A special type of Magnetic Resonance Imaging (MRI) , called a dynamic MRI is the preferred method for pituitary tumors/lesions/adenomas. With a dynamic MRI, a series of MRI images are taken quickly over several minutes after the infusion of gadolinium, a special contrast agent. Dynamic scans are used because pituitary tumors and normal gland tissue absorb the dye at different speeds. The contrast between the normal tissue and tumor is easier to delineate with this type of imaging.Dynamic MRIs may be especially important when a small tumor of the type that causes acromegaly or Cushing's disease is suspected.
According to recent research, a dynamic pituitary MRI has high sensitivity and specificity for the diagnosis of mild Cushing's syndrome and should be part of the initial workup. Because the pituitary gland is small, tumors/adenomas/lesions are even smaller and are difficult to spot. Therefore, it is essential the right type of MRI is ordered for you. A pituitary gland MRI is not the same as an MRI of the brain. A brain MRI does not show the pituitary nearly as well.
A closed MRI is preferred over an open MRI because the resolution is better. However, since obesity is a symptom of Cushing's, it is sometimes difficult to find a decent MRI big enough. There are some newer "open" MRI's which are 1.5 Tesla and above. (Click here for locations.)Typically, most MRI scanners have a strength of 1.5 Tesla but the newer, 3.0 Tesla scanners are becoming more prevalent and if available, should be used.
A computed tomography (CT) scan of the adrenal glands is usually done to localize an adrenal tumor if it has not been found and if a pituitary adenoma has been ruled out. If a discrete adenoma is seen in one adrenal gland and not on the other gland, then that with the biochemical evidence may be enough for a unilateral adrenalectomy. (Just the adrenal gland with tumor is removed.)
If the biochemical evidence is supportive of a pheochromocytoma, again a CT scan of the adrenal glands is performed. Most pheochromocytomas in the adrenal gland are easily visible with a CT scan. If the CT scan does not show one, then there is a chance of an "extra-adrenal" (outside the adrenal gland) pheochromocytomas. The literature supports using one or more of the following methods to find the tumor.
- iodine-131–labeled metaiodobenzylguanidine (131I-MIBG)
- positron emission tomography (PET) scan
- octreotide scan (done more in the UK than in the US)
- abdominal MRI
Often, adrenal tumors are found when CT or MRI scans are done for something else.
Ectopic tumors are probably the hardest to locate. Often, only after pituitary and adrenal tumors are ruled out are they pursued. Metomidate PET is a highly selective and potentially promising approach of adrenocortical tumours that has been introduced in only few PET centres up to now. A gold standard technique in localizing ectopic ACTH-producing tumors has not been determined, but CT, MRI, and somatostatin receptor scintigraphy (SRS) are all used in various combinations to try to find the tumor(s).
This is a bare-bones outline of possible imaging. The research is always telling diagnosticians more, and the biochemical evidence is very, very important in the diagnosis. Imaging is fallible and only as good as the equipment and the person reading the results. It is always good to get multiple opinions from respected surgeons who do a lot of pituitary or adrenal surgeries when reading imaging studies.
More on this series tomorrow.