title: Measurement of cone beam CT coincidence with megavoltage isocentre and image sharpness using the QUASAR Penta-Guide phantom.
abstract: For image-guided radiotherapy (IGRT) systems based on cone beam CT (CBCT) integrated into a linear accelerator, the reproducible alignment of imager to x-ray source is critical to the registration of both the x-ray-volumetric image with the megavoltage (MV) beam isocentre and image sharpness. An enhanced method of determining the CBCT to MV isocentre alignment using the QUASAR Penta-Guide phantom was developed which improved both precision and accuracy. This was benchmarked against our existing method which used software and a ball-bearing (BB) phantom provided by Elekta. Additionally, a method of measuring an image sharpness metric (MTF(50)) from the edge response function of a spherical air cavity within the Penta-Guide phantom was developed and its sensitivity was tested by simulating misalignments of the kV imager. Reproducibility testing of the enhanced Penta-Guide method demonstrated a systematic error of <0.2 mm when compared to the BB method with near equivalent random error (s=0.15 mm). The mean MTF(50) for five measurements was 0.278+/-0.004 lp mm(-1) with no applied misalignment. Simulated misalignments exhibited a clear peak in the MTF(50) enabling misalignments greater than 0.4 mm to be detected. The Penta-Guide phantom can be used to precisely measure CBCT-MV coincidence and image sharpness on CBCT-IGRT systems.
authors.affiliation: Medical Physics and Engineering, Leeds Teaching Hospitals Trust, St James's Hospital, Leeds, LS9 7TF, UK. johnathan.sykes@leedsth.nhs.uk
authors.forename: J R
authors.initials: JR
authors.lastname: Sykes
authors.forename: R
authors.initials: R
authors.lastname: Lindsay
authors.forename: C J
authors.initials: CJ
authors.lastname: Dean
authors.forename: D S
authors.initials: DS
authors.lastname: Brettle
authors.forename: D R
authors.initials: DR
authors.lastname: Magee
authors.forename: D I
authors.initials: DI
authors.lastname: Thwaites