Image Registration And Localization Accuracy Of A Micro Igrt System

The μIGRT systems bridge the technology gap between pre-clinical radiation research and clinical radiation therapy. Image registration enables accurate target localization based on reference data used for treatment plan design. The purpose of this research was to quantify the registration and localization accuracy of a μCBCT guided radiation therapy system. Images of a cylindrical phantom with five embedded fiducial markers and two mouse cadavers were acquired with a μCT scanner (eXplore Ultra, GE Healthcare, London, ON) and/or the CBCT scanner of a μIGRT system (XRad 225Cx, Precision Inc X-rays, North Branford, USA). Voxel sizes were 0.15x0.15x0.15 mm3 and 0.2x0.2x0.2 mm3 for the μCT scan and μCBCT scans respectively. Dicom images were imported into an in-house developed image registration software package. Fiducial markers for the phantom and naturally occurring calcifications in the abdomen for the mice were manually identified and their locations were stored in the coordinate system of the irradiator. Corresponding scans were aligned using local rigid registration with a shaped region of interest and correlation-ratio as a similarity measure. For the phantom, the fiducial markers were excluded from the region of interest. After registration the distance between the manually placed landmarks was calculated. To quantify the overall localization accuracy, a cylindrical phantom containing a Gafchromic film with a small pinched hole was scanned on the μIGRT system, imported into the image registration tool as a reference scan and the hole was marked as the treatment iso-center. The phantom was manually displaced, rescanned and imported as the localization scan. Following image registration the phantom was repositioned using a robotic couch adjustment. Finally the film was exposed to 8 equi-angular spaced beams and the distance between the pinched hole and the center of the beam intersection was manually determined. This was repeated 5 times. For the phantom the residual landmark distance was 19 μm ± 94 μm (1 SD) in the axial plane and 121 μm ± 86 μm in the longitudinal direction. For the mice cadavers this was about 40 μm ± 60 μm in all directions. The average localization accuracy was 167 μm ± 72 μm (1 SD). We quantified the registration and localization accuracy of a μIGRT system and observed an overall delivery accuracy better than 200 μm including manual landmark identification. Multi-modality image registration and quantification of the localization accuracy for living specimen are subject of further study.