Computed Tomography or CT Scan

X-ray computed tomography, also computed tomography (CT scan) or computed axial tomography (CAT scan), is a medical imaging procedure that utilizes computer-processed X-rays to produce tomographic images or 'slices' of specific areas of the body. These cross-sectional images are used for diagnostic and therapeutic purposes in various medical disciplines Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.

CT produces a volume of data that can be manipulated, through a process known as "windowing", in order to demonstrate various bodily structures based on their ability to block the X-ray beam. Although historically the images generated were in the axial or transverse plane, perpendicular to the long axis of the body, modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures. Although most common in medicine, CT is also used in other fields, such as non destructive materials testing. Another example is archaeological uses such as imaging the contents of sarcophagi. Usage of CT has increased dramatically over the last two decades in many countries. 

40 Slice CT Scan

The introduction of 40-slice CT scanners has opened up new possibilities for CT angiography of the supra-aortal vasculature. Imaging can be performed with even thinner slices, and more rapidly, than on 16-slice systems, and images have higher resolution. Conventional protocols for imaging the brain and its arterial supply must be adjusted to profit from these parameters.

Shorter scan times are expected to reduce motion artefacts, provide better contrast enhancement, reduce venous contamination, and require smaller dosages of contrast material. Extended coverage is especially useful for brain perfusion, but it can capture the entire extra- and intracranial pathway of the carotid arteries, instead of just the bifurcation. High spatial resolution along the patient axis (z-axis) makes it possible to evaluate the smallest aneurysms (2 mm). This capability puts 40-slice CTA in direct competition with MR angiography. Most indications in multi slice CT involve a three-part protocol: unenhanced CT scanning, CT perfusion imaging, and then CTA. Unenhanced CT excludes intra cerebral haemorrhage, extensive infarct, or hemorrhagic transformation of an infarct. If no abnormality is seen, perfusion CT may show early signs of ischemia. CTA is used as a final step to demonstrate most of the underlying vascular pathology. Forty-slice CTA has become a competitive technique for evaluation of the intra- and extra cranial arteries. The high spatial resolution, larger coverage, and reduced requirement for contrast material make the combination of CTA with perfusion CT an excellent strategy for many acute and sub acute situations. Important issues that remain and should be considered are options for dose reduction and optimization of image review.

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