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Flashcards in Xray Production, Tube And Beam Filtration Deck (68)
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The high energy collision of free electrons that were generated at the cathode/filament with the target on the anode results in what two things?

- heat and lots of it
- X-rays


Why is tungsten used as the filament wire?

it has lots of electrons to release and a very high melting point so it is durable


As the tungsten filament is heated, what happens?

the outer shell electrons speed up and move farther from the nucleus and become shared with adjacent atoms. This is called thermionic emission. Then they are flung out of the atom and form an electron cloud


Where is the filament located in the X-ray tube?

at the negatively charged end, the cathode


what is the target in an X-ray tube?

a slanted, smooth metal surface at the opposite end of the tube from the filament, at which the electrons will be aimed


The target is made of what metals?

It is embedded with a plate of tungsten and the remained is molybdenum and copper which help facilitate hear dissipation


What accelerates the electron cloud (aka space charge) toward the target?

the - to + flow of electricity created by the high voltage electrical circuit connecting the two ends of the tube


When are X-rays produced?

when the fast moving electrons strike the target, their kinetic energy is converted to heat (99%) and X-rays (1%)


There are two types of electron interactions at the target that produce X-rays. What are they?

- Bremsstrahlung "braking rays" are formed by sudden slowing of electrons that converts kinetic energy to other forms
- characteristic radiation is formed within target atoms as a results of electron to electron collision


Which type of electron interactions at the target produce the majority of the X-ray beam?

bremsstrahlung (85%)


As incoming electron nears the nucleus of a target atom, it slows and changes course. Its kinetic energy is released as a new X-ray photon. The "braking" will increase, the closer to the nucleus the incoming electron is and the ______ the energy of the resultant photon will be.



What produces a photon of characteristic radiation?

an inbound electron will cause a K-shell electron to be ejected and an L-shell electron will move to the K-shell. When it moves, it will release an X-ray photon that is equivalent in energy to the difference in binding energy between the two shells


What is the function of X-ray tube housing?

- protect tube
- absorb radiation
- provide mounting for tube attachments such as collimator


Most modern general purpose X-ray tube are ______..

"dual focus"


What is a "dual focus" X-ray tube?

it has 2 filaments, 2 focusing cups and 2 overlapping focal spots on the anode


In a dual focus X-ray tube, why does it have two focal spots on the anode?

- one is large for larger body parts and greater output
- one is small for lesser output but smaller beam and enhanced image detail


What is the function of the focusing cup in an X-ray tube?

the focusing cup is negatively charged and will repel electrons, thus focusing them on a small target cell ("focal spot"). Without the focusing cup, the electron stream spreads like a cone beyond target area


Focal spot sizes and filament sizes are associated with what?

mA of circuits. more mA = larger filament and focal spot


Where is the focusing cup located in an X-ray tube?

surrounding the filament (focusing cup is usually made of molybdenum)


The target face on an X-ray anode is bevels so that the target area is on a _____ slant.

10-20 degree


What is the composition of a typical anode?

tungsten target imbedded in molybdenum and copper disc to conduct heat away from target


What is the focal spot in an X-ray tube?

the area on the target where the electron stream is focused. Dual focus tubes have 1 large and 1 small


What is the effective focal spot?

the vertical projection of the actual/true focal spot. the electron stream is horizontal and the actual focal spot is slanted


In terms of actual focal spot, what size is better and why?

bigger is better because the larger surface area improves heat capacity and durability


In terms of effective focal spot, what size is better and why?

smaller is better because is means more "point source" for X-ray beam and therefore more image sharpness


The size of the effective focal spot determines image sharpness (smaller effective focal spot = sharper). The relative size of the effective focal spot is determined by what two things?

- target (anode) angle, steeper angle = smaller effective focal spot = sharper image
- actual focal spot size, smaller actual focal spot = smaller effective focal spot = sharper image


The anode angle is usually what.

12-15 degrees. the steeper the angle, the greater the differential between actual and effective focal spots (i.e. smaller effect focal spot and therefore sharper image)


Why can we not steepen the angle of the anode indefinitely in an attempt to have a smaller effective focal spot and therefore a sharper image?

- maximum field size limits: node angle determines the margin of field because it extends from the same angle. So the steeper the angle, the smaller the field size.
- anode heel effect


What is the anode heel effect?

X- rays are formed within the target material of the anode and absorbed by the target as they exit. The sloping of the target face results in uneven absorption of the primary beam. More sloping = more uneven absorption


How do you compensate for the anode heel effect with patient positioning?

place the thinner portion of the body part toward the anode end of the tube