Incorporation of preprocedural PET into CT-guided radiofrequency ablation of hepatic metastases: a nonrigid image registration validation study

Abstract

This study evaluates the accuracy of augmenting initial intraprocedural computed tomography (CT) during radiofrequency ablation (RFA) of hepatic metastases with preprocedural positron emission tomography (PET) through a hardware-accelerated implementation of an automatic nonrigid PET-CT registration algorithm. The feasibility of augmenting intraprocedural CT with preprocedural PET to improve localization of CT-invisible but PET-positive tumors with images from actual RFA was explored. Preprocedural PET and intraprocedural CT images from 18 cases of hepatic RFA were included. All PET images in the study originated from a hybrid PET/CT scanner, and PET-CT registration was performed in two ways: (1) direct registration of preprocedural PET with intraprocedural CT and (2) indirect registration of preprocedural CT (i.e., the CT of hybrid PET/CT scan) with intraprocedural CT. A hardware-accelerated registration took approximately 2 min. Calculated registration errors were 7.0 and 8.4 mm for the direct and indirect methods, respectively. Overall, the direct registration was found to be statistically not distinct from that performed by a group of clinical experts. The accuracy, execution speed, and compactness of our implementation of nonrigid image registration suggest that existing PET can be overlaid on intraprocedural CT, promising a novel, technically feasible, and clinically viable approach for PET augmentation of CT guidance of RFA.

Fig 1

Proposed workflow for incorporating preprocedural PET into intraprocedural CT guidance of hepatic RFA.

Fig 2

Illustration of the image subdivision scheme of the registration.

Fig 3

Schematic illustration of direct and indirect methods of registering preprocedural PET and intraprocedural CT.

Fig 4

Test landmark and the validation approach. CT_TEST test landmark in CT; T_E1, T_E2, T_E3 transformations of the test landmark determined by experts 1, 2, and 3, respectively; PET_E1, PET_E2, PET_E3 locations of the test landmark identified in PET image space independently by experts 1, 2, and 3, respectively; T_ALGO the test landmark transformation determined by algorithm; PET_ALGO the landmark location predicted by algorithm; PET_EXPERT calculated as average of PET_E1, PET_E2, and PET_E3.

Fig 5

Registration of PET and CT images acquired from separate scanners in a 56-year-old patient with a colorectal tumor and three hepatic metastases. a PET image. b Corresponding CT image before registration. c PET–CT fusion image before registration. d PET–CT fusion image after nonrigid registration. Green rectangle indicates the spatial alignment of edges.

Fig 6

Two examples illustrating the accuracy of nonrigid registration at an intrahepatic location. The case in the left column shows the center alignment of treatment scar. The case in the right column shows the edge alignment of treatment scar and residual tumor. a CT image. b Registered PET image. c PET–CT fusion after registration. The intersection of crosshairs indicates common structures.