Modeling liver motion and deformation during the respiratory cycle using intensity-based free-form registration of gated MR images

Mo delinglivermotionanddeformationduringtherespiratorycycleusingintensity-basedfree-formregistrationofgatedMRimagesTorstenRohl nga,Calvin R.Maurer,Jr.Walter G.O'DellbandJianhui ZhongcaDepartmentof Neurological Surgery,bof Radiation OncologycDepartmentof Radiology,UniversityofRo chester,NYABSTRACTInthispap er,wedemonstrateatechniqueformo delinglivermotionduringtherespiratory cycleusingintensity-based free-form deformation registration of gated MR images. We acquired 3-D MR image sets (multislice 2-D) ofthe ab domen of four volunteers at end-inhalation, end-exhalation, and eighttimepoints in b etween using respiratorygating. We computed the deformation eld b etween the images using intensity-based rigid and non-rigid registrationalgorithms. The non-rigid transformation is a free-form deformation with B-spline interp olation b etween uniformly-spaced control p oints. The transformations b etween inhalation and exhalation were visually insp ected. Much of theliver motion is cranial-caudal translation, and thus the rigid transformation captures much of the motion. However,there is still substantial residual deformation of up to 2 cm. The free-form deformation pro duces a motion eld thatapp ears on visual insp ection to b e accurate. This is true for theliver surface, internal liver structures such as thevascular tree, andtheexternal skinsurface. Weconcludethatab dominalorgan motionduetorespiration canbesatisfactorily mo deled using an intensity-based non-rigid 4-D image registration approach. This allows for an easierand p otentially more accurate and patient-sp eci c deformation eld computation than physics-based mo dels usingassumed tissue prop erties and acting forces.Keywords:non-rigidimageregistration,free-formspline-baseddeformation,livermotion,respiration-gatedMRimages1.INTRODUCTIONStereotactic radiosurgery has b een used to treat cranial lesions for more than a decade. This typ e of image-guidedpro cedurematchespreop erativeimagedataandthephysicalspaceo ccupiedbypatientusingfactthatthe brain inside a closed cranium is approximately rigid. We are interested in treating extracranial lesions suchasmetastases and primary tumors in the liver. Ab dominal targets typically move 10-30 mm during relaxed respiration.1,2One approach is to acquire images during respiration to obtain a blurred target and/or to draw a large margin aroundthe lesion to account for p osition uncertainty during respiration. Tissue motion tracking and respiratory gating3,4ofradiotherapy can p otentially allow for increased radiation dose to the tumor whileminimizing the dose to healthytissue.Weareinterestedusingkinematicmo delsoftissuemotiontodetermineanappropriategatingwindowduring which the p osition of the target is known within a sp eci ed excursion.Prior approaches have attempted to generate liver mo dels by registration of anatomical landmarks using thin-platesplines(TPS).5,6Individualpatient-sp eci c geometric mo dels of theliver could then b ematched to theresultingstandardliverusingthesameregistrationtechnique.Thesemetho dshavesomeobviousinherentlimitationsasthey are restricted to discrete landmarks rather than volumetric data. Other groups have derived kinematic mo delsusing nite-elementmo dels(FEMs)ofhumanorgans.Suctechniquesrequiredetailedassumptionsab outtissueprop erties and acting forces. Such information is usually dicult to obtain and also necessarily general rather thanpatient sp eci c. Heart motion can b e measured directly using tagged MR imaging.7,8However, this approachis notfeasible in the liver b ecause the tags do not last long enough (typically less than a second) to capture the relativelyFurther author information: (Send corresp ondence to T.R.)T.R..: Department of Neurological Surgery, Universityof Rochester, 601 Elmwood Avenue, Box 670, Ro chester, NY 14642,torstenrohl ng@urmc.ro chester.edu; C.R.M.: calvinmaurer@urmc.ro chester.edu; W.G.O.: waltero dell@urmc.ro chester.edu;J.Z.: jianhuizhong@urmc.ro chester.edu.