Can an MRI detect a haematoma in abdomen?

Yes and also other . Things like a post operative fluid collection or abscess. Mri does not use ionizing radiation and is also safe and can use safe contrast. read more about why MRI of the pelvis is a good test after an ultrasound. Ct scans are also good but use a lot of radiation and cause cancers in 1in 500 to 1 in 1000 patients ten years later.
Yes. Mri is an excellent way to detect blood. Changes in the signal that occur with time can actually give an idea how old the hematoma is.