20NOT ALL OF THE ELECTRONS THERMIONICALLY EMITTED FROM THE FILAMENT ARE ATTRACTED TO ANODE. SMALL % WILL EVAPORATE AND CAUSE THE TUBE ARCING. AS A RESULT OF THIS, THE TUBE BREAKS DOWN.
21ANOTHER MAJOR CAUSE OF TUBE FAILURE IS THE BRAKING OF THE FILAMENT ITSELF. FILAMENTS BECOME INCREASINGLY THIN AS VAPORIZATION TAKES PLACE. WHEN ABOUT 10% OF THE DIAMETER HAS VAPORIZED, FILAMENT BECOMES SUBJECT TO BREAKING.
22AN AVERAGE DIAGNOSTIC X-RAY TUBE LIFE IS ONLY ABOUT 6-9 HOURS (10,000-20,000 EXPOSURE) AT NORMAL FILAMENT HEATING LEVEL. ROUTINELY DELAYED EXPOSURES WHILE THE FILAMENT IS ENDURING MAX. CURRENT SHORTEN TUBE LIFE BY 50-60% ( DOWN TO 5,000-6,000 EXPOSURES)
23THE FOCUSING CUP IS THE SHALLOW DEPRESSION IN THE CATHODE ASSEMBLY DESIGNED TO HOUSE THE FILAMENT
24MOST X-RAY TUBES HAVE THE FOCUSING CUP AT THE SAME NEGATIVE POTENTIAL AS THE FILAMENT
25IT IS ALSO POSSIBLE TO USE HIGHER NEGATIVE POTENTIAL ON THE CUP TO EVEN FURTHER DECREASE THE SIZE OF ELECTRON BEAM. THIS TYPE OF FOCUSING CUP IS CALLED BIASED
26AS MORE AND MORE ELECTRONS BUILD UP IN THE AREA OF THE FILAMENT, THEIR NEGATIVE CHARGES BEGIN TO OPPOSE THE EMISSION OF ADDITIONAL ELECTRONS. THIS PHENOMENON IS CALLED THE SPACE CHARGE EFFECT AND LIMITS X-RAY TUBES TO MAXIMUM mA ranges of 1,000-1,200 mA
38TUNGSTEN IS THE MATERIAL OF CHOICE FOR THE TARGET OF GENERAL USE X-RAY TUBES. REASONS ARE: HIGH ATOMIC NUMBER ( Z#) 74. HIGH Z# INCREASED EFFICIENCY OF X-RAY PRODUCTION.HIGH MELTING POINT 3410 CHIGH THERMAL CONDUCTIVITY
39SPECIALTY X-RAY TUBES FOR MAMMO SPECIALTY X-RAY TUBES FOR MAMMO. HAVE MOLYBDENUM & RHODIUM TARGETS BECAUSE OF THEIR LOW K-SHELL CHARACTERISTIC X-RAY ENERGY
40DURING NORMAL USE FOCAL TRACK REACHES TEMP. BETWEEN 1,000-2000 C
41BECAUSE OF TUNGSTEN HIGH MELTING POINT, IT CAN WITHSTAND NORMAL OPERATING TEMPS.RHENIUM PROVIDES MECHANICAL STRENGTH & THERMAL ELASTICITY IN ROTATING ANODES
58TOTAL FILTRATION= INHERENT + ADDED AT LEAST 2. 5 mm AL equiv TOTAL FILTRATION= INHERENT + ADDED AT LEAST 2.5 mm AL equiv. FOR TUBES OPERATING ABOVE 70 kVp
59RADIATION COMING THROUGH THE HOUSING. NO MORE THAN 100mR/ hr at 1m LEAKAGE RADIATIONRADIATION COMING THROUGH THE HOUSING. NO MORE THAN 100mR/ hr at 1m
60Anode Heel EffectOne 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. Electrons interact with target atoms at various depths into the target.The x-rays that constitute the useful beam emitted toward the anode side must traverse a greater thickness of target material than the x-rays emitted toward the cathode direction. The intensity of x-rays that are emitted through the “heel” of the target is reduced because they have a longer path through the target, and therefore increased absorption. This is the heel effect.
62The difference in radiation intensity across the useful beam of an x-ray field can vary by as much as 45%. The central ray of the useful beam is the imaginary line generated by the centermost x-ray in the beam. If the radiation intensity along the central ray is designated as 100%, then the intensity on the cathode side may be as high as 120%, and that on the anode side may be as low as 75%.The heel effect is important when one is imaging anatomical structures that differ greatly in thickness or mass density. In general, positioning the cathode side of the x-ray tube over the thicker part of the anatomy provides more uniform radiation exposure of the image receptor. The cathode and anode directions are usually indicated on the protective housing, sometimes near the cable connectors.
63Off Focus RadiationX-ray tubes are designed so that projectile electrons from the cathode interact with the target only at the focal spot. However, some of the electrons bounce off the focal spot and then land on other areas of the target, causing x-rays to be produced from outside of the focal spot). These x-rays are called off-focus radiation
65Off focus radiation is undesirable because it extends the size of the focal spot. The additional x-ray beam area increases skin dose modestly but unnecessarily. Off focus radiation can significantly reduce image contrast.Finally, off focus radiation can image patient tissue that was intended to be excluded by the variable-aperture collimators. Examples of such undesirable images are the ears in a skull examination, the soft tissue beyond the cervical spine, and the lung beyond the borders of the thoracic spine