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A Monte Carlo framework for missing wedge restoration and noise removal in cryo-electron tomography

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HAL Id: hal-02424804

https://hal.inria.fr/hal-02424804

Submitted on 28 Dec 2019

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A Monte Carlo framework for missing wedge restoration

and noise removal in cryo-electron tomography

Emmanuel Moebel, Charles Kervrann

To cite this version:

Emmanuel Moebel, Charles Kervrann. A Monte Carlo framework for missing wedge restoration and

noise removal in cryo-electron tomography. Journal of Structural Biology: X, Elsevier, 2019, pp.1-18.

�10.1016/j.yjsbx.2019.100013�. �hal-02424804�

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A Monte Carlo framework for missing wedge restoration and noise

removal in cryo-electron tomography

Emmanuel

Moebel,

Charles

Kervrann

Inria - Centre de Rennes Bretagne Atlantique, Campus Universitaire de Beaulieu, 35042 Rennes (France)

Keywords

:

cryo electron tomography

missing wedge

restoration

inverse problems

Monte-Carlo simulation

A B S T R A C T

We propose a statistical method to address an important issue in cryo-electron tomography

im-age analysis: reduction of a high amount of noise and artifacts due to the presence of a missing

wedge (MW) in the spectral domain. The method takes as an input a 3D tomogram derived from

limited-angle tomography, and gives as an output a 3D denoised and artifact compensated

vol-ume. The artifact compensation is achieved by filling up the MW with meaningful information.

To address this inverse problem, we compute a Minimum Mean Square Error (MMSE) estimator

of the uncorrupted image. The underlying high-dimensional integral is computed by applying a

dedicated Markov Chain Monte-Carlo (MCMC) sampling procedure based on the

Metropolis-Hasting (MH) algorithm. The proposed MWR (Missing Wedge Restoration) algorithm can be

used to enhance visualization or as a pre-processing step for image analysis, including

segmen-tation and classification of macromolecules. Results are presented for both synthetic data and

real 3D cryo-electron images.

1. Introduction

Cryo-electron tomography (cryo-ET) is generally used to explore the structure of an entire cell and constitutes a rapidly

growing field in biology. The particularity of cryo-ET is that it is able to produce three-dimensional views of vitrified

samples at sub-nanometer resolution, which allows observing the structure of molecular complexes (e.g. ribosomes)

in their physiological environment. Nevertheless, observation of highly resolved cellular mechanisms is challenging:

i/ due to the low dose of electrons used to preserve specimen integrity during image acquisition, the amount of noise

is very high; ii/ due to technical limitations of the microscope, complete tilting of the sample (90

𝑜

) is impossible,

resulting into a blind spot. As a consequence, projections are not available for a determined angle range, hence the

term “limited angle tomography”.

The blind spot is observable in Fourier domain, where the missing projections appear as a missing wedge (MW).

This separates the Fourier spectrum into two regions: the sampled region (SR) and the unsampled regions (MW).

The sharp transition between these two regions is responsible for a Gibbs-like phenomenon: ray- and side-artifacts

emanate from high contrast objects (see Fig.

1

), which can hide important structural features in the image. Another

type of artifact arises from the incomplete angular sampling: objects appear elongated in the direction of the blind

spot (see Fig.

1

), in other words the data has an anisotropic resolution (e.g. linear features perpendicular to the tilt

axis disappear). This elongation erases boundaries and makes it difficult to differentiate neighboring features. The

(3)

quality of tomograms can be improved if sophisticated algorithms such as MBIR (

Yan et al.

,

2019

) are applied instead

of conventional methods (e.g. WPB (

Radermacher

,

1992

), SIRT (

Gilbert

,

1972

)).

Filling up the MW with relevant data enables to potentially reduce or completely suppress these artifacts.

Experi-mentally this can partially be achieved during data acquisition by using dual-axis tomography (

Guesdon et al.

