MRI brain and spinal cord
Magnetic resonance imaging uses the magnetic properties of protons to generate images of tissues.
It has the advantage of not exposing the patient to ion-ising radiation (particularly important in young infants, children and pregnant mothers). It is very versatile due to various processing which may be performed on the data. However, the main disadvantage is that any metal in the patient not only causes interference, but can become dislodged. Pacemakers may be reprogrammed by the electromagnetic pulses, and the magnetic field may induce a large current leading to burns so MRI is contraindicated. The MRI scanner can also be claustrophobic, and in the case of sick patients, is relatively unaccessible – although some units have facilities for ventilation in the MRI scanner.
MRI differentiates soft tissues much more clearly than CT.
T1 weighted images: Fat, brain tissue and old haematomas have high signal strength. Bone, CSF, cartilage and calcium have low signal strength (i.e. look darker).
T2 weighted images: High water content tissues and CSF have high signal strength (i.e. look white).
Tumours as small as 5 mm are clearly seen, even in the brainstem and cerebellum (these areas with a lot of surrounding bone are often poorly seen on CT). Acute infarction is seen earlier than on CT, at 6–12 hours. With new techniques (diffusion weighted MRI) changes can appear even earlier than this. MRI is excellent at detecting white matter lesions such as demyelination in multiple sclerosis, or intracerebral small vessel disease. Spinal cord MRI is useful for tumours, cord compression and vascular malformations. Intravenous gadolinium is used as a contrast to demonstrate areas of increased vascular supply and oedema. It is also useful in MR angiography (MRA), for example when looking for AV malformations.