Computed Tomography - Medical Imaging - Lecture Slides, Slides of Medical Genetics

Download Computed Tomography - Medical Imaging - Lecture Slides and more Slides Medical Genetics in PDF only on Docsity!

Computed Tomography

• Radiography: 3 problems
  • 3D collapsed to 2D
  • Low soft-tissue contrast
  • Not quantitative

• The mathematics behind X-Ray CT (reconstruction from projections) applies to other modalities as well (PET, Spect, etc).

• Computed tomography (CT) is in its fourth decade of clinical use and has proved valuable as a diagnostic tool for many clinical applications, from cancer diagnosis trauma to osteoporosis screening. - CT was the first imaging modality that made possible to probe the inner depths of the body, slice by slice.

• Because of the long acquisition times required for the early scanners and the constraints of cardiac and respiratory motion, it was originally thought that CT would be practical only for head scans.

• CT is one of the many technologies that was made possible by the invention of computer. - The clinical potential of CT became obvious during its early clinical use, and the excitement forever solidified the role of computers in medical imaging.

• The invention of the CT scanner earned Godfrey Hounsfield of Britain and Allan Cormack of the United States the Nobel Prize for Medicine in 1979. - CT scanner technology today is used not only in medicine but in many other industrial applications, such as nondestructive testing and soil core analysis.

BASIC PRINCIPLES

• The mathematical principles of CT were first developed by Radon in 1917. - Radon’s treatise proved that an image of an unknown object could be produced if one had an infinite number of projections through the object.

• With a conventional radiograph of the patient’s anatomy, information with respect to the dimension parallel in the x-ray beam is lost. - This limitation can be overcome, at least for obvious structures, by acquiring both a posteroanterior (PA) projection and a lateral projection of the patient.

For example, the PA chest image yields information concerning height and width, integrated along the depth of the patient, and the lateral projection provides information about the height and depth of the patient integrated over the width dimension.

• For objects that can be identified in both images, such as a pulmonary nodule on PA and lateral chest radiographs, the two films provide valuable location informarion. - For more complex or subtle pathology, however, the two projections are not sufficient.

• Imagine that instead of just two projections, a series of 360 radiographs were acquired at 1- degree angular intervals around the patient’s thoracic cavity. - Such a set of images provides essentially the same data as a thoracic CT scan.

• The tomographic image is a picture of a slab of the parient’s anatomy. - The 2D CT image corresponds to a 3D section of the patient, so that even with CT, three dimensions are compressed into two. - However, unlike the case with plain film imaging, the CT slice-thickness is very thin (1 to 10 mm) and is approximately uniform.

• The 2D array of pixels (short for picture elements) in the CT image corresponds to an equal number of 3D voxels (volume elements) in the patient. - Voxels have the same in-plane dimensions as pixels, bur they also include the slice thickness dimension.