Tungsten Alloy X-ray Tube in Various Generations of CT

First generation: CT scanners used a pencil-thin beam of radiation. The images were acquired by a "translate-rotate" method in which the x-ray source and the detector in a fixed relative position move across the patient followed by a rotation of the x-ray source/detector combination (gantry) by 1° for 180°. , The thickness of the slice, typically 1 to 10mm, is generally defined by pre-patient collimation using motor driven adjustable wedges external to tungsten alloy X-ray tube.

 

Second generation: The x-ray source changed from the pencil-thin beam to a fan shaped beam. The "translate-rotate" method was still used but there was a significant decrease in scanning time. Rotation was increased from one degree to thirty degrees. Because rotating anode tubes could not withstand the wear and tear of rotate-translate motion, this early design required a relatively low output stationary anode x-ray tube. The power limits of stationary anodes for efficient heat dissipation were improved somewhat with the use of asymmetrical focal spots (smaller in the scan plane than in the z-axis direction), but this resulted in higher radiation doses due to poor beam restriction to the scan plane. Nevertheless, these scanners required slower scan speeds to obtain adequate x-ray flux at the detectors when scanning thicker patients or body parts.

Third generation: Designers realized that if a pure rotational scanning motion could be used rather than the slam-bang translational motion, then it would be possible to use higherpower (output), rotating anode x-ray tubes and thus improve scan speeds in thicker body parts in which the 3rd generation become a Rotate-Rotate geometry. A typical machine employs a large fan beam such that the patient is completely encompassed by the fan, the detector elements are aligned along the arc of a circle centered on the focus of the X-ray tube. The X-ray tube and detector array rotate as one through 360 degrees, different projections are obtained during rotation by pulsing the x-ray source, and bow-tie shaped filters are chosen to suit the body or head shape by some manufacturers to control excessive variations in signal strength. Such filters generally attenuate the peripheral part of the divergent fan beam to a greater extent than the central part. It also helps overcome the effects of beam hardening and to minimize patient skin dose in the peripheral part of the field of view -A number of variants on this geometry have been developed, which include those based on offsetting the centre of rotation and the use of a flying focus tungsten alloy  X-ray tube.