CT generation

the general classification of computed tomography (CT) scanners based upon the arrangement of components and the mechanical motion required to collect the data. The term generation has been applied because of the order in which the CT scanner designs have been introduced, and each has a number associated with it. However, one should not assume that a higher generation number necessarily means a higher performance system. The geometries are illustrated in Fig. 1.

First generation: In the first CT scanner design, a single X-ray source and a single X-ray detector cell collect all the data for a single slice. The source and detector are rigidly coupled and the pencil beam is translated across the patient to obtain a set of parallel projection measurements at one angle. The source/detector pair is then rotated slightly and a subsequent set of measurements are obtained during a translation past the patient. This process is repeated once for each projection angle. Because of the translation and rotation process, this geometry is referred to as a translate/rotate scanner.

Second generation: Because the X-ray source emits radiation over a large angle, the efficiency of measuring projections was greatly improved by using multiple detectors. The detectors all lie within the scan plane but are not necessarily contiguous nor do they span the entire diameter of the object. The source and the array of detectors are translated as in a first generation system, but since the beam measured by each detector is at a slightly different angle with respect to the object, each translation step generates multiple parallel ray projections. Because multiple projections are obtained during each traversal past the patient, the 2nd generation scanner is significantly more efficient and faster than the original 1st generation scanner. This generation is also referred to as a translate/rotate scanner.

Third generation: With improvement in detector and data acquisition technology, it was possible to design a detector array with enough, high spatial resolution cells to allow the simultaneous measurement of a fan-beam projection of the entire patient cross-section. With such a large detector, it is no longer necessary for the detector-tube assembly to translate past the patient. Instead, the tube-detector assembly simply rotates around the object. The imaging process is significantly faster than 1st or 2nd generation systems. However, very high performance detectors are needed to avoid ring artefacts and the system is more sensitive to aliasing than 1st or 2nd generation scanners. Because the tube and detector both rotate, this generation is often referred to as a rotate/rotate scanner geometry.

Fourth generation: Contemporary with the development of viable third generation, rotate/rotate, systems and to avoid the sensitivity to ring artefacts, a design was developed using a stationary detector ring and a rotating X-ray tube. Because the reduced motion seemed consistent with a reduction in complexity, this geometry is known as the fourth generation. The stationary detector requires a larger acceptance angle for radiation, and is therefore more sensitive to scattered radiation than the 3rd generation geometry. Fourth generation geometries also require a larger number of detector cells and electronic channels (at a potentially higher cost) to achieve the same spatial resolution and dose efficiency as a 3rd generation system. This system is sometimes referred to as a rotate-stationary or rotate only geometry.

Several other CT scanner geometries which have been developed and marketed do not precisely fit the above categories. However, there is no agreed-upon generation designation for them.

In a fourth generation scanner, the detector ring is outside the circular path of the X-ray source. A CT system design was developed in which a circular detector ring is inside the source trajectory. This reduces the size of the detector array and may lead to a more compact system. In this system, the detector array nutates so that the detectors do not obstruct the X-rays as they pass from the source to the object (nutating detector ring). In some texts, this is referred to as a fifth generation system. It can also be called a rotate-nutate scanner.

The cine CT system has no mechanical scanning motion. In this system both the X-ray detector and the X-ray tube anode are stationary. The anode, however, is a very large semicircular ring that forms an arc around the patient scan circle, and is part of a very large, non-conventional X-ray tube. The source of X-rays is moved around the same path as a fourth generation CT scanner by steering an electron beam around the X-ray anode. Because the electron beam can be moved very rapidly, this scanner can attain very rapid image acquisition rates. In the literature, this system has been referred to variably as fifth generation and sixth generation. It has also been described as a stationary-stationary scanner. The terms millisecond CT, ultrafast CT and electron beam CT have also been used, although the latter can be confusing since the term suggests that the patient is exposed to an electron beam.

Slip-ring technology has had a great impact on CT system performance and utilization. Whereas most previous conventional CT systems used a cable-take-up mechanism to deliver electrical power to the X-ray tube (and could rotate through perhaps 400-600 degrees before it had to stop), use of a slip-ring allows the continuous rotation of the X-ray tube (and the detector assembly if appropriate). While not as fast as the cine CT scanner described above, these slip-ring scanners can attain sub-second image acquisition rates, zero interscan delay, and are compatible with helical scanning or spiral CT scanning (see helical CT scanner). They are generally referred to as slip-ring versions of their respective (e.g. third or fourth) generations. Speed and spatial resolution have been significantly improved recently with the development of multisection CT technology. The multisection capability has been created by dividing each detector element into several smaller sub-elements. Each sub-element has its own complete data acquisition electronics.

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