Longitudinal intensity normalization in the presence of multiple sclerosis lesions

This paper proposes a longitudinal intensity normalization algorithm for T₁-weighted magnetic resonance images of human brains in the presence of multiple sclerosis lesions, aiming towards stable and consistent longitudinal segmentations. Unlike previous longitudinal segmentation methods, we propose a 4D intensity normalization that can be used as a preprocessing step to any segmentation method. The variability in intensities arising from the relapsing and remitting nature of the multiple sclerosis lesions is modeled into an otherwise smooth intensity transform based on first order autoregressive models, resulting in smooth changes in segmentation statistics of normal tissues, while keeping the lesion information unaffected. We validated our method on both simulated and real longitudinal normal subjects and on multiple sclerosis subjects.

1. INTRODUCTION

Magnetic resonance (MR) imaging is a popular noninvasive imaging modality used to image the structure of human brains, for applications such as understanding and following the progression of normal aging [1] or diseases like multiple sclerosis (MS) [2]. Analysis of a series of 3D images of a subject taken at different times is valuable since the images provide a time varying analysis of the soft tissues as well as biomarkers for the disease. However, the longitudinal changes of cerbro-spinal fluid (CSF), gray matter (GM), and white matter (WM) are of interest in both normal aging and diseases such as MS [3]. With MS, WM lesions are also present in addition to the healthy brain tissues. New lesions could persist, change volume, or disappear in later time points depending on the type and state of the MS and the lesion it-self. An example is shown in Fig. 1 where four time-points (denoted by T) of magnetization prepared rapid gradient echo (MPRAGE) and -w fluid attenuated inversion recovery (FLAIR) scans are shown for an MS subject. It can be seen that a lesion appears (red arrow) at the third time-point and is gone at the fourth. Other lesions (blue arrow) are present in all the scans. Accurate segmentations of such lesions as well as stable and smooth longitudinal segmentation of the normal tissues (i.e., CSF, GM, WM) are important for understanding the progression of disease. Fig. 1. MPRAGE (top) and FLAIR (bottom) scans for four time-points of an MS subject are shown.