Computed tomography

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The most significant recent development has been volumetric data acquisition in times compatible with a single breath hold. This is achieved by a slip ring design allowing the patient to move through the gantry during scanning, often referred to as helical or spiral CT. The data can be presented as two-dimensional images (slices) in any desired plane and at any chosen slice interval, or in less familiar ways such as maximum intensity pixel projections or shaded surface renderings. The quality of the images depends upon, in particular, the tightness of the spiral and the number of data points collected. There is a time penalty for increasing image quality and also a reduction in the area covered; a major constraint is overheating of the X-ray tube. Whole regions such as the head or thoracic spine can now be imaged in seconds, and virtually the whole body can be imaged in a few minutes.

CT still has advantages over MRI. There are no contraindications, as there are to MRI, and the configuration of the gantry is generally far less claustrophobic. Life-support systems are not incompatible, provided that they are portable. CT is better than conventional tomography at delineating most fractures and bone destruction, and may be superior to MRI in correctly diagnosing recently extravasated clotted blood and subarachnoid hemorrhage. The disadvantage of CT is relatively low soft tissue contrast, particularly in areas enclosed by dense bone, such as the vertebral canal.

Special techniques using CT

CT angiography is capable of demonstrating the cervical and intracranial arteries and veins, is widely available, and has been shown reliably to demonstrate carotid stenoses and intracranial aneurysms larger than 3 to 4 mm. Data acquisition takes about 30 s (i.e. faster than magnetic resonance angiography), but the technique requires the rapid injection of about 90 ml of intravenous contrast medium.

CT myelography requires the introduction of contrast medium into the subarachnoid space via a spinal needle. In supine immobile patients this can be achieved via a fluoroscopy-guided lateral cervical puncture at C1-2, rather than a lumbar puncture. CT myelography may require little or no patient manipulation and imaging is much faster and of more reliable quality than conventional myelography.

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