Saori Koshino^{1,2}, Akiyuki Sakakibara^{3}, Christina Andica^{2}, Akifumi Hagiwara^{1,2}, Masaaki Hori^{2}, Osamu Abe^{1}, and Shigeki Aoki^{2}

Diffusion weighted imaging with shorter diffusion time using oscillating gradient spin echo (OGSE) may reveal microstructural features among brain disorders. Here we observed DTI eigenvalues in four patients with epidermoid cysts. The values measured by OGSE were higher than those measured by pulsed gradient spin echo (PGSE; used in conventional MRI), indicating restricted diffusion due to spatial restriction and/or viscosity. The results of our Monte Carlo simulation based on the pathological feature of epidermoid cysts suggest that spatial restriction of multiple ellipsoids formed by keratin filaments as well as viscosity and water exchange should influence the extent of restricted diffusion.

Four patients (1 man and 3 women; mean age, 49.0 years; age range, 39-59 years) were included in this study, and scanned on a 3T MR scanner (MAGNETOM Prisma, Siemens Healthineers, Erlangen, Germany) with a 64-channel head coil. DWI was performed with prototype sequences using b-values of 0 and 1000 s/mm^{2 }and six diffusion-encoding directions with uniform distribution for each PGSE (⊿_{eff }= 35.2 ms) and OGSE with trapezoid-cosine waveforms (frequency = 30 Hz; ⊿_{eff }= 6.5 ms). Other parameters were as follows: repetition time (TR), 4800 ms; echo time (TE), 101 ms; field-of-view, 200 x 200 mm^{2}; matrix size, 164 x 164; slice thickness, 5 mm; and number of slices, 30. DTI eigenvalues (λ_{1}, λ_{2}, λ_{3}), and ADC were measured using VOLUME-ONE, dTV.II.13k as a target for simulation.

On 3D simulation modelling, diffusion of water molecules was simulated using Monte Carlo simulation to find the optimum ellipsoid in epidermoid cysts. 10,000 water molecules started diffusion under the condition of uniform distribution in the ellipsoid. Considering water exchange, transmittance was set on the wall of the ellipsoid with its rate of 0%, 20% and 40%. Water molecules freely diffused with diffusion coefficients of 3.0, 2.5 and 2.0 µm^{2}/ms in the light of viscosity, and performed total reflection after hitting the wall of the ellipsoid. Stochastic gradient method using numerical differentiation was used to obtain the optimum ellipsoid size and DTI eigenvalues. In total, six Monte Carlo simulations were performed in the light of viscosity and water exchange. Each DTI eigenvalue for OGSE and PGSE was obtained in each simulation, and difference between target and simulation eigenvalues was calculated.

1. Novikov DS, et al. Revealing mesoscopic structural universality with diffusion. Proceedings of the National Academy of Sciences 2014;111(14):5088-93.

2. Baron CA, Beaulieu C. Oscillating gradient spin-echo (OGSE) diffusion tensor imaging of the human brain. Magn Reson Med 2014;72(3):726-36.

3. Andica C, et al. Spatial Restriction of Intracranial Epidermoid Cysts Analyzed Using Diffusion-Weighted Imaging with Shorter Diffusion Times. Magn Reson Med Sci 2018;17(3):269-72.

4. Van AT, et al. In vivo investigation of restricted diffusion in the human brain with optimized oscillating diffusion gradient encoding. Magn Reson Med, 2014;71(1):83-94.

5. Smith AB, et al. From the archives of the AFIP: lesions of the pineal region: radiologic-pathologic correlation. Radiographics 2010;30(7):2001-20.

6. Osborn AG, Preece MT. Intracranial cysts: radiologic-pathologic correlation and imaging approach. Radiology 2006;239:650-64.

7. Annet L, et al. Apparent diffusion coefficient measurements within intracranial epidermoid cysts in six patients. Neuroradiology 2002;44:326-28.

8. Santhosh K, et al. Diffusion tensor and tensor metrics imaging in intracranial epidermoid cysts. J Magn Reson Imaging 2009;29:967-70.

9. Koshino S, et al. Oscillating gradient spin echo (OGSE) diffusion weighted imaging of the epidermoid cysts: simulation application. Proceedings of the 27th Annual Meeting of ISMRM, 2018, p.3241.