What is an MRI?
• Imaging modality whereby images are produced by pulsing radio waves in a magnetic field.
MRI
• Radiowaves are then used to de-phase the protons.
• The timing and strength of the waves can be manipulated to produce different tissue weighting characteristics.
MRI Disadvantages
• Time consuming
• Acquire in one plane at a time (CT acquires volume)
• Inferior spatial resolution compared with CT
• Cramped space - claustrophobia
Standard Brain Pulse Sequences
• T1 weighted
• T2 weighted
• FLAIR
• DWI
• SWI
• T1 post contrast
• TOF MRA
T1
• Fat
• Methaemaglobin (sub-acute haemorrhage)
• Mineralisation
• Slow flow
• Contrast
• Proteinaceous fluid
• Melanin
SWI
• Susceptibility Weighted Imaging
• Gradient echo sequence
• Sensitive to
– Venous blood
– Haemorrhage
– Iron
DWI
• Diffusion Weighted Imaging
• Sensitive to restrictoin of Brownian motion of extracellular water
• Sensitive for
– Ischaemia
– Pus
– Epidermoid
Contrast Enhanced T1
• Contrast does not cross the normal BBB
• Contrast is useful for detecting and characterising diseases that disrupt the BBB
MR Angiography
• Contrast enhanced
• Time of Flight
• Lower resolution than CTA but can provide a 4D image (TRICKS/TWIST)
Prepontine Cistern
Quadrigeminal Plate Cistern
Fourth Ventricle
Foramen Magnum
Suprasellar Cistern
Third Ventricle
Cerebral Aqueduct
Anterior Cerebral Arteries
Middle Cerebral Artery
Internal Carotid Artery
Vertebral Arteries
Basilar Artery Posterior Cerebral Artery
Head of Caudate Nucleus
Lentiform Nucleus
Third Venticle
Splenium of Corpus Callosum
Genu of Corpus Callosum
Internal Capsule
Thalamus
Trigone of lateral Ventricle