Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

FIGURE 13-11. A schematic diagram of the fifth-generation, Imatron (South San Francisco,CA) cine-CT scanner is shown.X-rays are produced from the focal track as a high-energy electron beam strikes thetungsten. There are no moving parts to this scanner gantry. The electron beam isproduced in a cone-like structure (a vacuum enclosure) behind the gantry and iselectronically steered around the patient so that it strikes the annular tungsten target(Fig. 13-11). Cine-CT systems, also called electron beam scanners, are marketedprimarily to cardiologists. They are capable of 50-msec scan times and can producefast-frame-rate CT movies of the beating heart.Third-generation and fourth-generation CT geometries solved the mechanical inertialimitations involved in acquisition of the individual projection data by eliminatingthe translation motion used in first- and second-generation scanners. However,the gantry had be stopped after each slice was acquired, because the detectors(in third-generation scanners) and the x-ray tube (in third- and fourth-generationmachines) had to be connected by wires to the stationary scanner electronics. Theribbon cable used to connect the third-generation detectors with the electronics hadto be carefully rolled out from a cable spool as the gantry rotated, and then as thegantry stopped and began to rotate in the opposite direction the ribbon cable hadto be retracted. In the early 1990s, the design of third- and fourth-generation scannersevolved to incorporate slip ring technology. A slip ring is a circular contact withsliding brushes that allows the gantry to rotate continually, untethered by wires. Theuse of slip-ring technology eliminated the inertial limitations at the end of each sliceacquisition, and the rotating gantry was free to rotate continuously throughout theentire patient examination. This design made it possible to achieve greater rotationalvelocities than with systems not using a slip ring, allowing shorter scan times.Helical CT (also inaccurately called spiral Cn scanners acquire data while thetable is moving; as a result, the x-ray source moves in a helical pattern around thepatient being scanned. Helical CT scanners use either third- or fourth-generationslip-ring designs. By avoiding the time required to translate the patient table, thetotal scan time required to image the patient can be much shorter (e.g., 30 secondsfor the entire abdomen). Consequently, helical scanning allows the use of less contrastagent and increases patient throughput. In some instances the entire scan canbe performed within a single breath-hold of the patient, avoiding inconsistent levelsof inspiration.