Pineal region (an approach)
A systematic approach to the pineal region is crucial as it is at the confluence of many intracranial structures/regions and is the site of origin of a number of unique pathologies as well as playing host to many entities which are more frequently encountered elsewhere. As such an understanding of the normal anatomy of the region as well as a systematic approach to image interpretation is crucial.
The pineal region is best imaged with MRI although CT, angiography and ultrasound (in infants) also play a role.
The key to successful imaging of the region is the ability to clearly identify the relationship of any pathology to the surrounding structures and as such thin section high resolution imaging in all three planes is crucial. A typical protocol would include:
- sagittal T1 and T2 (high resolution)
- pre and post contrast T1 axial and coronal
- SWI/gradient echo (to assess for presence of calcification)
The pineal gland is a small (~7 mm AP diameter) structure located at the posterior-most aspect of the third ventricle below the internal cerebral veins and above and posterior to the tectum of the midbrain.
On CT it is of soft tissue attenuation with calcification seen in most individuals once they reach adulthood. Calcification is rare in children under 5 years of age.
On MRI it is of intermediate signal intensity, often heterogeneous due to the presence of calcification and cystic spaces, both of which are very common.
For more on the normal appearance refer to the article on the pineal gland.
Systematic approach to interpretation
The first step in assessing a lesion of the pineal region is determine the site of origin and the relationship to the pineal gland (if it can be found) and surrounding structure. Four key structures should be examined with respect to any lesion:
- internal cerebral veins
- tectum of the midbrain
- superior aspect of the cerebellar vermis
- the posterior aspect of the tentorial incisure
Assessing these structures will determine whether a lesion is actually of the pineal gland or whether it represents extension into the pineal region of an adjacent mass (e.g. tentorial meningioma or tectal plate glioma).
It is also important to try and identify the pineal gland, as a separate and normal gland essentially excludes a pineal parenchymal tumour from the differential. In adults, trying to find the pineal calcification is often the easiest way of achieving this.
The pineal gland is not calcified at birth, and in fact it is very rare for it to have any calcification before the age of 5 years of age. As the gland ages it gradually accumulates calcification such that by the age of 30 years or so, most glands have at least some calcium.
The pattern of involvement of calcification when the pineal gland is involved by a mass can be helpful. Pineal parenchymal tumours (e.g. pineocytoma or pineoblastoma) tend to peripherally disperse calcification where as germ cell tumours tend to engulf the calcifications. An easy way to remember this is that pineoblastomas tend to blast the calcifications appart.
As is the case with intracranial lesions elsewhere, the administration of contrast yields significant additional information, even if no abnormal enhancement is present. In the setting of a cystic lesion, identification of a significant nodular component or of a thickened (>2 mm) wall makes a pineal cyst unlikely and essentially excludes an arachnoid cyst or cavum velum interpositum.
It may also make identification of the normal pineal gland easier as it normally enhances, being out side the blood brain barrier.
Diffusion weighted imaging is also helpful as densely cellular tumours (e.g. pineoblastoma, meningioma and lymphoma) tend to demonstrate increased restricted diffusion compared to normal brain parenchyma.
Spread and seeding
Crucial to management and prognosis is identification of the degree of local invasion and distant CSF seeding.
Some pineal region masses are very well circumscribed and do not invade adjacent structures (e.g. pineal cyst, pineocytoma) whereas more aggressive masses can extend extensively into the adjacent brain and brainstem (e.g. pineoblastoma, some germ cell tumours).
Assessing post contrast T1 and FLAIR sequences are particularly useful to determine how well circumscribed the mass is and for the presence of oedema, respectively.
Due its anatomical location closely related to both the ventricular system (third ventricle) and the basal cisterns (quadrigeminal plate cistern) some aggressive pineal region masses have a predilection to seed the ventricular and subarachnoid space (e.g. pineoblastoma). In the case of germinomas, multifocal disease involving not only the pineal gland but also the floor of the third ventricle is relatively common, and it is not always clear whether this represents seeding or de novo multifocality.
In either case when a mass is identified with aggressive features (ill-defined, local invasion, high cellularity (restricted diffusion on DWI)) then a careful search of the whole intracranial compartment needs to be undertaken examining for leptomeningeal or intraventricular nodules. Imaging of the whole spine is also prudent in many instances as the presence of a spinal drop metastasis can affect surgical planning, making aggressive attempts at a curative resection pointless.
Pineal region masses
The pineal region is anatomically complex and plays host to a number of unique masses and tumours as well as potentially affected by many entities seen more frequently elsewhere in the brain.
- cystic non-neoplastic lesions
- pineal parenchymal tumours
- germ cell tumours
- tumours also encountered in the pineal region
- pineal gland metastases
- vascular lesions