Neuroradiology

Microadenoma, pituitary

intrasellar, intrapituitary benign tumour of less than 10 mm in diameter. Most frequently prolactin secreting (prolactinoma), microadenomas are also ACTH secreting or, less frequently, GH secreting.

The presenting clinical symptoms are exclusively endocrine, with the characteristic syndromes related to the type of hormone secreted (prolactinomas: amenorrhoeagalactorrhoea in women, impotence in men; ACTH secreting tumours: Cushing's syndrome).

Neuroradiological diagnosis, based in the past mainly on subtle erosive changes of the sellar floor as seen with polytomography, is nowadays made by MR. CT may also be used but due to the inferior resolution MR is preferred.

With both modalities the diagnostic protocol requires direct coronal images, thin slices (3 mm or less), without and with contrast. MR (Fig.1) is also performed in the sagittal plane.

The CT appearance of a microadenoma is that of an iso- or slightly hypodense nodule of few mm in diameter within the pituitary gland. The microadenoma is better seen after contrast injection since the pituitary tissue enhances, due to the absence of a bloodbrain barrier while the adenoma remains hypodense.

A similar effect is seen with MR; usually only T1-weighted sequences are used and the microadenoma is more evident following gadolinium injection since it appears hypointense within an hyperintense pituitary. Enhancement may be inhomogeneous.

Not infrequently, particularly with ACTH secreting tumours, the nodule is difficult to detect and dynamic techniques may improve the detection rate. Selective inferior petrosal sinus venous sampling may also be needed, in ACTH secreting microadenomas.

Not infrequently the microadenoma may be hyperintense on the baseline T1-weighted image, due to intratumoral bleed. Cystic components may either be the result of previous bleedings or be spontaneous, or may appear following drug treatment.

Cysts are usually hypointense on T1-weighted images but they may be hyperintense, and are hyperintense on T2-weighted images.

The differential diagnosis has to be made almost exclusively with Rathkes cleft cysts.

GS

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Fig.1

a. MR, coronal T1-weighted image without contrast. Mild focal depression of the sellar floor on the right side (associated, however, with an asymmetrical positioning of the intersphenoidal septum); mild focal enlargement and hypointensity of the right half of the pituitary gland. The patient has Cushing's disease. b. MR, coronal T1-weighted image following gadolinium injection. The normal pituitary gland enhances homogeneously while the microadenoma remains hypointense.
Microadenoma, pituitary, Fig.1 (a)
Microadenoma, pituitary, Fig.1 (b)