The ADC and MD values of ccRCCs were higher than those of RAMFs (p < 0.05), while comparable FA, MK and KA were found between ccRCCs and RAMFs (p > 0.05). Moreover, the RK values of RAMFs were higher than those of ccRCCs (p < 0.05). ROC curve analyses showed that MD values had the highest diagnostic efficacy in differentiating ccRCCs from RAMFs. For pairwise comparisons of ROC curves and diagnostic efficacy, ADC was worse than DKI analysis (p < 0.05).
DKI is a dimensionless measure that quantizes the deviation of the water diffusion displacement profile from the Gaussian distribution of unrestricted diffusion, providing a measure of the degree of diffusion hindrance or restriction[11]. It has been shown to offer superior sensitivity over conventional DTI[12]. An appealing aspect of the incorporation of DKI into routine clinical practice is that it can be performed in a straightforward manner, given that the sequence is performed in essentially the same manner as a standard DWI sequence[13], aside from the need for generally higher b values.
In a recent study, Lanzman et al[14]. could already highlight the potential of DTI for non-invasive functional assessment of transplanted kidneys. They also could show significant differences in FA values of the medulla between allograft recipients with heavily impaired renal function and those with moderate or mild impairment in renal function. Comparing MK values of normal kidneys with those of patients with various renal diseases may help to evaluate the clinical significance of renal kurtosis values and the role of the renal DKI[15]. For instance in RCC, DKI may provide additional diagnostic information. Since DKI has been proven to be more sensitive to tissue microstructure in comparison to FA measures, DKI of the kidney might be useful in evaluating conditions involving the renal tumors[16]. Notohamiprodjo et al[17]. reported that the higher the b-values and the number of directions, the more accurate the measurement of diffusion is. In our study, we could show that b-values in the range of about 0 to 2000 s/mm2, with 30 diffusion encoding directions are sufficient in abdominal DKI to observe the departure of the diffusion signal from mono-exponential behavior.
In our study, statistically significant differences were observed in the MD and ADC values between ccRCC and RAMF. Many authors attribute higher MD and ADC to higher cellularity. Tissue free water contents and structures can influence MD and ADC. Increase in MD and ADC due to micronecrosis or altered viscosity of the medium possibly counterbalances decreased MD and ADC values in ccRCC [18]. ccRCC is rich in lipid content of its cells; cholesterol, neutral lipids, and phospholipids are abundant on pathology [19].
Based on the viscosity that are greater in microstructure generally have a larger number of diffusion barriers [20], causing water diffusion to deviate more from a Gaussian distribution, a higher RK value typically implies a greater viscosity. As illustrated in our study, RAMF showed greater RK values than ccRCC, and showed a significant difference, compatible with the knowledge that the RAMF have a greater viscosity and the restriction of water diffusion by the walls of hemorrhage or haemosiderin deposition.
The MD and RK parameter showed greater discrimination of renal tumor types than the ADC parameter, perhaps because the latter includes microcirculation and tissue cellularity information [21]. These two sources of information may affect the ADC measurement in opposing ways, decreasing sensitivity and specificity[22]. Morover, the additional MD and RK parameters give specific information on non-Gaussian diffusion behavior, a more accurate measurement of tissue diffusion [23].
The main limitation to our study is that the small number of patients in each type of renal tumors, especially for RAMF. Recommend further studies with larger population to validate the results of our study. Furthermore, we did not evaluate comparison with other subtypes of RCCs or renal oncocytoma. Therefore, it may be too exploratory to apply our findings to the differentiation of renal oncocytoma from other types of renal tumors. But the papillary, chromophobe RCCs or renal oncocytoma are less likely to be confused with ccRCC or RAMF on CT and/or MRI. ccRCCs or RAMFs show hypervascularity and heterogeneous enhance-ment, whereas papillary or chromophobe RCCs are characteristically hypovascular. Renal oncocytomas central stellate scar, homogeneous enhancement and a spoke-wheel pattern of enhancement have been suggested as its characteristics.
Retest reliability was measured by an independent repeat assessment by two observers with 5 and 10 years’ experience and shown to be excellent. In addition, there was no statistically significant difference in retest reliability between the two observers, suggesting that the stability of DKI evaluation of microstructural differences in ccRCC or RAMF is not affected by work experience which is conducive to the clinical popularization and application of DKI technology.