QUESTIONS

1. Explain how the primary beam filter.
    
2. Does the collimator focus the primary beam?
    
3. In 1913, Dr. Gustave Bucky built the first grid.  Then Dr. Potter in Chicago found that if the grid moved at the time the film was taken, grid lines would not be apparent on the film.  What is a grid & why would anyone want to use one?
    
4. What does the term grid ratio refer to?
    
5. The amount of scatter when a film is taken is determined by what 3 things?
    
6. When should we use a grid?
   
   
   
    

ANSWERS

1. Whenever the primary beam passes through something, the lower energy photons get absorbed & don't come out on the other side. We want as uniform a primary beam as we can get so we want to filter out these low energy photons (they tend to be the one's causing scatter plus any photon that gets absorbed by the patient has the ability to produce radiation injury.)  So we build a filter into the machine near where the primary beam exits the tube housing.  This takes out any low energy photons that the tube & tube housing hasn't already absorbed.  (Filters are described in Aluminum equivalents - ie the thickness of aluminum required to obtain the amount of filtration desired.)
   
   
   
   
    
2. No.  Think of the primary beam as sort of like water coming out of a shower head.  Most water goes straight out but some kind of goes out at an angle.  We want our primary beam as straight as possible.  You might think the collimator is like a metal cylinder fit onto the shower head so that all the water goes straight but this is not exactly right.  In the shower analogy, all of the water exiting the shower head would still be exiting the cylinder.  Since we are talking about photons, the photons do not just get redirected down the cylinder; they actually get absorbed by the cylinder.  This means that fewer photons are leaving the cylinder than went into the cylinder.  We have removed photons going off in undesirable directions.
   
   
   
   
    
3. The function on the grid is to keep scatter radiation from fogging the film.  A grid is a plate with little lead strips sticking up.  Photons that do not navigate perfectly between the strips hit the strips, get absorbed, & do not contribute to film exposure.  What's in between the little strips that keeps them from bending around? plastic or fiber fills the gaps.  Grids can be linear(a bunch of parallel strips) or crossed (sort of a quadrille-ruled grid).  Grids can be parallel (the strips just stick straight up) or focussed (the strips are slightly angled  to more naturally meet the photons of the primary beam (some will come in at a slight angle).
   
   
   
   
    
4. Every grid has a grid ratio.  This is the ratio of the height of the strips to the distance between the strips.  The higher the ratio, the better scatter is absorbed but the more important it is to properly center the grid.  Other important grid qualities are:  lines per cm (thinner lines are less visible but this is not important except in stationary grids), lead content in gm/cm2.  (This is a measure of grid quality), Contrast Improvement Factor (the grid's ability to improve contrast - this is expressed as a number assuming kvp = 100, a large field & a 100 cm focal spot to film distance), Bucky factor (refers tot he change in mAs necessary to compensate for beam loss in the grid- the higher the Bucky factor, the higher you must set KVP).
   
   
   
   
    
5. The amount of scatter is determined by field size, patient thickness, & KVP.
   
   
   
   
    
6. Considering the above, scatter can really fog our film if we have a thick patient.  We thus want to use the grid for  solid structures >11 cm thick and for chests > 18 cm thick.