The Skull and Brain: Introduction & Imaging Methods — Anatomy Radiology

Introduction

Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) have become the absolute mainstays of modern cerebral imaging. Traditional skull radiography now plays very little part in diagnosis and has been largely replaced by multislice CT. Similarly, invasive catheter angiography is now strictly reserved for specialized diagnostic indications or interventional therapeutic procedures, having been superseded by non-invasive CT Angiography (CTA) and MR Angiography (MRA).

While both modalities are incredibly valuable in clinical practice, MRI generally provides far superior anatomical detail of the brain parenchyma compared to CT.

Tissue Appearances

⚡ High-Yield Snapshot

T1W MRI: Grey matter is darker than white matter because myelin lipids are bright.
T2W MRI: Grey matter is brighter than white matter.
CT Scans: White matter is paradoxically darker than grey matter due to the radiolucent lipid.

Bone, Air, and Fluids

Dense compact bone lacks free protons and therefore appears as a complete signal void (black) on MRI, whereas it appears solid white on CT. Air (e.g., in the paranasal sinuses) appears black on both CT and MRI. On MRI, hypointensity (black) is also seen in areas of severe iron deposition (such as the globus pallidus and substantia nigra).

Contrast Enhancement & Blood Flow

The blood-brain barrier (BBB) prevents intravenous contrast agents (iodine for CT, or gadolinium for MRI) from enhancing normal cerebral parenchyma. Enhancement is strictly confined to blood within cranial arteries/veins, dural sinuses, and highly vascular structures outside the BBB (e.g., the choroid plexuses, pituitary gland, and infundibulum).

🩸Fact to Remember: Flow Voids

A notable difference in MRI is the depiction of rapidly flowing blood or CSF (e.g., through the cerebral aqueduct), which appears as a 'signal void' (black) and does not enhance, unlike CT.