Elsevier

NeuroImage

Volume 50, Issue 2, 1 April 2010, Pages 446-455
NeuroImage

Rapid semi-automatic segmentation of the spinal cord from magnetic resonance images: Application in multiple sclerosis

https://doi.org/10.1016/j.neuroimage.2009.12.121Get rights and content

Abstract

A new semi-automatic method for segmenting the spinal cord from MR images is presented. The method is based on an active surface (AS) model of the cord surface, with intrinsic smoothness constraints. The model is initialized by the user marking the approximate cord center-line on a few representative slices, and the compact surface parametrization results in a rapid segmentation, taking on the order of 1 min. Using 3-D acquired T1-weighted images of the cervical spine from human controls and patients with multiple sclerosis, the intra- and inter-observer reproducibilities were evaluated, and compared favorably with an existing cord segmentation method. While the AS method overestimated the cord area by approximately 14% compared to manual outlining, correlations between cord cross-sectional area and clinical disability scores confirmed the relevance of the new method in measuring cord atrophy in multiple sclerosis. Segmentation of the cord from 2-D multi-slice T2-weighted images is also demonstrated over the cervical and thoracic region. Since the cord center-line is an intrinsic parameter extracted as part of the segmentation process, the image can be resampled such that the center-line forms one coordinate axis of a new image, allowing simple visualization of the cord structure and pathology; this could find wider application in standard radiological practice.

Introduction

In multiple sclerosis (MS), both focal and diffuse damage occurs in brain and spinal cord tissues, and magnetic resonance imaging (MRI) is the most sensitive technique for detecting changes in the integrity of tissue over time (Bakshi et al., 2008; Filippi et al., 2006). While quantitative measures have concentrated mainly on the brain, the relationship between spinal cord atrophy and development of disability has been the focus of growing interest (Kidd et al., 1993, Filippi et al., 1996, Lin et al., 2004) and a correlation between cord atrophy and disability has been shown (Losseff et al., 1996, Lin et al., 2004). For assessing atrophy, high-resolution T1- or T2-weighted MR images of the cord can be acquired either as multi-slice axial 2-D images, or with 3-D pulse sequences, although it is most common to use 3-D T1-weighted images. Spinal cord atrophy is a putative outcome measure that assesses the effects of emerging neuroprotective therapies (Kalkers et al., 2002), which represent a major unmet need in the current approach to managing MS. However, there is still a requirement for a fast, reliable post processing method to assess spinal cord atrophy from such scans, since the rate of atrophy is only on the order of 1% per year in the relapsing-remitting form of MS (Rashid et al., 2006a).

The most common method of assessing atrophy is to measure the cross-sectional area at specific anatomical levels, typically in the cervical region in the area from C2 to C5 (Filippi et al., 1996, Losseff et al., 1996, Tench et al., 2005). Normally, the images must be acquired perpendicular to the cord axis at these levels, or else the cord images must be reformatted to the correct plane orientation. The cord surface is then identified semi-automatically using a combination of intensity and gradient information to determine the location of the cord edge. The general method has proved reliable for assessing cross-sectional area (CSA) over a limited cord region. Since the cord curves in three dimensions, the method is not suitable for measuring the cord over an extended region. Other methods have been developed to segment the cord over an extended region, covering more of the cord length (Coulon et al., 2002; McIntosh and Hamarneh, 2006). Active surface (AS) models are an extension of the active contour (Kass et al., 1987) methodology to three dimensional images and, while they can give reproducible results, methods based on AS models can require extensive computer analysis time, on the order of several hours (Coulon et al., 2002). We report here on a new AS method that requires much less computation time, on the order of a minute. It is based on a compact parametrization of the cord surface, and uses the notion that the cord has a smooth surface with a cross-sectional shape that varies only slowly along the cord axis. All processing steps are implemented using locally-produced software.

We demonstrate that the method provides reproducible measures of cord cross-sectional area from the cervical region, and that it can be used to segment T1-weighted images of the cord from the foramen magnum to C5. The measures of cord atrophy are shown to have clinical relevance, with strong correlations between cord cross-sectional areas and clinical disability scores in MS patients. Furthermore, we illustrate that the method should also be applicable to 2-D multi-slice T2-weighted images, and that it may be possible to segment the whole spinal cord reliably, from the upper cervical origin to its inferior terminus in the lumbar region.

Section snippets

Model and initialization

The cord topology is that of a cylinder, and a convenient and compact representation of the surface can be achieved by specifying the center-line c of the cord, and a radius generator (Fig. 1). The center-line is parameterized by the through-plane distance, z:c(z)=(cx(z),cy(z),z)The functions cx(z) and cy(z) are implemented as cubic spline interpolators of the location of the cord at the slice centers, so that the unit vector tangential to the cord center-line curve (the center-line tangent

Reproducibility

Details of the cord area measured by the three observers using both the Losseff and AS methods are shown in Table 2. The mean inter-observer CoV was 1.07% for the AS method and 7.95% for the Losseff method. The mean intra-observer CoV was 0.44% for the AS method and 2.15% for the Losseff method. The intra-observer CoV for the AS method for areas measured just at C2 was 0.59% and the inter-observer CoV was 1.36%. Thus, if the same observer is used for all measurements, changes as small as 0.67 mm

Discussion

We have demonstrated a rapid method for segmenting the spinal cord from MR images, and have shown its high inter- and intra-observer reproducibility on 3-D T1-weighted images of the human cervical cord. The method needs only modest user interaction, requiring the user to identity landmarks at the extremes of the region of cord to be assessed, and to mark the approximate center-line of the cord on several slices between the landmarks. The algorithm is based on a constrained active surface model

Acknowledgments

Dr. Bakshi acknowledges support of this project by research grants from the National Multiple Sclerosis Society (RG3705A1 and RG3798A2) and National Institutes of Health (1R01NS055083-01).

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