Based on the introduction of slip ring technology, continuous rotation of the X-ray tube and the detector became possible. Larger volume coverage in shorter scan times and improved longitudinal resolution became feasible after the introduction of multislice CT systems in 1998. The gantry constitutes the frame on which the following are assembled (Fig. 1.1): the X-ray tube, the crown of detectors, the high voltage generator and the data measurement system. In these third-generation CT scanners, the X-ray tube and detector are built onto a rotation system that rotates around the patient, covering a scan field of view of 50 cm. The main challenge is the stability of both focal spot and detector position during rotation, in particular with regard to the high rotational speeds (0.20 s) and gravitational forces of latest CT systems. Also in the gantry are assembled the cooling system, the position and speed censors, the hydraulic apparatus for the inclination of the gantry, the system used for the ascent, descent, and braking of the CT table, and finally the microphones and loudspeakers used for intercommunication with the patient.

In the early 70s, Electrical and Musical Industries Ltd. (EMI), a company based in London, built the first conventional CT scanner, designed to obtain images of the head. Its prototype was installed in the Atkinson Morley’s Hospital, also in London, where on October 1st 1971 the first tomographic image with clinical ends was taken on a patient with suspicions of a frontal tumor [1, 2]. The EMI Mk1 took between 4.5 and 20 min to take each image, with a matrix of 80 × 80 pixels and a 3 mm × 3 mm × 13 mm voxel size. The processing of the images obtained during the day was carried out in the same factory during the night. This stage took approximately 20 min per image, which were returned to the center in the morning. Modifications in the image’s reconstruction algorithms and a rise in the number of detectors per slice permitted its successor, the CT1010, achieve images with a matrix of 160 × 160 or 320 × 320 pixels, on a field of vision of 210 mm and with an acquisition time of 1 min. The first images of the abdomen obtained on a prototype machine dedicated to body tomography, the CT5000, were achieved only in December of 1974, and Hounsfield himself joined the research team. This CT scanner generated an image with a matrix of 320 × 320 pixels in barely 20 s, and a field of vision of 240, 320, and 400 mm [3].

CT scanners from the first and second generations utilized a translation/rotation technology. In the first generation, a thick, collimated pencil X-ray beam faced a detector. Both were then moved covering the object of study, rotated 1°, and moved again and so on until covering a rotation of 180°. During the translation, X-rays are emitted and the data is collected. The resolution of the resulting image is low and the acquisition time very prolonged. In the second generation, the principle to achieve the image is similar; the difference lies in a wider X-rays beam, with a coverage angle of 5°, and a variable number of detectors. This modification permits the acquisition time of data to diminish to a couple of minutes. Another inconvenience with these devices was the instability of the detector, a sodium iodide crystal, and the need to calibrate at the end of each translation, which limited the scanning speed. The CT scanners of third and fourth generation appeared in 1976. The third generation systems are based on a rotation/rotation system, an X-rays beam with a wide coverage width of 30–60°, covering the entire region of study and an arch of detectors composed of multiple elements [3]. In these devices, the X-rays tube and the line of detectors assembled on the gantry turn 360° around the patient. The sweep times are reduced to less than 5 s, along with a better, and less wasteful, use of the radiation. The fourth generation consists of a rotation/stationary system, with a fixed crown of detectors inside of which turns the X-rays tube, reaching high speeds, and thus reducing the scanning time. Plus, the detectors yield a more homogenous result and greater stability (Fig. 1.5).

One of the most transcendent and important feats in CT history was the introduction of the spiral (helical) CT in the early 90s. This innovative system of volumetric image acquisition without a doubt constituted the fundamental base for future developments, and put the spotlight on a completely unknown spectrum of new diagnostic applications that generated a rebirth of the method, overshadowed slightly by the growth that magnetic resonance imaging began having around those years [19–24].

Taking into account the better temporal resolution, the image acquisition time lower than 1 s and the possibility of obtaining volumetric information of a certain region, allowed the spiral CT the chance of achieving, for the first time, a volume data without the danger of miss- or double-registration of anatomical details. These third generation CT scanners utilize a slip-ring technology to transmit electrical power for its functioning, disposing of the need for cables in the assembly of the gantry, and are able to achieve a volumetric acquisition of continuous images due to the constant spin of the X-rays tube and the detector system, mounted onto a rotating gantry. The temporal resolution is increased, with acquisition times below 1 s [25–27].

In parallel to the described technological development, an informatics advance of similar magnitude happened, with the appearance of workstations: specially created computers with programmes that offer diverse imaging post-processing techniques to visualize and analyze CT images, such as the multiplanar reformations, the maximum and minimum intensity projections, the three-dimensional surface shaded displays, the volume rendering techniques, and the virtual endoscopy images [28–30].

Nevertheless, there were still things to continue working on. The fundamental problem of the spiral CT is the inverse relationship which exists between the acquisition length and the space resolution on the longitudinal axis of the patient. The smallest unit that constitutes the CT image is the voxel. Isotropic images are those which are constituted by voxels, whose x, y and z axis are equal; this is a cubical or isotropic voxel [10]. In the spiral CT, images are constituted by voxels whose length in the x axis are longer in relation to the x and y axis (Fig. 1.6). Although this configuration yields a very good space resolution on the acquisition plane by generating axial images of high anatomical detail, it results insufficient in the creation of multiplanar or three-dimensional reconstructions with a high diagnostic impact. In the spiral CT era, the challenge of achieving isotropic images in a single apnea and large volume coverage were not yet possible. One way to increment the longitudinal resolution and obtain a cubic voxel at the same time is to acquire multiple images simultaneously, using a faster rotation speed of the gantry as well.