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IGRT: MVCBCT Calibration and Acceptance testing Procedure
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IGRT: MVCBCT Calibration and Acceptance testing Procedure



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  • 1. IGRT:     MV  CBCT  Calibra0on  and  ATP   on  Siemens  Oncor  Linac     By     Vibha  Chaswal,  Ph.D.  
  • 2. Ac0vi0es*   •  Flat  panel  2D  gain  calibra:on  and    dead  pixel   map   •  MVCBCT  calibra:on  using  clinically  used  and   custom  CBCT  protocols   •  MVCBCT  image  quality  assessment  using   clinically  used  and  custom  CBCT  protocols   *Performed at UIHC RadOnc as Medical Physics Resident
  • 3. Flat  panel  2D  gain  calibra0on     •  Correc:on  for  the  differences  in  flat   panel  diodes  response  in  2D  imaging   •  Without  correc:on,  an  obvious   banding  paHern  would  be  visible  on   the  image.     •  Should  be  done  every  4-­‐6  weeks.     •  Coherence  prac:ce  database,   SERVICE  PATIENT  is  used  to  acquire   port  during  gain  fields  at  different   photon  energies,  dose  rates,  clinically   used  SIDs,  monitor  units  and  field   sizes.   •  For  each  SID  a  treatment  site   containing  4  gain  fields  is  assigned,   and  each  site  can  be  delivered  in  a   auto-­‐sequence  group.      
  • 4. Flat  panel  2D  dead  pixel  map   •  Correc:on  for  non-­‐responding  ‘dead’  pixels.     •  The  grayscale  values  in  the  pixel  surrounding  the  dead  pixel  are   averaged  and  this  value  replaces  the  grayscale  of  the  dead  pixel   •  During  ATP  provided  by  the  manufacturer   Dead pixel map limits (table from Siemens ATP)
  • 5. MVCBCT  calibra0on   Using Geometry Calibration Phantom
  • 6. MVCBCT  calibra0on   •  •  Since the Linac rotation suffers with gantry sag, imager’s sag etc, the 3D projection matrix deviates from a projection matrix model calculated from transformations between the world and gantry co-ordinates system The calibration matrices are therefore, obtained from the projection images of the geometry calibration phantom for each Linac Geometry calibration co-ordinate system (source: Siemen’s ATP)
  • 7. MVCBCT  calibra0on   •  •  •  Geometry calibration is done every six months or whenever required A projection imaging dataset of the phantom is acquired using the clinical CBCT protocol. On Siemens Oncor machines, a 2000 arc-rotation starting from 2700 to an end-angle of 1100 is used for MVCBCT acquisition
  • 8. Acquiring  Projec0on  matrices   Posi:on  phantom  using  room  lasers     Fine-­‐tune  posi:oning  using  x-­‐re:c   Spend  sa:sfactory  amount  of  :me   Calibra:on  fails  oWen  due  to  poor  alignment   Take  a  cone-­‐beam  acquisi:on  using  clinical  CBCT   protocol   •  AWer  acquisi:on,  each  phantom  projec:on  image   is  processed  to  determine  the  ball-­‐bearings’   posi:ons  and  sizes  rela0ve  to  the  imager’s  co-­‐ ordinate  system     •  •  •  •  • 
  • 9. Post  acquisi0on  screens   Arrangement of projection images after calibration projection matrix is fitted. status message for successful or failed calibration is displayed.
  • 10. Failed  Calibra0on:  many  reasons   •  Phantom  misalignment   •  Incorrect  phantom  orienta:on  (gantry  side   opposite)   •  Object  in  image  (e.g.,  level  leW  on  the   phantom  base)   •  Incorrect  cone-­‐beam  protocol.     ………..  A  very  set-­‐up-­‐sensi:ve  procedure!  
  • 11. MV  CBCT  image  quality   MVCBCT Image Quality phantom and sections for image quality tests in IMA phantom. •  •  •  •  •  •  Geometry accuracy Uniformity Noise Spatial resolution Low contrast resolution, and High contrast resolution
  • 12. MVCBCT: Geometric Accuracy •  •  •  •  •  Tests the geometric accuracy of the MVCBCT reconstruction algorithm Phantom alignment very critical Check alignment usig x-retic all along the white engraved axes lines on the phantom. Axial, sagittal and coronal views of the Adaptive Targeting (AT) task-card are used to locate the beads of interest. The beads’ x, y and z positions should be within ± 2 mm of the actual physical co-ordinates.
  • 13. MVCBCT: Geometric Accuracy expected ranges for localization of beads Localized co-ordinates from a 270-110 cbct protocol
  • 14. MVCBCT: Geometric Accuracy (additional clinical protocols in use)
  • 15. MVCBCT: Image quality: Low contrast resolution section 1 (Clinical relevance of the visualization between this range is visualization of bone, air-cavities, and organs) Passing criteria
  • 16. MVCBCT: Image quality: Low contrast resolution section 2 (Clinical relevance of the visualization between this range is visualization of soft-tissue) Passing criteria
  • 17. MVCBCT: Image Quality: Spatial Resolution Determine smallest visible bar group Criteria for passing: group 6 (0.3 lp/mm)
  • 18. MVCBCT: Image Uniformity, Noise and Artifact Performed using uniform solid water insert standard deviation and across all ROIs uniformity w.r.t the central insert
  • 19. Thank  you!!!!!