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During laparoscopic surgery, insufflation introduces significant deformations to the liver through reshaping the abdominal cavity. Additionally, insufflation is associated with elevation of portal and hepatic venous pressures causing hemodynamic changes that may cause swelling or engorgement of the liver. Current methods for elastic soft tissue registration cannot account for these secondary effects, which have yet remained largely neglected. This paper presents preliminary work towards modeling these intraoperative physiological changes caused by insufflation to improve the accuracy of image-to-physical registration during laparoscopic procedures. Spatially localized modes of internal pressure-driven deformations are introduced to allow reconstruction of intraoperative changes to intrahepatic pressure gradients during registration. A pilot experiment was performed in one sheep to measure volumetric tissue changes induced by insufflation. Liver volume increased by 6.3 ± 0.7% between insufflated and preoperative states, according to three independent raters. Registrations were also performed between organ states, utilizing: (1) rigid registration, (2) deformable registration based on application of external forces to the organ, and (3) the same deformable registration extended to incorporate displacements driven by changes to intrahepatic pressure gradients. Target registration error (TRE) decreased from 15.2 ± 3.2 mm with rigid registration to 5.0 ± 1.9 mm with reconstruction of external boundary forces. When incorporating internal pressure changes, TRE further decreased to 3.0 ± 1.2 mm. Additional analysis comparing a pure scaling approach with the proposed mechanics-based method to account for volume change demonstrated that the method incorporating intrahepatic pressure changes leads to more accurate image-to-physical registrations between the preoperative and insufflated liver.
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