The X-Ray Tube Flashcards
What is the X-ray tube?
An electronic vacuum tube
What does the X-ray tube consist of?
an Anode and Cathode
What does the cathode consist of?
a filament made of tungsten wire
What is thermionic emission?
When the tungsten filament of the cathode heats up, causing electrons to be ‘boiled off’ the filament.
Electrons then accelerate across the tube towards the anode
What controls tube current? (mA)
The Cathode filament
How can we increase tube current?
By increasing the filament current
Why do anodes rotate in modern x-ray tubes?
in order to reduce the heating effect on the surface of the anode, larger target area for the beam
We can therefore use higher tube currents
What does the anode consist of
tungsten bonded on a bed of molybdenum and graphite
→ this reduces the weight of the anode allowing for high rotation speeds
What is the Focal Spot?
The area of the anode that produces the x-rays
The ____ the focal spot the better the _____ _________
The smaller the focal spot the better the spatial resolution
How do we reduce the focal spot size?
We angle the anode to produce a smaller effective focal spot
What is the heel effect?
Radiation intensity at the cathode side is greater than the anode side
This is because x-rays on the anode side of the tube must traverse a greater thickness of material
As a result of the heel effect the effective focal spot size is _____ on the anode side of the tube
Smaller
What does the heel effect allow for us to do with patients?
Thicker parts of the patient towards cathode end of the tube
Give an example of the heel effect in mammography?
the cathode is against the chest wall, because the shape of the breast is thicker by the best wall than the nipple side
