Automated computer quantification of breast cancer in small-animal models using PET-guided MR image co-segmentation

Care providers use complementary information from multiple imaging modalities to identify and characterize metastatic tumors in early stages and perform surveillance for cancer recurrence. These tasks require volume quantification of tumor measurements using computed tomography (CT) or magnetic resonance imaging (MRI) and functional characterization through positron emission tomography (PET) imaging. In vivo volume quantification is conducted through image segmentation, which may require both anatomical and functional images available for precise tumor boundary delineation. Although integrating multiple image modalities into the segmentation process may improve the delineation accuracy and efficiency, due to variable visibility on image modalities, complex shape of metastatic lesions, and diverse visual features in functional and anatomical images, a precise and efficient segmentation of metastatic breast cancer remains a challenging goal even for advanced image segmentation methods. In response to these challenges, we present here a computer-assisted volume quantification method for PET/MRI dual modality images using PET-guided MRI co-segmentation. Our aims in this study were (1) to determine anatomical tumor volumes automatically from MRI accurately and efficiently, (2) to evaluate and compare the accuracy of the proposed method with different radiotracers (¹⁸F-Z _HER2-Affibody and ¹⁸F-flourodeoxyglucose (¹⁸F-FDG)), and (3) to confirm the proposed method’s determinations from PET/MRI scans in comparison with PET/CT scans.

After the Institutional Administrative Panel on Laboratory Animal Care approval was obtained, 30 female nude mice were used to construct a small-animal breast cancer model. All mice were injected with human breast cancer cells and HER2-overexpressing MDA-MB-231HER2-Luc cells intravenously. Eight of them were selected for imaging studies, and selected mice were imaged with MRI, CT, and ¹⁸F-FDG-PET at weeks 9 and 10 and then imaged with ¹⁸F-Z _HER2-Affibody-PET 2 days after the scheduled structural imaging (MRI and CT). After CT and MR images were co-registered with corresponding PET images, all images were quantitatively analyzed by the proposed segmentation technique.

Automatically determined anatomical tumor volumes were compared to radiologist-derived reference truths. Observer agreements were presented through Bland-Altman and linear regression analyses. Segmentation evaluations were conducted using true-positive (TP) and false-positive (FP) volume fractions of delineated tissue samples, as complied with the state-of-the-art evaluation techniques for image segmentation. Moreover, the PET images, obtained using different radiotracers, were examined and compared using the complex wavelet-based structural similarity index (CWSSI). (continued on the next page) (continued from the previous page)

PET/MR dual modality imaging using the ¹⁸F-Z _HER2-Affibody imaging agent provided diagnostic image quality in all mice with excellent tumor delineations by the proposed method. The ¹⁸F-FDG radiotracer did not show accurate identification of the tumor regions. Structural similarity index (CWSSI) between PET images using ¹⁸F-FDG and ¹⁸F-Z _HER2-Affibody agents was found to be 0.7838. MR showed higher diagnostic image quality when compared to CT because of its better soft tissue contrast. Significant correlations regarding the anatomical tumor volumes were obtained between both PET-guided MRI co-segmentation and reference truth (R²=0.92, p<0.001 for PET/MR, and R²=0.84, p<0.001, for PET/CT). TP and FP volume fractions using the automated co-segmentation method in PET/MR and PET/CT were found to be (TP 97.3%, FP 9.8%) and (TP 92.3%, FP 17.2%), respectively.

The proposed PET-guided MR image co-segmentation algorithm provided an automated and efficient way of assessing anatomical tumor volumes and their spatial extent. We showed that although the ¹⁸F-Z _HER2-Affibody radiotracer in PET imaging is often used for characterization of tumors rather than detection, sensitivity and specificity of the localized radiotracer in the tumor region were informative enough; therefore, roughly determined tumor regions from PET images guided the delineation process well in the anatomical image domain for extracting accurate tumor volume information. Furthermore, the use of ¹⁸F-FDG radiotracer was not as successful as the ¹⁸F-Z _HER2-Affibody in guiding the delineation process due to false-positive uptake regions in the neighborhood of tumor regions; hence, the accuracy of the fully automated segmentation method changed dramatically. Last, we qualitatively showed that MRI yields superior identification of tumor boundaries when compared to conventional CT imaging.