,

2013

),

where the sample is tilted with respect to the second axis. Consequently the blind spot is smaller and the MW becomes

a missing pyramid, which results into a smaller missing spectrum. However dual-axis tomography is technically

chal-lenging and requires intensive post-processing in order to correct tilt and movement bias in the microscope. Another

reconstruction approach consists in exploiting the symmetry of the underlying structure (

Förster and Hegerl

,

2007

),

but this can only be applicable to a limited number of biological objects (e.g. virus with either helical or icosahedral

structure). Another common computational approach amounts to combining several hundred or thousands views of

the same object, but with distinct blind spots. This so-called sub-tomogram averaging technique (

Förster and Hegerl

,

2007

) is routinely used in cryo-ET, and is continuously improved for structure determination (

Xu et al.

,

2018

). To

im-prove sub-tomogram averaging and compensate the remaining MW artifacts, tomographic reconstruction algorithms

with dedicated regularization have also been proposed in (

Paavolainen et al.

,

2014

;

Leary et al.

,

2013

).

The objective of our work is to design a statistical approach for the problem of recovering missing Fourier

coef-ficients from a single volume in the situation where low and high frequency coefcoef-ficients are missing in a specific and

large region of the 3D spectrum. A simple way of handling MW artifacts is described in (

Kováčik et al.

,

2014

), where

a dedicated spectral filter is used to smooth out the transition between SR and MW; ray- and side-artifacts are reduced

with this filter, but the object elongation remains in the resulting image. Inspired from (

Maggioni et al.

,

2013

), we have

rather investigated MCMC methods to compute a MMSE estimator based on any non-local image denoiser to recover

the missing information. We show that our Monte-Carlo sampling algorithm performs as well as the iterative method

(

Maggioni et al.

,

2013

) but converges faster. Nevertheless, our concept is more general since any denoising method

can be applied, included denoising algorithms dedicated to cryo-ET images (

Frangakis and Hegerl

,

2001

;

Fernandez

and Li

,

2003

;

Darbon et al.

,

2008

;

Wei and Yin

,

2010

;

Kervrann et al.

,

2008

;

Moreno et al.

,

2018

). In this paper, we

focus on the cryo-ET restoration problem but the proposed algorithm could be potentially used to address a large range

of applications including medical and seismic imaging, and other inverse scattering problems.

Related work

We first focus on computational methods designed for spectrum restoration and Fourier coefficients

recovering. Most of methods have been designed for 2D images and very few of them for 3D imaging. In general,

the corruption process is supposed to be known and the artifacts observed in the input image, are due to a set of

missing Fourier coefficients, well localized in the spectrum. First, several methods have been investigated to retrieve

partially-missing phases of complex coefficients from modulus of coefficients in electron microscopy (

Fienup

,

1982

)

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Spatial domain

Fourier domain

Data

Corrupted

xy-view

yz-view

xz-view

Ray

artifacts

Object

elongation

Side

artifacts

Missing

wedge

Ground Truth

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Corrupted Data

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Ray artifacts

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Object elongation

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Side artifacts

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Missing wedge

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Figure 1:

The effects of the missing wedge on the Shepp-Logan phantom. Left: uncorrupted 3D data displayed as ortho-slices

(one central slice along each dimension). Right: data corrupted by the missing wedge. We display the data in spatial domain (top

row) and Fourier domain (bottom row).

and time-frequency signal analysis (

Krémé et al.

,

2018

). Here, we focus on another special case which consists in

ex-trapolating the band-limited spectrum of an image up to higher frequencies. Nevertheless, these problems are generally

formulated as denoising problems with specific reconstruction constraints. For instance,

Moisan

(

2001

) and

Guichard

and Malgouyres

(

1998

) investigated the Total Variation (TV) minimization to extend the band-limited spectrum of

an image. In (

Lauze et al.

,

2017

), the authors combine TV minimization and positivity constraints to reduce noise

and artifacts, providing an inpainting-like mechanism for the sinogram missing data in limited-angle tomography (see

also (

Frikel and Quinto

,

2013

;

Sentosum et al.

