Clinical utility and reproducibility of intravoxel incoherent motion MRI in rectal cancer: a structured narrative review.

Introduction: Magnetic resonance imaging (MRI) is a crucial imaging modality for the management of rectal cancer. Conventional MRI provides detailed anatomical information but has a limited capacity to evaluate tumor biology. Intravoxel Incoherent Motion (IVIM) is an advanced functional MRI technique that simultaneously measures tissue cellularity and microvascular perfusion without contrast agents, providing deeper insights into the tumor microenvironment. This narrative review critically examines the current evidence on the clinical usefulness of IVIM MRI in rectal cancer, focusing on three key areas: tumor characterization and staging, prediction and monitoring of treatment response, and the reproducibility and limitations of IVIM parameters. Methods: A literature search was conducted in vid MEDLINE, PubMed, Embase, Web of Science Core Collection, and Google Scholar from 2015 to October 2025. The search included keywords such as "intravoxel incoherent motion OR IVIM" and "rectal cancer OR colorectal cancer." Original studies with at least 10 patients were included; reviews, case reports, and animal studies were excluded. Results: The true diffusion coefficient D appears to be the most biologically interpretable and reproducible IVIM-derived parameter, with several studies linking lower D values to greater cellularity or more aggressive tumor features; however, findings are not uniform across all cohorts. While the perfusion fraction (f) has shown associations with markers of aggressiveness in some studies, its clinical utility is limited by poor reproducibility. For nodal staging, primary-tumor IVIM histogram models have shown promising diagnostic performance, although most evidence remains from single centers and is insufficiently validated. When assessing the response to neoadjuvant chemoradiotherapy (nCRT), a post-treatment increase in D appears to be the most consistent IVIM-derived marker of treatment response. However, its clinical use still requires standardized acquisition and external validation while perfusion parameters (f and D*) exhibit poor test-retest reproducibility. Combining IVIM with complementary techniques, such as diffusion kurtosis imaging or radiomics, may improve predictive performance compared with individual parameters. Conclusion: IVIM-DWI provides non-contrast quantitative information on diffusion and perfusion in rectal cancer. Current evidence suggests that D is the most reproducible and biologically interpretable IVIM parameter, whereas f and D* remain limited by poor repeatability and protocol dependence. IVIM may add value for tumor characterization, nodal risk assessment, and treatment-response evaluation, particularly when combined with histogram analysis, DKI, or radiomics; however, standardized acquisition protocols and external validation are required before routine clinical implementation.