Influence of thickening of the inner skull table on intracranial volume measurement in older people

Abstract

Introduction: It is generally assumed that intracranial volume (ICV) remains constant after peaking in early adulthood. Thus ICV is used as a 'proxy' for original brain size when trying to estimate brain atrophy in older people in neuroimaging studies. However, physiological changes in the skull, such as thickening of the frontal inner table, are relatively common in older age and will reduce ICV. The potential influence that inner table skull thickening may have on ICV measurement in old age has yet to be investigated.

Methods: We selected 60 (31 males, 29 females) representative older adults aged 71.1-74.3years from a community-dwelling ageing cohort, the Lothian Birth Cohort 1936. A semi-automatically derived current ICV measurement obtained from high resolution T1-weighted volume scans was compared to the estimated original ICV by excluding inner skull table thickening using expert manual image processing.

Results: Inner table skull thickening reduced ICV from an estimated original 1480.0ml to a current 1409.1ml, a median decrease of 7.3% (Z=-6.334; p<0.001), and this reduction was more prominent in women than men (median decrease 114.6 vs. 101.9ml respectively). This led to potential significant underestimations of brain atrophy in this sample by 5.3% (p<0.001) and obscured potential gender differences.

Conclusions: The effects of skull thickening are important to consider when conducting research in ageing, as they can obscure gender differences and result in underestimation of brain atrophy. Research into reliable methods of determining the estimated original ICV is required for research into brain ageing.

Fig. 1

(A) sagittal slice from a T2W MRI sequence showing the inferior limit of the intracranial volume at the foramen magnum (orange). (B) identification of the inferior boundary using the upper edge of the supraorbital ridge (blue).

Fig. 2

(A) ICV mask acquired using the MCMxxxVI method and providing the current ICV measurement. (B) ICV mask edited to provide the estimated original ICV excluding the effects of skull thickening.

Fig. 3

Example of the difficulty identifying the original inner skull table prior to the effects of skull thickening in those participants with some thickening (A, B), and those participants with little thickening (C, D).