,

2017

)). The common objective is to create new frequencies while

preserving discontinuities and details in the restored image. Instead of explicitly imposing some regularity (e.g. Total

variation, or robust regularization (

Geman and Reynolds

,

1992

;

Charbonnier et al.

,

1997

)) on the solution, another

successful restoration approach consists in exploiting the spatial redundancy of the input image. In (

Chambolle and

Jalalzai

,

2014

), a non-local method was suggested in the framework of variational methods for image reconstruction.

In this approach, a patchwise similarity measure based on atoms corresponding to pseudo Gabor filters is designed

(5)

to compare corrupted regions. Meanwhile,

Maggioni et al.

(

2013

) adapted the concept of BM3D for recovering the

missing spectrum applied to MRI imaging with very promising results on synthetic data. BM3D (

Dabov et al.

,

2007

)

is a popular denoising algorithm which combines clustering of noisy patches, DCT-based transform and shrinkage

operation to achieve the state-of-the-art results for several years. In our approach, we also focus on patch-based

meth-ods (

Kervrann and Boulanger

,

2008

;

Kindermann et al.

,

2005

;

Lou et al.

,

2010

;

Katkovnik et al.

,

2010

;

Pizarro et al.

,

2010

;

Milanfar

,

2013

;

Sutour et al.

,

2014

) to restore the input image corrupted by noise and non-linear transform.

Indeed, it has been experimentally confirmed that the most competitive denoising methods are non-local and exploit

self-similarities occurring at large distances in images, such as BM3D (

Dabov et al.

,

2007

), NL-Bayes (

Lebrun et al.

,

2013

), PLOW (

Chatterjee and Milanfar

,

2012

), S-PLE (

Wang and Morel

,

2013

), PEWA (

Kervrann

,

2014

) and many

other adaptative filters (

Kervrann and Boulanger

,

2006

,

2007

;

Van De Ville and Kocher

,

2009

;

Deledalle et al.

,

2009

;

Louchet and Moisan

,

2011

;

Duval et al.

,

2011

;

Deledalle et al.

,

2012

;

Kervrann et al.

,

2014

;

Jin et al.

,

2017

), inspired

from the seminal N(on)L(ocal)-means algorithm (

Buades et al.

,

2005

).

To complete the brief overview of non-local methods, we mention that a noisy image can also be restored from a

set of noisy or “clean” patches or a learned dictionary. The statistics of a training set of image patches serve then as

priors for denoising (

Elad and Aharon

,

2006

;

Mairal et al.

,

2009

;

Zoran and Weiss

,

2011

). Another approach based

multi-layer perceptron (MLP) exploiting a training set of noisy and noise-free patches was also able to achieve the

state-of-the-art performance (

Burger et al.

,

2012

). Very recently,

Buchholz et al.

(

2018

) proposed to train content-aware

restoration networks for denoising cryo-transmission electron microscopy data. While all these machine learning

methods are attractive and powerful, computation is not always feasible in 3D because very large collection of 3D

“clean” patches are required. In our study, we focus on unsupervised denoising methods since they are more flexible

for real applications. They are less computationally demanding and are still competitive when compared to recent

machine learning methods.

Our approach is mainly inspired from (

Maggioni et al.

,

2013

), but can use any competitive denoising methods for

restoring the Fourier coefficients. The method proposed by

Maggioni et al.

(

2013

) works by alternatively adding noise

into the missing region and applying the BM4D algorithm which is the extension of BM3D (

Dabov et al.

,

2007

) to

volumes. The authors interpret this iterative restoration method in the framework of compressed sensing with two

information theory concepts in mind: sparsity of the signal in the transformed domain, and incoherence between the

transform and the sampling matrix. Actually, BM4D does rely on a transform where the signal is sparse. Moreover,

it is not clearly established that this transform is incoherent with the sampling matrix, defined by the support of the

sampling region. Therefore, the proof of convergence is not clearly established, even though the authors presented

convincing experimental results on synthetic images corrupted with white Gaussian noise. It remains unclear how

the concept performs on experimental data and non Gaussian noise. To generalize this idea, we propose a statistical

(6)

approach well-grounded in the Bayesian and MCMC framework and applied to challenging real data in cryo-ET. Our

contributions are the following ones:

1. We present a MMSE estimator dedicated to the problem of MW restoration.

2. We propose an original Monte Carlo Markov Chain (MCMC) sampling procedure to efficiently compute the

MMSE estimator.

