IGRT:    MV  CBCT  Calibra0on  and  ATP  

on  Siemens  Oncor  Linac    

By    Vibha  Chaswal,  Ph.D.  

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

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.      

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)

MVCBCT  calibra0on  

Using Geometry Calibration Phantom

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)

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

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    

Post  acquisi0on  screens  

Arrangement of projection images after calibration projection matrix is fitted.

status message for successful or failed calibration is displayed.

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!  

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

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.

MVCBCT: Geometric Accuracy

expected ranges for localization of beads

Localized co-ordinates from a 270-110 cbct protocol

MVCBCT: Geometric Accuracy (additional clinical protocols in use)

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

MVCBCT: Image quality: Low contrast resolution section 2 (Clinical relevance of the visualization between this range is visualization of soft-tissue)

Passing criteria

MVCBCT: Image Quality: Spatial Resolution

Determine smallest visible bar group Criteria for passing: group 6 (0.3 lp/mm)

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

Thank  you!!!!!