Unidentified Bright Objects in the Internal Capsule in Neurofibromatosis Type 1: Microstructural Changes and Their Relationship to Fiber Tracts

Objective: In children with neurofibromatosis type 1 (NF1), the most common intracranial lesions are foci with signal hyperintensity in T2-weighted magnetic resonance (MR) images. These focal signal abnormalities are often described as an unidentified bright object (UBO), as only little is known about their nature. Diffusion-weighted imaging allows the in vivo characterization of tissue microstructure and can visualize fiber tracts. The aim of this work is to further characterize the microstructure of UBOs in the internal capsule and their relationship with fiber tracts.

Method: Three children with NF1 and UBOs in the internal capsule (6, 13 and 16 years of age) and a typically developing child (16 years of age) were investigated using diffusion-weighted MRI with a high angular resolution sequence on a 1.5 T Siemens Avanto. In addition to assessment of diffusion tensor parameters and fiber orientation distribution (FOD) in the UBO and the corresponding contra-lateral side, probabilistic fiber tracking was performed for generating the cortico-spinal tract and anterior thalamic radiation.

Results: Two patients showed UBOs unilateral at identical locations in the knee of the internal capsule, in one patient the UBOs were bilateral. Compared with the normal appearing side and to the healthy control, the UBO showed significantly different diffusion parameters with higher mean and axial diffusivity, lower fractional anisotropy. In addition, visual analysis of the FOD amplitude showed decreased fiber density in the UBO as well as in the anterior part of the thalamus. MR tractography revealed that corticospinal fibers traversed the internal capsule in the middle of the posterior limb, both in the patients and the control. Fibers passing through the UBO appeared less dense and mildly shifted. Patients had no corresponding neurological deficit.

Conclusion: These results of advanced diffusion imaging analyses indicate that UBOs consist of reduced fiber density of fibers passing through them, but still remaining pathways, accordingly without loss of neurological function.