Patients
This prospective study was approved by our hospital’s Institutional Review Board, and all participants had written informed consent. The inclusion and exclusion criteria are outlined in Fig. 1. This study included a cohort of 73 patients.
MRI Acquisition
MRI images were obtained using a 3.0T MRI scanner (MAGNETOM Skyra, Siemens Healthcare) equipped with an 18-channel body coil. The hybrid IVIM-DKI model sequence employed the echo planar imaging (EPI) technique with the following parameters: repetition time (TR) of 5700 ms, echo time (TE) of 76 ms, flip angle (FA) of 90°, field of view (FOV) of 200 mm × 200 mm, slice thickness of 3 mm, bandwidth of 1724 Hz/pixel, and b values of 0, 20, 40, 60, 80, 150, 400, 600, 800, 1000, 1500, 2000, and 2500 mm²/s. Diffusion was acquired in three directions, with a total scan time of 572 seconds. Routine MRI sequences included axial high-resolution T2-weighted imaging (TR/TE of 5300 ms/93 ms, FA of 150°, FOV of 250 mm × 250 mm, slice thickness of 2 mm, bandwidth of 248 Hz/pixel) with a scan time of 138 seconds. Additionally, coronal T2-weighted imaging, sagittal T2-weighted imaging, and axial T1-weighted imaging were performed as part of the protocol.
Histopathology
Pathological features were evaluated by specialized gastrointestinal pathologists using surgically resected specimens. The staging of rectal cancer follows the American Joint Committee on Cancer Tumor Node Metastasis Staging System [13]. Other histological features of rectal cancer were evaluated, including tumor differentiation degree (classified as well differentiated, moderate, or poor), lymphovascular invasion (LVI), and nerve infiltration (NI). Pathologists perform routine processing on the excised tissue blocks and select tumor areas for KRAS gene testing. Genomic DNA was extracted from tumor tissue sections fixed in formalin and embedded in paraffin using the QIAamp DNA FFPE tissue kit (Qiagen, Chatsworth, CA). The detection of KRAS using the mutation amplification refractory mutation system method is achieved through polymerase chain reaction (PCR) [14].
Quantitative hybrid IVIM-DKI model Analysis
Two radiologists with more than a decade of experience in abdominal diagnosis profiled each tumor using a polygonal area of interest (ROI) on b0 images from the DWI. Accurate delineation was ensured by referencing T1WI and T2WI images. High b-value imaging data were particularly used to identify the solid component of the tumor and align it with the ROI. The sections with the deepest tumor infiltration were selected for delineation and necrotic cystic lesion areas were excluded. The tumor ROI is then replicated on images corresponding to each b value.
The ADC is obtained by fitting all b values to the following equation [1]:
\(\:\frac{{S}_{\left(b\right)}}{{S}_{0}}=\text{exp}\left(-b\times\:ADC\right)\) [1]
Where S(b) is the average of the signal strength at 13 b values and \(\:{S}_{0}\) is the signal strength observed measured in the absence of a diffusion gradient.
Parameter estimation in the hybrid IVIM-DKI analysis is performed using nonlinear least squares (NLLS) optimization and parallel computation along with a custom MATLAB toolbox (version 9.1, MathWorks, Natick, MA, USA). The analysis utilized all 13 b values from 0 to 2500 s/mm², combining a reconstruction based on the iterated total variational (TV) penalty function with a hybrid IVIM-DKI model.
The parameters of the function are calculated using the signal strength derived from all 13 b values, including true diffusion coefficient (D), and kurtosis (K), perfusion fraction (f), pseudo-diffusion coefficient (D*). The calculation process is as follows [15]:
\(\:\frac{{S}_{\left(b\right)}}{{S}_{0}}=f\times\:\text{exp}\left(-b\times\:{D}^{*}\right)+(1-f)\times\:\text{e}\text{x}\text{p}(-b\times\:D+\frac{1}{6}\times\:{b}^{2}\times\:{D}^{2}\times\:K)\) [2]
In Equation [2], S(b) is the signal intensity at a given b value, S0 denotes the signal intensity at b = 0, and b is the b factor. The Levenberg-Marquardt algorithm was employed to perform the least squares fit of the signal intensities corresponding to the b values. All parameters were derived from the average signal intensity within the ROI. To enhance the fitting accuracy of the bi-exponential model and prevent overfitting, the following procedure was implemented: Initially, the data for b > 200s/mm2 were used to determine the parameters D and K. Subsequently, keeping D and K fixed, curve fitting was conducted for f and D∗ across all b values using Equation [2].
Statistical Analysis
Statistical analysis was performed using IBM SPSS Statistics version 26.0. Initially, the data were tested for normality. As the data did not follow a normal distribution, non-parametric tests were subsequently applied. The Mann-Whitney U test compared continuous variables, including IVIM-DKI model parameters and ADC values, between the KRAS mutation and wild-type groups. Receiver operating characteristic (ROC) curve analysis evaluated the discriminatory power of parameters showing significant differences between these groups. The ROC analysis provided the area under the curve (AUC), sensitivity, specificity, and optimal cut-off values for these significant parameters. Additionally, the Mann-Whitney U test and Kruskal-Wallis test examined the relationship between IVIM-DKI model parameters, ADC values, and clinicopathological features in rectal cancer patients.