Paper organization

The remainder of the paper is organized as follows. In the next section, we present an overview of

our computational method. In Section

3

, we give the theoretical background and formulate the reconstruction problem

as an inverse problem. We shortly describe the usual Bayesian approach to derive a MMSE estimator. Furthermore, a

Monte-Carlo Markov Chains method based on the popular Metropolis-Hastings algorithm is proposed to compute the

underlying high-dimensional integral. In Section

4

, we adapt this general Bayesian framework for MW restoration.

An original Metropolis-Hastings procedure is presented to explore the large space of admissible solutions and to select

relevant samples. Section

5

presents the experimental results obtained on simulated and real data. We illustrate the

potential of our MWR (Missing Wedge Restoration) algorithm with experiments on real cryo-tomogram images and

we compare our iterative approach to several competitive algorithms.

2. Overview of the MW restoration algorithm

In this section, we present our concept for MW restoration. First, we formulate the problem and introduce notation.

Second, we present the two-step algorithm which is embedded in a Monte-Carlo simulation framework.

2.1. Problem formulation and notation

Let us define a 𝑛-dimensional image 𝒙 ∶ 𝑆 ⊂ ℤ

3

→ ℝ

assumed to be periodic and defined over a cubic domain

Ω = [0, 1]

3

and 𝑛 = |Ω|. The discrete Fourier transform of 𝒙 = {𝑥(𝑠), 𝑠 ∈ 𝑆} is then as follows:

𝒙 ∶ 𝑘 →

𝑠

∈𝑆

exp (−2𝑖𝜋𝑘 ⋅ 𝑠) 𝑥(𝑠),

(1)

where 𝑠 is the coordinate of point in spatial domain 𝑆. In our problem, one considers a corrupted image denoted

𝒚

= {𝑦(𝑠), 𝑠 ∈ 𝑆}

defined as

𝑦

(𝑠) =

𝑘

∈𝑊

exp (2𝑖𝜋𝑘 ⋅ 𝑠)

𝒙[𝑘]

(2)

where 𝑊 is the sampled spectral region (SR) where the Fourier coefficients 𝒙[𝑘] are positive and non-zero. The

region 𝑊 is equivalent to the support of a binary mask 𝒎 ∈ {0, 1}

𝑆

such as 𝒎[𝑘] = 1 if 𝑘 ∈ 𝑊 and 0 otherwise:

(7)

𝑊

=

supp(𝒎). The so-called missing wedge 𝑊 is assumed to be symmetric with respect to the origin as illustrated

in Fig.

1

(bottom right), and 𝑆 = 𝑊 ∪ 𝑊 . In what follows, we assume that the clean image 𝒙 is known over the

region 𝑊 . Our objective is then to estimate 𝒙 ∶ 𝑆 → ℝ in the whole domain 𝑆 such that

∀𝑘 ∈ 𝑊 ,

𝒙[𝑘] = 𝒚[𝑘]

(3)

and ∀𝑠 ∈ 𝑆, 𝑥(𝑠) > 0. In other words, the set of known Fourier coefficients will be preserved by the restoration

procedure. The challenge is to recover the unknown set of low, middle and high frequencies in a large region in the

spectrum. This amounts to applying an interpolation operator 𝜑

𝑊

to the spectrum of 𝒚 to get an estimator ̂𝒙 of 𝒙:

̂

𝒙

=

−1

◦𝜑

𝑊

𝒚.

(4)

2.2. Principles of the iterative two step-algorithm

Our computational method takes as input an noisy image 𝒚 and iterates two steps as illustrated in Fig.

