Quantitative assessment of diffusion-weighted MR imaging in patients with primary rectal cancer: correlation with FDG-PET/CT

Abstract

Purpose: The aim of the study was to assess correlations between parameters on diffusion-weighted imaging and 2-deoxy-2-[(18)F]fluoro-D-glucose-positron emission tomography/computed tomography (FDG-PET/CT) in rectal cancer.

Procedures: Thirty-three consecutive patients with pathologically confirmed rectal adenocarcinoma were included in this study. Apparent diffusion coefficient (ADC) maps were generated to calculate ADC(mean) (average ADC), ADC(min) (lowest ADC), tumor volume, and total diffusivity index (TDI). PET/CT exams were performed within 1 week of magnetic resonance imaging. Standardized uptake values (SUVs) were normalized to the injected FDG dose and body weight. SUV(max) (maximum SUV), SUV(mean) (average SUV), tumor volume, and total lesion glycolysis (TLG) were calculated using a 50% threshold.

Results: Significant negative correlations were found between ADC(min) and SUV(max) (r = -0.450, p = 0.009), and between ADC(mean) and SUV(mean) (r = -0.402, p = 0.020). A significant positive correlation was found between TDI and TLG (r = 0.634, p < 0.001).

Conclusion: The significant negative correlations between ADC and SUV suggest an association between tumor cellularity and metabolic activity in primary rectal adenocarcinoma.

Fig. 1

MRI images of a 55-year-old man with moderately differentiated rectal adenocarcinoma. a Axial single-shot TSE T2-weighted image of the pelvis shows a large rectal mass (arrow). b Axial DWI image of the pelvis at the same slice location as a is shown with inverted gray scale to demonstrate a PET-like image of the rectal mass (arrow). c Fused image from a and b can be performed for easy viewing if desired. d On the ADC map generated from b, an ROI was manually drawn along the contour of the tumor (black line). Subsequently, ADC_mean, ADC_min, and the cross-sectional area of the tumor on this image were calculated by ImageJ software.

Fig. 2

PET/CT images of an 82-year-old woman with moderately differentiated rectal adenocarcinoma. a–c Fused PET/CT images in coronal and sagittal reformatted planes, as well as in the axial plane show a hypermetabolic lesion in the rectum. A 3D ROI (green box) was placed to cover the entire lesion on the Advanced AW Workstation. d From the corresponding PET images, SUV_max was measured. Subsequently, SUV_mean, tumor volume, and TLG were calculated automatically using a threshold of 50% SUV_max.

Fig. 3

Scatter plots show the significant negative correlations between ADC_min and SUV_max (a), ADC_mean, and SUV_mean (b).

Fig. 4

A Bland–Altman plot analysis shows a high correlation of volumes detected by DWI and PET/CT. X axis indicates the average tumor volumes measured by DWI and PET. Y axis indicates the difference in tumor volumes on DWI and PET. The solid horizontal line reflects the bias between the two measurements. The two dash lines denote 95% limit of agreement (mean difference ±1.96 × SD).