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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!!!!!