Non-parametric intravoxel incoherent motion analysis of the thyroid gland.

PURPOSE: To implement a protocol for intravoxel incoherent motion (IVIM) of the thyroid, to determine base parameters in healthy volunteers, and to provide preliminary experience on clinical applicability in one patient.

MATERIALS AND METHODS: Eight healthy volunteers underwent 3T MRI using a diffusion weighted echo-planar imaging sequence with 12 different b-values between 0-800 s/mm(2). The IVIM parameters diffusion coefficient D, pseudo-diffusion coefficient D*, perfusion fraction Fp, and the optimal b-values thresholds were calculated for each thyroid lobe, muscle tissue and the cerebrospinal fluid (CSF) using a non-parametric multi-step algorithm and compared with a Student's t-test. A p-value <0.05 was considered significant.

RESULTS: Mean values for healthy thyroid tissue were: D 1.01 ± 0.13 × 10(-3) mm(2)/s, D* 71.0 ± 52.5 × 10(-3) mm(2)/s and Fp 17.1 ± 4.2%; for muscle: D 0.50 ± 0.21 × 10(-3) mm(2)/s, D* 58.3 ± 99.2 × 10(-3) mm(2)/s and Fp 26.5 ± 9.3%; and for CSF D 2.18 ± 0.93 × 10(-3) mm(2)/s, D* 99.2 ± 41.2 × 10(-3) mm(2)/s and Fp 74.6 ± 12.7%. The optimal b-value threshold separating diffusion and perfusion effects in thyroid ranged between 0-70 s/mm(2). Healthy thyroid tissue showed similar Fp compared to muscle, both lower than CSF.

CONCLUSIONS: The proposed IVIM protocol provides surrogate markers on cellular diffusion restriction and perfusion; thereby providing a more comprehensive description of tissue properties compared to conventional DWI.