We found that a dual-parameter approach incorporating Ktrans of GRASP and established DWI allows better diagnosis of pLVI in rectal cancer than a traditional single-parameter assessment based solely on DWI. The Ktrans and Kep of pLVI-positive rectal cancer were significantly higher than those of pLVI-negative rectal cancer (p < 0.05).
Igarashi et al.[30] reported that the tumor ADC value is a significant predictor of LVI in breast cancer. In our study, the ADC of pLVI-positive rectal cancer was significantly lower than that of pLVI-negative rectal cancer (p < 0.05), which was consistent with previous studies[34, 35]. This results from the fact that with the increase in the degree of cancerous behavior of malignant tumor, the proliferation of tumor cells significantly accelerates. The ratio of the nucleus and the cell density per unit volume increases, leading to a decrease in the extracellular space distance and free diffusion of water molecules, limitation of distribution, and in turn, a lower ADC value[12]. Choi et al.[6] found no significant correlation for pLVI with either the minimum, maximum, or mean ADC values (p > 0.05). The reason for the discordance between our results and those of others may be attributed to several factors, including the use of different ADC measurements, inability to capture tumor heterogeneity when calculating mean diffusion parameters obtained from single-section regions or multiple small ROIs, and the choice of different b-values[35].
Ktrans reflects the ability of the contrast agent to be transported from the blood vessels to the interstitial space, the higher the degree of malignancy, the more capillaries there are, leading to a higher Ktrans value[5]. Similarly, a higher Kep value represents greater blood return to the vasculature. Therefore, a higher Kep value indicates more leakage of the contrast medium. Our study found that Ktrans and Kep of pLVI-positive rectal cancer were significantly higher than those of pLVI-negative rectal cancer (p < 0.05), which was consistent with previous studies[7]. This is because LVI strongly correlates with a high peritumoral lymphovascular density and more aggressive neovascularization, and these alterations induce differences in the volume and flow of blood in the tumor microcirculatory environment[5, 36–38]. However, according to Lai et al.[8], there was no significant difference in Ktrans between pLVI-positive and pLVI-negative groups (p > 0.05) in breast cancer. The reason could be due to the large sample size of our study and the complexity of the underlying pathophysiology of heterogeneous rectal cancer.
Quantitative DCE-MRI parameters and ADC values were reported to be closely correlated with clinical and histological grade, response to neoadjuvant chemoradiotherapy (CRT), and prognostic factors of various tumors[14, 39, 40]. There are some limitations of conventional DCE-MRI, however. First, the temporal resolution is approximately 5–18 s per phase. Second, the acquisitions require breath holds, which can be challenging in some patients and can restrict spatiotemporal resolution and volumetric coverage in dynamic imaging acquisitions[41–45]. In our study, we utilized radial acquisition with GRASP, which increased spatial and temporal resolutions compared to TWIST. According to the literature[46, 47], the higher the temporal resolution, the more accurate the semiquantitative and quantitative parameters obtained during the DCE scan will be.
Winkle et al.[48] revealed that GRASP had been shown to improve the diagnostic accuracy of multiparametric MRI examinations of the prostate when incorporated into a dual-parameter model that included diffusion and perfusion characteristics. Ao et al.[45] reported that the Ktrans and ADC values were independent predictors of extramural venous invasion in rectal cancer. According to the study by Oberholzer et al.[49], MR perfusion may serve as a complementary biomarker to ADC values to assess tumor characteristics associated with the effectiveness of chemoradiation before treatment initiation. We found that the dual-parameter analyses, which combined ADC values with the Ktrans from GRASP, provided a better diagnosis of pLVI-positive rectal cancers than the single-factor analysis of ADC. GRASP was the only technique used to calculate perfusion maps, resulting in a statistically significant difference in tumor detection. Thus, it may be hypothesized that the combined increased spatial and temporal resolution of the former acquisition method benefits the discrimination of pLVI-positive rectal cancer.
Our study had limitations. First, the TWIST and GRASP groups were scanned in MR scanners with different field strengths, which may affect the measurement of perfusion and diffusion parameters. Second, it is essential to note that this is a single institutional study without a validation cohort, so future studies are required to determine whether our results can be replicated in other medical institutions. Third, the retrospective design, which may predispose to selection bias.