Understanding Neurology

32ADVANTAGES AND DISADVANTAGES OF MRI(COMPARED TO CT)Advantages❏ Select any plane, e.g. oblique.❏ No ionizing radiation.❏ More sensitive (earlyischaemia/demyelination).❏ No bony artefacts (looking at sites adjacentto bone).Disadvantages❏ Cannot use with any ferromagnetic implant,e.g. pacemaker.❏ Claustrophobic.❏ Does show bony artefact.❏ Long-term hazards not known.T1 and T2 images can easily be distinguished. OnT1, CSF is black (hypointense) while on T2 it is white(hyperintense). Table 6 will help interpretation of MRimages (increase or decrease in signal is defined inrelation to normal grey matter). Figures 17–20present MRI images.Paramagnetic enhancementThe intravenous agents used (e.g. gadolinium) inducea strong local magnetic field effect that will shortenT1 signals. This will result in enhancement of theimage in areas of contrast leakage (across a damagedblood–brain barrier) or within the vascular compartment(arteries, microcirculation, and veins).Gadolinium and other such agents highlightischaemia, infection, and demyelination and can helpdistinguish tumours from surrounding oedema.Table 6 Guide to identifying the MRI sequenceT2-weighted image❏ CSF bright.❏ Grey matter brighter than white matter.Perfusion-/diffusion-weighted image❏ CSF grey.❏ Grey matter brighter than white matter.T1-weighted image❏ CSF dark.❏ White matter brighter than grey matter.CSF: cerebrospinal fluid.171817 T1 sagittal magnetic resonance image of the brain.18 T1 coronal magnetic resonance image of the brain.