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 dierences in at 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 elds at dierent photon energies, dose rates, clinically used SIDs, monitor units and eld sizes. For each SID a treatment site containing 4 gain elds 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, imagers 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: Siemens 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 imagers 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!!!!!
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