The application of a gamma distribution model to diffusion-weighted images of the orofacial region.

Objectives: This study evaluated the correlation among the diffusion-derived parameters obtained by monoexponential (ME), intravoxel incoherent motion (IVIM) and γ distribution (GD) models and compared these parameters among representative orofacial tumours.
Methods: Ninety-two patients who underwent 1.5 T MRI including diffusion-weighted imaging were included. The shape parameter (κ), scale parameter (θ), ratio of the intracellular diffusion (ƒ ₁ ), extracellular diffusion (ƒ ₂ ) and perfusion (ƒ ₃ ) were obtained by the GD model; the true diffusion coefficient (D) and perfusion fraction (f) were obtained by the IVIM model; and the apparent diffusion coefficient (ADC) was obtained by the ME model.
Results: ƒ ₁ had a strongly negative correlation with the ADC (ρ = -0.993) and D (ρ = -0.926). A strong positive correlation between f and ƒ ₃ (ρ = 0.709) was found. Malignant lymphoma (ML) had the highest ƒ ₁ , followed by squamous cell carcinoma (SCC), malignant salivary gland tumours, pleomorphic adenoma (Pleo) and angioma. Both the IVIM and GD models suggested the highest perfusion in angioma and the lowest perfusion in ML. The GD model demonstrated a high extracellular component in Pleo and revealed that the T _4a +T _4b SCC group had a lower ƒ ₂ than the T ₂ +T ₃ SCC group, and poor to moderately differentiated SCC had a higher ƒ ₁ than highly differentiated SCC.
Conclusions: Given the correlation among the diffusion-derived parameters, the GD model might be a good alternative to the IVIM model. Furthermore, the GD model's parameters were useful for characterizing the pathological structure.