Instrumentation and Principles of CT

The words computed tomography (CT) refer to a method of tomographic imaging in which a “tomographic,” or cross-sectional, slice is imaged with the aid of computer processing to obtain an exact representation of the slice. Tomographic imaging is important since in conventional projection imaging, such as plane-film x-ray imaging, a small feature may be difficult to visualize because of the confusing superposition of many overlying layers of different structures. X-ray tomographic imaging was developed more than 50 years ago as an approach to cross-section slice imaging. One of the most successful commercial devices was the axial tomograph developed by Takahashi,1 which in some ways is the precursor to modern CT scanners. The x-ray tube and a plane film positioned at a nearly perpendicular angle rotated around the body, exposing a single cross-section slice. The x-ray projections at each angle were recorded as crossing the film, and were accumulated. Today we call this a simple backprojection image. Although these images were useful, they were somewhat blurred because simple backprojection is only a first-order solution to the problem of reconstructing a cross-section image from rotational projections. This is where the computer part of the name computed tomography becomes critical: to provide digital processing to remove the tomographic blur. The projection data are gathered and backprojected using digital data rather than film. The resulting backprojection image is then “deblurred,” or reconstructed, by applying a simple sharpening filter, which is sometimes referred to as a convolution kernel. You can think of this process as a kind of edge-enhancement process, and it is sometimes referred to as filtered backprojection. In the end, the marriage of computer filtering and axial tomography resulted in the new field of computed tomography.