) = (1) e∈E c∈P(C) c ∈P(C ) e e zc,c = 1 with zc,c ∈ {0, 1} e zc,c = 1 and s. [sent-67, score-0.441]
34 (2) e∈E c ∈P(C ) e∈E c∈P(C) The constraints in the last line involve binary indicator variables z that reflect the consistency requirements: each candidate in the first frame must have a single fate, and each candidate from the second frame a unique history. [sent-69, score-0.296]
35 As an important technical detail, note that P(C) := C ∪ (C ⊗ C) is a set comprising each object candidate, as well as all ordered pairs of object candidates from a frame1 . [sent-70, score-0.325]
36 This allows us to conveniently subsume cell divisions, splits and mergers in the above equation. [sent-71, score-0.303]
37 the global affinity measure, states how well a set of associations z matches the observations f (x) computed from the raw data x, given the knowledge w from the training set. [sent-74, score-0.385]
38 The remaining tasks, discussed next, are how to learn the parameters w from the training data (section 2. [sent-75, score-0.113]
39 2); given these, how to find the best possible associations z (section 2. [sent-76, score-0.31]
40 2 Structured Max-Margin Parameter Learning In learning the parameters automatically from a training set, we pursue two goals: first, to go beyond manual parameter tweaking in obtaining the best possible performance; and second, to make the process as facile as possible for the user. [sent-80, score-0.249]
41 This is under the assumption that most experimentalists find it easier to specify what a correct tracking should look like, rather than what value a more-or-less obscure parameter should have. [sent-81, score-0.385]
42 ∗ Given N training frame pairs X = {xn } and their correct associations Z ∗ = {zn }, n = 1, . [sent-82, score-0.495]
43 , N , the best set of parameters is the optimizer of arg min R(w; X, Z ∗ ) + λΩ(w) (3) w Here, R(w; X, Z ∗ ) measures the empirical loss of the current parametrization w given the training data X, Z ∗ . [sent-85, score-0.181]
44 The empirical loss is given by N 1 ∗ ˆ ˆ R(w; X, Z ∗ ) = N i=1 ∆(zn , zn (w; xn )). [sent-93, score-0.25]
45 Here ∆(z ∗ , z) is a loss function that measures the discrepancy between a true association z ∗ and a prediction by specifying the fraction of missed events w. [sent-94, score-0.278]
46 Importantly, both the input (objects from a frame pair) and output (associations between objects) in this learning problem are structured. [sent-100, score-0.11]
47 We hence resort to max-margin structured learning [2] to exploit the structure and dependency within the association hypotheses. [sent-101, score-0.225]
48 In comparison to other aforementioned learning methods, structured learning allows us to directly learn the global affinity measure, avoid generating many artificial false association samples, and drop any assumptions on the signs of the features. [sent-102, score-0.263]
49 In particular, we attempt to find the decision boundary that maximizes the margin between the ∗ correct association zn and the closest runner-up solution. [sent-104, score-0.341]
50 3 ∗ that the score of zn be greater than that of any other solution. [sent-107, score-0.212]
51 1 N N n=1 ξn + λΩ(w) ∗ ∗ ˆ ˆ ˆ ∀n, ∀zn ∈ Zn : L(xn , zn ; w) − L(xn , zn ; w) ≥ ∆(zn , zn ) − ξn , (5) ∗ ˆ where Zn is the set of possible consistent associations and ∆(zn , zn ) − ξn is known as “marginrescaling” [24]. [sent-111, score-1.158]
52 Iteratively find, first, the optimum associations for the current ∗ ˆ ˆ w by solving, for all n, zn = arg maxz {L(xn , z; w) + ∆(zn , z)}. [sent-118, score-0.522]
53 Use all these zn to identify the most violated constraint, and add it to Eq. [sent-119, score-0.212]
54 For a given parametrization, the optimum associations can be found by integer linear programming (ILP) [16, 21, 13]. [sent-124, score-0.31]
55 Our framework has been implemented in Matlab and C++, including a labeling GUI for the generation of training set associations, feature extraction, model inference and the bundle method. [sent-125, score-0.138]
56 To reduce the search space and eliminate hypotheses with no prospect of being realized, we constrain the hypotheses to a k-nearest neighborhood with distance thresholding. [sent-126, score-0.144]
57 division and split) and resolve ambiguity in model inference, we need rich features to characterize different events. [sent-132, score-0.156]
58 In additional to basic features such as size/position [21] and intensity histogram [16], we also designed new features such as “shape compactness” for oversegmentation and “angle pattern” for division. [sent-133, score-0.221]
59 Shape compactness relates the summed areas of two object candidates to the area of their union’s convex hull. [sent-134, score-0.33]
60 Features can be defined on a pair of object candidates or on an individual object candidate only. [sent-136, score-0.363]
61 The two datasets show a certain degree of variations such as illumination, cell density and image compression artifacts (Fig. [sent-142, score-0.36]
62 action=show movie;query=243867 3 4 GFP stained cell nuclei were segmented using the method in [19], yielding an F-measure over 99. [sent-152, score-0.303]
63 Full ground truth associations for training and evaluation were generated with a Matlab GUI tool at a rate of approximately 20 frames/hour. [sent-154, score-0.385]
64 The Mitocheck sequence exhibits higher cell density, larger intensity variability and “blockness” artifacts due to image compression. [sent-163, score-0.422]
65 Task 1: Efficient Tracking for a Given Sequence We first evaluate our method on a task that is frequently encountered in practice: the user simply wishes to obtain a good tracking for a given sequence with the smallest possible effort. [sent-164, score-0.342]
