Multiple Sclerosis, Magnetic Resonance Imaging, Signal-to-Noise Ratio, Signal Intensity
The pathophysiologic changes of multiple sclerosis in the central nervous system present as changes in signal intensity of the white matter, particularly as hyperintensities in fluid attenuated inversion recovery (FLAIR) MRI sequences. The number of lesions and the presence of new lesions are critical factors in diagnosis, defining activity of multiple sclerosis, and in treatment decisions. Extrinsic contrast parameters as addition of fat saturation may increase the relative brightness of the lesions, increasing their conspicuity. The diagnostic performance of fat-saturated T2-weighted FLAIR MRI sequence is unknown.
Objectives: To compare the signal intensity of multiple sclerosis white matter lesions in fat-saturated T2-weighted FLAIR sequences and usual T2-FLAIR MRI sequences.
Methods: : Diagnostic test comparison study. After estimation of sample size, brain MRI studies of patients with confirmed clinical diagnosis of multiple sclerosis performed during the period from 2013 to 2014 were selected. These were performed according to the institutional protocol, in an HD23 General Electric 1.5 Tesla MRI scanner (GE Medical Systems, Milwaukee, Wisc. USA). Two radiologists reviewed the images in Barco Nio 3MP displays (E-3620 MA) (BOAT, Brussels, Belgium). Region-of-interest (ROI) with a diameter of 3mm was applied for measuring signal intensity in at least two lesions, normal white matter and air. Contrast to noise ratio (CNR) was calculated as the difference in intensity of the lesion and the white matter, divided by the standard deviation of the noise. For statistical analysis Wilcoxon matched-pairs signed-rank test and Somers´ D were used.
Results: Three studies were excluded because of artifacts, 102 lesions in 27 patients were reviewed, median lesions per patient were 4 (range 2-4). The mean contrast-to-noise was 99 (SD 46.58, range 24.87 - 268.64) in FLAIR and 116.99 (SD 62.49, range 30.63 - 330.48) in fat-saturated T2-Weighted FLAIR. The difference was statistically significant (p < 0.002). In this sample the positive difference was 38.39% (95% CI:14.36 - 55.34%).
The intensity of multiple sclerosis white matter lesion in Fat-Saturated T2-Weighted FLAIR is higher than in FLAIR MRI at 1.5 Tesla, the difference beeing statistically significant. Contrast-to-noise is a critical factor affecting image quality and appearance, the increased contrast increases conspicuity of white matter lesions.