Computed Tomography

Computed Tomography

The basic principle behind CT is that the internal structure of an object can be reconstructed from multiple projections of the object.

Scanning Motions 

CT scanners have gone through a number of design changes since the technology was first introduced in 1971. We will discuss geometry of design in five categories: First generation (translate-rotate, one detector) Second generation (translate-rotate, multiple detectors) Rotate-rotate (third generation) Rotate-fixed (fourth generation) Other geometries

Time reduction is the predominant reason for introducing new configurations. Scan time has been reduced in newer configurations by the reduction or simplification of mechanical motion. For example, the stop-start motion in the first two generations has been replaced by continuous rotation. We will discuss a device that has no moving parts and an extremely short scan time.

First Generation.

 The original EMI unit was a first-generation scanner. It employed a pencil-like x-ray beam and a single detector; that is, one detector per tomographic section. The x-ray tube-detector movements were both linear and rotary (usually termed "translate-rotate motion"). A five-view study of the head took about 25 to 30 minutes.

 Second Generation.

One major objective of second-generation scanners, and of all  later configurations, was to shorten the scanning time for each tomographic section. The increased speed was accomplished by abandoning the single detector and pencil beam of the original EMI scanner and adopting a fan-shaped beam and multiple detectors sec, depending on the manufacturer.

 Rotate-Rotate (Third Generation). 

In 1975, the General Electric Company introduced a CT scanner in which the translation motion was completely eliminated. Only rotation motion was required, with both the x-ray tube and detectors rotating around the patient. This scanning geometry came to be known as "fan beam" geometry, or "third-generation" geometry, and the original unit could produce a scan in 4.9 sec.  Multiple detectors are aligned along the arc of a circle whose center is the x-ray tube focal spot (the detectors are always perfectly aligned with the x-ray tube). The x-ray beam is collimated into a fan beam. Both the x-ray tube and detectors rotate about the patient (rotate-rotate geometry) in concentric circles whose centers approximately coincide with the center of the patient as illustrated in Figure 19-6. The original rotate-rotate scanner used 288 detectors, but newer versions of some units use over 700 detectors. The fan beam must completely cover the object to be imaged. Both xenon and scintillation crystal detectors can be used with this configuration because the detectors and x-ray tube are in precise alignment throughout the entire scan motion (more about this concept when we discuss detectors). In the original rotate-rotate scanners the x-ray tube was pulsed. Each pulse of the tube produced one projection, and the number of scan lines in each projection was equal to the number of detectors. A single image is computed from many projections, often more than 1 000 projections. In some rotate-rotate scanners the x-ray tube is continuously on, and individual projections are obtained by reading the output of the detectors at rapid intervals (more about this later). More than 1000 projections may be obtained in a time as short as 1 sec (or slightly less). The topic of the number of projections and the number of individual lines in each projection will require more attention when we discuss image quality. Rotate-rotate, or so-called third-generation, scanners continue to produce excellent images with short scan times. This emphasizes the point that "third-generation" units are not better or worse than "fourth generation" units

Rotate-Fixed (Fourth Generation)

. The detectors form a ring that completely surrounds the patient. The detectors do not move. The x-ray tube rotates in a circle inside the detector ring, and the x-ray beam is collimated to form a fan beam. A fan of detectors is always in the x-ray beam. Some designs have used more than 2000 detectors. When the x-ray tube is at predescribed angles, the exposed detectors are read. For example, an angular sampling rate of one projection each Yso will produce 1080 projections in one 360° rotation. Projections are taken at many angles during the rotation of the x-ray tube, with numbers of projections greater than 1000 being common. Thus, one CT scan will be made up of many projections, each projection taken at a slightly different angle. This is why continuous-on x-ray tubes are generally used today. It requires a less complex unit to read the detectors 1000 or more times in 1 sec. than to pulse an x-ray tube on and off 1000 times in 1 sec. Both rotate-rotate and rotate-fixed CT units continue to give excellent results, with no clear advantage of one over the other