Mono, bi- and tri-exponential diffusion MRI modelling for renal solid masses and comparison with histopathological findings.

Purpose: To compare diffusion tensor imaging (DTI), intravoxel incoherent motion (IVIM), and tri-exponential models of the diffusion magnetic resonance imaging (MRI) signal for the characterization of renal lesions in relationship to histopathological findings.

Methods: Sixteen patients planned to undergo nephrectomy for kidney tumour were scanned before surgery at 3 T magnetic resonance imaging (MRI), with T-weighted imaging, DTI and diffusion weighted imaging (DWI) using ten b-values. DTI parameters (mean diffusivity [MD] and fractional anisotropy [FA]) were obtained by iterative weighted linear least squared fitting of the DTI data and bi-, and tri-exponential fit parameters (D, fand D, ff) using a nonlinear fit of the multiple b-value DWI data. Average parameters were calculated for regions of interest, selecting the lesions and healthy kidney tissue. Tumour type and specificities were determined after surgery by histological examination. Mean parameter values of healthy tissue and solid lesions were compared using a Wilcoxon-signed ranked test and MANOVA.

Results: Thirteen solid lesions (nine clear cell carcinomas, two papillary renal cell carcinoma, one haemangioma and one oncocytoma) and four cysts were included. The mean MD of solid lesions are significantly (p < 0.05) lower than healthy cortex and medulla, (1.94 ± 0.32*10 mm/s versus 2.16 ± 0.12*10 mm/s and 2.21 ± 0.14*10 mm/s, respectively) whereas f is significantly higher (7.30 ± 3.29% versus 4.14 ± 1.92% and 4.57 ± 1.74%) and f is significantly lower (18.7 ± 5.02% versus 28.8 ± 5.09% and 26.4 ± 6.65%). Diffusion coefficients were high (≥2.0*10 mm/s for MD, 1.90*10 mm/s for D and 1.6*10 mm/s for D) in cc-RCCs with cystic structures and/or haemorrhaging and low (≤1.80*10 mm/s for MD, 1.40*10 mm/s for D and 1.05*10 mm/s for D) in tumours with necrosis or sarcomatoid differentiation.

Conclusion: Parameters derived from a two- or three-component fit of the diffusion signal are sensitive to histopathological features of kidney lesions.