2

. At the

initialization (𝑡 = 0), we set 𝒙

(0)

= 𝒚:

• In the proposal step and at iteration 𝑡, white Gaussian noise 𝜺 is added to the current image 𝒙

(𝑡−1)

. The Fourier

coefficients are then non-zero in the MW region 𝑊 . We substitute the original Fourier coefficients in the SR

region 𝑊 to the noisy Fourier coefficients to preserve information. Formally, this amounts to applying an

projection operator 

𝑊

(⋅)

to the artificially noisy image. The objective is to randomly initialize the Fourier

coefficients in the MW. A denoising algorithm is then used to efficiently remove the majority of the noise while

preserving image structure. Due to its random nature, a small amount of the added noise will be close to the

signal and is thus "saved" after denoising. The denoising algorithm (⋅), which promotes smoothness while

preserving contours, acts as a spatial regularizer. In summary, a candidate image 𝒛 = (

𝑊

(𝒙

(𝑡−1)

+ 𝜺))

is

obtained by applying an projection operator  in the Fourier domain and a denoising operator  in the spatial

domain.

• In the second evaluation step, the candidate image 𝒛 is compared and is potentially substituted to the current

image 𝒙

(𝑡−1)

depending on its ability to satisfies several constraints, including fidelity to the original input image

𝒛

. A cost function (or energy) is used to accept/reject this candidate image with a certain probability established

in the Metropolis-Hastings simulation framework as presented in the next section.

By repeating these two steps iteratively, we collect several hundreds of images {𝒙

(𝑡)

}

to compute an average

corre-sponding to the final restored image. The correcorre-sponding MWR (Missing Wedge Restoration) algorithm is actually able

to diffuse information from SR into MW. It cannot retrieve unobserved data, but it merely makes the best statistical

(8)

t

Proposal

step

Evaluation

step

Denoising

Noise

Recover known Fourier coefficients

+

from original volume

Spatial domain

Spectral domain

Accept/reject

according to

Metropolis scheme

Compute

energy

x

t

+ ✏

P

W

(x

t

+ ✏)

D (P

W

(x

t

+ ✏))

z

x

t+1

y

Unsampled region

(Missing Wedge)

Sampled region

(SR)

00

PW

(x

(t)

+ ")

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x

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+ "

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+ "))

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z

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x

(t+1)

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Figure 2:

The MCMC method flowchart. The 1

𝑠𝑡

icon row represents the data in the spectral domain, the 2

𝑛𝑑

in spatial domain.

guess of what the missing data could be, based on what has been observed. The underlying algorithm is controlled by

several parameters, especially the variance 𝜎

2

𝜀

of noise added to the current image and constant at each iteration. More

importantly, this iterative procedure will be successful provided the denoising algorithm is able to remove the artificial

additive Gaussian noise. Any state-of-the art algorithms can be then applied, especially the BM3D (

Dabov et al.

,

2007

)

and BM4D (

Maggioni et al.

,

2013

) algorithms. This stochastic procedure can be interpreted as a data-driven random

search in a large space of possible images. In the next section, we present the theoretical background required to justify

the rationale behind this concept, illustrated in Fig.

2

.

3. Theoretical background

In this section, we describe the theoretical background in the Bayesian framework to justify our original computational

method.

3.1. Bayesian estimator and Monte Carlo Markov Chain sampling

Solving inverse problems in image processing consists in estimating an unknown image 𝒙 ∈  given an image 𝒚 ∈ .

Different sources of distortion may cause damages on the ideal image, including noise, blur, and projections. In

the Bayesian framework, the whole information once the data have been collected, is represented by the posterior

Figure

Figure 1: The effects of the missing wedge on the Shepp-Logan phantom. Left: uncorrupted 3D data displayed as ortho-slices (one central slice along each dimension)
Figure 2: The MCMC method flowchart. The 1
Figure 3: Dataset A (2D): comparison of denoising algorithms. First, we compare conventional denoisers (left column) to our MH method (right column)
Figure 4: Dataset A: This figure shows the impact of algorithm parameters. We illustrate these effects in terms of PSNR through iterations
+7

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