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X-Ray Generator QC

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  14/03/20131 X ‐ Ray   Generator   QC Generator   Fundamentals Generator   Function ã  Converts   electrical   energy   from   power   supply   to   useable   x ‐ rays ã  Controls   key   beam   properties: ã  Quantity ã  Quality ã  Intensity ã  Duration ã  Recall   main   components   of    x ‐ ray   circuit  –   Primary   autotransformer   (PAT)   –   Primary   high   tension   transformer   (PHTT)   –   Secondary   high   tension   transformer   (SHTT)   –   Primary   filament   heating   transformer   (PFHT)   –   Secondary   filament   heating   transformer   (SFHT)   PAT ã  Compensates   for   voltage   drops   across   mains   by   maintaining   volts/turn   ratio. PHTT ã  Controls   voltage   input   to   primary   side   of    high   tension   transformer   thereby   controlling   voltage   gain   on   secondary   side  –   kV   selectors  –   Exposure   timer   switch    –   Voltage   meter   (calibrated   to   kV)   –   Voltage   Drop   under   load   compensator    –   Space   charge   compensator    14/03/20132 SHTT ã  Provides   rectified   high   voltage   across   X ‐ ray   tube ã  Contains   rectifier   circuitry ã  mA   meter PFHT ã  Controls   current   and   voltage   to   filament   heating   transformer   (thermionic   emission)   –   mAs,   focal   spot   selectors  –   Part   of    space   charge   compensation   and   VDUL   SFHT ã  Always   operating   at   low   level   current   to   maintain   cathode   filament   at   certain   temperature ã  Boosted   to   appropriate   current,   based   on   mAs   selction,   during   preparation   phase   of    exposure Overload   Protection ã  Circuitry   monitors   mAs/voltage/focal   spot   selection   to   ensure   no   excessive   power   loading   occurs.   ã  Exposure   locked   if    this   is   the   case. Steps   to   Taking   an   Exposure ã  Power   Up  –   Power   distributed   to   tube   locks,   movement   controls,   table,   collimator  –   Tube   filaments   given   standby   power  –   Voltage   line   compensation   occurs   if    necessary ã  Select   exposure   factors  –   Bucky   selection  –   mA/s  –   kVp   (space   charge   compensator   may   adjust   current   to   SFHT   if    low   kV   selected)   –   Focal   spot   size  –   Set   SID  14/03/20133 ã  Exposure   Prep.  –   Circuits   prevent   exposure   untill   certain   conditions   met: ã  Anode   rotation   rate   is   appropriate ã  Filament   boost   circuit   has   heated   filament   to   approx   2000   degrees ã  Voltage   applied   to   Ionization   chambers   (AEC)  ã  Grid   oscillation   initiated ã  Exposure  –   Circuit   closes   exposure   begins  –   Exposure   terminated   after   sufficient   time/mAs/dose   reached  –   X ‐ ray   machine   reverts   to   pre ‐ preparation   state Some   Examples   of    Problems ã  Line   Voltage   Compensation   failure  –   kVp   variability   which   affects   beam   quality ã  Voltage   Drop   Under   Load   Compensation   failure  –   Non ‐ congruence   between   kVp   selected   and   actual   kVp   across   tube   at   high   mAs  –   Beam   quality   affected ã  Space   Charge   Compensation   failure  –   Low   mAs   at   low   kVp  –   Beam   quantity   affected ã  Filament   Thinning  –   Inherent   filtration   increase   HVL   affected  –   Higher   mAs   than   that   selected  –   Quantity   of    beam   affected kVp   Related   Tests   ã  Reproducibility    –   Ability   to   produce   same   output   for   given   settings ã  Accuracy  –   How   closely   actual   output   meets   stated   output ã  Half    Value   Layer  –   Beam   spectrum   modified   to   improve   risk/benefit   Reproducability ã  An   X ‐ ray   generator   should   produce   the   same   intensity   of    radiation   whenever   the   same   set   of    technical   factors   are   used. ã  Reproducability   testing   evaluates   this   aspect   of    performance   through   monitoring   of    the   variance   in   x ‐ ray   output   over   a   series   of    identically   controlled   exposures.  14/03/20134 ã  Method  –   A   series   of    exposures   are   taken   in   succession   with   the   same   technical   factors   and   radiation   output   is   measured   by   dosimeter.  –   Variance   in   the   exposure   levels   is   then   calculated   and   compared   to   standards ã  Reproducability   failure   can   lead   to   inconsistent   image   quality   and   unnecessary   retakes kVp   Accuracy ã  Due   to   the   significant   effects   of    kVp on   image   quality   and   patient   dose,   selected   kVp setting   and   actual   kVp as   produced   by   the   machine   must   be   accurate. ã  Failure   in   accuracy   can   affect   image   quality   and   patient   dose ã  Method  –   A   series   of    exposures   is   made   at   different   kVp   levels   and,   with   the   aid   of    a   kVp   meter,   a   series   of    actual   kVp   levels   is   determined.  –   Comparison   is   made   with   reference   to   standards. Which   would   you   rather   work   with? ã  Machine   that   is   often   accurate   but   doesn't   always   reproduce   results ã  Machine   that   is   innacurate   but   reproduces   results   consistently   Half    Value   Layer ã  Low   keV   photons    –   Attenuated   entirely   within   patient    –   Never   have   probability   of    reaching   IR.    –   can   increase   a   patient’s   skin   dose   by   as   much   as   90%   ã  Inherent   and   added   filtration   remove   low   energy   photons    – beam   hardening ã  Photons   reaching   patient   should   have   some   probability   of    reaching   IR Half    Value   Layer ã  The   amount   of    aluminum   thickness   necessary   to   reduce   a   beam’s   intensity   to   one   half    of    its   initial   value.  –   Indicative   of    the   keV   spectrum   within   a   beam  –   Higher   HVL    – higher   average   beam   energy
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