XRAY TUBE

• Complete cooling of the housing is after a maximum heat capacity from 1 to 2 hours.
• X-ray tube is rarely seen by radiologic technologists.
• External structures of an x-ray tube include a support structure, protective housing, and glass or metal enclosure.
• The ceiling support system is the most frequently used support system for an x-ray tube, consisting of two perpendicular sets of ceiling-mounted rails that allow for both longitudinal and transverse travel of the x-ray tube.
• When the x-ray tube is centered above the examination table at the standard SID, it is in a preferred detent position.
• The floor-to-ceiling support system has a single column with rollers at each end, one attached to a ceiling-mounted rail and the other attached to a floor-mounted rail.
• Interventional radiology suites often are equipped with C-arm support systems, so called because the system is shaped like a C.
• Protective housing for an x-ray tube is made of lead and steel and reduces the intensity of leakage radiation to less than 1 mGya/hr at 1 m or 100 mR/hr at 1m.
• Protective housing functions as mechanical support for the tube & protects it from damage caused by rough handling, an insulator as it contains oil against electric shock, and a thermal cushion to dissipate heat.
• Glass enclosure for an x-ray tube is made of Pyrex glass to enable it to withstand the tremendous heat generated.
• Metal enclosure maintains a constant electric potential between the electrons of the tube current and the enclosure, advantages are less likely to fail and longer tube life.
• Cathode is the negative side of the x-ray tube and primary parts include filament and focusing cup.
• Filament is a coil of wire that emits electrons when it is heated, approximately 2 mm in diameter and 1 or 2 cm long, made of thoriated tungsten to enhance the efficiency of thermionic emission and prolong tube life.
• Focusing Cup is a metal shroud that surrounds the filament, it is negatively charged so that it electrostatically confines the electron beam to a small area of the anode, its effectiveness is determined by its size and shape, filament size and shape, and the position of the filament.
• Anode Heel Effect: One unfortunate consequence of the line-focus principle is that the radiation intensity on the cathode side of the x-ray field is greater than that on the anode side.
• X-ray Tube Failure: Factors Affecting the X-ray Tube Life include increased maximum radiographic factors such as mAs, kVp, and exposure time, and decreased excessive heat.
• Causes of Tube Failure: A single excessive exposure, long exposure time, and vaporization of the filament can cause tube failure.
• Three Ways of Heat Dissipation: Radiation is the transfer of heat by the emission of infrared radiation, conduction is the transfer of heat by the movement of a heated substance from one place to another, and convection is the transfer of energy from one area of an object to another.
• Radiographic Rating Chart: The most important as it conveys which radiographic techniques are safe and which techniques are unsafe for x-ray tube operation.
• Actual Focal Spot: The area on the anode target that is exposed to electrons from the tube current.
• The smaller the anode angle, the larger is the heel effect.
• Angles from 5 degrees to 20 degrees are available for biangular targets.
• X-ray Tube Rating Chart: It guides and aids the radiographer in using x-ray tubes and acceptable exposure levels to maximize x-ray tube life.
• Anode Cooling Chart: It is used to determine the thermal capacity of an anode and its heat dissipation characteristics, and is also used to determine the length of time required for complete cooling after any level of heat input.
• Three types of Rating Charts: Radiographic Rating Chart, Anode Cooling Chart, and Housing Cooling Chart.
• Line-Focus Principle: Design incorporated into x-ray tube targets allows for a large area for heating while maintaining a small focal spot.
• Effective Focal Spot: The area projected onto the patient & image receptor.
• Target Angle: The smaller the target angle, the smaller the effective focal spot.
• Geometric Solutions: Designing a fixed diaphragm in the tube housing near the window of the x-ray tube or using a metal enclosure x-ray tube can help reduce off-focus radiation.
• Off-Focus Radiation: X-rays produced in the anode but not at the focal spot.
• Space charge is a cloud of electrons that repel one another and tend to form a cloud around the filament.
• Higher tube currents and shorter exposure times are possible with the rotating anode.
• Most common anode materials are copper (Cu-29), molybdenum (Mo-42), and graphite.
• Large focal spot is used when large body parts are imaged.
• Anode has three functions: electrical conductor, mechanical support for the target, and thermal dissipator.
• X-ray tube current is adjusted by controlling the filament current.
• Focal spot is a region of the anode target in which electrons interact to produce x-rays, and is the actual x-ray source.
• Rotating Anode has a target area of typical stationary anode (4 mm2) and rotating anode (1800 mm2) x-ray tubes with 1-mm focal spots.
• Target is the area of the anode struck by the electrons.
• Space Charge Effect is a phenomenon of the space charge that makes it difficult for subsequent electrons to be emitted by filament because of electrostatic repulsion.