Brain stones revisited—between a rock and a hard place

Tiberin [1] defined “brain stones” or “cerebral calculi” as general terms referring to “large, solitary or multiple, well-circumscribed bony hard areas of pathological intracerebral calcification”, the aetiopathologies of which were thought to be the end result of chronic, insidious “non-neoplastic space-occupying lesions”. The lack of specific data regarding the amount and configuration of calcium salts that would allow detection by plain x-ray is compounded by the fact that no consensus exists on how histological characteristics of intracranial calcifications explain the appearance of intracranial calcifications on plain x-rays [3, 5, 6]. Similarly, there is no clear cutoff value as to what “large” pertains to in the definition of brain stones. The superiority of CT in detecting macroscopic intracranial calcifications as compared to plain x-rays has resulted in routine visualisation of various forms and sizes of intracranial calcification [7]. Despite the ability of CT to quantify calcification, the subjective designation of intracranial calcifications as brain stones has been considered of less importance lately, although the challenge of reaching a radiological diagnosis on the basis of their presence has remained an intellectual curiosity [2].

In a historical context, conventional skull x-rays have been most instrumental in identifying brain stones. The use of various projections has helped localise brain stones and, in some instances, narrow the differential diagnosis [6, 8, 9]. The introduction of the Potter-Bucky grid in the 1920s further increased the detection rate of skull x-rays [5]. CT (and especially the multidetector CTs), because of their multiplanar reconstruction (MPR) and 3D reconstruction capabilities, have supplanted the use of skull x-rays in the diagnosis of intracranial pathology. They are also considered to be better than magnetic resonance imaging (MRI) in identifying and characterising intracranial calcification [4, 10], although newer MRI sequences such as gradient echo T2* and susceptibility-weighted imaging have been shown to be promising in detecting intracranial calcification [11‐13]. In certain clinical situations, neurological symptoms have even been shown to correlate better with MRI findings than with the corresponding CT-identified calcifications [14‐16].

As with many other intracranial pathologies, brain stones can be localised and classified as either extra- or intra-axial. Various imaging findings have been found to suggest an extra-axial localisation of brain lesions such as buckling of adjacent white matter, expansion of the ipsilateral subarachnoid space, presence of bony reactions and the “dural tail” sign [17].

The most common extra-axial aetiologies of brain stones include meningiomas, dural osteomas, calcifying tumours (e.g. craniopharyngiomas) as well as exaggerated physiological calcifications [5, 8, 18]. Macroscopic calcifications are present in meningiomas in up to 60 % of cases, the pattern of which may range from diffuse, rim or sand-like to focal or even globular (Fig. 1) [8, 18, 19]. Calcified meningiomas can be associated with focal thickening of the overlying skull bones or with dilatation of the adjacent air-containing sinus of the skull base [20]. Dural osteomas are believed to be a result of new bone formation arising from the dura and falx; conversely, intra-axial intracerebral osteomas are postulated to originate from primitive mesenchymal cells that have migrated from connective tissue (Fig. 2). Unique to these rare ossified tumours is the fact that they do not possess a laminated bony architecture, but present as a solid mass of calcification [21, 22]. Craniopharyngiomas are frequently located in the suprasellar region and usually present with haemorrhage and calcification mainly in an amorphous and sometimes lobulated pattern [8, 23]. Calcifying pseudoneoplasms of the neuraxis (CAPNON) are extremely rare fibro-osseous lesions that can be extra- or intra-axial. Histologically, these benign lesions are composed of a pathognomonic chondromyxoid matrix in an amorphous or nodular pattern as well as palisading spindle or epithelioid cells and varying proportions of fibrous stroma [24‐26]. When present in the skull base, they are known to symptomatically encase cranial nerves. Lastly, exaggerated physiological calcifications can theoretically lead to identification of brain stones in radiological examinations. Structures known to calcify include the pineal gland, choroid plexus, habenula, dura and arachnoid, falx cerebri, tentorium cerebelli, superior sagittal sinus, petroclinoid and interclinoid ligaments, arachnoid granulations and intra-axially the basal ganglia and cerebellum [9, 27]. Although physiological calcifications are usually considered of no clinical significance, the presence of pathology may be suspected in cases of abnormal increase in their usual size [e.g. normal choroid plexus calcification versus a choroid papilloma (Fig. 3) or carcinoma], location (e.g. displaced pineal gland due to tumour) or age at presentation [e.g. age-related basal ganglia calcification versus Fahr’s disease in younger individuals (Fig. 9)] [8, 18, 19].

Dural calcifications and ossifications, especially of the falx cerebri, can also appear as “brain stones”. They generally do not have any clinical significance and are often incidental findings during CT scans of the brain [28]. Dural calcifications, which are a result of calcium salt deposition, should not be confused with dural ossifications, which actually involves new bone formation [29]. As the falx is derived from multipotential mesenchymal cells, they may become osteogenic after exposure to friction, haemorrhage or trauma [29, 30]. Ossifications in the falx cerebri are considered to be a rare phenomenon [9, 31, 32]. However, their incidence may be underestimated as it has been shown that a percentage of previously identified dense calcification on conventional radiography and CT may actually possess an outer shell of cortical bone surrounding an inner core of (fatty) bone marrow using modern high-resolution CT and MR imaging [28, 31, 32]. They can range in size from small islands of bone to large bony structures, can involve the entire falx [29, 33] and can be associated with certain endocrine and congenital disorders [30, 33].

Intra-axial aetiologies of calcifications potentially resulting in formation of brain stones comprise a broad range of pathologies that can be subdivided into neoplastic, vascular, infectious, congenital and endocrine/metabolic [2, 8, 18, 34].