The X-ray Tube a Major component of CT Scanner.

The x ray tube is one of most component of CT System. Indeed, x ray tubes supply the necessary x ray photons to performs a scan. In the early days of CT, pulsed x ray tube was generally used. In the pulsed mode, x ray tube produced x ray photons in pulse of short duration. This pulsed time is varied between 1 ms and 4 ms. This was mainly to accommodate the fact that x ray detector could not Take measurement while signals were sampled.

Although the size and appearance of the x ray tube have changed significantly since its invention by Rontgen in 1895, the fundamental principles of the x ray generation have nit changed. The basic component of the x ray tube consist of a cathode and anode. The cathode supplies high speed electrons and the anode provides the target. X ray photons are produces by bombarding a target with high speed electron. The intensity of the x rays produced is proportional to the atomic number of the target material and to number of electrons bombarding the target. The energy of the generated x ray photons on other hand, depend on the electric potential difference between the cathode and anode. 

 Most of the x ray tubes used in CT Scanner today employ the heated cathode design, which date back to Coolidge, who built the first high voltage tube in 1913. The figure above shows a photograph of a glass envelope x ray tube. The glass frame provides a housing and support to the anode and cathode assemblies and sustain a vacuum level in the early operating life tends. The glass frame is a composite of several types of glass. The main section is borosilicate glass with good thermal and electrical insulation properties. The thickness of the glass typically between 0.18 mm and 0,30 mm. The glass seal at the end of tube are made with splice rings of various grades of glass to mach the thermal expansion coefficient between the metal and the borosilicate glass. In more advanced tube design, the glass frames are replaced with metal frames. The metal frame has the advantage of being able to operate at or near ground potential to improve the efficiency of the motor that drives the anode assembly. Another advantage of the metal frame is the reduced spacing between the frame and anode to allow a larger size anode with out significantly increasing the size of the tube envelope. The metal case can also collect back scattered electron off the target. The production of x rays by electron bombardment of the target is highly inefficient. Only less 1 % of the input energy to an x ray tube is converted to x ray photons. Over 99 % energy becomes heat. The temperature at the impacting point can reach between 2600^oC and 2700^oC. The melting temperature of the tungsten is 3300^oC. To prevent melting of the target the anode rotated at a very high speed, typically between 8000 rpm and 10,000 rpm. This allow the cooler part of the focal track to be brought under the electron beam and the heat distributed over a larger area on the target. The rotating part of the tube are located and sealed inside the vacuum tube envelope. Anode target connected to molybdenum shaft that is surrounded by a rotor. A bearing is also located inside the vacuum so that the anode, the shaft and the rotor can rotate freely inside the tube envelope. A stator is placed outside the envelope to provide an alternating magnetic field, which causes the rotators assembly to rotate.

Because of the enormous heat in the target, special attention has to be paid to the target design. Traditionally, the tube target is made of molybdenum alloy and the focal track  is made of a layer of tungsten-rhenium. The advantage of the design is the quick heat transfer from the focal track to the bulk of the target. The newer design uses brazed graphite, in which the metal target is brazed to graphite body to increase the heat storage per unit weight.

He heat storage per unit weight. The design combines the good focal track characteristics of the metal tube with the heat storage capability of the graphite. An alternatives design uses chemical vapor deposition to deposit a thin layer of the tungsten-rhenium on high purity graphite target. The advantages of this approach are light target weight and high heat storage capacity. The disadvantages are the higher cost, increased potential of particle and limited focal spot loading.

Because heat management is an important and complicate issue, many commercial CT Scanner employ computer algorithms or called tube cooling algorithms to estimate the clinical protocols allowable under different conditions to prevent premature damage to the tube. For example a protocol may be safely run when the x ray tube is cold. The some protocols cause damage to the tube when it is hot. Under condition, the algorithm will recommend either a reduced technique of tube current or scan time or cooling period before star the scan. The tube cooling algorithm is essence derive a compromised solution between performance and tube life.