Brain computed tomography
- The routine study of the head is made in the axial plane.
- Slice thickness was originally 5–10 mm for the supratentorial compartment and 5 mm or less for the posterior fossa, mainly in an attempt to reduce beam-hardening artefact.
- Multidetector CT can provide thinner cuts and many more options, including multiplanar reconstructions.
- Window widths and levels are set to maximize contrast between grey and white matter, and are kept constant from patient to patient.
CONTRAST
- A plain unenhanced study always should be performed first.
- IV contrast enhancement shows areas of blood–brain barrier breakdown within the brain, which is a very nonspecific phenomenon; it can make small lesions much more conspicuous.
- Guidelines for contrast medium use include:
- (A) when plain CT is abnormal and there is a reasonable expectation that enhancement may improve diagnostic accuracy;
- (B) when lesions are suspected close to the skull base or in the posterior fossa (this includes pituitary and imaging for visual failure);
- (C) when staging for carcinomas known frequently to metastasize to the brain;
- (D) when suspecting focal intracranial infections; and
- (E) when meningeal disease is suspected such as caused by sarcoidosis or metastases (e.g. cranial nerve palsies, especially if multiple).
- An IV injection of iodinated contrast medium------ dose equivalents of 15–30 g of iodine generally are given; some clinics use two or even three times that dosage.
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