Both CT and MRI will provide not only an excellent anatomic overview, but also very detailed information about the various complex structures and will in most cases provide enough information to rule out various diseases and pathological conditions. Use of contrast media (CM) to enhance pathological findings is recommended in many diagnostic settings. If significant pathology is found a detailed characterisation of the lesion can be done.
General CNS
In order to differentiate between tumours and other focal abnormalities, i.v. contrast media are used in CT. Iodinated contrast media normally remain in the blood stream because the so-called blood-brain barrier (BBB) is intact and does not allow large molecules to leak through the walls of the capillaries. If, on the other hand, the barrier is damaged because of a pathological process like tumour, infarction or infection, leakage of contrast media through the BBB will occur and so the structure will be readily seen on the CT image.
CT is a sensitive method to detect focal abnormalities in central nervous system (CNS), and is used world-wide.
Because of the absence of bone artefacts as seen on CT, MRI is preferable in the posterior fossa of the skull; i.e. the cerebellum, brain stem and basal parts of the cerebrum.
In coming years, MRI will be used in functional imaging, reflecting subtle changes in cerebral function due to biochemical/physiological and not necessarily structural changes.
Brain tumours
Large and medium-sized tumours are seen equally well on CT and MRI, but smaller tumours are usually easier to detect with MRI.
A focal abnormality detected on CT may in some instances raise the question: is this lesion a tumour or other type of lesion, e.g. an infarcted area (loss/diminished blood supply)? This clinically very important question can often be answered by an MR-examination.
Acoustic neurinoma (Schwannoma)
Medium and larger tumours are detected equally well by both modalities. Very small tumours situated in the bony channel (porus acusticus) is better visualised by MRI.
Pituitary tumours
Both CT and MRI is very sensitive to detect structural changes. MR is better to define the precise extent of the lesion, and also has advantages in detecting microadenomas because lack of bony artifacts.
Cerebrovascular diseases
Within the first few hours of the stroke, MR is able to detect and define an area of cerebral infarction. CT usually is negative during the first 24 hours. In later stages, CT and MRI are equivalent. In stroke and transitory ischaemic attacks (TIA) NM (cerebral blood flow SPECT) and PET are also sensitive and frequently used techniques. When it comes to heamorrhage CT is better to detect bleeding during the first 24-48 hours. On the other hand, subacute haemorrhages (10-20 days) are often seen better with MRI.
Trauma
CT is preferable during acute phase because shorter examination time and a higher detectability of bleeding and haematoma.
Demyelinating disorders (Multiple Sclerosis)
These disorders manifest themselves as focal spots and are quite easy detected by MRI. However, there are conditions which can mimic demyeliniation on MRI, so the clinical findings must be compared with MRI scan.
Dementia
This diagnosis rest upon a clinical neurological examination.
Both CT and MRI can show multiple lesions as in multi-infarct dementia; MRI has somewhat higher sensitivity (ability to detect) lesions in white matter but the specificity is somewhat lower.
Atrophic changes, especially in frontal and temporal regions, are associated with, but not conclusive for Alzheimer disease.
CBF SPECT may differentiate between Alzheimer and other forms of dementia.