Susceptibility-Weighted Imaging at 3 Tesla Delineates the Optic Radiation

Abstract

Some white matter tracts, including the optic radiation (OR), have recently been reported to be delineated as low signal intensity bands (LSBs) on T2*-weighted images at 7 T, presumably because of susceptibility effects caused by myelin.Susceptibility-weighted imaging (SWI) is more sensitive to magnetic susceptibility than T2*-weighted imaging. This study examined whether the LSBs, lateral to the lateral ventricles (LVs) on 3-T SWI, represent the OR. Subjects comprised 17 healthy volunteers. Transaxial and oblique coronal SWIs orthogonal to the long axes of the inferior horns of the LVs were acquired covering the entire OR at 3 T. For quantification of the LSBs, breadth and thickness of LSBs were measured on planes including: (a) the tip of the inferior horn of the LV, (b) the lateral geniculate body, (c) the trigone of the LV, and (d) the posterior horn of the LV. Distances between the temporal pole and most anterior tip (D) of each LSB were also measured. Diffusion tensor tractography of the OR was also compared with the LSB on SWI in 10 of the 17 subjects. Mean and SD of LSB measurements were breadth: (a) 8.8 +/- 1.8 mm, (b) 18.6 +/- 1.7 mm, (c) 20.5 +/- 3.1 mm, (d) 23.9 +/- 4.1 mm; and thickness: (a) 1.7 +/- 0.4 mm, (b) 2.4 +/- 0.5 mm, (c) 3.7 +/- 0.5 mm, (d) 3.3 +/- 0.4 mm, respectively. Mean D was 32.0 +/- 4.0 mm. These measurements were consistent with the anatomic literature regarding the OR. D on the diffusion tensor tractography was 40.3 +/- 7.7 mm, which was significantly longer than that of the LSB on SWI (31.8 +/- 3.4) for the 10 subjects (P = 0.0002). SWI at 3 T can constantly delineate the OR. The present study includes novelty in reporting that the entire length of the OR is constantly depicted on SWI, a 3-dimensional high-resolution imaging sequence, at a clinically more widely available 3-T magnetic field.