Anatomy of the X-ray Tube

The x-ray tube takes the stepped up voltage from the generator and converts it to x-rays. There are 4 major internal components to the x-ray tube.

  1. Cathode
  2. Filaments
  3. Focusing Cup
  4. Anode

x-ray tube

This is what an x-ray tube looks like when removed from the housing.  The anode and cathode are encased in a glass envelope that has Lead (Pb) infused in it.  The Lead acts as a filter to absorb the lower energy x-rays.  These lower energy x-rays would not contibute to the image at all, but would contribute to patient dose.  In all cases, the main function of a filter is to reduce patient dose. We'll get into this in greater detail later.

 


Cathode

cathode

This is an image of the cathode and anode removed from x-ray tube.  The anode and cathode go hand in hand.  Why?  Because opposites attract.  Just like a magnet, there is a positive and negative side.  The reasons will be come evident as we go along.  The cathode contains the NEGATIVE side of the system and is wired to the electricity from the generator. 


 

Filament

filament

The filament is located on the cathode side and can be thought of much like the filament in a light bulb.  When the electricity is turned on, the filament is super heated.  Just like in most light bulbs, the filament is usually made of Tungsten.  Tungsten is used because it is a heavy metal and has a very high melting point.  It gets so hot that electrons in the tungsten atom are "boiled" off.  This process is known as thermionic emission.  In most x-ray systems used for medical imaging, there are 2 filaments, one large and one small. The larger filament is able to withstand more heat, so it is used for exams where higher energy x-ray production is required, such as a lumbar spine.  So why do we also have a small filament?  Small filaments create small focal spots (the area that strikes the anode).  The smaller focal spot is beneficial in exams where better detail in bony structures is desired, such as hand x-rays.  You can select filament size by selecting the focal spot size on the control console.


Focusing Cup

focus cup

As you can see from this view, the filaments reside within the focusing cup.  Without the focusing cup, electrons would fly off in every direction.  The focusing cup is responsible for harnessing the electrons boiled off of the filament.  It collects and concentrates the electrons for the next step in the production of x-rays.


 

Anode

anode

The Anode is the positive side of the x-ray system.  When an x-ray operator pushes the button on the x-ray control console, several things happen. 

  1. The anode begins to rotate at a very high rpm. This is the "wind up" noise that you here when taking the x-ray.
  2. The generator creates the voltage necessary for x-ray production.
  3. The filament, in response to the generator voltage, is superheated by the high voltage and boils off electrons within the filament.
  4. The focusing cup collects and concentrates the electrons.
  5. When all of this has been accomplished, the electrons are accelerated and an extremely high speed towards the anode, where it strikes the part of the anode called the target.  It is the interaction of the electrons with the target  that creates the x-rays. 

The anode is usually made of Tungsten for the same reason the filament is.  Tungsten has a very high melting point.  You will notice 2 copper holes in the shaft of the anode.  Those were drilled to balance the rotating anode.  The anode spins as fast as 10,000 revolutions per minute.  If it wasn't perfectly balanced, it would literally shake itself apart.  The anode rotates for one reason, heat dissipation.  By constantly spinning, the electrons interact with a much larger surface area.  This prevents heat related damage and promotes longevity in the tube.

WHEN HIGH SPEED ELECTRONS STRIKE THE TARGET ON THE ANODE, THEY TRANSFER ENERGY IN 2 FORMS, X-RAYS AND HEAT.  ONLY 1% OF THE ENERGY PRODUCED OCCURS IN THE FORM OF X-RAYS.  THE REMAINING 99% IS IN THE FORM OF HEAT.