Published on March 2, 2014
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
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