QUESTIONS

1. Inside the x-ray tube, there is an anode & a cathode.  Which is negatively charged & which is positively charged?
    
2. What is the role of the anode & cathode in x-ray production?
    
3. Why is the anode made out of tungsten & why is is mountied on copper.  For that matter, why is it spinning?
    
4. What is the low tension circuit of the x-ray machine?
    
5. What in the hell is your x-ray machine's focal spot?
    
6. What is the heel effect?
    
7. What is "off focus" or "stem" radiation?
    
8. What causes most x-ray machines to poop out?  (What part usually goes whacky first)?
    
9. Ideally you want a high KVP low mAs technique.  (less seconds = less motion artifacts & spares the filament extra wear).  Okay, so say we are using 60 kvp & 10 mA at 0.4 sec.  How could we get the same film but with a higher kvp & lower mAs?
    
10. When talking about an x-ray machine, what the hell is "rectification?"
    
    
    
    
     

ANSWERS

1. This is a dyslexic type of question because the electrodes are not named for how they are charged but they are named for how particles move towards them. "Cath" means positive meaning that positively charged particles go toward the cathode.  "An" means negative & negatively charged particles go toward the anode.  This means the cathode is negatively charged & the anode is positively charged.
   
   
   
   
    
2. The cathode is a filament shape that is heated up as the machine charges.  In other words, we're putting a bunch of electrons on the cathode.  The anode is the target (the field of atoms that we will be running electrons through/at.  When we push the pedal all the way down, we are putting a potential difference across the anode & cathode & at this moment, the electrons zoom across at the target & x-rays are produced.
   
   
   
   
    
3. It gets pretty hot in there with all these electrons flying around & xrays being generated.  In fact, when the electron beam smacks the anode, 99% of the energy lost is lost as heat & not as photons (I hate when that happens.)  we spin the anode to help dissipate heat & the copper helps conduct heat away. The actual target is made of tungsten because of tungsten's high Z number (we get x-rays & not some other kind of light.)
   
   
   
   
    
4. The low tension circuit heats the filament (cathode) - that is it puts electrons on the cathode.  Note the cathode is shaped like a cup so as to focus the electrons.  There is a variable resistor in the circuit that allows you to set how many electrons you put on the cathod (setting mA). There is also a "step down transformer" - we want about 10 volts in this circuit & the wall has about 220.  There is also a high voltage circuti which produces a high potential across the electrodes.  This has a step-up transformer so we can get kilovolts across our electrodes & not just the measly 220 volts of the wall.
   
   
   
   
    
5. The focal spot is the area on the anode target where the electron beam actually hits.  A larger focal spot can accommodate more heat & thus helps the machine last longer.  You especially need this if you are going to take films rapidly or if you are tyring to penetrate a big animal.  A small focal spot however gives you better detail on the radiograph.  There's actually more to it but without pictures I can't really show you.
   
   
   
   
    
6. Okay, I'll try to explain without pictures.  Picture a rectangle that's kind of tall & skinny.  Cut off the lower left corner.  This is sort of what an anode looks like with the target being the cut off corner.  Say the electron beam is heading right into this cut off corner.  X-rays are being spit off from that point in all directions 360 degrees around the focal spot.  But we don't get a primary beam that is 360 degrees around because the anode is made of metal & metal absorbs x-rays well.  Erase the part of the 360 degree bean then that is taken out by the anode.  The primary beam is thus heavier on x-rays on the side toward the cathode & weaker (less photons) on the anode side.  You should put the thickest part of your patient thus toward the cathode to take advantage of the heel effect.  (Did that make sense?  If  not, I'll try to upload a graphic.)
   
   
   
   
    
7. This type of radiation refers to x-rays resulting from the interaction of electrons & parts of the tube other than the target.  We cannot control the direction of the resulting photons thus they will contribute to film fogging & not the primary beam.  We'd like to take them out of the picture (no pun intended).  Embedding the target in graphite, which has a low Z number, helps insure that the resulting off focus photons are low energy & will be absorbed by the tube housing long before they see the film below.
   
   
   
   
    
8. The filament (cathode) usually goes whacky first.  When you buy an x-ray machine, your filament gets 500 minutes at max temp before it breaks.  Heat stress, you know.  You can screw up the machine much earlier by setting up for the film, pressing the standby switch (which heats the filmament & spins the anode) & then not taking the film.  (You waste your valuable minutes of filament life.)  When your filament fails, the ammeter on the machine does not move when you snap the film & the film is unexposed when it comes out of the processor.The next most likely cause of failure is overheated anode bearings.  The whir of the anode just plain sounds funny or just doesn't sound at all.  Without spinning, the anode surface will pit & the beam will not reliably conform to kvp settings.
   
   
   
   
    
9. When you increase kvp by 10, you can cut mAs in half & get the same film.  So we could go with 70 kvp & 10mA at 0.2 sec.  We could also go with 90 kvp and 5mA &0.1 sec.
   
   
   
   
    
10. The wall outlet supplies an AC current.  This means that the potential making electrons move along the copper wire of your computer plugged into the wall makes the electrons move forward then they stop then they move backward & they keep alternating in this way.  Well, your x-ray machine has no use for electrons moving backwards so we need to make some modification (rectification.)
    
    By using a gadget called a "rectifier" the backward current flow is blocked.  This only sort of solves the problem because now only half the electrons are useable (we've blocked the ones going backward.)  Well this is inefficient.  :(  It is also called "half wave rectification."
    
    So let's try "full wave rectification."  By using 4 rectifiers, we can keep all the electrons moving forward.  The only trouble is 120 times per second they still stop moving. 
    
    So let's get a three phase generator.  Now we superimpose 3 generators of current 120 degrees out of sync.  This means we get a fairly constant forward motion & allows for very short exposure times.  (This is another thing that seems to want an illustration.  Let me know if you need one.)