Hello, Alfonso, 

Thank you so much for your response. I created the `.json` file with the required lines, but I’m still receiving a message stating that there is no unit of Hz.

I used `fslmerge` and `topup` to prepare the images, but it doesn’t seem to be correcting the distortions. Am I correct in understanding that I should use the `topup` command with the raw field map images? I merged the two AP and PA field map images using `fslmerge` and then applied `topup` to the result.

I’ve attached the images after running `topup`. Are these the correct images to use for fieldmap correction?

Thank you for your guidance.

Best,

Hyeonmin