How X-rays are Created

X-rays are created by bombarding a tungsten target with electrons inside a device known as the x-ray tube. To generate this stream of electrons inside the x-ray tube, a powerful x-ray generator first takes the regular alternating current (AC) electricity from the power line at about 120 to 480 volts and transforms it into power in the range of 35 to 150 kilo volts (kV or thousands of volts). When this very high voltage potential is applied to the x-ray tube, a tight beam of electrons is fired out of a small wire (called the cathode) and strikes a metal disk (called the anode). When this stream of electrons hits the special metal compound of the anode (often tungsten or alloys including tungsten), it causes x-ray energy to be released from the metal's atomic structure. These x-rays are often filtered and collimated (or focused) as they leave the x-ray tube. The rays pass through the body part of interest in a straight line and are then recorded onto film or captured by an image intensifier and TV system to make the final image.
X-ray tubes are precision designed and manufactured and have evolved tremendously over the past 100 years. X-ray tubes are often the most expensive component in an x-ray system and can cost more than $50,000 each (for the ones used in high speed CT scanners or cardiac catheterization labs). The x-ray tube is a glass or metal envelope with a vacuum seal inside. X-ray tubes create tremendous heat while the beam of electrons is bombarding the cathode to produce the x-ray. Like a light bulb, an x-ray tube requires replacement up to a few times per year, depending on use.

sten Alloy X-ray Tube in Various Generations of CT-3

Sampling Geometries of the beam: The sampling geometry of the beam in CT scanners along the different generations can be described as three configurations: Further advances: Another limiting factor in image acquisition was tungsten alloy X-ray tube. The need for long, high intensity exposures and very stable output placed enormous demands on:

Further advances: Another limiting factor in image acquisition was tungsten alloy X-ray tube. The need for long, high intensity exposures and very stable output placed enormous demands on both the tube and generator (power supply). Very high performance rotating anode tubes were developed to keep up with demand for faster imaging, as were the regulated 150 kV switched mode power supplies to drive them. Modern systems have power ratings up to 100 kW.

Tungsten Alloy X-ray Tube in Various Generations of CT-2

Fourth generation of CT scanner uses Rotate-Fixed Ring geometry where a ring of fixed detectors completely surrounds the patient. Tungsten alloy X-ray tube rotates inside the detector ring through a full 360 degrees with a wide fan beam producing a single image. Due to the elimination of translate-rotate motion the scan time is reduced comparable with third generation scanner, initially, to 10 seconds per slice but the radiographic geometry is poor because the X-ray tube must be closer to the patient than the detectors, i.e. the geometric magnification is large also scatter artifact is more than third generation since they cannot use anti-scatter grid. -The disadvantages of poor geometry noted above have been alleviated very neatly by the so called nutating geometry. Tungsten alloy X-ray tube is external to the detector ring but slightly out of the detector plane, this change resulted in increasing both the acquisition speed, and image resolution. The method of scanning was still slow, because the X-ray tube and control components interfaced by cable, limiting the scan frame rotation. Further, they were more sensitive to artifacts because the non-fixed relationship to the x-ray source made it impossible to reject scattered radiation.

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.

 

Design of CT scanner for Tungsten Alloy Tube

The CAT scanner is made up of three primary systems, including the gantry, the computer, and the operating console. Each of these is composed of various subcomponents. The gantry assembly is the largest of these systems. It is made up of all the equipment related to the patient, including the patient support, the positioning couch, the mechanical supports, and the scanner housing. It also contains the heart of the CAT scanner, tungstenalloy x-ray tube, as well as detectors that generate and detect x- rays.

History of Tungsten Alloy X-ray Tube

William Roentgen, who discovered x rays in 1895. Around this time, various scientists were investigating the movement of electrons through a glass apparatus known as a Crookes tube. Roentgen wanted to visually capture the action of the electrons, so he wrapped his Crookes tube in black photographic paper. When he ran his experiment, he noticed that a plate coated with a fluorescent material, which just happened to be lying nearby the tube, fluoresced or glowed. This was unexpected because no visible light was being emitted from the wrapped tube. Upon further investigation, he found that indeed there was some kind of invisible light produced by this tungsten alloy tube, and it could penetrate materials such as wood, aluminum, or human skin.

Electron Beam Scanner and Tungsten Target

The electron beam gun is a triode system operated with a small-size tungsten bolt cathode that is indirectly heated by electron bombardment. The Wehnelt electrode and the anode are shaped as a Rogowski transducer with a bell-shaped through-hole anode. This configuration has been chosen since it greatly reduces the dependence of focal spot size on electron beam current. The electron beam gun is operated at about 10−6 mbar gas pressure which is provided by a vacuum pump system, consisting of a scroll pre-pump and a turbo molecular main pump operated in series. The gas pressure in the beam column and the scanner head is somewhat higher in the range of 10−5 mbar.