CT GENERATIONS

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For example the stop –start motion in the first two generations has been replaced by continuous rotation.

First generation 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 rotatory ( 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 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 scanners and adopting a fan –shaped beam and multiple detectors.

Second generation The movements of the x-ray tube-detector array are both linear and rotatory as just like the first generation , but the rotatory steps larger. The 30 detectors gather more data per linear scan ,so fewer linear movements are needed to gather an adequate data base.

Second generation Instead of moving 1 angle at the end of each linear scan , the gantry rotates through a greater arc ,up to 30 angle. The number of repetitions is determined by the number of detectors in the detector array.

Second generation With 30 detectors the linear movements only have to be repeated six times rather than 180 linear movements of the original . Second generation scanners produced a tomographic section in between 10 and 90 seconds depending on the manufacturer.

Third generation Its called rotate-rotate . 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 and the detectors rotating around the patient .

The scanning geometry came to be known as fan beam geometry. It could produce a scan in 4.9 seconds. Both the x-ray tube and the detectors rotate around the patient in concentric circles whose centers approximately coincide with the center of the patient.

The original rotate-rotate scanner used 288 detectors, but newer versions of some units use over 700 detectors. The third generation scanners continue to produce excellent images with short scan time , this emphasis the point that third generation units are not better or worse than fourth generation.

Fourth generation Its called rotate- fixed. 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 design used 2000 detectors .

When the x-ray tube is at the prescribed angles only the exposed detectors read.

Advantages and disadvantages The advantage of a fan-beam / multiple –detectors array is speed . Obviously , multiple detectors can gather data faster than a single detector. But one disadvantages of the fan beam is the increased amount of scattered radiation . The scattered photons generated from the same volume of tissue , strike a detector and are recorded as noise.

Rotate –rotate and rotate-fixed CT scanners cannot achieve scan times much shorter than 1 second because of mechanical constrains as rotating a heavy x-ray tube takes time. Interest of in faster scan times evolves from desire to image moving structures , such as the wall of the heart or contrast material in blood vessels and the heart chambers.