Signal Intensity Changes of Normal Brain at Varying High b-Value Diffusion-Weighted Images Using 3.0T MR Scanner

ABSTRACT

PURPOSE: Using diffusion-weighted MR imaging (DWI), to evaluate the signal intensity characteristics of normal adult brain as diffusion gradient strength (b value) increases from 1,000 to 3,000 s/mm2. MATERIALS AND METHODS: Twenty-one healthy volunteers with neither neurologic symptoms nor pathologic findings at axial and sagittal T2-weighted MR imaging were involved in this study. All images were obtained with a 3.0T MR scanner. Six sets of spin-echo echo-planar images were acquired in the axial plane using progressively increasing strengths of diffusion-sensitizing gradients (corresponding to b values of 0, 1,000, 1,500, 2,000, 2,500, and 3,000 s/mm2). All imaging paremeters other than TE remained constant. Changes in normal white-gray matter signal intensity observed at variable b-value DWI were qualitatively analysed, and the signal-to-noise ratios (SNRs) in six anatomic regions (frontal and parietal white matter, genu and splenium corporis callosi, the posterior limb of the internal capsule, and the thalamus) quantitatively, and the ratios were averaged and compared with the average SNR of 1,000 s/mm DWI. RESULTS: As gradient strength increased from 1,000 to 3,000 s/mm2, both gray-and white-matter structures diminished in signal intensity, and images obtained at a b value of 3,000 s/mm2 appeared very noisy. White matter became progressively hyperintense to gray matter as the diffusion sensitizing gradient increased, especially at the centrum semiovale, the posterior limb of the internal capsule, and the splenium corporis callosi, but the genu corporis callosi, showed exceptional intermediate low signal intensity. At quantitative assessment, the signal-to-noise ratio decreased as the diffusion sensitizing gradient increased. Relative to the images obtained at a b value of 1,000 s/mm2, average SNRs were 0.71 (b=1,500 s/mm2), 0.52 (b=2,000 s/mm2), 0.41 (b=2,500 s/mm2), 0.33 (b=3,000 s/mm2). CONCLUSION: As the diffusion sensitizing gradient increased, the signal-to-noise ratio of brain structures diminished, especially at a b value of 3,000 s/mm2, and white matter became relatively hyperintense compared to gray matter. In order to avoid misdiagnosis, it is important to be aware of the nature of normal changes in the signal intensity of gray-white matter occurring at high-b-value DWI.