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time series analysis
Time Series Analysis is a term created to denote the direct analysis of MRI intensity signals over time in long-term follow-up. In contrast to the morphometric approaches, where an MRI image is first segmented into specific tissue regions or classes, which are then measured in size, the TSA approach studies the signal of each image voxel directly. The metric of interest here is not so much the MRI intensity per se, as are the inherent dynamics of the signal, which provide additional insight and specificity toward the underlying pathological processes 1,2.
Attached media: Figure 1
Time Series Analysis of MRI intensity profiles to explore WM signal dynamics across a series of follow-up exams. A ROI comprising a single lesion (box) is shown in a series of small thumbnail images, with the exam time in weeks past baseline shown in yellow. The heat-map profile below shows a cross-section through the lesion along the yellow line indicated, with MRI intensity shown in color and time in weeks along the x-axis. The profile directly visualizes temporary versus permanent changes of tissue appearance. For each voxel change can be defined directly as a temporal gradient in signal intensity.
Attached media: Figure 2a, Figure 2b
Example of lesion evolution over a 1-year follow-up, extracted from frequent MRI using methods of time series analysis. Lesion appearance and resolution are shown on a single slice of a proton-density weighted scan, with temporal profiles extracted for three regions of interest (ROI 1-3). Note the partial and full recovery in central and peripheral parts of the lesion, respectively.
Attached media: Figure 3
Example temporal intensity profiles extracted from a small point sample regions within an active lesion. Three channels of T1-Gd enhancement, proton-density weighting (PDw) and T2weighting T2w). Follow up occurred in weekly to monthly exams over a 1-year period. Note that the T1-Gd signal recovers to almost baseline, whereas lesion center portions of the dual echo signal remains hyperintense. Spatiotemporal propagation pattern of this lesion (see small thumbnails) further suggests that the majority of the signal change is coming from edematous water, rather than destructive tissue damage.
Examples of movies created from TSA processing are available here. They show in parallel the development of T1-Gd-DTPA contrast enhancement (red), proton-denity (PD) and T2-weighted scans. Follow-up was over a period of 1-year, with weekly to monthly scans. Qualitative dynamics of lesion waxing and waning is apparent, as well as the different levels of recovery. Peripheral portions of the lesion return to isointensity, whereas central regions retain residual hyperintensity levels, indicating tissue changes other than the transient inflammatory and edematous portion.
Attached media: MPEG Movie 1, MPEG Movie 2, MPEG Movie 3
References:
- Meier, D.S. & Guttmann, C.R. Time-series analysis of MRI intensity patterns in multiple sclerosis. Neuroimage 20, 1193-209. (2003).
- Guttmann, C.R.G., Ahn, S.S., Hsu, L.G., Kikinis, R. & Jolesz, F.A. The Evolution of Multiple-Sclerosis Lesions On Serial Mr. American Journal of Neuroradiology 16, 1481-1491 (1995).