66 For a fair comparison, we extended Padfield’s method [21] to account for the six events described in section 2. [sent-165, score-0.111]
67 A detailed analysis of the error counts for specific events shows that the method accounts well for moves, but has difficulty with disappearance and split events. [sent-172, score-0.231]
68 To study the difference between manual tweaking and learning of the parameters, we used the learning framework presented here to optimize the model and obtained a reduction of the total loss from 1. [sent-174, score-0.212]
69 Note that the learned parametrization actually deteriorates the detection of divisions because the learning aims at minimizing the overall loss across all events. [sent-178, score-0.253]
70 With 37 features included and their weights optimized using structured learning, our model fully profits from this richer description and achieves a total loss of only 0. [sent-180, score-0.192]
71 30% (4th row) which is a significant improvement over [21, 16] (2nd/7th row) and manual tweaking (6th row). [sent-181, score-0.174]
72 Though a certain amount of efforts is needed for creating the training set, our method allows experimentalists to contribute their expertise in an intuitive fashion. [sent-182, score-0.118]
73 They afford the following observations: Firstly, features on cell size and shape are generally of high importance, which is in line with the assumption in [21]. [sent-187, score-0.417]
74 Secondly, the correlations of the features with the final association score are 5 Table 3: Performance comparison on the DCellIQ dataset. [sent-188, score-0.187]
75 The header row shows the number of events occurring for moves, divisions, appearance, disappearance, splits and mergers. [sent-189, score-0.246]
76 mov div app dis spl mer total loss 10156 104 78 76 54 55 Padfield et al. [sent-191, score-0.414]
77 Figure 3: Some diverging associations by [21] (top) and our method (bottom). [sent-204, score-0.31]
78 For example, shape compactness is positively correlated with split but negatively with division. [sent-207, score-0.182]
79 This is in line with the intuition that an oversegmentation conserves compact shape, while a true division seemingly pushes the daughters far away from each other (in the present kind of data, where only DNA is labeled). [sent-208, score-0.178]
80 Task 2: Tracking for High-Throughput Experiments The experiment described in the foregoing draws both training and test samples from the same time lapse experiment. [sent-210, score-0.167]
81 To emulate this situation, we have used the parameters w trained in the foregoing on the DCellIQ sequence [16] and used these to estimate the tracking of the Mitocheck dataset. [sent-212, score-0.385]
82 The main focus of the Mitocheck project is on accurate detection of mitosis (cell division). [sent-213, score-0.122]
83 Despite the difference in illumination and cell density from the training data, and despite the segmentation artifacts caused by the compression of the image sequence, our method shows a high generalization capability and obtains a total loss of 0. [sent-214, score-0.517]
84 2% of 384 mitosis events which is a significant improvement over the mitosis detection rate reported in [12] (81. [sent-217, score-0.307]
85 We sample positive associations from the ground truth and randomly generate false associations. [sent-221, score-0.31]
86 The predicted probabilities by the RF classifiers are used to compute the overall association score as in Eq. [sent-223, score-0.129]
87 Since we have multiple competing events (one cell can only have a single 6 mov div app dis spl mer Feature Importance (L2) Importance 0. [sent-226, score-0.747]
88 su fa fa an in d me m th th gl te ev an er er e n. [sent-244, score-0.209]
89 p e i a s sh s cc nt tt um ap iz en en er e e tr si n ov er m co ev ic ty di la as mp en it st p s ac ne y an wi ev tn ss ce th en es n s d t b e diiff d o or ss di f . [sent-246, score-1.374]
90 1 di di ff Importance Feature Importance (L1) 0. [sent-273, score-0.438]
91 Parameters weighing the features for different events are colored differently. [sent-275, score-0.169]
92 The header row shows the number of events occurring for moves, divisions, appearance, disappearance, splits and mergers. [sent-281, score-0.246]
93 mov div app dis spl mer total loss 22520 384 310 304 127 132 Padfield et al. [sent-283, score-0.414]
94 To test the sensitivity of the results to the training data used, we drew different numbers of training image pairs randomly from the entire sequence and used the remaining pairs for testing. [sent-296, score-0.199]
95 According to the one-standard-error-rule, associations between at least 15 or 20 image pairs are desirable, which can be accomplished in well below an hour of annotation work. [sent-300, score-0.359]
96 L2) 25 20 L1 Regularization L2 Regularization 15 10 5 0 10 20 30 40 50 60 70 Number of constraints Number of frame pairs for training Figure 5: Learning curve of structured learning Figure 6: Convergence rates of structured learn(with L2 regularization). [sent-315, score-0.377]
97 4 Conclusion & Future Work We present a new cell tracking scheme that uses more expressive features and comes with a structured learning framework to train the larger number of parameters involved. [sent-318, score-0.756]
98 We currently work on further improvement of the tracking by considering more than two frames at a time, and on an active learning scheme that should reduce the amount of required training inputs. [sent-320, score-0.486]
99 CellCognition: time-resolved phenotype annotation in highthroughput live cell imaging. [sent-404, score-0.26]
100 Cell population tracking and lineage construction with spatiotemporal context. [sent-440, score-0.391]
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