BlindHarmony: "Blind" Harmonization for MR Images via Flow model

In MRI, images of the same contrast (e.g., T1) from the same subject can show noticeable differences when acquired using different hardware, sequences, or scan parameters. These differences in images create a domain gap that needs to be bridged by a step called image harmonization, in order to process the images successfully using conventional or deep learning-based image analysis (e.g., segmentation). Several methods, including deep learning-based approaches, have been proposed to achieve image harmonization. However, they often require datasets of multiple characteristics for deep learning training and may still be unsuccessful when applied to images of an unseen domain. To address this limitation, we propose a novel concept called "Blind Harmonization," which utilizes only target domain data for training but still has the capability of harmonizing unseen domain images. For the implementation of Blind Harmonization, we developed BlindHarmony using an unconditional flow model trained on target domain data. The harmonized image is optimized to have a correlation with the input source domain image while ensuring that the latent vector of the flow model is close to the center of the Gaussian. BlindHarmony was evaluated using simulated and real datasets and compared with conventional methods. BlindHarmony achieved a noticeable performance in both datasets, highlighting its potential for future use in clinical settings.