What does heterogenous in signal on an MRI mean?

Not the same. Homogenous means the appearance is all the same, like a bowl of milk is all white or charcoal is all black. Heterogenous means the appearance is different like salt and pepper.
Its just a descripti. Description of what the doctor saw. You can have even signal intensity or differing signal intensities. Most extra cranial lesions are hetergeneous in their enhancement. But in the brain they tend to be more often homogeneous. Anyhow it is what the reader interprets this to mean thats important not the term itself. Its a descriptor - it kinda means what the reader says later.... In the report.