Accurate staging and grading of rectal tumors can facilitate the treatment process and improve the prognosis of patients. DWI is a non-invasive functional imaging technology that provides information about the diffusion of water molecules in the tumor microenvironment. DKI provides more effective clinical information about lesions. Studies have shown that DKI can be used to assess the non-gaussian distribution of molecules in patients with brain diseases such as stroke and Parkinson’s disease. The purpose of this study was to compare the application value of DKI and DWI in the clinical evaluation of rectal carcinoma.
ADC values reflect the state of limited diffusion of water molecules. The higher the density of tissue cells, the more limited the diffusion of molecules, the lower the ADC value and vice versa. Malignant cells elicit a fast differentiation rate, thus the diffusion of water molecules in such lesions are limited by the restriction of macromolecular substances and cell membranes,resulting in a decrease in the ADC value. This study’s results reveal that ADC values in the rectal carcinoma group were lower than those depicted in the control group(P < 0.001)༌which was consistent with the previous theoretical hypotheses.Sun YS et al  confirmed that the optimal b value of DWI in rectal cancer was 1000 s/mm2༌which could better overcome the influence of perfusion and T2 penetration effects. However, measurement of the ADC value is based on the fact that the diffusion of water molecules conforms to the gaussian distribution. Withal, the human body is composed of several tissues, hence the cell types, cell density and blood supply of each tissue are different. The diffusion of water molecules follows a non-gaussian distribution。DKI represents the plane echo sequence of a single excitation, with 7 different b values and 3 scanning directions perpendicular to each other;The MK value is calculated by the apparent kurtosis coefficient (AKC) in each direction. The larger the MK value is, the more complex the structure is in the target organization, so it is used to measure the complexity of the organizational structure༛ the larger the MK value is, the more complex the structure is in the target organization, so it is used to measure the complexity of the organizational structure. Moreover, malignant lesions have an abundant supply of interstitial blood vessels, so it was speculated that the MK values of malignant lesions should be higher than those of normal tissues, which was confirmed by the results of this study. On the other hand, MD values were similar to the ADC values of the DWI single index model, which reflects the diffusion state of water molecules in the human body. Malignant cells are compactly arranged, which limits the diffusion of water molecules, leading to a subsequent decrease in their MD values. Based on the aforementioned observations, we speculate that the MD values of malignant lesions should be lower than those of normal tissues. In this study, differences in ADC, MD and MK values between the two groups were statistically significant, indicating that the three parameters were all effective parameters for the diagnosis of rectal carcinoma and could reflect the changes of lesion tissue structures at the microstructure level.
The ROC analysis conducted in this study suggested that the AUC for the MK value was the largest and that of the ADC value was the smallest. The sensitivity and specificity values of MK value were higher than those of the MD and ADC values, so DKI was more effective in diagnosing rectal carcinoma. At present, the preoperative diagnosis of rectal cancer is mainly based on the combination of DWI and T2WI, and DKI can also provide similar diffusion images. Rectal tumors delineate a significantly high signal on images with high b values, while the normal intestinal wall, bezoars and surrounding normal structures exhibit a low signal when inhibited, and the higher b the value, the more obvious the inhibition;On the image with a b value of 2000 s/mm2, only the tumor elicited a significantly high signal. Thence, DKI sequences can also highlight the tumor lesions and has a higher SNR, which can display a more intuitive image of the tumor compared to conventional MRI images, making up for the deficiency of conventional scan sequences in the diagnosis of rectal cancer.
DKI and DWI are both magnetic resonance functional imaging technologies, and the diffusion limitation degree of water molecules has an impact on the MK, MD and ADC values. The results obtained in this study showed that ADC values were positively correlated with MD values, and negatively correlated with MK values. Due to the rapid proliferation of malignant cells, the extracellular volume is rapidly depleted, leading to a drop in both the ADC and MD values. However, the more complex the arrangement structure of tumor cells, the further the diffusion of water molecules from the gaussian distribution, hence increasing the MK value.
Limitations of this study: ①This study used a retrospective study design, with the possibility of selection bias. ② Rectal carcinoma was not subdivided according to the pathological grading, and different differentiation grades may affect the DKI model parameters and ADC values;③This study’s sample size is relatively small, which may be affected by validation bias. ④Intrarectal coils were not used in this study to increase the SNR. Thus, future studies should expand the sample size in order to increase the reliability of the conclusions.