CT Teaching Manual A Systematic Approach to CT Reading
Computed tomography is a special type of x-ray procedure that involves the indirect measurement of the weakening, or attenua-tion, of x-rays at numerous positions located around the patient being investigated. Basically speaking, all we know is • what leaves the x-ray tube, • what arrives at the detector and • the position of the x-ray tube and detector for each position. Simply stated, everything else IS deduced from this Information. Most CT slices are oriented vertical to the body's axis. They are usually called axial or transverse sections. For each section the x-ray tube rotates around the patient to obtain a preselected section thickness (Fig. 6.1). Most CT systems employ the continuous rotation and fan beam design: with this design. the x-ray tube and detector are rigidly coupled and rotate continuous-ly around the scan field while x-rays are emitted and detected. Thus, the x-rays, which have passed through the patient, reach the detectors on the opposite side of the tube. The fan beam opening ranges from 40° to 60°, depending on the particular system
design, and is defined by the angle originating at the focus of the x-ray tube and extending to the outer limits of the detector array. Typically, images are produced for each 360° rotation, permitting a high number of measurement data to be acquired and sufficient aadose to be applied. While the scan is being performed, attenuation profiles, also referred to as samples or projections, are obtained. Attenuation profiles are really nothing other than a collection of the signals obtained hunt all We detectut eluoinels at a given angular position of the tube-detector unit. Modern CT systems (Fig. 6.4) acquire approximately 1400 projections over 360°. or about four projections per degree. Each attenuation profile comprises the data obtained from about 1500 detector channels, about 30 channels per degree in case of a 50° fan beam. While the patient table is moving continuously through the gantry, a digital radio-graph rscanogramm" or "localizer", Fig. 6.2) is produced on which the desired sections can be planned. For a CT examination of the spine or the head, the gantry is angled to the optimal orientation (Fig. 6.3).