Quantitative comparison of algorithms for inter-subject registration of 3D volumetric brain MRI scans

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Abstract

The objective of inter-subject registration of three-dimensional volumetric brain scans is to reduce the anatomical variability between the images scanned from different individuals. This is a necessary step in many different applications such as voxelwise group analysis of imaging data obtained from different individuals. In this paper, the ability of three different image registration algorithms in reducing inter-subject anatomical variability is quantitatively compared using a set of common high-resolution volumetric magnetic resonance imaging scans from 17 subjects. The algorithms are from the automatic image registration (AIR; version 5), the statistical parametric mapping (SPM99), and the automatic registration toolbox (ART) packages. The latter includes the implementation of a non-linear image registration algorithm, details of which are presented in this paper. The accuracy of registration is quantified in terms of two independent measures: (1) post-registration spatial dispersion of sets of homologous landmarks manually identified on images before or after registration; and (2) voxelwise image standard deviation maps computed within the set of images registered by each algorithm. Both measures showed that the ART algorithm is clearly superior to both AIR and SPM99 in reducing inter-subject anatomical variability. The spatial dispersion measure was found to be more sensitive when the landmarks were placed after image registration. The standard deviation measure was found sensitive to intensity normalization or the method of image interpolation.

Introduction

An important methodological consideration for analysis of human brain imaging data is inter-subject registration or spatial normalization of images acquired from different individuals. The aim of inter-subject registration is to reduce the anatomical variability in three-dimensional (3D) volumetric brain scans obtained from different subjects. For example, inter-subject registration allows voxelwise group analysis of functional magnetic resonance imaging (fMRI) data (Svensen et al., 2002, Zeffiro et al., 1997), and studies of brain white matter using diffusion tensor imaging (Ardekani et al., 2003, Jones et al., 2002). Another class of applications of inter-subject registration can be categorized as automatic ‘image understanding’, where higher order information (tissue type, locations of anatomical landmarks, structural boundaries, specific sulci/gyri, etc.) that are known about a template image are obtained about a subject or test image after the subject image is registered to the template by non-linear spatial transformation or deformation (Collins et al., 1995, Marroquin et al., 2002, Webb et al., 1999). An additional important application is the quantification of small changes in volume observed in anatomical structures over time (Holden et al., 2002, Rey et al., 2002) that can be used in diagnosis and evaluation of disease progression and treatment.

Spatial registration is specified in terms of a 3D transformation or displacement field w: 33 which is applied to the subject image, Is(r), to obtain the spatially normalized or warped image Iw(r) = Is(r + w(r)). The main objective of most inter-subject image registration algorithms is to find a displacement field w such that the warped image Iw(r) is as ‘similar’ as possible to a template image, It(r). For a given pair of subject and template images Is and It, algorithms mainly differ in their approach to modeling and estimation of w. In the SPM99 software package (Friston et al., 1995), the displacement field w is modeled by a finite orthogonal series with trigonometric basis functions. The algorithm computes a displacement field by estimating the coefficients of the series using an iterative linearized least-squares approach. In the automated image registration (AIR) software package (Woods et al., 1998), the displacement field is modeled as a polynomial, and the algorithm computes the displacement field by estimating the polynomial coefficients using non-linear least-squares optimization. Other methods of inter-subject registration model the displacement field as the displacement field in an elastic object (Bajcsy and Kovačič, 1989) or a viscous fluid (Christensen et al., 1997, Christensen and Johnson, 2001) reacting to internal forces proportional to ‘image mismatch’ between template and subject volumes. These methods are governed by partial differential equations that model the physical phenomenon. In several other methods, the displacement fields are estimated as non-parametric vector fields subject to regularity constraints (Collins et al., 1995, Kjems et al., 1999, Kosugi et al., 1993).

Although many methods have been proposed for spatial normalization, there are few studies comparing the performance of various algorithms on a common set of real MRI data. At least two important questions can be raised: (1) how do the algorithms compare in terms of their ability to reduce anatomic differences between subjects? And (2) how does image registration accuracy impact the final analysis results (e.g., activation maps in fMRI)? The present paper is concerned with the first question. The latter question has been previously addressed primarily in the context of positron emission tomography (PET) functional imaging studies (Crivello et al., 2002, Kjems et al., 1999, Senda et al., 1998). Overall, little difference has been found between the functional activation maps obtained by processing PET activation data using different inter-subject registration methods. Crivello et al. (2002) attributed this finding to the limited spatial resolution of PET and the inherent functional variability across subjects. However, this conclusion may not be automatically extended to higher resolution fMRI studies or group analysis of diffusion tensor imaging data. Recently, Ardekani et al. (2004) studied the impact of inter-subject registration on group analysis of fMRI data. They showed that increased accuracy of inter-subject registration in removing anatomic variability between subjects results in significant increases in the sensitivity of activation detection and the reproducibility of activation maps. Thus, at least in fMRI studies, it is important for researchers to understand the relative accuracy of inter-subject registration tools available to them.

The first objective of the present paper was to present details of our implementation of an inter-subject registration algorithm included in our automatic registration toolbox (ART). This algorithm is a non-parametric method similar to those proposed by Collins et al. (1995), Kjems et al. (1999), and Kosugi et al. (1993) with some new components. The second objective was to quantitatively compare the performance of three different inter-subject registration programs: (1) SPM99; (2) AIR; and (3) ART. The third aim was to evaluate different independent criteria for assessment of registration accuracy. One measure is the post-registration spatial dispersion of sets of homologous landmarks located manually on the images. Another criterion compares the sample standard deviation (S.D.) of voxel intensity maps computed from registered image sets. The precise mathematical definitions of these measures are presented in the following section.

Section snippets

Implementation of ART inter-subject registration

Without loss of generality, we assume that the template image It(r) and the subject image Is(r) are of the same matrix and voxel dimensions. If not, the subject image is resized and interpolated so that its voxel and matrix dimensions match those of the template image. The objective of ART is to find a displacement vector w(r) = (ux(r), uy(r), uz(r)) at each voxel r. To achieve this, each voxel is visited in a raster scan fashion. Let Ωr be a neighborhood around and including voxel r. The

Results and discussion

The 16 MP-RAGE volumes were registered to the template volume using ART, AIR, and SPM99, resulting in a total of 48 spatially normalized volumes (16 per method). The 16 registered volumes obtained from applying each algorithm were averaged. Selected slices from the resulting three average volumes are shown in Fig. 1. Qualitatively, it can be clearly seen that the average registered volume corresponding to ART (first column) has higher resolution than those corresponding to AIR (second column)

Conclusions

Details of implementation of a non-parametric method (ART) for estimating displacement fields for inter-subject registration of high-resolution volumetric MRI images were presented. The ability of ART in reducing anatomical variability between subjects was compared to the registration accuracy of two other popular registration programs: AIR and SPM99. Homologous landmark sets manually identified on the images before registration were significantly less dispersed in ART after registration, as

Acknowledgement

This research was supported by Biomedical Engineering Research Grant RG-00-0350 from the Whitaker Foundation to BAA. MJH gratefully acknowledges the support of NIH R01 MH64783 and a NARSAD Young Investigator award.

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