Quantification of spinal cord atrophy from magnetic resonance images via a B‐spline active surface model

Abstract

Quantitative assessment of disease progression in multiple sclerosis (MS) using MRI is an important issue for therapeutic monitoring and understanding the development of disability. In particular, the relationship between spinal cord atrophy and development of disability has been the subject of recent interest (5,7,13,16,17) and a correlation between atrophy and disability has been demonstrated.Although imaging techniques have improved significantly and provide convenient images to study the spinal cord, reliable and reproducible postprocessing and image analysis techniques to measure spinal cord atrophy are still limited. Atrophy is mostly assessed by measuring cross-sectional area at specific levels, typically C2 and C5 (5,17), along the cervical cord. This creates several problems, including the choice of the level at which the measurements are performed, the cord orientation, and the cord segmentation process. To provide nonbiased area measurements, the image, or part of the image, needs to be reformatted (17), or acquired (5), with slices perpendicular to the cord. The concept of "perpendicular to the cord" is somewhat imprecise and operator-dependent for a structure which is not a strict cylinder or a surface of revolution. It has not been formally defined in the above-mentioned publications. Moreover, spinal cord cross-sectional area has often been measured either manually or using intensity-based 2D processing techniques. The limitations of such methods are various: because the orientation of the cord changes along its axis, measurements are restricted to a predefined level at which cord and slices are orthogonal; intensity-based segmentation is hindered by the significant intensity variations caused by surface coils typically used during acquisition; 2D measurements are more prone to being biased by partial volume effect than 3D measurements; manual analysis are more time-consuming and more sensitive to intra-and interoperator variability.We present here a method that aims to solve those problems. The method is based on a 3D extraction of the cervical cord surface and the computation of a medial axis, used to define orthogonal cross-sections anywhere along the cord. The segmentation method is semiautomated with very few interventions by the operator, based on the optimization of a parameterized active surface. Cross-sectional area measurements can be provided globally as well as locally at any point along the cord. We show that the method provides accurate and reproducible measurements. MATERIALS AND METHODSMR Images were acquired on a Signa 1.5T system (GE Medical Systems, Milwaukee, WI), with a phased array spinal coil for data reception. Volume-acquired inversionprepared fast spoiled gradient echo (IR-FSPGR) acquisition was performed (60 ϫ 1 mm slices, TI 450 ms, TE 4.2 ms, TR 17.8 ms, flip angle 20°, matrix 256 ϫ 256, field of view 25 ϫ 25 cm, 7-min acquisition time). A group of nine control subjects were scanned with scan-rescan (two sessions on the same day). SegmentationIn IR-FSPGR images, the ce...

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