In Cushing's syndrome, MRI identification of a pituitary mass in conjunction with positive non-invasive endocrine tests (such as CRH stimulation) obviates the need for invasive diagnostic tests such as inferior petrosal sinus sampling. Localization of the tumor by MRI also allows the neurosurgeon to target a specific area during TSS and so may improve the success of surgical treatment.1, 2
Thus, efforts to improve MRI detection of these tumors may reduce diagnostic costs and improve treatment outcomes for patients with Cushing's disease. To that end, we sought to identify MR protocol parameters associated with improved detection of an adenoma.
Spin echo (SE) MRI protocols were the first to gain widespread popularity and continue to be the most commonly used pulse sequence in the evaluation of the pituitary gland.7
The SE sequence is made up of two radiofrequency pulses - one pulse that excites the spins in the tissue and a subsequent 180 degree pulse that refocuses a resultant `echo.' T1-weighted images use a short TR and TE. As a result, tissues that relax more quickly (such as fat) present as bright signal. Tumors have longer T1 relaxation times and show as a dark signal. 5
Based on this, T1-weighted spin echo MRI has been recommended for the routine evaluation of pituitary adenomas. 8–10
Besides the spin echo technique, other MRI protocols have been used for the detection of corticotropinomas. In one study, dynamic MRI, in which a gradient echo technique was obtained within seconds of contrast administration, identified tumor in 11 out of 14 patients with microadenoma, compared to 8 positive studies with conventional spin echo imaging. However, three falsely positive tumors were identified, suggesting in this report, an important loss of specificity.11
We previously compared the performance of a spoiled gradient recalled acquisition in the steady state (SPGR) protocol with conventional spin echo MR where SPGR had superior sensitivity (80% vs. 49%) but a higher false positive rate (2% vs. 4%). 3
The current study demonstrated that not all T1-weighted SE protocols are alike, and suggests specific MRI parameters that may influence results. The two most important factors were the field of view and TR/TE. Other parameters such as the use of thin (3 mm or less) slices, use of contrast, and a fine matrix size are known to influence resolution. However, these were not different in the non-NIH and NIH studies, so their relative contribution could not be evaluated.
The field of view appeared to be crucial for tumor detection, probably because a large field of view has less resolution for a given matrix.12
Thus the resolution is inherently superior in a pituitary study performed with a FOV of 12 ×12 cm as compared to a FOV of 18 ×18 cm for the same matrix of 192 × 256.
A second influential MRI parameter was the TR/ TE value. Previous publications recommend TR/TE values in the range of 500–700/15–25 ms.7, 9, 13, 14
Since the contrast resolution of the soft tissues is strongly influenced by short TR/TE values, we have adopted the shortest possible values for the available gradient strength of our magnet. In this study, 86% of outside scans had TR values greater than 416 ms, while 90% of TE values were greater than 10 ms.
Magnetic field strength also may affect the detection of corticotropinomas. We used a 1.5 Tesla magnet, as described in a number of previous studies. 4, 9, 11
. Twenty-five percent of the outside scans used a magnet field strength less than 1.5 Tesla, suggesting that a lower magnetic field strength may decrease the ability to detect these tumors. While it is possible that this parameter was important, it was not statistically different between the two sets of studies.
Another important MRI parameter is the use of a contrast agent, which improves the detection of ACTH-secreting adenomas by taking advantage of the different signal dynamics of contrast enhancement between the normal tissue and tumor.11
The T1-weighted images of the normal pituitary gland, stalk, and cavernous sinus all increase in signal rapidly following administration of gadolinium.15
A pituitary adenoma enhances less intensely than the adjacent normal tissue and appears as a focal hypointense area. Steiner et al. demonstrated that the detection of pituitary adenomas on T1-weighted SE images improved from 47% to 91% after administration of gadolinium. 16
In the current study, all scans, both at NIH and other institutions, used contrast material.
Other MRI parameters such as thin sections (3 mm or less) and a fine matrix are important for high resolution. 15
However, these two parameters were similar between the studies performed at the NIH and the studies done at other institutions. Thus, it seems unlikely that contrast administration, section thickness, or matrix size contributed to the differences in detection of the pituitary adenoma in our patients.
One explanation for the ability to detect tumor at the (later) NIH study might be that the tumors grew sufficiently in the interval between the studies to allow detection. We doubt that this factor influenced the difference in detection rate of these 3 – 15 mm adenomas. The median interval of 4.0 months was almost certainly not sufficient for dramatic growth, as ACTH-secreting tumors typically grow extremely slowly. Another explanation is that of a referral bias in favor of the NIH technique, since tumors seen on MRI at another institution were not included in the study.
Observer error also may explain differences in tumor identification. Our neuroradiologist identified three pituitary lesions on outside films that were initially read as negative at another institution. These were not small tumors, ranging from 5 to 7 mm in size. Thus, optimizing MRI techniques is essential, but an experienced reader of the MRI films also is required.
In conclusion, we have identified MR protocol parameters associated with improved identification of a pituitary tumor on a T1-weighted spin echo study. Preoperative localization of the pituitary lesion by MRI has been associated with a better outcome, presumably because it facilitates operative identification of the pathological tissue.1, 2
We recommend that endocrinologists understand and discuss optimal parameters on pituitary MR scans with their radiology colleagues, to increase their diagnostic utility and to potentially improve surgical outcomes.